Topic 5A - Photosynthesis And Respiration Flashcards
2nd stage of photosynthesis - The light independent reaction
(The Calvin cycle)
Summary
This is also called the Calvin cycle and it does not use light energy.
But it does rely on the products of the light-dependent reaction.
It takes place in the stroma of the chloroplast.
Here, the ATP and reduced NADP from the light dependent reaction supply the energy and hydrogen to make simple sugars from CO2.
E.g. Glucose
1st stage of photosynthesis - The light dependent reaction
Summary
This reaction needs light energy.
It takes place in the thylakoid membranes of the chloroplasts.
Here, the light energy is absorbed by chlorophyll in the photosystems. The light energy excites the electrons in the chlorophyll, leading to their eventual release from the molecule. The chlorophyll has been photoionised.
Some of the energy from the released electrons is used to add a phosphate group to ADP to form ATP, and some is used to reduce NADP to form reduced NADP. ATP transfers energy and reduced NADP transfers hydrogen to the light independent reaction.
During this process H2O is oxidised to O2.
Light dependent reaction-
Uses of energy from the photoionisation of chlorophyll
1) Making ATP from ADP and inorganic phosphate. This reaction is called photo phosphorylation
2) Making reduced NADP from NADP.
3) Splitting water into protons (H+ ions), electrons and oxygen. This is called photolysis.
Light dependent reaction
1) Light energy excites electrons in chlorophyll
Light energy is absorbed by photosystem II.
The light energy excites electrons in chlorophyll.
The electrons move to a higher energy level.
These high energy electrons are released from the chlorophyll and move down the electron transport chain to photosystem I.
Light dependent reaction
2) Photolysis of water produces protons (H+ ions), electrons and oxygen
As the excited electrons from chlorophyll leave PSII to move down the electron transport chain, they must be replaced.
Light energy splits water into protons (H+ ions), electrons and oxygen. This is called photolysis.
Light dependent reaction
3) Energy from the excited electrons makes ATP
The excited electrons lose energy as they move down the electron transport chain.
This energy is used to transport protons into the thylakoid, so that the thylakoid has a higher concentration of protons than the stroma. This forms a proton gradient across the thylakoid membrane.
Protons move down their concentration gradient, which is embedded in the thylakoid membrane. The energy from this movement combines ADP and inorganic phosphate to form ATP.
Light dependent reaction
4) Generate reduced NADP
Light energy is absorbed by PSI, which excites electrons again to an even higher energy level.
Finally, the electrons are transferred to NADP, along with a proton (H+ ion) from the stroma, to form reduced NADP.
Light independent reaction (Calvin cycle)
1) Carbon dioxide is combined with ribs lose biphosphate to form two molecules of glycerate 3-phosphate.
Carbon dioxide enters the leaf through the stomata and diffuses into the stroma of the chloroplast.
Here it’s combined with ribulose phosphate (RuBP), a 5-carbon compound. This reaction is catalysed by the enzyme rubisco.
This gives an unstable 6-carbon, which quickly breaks down into two molecules of a 3-carbon compound called glycerate 3-phosphate (GP)
Calvin cycle
2) ATP and reduced NADP are required for the reduction of GP to triose phosphate
The hydrolysis of ATP from the LDR provides energy to turn the 3-carbon compound GP, into a different 3-carbon compound called TP.
This reaction also requires H+ ions which come from reduced NADP also in the LDR. Reduced NADP is recycled to NADP.
Some triose phosphate is then converted into useful organic compounds (glucose) and some continues in the Calvin cycle to regenerate RuBP.
Calvin cycle
3) Ribulose biphsophate is regenerated
Five out of every six molecules of TP produced in the cycle aren’t used to make hexose sugars, but to regenerate RuBP.
Regenerating RuBP uses the rest of the ATP produced by the LDR.
Two types of respiration
Aerobic and anaerobic
Both produce ATP although anaerobic produces less
Both start with the process of glycolysis but after glycolysis the stages differ.
What glycolysis involves
Glycolysis involves splitting one molecule of glucose with 6 carbon atoms into two smaller molecules of pyruvate with 3 carbons.
This process happens in the cytoplasm of cells.
Glycolysis is the first stage of both aerobic and anaerobic respiration and doesn’t need oxygen to take place, therefore it’s an anaerobic process.
Two stages of glycolysis?
Phosphorylation
Oxidation
Stage one of glycolysis - Phosphorylation
Glucose is phosphorylated using a phosphate from a molecule of ATP.
This creates one molecule of glucose phosphate and one molecule of ADP.
ATP is then used to add another phosphate, forming hexose biphosphate.
Hexose biphosphate is then split into two molecules of triose phosphate.
Stage two of glycolysis - Oxidation
Triose phosphate is oxidised (loses hydrogen), forming 2 molecules of pyruvate.
NAD collects the hydrogen ions forming 2 reduced NAD.
4 ATP are produced, but two were used up in stage one, so there’s a net gain of 2ATP.
