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.

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.

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.

Small intestine exchange adaptations

The small intestine absorbs soluble products of digestion, such as glucose and amino acids, into the body. Its lining is adapted to absorb these substances efficiently.

The lining has many small projections called villi. The cells covering each villus have even smaller projections called microvilli. Together, these provide a large surface area, so more absorption can occur at the same time.

Each villus is covered by a thin epithelial layer. Substances therefore have only a short distance to cross before reaching the blood in the capillaries inside the villus.

Each villus has a rich capillary supply. Blood carries absorbed substances away, helping to keep their concentration lower in the blood than in the small intestine. This maintains a concentration gradient, allowing substances to continue moving into the blood by diffusion.

Diffusion is not the only way substances are absorbed. Some can also be absorbed by other mechanisms, including active transport.

Fish gill gas exchange

Fish gills exchange gases between water and blood. Oxygen diffuses from the water into the blood, while carbon dioxide diffuses from the blood into the water. Each gas moves from a region where its concentration is higher to one where its concentration is lower.

Gill structure and diffusion

Gills contain filaments, which bear many small structures called lamellae. Together, the lamellae provide a large surface area across which gases can diffuse. Their thin exchange surfaces also give gases a short distance to travel between the water and the blood.

Maintaining gas exchange at the gills

Water flows over the gills, ventilating the exchange surfaces. This brings oxygen-containing water to the gills and carries carbon dioxide away.

The lamellae have a rich blood supply. Blood carries absorbed oxygen away and brings carbon dioxide to the gills. Water flow and blood flow therefore help maintain the concentration differences that allow oxygen and carbon dioxide to continue diffusing in opposite directions.

Root hair cell exchange adaptations

Root hair cells absorb water and mineral ions from the soil. Their structure helps these substances enter the root efficiently.

Each root hair cell has a long, thin projection that extends between soil particles. This increases the surface area in contact with the soil, providing more area for water and mineral ions to enter. The cell also has a thin cell wall, giving substances a short path to the cell membrane.

Water and mineral ion uptake

Water enters a root hair cell by osmosis: it moves across the partially permeable cell membrane from a more dilute solution in the soil to a more concentrated solution inside the cell.

Mineral ions can enter by active transport when their concentration is lower in the soil than inside the cell. This moves the ions against their concentration gradient and requires energy.

Leaf gas exchange adaptations

A leaf exchanges gases with the surrounding air through small pores called stomata. Each stoma opens into air spaces within the leaf. These spaces provide routes between the outside air and the moist surfaces of the mesophyll cells inside the leaf.

During photosynthesis, mesophyll cells use carbon dioxide and produce oxygen. Carbon dioxide diffuses from the air through the stomata and air spaces to the mesophyll cell surfaces. Oxygen diffuses in the opposite direction, from the cells through the air spaces and out through the stomata.

Leaf structure and diffusion

A broad leaf has a large surface area across which gas exchange can occur. Its thin structure gives gases a short diffusion path between the air and the mesophyll cells. The internal air spaces also allow gases to reach many moist cell surfaces.

Stomata and guard cells

Each stoma is controlled by guard cells, which change the size of its opening. When a stoma is open, carbon dioxide and oxygen can diffuse through it. Water vapour can also diffuse out of the leaf through the opening. Closing the stoma reduces gas exchange and the loss of water vapour.