Respiration and Photosynthesis

Question 1

What are photosynthetic pigments?

Answer 1

• photosynthetic pigments are found in photosystems which are embedded on the thylakoid membranes and their role is to absorb light energy and convert it to chemical energy

Question 2

Describe and explain how light energy is harnessed and converted into chemical energy during the light-dependent reactions of photosynthesis (non-cyclic phosphorylation)

Answer 2

• light-dependent reactions occur in the grana and the objective of these reactions is to provide ATP and reduced nicotinamide adenine dinucleotide phosphate (reduced NADP/NADPH + H+) for the light-independent reactions
• light of particular wavelengths strike an accessory pigment molecule in the light harvesting complex of PSI and PSII and this energy is relayed to neighbouring accessory pigment molecules, until it accumulates and reach one of the two P680 chlorophyll a molecule in the reaction centre of PSII
• the same occurs for the P700 chlorophyll a molecules in the reaction centre of PSI
• light excites one of the P680 electrons or one of the P700 electrons to a higher energy state, which subsequently gets emitted and captured by the primary electron acceptor within each PS, leaving behind a positive ‘hole’ in the chlorophyll a molecules
• photolysis of water occurs when an enzyme catalyses the splitting of water molecule into protons, electrons and molecular oxygen
• the electrons are used to fill up the positive ‘holes’ in the reaction centre of PSII to return P680+ to ground state
• the photoexcited electrons passes from primary electron acceptor of PSII to P700+ in PSI to fill up the positive ‘hole’ in P700+ via an electron transport chain made up of electron carriers, each with an energy level lower than the one preceeding it
• energy from the electron transfer down the chain of electron carriers is used to actively pump protons from the stroma into the thylakoid space, generating an electrochemical proton gradient for the synthesis of ATP, as the protons diffuse through stalked particles containing ATP synthase, back into the stroma
  -electrons are subsequently passed from the primary electron acceptor of PSI to the protein Ferredoxin
• the enzyme NADP reductase catalyses the transfer of electrons from ferredoxin to oxidised NADP to form reduced NADP

Question 3

Describe and explain how light energy is harnessed and converted into chemical energy during the light-dependent reactions of photosynthesis (cyclic phosphorylation)

Answer 3

• PSI is now both a donor and acceptor of electons
• the excited electrons in the primary electron acceptor of PSI pass to ferredoxin and back to the cytochrome complex in the electron transport chain
• the electrons eventually return to the PSI reaction centre
• the energy released during the cycle of electrons down the chain of electron carriers allows protons to be actively pumped from the stroma into the thylakoid space, generating an electrochemical proton gradient across the thylakoid membrane, just like in non-cyclic phosphorylation

Question 4

Role of oxygen in aerobic respiration

Answer 4

• Oxygen acts as final electron and proton acceptor in oxidative phosphorylation to form water, catalyses by cytochrome oxidase
• Allows flow of electrons down the electron transport chain, regenerating oxidised FAD and NAD
• Oxidised NAD and FAD are raw materials for glycolysis, link reaction and Krebs cycle, allowing these processes to continue

Question 5

Why anaerobic respiration cannot be used continuously by the athlete to generate ATP during the long-distance run

Answer 5

When muscle cells undergo lactate fermentation under anaerobic respiration, pyruvate is converted directly to lactate which is accumulated. Accumulation of lactic acid is hazardous to tissues as it is acidic and will lower the pH of the cells, which is lower than optimum pH cellular enzymes function in.

Question 6

Explain how carbon dioxide is produced in respiration

Answer 6

Aerobic respiration:
- Produced during link reaction and Krebs cycle in mitochondrial matrix
- CO2 is produced via oxidative decarboxylation when pyruvate is converted to acetyl coA
- 2 molecules of CO2 produced from 1 molecule of citrate via decarboxylation
- When 1 molecule of glucose is completely oxidised, 6 molecules of CO2 is produced

Anaerobic respiration:
- During alcoholic fermentation in yeast, CO2 is released when pyruvate is converted to ethanol

Question 7

How energy released from flow of electrons in ETC during OP result in formation of ATP

