Exchange of Materials

Cells and Diffusion

Diffusion is the process by which particles move from an area of high concentration to an area of low concentration.
This is a passive process, which means it’s uncontrolled and doesn’t require energy.
Factors affecting the rate of diffusion include temperature, concentration gradient and surface area.
In cells, diffusion occurs across a semi-permeable membrane, moving substances necessary for cell function in or out of the cell.

Surface area to Volume Ratio

For efficient exchange of materials, an organism or cell should have a high surface area to volume ratio.
A larger surface area allows more material to be exchanged at once.
Small cells have a higher surface area to volume ratio than large cells, making exchanges more efficient.

Gaseous Exchange in Humans

In humans, gaseous exchange occurs in the lungs.
When we breathe in, oxygen enters the lungs and moves by diffusion across the thin walls of the alveoli into the bloodstream.
At the same time, carbon dioxide, a waste product of metabolism, diffuses out of the blood and into the alveoli, to be exhaled.
The efficiency of gaseous exchange is increased by having millions of alveoli providing a large surface area, a moist lining for dissolving gases, and a rich supply of blood capillaries.

Exchange in Plants

In plants, the main gas exchange occurs through stomata found on the underside of leaves.
Carbon dioxide is diffused into the leaf for photosynthesis, while oxygen, a by-product of photosynthesis, diffuses out.
Water vapour is also lost through the stomata in a process known as transpiration.

Exchange in Fish

Fish extract oxygen from water through their gills.
As water passes over the gill filaments, oxygen diffuses into the blood and carbon dioxide diffuses out.
This process is efficient due to the large surface area of the gills, the thinness of the gill membranes, and the constant flow of water.

Remember, the speed and efficiency of the exchange of materials are essential for maintaining the life processes within all living organisms. High surface area, thin barriers to diffusion, and efficient transport systems all enhance this process.