AQA Biology

3.5.1 Photosynthesis

Name: ______________________

Lesson Exam Question Marks

Homework Checked

Date

Lesson 1 – Chloroplast Ultrastructure /24

Lesson 2 –Photosynthetic Pigments and Chromatography

/25

Lesson 3 – Light Dependent Reactions: Photolysis and Photophosphorylation

/14

Lesson 4 – Light Dependent Reactions: The Electron Transport Chain

/18

Lesson 5 – Light Independent Reactions: Calvin Cycle

/22

Lesson 6 – Factors that affect Photosynthesis /22

Specification Section

Key word list for 3.5.1

ATP (adenosine

triphosphate)

Nucleotide found in all living organisms, which is produced during respiration and is important in the transfer of energy

Calvin cycle A biochemical pathway that forms part of the light-independent reaction of photosynthesis, during which carbon dioxide is reduced to form carbohydrate

Hydrogen bond Chemical bond formed between the positive charge on a hydrogen atom and the negative charge on another atom of an adjacent molecule

Hydrolysis The breaking down of large molecules into smaller ones by the addition of water molecules

Granum A stack of thylakoids in a chloroplast that resembles a pile of coins, this is the site of the light-dependent reaction of photosynthesis

Krebs cycle Series of aerobic biochemical reactions in the matrix of the mitochondria of most eukaryotic cells by which energy is obtained through the oxidation of acetylcoenzyme A produced from the breakdown of glucose

Light-dependent

reaction

Stage of photosynthesis in which light energy is required to produce ATP and reduced NADP

Light-independent

reaction

Stage of photosynthesis which does not require light energy directly but does need the products of the light-dependent reaction to reduce carbon dioxide and so form carbohydrate

Limiting factor A variable that limits the rate of a chemical reactionNADP

(nicotinamide adenine

dinucleotide phosphate

A molecule that carries electrons produced in the light-dependent reaction of photosynthesis

Oxidation Chemical reaction involving the loss of electronsOxidation-reduction

A chemical reaction I which electrons are transferred from one substance to another substance. Losing electrons is oxidised, gaining electrons is reduced

Oxidative phosphorylation

The formation of ATP in the electron transport system of aerobic respiration

Palisade cells Long, narrow cells packed with chloroplasts that are found in the upper region of a leaf and which carry out photosynthesis

Pentose A sugar that possesses five carbon atomsPhloem Plant tissue that transports the products of photosynthesis from leaves to the

rest of the plantPhotolysis Slitting of a water molecule by light such as occurs during the light-dependent

reaction of photosynthesisStroma Matrix of a chloroplast where the light-independent reaction of

photosynthesis takes placeSubstrate-level phosphorylation

The formation of ATP by the direct transfer of a phosphate group from a reactive intermediate to ADP

Thylakoid Series of flattened membranous sacs in a chloroplast that contain chlorophyll and the associated molecules needed for the light-dependent reaction of photosynthesis

Yield to supply or produce something positive such as a profit, an amount of food or information.

Lesson 1 – Chloroplast UltrastructureBy the end of this lesson you should be able to:

Identify and explain the function of the ultra-structures of chloroplasts

Notes:Chloroplasts are small flattened organelles between 2-10 µm long, they absorb light to carry out photosynthesis which produces sugar the plant can use for respiration to provide energy. They have a double membrane, the outer membrane is permeable to small molecules (water, carbon dioxide, oxygen) and ions, which diffuse easily but is not permeable to larger proteins. The inner membrane regulates the passage of large substances (sugars and proteins) in and out of the chloroplast through membrane bound transport proteins. The double membrane is evidence for the endosymbiotic theory.

The inside of a chloroplast is filled with a gel-like fluid called stroma, it contains enzymes, starch granules, proteins and chloroplast DNA and ribosomes. Starch granules are used to store the products of photosynthesis.

The stroma contains disc shaped fluid filled sacs made of thylakoid membrane called thylakoids. The inside of the sacs is hollow and known as the thylakoid space, it contains the chlorophyll and other pigments required for absorbing light for photosynthesis as well as the enzymes required for the light dependent reactions. The thylakoids are stacked like coins into structures called grana (sing. granum). The grana are linked by bits of thylakoid membrane called lamellae (sing. Lamella). The thylakoid membranes contain the ATP synthase enzymes required to make ATP in the light dependent reactions.

The light dependent stage occurs in the thylakoids, and the light independent stage occurs in the stroma.

Recall Questions: 1. What type of membrane does the chloroplast have?2. What structures link the grana?3. What is found within the stroma?4. Where does the light dependent stage of photosynthesis take place?5. Where does the light independent stage of photosynthesis take place?6. What structural features of the thylakoids increase rate of photosynthesis?7. What is found within the inner chloroplast membrane?8. Describe the outer chloroplast membrane 9. What is a thylakoid disc?10. What is the function of the starch grain?11. Describe the stroma

12. Calculate the actual size of the chloroplast in the micrograph image.13. What is the function of the stroma?14. What is the function of the thylakoid space?15. Why are lots of ATP synthase molecules present in the thylakoid membranes?

Exam Questions: 1.

2.

3. The diagram shows a chloroplast as seen with an electron microscope.

 

(a)     Name X and Y.

X _________________________________________________________________

Y _________________________________________________________________(2)

(b)     Describe the function of a chloroplast.

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(c)     Calculate the maximum length of this chloroplast in micrometres (μm). Show your working.

 

Answer ____________________ μm(2)

4. The electron micrograph shows part of a chloroplast.

 

(a)     Name the parts labelled A and B and, for each, describe one role in the process of photosynthesis.

A Name ___________________________________________________________

Role ______________________________________________________________

B Name ___________________________________________________________

Role ______________________________________________________________(4)

(b)     (i)      Name the main substance present in the part labelled C.

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(ii)     How is this substance formed?

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5.(a)     Describe how you would use cell fractionation techniques to obtain a sample of

chloroplasts from leaf tissue. Do not include in your answer information about any solutions.

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(b)     The table shows features of a mitochondrion and a chloroplast. Complete the table with ticks where a feature is present.

 

Feature Mitochondrion Chloroplast

Double outer membrane    

Starch grains    

Diffusion of oxygen into the organelle    

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Homework Read Green Genes article and answer questions 1-4:

Questions: 1. Given that chloroplasts evolved from freeliving organisms, what characteristics (in addition to

biosynthesis) would you expect them to display?

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2. Although many cultivated plants have green and white variegation (for example many garden varieties of ivy, holly and privet), very few wild plants have this characteristic. Suggest two reasons for this observation?

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3. Biologists believe that mitochondria also evolved from engulfed bacteria (of a different type, called the ‘purple bacteria’). Considering the structure of plant and animal cells, is it more likely that the chloroplasts evolved before mitochondria, or vice versa? Give reasons for your answer.

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4. What are the advantages of sexual reproduction, and therefore of having chloroplast genes in the nucleus?

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Recall Question Answers: 1. What type of membrane does the

chloroplast have? Double membrane2. What structures link the grana? Lamellea (part of thylakoid membrane)3. What is found within the stroma? Starch and enzymes4. Where does the light dependent stage of

photosynthesis take place? Thylakoid membrane5. Where does the light independent stage of

photosynthesis take place? Stroma6. What structural features of the thylakoids

increase rate of photosynthesis?Stacked into granum/increased surface area/maximum absorption of light in membrane

7. What is found within the inner chloroplast membrane?

Contains many membrane bound transport proteins to control the passage of substances (sugars and proteins) in and out of the chloroplast.

