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Monday, 26 November 2018

Cell Ultrastructure and Function





Scientists viewing the interior of a cell through an electron microscope described the ultrastructure of the cell. In addition, they ruptured the cell and studied all the ultracellular  organelles in it. To do the studies, the different types of organelles were separated by centrifugation. Then, biochemical experiment were carried out by each type of cell organelle to study its function. Chromatography was used to identify many of the substances in the reaction systems. From the results, scientist verified the function of most of the cell organelles.

     From the above studies, cells were found to have many features in common. However, a generalized cell does not exist. Plant and animal cells have important differences. Cells within an organism also show many differences. The precise structure of a cell is closely related to the work it does. Bearing this in mind, we shall study some important cell structures and their functions.

Plasma membrane

This is a flexible membrane made up of mainly proteins and lipids. It prevents the cell contents from escaping. It also controls  the materials which are allowed to enter or leave the cell, I.e. it is selectively permeable.

Cell wall

In plants, a cellulose wall surrounds the plasma membrane. It is a tough, fairly rigid structure that is freely permeable, unlike the cell membrane. It provides mechanical support and protection to the cell, and hence to the plant as a whole.

Protoplasm

This is the living material inside the cell membrane. It consist of the nucleus and the cytoplasm.

Nucleus

Easily stained by dyes, the nucleus (plural : nuclei) is the largest and most important cell organelle.  It is enclosed by a double layer nuclear membrane with many pores. This nuclear pores allow many exchange of materials between the nucleus and the cytoplasm.

    The nucleus contains a thread like network of chromatin granules, which is the extended form of the chromosomes. The chromosomes contain deoxyribonucleic acid or DNA, the molecule that contain hereditary information in a code form. This information include all the genetic instructions for controlling

•  the production of chemicals for normal cell functioning;

•  the production of a new cell or the whole organism itself; and

•  the development of genetically acquired traits.

    Among the chromatin are one or more darkly stained bodies called nucleoli. They Ade rich in proteins and nucleic acids. These material are used to manufacture molecules which acts as messengers and carry information for making proteins from the nucleus to the cytoplasm. Ribosomes in the cytoplasm receive this information and make proteins (mainly enzymes).

   The structure of the nucleus makes it the control centre of the cell. Thus, in a given cell, the nucleus controls the type and quantity of proteins (enzymes) produced in the cytoplasm, thereby determining what the cell will be : a hormone producing cell in the testis, a sensory cell in the eye  or an acid producing cell in the stomach lining. It also controls the cell division (reproduction) of the cell.

Cytoplasm 

This is a jelly like, transparent material composed mainly of water. It can exist in liquid (Sol) or semi solid (gel) state. Many organic and inorganic substances are dissolved in it. All metabolic processes occur here.

    The cytoplasm of the cell is the part that is capable of specialization, not the nucleus. This is why the structure and function of the cytoplasmic components will differ to a certain extent in various types of cells.

   In the cytoplasm there are many ultracellular components known as inclusions and organelles. Inclusions are mainly starch grains   oil globules and excretory crystals (plants). These are temporary features. Organelle are permanent structures needed for cell function. Each organelles does a special job. Some important organelles are discussed below :

Mitochondrion

 This is a sausage shaped body surrounded by a double membrane layer. The inner membrane is folded to form partitions or cristae (singular: crista) , which project into the side of Mitochondrion   the energy producing reactions of cellular respiration take place in the Mitochondrion. It is thus often called the cells power house. Cells that are working actively and need a lot of energy have numerous Mitochondrion in their cytoplasm.

Chloroplast

Found only in the cells of green plants like protists, a chloroplast contains the green pigment chlorophyll. It is the site of photosynthesis.

Ribosomes

These are small round bodies that are found in free in the cytoplasm or attached to the endoplasmic reticulum. Ribosomes are the sites of protein synthesis.

Endoplasmic Reticulum 

This is a system of membrane lined sacs that forms channels throughout the cytoplasm. It's membrane is continuous with the nuclear membrane.


   These are two types if endoplasmic reticulum: rough and smooth. Ribosomes are found on the surfaces of rough endoplasmic reticulum. Such endoplasmic reticulum packs and transports protein made by ribosomes through the channels from one part of the cell to another. Smooth endoplasmic reticulum is involved in lipid and steroid synthesis, packaging and transport.

Golgi apparatus

This is a stack of membrane lined sacs. It is continuously formed at one end and broken off as Bibles or vesicles at the other end. These vesicles carry export materials like enzymes and hormones which the cell makes. They move towards the plasma membrane and transport the material towards the cell. This is known as exocytosis. Thus, Golgi apparatus is said to pack and export materials from the cell.

Lysosomes

These are small rounds sacs that contain digestive enzymes which breaks Down structures and substances. They are also involved in the transport of materials in and out of the cell.

Centrioles

Two rod like Centrioles are found at right angles to each other in an animal cell. They are found near the nucleus and seem to be concerned with cell division.

Vacuole

In plants, a large central vacuole is usually found in a mature cell. It is lined with a membrane and filled with cell sap. Cell sap acts as a store house for many substances including excretory products.

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