Biology Topics related to disease and health provide critical insights into human physiology and medicine.
Introduction to Cells
In biology, a cell is the smallest unit of life. All the living things (plants and animals) are made from cells. Just as a house is made of small units called bricks, in the same way, the bodies of living organisms are made of tiny units called cells. Thus, cells are the building blocks of plants and animals. Cells are very, very small in size which cannot be seen with naked eyes. Cells can be seen only under a microscope. We can, however, see the plants and animals around us because they are made up of millions and millions of tiny cells joined together.
Some organisms are made of only one cell but others are made of many, many cells joined together. For example, the simplest animal called Amoeba is made up of only one cell but a complex organism like a rose plant or a man is made up of millions of cells. All living cells come only from other living cells. The cells are mainly of two types : Animal cells and Plant cells. Though many things are common between animal cells and plant cells but they differ in some ways.
A cell consists of a jelly-like material enclosed in a thin membrane (see Figure). The jelly like material which fills the cell is called cytoplasm. The thin outer covering of the cell is called cell membrane. There is a large spherical structure floating in the centre of a cell which is called the nucleus. The nucleus is surrounded by jelly-like material called cytoplasm. A number of small bodies called mitochondria are also present in the cell. There are tiny air spaces in the cytoplasm of an animal cell which are called vacuoles. The general diagram of an animal cell is shown in Figure.
We will now give the functions of all the parts of a cell. The cell membrane protects the cell and also controls the passage of materials which go into the cell’ or go out from the cell’. The function of nucleus is to control all the activities of the cell (like cell growth, etc.). The function of mitochondria is to carry out respiration for releasing energy from food. The function of small vacuoles is to hold air, water or particles of food. And the function of cytoplasm is to carry out all the activities of life processes (or metabolism).
All the parts of animal cells such as cell membrane, cytoplasm, nucleus and mitochondria are also present in plant cells. In addition to these, the plant cells have three more parts in them. These are cell wall, chloroplasts and large vacuoles. These parts make the plant cells different from animal cells. The general diagram of a plant cell is given in Figure. All the plant cells have a thick cell wall around the cell membrane. The cell wall protects the cell, gives it a fixed shape and makes it rigid (strong). The chloroplasts contain chlorophyll and make food in green plants by the process of photosynthesis. The large vacuoles in plant cells are filled with cell sap (which is a solution of sugars and other substances).
The animals are made up of animal cells joined together whereas plants are made up of plant cells combined together. Since the animals and plants are made up of different types of cells, therefore, animals and plants look different and behave in different ways.