Answer 7

• Energy released from flow of electrons is used to actively pump protons from the mitochondrial matrix into the inter membrane space, through conformational change of proteins in ETC
• Produces a high concentration of H+ in intermembrane space, setting up a steep electrochemical proton gradient and generating the proton motive force
• H+ diffuse through the stalked particles containing ATP synthases embedded in the inner mitochondrial membrane, down the electrochemical proton gradient , back into the matrix
• This provides energy for ATP synthase to catalyse the synthesis of ATP by the phosphorylation of ADP with inorganic phosphate

Question 8

Difference in flow of electrons in photophosphorylation and oxidative phosphorylation

Answer 8

• Light energy required for photolysis of water and photoactivation of primary pigment molecule in the reaction centre of photosystem, where one of the electrons of the P680/P700 chlorophyll a molecule gets excited to a higher energy state and gets emitted and then captured by primary electron acceptor whereas light energy is not required in OP
• The electron donors for non-cyclic phosphorylation are water, PSI, PSII compared to reduced NAD and reduced FAD for OP
• Oxidised NADP is the final electron and proton acceptor in the non-cyclic pathway and is reduced to reduced NADP catalyses by NADP reductive whereas oxygen is the final electron and proton acceptor and is reduced to water, catalyses by cytochrome oxidase

Question 9

OP regenerates FAD and NAD from the reduced FAD and reduced FAD, outline how this results in the production of ATP

Answer 9

• High energy electrons from reduced coenzymes are passed down a series of electron carriers on the ETC, each with an energy level lower than the one preceding it
• Energy from the flow of electrons is used to actively pump protons from matrix to intermembrane space, through conformational change of proteins in ETC
• This generates an electrochemical proton gradient across the inner mitochondrial membrane
• Protons diffuse down the gradient from the intermembrane space back into mitochondrial matrix through stalked particles and ATP synthase synthesizes ATP from ADP and phosphate

Question 10

Role of NAD in Krebs cycle

Answer 10

• NAD is a coenzyme to dehydrogenase
• NAD removes electrons and protons from the Krebs cycle to form reduced NAD
• Reduced NAD transfers high energy protons and electrons to electron transport chain which is embedded in the inner mitochondrial membrane, where oxidative phosphorylation takes place
• Resulting in oxidised NAD being regenerated

Question 11

Main stages of Krebs cycle

Answer 11

• Occurs in mitochondrial matrix
• During Krebs cycle, acetyl-CoA (2C) is attached to oxaloacetate (4C) to for citrate (6C) which is gradually converted back to oxaloacetate (4C)
• At 2 stages in the Krebs cycle, carbon is removed from the intermediate compounds via oxidative decarboxylation
• 2 molecules of carbon dioxide are produced per cycle and carbon dioxide diffuses out of the mitochondrion, and out of the cell
• 1 molecule of ATP is produced per cycle via SLP where the inorganic phosphate is obtained from guanosine triphosphate (GTP)
• Intermediate compounds undergo oxidation via dehydrogenation whereby electrons and protons are transferred to oxidised NAD and oxidised FAD
• Oxidised NAD and oxidised FAD are reduced to reduced NAD and reduced FAD respectively
• These coenzymes subsequently transfer these high energy protons and electrons to the electron transport chain for ATP synthesis
• Since 2 moles of acetyl CoA are formed per glucose molecule during the link reaction, the Krebs cycle runs twice to completely utilise them
• For each glucose molecule, the products of Krebs cycle are 4 molecules of carbon dioxide, 6 molecules of reduced NAD, 2 molecules of reduced FAD and 2 molecules of ATP

Question 12

Distinguish oxidative decarboxylation from oxidative phosphorylation

Answer 12

• Oxidative decarboxylation occurs in matrix of mitochondrion (LR & KC) whereas oxidative phosphorylation occurs in inner mitochondrial membrane, carried out by electron transport chain and stalked particles embedded within
• OD involves a single reaction while OP involves a series of redox reactions where the electron carriers alternate between reduced and oxidised state as they accept and donate electrons
• OD is catalysed by dehydrogenases whereas