Photosynthesis

Watch first

MrExhamPhotosynthesis — equation and limiting factors

CognitoLimiting factors and interactions (Higher)

Key facts & methods

Photosynthesis equation and basics

Word equation: carbon dioxide + water → glucose + oxygen
Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Photosynthesis is an endothermic reaction — energy is transferred FROM the environment.
Light energy absorbed by chlorophyll in chloroplasts, mainly in leaf mesophyll cells.
CO₂ enters through stomata; water absorbed by roots and transported in xylem.

Limiting factors

A limiting factor is the factor preventing the rate from increasing further.
Light intensity — more light → faster rate (up to a point). Inverse square law: double distance → quarter intensity.
CO₂ concentration — more CO₂ → faster rate until another factor limits.
Temperature — increases rate up to optimum (~25–35°C), then enzymes denature and rate falls.
Chlorophyll concentration — more chlorophyll → more light absorbed.

Interactions of limiting factors (Higher)

More than one factor can interact. Graphs show rate vs light intensity at different CO₂/temperature levels.
At low light: light is the limiting factor regardless of CO₂ or temperature.
At high light + high CO₂ + high temperature: maximum rate achieved — all factors optimal.
Top line on graph = high CO₂ AND high temperature. Lines plateau when light is no longer limiting.
Greenhouses: horticulturalists control all factors (add CO₂, use heaters, artificial lights) to maximise growth.

Uses of glucose from photosynthesis

Respiration — releases energy for life processes.
Starch — insoluble storage in leaves, stems and roots.
Cellulose — for cell walls (structural).
Proteins — combined with nitrate ions for growth/repair.
Fats and oils — storage, especially in seeds.
Starch test: iodine solution → turns blue-black in presence of starch.

Exam questions — 5 questions · 10 marks · AQA 8461 style

4 marksLimiting factors graphAQA 8461 P2 style

A student measures the rate of photosynthesis of pondweed at different light intensities, first at 15°C and then at 25°C. At low light intensities the two lines are identical, but at high light intensities the 25°C line gives a higher rate. Explain these results. (4 marks)

Hint: Two separate regions to explain: low light (same) and high light (different). Think about which factor is limiting in each region.

+40 XP

4 marksUses of glucoseAQA 8461 style

Glucose produced in photosynthesis can be used in several ways. Describe THREE ways a plant uses glucose, other than respiration. For each, state the purpose. (3 marks)

Hint: Name the molecule glucose is converted to AND explain what it is used for. Three separate points needed.

+40 XP

1 markPhotosynthesis equationAQA 8461 style

Which gas is produced during photosynthesis and used in aerobic respiration?

A Carbon dioxide

B Nitrogen

C Oxygen

D Hydrogen

3 marksStarch test required practicalAQA 8461 style

A student tests a leaf for starch using iodine solution. (a) What colour change indicates starch is present? (b) Why must the leaf be boiled in ethanol first? (c) Why is starch tested rather than glucose? (3 marks)

Hint: (a) Specific colour change. (b) Purpose of ethanol step. (c) Chemical property that makes starch detectable.

+30 XP

1 markLimiting factor identificationAQA 8461 style

A plant is given high light intensity and high CO₂ but the rate of photosynthesis does not increase beyond a certain point. What is most likely to be the limiting factor?

A Light intensity

B CO₂ concentration

C Temperature

D Water

Photosynthesis equation and basics

Limiting factors

Interactions of limiting factors (Higher)

Uses of glucose from photosynthesis

Module complete! 🎉