8. Describe the outer chloroplast membrane It is freely permeable to molecules such as CO2 and H20 but not large proteins

9. What is a thylakoid disc?A system of interconnected, flattened fluid filled sacs with proteins embedded in the membranes

10. What is the function of the starch grain? Stores the product of photosynthesis

11. Describe the stroma

Gel-like fluid surrounding the thylakoids. contain all the enzymes, sugars and organic acids needed for the light independent reaction.

12. Calculate the actual size of the chloroplast in μm from the micrograph image.

Actual size = Image size/magnificationOr calculate using scale bar

13. Why do thylakoids have a large surface area?

To allow as much light energy to be absorbed as possible

14. What is the function of the stroma?fluid matrix inside a chloroplast, contains enzymes for light independent stage

15. What is the function of the thylakoid space?

Contains photosynthetic pigments and enzymes for light dependent reactions.

16. Why are lots of ATP synthase enzymes present in the thylakoid membranes? To produce ATP in the light dependent reaction

Lesson 2 –Photosynthetic Pigments and ChromatographyBy the end of this lesson you should be able to:

State the precise location of the pigments involved in photosynthesis within the leaf Outline the role of the leaf pigments in photosynthesis Explain why many plants have a variety of photosynthetic pigments Describe how to investigate photosynthetic pigments using chromatography

Notes:Photosynthetic Pigments

Photosynthetic pigments are arranged in structures called photosystems. These can also be referred to as light harvesting systems or reaction centres. Photosystems are transmembrane protein complexes found in the thylakoid membranes (thylakoids (grana) and lamellae)

Photosystems contain coloured photosynthetic pigments which absorb the light energy needed for photosynthesis, there is a chlorophyll a molecule at the centre of each photosystem, chlorophyll absorbs red wavelengths of light. There are two main photosystems – Photosystem I (PSI) contains a chlorophyll molecule which absorbs light with a wavelength of 700nm best, photosystem II (PSII) contains a chlorophyll molecule which absorbs light with a wavelength of 680nm best.

Plants have several photosynthetic pigments (other than chlorophyll) in their leaves because each pigment absorbs different wavelengths of light. Having more pigments increases the range of wavelengths that plants can absorb so it increases their ability to photosynthesise. In the photosystems these accessory pigments are not directly involved in the light dependent reactions but they channel more captured light energy to chlorophyll so more electrons can be excited.

Some plants produce high levels of anthocyanins, dark red and purple pigments which can help protect the plant from high UV radiation. These pigments are also used to colour flowers and fruits e.g blueberries. Different species of plants will have different proportions of photosynthetic pigments in their leaves.

All photosystems have a similar structure but all absorb different wavelengths of light. Pigments include: • chlorophyll a• chlorophyll b• Xanthophyll• Carotene

• Pheophytin (a and b) – electron carriers, part of the electron transport chain. Chromatography is a method used to separate molecules in mixtures. The molecules are dissolved and move through the mobile phase (a liquid solvent) and they get fixed to a stationary phase. In thin-layer chromatography the stationary phase is a thin layer of solid coating on a plastic plate. Different molecules will spend different amounts of time in the mobile or stationary phase. The longer the components spend in the mobile phase the faster or further up the plate they move, this allows them to be separated out. The rates of migration of individual pigments will depend on three things:

1. Solubility - Pigments (solutes) dissolve in the solvent - The more soluble the pigment is in the solvent the further it will travel2. Affinity to stationary phase - Pigment molecules interact with the plate - Molecules which interact more strongly with the plate will not travel as far3. Mass - Pigment molecules are also separated by size - The smaller the molecule the further it will travel (this is less important)

The distance the substance moved through the stationary phase relative to the distance the solvent moved can be calculated to give an Rf value. Each photosynthetic pigment will have a specific Rf value when run under certain conditions (solvent and stationary phase used). Pigments that travel further up the paper will have a higher Rf value. Rf values can be compared to known Rf values in a database to identify the pigment. Other characteristics of the chemical may also help identification – e.g. the colour of photosynthetic pigments.

However, more than one substance can have the same Rf value for a particular solvent and chromatography paper and so it’s possible that multiple chromatograms will need to be run with different solvents (and/or chromatography paper) in order to find out the exact identity of the substance.

Solvents will compete with compounds for sites on the stationary phase, chromatography works because the stationary phase will always be more polar than the mobile phase. This is why it is important to use a non-polar solvent where possible because a less polar solvent will not compete well, allowing the compounds to remain bound to the stationary phase. A polar solvent will compete well with molecules and will occupy sites on the stationary phase. This will force compounds into the mobile phase, and result in an increased travel distance.

A chromatogram with measurements needed to calculate an Rf value

Chromatography Method

1) Tear up a sample of plant leaves into small pieces, place in a mortar with a pinch of sand, and grind with a pestle.

2) Add solvent and continue to grind with a pestle. It is important that the tissue be ground up in order to break open plant cells and release the chloroplasts.

3) Gently draw a pencil line 1cm from the base of the chromatography paper. The pencil line marks the starting position of the pigments. It is important it is in pencil so that the line doesn’t move with the solvent and won’t mask the results of the chromatography.

4) Repeatedly spot a small quantity of the leaf extract onto the centre of the line, allowing the spot to dry between each application. The spot is made with small quantities through repeated spotting and drying in order to build up a concentrated, but still small, spot.

5) Suspend the chromatography plate/paper from a bung in the glass vial with 1cm depth of solvent in the bottom. Ensure that the chromatography plate dips into the solvent but the spot of leaf extract remains above the surface of the solvent. The spot needs to be above the surface of the solvent so that the pigments travel up the plate and don’t just dissolve out into the solvent in the vial.

6) Place a lid on the container. Chemicals used for solvents are very volatile and the vapours can be hazardous.

7) Allow the solvent to run up the chromatography plate until the solvent front is near the bung then remove the plate, mark the location of the solvent front with a pencil, and allow to dry. The solvent evaporates quickly and so the extent to which it has travelled up the plate needs to be marked whilst it can be seen otherwise the distance will be unknown and the RF value cannot be calculated. Drying ensures that the movement of solvent and pigments stops.

8) The resultant chromatography plate is called a chromatogram which can then be photographed or the location of each separated pigment can be marked with pencil. The pigments can be affected by light and fade so the measurements need to be taken quickly and the spots marked so they can be identified once the spot fades.

Recall Questions: 1. What is a photosynthetic pigment?2. Where are photosynthetic pigments found?3. How are photosynthetic pigments arranged?4. What is the main photosynthetic pigment?5. Why do photosystems contain accessory pigments?6. Why does chlorophyll appear green?7. Why is it important to use an organic (non-polar) solvent and not a polar one like water?8. Why is it important to mark positions in pencil rather than pen (particularly the starting position of the

concentrated extract)?9. Why is repeated application of the plant extract onto the same spot required? 10. Why is it important that the spot of plant extract is above the surface of the solvent when the

chromatography plate is placed in the vial?11. Why is it important that the chromatogram is stopped before the solvent front reaches the top? 12. Why is it important to mark the solvent front quickly?13. Why is it useful to mark the positions of the pigment spots or take a photograph of the

chromatogram soon after it has been run?

