Diffusion and size
Gas exchange
Gas exchange in plants
Gas exchange in fish
Gas exchange in humans



All organisms need to exchange substances such as food, waste, gases and heat with their surroundings. These substances must diffuse between the organism and the surroundings. The rate at which a substance can diffuse is given by Fick's law:

So rate of exchange of substances depends on the organism's surface area that's in contact with the surroundings. Requirements for materials depends on the volume of the organism, So the ability to meet the requirements depends on the surface area : volume ratio. As organisms get bigger their volume and surface area both get bigger, but volume increases much more than surface area. This can be seen with some simple calculations for different-sized organisms. Although it's innacurate lets assume the organisms are cube shaped to simplify the maths - the overall picture is still the same. The surface area of a cube with length of side L is LxLx6, while the volume is LxLxL.



SA (m²)

Vol. (m³)



1 mm 6 x 10-12 10-18 6,000,000:1


100 mm 6 x 10-8 10-12 60,000:1


10 mm 6 x 10-4 10-6 600:1


1 m 6 x 100 100 6:1


100 m 6 x 104 106 0.06:1

So as organisms get bigger their surface area/volume ratio gets smaller. Bacteria are all surface with not much inside, while whales are all insides without much surface. So as organisms become bigger it is more difficult for them to exchange materials with their surroundings.

Organisms also need to exchange heat with their surroundings, and here large animals have an advantage in having a small surface area/volume ratio: they lose less heat than small animals. Large mammals keep warm quite easily and don't need much insulation or heat generation. Small mammals and birds lose their heat very readily, so need a high metabolic rate in order to keep generating heat, as well as thick insulation. So large mammals can feed once every few days while small mammals must feed continuously. Human babies also loose heat more quickly than adults, which is why they need woolly hats.

Systems that increase the rate of exchange

Fick's law showed that for a fast rate of diffusion you must have a large surface area, a small distance between the source & the destination, and maintain a high concentration gradient. All large organisms have developed systems that are well-adapted to achieving these goals, as this table shows. For comparison, a tennis court has an area of about 260 m² and a football pitch has an area of about 5000 m².





Human lungs

600 million alveoli with a total area of 100m²

each alveolus = 1 cell thick

constant ventilation replaces the air

Fish gills

feathery filaments with secondary lamellae

lamellae = 2 cells thick

water pumped over gills in countercurrent to blood


For a tree

 - SA of leaves =200mē; 

 - SA of spongy cells inside leaves = 6000mē.

gases diffuse straight into leaf cells

wind replaces air round leaves



Gas exchange takes place at a respiratory surface - a boundary between the external environment and the interior of the body. For unicellular organisms the respiratory surface is simply the cell membrane, but for large organisms it is part of specialised organs like lungs, gills or leaves. This name can cause problems - in biology the word "respiration" means cellular respiration (ATP generation inside cells), however sometimes (such as here) it can also refer to breathing, which is what most non-biologists mean by it anyway.

Gases cross the respiratory surface by diffusion, so from Fick's law we can predict that respiratory surfaces must have:

Many also have

We shall examine how these requirements are met in the gas exchange systems of humans, fish and plants.


All plant cells respire all the time, and when illuminated plant cells containing chloroplasts also photosynthesise, so plants also need to exchange gases. The main gas exchange surfaces in plants are the spongy mesophyll cells in the leaves. Leaves of course have a huge surface area, and the irregular-shaped, loosely-packed spongy cells increase the area for gas exchange still further. You are expected to know leaf structure in the detail shown in the diagram

Gases enter the leaf through stomata -usually in the lower surface of the leaf. Stomata are enclosed by guard cells that can swell up and close the stomata to reduce water loss. The gases then diffuse through the air spaces inside the leaf, which are in direct contact with the spongy and palisade mesophyll cells. Plants do not need a ventilation mechanism because their leaves are exposed, so the air surrounding them is constantly being replaced in all but the stillest days. In addition, during the hours of daylight photosynthesis increases the oxygen concentration in the sub-stomatal air space, and decreases the carbon dioxide concentration. This increases the concentration gradients for these gases, increasing diffusion rate.

The palisade mesophyll cells are adapted for photosynthesis. They have a thin cytoplasm densely packed with chloroplasts, which can move around the cell on the cytoskeleton to regions of greatest light intensity. The palisade cells are closely packed together in rows to maximise light collection, and in plants adapted to low light intensity there may be two rows of palisade cells.

The spongy mesophyll cells are adapted for gas exchange. They are loosely-packed with unusually large intercellular air spaces where gases can collect and mix. They have fewer chloroplasts than palisade cells, so do less photosynthesis.



Gas exchange is more difficult for fish than for mammals because the concentration of dissolved oxygen in water is less than 1%, compared to 20% in air. (By the way, all animals need molecular oxygen for respiration and cannot break down water molecules to obtain oxygen.) Fish have developed specialised gas-exchange organs called gills, which are composed of thousands of filaments. The filaments in turn are covered in feathery lamellae which are only a few cells thick and contain blood capillaries. This structure gives a large surface area and a short distance for gas exchange. Water flows over the filaments and lamellae, and oxygen can diffuse down its concentration gradient the short distance between water and blood. Carbon dioxide diffuses the opposite way down its concentration gradient. The gills are covered by muscular flaps called opercula on the side of a fish's head. The gills are so thin that they cannot support themselves without water, so if a fish is taken out of water after a while the gills will collapse and the fish suffocates.

Fish ventilate their gills to maintain the gas concentration gradient. They continuously pump their jaws and opercula to draw water in through the mouth and then force it over the gills and out through the opercular valve behind the gills. This one-way ventilation is necessary because water is denser and more viscous than air, so it cannot be contained in delicate sac-like lungs found in air-breathing animals. In the gill lamellae the blood flows towards the front of the fish while the water flows towards the back. This countercurrent system increases the concentration gradient and increases the efficiency of gas exchange. About 80% of the dissolved oxygen is extracted from the water.



In humans the gas exchange organ system is the respiratory or breathing system. The main features are shown in this diagram.

The actual respiratory surface is on the alveoli inside the lungs. An average adult has about 600 million alveoli, giving a total surface area of about 100m², so the area is huge. The walls of the alveoli are composed of a single layer of flattened epithelial cells, as are the walls of the capillaries, so gases need to diffuse through just two thin cells. Water diffuses from the alveoli cells into the alveoli so that they are constantly moist. Oxygen dissolves in this water before diffusing through the cells into the blood, where it is taken up by haemoglobin in the red blood cells. The water also contains a soapy surfactant which reduces its surface tension and stops the alveoli collapsing. The alveoli also contain phagocyte cells to kill any bacteria that have not been trapped by the mucus.

The steep concentration gradient across the respiratory surface is maintained in two ways: by blood flow on one side and by air flow on the other side. This means oxygen can always diffuse down its concentration gradient from the air to the blood, while at the same time carbon dioxide can diffuse down its concentration gradient from the blood to the air. The flow of air in and out of the alveoli is called ventilation and has two stages: inspiration (or inhalation) and expiration (or exhalation). Lungs are not muscular and cannot ventilate themselves, but instead the whole thorax moves and changes size, due to the action of two sets of muscles: the intercostal muscles and the diaphragm.


Normal expiration

Forced expiration

These movements are transmitted to the lungs via the pleural sac surrounding each lung. The outer membrane is attached to the thorax and the inner membrane is attached to the lungs. Between the membranes is the pleural fluid, which is incompressible, so if the thorax moves, the lungs move too. The alveoli are elastic and collapse if not held stretched by the thorax (as happens in stab wounds).


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