photosynthesis uses light energy to synthesise organic molecules

What is photosynthesis?

A process by which plants, algae, and some bacteria convert light energy into chemical energy, producing organic molecules (like glucose) from carbon dioxide and water.

Give the general equation for photosynthesis

6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂

Where does photosynthesis occur?

In the chloroplasts of plant cells.

What are the two main stages of photosynthesis?

1. Light-dependent reactions (light reactions)

2. Light-independent reactions (Calvin cycle or dark reactions)

Describe the structure of a chloroplast.

Double membrane organelle with:

• Thylakoids: stacked into grana, containing chlorophyll for light absorption

• Stroma: fluid surrounding thylakoids, site of the Calvin cycle

• Intergranal lamellae: connections between grana

How are chloroplasts adapted for photosynthesis?

• Large surface area of thylakoid membranes for pigments

• Contain enzymes in stroma for Calvin cycle

• Fluid stroma allows diffusion of substances

Where do light-dependent reactions occur?

In the thylakoid membranes of chloroplasts.

What is the main purpose of light-dependent reactions?

To capture light energy to produce ATP, NADPH, and O₂.

What is photolysis?

The splitting of water molecules by light to produce electrons, protons (H⁺), and oxygen.

What are the main products of the light-dependent reactions?

ATP, NADPH (reduced NADP), and oxygen gas (O₂).

How is ATP produced in these reactions?

Via photophosphorylation, using energy from electrons moving through the electron transport chain.

Where do the light-independent reactions occur?

In the stroma of chloroplasts.

What is fixed in the Calvin cycle?

Carbon dioxide is fixed into organic molecules.

What enzyme catalyses carbon fixation?

Ribulose bisphosphate carboxylase/oxygenase (RuBisCO).

What are the main products of the Calvin cycle?

Glycerate-3-phosphate (G3P), which can be converted into glucose, and other organic molecules.

What are the inputs required for the Calvin cycle?

CO₂, ATP, and NADPH from the light-dependent reactions.

Name factors that can limit the rate of photosynthesis.

• Light intensity

• Carbon dioxide concentration

• Temperature

• Water availability

How does light intensity affect photosynthesis?

Higher light increases the rate up to a point; beyond this, another factor may become limiting.

How does CO₂ concentration affect photosynthesis?

Increased CO₂ raises the rate until light or temperature becomes limiting.

How does temperature affect photosynthesis?

Low temperature slows enzyme activity; very high temperatures can denature enzymes like RuBisCO.

How are leaves adapted for photosynthesis?

• Large surface area to absorb more light

• Thin for short diffusion distance

• Transparent cuticle and epidermis allow light penetration

• Stomata regulate gas exchange

How are chloroplasts adapted for photosynthesis?

• Granal membranes with pigments maximise light absorption

• Stroma contains enzymes and substrates for the Calvin cycle

• Thylakoid lumen provides a proton gradient for ATP synthesis

Why is photosynthesis essential for life on Earth?

• Produces glucose for energy and growth

• Releases oxygen for respiration

• Forms the base of food chains

• Converts solar energy into chemical energy

How does photosynthesis link to the carbon cycle?

It removes CO₂ from the atmosphere and stores it in organic compounds, balancing carbon levels.