Exam Questions:1.

2. Beech trees have two types of leaves called sun leaves and shade leaves. Sun leaves grow on branches exposed to direct sunlight, shade leaves grow on branches exposed to light that has passed through leaves. An ecologist collected sun leaves and shade leaves from beech trees and determined the mean mass of each photosynthetic pigment in both types of leaf. His results are shown the table below. 

Photosynthetic pigment

Mean mass of each pigment per m2 of leaf area / μg(± standard deviation)

Sun leaves Shade leaves

Chlorophyll a 299.3 (± 2.1) 288.9 (± 0.1)

Chlorophyll b 290.7 (± 2.1) 111.1 (± 0.1)

Chlorophyll c 0.10 (± 0.01) 0.07 (± 0.01)

(a)     Describe how you would present the data in the table as a graph.

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(b)     The ecologist collected shade leaves at random from a branch.Suggest a method he could have used to collect shade leaves at random from a branch.

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I     The ecologist concluded that there is a significant difference between the amounts of chlorophyll b in sun leaves and shade leaves of beech trees.

Do you agree with this conclusion?

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3. The graph shows the absorption of different wavelengths of light by three photosynthetic pigments in a red seaweed.

 

(a)     Describe what the graph shows about the properties of chlorophyll a.

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(b)     The red seaweed lives under water at a depth of 2 metres. Suggest an advantage to the red seaweed of having other pigments in addition to chlorophyll a.

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4. A student investigated the colours in three different flowers, A, B and C.

The colours are soluble in ethanol but are insoluble in water.

This is the method used.

1.   Crush flower A.2.   Add ethanol to flower A.3.   Filter the mixture.4.   Put spots of the coloured filtrate on to the chromatography paper.5.   Repeat steps 1-4 with flowers B and C.

Figure 1 shows the apparatus used.

 

(a)  The student made two mistakes in setting up the apparatus.

Give one problem caused by each mistake.

Mistake 1 __________________________________________________________

Problem caused _____________________________________________________

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Mistake 2 __________________________________________________________

Problem caused _____________________________________________________

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(b)  Another student set up the apparatus correctly.

Figure 2 represents the student’s results.

 

Give two conclusions you can make from Figure 2.

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I  Colour A has an Rf value of 0.65

Colour A moves 3.2 cm

Calculate the distance moved by the solvent.

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Distance moved by the solvent = __________________ cm(2)

5. The dinitrobenzenes shown were investigated by thin layer chromatography (TLC).

In an experiment, carried out in a fume cupboard, a concentrated solution of pure 1,4-dinitrobenzene was

spotted on a TLC plate coated with a solid that contains polar bonds. Hexane was used as the

solvent in a beaker with a lid.

The start line, drawn in pencil, the final position of the spot and the final solvent front are shown on the chromatogram in the diagram below

 

Use the chromatogram in the diagram above to deduce the Rf value of 1,4-dinitrobenzene in this experiment.

Tick (✔) one box. 

A 0.41

B 0.46

C 0.52

D 0.62

(d)     State in general terms what determines the distance travelled by a spot in TLC.

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I     To obtain the chromatogram, the TLC plate was held by the edges and placed in the solvent in the beaker in the fume cupboard. The lid was then replaced on the beaker.

Give one other practical requirement when placing the plate in the beaker.

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Homework:Read the extracts from the paper below:Panagiota Karageorgou, Yiannis Manetas, The importance of being red when young: anthocyanins and the protection of young leaves of Quercus coccifera from insect herbivory and excess light, Tree Physiology, Volume 26, Issue 5, May 2006, Pages 613–621,

Introduction: It is generally accepted that anthocyanins (together with other pigments) in flowers and fruits provide optical guides to animals facilitating pollination and seed dispersal. Anthocyanins are also present in leaves and in some cases their concentrations are high enough to mask the green coloration due to chlorophyll. Because anthocyanins absorb visible light without participating in photosynthesis, their presence in leaves should reduce the probability of photon capture by chlorophylls thereby lowering photosynthesis, an effect that may be of adaptive significance. However, the possible role(s) of anthocyanins in leaves is obscure as is reflected in the numerous hypotheses that have been proposed concerning their function(s). Foliar anthocyanins have been correlated with resistance to biotic and abiotic agents like fungi, herbivores, cold and excess radiation, both UV and visible (see reviews by Chalker-Scott 1999, Gould et al. 2000, Hoch et al. 2001, Gould et al. 2002, Steyn et al. 2002, Close and Beadle 2003).

A second hypothetical function of foliar anthocyanins is that they participate in defense mechanisms against herbivory. Insects may show preferences for healthy green leaves for food or oviposition (Prokopy and Owens 1983). In addition, anthocyanin-rich leaves may contain high concentrations of other phenolic compounds, as found in young leaves of some tropical plants (Lee and Lowry 1980) and Eucalyptus seedlings (Close et al. 2001), because anthocyanins share common initial steps with other phenolics in the phenylpropanoid biosynthetic pathway (Winkel-Shirley 2002). Some phenolics serve as potent deterrents against generalist herbivores (Feeny 1970, Bennett and Wallsgrove 1994) and it has been proposed that the presence of anthocyanins in old, senescing leaves may be an optical warning signal (handicap signal) against consumers, indicating the co-occurrence of potentially defensive phenolic compounds (Hamilton and Brown 2001, Lev-Yadun 2001).

Young leaves of many plants are transiently red because of the accumulation of anthocyanins, with the redness disappearing as leaves mature. Among the many hypothetical functions of foliar anthocyanins, two are tested in this field study: the sunscreen photoprotective function against excess visible light and the handicap signal against herbivory. We took advantage of intraspecies variation in anthocyanin concentrations of young leaves of Quercus coccifera L. to compare in vivo chlorophyll fluorescence parameters, reflectance spectra, total phenolics and the extent of herbivory of leaves of red- and green-leaved phenotypes occupying the same habitat.

Methods and Hypothesis: We conducted field tests of both the photoprotective and the handicap signal hypotheses in the Mediterranean evergreen sclerophyll Quercus coccifera L., which displays intraspecies variation in anthocyanin concentrations in young, developing leaves, whereas mature leaves are invariably green. Red- and green-leaved individuals occupy the same habitat, providing the opportunity to compare aspects of photosynthesis as well as the extent of herbivory and phenolic concentrations in plants subjected to the same set of environmental parameters. We predicted that, if the primary role of anthocyanins were photoprotective, red leaves would have higher PSII photochemical efficiencies than green red leaves. Alternatively, less damage by insects to red leaves than to green leaves would favour the antiherbivore hypothesis.

