The lipids and protiens that make up the cell membrane are synthesised in the endoplasmic reticulam. Movement of substances in the cell: There is movement of different types of substances within the cells. But the cell membrane or plasma membrane allows only specialised substances to pass through it. Therefore, the plasma membrane is called a selectively permeable membrane. The movement of substances in cells takes place in two ways:

(i) Diffusion (ii) Osmosis

(1) Diffusion: The movement of substances through cells is accomplished by the process of diffusion. When a substance (like CO2 or O2) moves from a higher concentration to a lower concentration, that process is called diffusion. The exchange of gases between the leaves of plants and the atmosphere takes place by the process of diffusion.

Importance of diffusion:

(a) Different substances in the cytoplasm move from one place to another due to diffusion.

(b) Flowers attract pollinators by spreading their fragrance by diffusion.

(c) When a perfume bottle is opened, its molecules are evenly dispersed in the air.

(d) The rate of diffusion higher in the gases than in solids and liquids.

(e) Diffusion helps in transpiration of water from plants in the form of water vapour. It is beneficial for the plants.

(f) Diffusion helps in the exchange of substances required by the organism.

(2) Osmosis: The movement of substances in the liquid state takes place in the cells by another process, which is called osmosis. When water molecules move from high water concerntration to low water concentration through a selectively permeable membrane, this process is called osmosis. That is, "Osmosis is the process in which molecules of a a solvent (water) move from a region of pure water or a solution of less solute to a solution of more solute through a semi permeable membrane."

Importance of Osmosis:

(a) The absorption of water by the roots of plants takes place through osmosis.

(b) The crunching speed of some plants is based on osmosis.

(c) Stomata of leaves open and close by osmosis.

(d) Unicellular, saltless (fresh) aquatic organisms take in water by osmosis.

(e) Cells absorb water by osmosis.

Solutions are divided into three parts on the basis of rate of osmosis:

1. Low osmotic pressure solution.

2. Semi osmotic pressure solution.

3. Super osmotic pressure solution.

1. Low osmotic pressure solution: The solution which produces low osmotic pressure is called low osmotic pressure solution, for example: if a cell is placed in such a solution of sugar or salt in which the amount of sugar or salt is very less relative to amount of water, then water starts going inside the cell by osmosis. Such solution of water are called low osmotic pressure solution. Cell starts swelling by this phenomenon.

2. Semi osmotic pressure solution: The solution whose concentration is exactly equal to the concentration of the substance of the cell, are called semi osmotic pressure solution.

3. Super osmotic pressure solution: If the solution outside the cell is more concentrated or thicker than the substance inside the cell, the water inside the cell, will start coming out by osmosis. Such solution is called super osmotic pressure solution. When cell is placed in highly osmotic pressure solution, excess amount of water comes out from the cell, hence the cell gets compressed. It can be understood by the following examples:

(a) The outer shell of the egg which is made up of calcium carbonate can be removed by soaking with dilute hydrochloric acid. There is a membrane inside it. In this state, if the egg is immersed in pure water for 5 minutes then egg get swelled due to osmosis.

(b) If a shellless egg is placed in a concentrated solution of salt, then the egg shrinks because the water of egg comes out in the salt solution.

(c) If raisins are kept in a normal water, they swell after absorbing water. This process is also called imbition. Conversely, if they are kept in a concentrated solution, their water gets into the solution and they shrink.