Honors Biology

Photosynthesis

What you will learn…1. How do plants get food?2. Photosynthesis overview3. Leaf structure4. Chloroplast structure5. Pigments6. Overview of Two Stages

6a. Overview of Light Reactions6b. Overview of Dark Reactions

7. Light Reactions8. Dark Reactions (Calvin Cycle)9. Relationship Between the 2 Stages10. Factors Affecting the Photosynthetic Rate11. Alternate Pathways

1.How do plants get food?Plants are autotrophs (meaning “self-

feeders” in Greek)

Often referred to as the producers of the biosphere because they produce its food supply

All organisms that produce organic molecules from inorganic molecules using the energy of light are called photoautotrophs.

2.Photosynthesis OverviewPhoto, from the Greek word for light, refers

to the first stage.Synthesis, meaning “putting together”

refers to the sugar construction in the second stage

2.Photosynthesis: OverviewThe main purpose of photosynthesis is to

make organic molecules (carbohydrates).

Overall equation:6 CO2 + 6 H20 C6H12O6 + 6 O2

Occurs in the leaves of plants in the chloroplasts.

Oxygen is also produced in this process.

3.Leaf StructureMost photosynthesis occurs in the mesophyll

layer of the leaf.

Gas exchange of CO2 and O2 occurs at openings called stomata surrounded by guard cells on the lower leaf surface.

Stomata are able to open and close because water is also evaporated through them into the atmosphere from the plant.

3.Leaf Structure

4.Chloroplast Structure• Similar to mitochondria, chloroplast has an outer

membrane and an inner membrane, with an intermembrane space between them.

•Inner membrane is filled with a thick fluid called stroma

*Stroma is where sugars are made from carbon dioxide and water

4.Chloroplast StructureWithin stroma is a system of interconnected

membranous sacs called thylakoidscontains thylakoid spaceBuilt into thylakoid membranes are the

chlorophyll molecules that capture light energy

concentrated in stacks called grana.

5. PigmentsPigment is any molecule that is able to absorb light

. Only light that is absorbed by pigments is useful for

photosynthesis.Chlorophyll a is the most important photosynthetic

pigment. Other pigments called antenna or accessory

pigments are also present in the leaf.Chlorophyll bCarotenoids (orange / red)Xanthophylls (yellow / brown)

These pigments are embedded in the membranes of the thylakoid in groups called photosystems.

6.Two Stages of Photosynthesis1. Light Reactions2. Dark Reactins (Calvin Cycle)

6a.Light Reactions overview1. Light reactions

Include the steps that convert light energy to chemical energy stored in ATP and NADPH and produce O2 gas as a waste product.

Occur in thylakoid membranes Light energy absorbed by chlorophyll is used

to make ATP from ADP and phophate. Also used to drive a transfer of electrons from

water to NADP+, an electron carrier similar to NAD+ that carries electrons in cellular respiration.

NADP+ gets reduced to NADPH via enzymes by adding a pair of light-excited electrons along with an H+ Reaction temporarily stores energized electrons

which originally came form water that is split and O2 is released.

6b.Dark Reactions overviewDark reactions, or Calvin Cycle

Occurs in the stroma Does not require light directly Cyclic series of reactions that assembles

sugar molecules using CO2 and the energy-containing products (NADPH and ATP) of the light reactions.

Incorporation of carbon from CO2 into organic compounds is called carbon fixation.

After carbon fixation, enzymes of the cycle make sugars by further reducing the carbon compounds.

7. Light Reactions

7. Light ReactionsLight energy is transformed into the chemical

energy of ATP and NADPHIn this process, electrons removed from

water molecules pass from photosystem II to photosystem I to NADP+

Between the two photosystems, the electrons move down an electron transport chain and provide energy for ATP production.

7. Light ReactionsConsists of two Photosystems:

Contain clusters of chlorophyll molecules along with other pigments and proteins in the thylakoid membrane

Consists of a number of light-harvesting complexes surrounding a reaction center.

Have chlorophyll a, chlorophyll b, and carotenoid pigments that function collectively as a light-gathering antenna.

Pigments absorb photons and pass the energy from molecule to molecule until it reaches the reaction center. A protein complex that contains a chlorophyll a molecule and a

molecule called the primary electron acceptor:Captures a light-excited electron from the reaction-center

chlorophyll molecule and passes it to an electron transport chain

7. Light ReactionsTwo types: Photosystem I and Photosytem II:

Photosystem I:Occurs second in light reactionsReaction center is called P700 because the

wavelength of light it absorbs best is 700 nmPhotosystem II:

Occurs first in light reactionsChlorophyll a molecule in reaction center is called

P680 because the light it absorbs best is red light with a wavelength of 680nm

7. Light ReactionsFlow of electrons in light reactions (Figure 7.8A):

1. A pigment molecule in a light-harvesting complex absorbs a photon of light. The energy is passed to other pigment molecules and finally to the reaction center of Photosystem II, where it excites an electron of chlorophyll P680 to a higher energy level.

2. The electron is captured by the primary electron acceptor.

3. Water is split, and its electrons are supplied one by one to P680, replacign those lost to the primary electron acceptor. The oxygen atom compbines with an oxygen from another split water molecule to form a molecule of O2.

7. Light Reactions4. each photoexcited electron passes from

photosystem II to photosystem I via an electron transport chain. The exergonic “fall” of electrons provides energy for the synthesis of ATP.

