What is a Cell Made Up of? What is the Structural Organisation of a Cell?

Cell has special components called organelles. Cell under a microscope, we would come across three features in almost every cell; plasma membrane, nucleus and cytoplasm.

PLASMA MEMBRANE OR CELL MEMBRANE

This is the outermost covering of the cell that separates the contents of the cell from its external environment. The plasma membrane allows permits the entry and exit of some materials in and out of the cell. It also prevents movement of some other materials. The cell membrane, therefore, is called a selectively permeable membrane.

Diffusion: Some substances like carbon dioxide or oxygen can move across the cell membrane by a process called diffusion. Substance like CO₂ (which is cellular waste and requires to be excreted out by the cell) accumulates in high concentrations inside the cell. In the cell’s external environment, the concentration of CO₂ is low as compared to that inside the cell. As soon as there is a difference of concentration of CO₂ inside and outside the cell, CO₂ moves out of the cell, from a region of high concentration, to a region of low concentration outside the cell by the process diffusion. Similarly, O₂ enters the cell by the process of diffusion when level or concentration of O₂ inside the cell decreases. Thus, diffusion plays an important role in gaseous exchange between the cells as well as the xell and its external environment.

Water also obeys the law of diffusion. The movement of water molecules through such a selectively permeable membrane is called osmosis. Osmosis is the passage of water from a region of high concentration through a semi- permeable membrane to a region of low water concentration.

There are three following things could happen when we put an animal cell or plant cell into a solution of sugar or salt:-

• If the medium surrounding the cell has a higher water concentration that the cell, meaning that th4e outside solution is very dilute, such a solution is known as a hypotonic solution. Water molecules are free to pass across the cell membrane in both directions, but more water will come into the cell that will leave. The net (overall) result is that water enters the cell. The cell is likely to swell up.
• If the medium has exactly the same water concentrate as the cell, there will be no net movement of water across the cell membrane. Such a solution is known as an isotonic solution. Water crosses the cell membrane in both directions, but the amount going in is the same as the amount going out, so there is no overall movement of water. The cell will stay the same size.
• If the medium has a lower concentration of water than the cell, meaning that is a very concentrated solution, the cell will lose water by osmosis. Such solution is known as a hypertonic solution. Again, water crosses the cell membrane in both directions, but this time more water leaves the cell than enters it. Therefore the cell will shrink.

The plasma membrane is flexible and is made up of organic molecules called lipids and proteins. However, we can observe the structure of the plasma membrane through an electron microscope. The flexibility of the cell membrane also enables the cell to engulf in food and other material from its external environment. Such processes are known as endocytosis. Amoeba acquires its food through such processes.