La explicación
Particles in constant motion
All matter is made of tiny particles, and these particles are never still. In a gas the particles are far apart and move quickly in straight lines until they collide with another particle or a wall, then bounce off in a new direction. In a liquid the particles are close together but can slide past one another, so they also move about continuously, though more slowly and over shorter distances than in a gas.
This motion is random: a particle has no target and no preferred direction. Diffusion is a direct result of this random motion, and no push, pump or energy supply from outside is needed for it to happen.
What diffusion is
Diffusion is the net movement of particles from a region where they are at a higher concentration to a region where they are at a lower concentration. Concentration means how many particles of a substance there are in a given volume.
In gases and liquids, diffusion is the intermingling of substances by the movement of their particles. The particles of each substance spread out among the particles of the other until the mixture is evenly spread.
A familiar gas example is perfume. When a bottle is opened in one corner of a room, the perfume particles leave the bottle and move among the air particles. They collide and bounce randomly, and gradually spread through the whole room, so the smell is eventually noticed far from the bottle. A liquid example is a drop of food colouring placed in still water. The colouring particles spread slowly through the water until the whole beaker is evenly coloured, even without stirring.
Diffusion is faster in gases than in liquids because gas particles move faster and are further apart, so they travel greater distances between collisions. It does not take place in the same way in solids, where particles only vibrate about fixed positions.
Why there is a direction: net movement
Individual particles move randomly in every direction, including from low to high concentration. Direction appears only when the whole population is considered.
Imagine a region with many particles beside a region with few. In any short time, far more particles leave the crowded region than leave the sparse one, simply because there are more of them to move. The result is that more particles travel from the high-concentration side to the low-concentration side than the other way. This overall difference is the net movement.
The difference in concentration between two regions is the concentration gradient. Particles move down the gradient, from high to low. When the particles are evenly spread, the concentration is the same everywhere and there is no net movement. Particles are still moving randomly at this point, but equal numbers travel in each direction, so the mixture stays evenly spread. This steady state is called equilibrium.
What changes the rate of diffusion
Several factors change how quickly diffusion happens.
- Temperature: at a higher temperature particles have more energy and move faster, so they spread out more quickly. Food colouring spreads faster in warm water than in cold water.
- Concentration difference: the steeper the concentration gradient, the faster the net movement. A large difference means many more particles move one way than the other.
- Distance: the shorter the distance the particles must travel, the sooner they arrive. Diffusion works well over very short distances, such as the width of a cell, and is slow over long distances.
- Surface area: the larger the area across which particles can pass, the more particles cross in a given time.
The type of substance and the state of matter also matter. Smaller, lighter particles generally diffuse faster than larger, heavier ones, and diffusion is faster in gases than in liquids.
Diffusion across the cell membrane
Every living cell is surrounded by a thin cell membrane. The membrane controls which substances enter and leave the cell. Small particles such as oxygen, carbon dioxide and water can pass through it, while large particles cannot pass easily. For this reason the membrane is described as partially permeable.
Small dissolved particles move through the membrane by diffusion whenever there is a concentration gradient across it, and the direction depends on where the concentration is higher.
- Oxygen: a cell uses oxygen in respiration, so the oxygen concentration inside the cell is lower than outside, where it is supplied by the air or by the blood. Oxygen therefore diffuses into the cell.
- Carbon dioxide: respiration produces carbon dioxide, so its concentration is higher inside the cell than outside. Carbon dioxide diffuses out of the cell.
- Dissolved nutrients: substances such as glucose are used up inside the cell and are at a higher concentration outside, so they diffuse in.
- Dissolved wastes: waste substances build up inside the cell and diffuse out into the surrounding fluid.
In a leaf, the pattern changes with the process taking place. In light, photosynthesis uses carbon dioxide and releases oxygen, so carbon dioxide diffuses into the leaf cells and oxygen diffuses out.
This movement continues only while the gradient is maintained. A cell keeps using oxygen and the blood keeps delivering more, so the gradient stays steep and oxygen keeps diffusing in.
Single-celled organisms and the need for exchange surfaces
A single-celled organism such as an amoeba is so small that every part of it is very close to its surroundings. Oxygen from the water only has to diffuse a tiny distance to reach the centre of the cell, so diffusion alone supplies the whole cell and removes its carbon dioxide.
Larger organisms are different. Cells deep inside the body are far from the outside, and diffusion over such a long distance would be far too slow to meet their needs. Large organisms therefore have specialised exchange surfaces where substances pass between the organism and its environment. These surfaces have features that make diffusion fast:
- a large surface area, often achieved by folding or branching;
- a thin wall, often only one cell thick, giving a short diffusion distance;
- a steep concentration gradient, maintained by a good blood supply or by movement of air or water.
The lungs show all three features. Each lung contains millions of tiny air sacs called alveoli, which together give a very large area. The wall of an alveolus is very thin and is surrounded by tiny blood vessels. Breathing keeps bringing in air that is rich in oxygen, and blood keeps carrying oxygen away, so oxygen diffuses from the air in the alveoli into the blood, and carbon dioxide diffuses from the blood into the air to be breathed out.
