Introduction to Exchange and Transport

In order for organisms to function, biological molecules need to be moved around. The processes by which biological molecules are moved can be put into two categories: exchange and transport.

Exchange is the movement of substances in and out of an organism

All organisms need to take in certain substances from their surroundings, and remove waste products into the surroundings. The movement of substances in and out of an organism is called exchange, because the organism is exchanging substances with its surroundings.

Exchange is the movement of substances in and out of an organism.

Exchange does not have to involve a direct trade of one substance for another. Even if a process only involves taking substances in, or only involves removing substances, it is still considered exchange. This is because all of these individual processes are part of a bigger picture in which the organism is exchanging substances with its environment.

For example, in animals, nutrients from food are absorbed into the blood from the intestines. This process only involves taking substances in, but it is still considered an exchange process because it happens alongside other processes in which substances are removed - such as the removal of urea through urination and the removal of carbon dioxide by breathing out.

Transport is the movement of substances within an organism

Biological molecules also need to be moved around within organisms. In unicellular organisms, biological molecules often need to be moved from one part of the cell to another. In multicellular organisms, there is not just a need to move substances between different parts of cells, but also to move substances between different cells, tissues and organs.

The movement of biological molecules within an organism is referred to as transport.

Transport is the movement of substances within an organism.

Exchange and transport work together

Exchange processes and transport processes work together to provide for the needs of an organism.

Exchange and transport work together.

An example of this is gas exchange in animals. First, oxygen is taken in from the surroundings by the lungs or gills (exchange). Then this oxygen is carried in the blood to the places where it is needed (transport). The oxygen is used in aerobic respiration, which generates carbon dioxide as a waste product. The carbon dioxide is then carried in the blood to the lungs or gills (transport) where it is removed from the body (exchange).

Another example can be found in the processes of feeding and excreting in animals. Animals take in useful biological molecules such as carbohydrates and proteins when they eat food (exchange). These useful biological molecules are taken around the body by the blood so that they can be absorbed by cells that need them (transport). If there is too much protein, the liver breaks down the excess, which produces a waste product called urea. The urea is carried in the blood from the liver to the kidneys (transport), where it is filtered out of the blood and into the urine, which is then removed from the body through urination (exchange).

Diffusion, osmosis and active transport

Diffusion, osmosis and active transport are three important processes by which substances are moved. They can be used for both exchange (moving substances in and out of organisms) and transport (moving substances around within organisms).

The details of these three processes will be explained over the next few pages.

Multicellular organisms need exchange surfaces

For unicellular organisms, exchange is simply a processes of moving substances in and out of the cell. This involves moving substances from one side of the cell membrane to the other.

However, for multicellular organisms, exchange is more complicated. If a multicellular organism had a simple shape like a unicellular organism then its surface area would not be large enough to carry out enough exchange to supply the needs of its large volume.

Therefore, multicellular organisms have specialised structures called exchange surfaces which are used to carry out exchange. An exchange surface has a folded structure with a very large surface area, greatly increasing the rate of exchange that is possible. Examples of exchange surfaces include lungs, gills and intestines in animals, and roots and leaves in plants.

Multicellular organisms need transport systems

Diffusion, osmosis and active transport are fairly slow processes. For example, it would take a glucose molecule over 27 hours just to move 1cm by diffusion.

For unicellular organisms, this is not necessarily a problem. The distances that substances need to move within the cell are very small. Most unicellular organisms have a diameter somewhere between a few micrometers and a few hundred micrometers (for a glucose molecule to diffuse 100 micrometers takes about 10 seconds).

However, within multicellular organisms, it is often necessary to move substances over many centimeters or even many meters - for example, moving water from the roots to the leaves of a tree.

Multicellular organisms use diffusion, osmosis and active transport to move substances in and out of cells and to move substances around within cells, but for longer distance transport they have specialised transport systems, such as the xylem and phloem in plants and the circulatory system in animals.

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