Multicellular Organisms And Transport Systems - OCR 21st Century

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Pages

1. The need for diffusion
2. Limitation of diffusion in multicellular organisms
3. Multicellular organisms and transport systems
4. Adaptations to maximise exchange surface exchange
5. Types of transport
6. Blood
7. Blood vessels
8. The heart

Multicellular organisms and transport systems

Multicellular organisms have a small surface area to volume ratio. They need transport systems to be able to get all of the substances that cells need to the correct place.

Features of transport systems

Common features of transport systems:

• they are made up of tubes or 'vessels' that carry materials from one part of the organism to another
• they make close contact with cells, such as those of exchange surfaces

The circulatory system

The circulatory system transports substances, such as oxygen, around the body in the bloodstream. It links together all the other systems in the body.

The digestive system

Multicellular organisms take in food by eating. This is broken down in the digestive system. The dissolved food molecules are transferred into the bloodstream at the small intestine by diffusion and other transport processes.

Once the dissolved food molecules are in the bloodstream, they can be transported to all the cells in the body. This is particularly important for glucose as it is needed by all cells for the process of respiration.

The gaseous exchange system

When we breathe in air, it goes into lungs and oxygen diffuses from the lungs into our bloodstream. The oxygen is then carried around the body by red blood cells in the bloodstream to the cells that need it for respiration. At the respiring cells, waste carbon dioxide diffuses into the bloodstream to be taken back to the lungs to be exhaled. Like with the digestive system, the success of the gaseous exchange system relies on the circulatory system.

The excretory system

The liver is an organ that processes the body's waste products, for example, urea, which is made when excess amino acids are broken down. Too much urea is toxic so the body must get rid of it. The urea is transported from the liver to the kidneys using the circulatory system. Here, urea is filtered out of the blood and ends up in the bladder as part of urine.

What substances are transported in the blood?

Substance transported	From	To
Oxygen	Lungs	All the body's cells
Carbon dioxide	All the body's cells	Lungs
Glucose	Digestive system	Liver, then all the body's cells
Urea	Liver cells	Kidneys

Substance transported	Oxygen
From	Lungs
To	All the body's cells

Substance transported	Carbon dioxide
From	All the body's cells
To	Lungs

Substance transported	Glucose
From	Digestive system
To	Liver, then all the body's cells

Substance transported	Urea
From	Liver cells
To	Kidneys

Other substances transported in the blood include:

• antibodies
• hormones
• amino acids (from the digestion of protein)
• mineral ions
• water

Next pageAdaptations to maximise exchange surface exchangePrevious pageLimitation of diffusion in multicellular organisms

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