LEARNING OBJECTIVES
By the end of this chapter, you should be able to:
- state photosynthesis equations
- explain carbohydrate uses
- describe mineral requirements
- investigate limiting factors
- relate leaf structures to photosynthesis
THE BIG IDEA
How leaf structure, light and mineral ions allow plants to manufacture carbohydrates.
This chapter builds each process from its structures, inputs and outputs. Follow the connections rather than memorising isolated facts, then use precise biological terms when explaining cause and effect.
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
- Glucose is respired, converted to starch for storage, made into cellulose for walls, or converted to sucrose for phloem transport.
- 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
- Main limiting factors are light intensity, carbon dioxide concentration and temperature.
- 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
- Transparent cuticle and upper epidermis allow light through; palisade cells near the top contain many chloroplasts.
- 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.
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.