Summary of Results:

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Photosystem II (PSII) photochemical efficiencies obtained at various photon fluence rates of red light were similar in green and red leaves. In white light, PSII efficiencies were slightly higher in red leaves than in green leaves, indicating a slight photoprotective role of anthocyanins in the field. However, compared with red phenotypes, green phenotypes suffered greater herbivore damage, as judged by the number of leaves attacked and the area lost to herbivory. In addition, there was a positive correlation between the concentrations of anthocyanins and total phenolics. We suggest that the importance of a photoprotective anthocyanic screen is low in thin, young leaves with low chlorophyll concentrations because the green light attenuated by anthocyanins is less significant for chlorophyll excitation. However, the decreased reflectance in the green spectral band and the concomitant levelling of reflectance throughout the 400–570 nm spectral range may either make red leaves less discernible to some insect herbivores or make insect herbivores more discernible to predators, or both. Moreover, excessive herbivory may be additionally discouraged by the high phenolic concentrations in red leaves.

Answer the questions: 1. How did the researchers try to reduce

environmental effects?

2. Describe what Figure 4a shows you must discuss error bars and P values in your answer

3. Do the results support the researchers conclusions?

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4. What reasons do the researchers give for decreased herbivory in red leaves.

Recall Question Answers:

1. What is a photosynthetic pigment?Coloured molecules that absorb the light energy needed for photosynthesis

2. Where are photosynthetic pigments found? In the thylakoid membrane

3. How are photosynthetic pigments arranged? Into photosystems in the thylakoid membane

4. What is the main photosynthetic pigment? Chlorophyll a

5. Why do photosystems contain accessory pigments?

To help absorb the wavelengths of light that are not easily absorbed by chlorophyll (maximise light absorption)

6. Why does chlorophyll appear green?

It absorbs light at two wavelengths 430nm (blue) and 662nm (red). It reflects green light strongly so it appears green.

7. Why is it important to use an organic (non-polar) solvent and not a polar one like water?

A polar solvent could compete with the dissolved molecules for space on the stationary phase and could cause them to move further than they should do, giving the incorrect Rf value.

8. Why is it important to mark positions in pencil rather than pen (particularly the starting position of the concentrated extract)?

Ink may dissolve in the solvent move up the stationary phase. Colours could affect the results and there will be no visible start line to measure RF value from.

9. Why is it important that the spot of plant extract is above the surface of the solvent when the chromatography plate is placed in the vial?

To ensure the pigments are carried up by the solvent and don’t just dissolve out into the solvent at the bottom.

10. Why is repeated application of the plant extract onto the same spot required? To increase the concentration of the pigments

11. Why is it important that the chromatogram is stopped before the solvent front reaches the top?

So that the distance travelled by the solvent (solvent front) can be identified and drawn – needed for the Rf value calculatoin

12. Why is it important to mark the solvent front quickly?

The solvent is volatile and will evaporate quickly, once dry it can’t be seen.

13. Why is it useful to mark the positions of the pigment spots or take a photograph of the chromatogram soon after it has been run? Light can cause them to fade.

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Lesson 3 – Light Dependent Reactions: Photolysis and Photophosphorylation

By the end of this lesson you should be able to: • Describe how light energy is used to excite electrons in photosynthetic pigments• Describe the importance of photolysis of water to replace electrons in PSII

Notes: Photosynthesis is a metabolic pathway (it takes place in a series of enzyme-controlled reactions) it can be split into two main parts that take part in two different places in the chloroplast.

The light-dependent reactions take place in the thylakoid membrane (grana/thylakoids and lamellae), they require light energy absorbed by chlorophyll to excite electrons which raises their energy level and that energy is released to produce a small amount of ATP from ADP (photophosphorylation).

The light-independent reactions take place in the stroma and it uses the products of the light-dependent reactions along with CO2 to make glucose (chemical energy).

Photoionisation describes the process of using light energy to excite electrons in chlorophyll enough so that they can leave the molecule. It leaves the chlorophyll as a positively charged ion.

Light energy is passed to the reaction centre chlorophyll molecule in the photosystem. Electrons are raised to higher energy level – they are said to be in an ‘excited state’. The electrons leave and they can get taken up by a molecule called an electron carrier or electron acceptor.As the chlorophyll has lost electrons it has been oxidisedThe electron carrier has gained electrons – it has been reduced.

When photosystem II is left positively charged – the electrons must be replaced. They are provided from the photolysis of water. The energy from light is used to split water into protons (H+ ions), electrons and oxygen. Only PSII can split water as it has the correct enzyme complex.

Recall Questions: 1. What happens to light energy when it enters the

chloroplast?2. What happens when the light energy is absorbed by PSI and PSII ?3. What is photoionisation?

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4. What charge will chlorophyll molecules have after they have been “excited” by light absorption?5. What happens to excited electrons?6. What is produced from the photolysis of water?7. What do the electrons produced by photolysis of water do? 8. Why does photolysis only occur in PSII?

Exam Questions: Q1. The diagram shows the structure of a chloroplast.

 

(a)     Label the diagram with an X to show where the light-dependent reactions take place and with a Y to show where the light-independent reactions take place.

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(b)     The photolysis of water is an important part of the process of photosynthesis. Describe what happens in the photolysis of water.

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Q2. Describe the effect of light energy in the light-dependent reaction of photosynthesis.

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Q3.(a)     (i)      Give two products of the light-dependent stage of photosynthesis.

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(ii)     The products of the light-dependent stage are used in the light-independent stage of photosynthesis. What are these products used for?

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Q4. The diagram shows the light-dependent reactions of photosynthesis.

 

(a)     In which part of a chloroplast do the light-dependent reactions occur?

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(b)     Use information in the diagram to explain the role of chlorophyll in photolysis;

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Q5. Some bacteria use hydrogen ulphide, H2S, to produce organic compounds.The hydrogen ulphide has a similar role to that of water in photosynthesis.

A simple equation for this process in bacteria is shown below:

hydrogen ulphide + carbon dioxide → glucose + sulfur + water

Suggest what the hydrogen ulphide is used for in these bacteria.

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Recall Question Answers: 1. What happens to light energy when it enters

the chloroplast?Absorbed by the photosynthetic pigments in the photosystems and converted to chemical energy

2. What happens when the light energy is absorbed by PSI and PSII ?

Energy from light raises two electrons in each chlorophyll molecule to a higher energy level. The chlorophyll molecules are now in an excited state

3. What is photoionisation? When light causes electrons to become excited and leave chlorophyll

4. What charge will chlorophyll molecules have after they have been “excited” by light absorption?

Positive (they have been oxidised as they have lost electrons)

5. What happens to excited electrons? They leave the excited PSII and pass along the electron transport chain via electron carrier molecules in a series of redox reactions, losing energy in the process. They move towards PSI

6. What is produced from the photolysis of water? Two hydrogen ions, one oxygen, two electrons

7. What do the electrons produced by photolysis of water do? Replace the electrons that have left PSII

8. Why does photolysis only occur in PSII? Because only PSII has the enzymes

Homework:

Part I Watch the video: https://www.bbc.co.uk/programmes/p011sq1m and answer the questions below:

1. Why do plants not produce oxygen when in the dark?

2. How did Ingenhousz confirm the gas produced by plants was oxygen?

3. How did Ingenhouszs’ experimental design help make his conclusions reliable?

4. Explain how plants produce oxygen during photosynthesis.

Part II Watch the video: https://www.bbc.co.uk/programmes/p011mdbm and answer the questions below:

1. What part of the photosynthetic process are the scientists trying to replicate in the lab?

2. What chemical method are the scientists using to produce hydrogen and oxygen in the video?

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3. How is the method the scientists are using different from photosynthesis?

