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PHOTOSYNTHESIS

SUMMARY

Chloroplast structure Stroma - The matrix surrounding the grana in

the inner membrane of chloroplasts. The area

between membranes (thylakoids, grana) inside

the chloroplast

Grana- A series of stacked thylakoid disks

containing chlorophyll; found in the inner

membrane of chloroplasts

Thylakoids- Internal membranes in the

chloroplast where the light reaction chemicals

are embedded. Collections of thylakoids form

the grana

LIGHT AND DARK REACTIONS Light reactions- The photosynthetic process in

which solar energy is harvested and transferred

into the chemical bonds of ATP; can occur only in

light. In the Light Dependent Processes light

strikes chlorophyll a in such a way as to excite

electrons to a higher energy state. In a series of

reactions the light energy is converted into ATP

and NADPH. Water is split in the process,

releasing oxygen as a by-product of the

reaction. The ATP and NADPH are used to make

C-C bonds in the Light Independent Process.

Dark reactions - The photosynthetic process in

which sugar molecules are formed from carbon

dioxide from the atmosphere with the use of ATP;

can occur in the dark as long as ATP is present.

In the Light Independent Process, carbon

dioxide from the atmosphere is captured and

modified by the addition of Hydrogen to form

carbohydrates. The incorporation of carbon

dioxide into organic compounds is known as

carbon fixation. The energy for this comes from

the first phase of the photosynthetic process

Calvin-benson cycle - Series of biochemical,

enzyme-mediated reactions during which

atmospheric carbon dioxide is reduced and

incorporated into organic molecules, eventually

some of this forms sugars. In eukaryotes, this

occurs in the stroma of the chloroplast.

http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookPS.html

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Question 7 -2007 exam

A student was asked to identify differences between the overall processes of photosynthesis and aerobic

respiration in eukaryotic cells. The student prepared the table below to outline the differences.

The only correct comparison listed by the student is- answer D

Question 8

During periods of strenuous exercise or prolonged fasting, glycogen stored in liver and muscle cells is depleted.

All glucose in the blood and cells may be completely depleted as well.

If this occurs, the compound that is next most accessible for use in glycolysis is

A. lipid.

B. DNA.

C. starch.

D. protein.

Question 14

Nitrosomonas europa is a species of bacteria that obtains its energy by oxidising ammonium ions to nitrite

ions.

It is reasonable to argue that

A. the reaction is anaerobic.

B. the reaction is endergonic.

C. no heat is produced in this reaction.

D. ATP is produced as a result of this reaction.

Question 4

The bird’s-nest fern, Alocasia marcrorhizza, usually grows in deeply shaded rainforests and has dark green

fronds. Sometimes it is found in open, sunny locations by roadsides where it tends to have lighter coloured

fronds. Two bird’s-nest ferns, one from each of the two habitats described above, were examined. A sample of

cells from a frond of each of the ferns was collected.

These cells were examined under an electron microscope and a typical chloroplast from each habitat (deeply

shaded rainforest and sunny location) was drawn. These drawings are shown below.

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a. Which of the labelled parts, X, Y or Z, absorb light energy? Name the part.

Part Y –grana absorbs light energy

b. Which drawing, A or B, shows a chloroplast from the rainforest habitat? Explain the reason for your choice

in terms of the relationship between structure and function.

Drawing B is from a plant living in a shaded rain forest as it has many more grana (as indicated by the

information that there are dark green fronds) and this gives a greater ability to absorb any available

light.

c. Name the process by which the chemical reactants for the light-dependent reactions of photosynthesis

enter a chloroplast.

Either of:

osmosis (of water)

diffusion (of water/carbon dioxide).

Chloroplasts contain large numbers of ribosomes.

d. Explain the importance of the presence of these ribosomes.

Ribosomes synthesise protein to:

produce the enzymes required for photosynthesis

assist in the production of chlorophyll

make the membranes of the chloroplast or within it.

