Exchange Surfaces

Exchange surfaces enable organisms to interact with their environment and exchange substances like oxygen, carbon dioxide, and nutrients.
These surfaces have properties that optimise exchange: they are thin, have a large surface area, and possess a plentiful supply of blood vessels for efficient gas and nutrient transport.
The thinness of the surface allows for a shorter diffusion pathway, thus speeding up the exchange process.
Larger surface areas enable a larger quantity of molecules to be exchanged at any one time, enhancing the efficiency of the process.

Multicellular organisms, like humans, have specialised exchange surfaces to deal with their increased demands.
Lungs, for example, are exchange surfaces that allow for the transmission of oxygen into the blood and removal of carbon dioxide.
The walls of the alveoli, small air sacs in the lungs, are the primary sites of gas exchange and embody all the aforementioned properties - they are thin, have a large surface area, and are surrounded by a network of capillaries.
The small intestine in humans is designed for the efficient absorption of digested products. The inner surface is lined with tiny finger-like projections called villi which further increase the surface area for absorption.

Unicellular organisms, like amoebae, don’t require specialised exchange surfaces; rather, they exchange substances directly with their environment.
Because the organism is single-celled, they have a high surface area to volume ratio which means they can meet their requirements through the cell membrane.
Diffusion is the primary mode of exchange in these organisms, and it occurs across the whole of the cell surface membrane.

Speed of exchange is influenced by other factors apart from surface area and thickness of the membrane.
Concentration gradients can affect the rate of exchange: a larger gradient (difference in concentration of molecules) will increase the speed at which molecules are transferred.
Finally, the presence of transport systems like active transport can greatly increase exchange efficiency by moving molecules against their concentration gradient, faster than passive diffusion alone would permit.