Photosynthesis

Question 1

How are leaves adapted for photosynthesis?

Answer 1

• large S.A. of leaf
• thin leaf
• waxy cuticle - waterproof and transparent, prevents excess water loss via osmosis and traps moisture
• many stomata - allow diffusion of gases and conserve amount of water lost
• upper epidermis - one cell thick and transparent
• palisade mesophyll - tightly packed together and contain lots of chloroplasts
• spongy mesophyll - air spaces for efficient diffusion
• vascular bundle - xylem (water) and phloem (carbohydrates)

Question 2

What are the main stages of photosynthesis?

Answer 2

1. Capturing light energy (grana)
2. Light dependent reaction - light energy to chemical energy (grana)
3. Light independent reaction (stroma)

Question 3

What are the stages of LDR?

Answer 3

1. Photolysis
2. Photoionisation
3. Chemiosmosis
4. Production of ATP and reduced NADP

Question 4

Describe photolysis

Answer 4

• light energy absorbed in chlorophyll splits H2O into H+, O2 and e-
• occurs in thylakoids in chloroplasts

Question 5

Describe the photoionisation of chlorophyll

Answer 5

• when chlorophyll absorbs light energy it boosts a pair of e- (raising to a higher energy level)
• e- are in an excited state and leave the chlorophyll molecule
• chlorophyll is ionised

Question 6

Describe chemiosmosis

Answer 6

• e- that leave the chlorophyll molecule are taken up by e- carriers in thylakoid membrane
• chlorophyll has been oxidised
• e- carrier has been reduced
• e- passed along electron carriers in oxidation-reduction reactions and e- lose energy at each stage
• H+ from photolysis pumped from stroma and electrochemical gradient is made
• protons can only cross membrane through ATP synthase / stalked protein

Question 7

Describe production of ATP and reduced NADP

Answer 7

• protons catalyse formation of ATP in stalked protein
• in thylakoid membrane protons are taken up by NADP and combine with electrons from final electron carrier to make NADPH

Question 8

What are the products of the LIR?

Answer 8

• reduced NADP
• ATP
• oxygen

Question 9

Describe the Calvin cycle (LIR)

Answer 9

• CO2 diffuses into leaf via stomata
• CO2 combines with ribulose bisphosphate (RuBP) (5c) catalysed by ribulose bisphosphate carboxylase (RuBisCo)
• 6c breaks down into 2x glycerate 3-phosphate
• GP is reduced to form triose phosphate and NADPH is oxidised, energy from ATP also required
• TP is used to:
• 1C removed from one molecule to make hexose sugars
• 2C and 3C use energy from ATP to recombine into 5c ribulose bisphosphate and cycle continues

Question 10

What are the adaptations of chlorophyll for the LIR?

Answer 10

• stroma contains all enzymes needed separate from cytoplasm maintaining high concentration
• stroma fluid surrounds the grana and products of LIR in grana can diffuse into stroma
• chloroplasts contain DNA and ribosomes so they can manufacture proteins involved in LIR