LEARNING OBJECTIVES
What you will be able to do
- state photosynthesis equations
- explain carbohydrate uses
- describe mineral requirements
- investigate limiting factors
- relate leaf structures to photosynthesis
AT A GLANCE
INTRODUCTION · THE BIG IDEA
How leaf structure, light and mineral ions allow plants to manufacture carbohydrates.
Plants manufacture carbohydrate by transferring light energy into chemical energy. Leaf structure brings light, carbon dioxide and water together while allowing products to move away.
Photosynthesis questions often combine equations, limiting-factor graphs and practical evidence, so connect each observation to the underlying process.
SECTION 01
Photosynthesis and carbohydrate use
Photosynthesis transfers light energy into chemical energy by making glucose from carbon dioxide and water in chloroplasts containing chlorophyll.
DETAILED EXPLANATION
- Carbohydrate also contributes to nectar, which attracts pollinators.
- Nitrate ions are needed for amino acids; magnesium ions are needed for chlorophyll.
SECTION 02
Limiting factors and investigations
The photosynthesis rate is controlled by the factor in shortest supply. Increasing that factor raises the rate until a different factor becomes limiting.
DETAILED EXPLANATION
- Destarch a plant before testing a leaf. Use iodine after killing the leaf and removing chlorophyll.
- Use appropriate controls to test the need for light, chlorophyll or carbon dioxide.
- Hydrogencarbonate indicator shows net gas exchange in light and darkness.
SECTION 03
Leaf structure
A broad, thin leaf captures light over a large area and provides a short diffusion path. Its tissues organise light capture, gas exchange and transport.
DETAILED EXPLANATION
- Spongy mesophyll air spaces create a large internal diffusion surface.
- Stomata controlled by guard cells regulate gas exchange and water loss.
- Xylem supplies water and minerals; phloem carries sucrose and amino acids away.
STEP-BY-STEP EXAM EXAMPLE
Identifying a limiting factor
- A plant's photosynthesis rate rises as light intensity increases, then becomes constant.
- The initial rise shows that light was limiting.
- The plateau means light is no longer the factor in shortest supply.
- Consider carbon dioxide concentration or temperature as the new limitation.
Answer: Light limits the first part; another factor limits the plateau.
QUICK CHAPTER SUMMARY
The ideas to carry forward
- Photosynthesis makes carbohydrate using light energy.
- Plants use glucose in several pathways.
- Rates depend on limiting factors.
- Leaf tissues balance light capture, diffusion, transport and water conservation.
QUICK REVISION CHECKLIST
Can you do each of these without your notes?
- state photosynthesis equations
- explain carbohydrate uses
- describe mineral requirements
- investigate limiting factors
- relate leaf structures to photosynthesis