AQA GCSE Diffusion for multicellular plants (Biology)
Diffusion for multicellular plants.
Diffusion is slower in multicellular organisms as they have a smaller surface area to volume ratio. This is because less of the surface is exposed to the environment, also there are multiple cell layers deep which increases the diffusion distance, slowing down diffusion.
In the diagram below, Water moves from the soil, into the root hair cell, then across the structure of the root passing through several layers of cells to reach the xylem. This is an example of multiple cell layers deep.
To solve these problems multicellular organisms have exchange surfaces and a transport system. Using both an exchange surface and transport system allows diffusion to be fast enough to meet the needs of the organism in terms of exchanging substances.
Without specialist exchange surfaces and transport system diffusion alone would not be sufficient to meet the needs of the multicellular organism
Exchange surfaces
In multicellular organisms, surfaces and organ systems are specialised for exchanging materials. This is to allow sufficient molecules to be transported into and out of cells to meet the organism’s needs.
Multicellular organisms have a specialist exchange surface to increase the rate of diffusion
A good exchange surface should have the following features:
1.Large Surface area
2.Thin membrane to provide short diffusion pathway
Exchange in Leaves
The leaf is the main photosynthetic organ in the plant. On the underside of the leaf there are stomata (pores). When the stomata are open it allows gas exchange to occur.
Carbon dioxide will diffuse from a high concentration outside the leaf to inside the leaf and then into the chloroplasts of mesophyll cells for photosynthesis.
Oxygen which is produced in photosynthesis will diffuse from the mesophyll cells to outside of the leaf, through the stomata.
Leaves are thin, this provides a short diffusion distance for the gases.
Within the spongy mesophyll layer of the leaf there are air spaces to allow the gases to circulate.
Exchange in roots
Roots of a plant anchor the plant to the soil. They absorb mineral ions using active transport and absorb water by osmosis.
The root system of a plant is highly branched to increase surface area and contains many root hair cells.
Root hair cells have a root hair which increases surface area for absorption.
Root hair cells have thin walls, short diffusion distance.
Practice Question
1.Explain why diffusion is slower in multicellular organisms compared to single celled organisms
2. State the names of two exchange surfaces in plants
3. How are exchange surfaces adapted for their role in diffusion.