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Cell Physiology

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Diffusion

From kinetic theory, matter is made up of particles that are in continuous random motion.In solids, the particles are at fixed positions and can only vibrate at these fixed positions.

In liquids and gases, the particles are loosely held and are free to move from one region to another randomly. This movement of gas or liquid particles is observed to be from regions of high concentration to a region of low concentration. The process by which particles move from a region of high concentration to a region of low concentration is known as diffusion.

In particular, the scent of a flower or perfume experienced by an individual is as a result of the flower scent particles or perfume particles move from a region of high concentration.

Diffusion occurs until the regions have an even concentration of the liquid or gas particles.

The difference in concentration of particles between the region of high concentration and region of low concentration is known as the diffusion gradient/concentration gradient.

Demonstration of the process of diffusion using potassium manganate (VII)

The Role of Diffusion in Living Organisms

a) In Plants

Diffusion plays an important role in plants in that:

•It helps in absorption of mineral salts from the soil to the plant. Most salts dissolve in soil water. For those salts whose concentration in soil water is higher that their concentration in the cell sap of root hair cells, they move into the root hair cells through diffusion.Plants require mineral salts for numerous life processes.

•Diffusion plays a role in gaseous exchange in plants. The respiratory gases (oxygen and carbon (IV) oxide) diffuse across the stomata and lenticels of plants.

•Diffusion also contributes to the transportation of manufactured food materials from the leaves to other parts of the plant.

b) In Animals

In animals diffusion plays the following important roles:

•It helps in the absorption of digested food materials in the alimentary canal. End products of digestion such as amino acids and glucose diffuse across the wall of the ileum into the blood for transport to other parts of the animal body.

•Diffusion also plays a significant role in gaseous exchange in animals. In animals,gaseous exchange occurs at certain structures known as respiratory surfaces. These include the skin, gills, lungs, tracheal system and the cell membrane (in unicellular organisms). Gaseous exchange at these surfaces occurs through the process of diffusion.

•Diffusion is important in excretion of nitrogenous wastes especially in unicellular animals.

Factors affecting the rate of Diffusion

Factors affecting the rate of Diffusion:

a) Diffusion gradient

A greater diffusion gradient between two points increases the rate of diffusion. Increasing the concentration of diffusing molecules also increases diffusion gradient with corresponding regions hence increases the rate of diffusion.

b) Surface area to volume ratio

Rate of diffusion directly depends on the surface area to volume ratio. The greater the surface area to volume ratio, the greater the rate of diffusion will be. Conversely, low surface area to volume ratio results in a low diffusion rate.

This implies that diffusion rate is greater in small organisms than the large organisms.This is because the small organisms have a large surface area to volume ratio. As a result,most of their body parts are closer to the external surrounding leading to faster diffusion.

Small organisms can, therefore, depend on diffusion alone as a means of transporting foods, respiratory gases and waste products.

To large organisms, diffusion alone is inadequate as a means of transport of foods and excretion. They have an additional transport system.

Organisms always lose heat to the surrounding through diffusion. This implies that small animals lose a lot of heat to the surrounding compared to the large animals.

c) Thickness of membranes and tissues

The thicker the membrane or tissue, the lower the rate of diffusion. This is because the distance covered by the diffusing molecules is greater through the thicker membranes. The rate of diffusion is higher in thinner membranes.

d) Size of molecules

Small and light molecules diffuse much faster than the heavy and large sized particles.

e) Temperature

An increase in temperature increases the energy content of the diffusing particles; thereby causing them to move faster, this implies that the rate of diffusion increases with increase in temperature.
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