EDEXCEL SNAB BIOLOGY Photosynthesis

Topics 5.5 to 5.8

Where do the light dependent reactions take place?

Thylakoid membrane

State the 7 main components of the light dependent reactions? (some stages repeated)

• electron excitement

• photolysis

• electron transport chain

• chemiosmosis

• PS I excitement

• chemiosmosis

• production

Initial electron excitement

• light energy hits photosystem II

• two electrons are excited to a higher energy level

• The electrons leave photosystem II

Photolysis

• one molecule of water is split

• Useful products: 2H⁺ and 2e^-

• Waste: oxygen (one atom)

What are the waste products of the Light Dependent Reactions?

oxygen

What are the electrons in photolysis used for?

They replace the electrons lost from PS II.

Electron Transport Chain

Energy is released as electrons pass down the electron transport chain → used to pump protons into thylakoid space

Chemiosmosis

• H⁺ ions are pumped from a high to low concentration from the stroma to the thylakoid membrane

• a protein gradient is formed

• ATP synthase facilitates the diffusion of H⁺ ions back into the stoma

• the movement of H⁺ ions causes ATP synthase enzyme to catalyse the production of ATP.

Second Excitement of Electrons

• At the end of the ETC, electrons are passed onto PS I.

• light energy hits PS I.

• a pair of electrons are excited to a higher energy level

• the two electrons leave PS I and pass along the ETC

• chemiosmosis takes place here

Production

These two electrons react with the hydrogen ions from the photolysis of water and the coenzyme NADP to produce NADPH (or reduced NADP)

Equation for Light Dependent Reactions

H⁺ + 2e^- + NADP⁺ → NADPH

Cyclic Phosphorylation

• Light hits Photosystem I

• electrons are excited to a higher energy level

• energy is released as the electrons pass through ETC

• chemiomosis takes place

• At the end of ETC, electrons rejoin PS I

• no NADPH produced

When is cyclic phosphorylation used?

when higher volumes of ATP are required.

where do the light Independent Reactions take place?

Stroma

RuBP

ribulose bisphosphate

State the three main components of Light Independent Reactions

1. CO₂ and RuBP react (carbon fixation)

2. Reduction of glycerate 3-phosphate

3. Regeneration of RuBP

carbon fixation

• carbon dioxide reacts with a 5C sugar RuBP

• this is catalysed by the enzyme rubisco

• an unstable 6C carbon compound is formed which immediately splits

• 2 GP are produced

2. Reduction of glycerate 3-phosphate

• Energy from ATP and hydrogen from NADPH

→ reduce 2GP → 2GALP

• ADP and NADP are left

3. Regeneration of RuBP

• energy from ATP facilitates:

• 5 carbons regenerate RuBP

• 1 carbon is used to form glucose

• 6 turns of the Calvin cycle produce one mole of glucose

GALP

glyceraldehyde 3-phosphate

GP

glycerate 3-phosphate

light intensity as a limiting factor

• high rate of light independent reactions

• high levels of ATP and reduced NADP

• high rate of light independent reactions

• until CO₂ becomes limiting factor

Temperature as a limiting factor

• rubisco enzyme functions at a low rate

• low rate of light independent reactions

• low rate of photosynthesis

• until light intensity becomes limiting factor

• NOW limited by light dependent reactions

Water as a limiting factor

• stomata close - to prevent water loss by transcription

• less CO₂ diffuses into leaves

• CO₂ becomes limiting factor