6. Photosynthesis- Light-independent reactions

Question 1

Where does the light-independent stage of photosynthesis take place?

What products from the light-dependent stage does this stage?

Answer 1

Stroma

Enzymes in the stroma use the energy in ATP and NADPH to reduce CO2

Production of ATP and NADPH is light-dependent therefore CO2 fixation must also take place in light

Question 2

What did Calvin and Benson do to discover the Calvin-Benson cycle?

Answer 2

Used the 14C radioisotope to determine the sequence of reactions in CO2 fixation

Exposed Chlorella to 14CO2, then extracted the organic compounds and spared them by paper chromatography

Showed that it produces organic compounds

Question 3

What is the first stage of the Calvin-Benson cycle?

Answer 3

1) Carboxylation of the CO2 acceptor molecule
6CO2 is used per cycle
Rubisco enzyme adds CO2 to 5 carbon sugar ribulose 1,5-biphosphate
Forms: 12x 3-carbon containing 3-phosphoglycerate

Question 4

What is the second stage of the Calvin-Benson cycle?

Answer 4

2) Reduction of the carboxylation production
Overall: 12x 3-phosphoglycerate is transformed to triose phosphates using ATP and NADPH

3-Phosphoglycerate —> 1,3-Biphosphoglycerate using 12 molecules of ATP

1,3-Biphosphoglycerate —>Glyceraldehyde 3-Phosphate using 12 molecules of NADPH and it also loses 12 molecules of Pi

Question 5

What is stage 3 of the Calvin-Benson cycle?

Answer 5

3) Regeneration of the CO2 acceptor molecule
5/6ths of the triose phosphates are used to regenerate ribulose-1,5-biphosphates while the remaining 1/6th is used to supply carbon skeletons for biosynthesis, transport and storage

Glyceraldehyde 3-Phosphate —> 2 molecules is transported to the cytosol and converted to sucrose

10x Glyceraldehyde 3-Phosphate —> Ribulose 5-phosphate using 7 enzymes and loses 4Pi

6x Ribulose 5-phosphate –> 6x Ribulose-1,5-biphisohate

Question 6

How is the Calvin cycle stimulated by light?

Answer 6

Protons which are pumped from the stroma into thylakoids increase the pH which favours the activation of rubisco

Electron flow from photosystem I reduces disulphide bonds to activate Calvin cycle enzymes

Question 7

How is the Calvin-Benson cycle regulated?

Answer 7

Maximum efficiency and economy, the rate of the Calvin-Bension cycle matches that of the light reactions and the cycle responds sensitively to changes in light flux

In the dark: Calvin cycle enzymes are rapidly inactivated in the dark= Conserves ATP

Also controlled by pH: Protons are transported from the stroma into the thylakoid lumen during photoelectron transport, pH increases from around 7 in the dark to 8 in light. Increase activity of several enzymes at higher pH levels

Question 8

How does CO2 affect the function of rubisco?

Answer 8

It is both an activator and a substrate for rubisco

Rubisco activase complements the CO2 dependent activation of rubisco by lowering the inhibition caused by sugar phosphates

Question 9

What is photorespiration?

Answer 9

Consumes O2, releases CO2

Rubisco can catalyse a competing reaction with the same substrate ribulose-1,5-bipshohate but with O2 instead of CO2 BUT it does have 10x more affinity for CO2

It is an oxygenate as well as a carboxylase

It reduces the amount of CO2 converted to carbohydrates which may limit plant growth

Products: 1x 3-phosphoglycerate + 1x phosphoglycolate (2C)

The phosphoglycolate forms glycolate and is converted to glycine= diffuses into mitochondria= converted to glycerine + CO2

Both processes form 3-phosphoglycerate

Question 10

When is the carbon-fixing reaction favoured?

When is photorespiration favoured?

Answer 10

When CO2 levels are relatively high

Photorespiration: Is favoured when CO2 is low and O2 is high + takes place in light + more likely at high temperatures when stomata are closed

Question 11

Why is photorespiration bad?

Answer 11

Excessive photorespiration could become a problem for plants in hot, dry environments as their stomata will remain closed

Result: CO2 levels inside leaf can fall below the Km of rubisco for CO2= Rate of photosynthesis is slowed

Question 12

What is the C4 carbon cycle?

Answer 12

2-step pathway of CO2 fixation in which a CO2-hoarding step precedes the Calvin cycle

Fixes atmospheric CO2 into carbon skeleton in one compartment and releases CO2 in another compartment to increase CO2 concentration for rubisco

Question 13

What are the two types of cells involved in the C4 pathway?

Answer 13

Mesophyll cells: Adjacent to the air spaces in the leaf interior

Bundle sheath cells: Surround the vascular tissue and are hidden from high oxygen levels whereas mesophyll cells are exposed

Question 14

What are the stages of the C4 pathway?

Answer 14

1) In mesophyll cells, the enzyme PEP carboxylase (which is insensitive) to O2) adds carbon dioxide to PEP (Phosphoenolpyruvate) to produce oxaloacetate which uses the hydrolysis of 1xATP
2) Oxaloacetate is reduced to malate which is then transferred to bundle-sheath cells
3) CO2 is released and enters the Calvin cycle

Question 15

How does the C4 cycle affect photorespiration?

Answer 15

Bundle sheath cells lack PSII and carry out only cyclic electron flow

High CO2 and low O2 concentrations in the cells= favours fixation of CO2 by rubisco to form 3-phosphoglycerate and inhibits utilisation of ribulose 1-5-biphosphate in photorespiration

Rate of photorespiration in C4 plants is much lower than it is in C3 plants

Question 16

What is CAM?

Answer 16

Crussulacean Acid Metabolism
Another photosynthesis pathway used by some plants adapted to arid conditions

Functions to capture atmospheric CO2 and savings respiratory CO2 in arid environments

Generally associated with anatomical features that minimise water

The initial capture of CO2 and its final incorporation into carbon skeletons are temporarily separated

Up to 99% CO2 assimilation occurs during the night= need less evaporative demand= Limiting water loss

Exactly the same process as C4 plants