light-independent reactions

Question 1

what is the second stage of photosynthesis?

Answer 1

the light-independent reactions

Question 2

do the light independent reactions require light?

Answer 2

this stage does not require light directly however in practise it stops in the absence of light as it requires the products of the light dependent reactions ATP and reduced NADP

Question 3

what are the ATP and reduced NADP from the LDRs used for in this stage?

Answer 3

they are used now to reduce carbon dioxide from the atmosphere, building it into carbohydrates

Question 4

where do the light independent reactions occur and what catalyses them?

Answer 4

the light independent reactions occur in the stroma of the chloroplast, and are catalysed by enzymes

Question 5

what are the light independent reactions often referred to as?

Answer 5

the calvin cycle

Question 6

explain the light independent reactions

Answer 6

• CO2 combines with a five carbon molecule called ribulose biphosphate (RuBP) to form two molecules of the three carbon compound, glycerate 3-phosphate (GP). this reaction is catalysed by the enzyme RuBisCo.
• in the next step, the ATP and the reduced NAPD produced in the light dependent reactions is used to reduce the glycerate 3-phosphate (GP) to triose phosphate (TP). the ATP provides the necessary energy for this reaction (so activates GP) and the reduced NADP provides the hydrogen for the reduction reaction.
• the NADP is reformed and goes back to the light dependent reaction to be reduced again by accepting more hydrogen.
• some (about one sixth) of the triose phosphate is converted into carbohydrates such as glucose, sucrose, starch or cellulose and from there converted into other useful organic substances eg amino acids, nucleic acids, lipids etc.
• the rest of the triose phosphate (about five sixths) is used to regenerate RuBP using ATP from the light dependent reaction. ATP is required as it supplies the phosphate necessary to convert ribulose phosphate into ribulose biphosphate.