chapter 17 p3 Flashcards
The light-dependent stage of photosynthesis:
Non-cyclic photophosphorylation:
part 1
Two photosystems are involved in non-cyclic photophosphorylation, photosystem II (PSII) followed by photosystem I (PSI).
The reaction centre of PSI absorbs light at a higher wavelength (700nm) than PSII (680nm).
The light absorbed excites electrons at the reaction centres of the photosystems.
The excited electrons are released from the reaction centre of PSII and are passed to an electron transport chain.
ATP is produced by the process of chemiosmosis
The electrons lost from the reaction centre at PSIl are replaced from water molecules broken down using energy from the Sun
The light-dependent stage of photosynthesis:
Non-cyclic photophosphorylation:
part 2
Excited electrons are released from the reaction centre at PSI, passed to another electron transport chain, and ATP is again produced by chemiosmosis.
The electrons lost from this reaction centre are replaced by electrons that have just travelled along the first electron transport chain after being released from PSI.
The electrons leaving the electron transport chain following PSI are accepted, along with a hydrogen ion, by the coenzyme NADP, forming reduced NADP.
Reduced NADP provides the hydrogen or reducing power in the production of organic molecules, such as glucose, in the light-independent stage which follows.
diagram of cyclic and Non-cyclic photophosphorylation
Photolysis:
Water molecules are split into hydrogen ions, electrons, and oxygen molecules using energy from the Sun in a process called photolysis.
The electrons released replace the electrons lost from the reaction centre of PSII.
This is why water, along with light and carbon dioxide, is a raw material of photosynthesis.
The oxygen-evolving complex which forms part of PSII is an enzyme that catalyses the breakdown of water.
Oxygen gas is released as a by-product.
The protons are released into the lumen of the thylakoids, increasing the proton concentration across the membrane.
As they move back through the membrane down a concentration and electrochemical gradient, they drive the formation of more ATP.
Once the hydrogen ions are returned to the stroma, they combine with NADP and an electron from PSI to form reduced NADP.
This is used in the light-independent reactions of photosynthesis.
This process removes hydrogen ions from the stroma so it helps to maintain the proton gradient across the thylakoid membranes.
The photolysis reaction is summarised as:
photolysis diagram
Cyclic photophosphorylation:
The electrons leaving the electron transport chain after PSI can be returned to PSI, instead of being used to form reduced NADP, leading to cyclic photophosphorylation.
This means PSI can still lead to the production of ATP without any electrons being supplied from PSII.
Reduced NADP is not produced when this happens.
diagram of cyclic photophosphorylation:
The light-independent stage of photosynthesis:
The light-independent stage of photosynthesis takes place in the stroma of chloroplasts and uses carbon dioxide as a raw material.
The products from the light-dependent stage - ATP and reduced NADP, are also required.
Organic molecules, like glucose, are produced in a series of reactions collectively known as the Calvin cycle.
Calvin cycle: part 1
Carbon dioxide enters the intercellular spaces within the spongy mesophyll of leaves by diffusion from the atmosphere through stomata.
It diffuses into cells and into the stroma of chloroplasts where it is combined with a five-carbon molecule called ribulose bisphosphate (RuBP).
The carbon in carbon dioxide is therefore fixed, meaning that it is incorporated into an organic molecule.
The enzyme ribulose bisphosphate carboxylase (RuBisCO) catalyses the reaction and an unstable six-carbon intermediate is produced.
Calvin cycle: part 2
RuBisco is the key enzyme in photosynthesis.
It is a very inefficient enzyme as it is competitively inhibited by oxygen so a lot of it is needed to carry out photosynthesis successfully.
Biologists estimate that it is probably the most abundant enzyme in the world.
The unstable six-carbon compound formed immediately breaks down, forming two three-carbon glycerate 3-phosphate (GP) molecules.
Calvin cycle: part 3
Each GP molecule is converted to another three-carbon molecule, triose phosphate (TP), using a hydrogen atom from reduced NADP and energy supplied by ATP, both supplied from the light-dependent reactions of photosynthesis.
Triose phosphate is a carbohydrate, a three-carbon sugar, the majority of which is recycled to regenerate RuBP so that the Calvin cycle can continue.
It is the starting point for the synthesis of many complex biological molecules, including other carbohydrates, lipids, proteins, and nucleic acids.
The Calvin cycle can be summarised in three steps:
Fixation - carbon dioxide is fixed (incorporated into an organic molecule) in the first step.
Reduction - GP is reduced to TP by the addition of hydrogen from reduced NADP using energy supplied by ATP.
Regeneration - RuBP is regenerated from the recycled TP.
diagram of the Calvin cycle
Regeneration of RuBP:
For one glucose molecule to be produced six carbon dioxide molecules have to enter the Calvin cycle, resulting in six full turns of the cycle.
This will result in the production of 12 TP molecules, two of which will be removed to make the glucose molecule.
This means that 10 TP molecules are recycled to regenerate six RuBP molecules (used in the six turns of the cycle).
10 x three-carbon TP = 30 carbons ‘shuffled’ gives 6 x five-carbon RuBP = 30 carbons
Energy is supplied by ATP for the reactions involved in the regeneration of RuBP.
diagram of photorespiration
diagram of summary of photosynthesis
