The multicellular plants and animals are made up of large numbers of such unicellular structures of unit called cells. Therefore, it may be said that a cell is a single unit of a living orga
nism. Since plant and animal differ from one another, the cells that compose the body of a plant or an animal must also be different from each other in certain respects.
PLANT CELLS
Plant cells are of different types and sizes. Most plant cells have regular outlines and fixed shapes. They may be rectangular, polygonal or cylindrical in shape.
A typical plant cell taken from the surface of a leaf would clearly show the various parts which most plant cells are composed of. There is a distinct cell wall composed of non living material and divided into two layers namely the primary wall and the secondary wall. These two layers are made up of different types of substance such as lignin, pectin, tannin and cellulose. The most important of all these substances is the carbohydrate cellulose which is found in all plant cell. The cell wall is generally thick and rigid and does not change it's shape. However, in some plant cells it is thin and flexible. In some plants, the cell wall is permeable. In between adjacent cells lies a very thin layer of cementing material known as the middle lamella. This also is permeable. Enclosed by this non living cell wall is the living material known as protoplasm. The outer surface of the protoplasm is thicker than the rest of it and is called the plasma membrane or the plasmalemma. It is semi permeable. Protoplasm is semi fluid in nature and in a living cell is the continuous motion. When you view the tip of a thin leaf of a water plant under a microscope you will be able to notice this protoplasmic movement.
The protoplasm in a cell is composed of two parts , namely , the cytoplasm and the nucleus. The cytoplasm occupies a large part of the space within the cell wThe nucleus may be situated at the centre or periphery of the cell.
There are open spaces called vacuoles in the cytoplasm. In a young cell the cytoplasm is dense and there may be several tiny vacuoles. As the cells grow older, all these vacuoles finally join up to form a large central vacuole. The vacuoles are filled with a fluid known as cell sap. This fluid consists of a high percentage of water with sugars and some mineral salts dissolved in it.
Cytoplasm is collodal in nature and consists of solutions of many different types of peoteins. Sometimes reserve food substances such as starch or sugar may be present in the cytoplasm in addition to these proteins.
Embedded in the cytoplasm are little ovoid organelles known plastids. Most of these plastids are green in colour and are known as chloroplasts. They contain chlorophyll and their presence gives rise to the green colour of leaves and stems. Chloroplasts are essential for the manufacture if carbohydrateSome plants however have leaves which are red and yellow in colour. This is due to the presence of coloured plastids(known as chromoplasts) in the cytoplasm.
Chromoplasts contain red and yellow pigments in them, e.g carotene and xanthophyll.
Also embedded in the cytoplasm are tiny organells called mitochondria which may be elongated, round or oblong in shape. They are intimately connected with the respiration of the cell. There are also minute granular microsomes which are essential for the formation of proteins. Besides, they play an important role in cell metabolism.
The nucleus is generally spherical or ovoid in shape. It is denser than the cytoplasm and is very prominent. It is enclosed in a cell membrane, the nuclear membrane or neurilemma, and contains a few long, coiled thread like structures called chromosomes. Besides, the chromosomes, the nucleus also contains a small, dense, spherical structure called the nucleolus. The contains nucleo proteins.
Each chromosomes bears numerous tiny structures arranged in a neat row along its length. These structures are known as genes.
Each gene contains the information for the expression of a particular characteristic such as the height if a plant, the colour if a flower or the shape of a fruit. Genes are believed to be the unit of inheritance; that is, the characteristics of the parent plant are passed on to the young seedlings through the genes which are contained in the seeds of the parent plant.
It may, therefore, be said that the genes on a chromosome are the carriers of parental characteristics.
The cytoplasm and the nucleus contain two special type of acids known as deoxyribonecleic acid(DNA) and ribonucleic acid(RNA).These substances consist of sugars, phosphates and certain types of nitrogenous ANIMAL CELL
Animal cells, like plant cells are of different shapes and sizes. They may be spherical, rectangular, cuboidal, spindle shaped or irregular. In general, an animal cell is much smaller than a plant cell. It does not have a thick, rigid cell wall as in the case of a plant cell. The other outer wall, or cell membrane, is rather thin and delicate and is not made up of cellulose but of lipids and protein. Owing to the delicate and flexible nature of the cell membrane, the shape of an animal cell can change and does so in different cases.
Within the thin cell membrane lies the protoplasm which is made up of the cytoplasm and the nucleus. The cytoplasm is semi fluid in nature and as in a plant cell, contains dissolved proteins and sugars. Most animal cell have no plastids, if pigments are present, they occur in the form of granules scattered throughout the cytoplasm. Unlike the plant cell, there are no large vacuoles in the cytoplasm. However, there may be a few tiny vacuoles as well as little oil droplets containing fat or oil. Rod like, granular or filamentous mitochondria which help in the oxidation of food substances are also present. The cytoplasm also contains microsomes which help in the manufacture of proteins and Iysosomes which are the sites if the enzymes concerned with the breakdown of large molecules. Coiled thread like Golgi bodies are also present near the nucleus. The nucleus contains the same structures as those found in a plant cell, but the number of chromosomes and types of genes as well as the characteristics which the genes carry are differenCELL DIVISION
Both plant and animal cells grow to a maximum size after which all cells divides into two equal cells. The division starts first with the nucleus and passes on to the cytoplasm so as to produce two new identical cells with an equal number of chromosomes. This type of cell division is known as mitosis.
Another type of cell division occurs prior to sexual reproduction. This is known as meiosis.t.bases.s.all.
0 Comments