photosynthesis and respiration

Photosynthesis: Light dependent reaction

• → non-cyclic photophosphorylation

• light energy is absorbed by accessory pigments in PSII and PSI

• light energy is passed via accessory pigments to special chlorophyll a in reaction centres (P680 and P700)

• electrons are excited and displaced from primary pigments of respective reaction centres and transferred to primary electron acceptor

2. from primary electron acceptors, electrons are transferred along electron transport chain with electron carriers with progressively lower energy levels (redox reactions)

3. Energy released during this process is used by proton pumps to transport protons from from stroma into thylakoid space against a concentration gradient via active transport→ creates a proton gradient across thylakoid membrane

4. Protons move from thylakoid space back into stroma through ATP synthase via facilitate diffusion down a concentration gradient

• > ATP synthase synthesizes ATP from ADP and Pi

• > chemiosmosis → process where proton gradient is used to drive ATP synthesis

• > ATP synthesized is used in light independent reactions (Calvin cycle)

5. Electrons flow down electron transport chain of PSII and PSI, combine with NADP+ and H+ in stroma to form NADPH → catalysed by NADP reductase

6. PSI receive replacement electrons from PSII

7. PSII receives a replacement of electrons from photolysis of water which produces electrons, H+ and oxygen → water is electron donor for non-cyclic photophosphorylation

Photosynthesis: Light dependent reaction

→ cyclic photophosphorylation

• light energy is absorbed by accessory pigments in PSII and PSI

• light energy is passed via accessory pigments to special chlorophyll a in reaction centres (P680 and P700)

• electrons are excited and displaced from primary pigments of respective reaction centres and transferred to primary electron acceptor

2. from primary electron acceptors, electrons are transferred along electron transport chain with electron carriers with progressively lower energy levels (redox reactions)

3. Energy released during this process is used by proton pumps to transport protons from from stroma into thylakoid space against a concentration gradient via active transport→ creates a proton gradient across thylakoid membrane

4. Protons move from thylakoid space back into stroma through ATP synthase via facilitate diffusion down a concentration gradient

• > ATP synthase synthesizes ATP from ADP and Pi

• > chemiosmosis → process where proton gradient is used to drive ATP synthesis

• > ATP synthesized is used in light independent reactions (Calvin cycle)

5. The electrons are passed back to special chlorophyll a (final electron acceptor) in reaction centre P700, no NADPH synthesized

Photosynthesis: Light independent reactions

→ Calvin cycle

1. carbon dioxide fixation

• 1 molecule of carbon dioxide combines with a 5C ribulose bisphosphate molecule to give 6C intermediate → catalysed by rubisco

• 6C intermediate is unstable, breaks down into 2 3C molecules of glycerate-3-phosphate (GP)

2. PGA reduction

• each molecule of GP receives 1 additional phosphate group from an ATP molecule to form glycerate-1,3-bisphosphate

• each molecule of glycerate-1,3-bisphosphate is reduced by NADPH from LDR

• gylcerate-1,3-bisphosphate also loses a phosphate group to form glyceraldehyde-3-phosphate (GALP) / triose phosphate (TP)

• GALP/ TP is first carbohydrate product synthesized during photosynthesis

• NADP+, ADP, Pi are recycled at thylakoid membrane for LDR

2. ribulose bisphosphate regeneration

• small proportion of GALP/ TP exits Calvin cycle and is used as starting material for metabolic pathways that synthesize organic compounds

• majority of GALP/ TP remains in Calvin cycle to regenerate RuBP. ATP from LDR provides energy and phosphate for rearrangement of carbon atoms between sugar phosphates to regenerate a 5C RuBP from 3C molecules

• ADP, Pi recycled at thylakoid membrane for LDR

Respiration: glycolysis

1. Phosphorylation of Hexose (2 ATP used)

• glucose is phosphorylated by 1 molecule of ATP to give glucose-6-phosphate (6C) → make glucose more reactive

• glucose-6-phosphate is isomerized to give fructose-6-phosphate (6C)

• fructose-6-phosphate (6C) is further phosphorylated by 1 molecule of ATP to give fructose-1,6-bisphosphate (6C)

2. Splitting of 6C bisphosphate

• fructose-1,6-bisphosphate splits into 2 molecules of TP (3C) [dihydrixyacetone phosphate + glyceraldyhyde-3-phosphate]

• dihydroxyacetone phosphate converts into glyceraldehyde-3-phosphate

3. Oxidation

• oxidation of glyceraldehyde-3-phosphate by NAD+ forms 1,3-bisphosphoglycerate

• NAD+ reduced to NADH → 1 glucose gives 2 GALP molecules, 2 NADH molecules from 1 glucose molecule

• NADH produced goes to electron transport chain

4. Substrate level phosphorylation (total 4 ATP molecules produced)

• substrate level phosphorylation is formation of ATP by direct enzymatic transfer of high-energy phosphate group to ADP from organic substrate

• 1,3-bisphosphoglycerate undergoes a series of steps to get pyruvate and generate 2 molecules of ATP (1 molecule of 1,3-bisphosphoglycerate)

Products

• net 2 ATP

• 2 NADH

• 2 pyruvate

Respiration: Krebs cycle