Lesson (4) Cell processes

Cells perform many vital processes such as respiration, movement, nutrition, reproduction…etc. In this lesson, we'll study some of these cellular processes

à Cells get substances from the surrounding medium, and get rid of wastes by expelling them to the surrounding medium.

à Cell membrane has a property called selective permeability. This means that some substances (such as water, oxygen, carbon dioxide) can pass through it, while other substances (such as big molecules and salts) cannot do so. This depends on many factors such as:-

1- Size of particles

2- Charge of particles

3- The difference of concentration between the two sides of cell membrane

Substances are transported to and from cells by the following methods:-

Passive transport

Passive transport: The movement of substances through cell membrane where cell doesn't consume any energy.

Passive transport has the following methods:-

Diffusion

Diffusion: The movement of molecules through membrane from a region of high concentration to a region of low concentration, which makes the concentration of molecules on both sides equal.

Example:-

à The exchange of carbon dioxide and oxygen

gases between the inner and outer mediums

of the cell during respiration process.

à If the concentration of molecules in cell is

greater that that of the external medium,

molecules move from the cell to the external

medium to balance the concentration of both sides

Fig. (35) diffusion process

Osmosis

Osmosis: The movement of pure water molecules through a membrane from a region of low concentration to a region of high one, which makes the concentration of both sides equal.

à Water transports from and to cell due to the difference between the concentration of cytoplasm (fluid inside the cell) and the external medium.

Fig. (36) Red blood cells and osmotic pressure

1- When we put cells in a solution of low concentration, water transports from the external medium (low concentration) into the cell (high concentration), which tears up cells.

2- When we put cells in a solution of equal concentration, water transports equally between the two mediums, and cells sizes don't change

3- When we put cells in a solution of higher concentration, water transports from the cell (low concentration) to the external medium (high concentration), which makes cells shrink.

Дата добавления: 2015-10-23