It is now widely accepted by biologists that chloroplasts and mitochondria were once independent prokaryotic

organisms which came to live symbiotically inside larger eukaryotic cells. This idea is known as the

endosymbiotic theory.

e. Name one structural feature of chloroplasts or mitochondria and outline how it supports the endosymbiotic

theory for the origin of these organelles.

Possible answers included:

both contain DNA and are capable of self-replication

both have ribosomes, which indicates they can make their own proteins

both have a double membrane, one of the prokaryote and one of the host.

The simplified diagram below summarises the light-independent reactions (Calvin cycle) of photosynthesis.

f. Name reactant Q. is carbon dioxide

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You will note that during the light-independent reactions many energy-carrier molecules of NADPH and ATP

are used.

g. What is the source of these energy carriers?

The source of these carriers is:

the light dependent stage

the electron transport chain of photosynthesis

the splitting of water

h. What is compound P? Glucose or water

3. A student predicted that if a temperature graph was prepared for carrot catalase activity, the optimal

temperature would be expected to be much lower than that shown by catalase from humans.

b. Do you agree or disagree with the student.s prediction? Explain the reason for your choice.

Agree - As the carrot grows at a soil temperature of 16°C, it would be expected that the carrot’s catalase

optimum temperature would be around this temperature, not 37°C as for humans.

Disagree - It would be expected that the enzyme is the same in both organisms and would therefore have

the same optimum temperature.

Either of the above answers was acceptable.

c. Describe (or outline) an experiment you would carry out with pieces of carrot to test the accuracy of the

student’s prediction. Hydrogen peroxide is available as a 3% in water solution. Explain fully what results would

support or negate the student.s prediction.

To gain 3 marks, the following points should have been included in students’ responses.

The use of at least 2 groups of identical pieces of carrot, placed at various temperatures (for

example, 16°C and 37°C) in the same concentration of hydrogen peroxide. A specific number of carrot

pieces could have been given (for example, 10). Students who mentioned the variable and other factors which

were controlled adequately demonstrated their understanding of the experimental design and were awarded the

first mark.

For the second mark, students needed to discuss how catalase activity would have been measured. For

example, collecting the gas to measure the production of oxygen gas, or observing the bubbles being

produced.

The third mark was awarded for a discussion of the expected results and a conclusion based on the

student’s prediction. For example, more oxygen gas produced at 16°C compared to 37°C would support the

prediction that carrot catalase optimum temperature was 16°C. The idea of replicating the experiment

could also have been mentioned.

2006 exam 2. Anabolic reactions, also called endergonic reactions, include

A. glycolysis.

B. Kreb’s cycle.

C. photosynthesi

D. protein digestion.

6. The chloroplast is the organelle responsible for photosynthesis in eukaryotic cells. In chloroplasts

A. the light independent reactions require water as the initial reactant.

B. the light independent reactions occur in the inner membrane area.

C. the final product of the light reaction is glucose.

D. the light reactions occur in the grana.

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15 Consider the following diagram that is a summary of an important biochemical process.

The product, X, in this biochemical process is

A. ADP.

B. a polymer.

C. an amino acid.

D. a monosaccharide.

18 The following image shows a portion of an electron photomicrograph of a chloroplast.

Light-dependent reactions occur in region P and the Calvin cycle reactions occur in region Q.

Considering events that occur in a chloroplast during photosynthesis it is reasonable to claim that

A. oxygen is an input to reactions at P.

B. carbon dioxide is an input to reactions at Q.

C. chlorophyll is essential for reactions that occur at Q.

D. ADP produced during the events at P is used by events at Q.

3c Before a drug is used for human treatment it is usually tested on animals. This is because results for animals

often give some indication of how effective a drug may be in humans. In this case the virus under investigation

also infects mice.Design an experiment, using mice, to test the effectiveness of the drug you have designed.

