Substances moving into and out of cells are always in solution. Four ways by which substances move into cells are diffusion, osmosis, active transport and facilitated transport.

All this substances passed in, out though the cell membrane. This cells membrane allows only some substances to pass through not others. Such a membrane is set to be selectivity permeable or differentially permeable. Substances in the body such as salt, sugar, carbon dioxide dissolves in water to make solution. These substances are called solutes the water is the solvent.

1. DIFFUSION:  

This refers to the movement of molecules from a region of higher concentration to a region of lower concentration. This movement continues until the molecules are evenly distributed. Only liquid and gases can diffuse because their molecules are not held so tight together and so they can move away from each other. The diffusion occurs only when a concentration gradient or when a concentration exist. Diffusion takes place through living and non living membranes.

1. Factors That Affect The Rate Of Diffusion

• Size of Particles: In general, small molecules move faster than larger ones and those of gases diffuse faster than those of liquids.
• Temperature: Increasing the temperature makes the particles in a solution of gas to move faster. So increasing the temperature will increase rate of diffusion.
• Surface Area: The bigger the surface areas of the cell surface membrane are available for diffusion, the faster the rate of diffusion.
• Difference in Concentration: The bigger the difference of concentration between the two points, the faster the rate of diffusion.

2. Importance Of Diffusion to Living Organisms

• Oxygen diffuses out and carbon dioxide diffuses I through the stomata in leaves during the day.
• Unicellular organisms. Eg: Amoeba takes in oxygen and sends out carbon dioxide by diffusion across the general body surface.
• Oxygen diffuses in the blood and carbon dioxide diffuses out of the blood in the lungs of mammals and in gills of fish.
• Oxygen diffuses out of the blood and carbon dioxide diffuses in the blood in the tissue of mammals.
• Digested food like sugars and amino acids enter the blood from the gut by diffusion.
• Waste products of metabolic activities diffuse out of animal cell into the blood, where they are transported to the animal’s kidney for excretion in urine.

2. OSMOSIS:

Osmosis is the movement or passage of water molecules (solvent) from a region of high concentration (dilute solution) to a region of low concentration (concentrated solution) through a selectively or differentially permeable membrane until equilibrium is reached. When the difference in concentration between solutions increases, the rate of osmosis increases. An increase in temperature also increases the rate of osmosis.