Multicellular Organisms
Biology: AQA A Level, topic 3.2.2
Insects
Insects are multicellular organisms that use gas exchange surfaces. Gas exchange surfaces are parts of the body that are specialised for gas exchange.
Tracheal system
- Insects use a gas exchange surface called a tracheal system.
- The tracheal system consists of a network of small tubes called tracheoles that carry oxygen around the body.
Spiracles
- Insect bodies have openings along the thorax and abdomen called spiracles.
- Gases (e.g. oxygen) diffuse through the spiracles and into the tracheoles.
- Gases (e.g. carbon dioxide) also diffuse out of the spiracles.
Efficiency
- Gas exchange systems have evolved to maximise the rate of gas exchange.
- The tracheal system is the most efficient gas exchange system in active animals.
Fish
Fish are multicellular organisms that use gas exchange surfaces. Gas exchange surfaces are parts of the body that are specialised for gas exchange.
Gills
- Fish use an exchange surface that is specialised for obtaining oxygen from the water.
- Gills are filaments of thin tissue that are highly branched and folded.
- The structure of gills creates a large surface area for gas exchange.
- The gills are also covered in lamellae which further increase the surface area.
Diffusion of oxygen
- When water flows through the gills, oxygen in the water diffuses quickly into the bloodstream.
- Oxygen can diffuse from the water into the blood because of the counter-current system.
Counter-current system
- The circulatory system of fish is a counter-current system.
- In the counter-current system, blood flows through the lamellae in the opposite direction of the flow of water through the gills.
- The counter-current system ensures there is always a steep concentration gradient between the water and the blood.
Plants
Dicotyledonous plants are multicellular organisms that use gas exchange surfaces in their leaves. Gas exchange surfaces are parts of the body that are specialised for gas exchange.
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Mesophyll cells
- Gas exchange takes place in the mesophyll cells.
- Mesophyll cells are the site of photosynthesis and are located in the middle layer of plant leaves.
- Mesophyll cells have a large surface area to allow rapid exchange of carbon dioxide and oxygen with the environment.
Stomata
- Carbon dioxide diffuses into plant leaves through pores on the underside of the leaf called stomata (singular = stoma).
- The underside of the leaf is called the epidermis.
Guard cells
- Stomata also play a role in controlling water.
- There is a guard cell on either side of a stoma that can open and close the stomata to control water loss.
- When the guard cells swell, the stomata open.
Test yourself on Multicellular Organisms
Which of these is NOT a feature of the insect gas exchange system?
Which of the following statements about insect gas exchange are true?
Which of the following statements about gas exchange in fish are true?
What is the benefit of the counter-current system?
Features of the gills