Unit 5 - Photosynthesis

Question 1

Site of the Light Dependent Reaction

Answer 1

Thylakoid

Question 2

Stages of the Light Dependent Reaction

Answer 2

• Photoexcitation, Photoionisation and ETC
• Photolysis of Water
• Chemiosmosis to generate ATP
• Generation of NADPH

Question 3

Photoexcitation, Photoionisation and ETC

Answer 3

• Light is absorbed by the chlorophyll pigments in PSII and this causes the electrons to become excited
• They move to a higher energy electron carrier and through as series of redox reactions they move down the ETC
• As the electrons move to a lower energy electron carrier they release small amounts of energy

Question 4

Photolysis of Water

Answer 4

• Light energy is used to split up water into H⁺ ions, electrons and O₂
• 2H₂O –> 4H⁺ + 2O₂ + 4e-
• The electrons replace the electrons lost from PSII from photoexcitation and photoionisation
• The protons are used in chemiosmosis
• O₂ is a by-product

Question 5

Chemiosmosis

Answer 5

• The energy released from the electrons in the ETC is used to actively transport H⁺ ions from photolysis from the thylakoid membrane into the thylakoid space
• This creates a proton gradient
• The protons diffuse through ATP synthase into the stroma and this movement synthesises ATP from ADP and P(i)
• NADP is reduced by the protons in the stroma to form NADPH

Question 6

Generation of NADPH

Answer 6

NADP is reduced by the protons in the stroma and the addition of electrons from the end of the ETC to form NADPH

Question 7

Cyclic Phosphorylation

Answer 7

• Only involves PS1
• No NADPH is produced
• Still uses same mechanisms as non-cyclic phosphorylation

Question 8

Cyclic Phosphorylation Process

Answer 8

• Light energy absorbed by chlorophyll in PS1 excites the electrons so they move to a higher energy e- carrier
• Move down the ETC through redox reactions releasing energy as they move to a lower energy e- carrier
• However the electrons return to PS1 after reaching the end of the chain, unlike in non-cyclic phosphorylation
• Chemiosmosis happens as normal except H⁺ ions are actively transported from the stroma to the thylakoid space and then diffuse back to the stroma
• Proton gradient set up
• Movement of H⁺ through ATP synthesises ATP from ADP + P(i)

Question 9

Adaptations of the thylakoid for LDR

Answer 9

• Large Surface Area: Allows space for more electron carriers and chlorophyll
• ATP Synthase Channels : Allows ATP synthesis
• Selectively Permeable Membrane: Allows a proton gradient to be set up

Question 10

Site of the Light Independent Reaction

Answer 10

Stroma

Question 11

Stages of the Light Independent Reaction

Answer 11

• Carbon Fixation
• Reduction of GP
• Regeneration of RuBP

Question 12

Carbon Fixation

Answer 12

• CO₂ reacts with ribulose bisphosphate (RuBP), a 5C molecule to form a 6C molecule
• This 6C molecule is unstable and splits to form 2 3C molecules called glycerate phosphate
• The enzyme that catalyses this reaction is ribulose bisphosphate carboxylase

Question 13

Reduction of GP

Answer 13

• GP gets reduced into triose phosphate (TP)
• This uses energy from the hydrolysis of ATP to form ADP and P(i) and the enzyme ATP hydrolase
• This also uses H⁺ from NADPH so that is oxidised to form NADP
• NADP returns to the LDR to be reused

Question 14

Regeneration of RuBP

Answer 14

• For each turn of the Calvin Cycle, 5 out of the 6 carbon are used to regenerate RuBP
• One molecule is used to form glucose, therefore it takes 6 turns of the cycle to form 1 glucose molecule

Question 15

Uses of TP

Answer 15

Can be used to make:
- Simple sugars (e.g glucose)
- Larger Carbohydrates (e.g starch, cellulose, and sucrose)
- Amino Acids
- Lipids
- Nucleotides

Question 16

Limiting Factors of Photosynthesis

Answer 16

• Light Intensity
• CO₂ Concentration
• Temperature

Question 17

How does Light Intensity impact Photosynthesis

Answer 17

• Lower light intensity limits the LDR as it means there is a lower rate of photolysis and photoexcitation so less ATP and NADPH is formed
• This limits the LIR in itself as the products won’t be made so can’t be used in the LDR

Question 18

How doe CO₂ Concentration impact Photosynthesis

Answer 18

• Limits the LIR as it means less CO₂ is fixed so less GP and therefore less TP is produced as a result

Question 19

How does Temperature impact Photosynthesis

Answer 19

• A lower temperature means any enzyme catalysed reactions with happen at a slower rate due to lack of kinetic energy which decreases the rate of photosynthesis
• A temperature past the optimim will denature any enzymes so the rate of photosynthesis is lower