Exchange Surfaces

All organisms need to exchange substances with their environments. Useful substances need to be taken in and waste products need to be removed.

Multicellular organisms need to have specialised exchange surfaces (such as lungs, gills, intestines, roots and leaves) in order to carry out exchange.

Unicellular organisms do not need specialised exchange surfaces.

The reason for this difference has to do with surface area to volume ratio.

The greater an organism's volume, the more exchange it needs per minute

The rate at which an organism needs to carry out exchange depends on its volume. The greater the volume of the organism, the greater the amount of exchange it needs to carry out per minute.

For example, many organisms need to take in oxygen for aerobic respiration. The greater the volume of the organism, the more oxygen it needs to take in per minute. This is because as an organism gets bigger, the amount of energy it uses per minute increases, and therefore the amount of aerobic respiration it needs to carry out per minute increases.

The greater an organism's surface area, the more exchange it can carry out per minute

The rate at which an organism is able to carry out exchange depends on the surface area. The greater the surface area over which exchange is happening, the higher the rate of exchange.

A high surface area to volume ratio is needed for exchange

Having a high surface area to volume ratio is very important for exchange.

As we have seen, an organism's volume determines the amount of exchange it needs to carry out per minute, whereas its surface area determines the amount of exchange it actually can carry out per minute.

Surface area to volume ratio tells us how much surface area there is per unit of volume. In other words, how much surface area is available to supply the exchange needs of each unit of volume.

If an organism has a low surface area to volume ratio then it may not be able to carry out exchange at a fast enough rate to meet its needs.

Unicellular organisms do not need specialised exchange surfaces

Unicellular organisms are generally much smaller than multicellular organisms.

They usually have very simple shapes such as a sphere, a rod with rounded ends, or some other round shape.

Because they are small, they have high surface area to volume ratios (for any given object, increasing its size causes its surface area to volume ratio to decrease). This means that they can easily meet their exchange needs simply by moving substances across their cell membrane.

Multicellular organisms need specialised exchange surfaces

Multicellular organisms are generally much bigger than unicellular organisms.

Therefore, if multicellular organisms were shaped like unicellular organisms they would have very low surface area to volume ratios.

For example, imagine an organism the size of an elephant but shaped like a sphere or a rod, and without any specialised exchange surfaces like lungs or intestines. It would have such a low surface area to volume ratio that it would never be able to meets its exchange needs and therefore it would not survive.

It is for this reason that multicellular organisms have exchange surfaces.

Exchange surfaces usually have highly folded structures. For example, roots are covered in root hairs, intestines have folds called villi and microvilli, and lungs contain many tiny air sacs called alveoli.

These structures massively increase the surface area available for exchange (although some exchange surfaces like lungs and intestines are internal to the body, they still count as part of the organism's surface area). This enables multicellular organisms to have much higher surface areas than they would have if they were shaped like unicellular organisms and therefore ensures that multicellular organisms have the high surface area to volume ratios needed to meet their exchange needs.

Features of effective exchange surfaces

In order to be effective, an exchange surface needs to have the following features:

• A large surface area.
• A thin membrane for substances to diffuse across (the thinner it is, the shorter the diffusion distance is and therefore the higher the rate of exchange).
• An effective way of moving substances that have been exchanged away from the exchange surface and bringing new substances to be exchanged to the exchange surface (this helps to maintain a high concentration gradient, which increases the rate of exchange). In animals, this means having an efficient blood supply to all exchange surfaces, and it means having lungs that are well ventilated.

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