Movements of Substances Across the Plasma Membrane

Passive Diffusion
Simple diffusion is the movement of a substance from a region of high concentration to a region of low concentration. This is known as moving down the concentration gradient. For example; a pinch of table sugar placed at the bottom of a glass containing water will dissolve in water completely to form a solution of uniform concentration. The sugar is referred to as the solute and water as the solvent. Compounds in a tea bag placed in a cup of hot water diffuse uniformly throughout the water until the colour of the water is brown. You can also use potassium permanganate instead of tea leaves.
Diffusion is faster at high temperatures. It is also faster in gases than in liquids and is slowest in solids.

Passive diffudion

Osmosis
Osmosis is defined as the movement of water molecules from a region of higher concentration of water to a region of lower concentration of water through a semi-permeable membrane.
The plasma membrane is selectively permeable to molecules. Smaller molecules cross the cell membrane more easily than large molecules. Water, carbon dioxide and oxygen cross the membrane easily. Glucose and other large compounds are too large to cross easily.

In osmosis water moves from a region of low solute concentration to a region of high solute concentration. In the diagram above water moves from A to B through the semi-permeable membrane(SP). The glucose molecules in B are too large to pass across the semi-permeable membrane, SP and enter A. Incoming water molecules cause the level of the glucose solution in B to rise while the level of water in A drops.

Osmosis can also be demonstrated using a peeled potato. A hollow is made in a potato; the potato is placed on a petri dish containing distilled water. Sugar crystals are placed in the hollow. Twenty four hours later, it is found that the sugar crystals have formed a solution and that the water level in the petri dish has decreased because water has moved to the hollow by osmosis.

 

Types of Solutions
Most animal cells are isotonic with their environment. This means that the concentrations of the salts inside the cells are equal to the concentrations of the salts in the blood so that there is no net flow of water in either direction. A solution whose osmotic concentration is lower than the salt concentration of the blood or that of the body fluids is referred to as hypotonic. On the other hand, a hypertonic solution has a higher osmotic concentration than that of the body.

If red blood cells are placed in a hypotonic solution, water will diffuse into the red blood cells by osmosis . This water will accumulate in the cells and the cells  will expand in size and finally the cells will burst. On the other hand, if the red blood cells are put in a hypertonic solution, water will diffuse out from the cells to the hypertonic solution, causing the cells to shrink.

 

Facilitated Diffusion
Some molecules are too big to pass through the plasma membrane. In order to cross the membrane, they attach themselves to carrier proteins on the plasma membrane, which will carry them across the membrane, just as a river ferry does when it carries people with their properties across a river. Such carrier proteins transport their substances from a region of high concentration to a region of low concentration. Facilitated diffusion does not require energy.

 





 Active Transport

A cell will sometimes accumulate a certain compound at higher concentration than it is in the surrounding medium. A liver cell, for example, will continue to pick up glucose from the blood even when the concentration of glucose in the liver far exceeds that present in the blood. For human, the sodium- 

    potassium pump is a very good example to explain the active transport. Sodium ions(Na+) are pumped  

    out of te cell and the potassium ions(K+) are pumped into the cell. Energy is needed for this process in  

    the form of ATP.

The movement of charged molecules(ion) in active transport uses both a carrier protein and the energy provided by ATP to pump the molecules against the concentration gradient. Active transport is also important in nerve action, muscle contraction and kidney function. Active transport requires energy.