Describe why diffusion is important in plants and animals. For both plants and animals you need to give examples of the substances that are diffusing. (6 marks)

Diffusion is important in plants and animals because it ensures that the metabolic processes in both types of cells / organisms can develop normally.

Diffusion can be defined as the spontaneous movement of particles as a consequence of their thermal energy from areas of high concentration to areas of low concentration.  

Plant and animal cells are similar, plant cells have a rigid cellulose wall, which provides protection, without preventing the diffusion of water and ions from the environment to the plasma membrane.

For example, photosynthesis in plants depends on the diffusion of water and CO2; likewise, that of water vapor by perspiration is a diffusive process.

The absorption of minerals from the soil solution by the roots is partly dependent on diffusion; likewise, all chemical processes, including those catalyzed by enzymes, depend on collisions produced by diffusing molecules.

Animal cells also undergo a series of changes when subjected to different conditions of water content.

For example, the movement of water through the plasma membrane determines the cell volume.

The regulation of this volume is very important since animal cells cannot withstand the entry of large amounts of water due to the fact that they enter a state of turgor.

The regulation of the movement of water under these conditions is called osmosis, a special kind of diffusion that depends on the relative concentration of water molecules.

Therefore, we can conclude that diffusion is important in plants and animals because it ensures that the metabolic processes in both types of cells / organisms can develop normally.

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Diffusion is a passive process involving the movement of molecules from a region of a higher concentration to one of a lower concentration (the term passive means that the process does not require an input of energy to take place).  

Diffusion can occur across partialy permeable membranes, such as those surrounding cells. Therefore, diffusion is involved in the movement of important molecules into and out of cells. It is important for the uptake of substances needed by cells, and also the removal of waste products produced by the cells.

In animals:

Respiration - Oxygen and glucose react to form carbon dioxide and water along with ATP (a source of energy) in the process of aerobic respiration. Therefore, oxygen and glucose must be taken up by the cell, and typically the concentration of these molecules outside the cell is greater than inside. Therefore, the overall net movement of these molecules will be down the concentration gradient, and they will move into the cell via diffusion. Similarly, the carbon dioxide produced is a waste product and moves out of the cell, again via diffusion down its concentration gradient.

In Plants:

Mineral uptake - Useful minerals and ions need to be taken up from soil into plants via root hair cells. These cells are adapted (through a large surface area and large number) to maximise the rate of diffusion. Therefore, the useful molecules in the soil move down a concentration gradient and into the roots to be taken up by the plant. Many molecules found in the soil are essential for the growth and survival of plants, making diffusion a very important process.

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