Diffusion in living organisms

Cells exchange substances with their surroundings across the cell membrane. Some substances cross by diffusion when their concentration differs on the two sides of the membrane and the membrane allows them to pass.

A difference in concentration is called a concentration gradient. Particles move randomly in both directions, but more move from the region of higher concentration to the region of lower concentration. This produces a net movement down the concentration gradient. The cell does not direct the particles, and diffusion does not require the cell to supply energy.

Diffusion helps cells obtain substances they need and remove substances they produce. For example, oxygen can diffuse into a cell when its concentration is higher outside. Carbon dioxide produced by the cell can diffuse out when its concentration is higher inside.

Dissolved solutes can also diffuse across cell membranes. For example, glucose can diffuse into some cells through membrane proteins when its concentration is higher outside. Not every substance can cross every membrane: some pass through the membrane directly, some require suitable membrane proteins, and others cannot pass through it.

Factors affecting the rate of diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. Its rate depends on the concentration gradient, temperature and surface area available for exchange.

Concentration gradient

The concentration gradient describes the difference in concentration between two regions. A steeper gradient means a greater difference in concentration. Particles move in both directions, but a steeper gradient produces a greater net movement from the higher-concentration region to the lower-concentration region, so diffusion is faster.

Temperature

At a higher temperature, particles have more kinetic energy and move faster. This increases the rate of diffusion.

Surface area

A larger membrane surface area provides more space for particles to cross at the same time. More particles can therefore diffuse across the membrane in a given time, increasing the overall rate of diffusion.

Alveolar gas exchange

Gas exchange takes place between the air in the alveoli and the blood in the capillaries surrounding them. An alveolus is a tiny air sac in the lungs. Gases cross the alveolar and capillary walls by diffusion: their net movement is from a region of higher concentration to a region of lower concentration.

Oxygen is at a higher concentration in alveolar air than in the blood arriving at the alveoli, so it diffuses into the blood. Carbon dioxide is at a higher concentration in the arriving blood than in alveolar air, so it diffuses in the opposite direction, into the alveoli.

Alveolar adaptations for gas exchange

The lungs contain numerous alveoli, providing a large surface area across which gases can diffuse. The alveolar epithelium is very thin, and the capillary walls are also thin. This gives gases a short diffusion distance between air and blood. The alveolar surface is moist, allowing gases to dissolve before diffusing.

A rich capillary supply continually brings blood to the alveoli and carries it away. This helps maintain the concentration differences needed for oxygen and carbon dioxide to diffuse. Ventilation also maintains these gradients: fresh air replenishes oxygen in the alveoli, while air containing carbon dioxide is breathed out.

Surface area to volume ratio in organisms

An organism exchanges substances with its environment across its surface. Its surface area to volume ratio compares the area available for exchange with the volume of the organism. A larger ratio means there is more surface area available for each unit of volume.

To calculate the ratio, divide the surface area by the volume:

The result can be written as a ratio with volume as one. For example, a surface area of $$24,\text{cm}^2$$ and a volume of $$8,\text{cm}^3$$ give a ratio of $$3:1$$. Writing ratios this way makes them easier to compare, provided the same units are used.

Effect of size on surface area to volume ratio

As an organism increases in size, its volume generally increases faster than its surface area. Its surface area to volume ratio therefore decreases: there is less surface area available for each unit of volume.

For example, a cube with sides of $$1,\text{cm}$$ has a surface area of $$6,\text{cm}^2$$ and a volume of $$1,\text{cm}^3$$, giving a ratio of $$6:1$$. A cube with sides of $$2,\text{cm}$$ has a surface area of $$24,\text{cm}^2$$ and a volume of $$8,\text{cm}^3$$, giving a smaller ratio of $$3:1$$.

Exchange in single-celled and multicellular organisms

A single-celled organism has a relatively large surface area compared with its volume. Substances can be exchanged across its surface quickly enough to meet its needs.

A multicellular organism has a smaller surface area relative to its volume, and many of its cells are far from the external surface. Exchange across the external surface alone cannot supply and remove substances fast enough for all the cells. Specialised exchange surfaces provide areas for exchange with the environment, while a transport system carries substances between those surfaces and the cells.

For example, urea diffuses from cells into blood plasma. The plasma carries the urea to the kidneys, where it is removed from the blood for excretion.

Exchange adaptations in larger organisms

Cells need substances such as oxygen and nutrients, and they must remove waste substances. These substances must cross a surface to enter or leave the organism. As an organism becomes larger, its volume increases faster than its external surface area. Its surface area-to-volume ratio therefore decreases: there is less external surface available for exchange relative to the volume of living tissue it supplies.

In a larger body, many cells are also farther from the external surface. Substances take longer to diffuse to and from these cells, so exchange across the body surface alone may not meet their needs.

Body shape and exchange

A thin or flattened body has a larger surface area relative to its volume than a more compact body of the same volume. It also keeps cells closer to the body surface, shortening the distance over which substances must diffuse. These features make exchange across the body surface more effective.

Specialised exchange surfaces and transport

Larger organisms can have specialised exchange surfaces that provide a large area for substances to enter or leave the body. They can also have transport systems that carry substances between these surfaces and cells throughout the body.

For example, a transport system can carry oxygen from an exchange surface to internal cells and carry waste substances from cells towards an exchange surface. This reduces the distance substances must travel by diffusion alone.