4. What could this help with in the future if they achieve it?

Lesson 4 – Light Dependent Reactions: The Electron Transport Chain

By the end of this lesson you should be able to: • Describe the process of cyclic and non-cyclic photophosphorylation

Notes:Redox reactions reminder! – OILRIG

oxidation is loss of electrons or protons OR gained oxygen reduction is gain of electrons or protons OR lost oxygen redox reactions occur together, oxidation of one molecule always involves reduction of another.

Co-enzymesHydrogen or electrons are transported by the coenzyme NADP, which transfers them from one molecule to another. NADP becomes NADPH or reduced NADP when it collects a hydrogen and it can then reduce other molecules.

Phosphorylation and ATPAdenosine triphosphate (ATP) is soluble and easily diffuses around the cell to provide energy for cellular processes e.g active transport, DNA replication, cell division, protein synthesis photosynthesis. It can be broken down and remade with a simple reaction.

ATP is synthesised using an enzyme called ATP synthase which catalyses the condensation reaction between ADP and an inorganic phosphate. Adding a phosphate molecule is phosphorylation. In this case, light is used as an energy source, so the process is called photophosphorylation.

The process of photophosphorylationATP is produced by adding an inorganic phosphate to ADP. In photosynthesis energy from excited electrons and H+ ions generated from photolysis are used to phosphorylate ADP hence the “photo”. The two photosystems are linked by electron carriers, these are proteins which can

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transfer electrons between them using a series of redox reactions. The chain of photosystems and proteins that electrons flow through in the thylakoid membranes are known as an electron transport chain. There are 2 types of photophosphorylation: cyclic and non-cyclic: Non-cyclic involves PSII and PSI and cyclic involves in just PSI.

Non-Cyclic: 1. Chlorophyll in PSII absorbs light energy which excites electrons. The electrons leave and move

along the electron transport chain to PSI releasing energy. (photolysis of water replaces electrons and also produces O2 and protons)

2. The energy lost from electrons is used to transport H+ ions from the stroma into the thylakoid space to create a proton gradient across the thylakoid membrane. The protons diffuse down their concentration gradient into the stroma via the ATP synthase enzyme. This process is known as chemiosmosis, or the chemiosmotic theory. (Unless specifically stated in the question you will not need to describe this process in exam questions relating to photosynthesis)

3. Photosystem I absorbs the electrons that came from photosystem II, they are excited by light and move through some more electron carrier proteins.

4

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4. The electrons are finally taken up by NADP along with protons to form reduced NADP which moves into the stroma for the light independent reactions.

Cyclic photophosphorylation: The same process applies where electrons leave PSI and move down the ETC releasing energy used to actively transport H+ ions. It is cyclic because the electrons at PSI cannot be replaced by photolysis, so some are not passed to NADP and they flow back to PSI through the ETC. The electrons are constantly recycled and can repeatedly flow back through PSI. This process doesn’t produce any reduced NADP or Oxygen and it only produces small amounts of ATP compared to non-cyclic.

To summarise: chlorophyll absorbs light energy which is used to excite electrons the electrons move along the electron transport chain releasing energy the electrons are replaced through the photolysis of water which also produces O2 and protons energy from the electrons is used to join ADP and inorganic phosphate to generate ATP NADP is reduced (becomes NADPH) when it accepts electrons and protons the products of the light dependent reactions are therefore:

o ATP (from photophosphorylation, goes to the light-independent reactions)o oxygen (from photolysis, leaves the leaf or is used in respiration)o reduced NADP (from photophosphorylation, goes to the light-independent reactions)

Recall Questions:1. What are electron carriers?2. What is the electron transport chain?3. How do electrons move along the electron transport chain in photosynthesis?4. What happens to the electrons as they move along the electron transport chain?5. What happens to the electrons when they leave the electron transport chain (photosynthesis)? 6. Name the coenzyme in the light dependent reactions. 7. How does energy from the excited electrons generate NADPH?8. What is the structure of ATP?9. How is ATP made from ADP?10. What is photophosphorylation?11. State the name for the synthesis of ATP using light12. What does the hydrolysis of ATP provide?13. Why are lots of ATP synthase molecules present in the thylakoid membranes?14. How do H+ help to generate ATP? 15. What are the products of the light dependent reaction?16. Where do the products of the light dependent reaction go?

Exam Questions:

Q1. (a)     Write a simple equation to show how ATP is synthesised from ADP.

___________________________________________________________________(1)

(b) Give two ways in which the properties of ATP make it a suitable source of energy in biological processes.

1. _________________________________________________________________

2. _________________________________________________________________ (2)

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Q2. The diagram shows some of the stages in one of the processes that produce ATP.

          In Process 1, what causes substance X to lose electrons (e–)?

___________________________________________________________________

(1)

Q3. Describe the role of electron transport chains in the light-dependent reactions of photosynthesis

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Q4. The diagram shows the light-dependent reactions of photosynthesis.

 

(a)     Name the substances in boxes A, B and C.

A ________________________________

B _______________ + _______________

C ________________________________(3)

(b)     Use information in the diagram to explain how the energy of light is converted into chemical energy in the light-dependent reactions.

______________________________________________________________

______________________________________________________________

______________________________________________________________

______________________________________________________________ (3)

Q5. Energy enters most ecosystems through the light-dependent reaction of photosynthesis. Describe what happens during the light-dependent reaction.

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

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___________________________________________________________________(5)

Recall Question Answers:1. What are electron carriers? Proteins that transfer electrons2. Name the coenzyme in the light dependent

reactionsNADP

3. What is the electron transport chain? A chain of proteins through which excited electrons flow. Formed from the photosystems and electron carriers.

4. How do electrons move along the electron transport chain in photosynthesis? A series of redox reactions

5. What happens to the electrons as they move along the electron transport chain?

They lose energy which is used to transport H+ into the thylakoids so that the thylakoid has a higher conc of H+ than the stroma, forming a proton gradient.

6. What happens to the electrons when they leave the electron transport chain?

Combine with one hydrogen ion and NADP to form NADPH

7. How does energy from the excited electrons generate NADPH ?

PSI absorb light energy which excites the electrons to an even higher energy level. The electrons and H+ from the stroma are transferred to NADP to make NADPH

8. What is the structure of ATP? An adenine base, a ribose sugar and 3 phosphate groups

9. How is ATP made from ADP? By adding an inorganic phosphate – condensation reaction

10. What is phosphorylation? The transfer of a phosphate group form one molecule to another

11. State the name for the synthesis of ATP using light Photophosphorylation

12. What does the hydrolysis of ATP provide? An immediate supply of energy for biological processes

13. Why are lots of ATP synthase molecules present in the thylakoid membranes?

To produce ATP in the light dependent reaction

14. How do H+ help to generate ATP? H+ move down their concentration gradient, into the stroma via the enzyme ATP synthase. The energy from this movement combines ADP with Pi to form ATP

15. What are the products of the light dependent reaction?

ElectronsH+ ionsOxygen moleculeATPNADPH

16. Where do the products of the light dependent reaction go?

Electrons (from photolysis) go to PSII H+ ions go across membrane and through ATP synthase and used to reduce NADPOxygen diffuses out of the leaf or is used in respirationATP used in light independent reactionNADPH goes to light independent reaction (stroma)

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Homework:Why is the Hill reaction important in the elucidation of the mechanism of photosynthesis?