5. Meanwhile, light energy excites an electron of chlorophyll P700 in the reaction center of photosystem I. The primary electron acceptor captures the excited electron and an electron from the bottom of the electron transport chain replaces the lost electron in P700.

6. The excited electrons of photosystem I is passed through a short electron transport chain to NADP+, reducing it to NADPH

7. Light ReactionsChemiosmosis

Drives ATP synthesis using the potential energy of a concentration gradient of hydrogen ions across a membrane

Gradient is created when an electron transport chain pumps hydrogen ions across a membrane as it passes electrons down the chain.

Relationship between chloroplast structure and function in light reactions:

The two photosystems and e.t.c. are all located in the thylakoid membrane of a chloroplast.

As photoexcited electrons are passed down the e.t.c. connecting the two photosystems, H+ are pumped across the membrane from the stroma into the thylakoid space. This generates a concentration gradient across the membrane.

7. Light ReactionsChemiosmosis (continued):

Similar ATP synthase complex in mitochondriaEnergy of concentration gradient drives H+ back

across the membrane through ATP synthaseATP synthase couples the flow of H+ to the

phosphorylation of ADP: called photophosphorylation

7. Light ReactionsPhotosynthesis vs. Cell Respiration:

In photosynthesis, light energy is used to drive electrons to the top of the transport chain (whereas, cell respiration, high-energy electrons pass down the e.t.c. coming from oxidation of food molecules)

Chloroplasts transform light energy into the chemical energy of ATP (whereas, mitochondria transfer chemical energy from food to ATP)

In photosynthesis, the final electron acceptor is NADP+ (whereas, in cell respiration, O2 is)

In photosynthesis, electrons are stored in at a high state of potential energy in NADPH (whereas, in cell respiration, they are at a low energy level in H20)

8. Dark ReactionsDuring this process, carbohydrates are formed.

This is the only process on the earth that can

form organic molecules from inorganic ones. All other organic molecules (big 4) form from carbohydrates!

This cycle requires ATP, NADPH and CO2 to take place in the stroma of the chloroplast.

ATP, NADPH are from the light reaction, while CO2 has to be taken in from the atmosphere through the stomata of the leaves.

8. Dark ReactionsFigure 7.10A: Overview of Calvin Cycle

CO2 (from air), energy from ATP and high energy electrons from NADPH (both generated by light reactions) , the Calvin Cycle constructs an energy-rich, three-carbon sugar, glyceraldehyde-3-phosphate (G3P).

A plant cell uses G3P to make glucose and other organic molecules as needed.

8. Dark ReactionsFigure 7.10B: Details of the Calvin Cycle

1. Carbon fixation: the enzyme rubisco attaches CO2 to RuBP (5-C). The unstable 6-C product splits into two molecules called 3-PGA.

1. For three CO2, six 3-PGA result

2. Reduction: NADPH reduces the organic acid six 3-PGA to six molecules G3P with the assistance of ATP

8. Dark Reactions3. Release of one molecule of G3P:

1. Five G3Ps remain in the cycle, and one G3P will leave. Plant cells use two G3P molecules to make one molecule of glucose.

4. Regeneration of RuBPenergy from ATP drives a

series of chemical reactions to rearrange the atoms in the five G3P molecules to form three RuBP molecules. These can start another turn of the cycle.

8. Dark Reactionshttp://www.science.smith.edu/departments/Biology/Bio231/calvin.html

9. Relationship between the 2 Stages

10. Abiotic Factors Affecting Photosynthetic RatePhotosynthetic rate is depended on

environmental factors:Amount of light availableLevel of carbon dioxidetemperature

10. Abiotic Factors Affecting Photosynthetic Rate

Light intensityUp to a certain intensity, photosynthesis

increases as more light is available to the chlorophyll.

When all the chlorophyll molecules are activated (saturated) by the light, more light has no further effect.

10. Abiotic Factors Affecting Photosynthetic RateTemperature:

Increased temperature increases photosynthetic rate until an optimal temperature is reached.

Above the optimal temperature, enzymes cannot function properly and photosynthesis will decrease.

10. Abiotic Factors Affecting Photosynthetic RateCarbon Dioxide Levels:

Increased carbon dioxide levels increases photosynthesis, unless limited by another factor, then levels off.

11. Alternate PathwaysThese pathways adapt to perform

photosynthesis in dry and hot environmentThey are more efficient than the traditional

C3 pathway which use CO2 directly from the air

Plants with alternative pathways have a slightly different Calvin cycle.In C4 plants the location of the Calvin cycle

is differentIn CAM plants the timing is different

11. Alternate PathwaysC4 Pathway:

CO2 fixation and the Calvin cycle take place in two separate location.

CO2 fixation is in the mesophyll cells of the leaf, even when CO2 levels are low, producing a C4 product used for the Calvin CycleC4 product acts as a carbon shuttle to…

The Calvin cycle takes place in the bundle sheath cells (around the veins of the leaf) where sugars are made

Examples of C4 plants: corn, sugar cane

11. Alternate PathwayCAM

Occurs in succulent plants (cacti)Carbon fixation (trapping CO2) takes place at

night when the stomata are openCalvin cycle takes place during the day,

when stomata are closedThis way plants do not lose much water

during hot and dry days.

11. Alternate Pathway