AQA GCSE Photosynthesis (Biology)
Photosynthesis
Photosynthesis is how plants and algae make their own food.
Algae are a unicellular organism, whilst plants are multicellular.
We will mostly focus on photosynthesis in plants.
Photosynthesis in plants
Photosynthesis happens mainly in leaves.
Inputs (raw materials): carbon dioxide & water
Outputs(products): glucose & oxygen
Where Photosynthesis occurs within plants
Photosynthesis is an endothermic chemical reaction that occurs within chloroplasts.
Chloroplasts are found in leaf cells, such as palisade mesophyll cells and spongy mesophyll cells.
An image of a leaf is show below with both of these cell types.
How leaves are adapted for photosynthesis
Leaves are the main photosynthetic organs. An organ is a group of tissues working together to carry out a specific function.
Leaves are adapted for photosynthesis by having:
1. Large surface area to absorb light energy.
2. Leaves are thin, short diffusion distance for gases.
3. Leaves are flat to absorb the maximum amount of light.
Within the leaf there are various tissues such as palisade mesophyll and spongy mesophyll tissues.
A tissue is a group of cells with a similar structure and function.
Palisade mesophyll tissue
(see this tissue in the image above)
Located near the top of the leaf
Cells contain many chloroplasts
This is where most photosynthesis happens
Spongy mesophyll tissue
(see this tissue in the image above)
Contains air spaces
Allows carbon dioxide to diffuse to photosynthesising cells
Transport tissues for photosynthesis
Xylem – transports water to the leaf, water is a reactant in photosynthesis
Phloem – transports sucrose away from the leaf
Leaf Surfaces and Gas Exchange
Stomata are pores mainly on the lower leaf surface. Guard cells, either side of the stomata will open or close the stomata for gas exchange.
When light shines on the leaf, the stomata open.
Carbon dioxide diffuses into the leaf for photosynthesis
Oxygen diffuses out of the leaf after photosynthesis
Photosynthesis as an Endothermic reaction.
Photosynthesis needs energy, the energy comes from sunlight.
As photosynthesis takes in energy, the reaction is endothermic.
Light energy is transferred from the environment to the chloroplasts.
Energy is needed to make glucose.
Role of Chlorophyll
Chlorophyll is a green pigment, which absorbs light energy. It is found inside chloroplasts.
Equation for photosynthesis
Practice Questions
1.What is photosynthesis?
2.Which two types of organisms can carry out photosynthesis?
3.Where in a plant does photosynthesis mainly occur?
4.Name two raw materials needed for photosynthesis.
5.Name two products of photosynthesis.
6.Explain why photosynthesis happens in leaves rather than roots.
7.Explain why photosynthesis is described as an endothermic reaction.
8.Explain why chloroplasts are found in palisade mesophyll cells.
9.Explain why palisade mesophyll cells are located near the top of the leaf.
10.Explain how a large surface area helps a leaf carry out photosynthesis.