The Hill Reaction of the Photosynthesis was found by Robert Hill (1939).He found that isolated chloroplasts from plants can discharge oxygen (O2 ) when they are illuminated by sunlight (or by a light source same to sunlight) in the presence of suitable electron acceptor such as Ferricyanide. Dichlorophenolindophenol (DCPIP) can be used to demonstrate the hill reaction in the laboratory. During this reaction ferricyanide is reduced to ferrocyanide while H2O is oxidized into O2 .. By using this reaction, Robert Hill revealed some valuable facts on Photosynthesis and these facts can be described well by this experiment.

The Hill reaction confirms that Oxygen is discharged by a separate reaction from CO2 fixation. This Oxygen discharging reaction needs solar energy to discharge oxygen. Therefore it can be described as Light Dependant Reaction of photosynthesis. Chloroplasts are responsible for taken place this Oxygen emitting reaction and Hill reaction further confirms that O2 emission reaction is only one step (or only partial reaction) of photosynthesis which is happened before the carbon dioxide fixation step. So Oxygen evolving can happen without reduction of CO2.

The Hill reaction suggests that Light Dependent Reaction of photosynthesis is a result of a series of redox reactions. There should be a suitable electron acceptor to occur the reaction. So plants have natural electron acceptors to reduce water in to oxygen in order to remove oxygen. Another important fact is that the Hill reaction tells us that these natural electron acceptors can be substituted by artificial electron acceptors (such as DCPIP) and the oxygen evolving reaction can occur with the artificial electron acceptors also. Another fact that can be described by using the Hill Reaction is evolved oxygen is coming from H 2O not from CO2.

The Hill reaction also explains that the first reaction of the photosynthesis is Light Dependant redox reaction. An Electron Transport System (ETS) is used to gain the excited energy of electrons during a series of oxidation and reduction reactions along the ETS. Electrons are travelled against a chemical potential gradient by occurring the Chemiosmosis for photophosphorylation. Solar energy is converted into chemical energy of reduced coenzymes (such as NADPH) at the Light Dependent reaction while Oxygen is evolved as a by-product of the light dependent step. All the above facts could be clearly described by the Hill experiment.

The Theory behind the Hill Reaction

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The Hill reaction is an experiment which investigates the light dependent reactions of photosynthesis which take place in the thylakoid membranes of chloroplasts. Chloroplasts are isolated from plant cells and exposed to light. The name ‘Hill’ comes from the scientist Robert Hill who developed the procedure and showed that isolated chloroplasts would continue to perform some of the reactions of photosynthesis if provided with the correct conditions in vitro. The Hill Reaction depends on electrons released during the light dependent stage of photosynthesis being picked up by the blue electron acceptor DCPIP. The reaction can only occur if the thylakoid membranes are illuminated as the light dependent stage stops in the dark. DCPIP is blue when oxidised (at pH 7.0) and colourless when reduced, so it is possible to monitor the loss of blue colour as an indication that DCPIP has accepted electrons. It can be used to participate in, and monitor, redox reactions. In this experiment the DCPIP takes the place of NADP, allowing photolysis to continue even when the supply of NADP has been exhausted because the DCPIP can continue to accept the electrons from the electron transport chain.

The process of isolating the chloroplasts will inevitably cause some damage to the chloroplasts. The experiment uses a mixture of intact chloroplasts and thylakoid membranes without the surrounding stroma and outer membranes. This is actually an advantage because it means that the DCPIP can access the thylakoid membranes directly, without having to pass through the outer membranes, to accept electrons directly from the electron transport chain.

Read the two extracts and answer the following questions: 1. Describe how you could collect a solution of chloroplasts to use for the Hill reaction experiment.

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

2. Explain conditions that must be controlled in order for the light dependent reactions to continue during the experiment.

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

3. Explain the role of DCPIP in the experiment

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

4. Why would an ice cold buffer need to be used in this experiment? (N.B you must talk about both temperature and pH in your answer)

___________________________________________________________________

___________________________________________________________________

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___________________________________________________________________

___________________________________________________________________

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5. The percentage of light absorbed by an aquatic plant was measured when it was exposed to different wavelengths. The rate of photosynthesis was also measured at each wavelength of light.

(b)     Give one dependent variable you could measure in order to determine the rate of photosynthesis in an aquatic plant.

___________________________________________________________________(1)

(d)     A suspension of chloroplasts was isolated from an aquatic plant and a reagent was added. The reagent is blue when oxidised and is colourless when reduced.

(i)      The suspension of chloroplasts in blue reagent was exposed to sunlight. The blue colour disappeared. Use your knowledge of the light-dependent reactions of photosynthesis to explain why.

______________________________________________________________

______________________________________________________________

______________________________________________________________

______________________________________________________________(2)

(ii)     Another suspension of chloroplasts was set up as before. Small quantities of ADP and phosphate ions were added and then the tube was exposed to light. The blue colour disappeared more quickly. Explain why.

______________________________________________________________

______________________________________________________________

______________________________________________________________

______________________________________________________________(2)

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Lesson 5 – Light Independent Reactions: Calvin Cycle

By the end of this lesson you should be able to: • State the three stages of the Calvin cycle• Describe the process of carboxylation and state that the reaction is catalysed by rubisco. • Explain that the reduction of GP to triose phosphate is caused by products from the light-

dependent reaction.

Notes: The light-independent reaction is the second stage of photosynthesis, the light independent reactions are also known as the Calvin cycle and take place in the stroma of chloroplasts. It can happen without light, but it does require the products of the light dependent reaction. The Calvin cycle uses carbon dioxide to produce two molecules of triose phosphate (TP) which can be used to make glucose and other organic substances.

CarboxylationCO₂ enters the leaf through the stomata and diffuses into the stroma. It combines with RuBP (a 5C compound). This requires the use of an enzyme rubisco This produces an unstable 6C compound. The 6C compound quickly splits into two molecules of a 3C compound called glycerate 3-phosphate (GP).

ReductionGP in reduced into a different 3C compound called triose phosphate (TP). ATP and the H+ ions (provided by reduced NADP) from the light-dependent reaction are needed to do this.

Regeneration5 out of 6 molecules of TP produced are used to regenerate RuBP. The rest of the ATP produced in the light-dependent reaction is used to regenerate RuBP.

In sumamry: 1. CO2 combines with RuBP this is

catalysed by the enzyme rubisco2. Two molecules of GP are produced3. GP is reduced to TP using energy

from ATP and reduced NADP from the light dependent reaction

4. TP is converted into glucose and other useful organic compounds

5. RuBP is regenerated using some TP and another ATP

Products of the Calvin cycle:

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1. NADP (passes back into the light-dependent reaction)2. ADP (passes back into the light-dependent reaction)3. Inorganic Phosphate (passes back into the light-dependent reaction)4. TP (used to build useful organic compounds such as glucose, amino acids and lipids)5. RuBP is regenerated and reused in the Calvin cycle

Efficiency of the system: One hexose sugar (e.g glucose) is made by joining 2 molecules of TP. Three turns of the Calvin cycle produces 6 molecules of TP, but 5 of these are needed to regenerate RuBP. This means that three turns actually only produces one molecule of TP that can be used to make glucose so the Calvin cycle actually needs to happen 6 times to make one hexose sugar.

