All living things have cells which have a diameter of about 10 µm. They comprise of different structures called organelles which perform specific functions within the cell.
Cell Membrane
Also known as the plasma membrane or the plasmalema. It encloses the cell contents separating them from the external environment of the cell. It is semi permeable and has pores which allow movement of certain substances in and out of the cell but prevents the passage of waste materials.
Cytoplasm
It is a jelly-like medium in which most of the chemical reactions essential for life take place. It contains organelles and other inclusions such as starch, glycogen and fat droplets. The cytoplasm is not static but has a movement known as cytoplasmic streaming.
Nucleus
It is the largest and the most important of the cell organelles. It is situated at the centre of the cell and controls all the activities that go on in the cell. It is enclosed by a double membrane known as the nuclear membrane. The membrane is perforated by pores which allow movement of materials in and out of the nucleus. The nucleus is made up of viscous fluid known as nucleoplasm or the nuclear sap in which the nucleolus and the chromatin materials are suspended. The nucleolus is responsible for the manufacture of ribosomes while chromatin is made up of deoxyribonucleic acid which contains hereditary materials.
Mitochondria
They are sausage-shaped organelles that are bound by an outer and an inner membrane. The outer membrane is greatly folded into cristae to increase the surface area for respiration and attachment of the enzymes. The interior is filled with a gel-like matrix. The mitochondria are sites of respiratory reactions. They contain many enzymes which catalyze the oxidation of food (respiration) with the release of chemical energy which is used for the formation of a high-energy chemical compound called adenosine triphosphate (ATP). Highly active cells like spermatozoa and muscle cells have many mitochondria while less active cells like fat cells have less mitochondrion.
See the function of mitochondria
Endoplasmic Reticulum (ER)
The endoplasmic reticulum appears as a series of parallel interconnected channels, lined with a thin unit membrane, running throughout the cytoplasm. There are two types of ER, the rough ER and the smooth ER. The rough ER has granules called ribosomes on its surfaces and transports proteins. The smooth ER is more tubular in structure and has no ribosomes. It is involved in the synthesis of various types of lipids and in the breakdown of foreign chemicals such as drugs.
Ribosomes
They are spherical in shape. Some are scattered within the cytoplasm while others are bound to the surface of the rough endoplasmic reticulum. They form sites for protein synthesis in the cell.
Lysosomes
They are dark spherical sac like vesicles bound by a single membrane. They contain lytic enzymes which break down large molecules, destroy worn out organelles or even the entire cells. The unwanted materials are segregated in a membrane-lined vacuole into which several lysosomes discharge their contents. The materials are digested to their simple building blocks which are released back into the cytoplasm.
Golgi bodies
These are stacks of membrane-bound tube like sacs which are found close to the cell membrane. At their ends are spherical vesicles that are pinched off from the flattened vesicles. They receive certain materials like glyco-proteins from the endoplasmic reticulum and package them. They usually move to the cell membrane, fuse with it and secrete the synthesized proteins and carbohydrates to the exterior of the cell. This process is called exocytosis. Golgi bodies are abundant in cells of granular organs such as the liver where they are involved in the processing and release of secretions.
Centrioles
They are rod-shaped structures located outside the nuclear membrane. They take part in cell division and in the formation of cilia and flagella in cells that have these structures. Plant cells luck centrioles.
Cell Wall
It is a tough, rigid outer cover which encloses the plant cells and acts as a protective coat. The cell wall is made up of a chemical substance called cellulose. It gives the plant cell a definite shape as well as providing mechanical support and protection against mechanical injury. It readily allows gases, water and other substances to pass through but resists swelling or expansion of the enclosed cell. Cell walls of woody portions in trees are strengthened by the deposition of a tough chemical substance called lignin to enable them to serve a supportive function as well.
Vacuoles
These are sacs that are enclosed by a membrane called tonoplast and are filled with fluid called cell sap. The cell sap contains ions and sugars hence contribute to the osmotic properties of the cell. The vacuoles vary in size; animal cells contain small vacuoles while plant cells have large ones. In some unicellular organisms, food vacuoles store and digest food while contractile vacuoles excrete waste materials from the cell.
