IGCSE Plants

Photosynthesis

lightcarbon dioxide+water glucose+oxygen

chlorophyll

6H2O + 6CO2 C6H12O6 + 6O2

Leaf Structure

• The leaf has a waxy cuticle to stop it losing water.

• The epidermis is a protective layer of cells and contains no chloroplasts.

• The palisade layer contains the most chloroplasts as it is near the top of the leaf. The chloroplasts contain the pigment chlorophyll. It is here that photosynthesis takes place.

• The palisade cells are arranged upright. This means the light has to pass through the cell lengthways and so increases the amount of light absorbed.

• The spongy layer allows for diffusion and osmosis.

Stomata

Water moves into the guard cells by osmosis and the stoma opens

Guard cells

stoma

During the daytime the rate of photosynthesis is greater than the rate of respiration

Day

During both the day and night respiration occurs in plants.

Night

Limiting Factors

• Photosynthesis is a chemical reaction, its rate depends upon temperature, how much CO2 is available, light intensity, amount of chlorophyll or water.

• Without enough light a plant cannot photosynthesise very fast, even if there is plenty of water and carbon dioxide. Increasing the light intensity will make photosynthesis faster.

• Sometimes photosynthesis is limited by the level of carbon dioxide. Even if there is plenty of light a plant cannot photosynthesise if it has run out of carbon dioxide.

• Temperature can be a limiting factor too. If it gets too cold the rate of photosynthesis will slow right down; equally, plants cease to be able to photosynthesise if it gets too hot.

• If you plot the rate of photosynthesis against the levels of these three limiting factors you get graphs like the ones below.

Maximising growth

• Understanding the factors that limit photosynthesis enables greenhouse farmers to maximise the conditions for plant growth. They often use paraffin lamps inside the greenhouse because burning paraffin produces carbon dioxide as well as heat, and so makes photosynthesis proceed faster. They may also use artificial light to enable photosynthesis to continue beyond daylight hours.

Uses of Glucose

• Turned into starch for storage• Converted into lipid/fat for storage – energy

rich• Nitrogen can be added and turned into

protein• Stored in fruit• Used in respiration

Mineral Requirements

Magnesium for chlorophyll

Nitrogen for growth

Phosphorus for cell membranes and DNA

Remember how to test leaves for starch