Gas exchange in single-celled organisms

A single-celled organism exchanges gases directly with its surroundings across its cell surface membrane. The gases move by diffusion: their particles move from a region of higher concentration to a region of lower concentration.

Oxygen diffuses into the cell when its concentration is higher outside than inside. Carbon dioxide diffuses out when its concentration is higher inside than outside. Each gas therefore moves down its own concentration gradient.

Why the cell surface is sufficient

A single-celled organism has a large surface area relative to its volume. This means there is plenty of membrane surface available for gas exchange compared with the volume of the cell that needs gases exchanged.

The cell is also small, so gases have only short distances to diffuse between the membrane and the parts of the cell where they are used or produced. Together, the large relative surface area and short diffusion distances allow exchange across the cell surface to meet the organism’s needs without a specialised gas exchange system.

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.

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.