Therefore, for every three turns of the cycle only one molecule of TP is produced.Recall Questions:

1. Where does the Calvin cycle occur? 2. What are the products of the Calvin cycle? 3. What is the 5 carbon sugar which starts the Calvin cycle?4. How many times does the Calvin cycle have to happen to form one glucose molecule?5. How does GP turn into TP?6. What are 10 out of the 12 molecules of TP used for?7. What are 2 out of the 12 molecules of TP used for?8. What does the enzyme rubisco do?9. How many molecules of ATP does the Calvin cycle use?10. How many molecules of NADPH does the Calvin cycle use?

Exam Questions:Q1. (a)     Where precisely in a cell does the Calvin cycle take place?

___________________________________________________________________(1)

(b)     ATP and reduced NADP are two products of the light-dependent reactions. Describe one function of each of these substances in the light-independent reactions.

ATP _______________________________________________________________

___________________________________________________________________

Reduced NADP ______________________________________________________

___________________________________________________________________ (2)

Q3. During the light-independent reaction of photosynthesis, carbon dioxide is converted into organic substances. Describe how.

_______________________________________________________________________

_______________________________________________________________________

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_______________________________________________________________________

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Q2. Heat stress is a condition that often occurs in plants exposed to high temperatures for a prolonged period of time. Heat stress is a major factor in limiting the rate of photosynthesis.

(a)     Heat stress decreases the light-dependent reaction of photosynthesis.

Explain why this leads to a decrease in the light-independent reaction.

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________(2)

(b)     Another effect of heat stress is a decrease in the activity of the enzyme rubisco. A decrease in the activity of an enzyme means that the rate of the reaction it catalyses becomes slower.

A decrease in the activity of the enzyme rubisco would limit the rate of photosynthesis.

Explain why.

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________(2)

I     Where precisely is rubisco found in a cell?

___________________________________________________________________

___________________________________________________________________(1)

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Q4 . (a)     Describe how NADP is reduced in the light-dependent reaction of photosynthesis.

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________(2)

(b)     In an investigation of the light-independent reaction, the amounts of glycerate3-phosphate (GP) and ribulose bisphosphate (RuBP) in photosynthesising cells were measured under different environmental conditions.

Figure 1 shows the effect of reducing the carbon dioxide concentration on the amounts of glycerate 3-phosphate and ribulose bisphosphate in photosynthesising cells.

Figure 1

 

(i)      Explain why there is twice the amount of glycerate 3-phosphate as ribulose bisphosphate when the carbon dioxide concentration is high.

______________________________________________________________

______________________________________________________________(1)

(ii)     Explain the rise in the amount of ribulose bisphosphate after the carbon dioxide concentration is reduced.

______________________________________________________________

______________________________________________________________ (1)

(c) Figure 2 shows the results of an experiment in which photosynthesising cells were kept in the light and then in darkness.

Figure 2

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(i)      In the experiment the cells were supplied with radioactively labelled 14CO2. Explain why the carbon dioxide used was radioactively labelled.

______________________________________________________________

______________________________________________________________(1)

(ii)     Explain how lack of light caused the amount of radioactively labelled glycerate 3-phosphate to rise.

______________________________________________________________

______________________________________________________________

______________________________________________________________

______________________________________________________________(2)

(iii)     Explain what caused the amount of radioactively labelled glucose to decrease after the light was switched off.

______________________________________________________________

______________________________________________________________ (1)

Recall Question Answers: Where does the Calvin cycle occur? StromaWhat are the products of the Calvin cycle? NADP, ADP and glucoseWhat is the 5 carbon sugar which starts the Calvin RuBP

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cycle?How many times does the Calvin cycle have to happen to form one glucose molecule? Six

How does GP turn into TP?

It is reduced. The H atom comes from NADPH and ATP provides energy for the endothermic reaction

What are every 5 out of 6 molecules of TP used for? To regenerate RuBP

What are 2 out of the 12 molecules of TP used for?

The creation of 6 carbon sugars which can be converted to other organic compounds e.g. amino acids, lipids, nucleic acids and carbohydrates

What does the enzyme rubisco do?Combines CO2 with RuBP to make 2 molecules of GP

How many molecules of ATP does the Calvin cycle use? ThreeHow many molecules of NADPH does the Calvin cycle use? Two

Homework:

Watch the video about Calvin and Benson’s research to discover the photosynthesis: https://www.bbc.co.uk/programmes/p011lymf

1. How was Calvin able to follow the carbon dioxide used by the algae?

2. What method did Calvin use to visualise molecules made by the algae that contained carbon from carbon dioxide?

3. What is the protein Benson discovered that was common to all plants that is required for the Calvin cycle to take place?

Read the article: https://www.sciencemag.org/news/2020/05/artificial-chloroplasts-turn-sunlight-and-carbon-dioxide-organic-compounds?utm_campaign=ScienceNow&utm_source=Jhubbard&utm_medium=Facebook

1. What have the scientists managed to do?

2. What part of photosynthesis have they improved and how?

3. What could artificial chloroplasts help with in the future?Lesson 6 – Limiting Factors of Photosynthesis

By the end of this lesson you should be able to: • Identify the environmental factors that limit the rate of photosynthesis• Evaluate common agricultural practices used to overcome the limiting effects of these factors.

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Optimum growing conditions for photosynthesis:

1. Light Intensity and WavelengthThe higher the light intensity the more energy it provides to excite electrons, however at extremely high light intensities e.g high UV chlorophyll may also be damaged. Photosynthetic pigments in plants mostly only absorb red and blue light from sunlight (green light is reflected) this can be low in the shade/low light.

2. TemperaturePhotosynthesis is a series of reactions that rely on enzymes (ATP synthase and Rubisco). If the temperature is too low (below 10°C) the enzymes will work too slowly, too high (above 45°C) the enzymes may start to denature. Remember: denaturing causes bonds holding the protein chains in an enzyme to break, thus changing its’ tertiary structure so that it is no longer a complimentary shape to the substrate. No enzyme-substrate complexes can occur.

These temperatures can be different for different plant species, as some plants are adapted to live in conditions that are outside this range e.g desert. At higher temperatures stomata also close to avoid water loss which decreases CO2 availability which slows down photosynthesis.

3. Carbon dioxide levelsAtmospheric CO2 concentrations are low (0.04%), photosynthesis in plants works best when this level is increased to 0.4%. If this is increased further however, stomata will begin to close.

4. Availability of waterPlants need a constant supply of water but not too much as their roots can become waterlogged. This can cause the roots to “drown” as they are not able to respire underwater (some plants can do this!) so no ATP can be made for active transport of minerals. This can affect the plants health and often leads to chlorosis (yellowing of the leaves) because not enough magnesium is being absorbed to make chlorophyll. This can reduce photosynthesis

How Limiting Factors Affecting the Rate of Photosynthesis: If light, CO2 concentration or temperature are not at the correct levels they can all limit the rate of photosynthesis (slow it down) even if the other two are at the perfect level the speed of photosynthesis will be limited for as long as one of the factors is

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incorrect.