Points for consideration in the experiment that students were asked to design included:

• the selection of the mice. These should have been two large groups of identical mice kept in the same

environmental conditions

• the number of mice in each group. It was preferable for students to state a specific number (of reasonable

magnitude) in each group, instead of simply describing a ‘large’ group. If no mention was made of the

size of the group, then the idea of replication of the experiment needed to be mentioned

• the infection of both groups with the virus against which the drug has been designed. One of the groups

then needed to receive no further treatment (the control group), the other group (the trial group) receives

the drug under investigation

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• after a few days, each of the groups needs to be examined and the number of mice that have developed

the viral disease in each group counted. If the number of mice in the trial group is significantly less than

the number in the control group, the drug has been effective.

Common errors in the experiments described included selecting only two mice without referring to

repetition of the experiment; not mentioning the similarity of the mice and/or environment; injecting

mice with the virus and then waiting days or weeks before the drug was used; administering the drug

first and then exposing the mice to the virus days or weeks later; and general statements about

comparing the results, without any reference to what result would indicate effectiveness of the drug.

Experimental design is integral to science and this question exposed many deficiencies in students’

knowledge and understanding of the process. Students need more practical experience with appropriate

examples.

4. 2,4-dinitrophenol is a chemical that is toxic to mitochondria. When added to mitochondria this chemical

allows electron transport to occur but prevents the phosphorylation of ADP to ATP. The chemical achieves this

by breaking the essential link between electron transport and ATP synthesis. This toxin causes mitochondria to

produce heat instead of ATP. The greater the amount of toxin added, the quicker is its action.

a. If mitochondria are poisoned with 2,4-dinitrophenol by what process could a plant cell produce more

ATP? Acceptable answers were anaerobic respiration, glycolysis or fermentation.

b. Where in the mitochondria does electron transport and ATP production occur?

Cristae, inner membranes or membrane folds needed to be specified; simply stating ‘membranes’ was

not sufficient A researcher wanted to study cellular respiration in insect cells. She cultured some muscle cells from the

common field cricket, Teleogryllus oceanicus, then studied the effects of adding 2,4-dinitrophenol to these cells.

An agricultural company may want to fund this research.

c. Give one reason why an agricultural company might want to fund research on the effects of this toxin on

field crickets.

Any of the following answers were accepted:

• field crickets eat crops

• the compound may be effective as a pesticide/insecticide against crickets

• the chemical may be used to get rid of crickets

The experiment is summarised in the table below. Temperature observations in each trial were made at equal

time intervals.

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d. In terms of energy production, why did the temperature go up in trial 1 and not in the control?

The presence of 2,4-dinitrophenol caused trial 1 to produce heat (instead of ATP). There was no chemical

in the control group, therefore ATP was produced, not heat.

e. Explain why the temperature went down after the fifth observation in trial 1.

The enzyme denatured, or an increase in heat killed/damaged the cells.

f. Trial 2 had twice the concentration of 2,4-dinitrophenol added. Complete the table by writing in temperatures

in the spaces provided to predict the trend.

Students needed to indicate an initial rise in the temperature occurring faster than in trial 1 and a decline

that commenced no later than time interval 5. Two examples of appropriate sequences for trial 2 (from

the commencement) include 28, 29 30, 36, 23, 21, 19 and 28, 29, 36, 30, 28, 24, 19. Another researcher

suggested adding pyruvate to the cells to cancel out the effects of this toxin.

g. Explain what effect adding pyruvate would have on cancelling out the effect of this toxin.

Various answers to this question were possible, depending on when the pyruvate was to be added.

• If it was added at the beginning of the experiment, there would be no effect because pyruvate is used

before the electron transport process.

• If the pyruvate was added after the experiment, there would be no effect because the toxin has

already destroyed the enzymes or cricket cells (many students wrote crickets in error).

If the answer referred to Krebs Cycle, there may have some ATP as replacement by the addition of

pyruvate. Stating that some ATP would be produced independent of electron transport was also

acceptable.