Diffusion is the process by which molecules move from an area of higher concentration to an area of lower concentration. It occurs due to the random motion of particles. This process does not require any energy input and happens naturally until equilibrium is reached.

Unicellular organisms, which consist of a single cell, have a relatively large surface area to volume ratio. This means that they have a high surface area through which exchange of substances can occur. As a result, they can obtain the necessary oxygen and release carbon dioxide through diffusion.

- Oxygen uptake:
When unicellular organisms are in an environment with a higher concentration of oxygen, such as water or air, oxygen molecules will naturally diffuse across their cell membranes into the cytoplasm. This is because the concentration of oxygen outside the cell is higher than inside the cell. The oxygen molecules move from an area of higher concentration (outside the cell) to an area of lower concentration (inside the cell) until equilibrium is reached.

- Carbon dioxide release:
On the other hand, carbon dioxide is produced as a waste product during cellular respiration. It accumulates inside the cell, creating a higher concentration of carbon dioxide compared to the surrounding environment. As a result, carbon dioxide molecules will diffuse out of the cell, moving from an area of higher concentration (inside the cell) to an area of lower concentration (outside the cell).

- Cell membrane and diffusion:
The cell membrane plays a crucial role in facilitating diffusion. It is selectively permeable, allowing only certain substances to pass through. Small molecules like oxygen and carbon dioxide can easily diffuse across the phospholipid bilayer of the cell membrane. The lipid-soluble nature of these gases allows them to dissolve in the lipid layer and pass through the membrane.

In conclusion, unicellular organisms rely on diffusion to obtain oxygen and release carbon dioxide. The high surface area to volume ratio of these organisms allows for efficient exchange of gases through their cell membranes. Diffusion occurs passively and does not require any energy expenditure by the organism.

In unicellular organisms like Amoeba, gaseous exchange takes place through the process of diffusion.