Remember: On graphs that show ‘rate’ on the y axis the levelling off of the graph means the rate is no longer increasing, not that the reaction has stopped – that would mean the rate would be 0.

Light IntensityFrom A-B light intensity is the limiting factor (e.g night to daylight) as it increases so does the rate they are directly proportional. Point B represents the saturation point – increasing the light intensity will not make a difference to the rate, something else has become the limiting factor.

TemperatureBoth graphs level off when light intensity is no longer the limiting factor but the graph at 25°C levels off at a higher point. The lower temperature must have been a limiting factor as the rate was not as fast at 15°C.

CO2 ConcentrationAs with temperature above, both graphs level off when the light intensity is no longer the limiting factor. The temperature was the same for both so the difference in the graph heights must have been caused by the CO2 concentration. The graph at 0.4% CO2 levels off at a higher point so CO2 concentration must have been the limiting factor in the bottom graph where the rate was not as fast.

Farmers try to optimise conditions to improve growth: Growers can create optimum conditions in glasshouses to increase the rate of photosynthesis and improve yield of their crops. Higher levels of photosynthesis = more sugars/starch = more energy = more growth/fruit formation:

They increase carbon dioxide concentration in the air using CO2 generators The increase the light intensity during the winter/cloudy days using lamps, they can use LEDs to

only give the plants blue/red wavelengths of lights. The glasshouses trap heat energy from the sun which warms the air but to keep the temperature

optimum heating or cooling systems such as fans, automatic windows, and air circulation systems can be used.

In order to make sure that any changes to a limiting factor are having an effect all three factors must be measured and the two not being changed must be controlled to make sure they are not affecting the results. Growers will also want to consider cost of these technologies vs the profit they will make with them.Recall Questions

1. What is a limiting factor?2. What are the three limiting factors of photosynthesis?3. What is the saturation point? 4. What can growers do to prevent light intensity limiting the rate of photosynthesis in a glasshouse?

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5. What can growers do to prevent CO2 concentration limiting the rate of photosynthesis in a glasshouse?

6. What can growers do to prevent temperature limiting the rate of photosynthesis in a glasshouse?7. How can you be sure when changing one limiting factor in an experiment that your results are only

caused by that factor?

Exam QuestionsQ1. Scientists investigated the effects of temperature and light intensity on the rate of photosynthesis in creeping azalea. They investigated the effect of temperature on the net rate of photosynthesis at three different light intensities. They also investigated the effect of temperature on the rate of respiration. The graph shows the results.

 

(a)     (i)      Name the factors that limited the rate of photosynthesis between X and Y.

______________________________________________________________(1)

(ii)     Use information from the graph to explain your answer.

______________________________________________________________

______________________________________________________________

______________________________________________________________

______________________________________________________________

______________________________________________________________(2)

(b)     Use information from the graph to find the gross rate of photosynthesis at 20°C and medium light intensity.

 

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Answer ____________________(1)

(c)     Creeping azalea is a plant which grows on mountains. Scientists predict that in the area where this plant grows the mean summer temperature is likely to rise from 20 °C to 23 °C. It is also likely to become much cloudier. Describe and explain how these changes are likely to affect the growth of creeping azalea.

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________(3)

Q2. Tomato plants were grown in two glasshouses, each with an area of 2000 m2. The table shows the mean number of hours of sunshine per month during fruit production.

 

  1995 – 1997 (no extra carbon dioxide

1998 – 2000 (extra carbon dioxide)

Mean number of hours of

sunshine per month148.91 147.00

•        The scientists used heating to maintain the temperature inside the glasshouses above 18 °C. They opened the windows to keep the temperature below 30 °C.

•        From 1998 to 2000 they maintained the carbon dioxide concentration between 0.06 % and 0.08 % when the windows were closed and between 0.04 % and 0.05 % when the windows were open.

•        The carbon dioxide concentration in the air outside the glasshouse was 0.04 %.

(a)     The scientists monitored the number of hours of sunshine per month. Explain why they monitored the number of hours of sunshine.

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________ (2)

(b)     The temperature, the use of fertiliser and the number of insect pests were controlled during this investigation. Name one other factor which should have been controlled during the investigation. Explain why variation in this factor would affect yield.

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Factor _____________________________________________________________

Explanation _________________________________________________________

___________________________________________________________________(2)

Q3. An investigation was carried out into the effect of carbon dioxide concentration and light intensity on the rate of photosynthesis in a species of plant.

(a)     The temperature was kept constant during this investigation. Explain why.

___________________________________________________________________

___________________________________________________________________(2)

(b)     The table shows the effect of increasing carbon dioxide concentration on the rate of photosynthesis in maize.

 

Carbon dioxideconcentration / arbitrary units

Rate of photosynthesis / arbitraryunits

30 10

60 20

100 30

150 40

230 50

300 60

400 60

Describe and explain the effect of increasing carbon dioxide concentration on the rate of photosynthesis.

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________(3)

Q4. An investigation was carried out to find the effect of increasing carbon dioxide concentration on the rate of photosynthesis in a particular type of plant. The graph shows the results.

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(a)     (i)      In this investigation, temperature was kept constant. Explain why.

______________________________________________________________

______________________________________________________________(1)

(ii)     Suggest suitable units for measuring the rate of photosynthesis in this investigation.

______________________________________________________________(2)

(b)     (i)      Give the evidence from the graph that carbon dioxide is limiting the rate of photosynthesis between A and B.

______________________________________________________________

______________________________________________________________(1)

(ii)     Explain the shape of the curve between B and C.

______________________________________________________________

______________________________________________________________

______________________________________________________________

______________________________________________________________(2)

Recall Question AnswersQuestion Answer

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1. What is a limiting factor? A variable that limits the rate of a chemical reaction2. What are the three limiting factors of

photosynthesis?Carbon dioxide concentration, temperature and light intensity.

3. What is the saturation point? Where the factor being changed no longer limits the reaction

4. What can growers do to prevent light intensity limiting the rate of photosynthesis in a glasshouse?

Increase light intensity in low light with lamps and use LEDs to boost blue/red wavelengths.

5. What can growers do to prevent CO2

concentration limiting the rate of photosynthesis in a glasshouse?

Increase CO2 concentration in the air to 0.4% using carbon dioxide generators

6. What can growers do to prevent temperature limiting the rate of photosynthesis in a glasshouse?

Maintain constant optimum temperature using heaters in the winter and cooling methods such as fans and air circulation systems in the summer.

7. How can you be sure when changing one limiting factor in an experiment that your results are only caused by that factor?

By controlling and measuring the other two factors.

Homework: How can improving photosynthesis help feed the world in the future? Read the article and watch the video clip: https://scitechdaily.com/photosynthesis-hacks-boost-yield-and-conserve-water/

1. What stages of photosynthesis have the scientists improved?

2. What limiting factors are these related to?

3. What percentage increase in yield have the researchers achieved?

4. What is the extra benefit of this discovery which is important to climate change?

5. What other discoveries have the team previously made to help reduce the effect of limiting factors on growth?

6. How do these changes help plants increase their rate of photosynthesis?

7. How will this research help feed more people in the future?

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