3.5.1 Photosynthesis

What are the stages of photosynthesis?

1. Light dependent reaction ( In thylakoid membrane of chloroplast )

2. Light independent ( In stroma of chloroplast )

Describe the photoionisation in the LDR:

• Chlorophyll absorbs light energy which excites electrons

• So electrons are released from chlorophyll and become positively charged

What happens after photoionisation in LDR?

1. Electrons move along electron transfer chain, releasing energy

2. This energy is used to actively transport protons from stroma into thylakoid

3. Protons move by facilitated diffusion down electrochemical gradient into stroma by ATP synthase.

4. Energy used to join ADP+Pi to form ATP

5. NADP accepts a proton and electron to become reduced NADP

Describe photolysis of water in LDR:

Water splits to produce protons, electrons and oxygen

Electrons replace those lost from chlorophyll

Describe the electron transfer chain

Electrons pass along electron transfer chain

Via a series of oxidation/reduction reactions

What is an oxidation reaction?

When a substance loses hydrogen & electrons.

What is a reduction reaction?

When a substance gains hydrogen & electrons.

Describe how proton gradient is made

There is active transport of protons from stroma into thylakoid space

Ensures there’s a higher concentration of protons in thylakoid space

How is the proton gradient maintained?

Active transport

Describe the LIR of photosynthesis (Calvin cycle).

1. CO2 reacts with ribulose bisphosphate (RuBP) → catalysed by the enzyme rubisco

2. Forming 2 glycerate-3-phosphate (GP) molecules

3. GP reduced to triose phosphate (TP) → using reduced NADP and energy from ATP

4. Some TP converted to useful organic substances (eg. glucose, amino acids & glycerol)

5. Some TP used to regenerate RuBP in the Calvin cycle (using energy from ATP)

Describe and explain how temperature affects rate of photosynthesis.

1. As temperature increases, rate increases ( → Enzymes eg. rubisco gain kinetic energy, so more enzyme-substrate complexes form )

2. Above an optimum temperature, rate decreases (→ Enzymes denature as H bond in tertiary structure break, so fewer enzyme-substrate complexes form )

Describe and explain how light intensity affects rate of photosynthesis.

1. As light intensity increases, rate increases ( → LDR increases (more photoionisation of chlorophyll) so more ATP & reduces NADP produced so LIR increases as more GP reduced to TP and more TP regenerates RuBP )

2. Above a certain light intensity, rate stops increasing ( → Another factor is limiting eg. temp/CO2 concentration

Describe and explain how CO2 concentration affects rate of photosynthesis.

1. As CO2 concentration increases, rate increases ( → LIR increases as more co2 combined with RuBP to form GP so more GP reduced to TP so more TO converted to organic substances and more RuBP regenerated )

2. Above a certain CO2 concentration, rate stops increasing ( → Another factor is limiting )