CELLS & DIFFERENT BETWEEN PLANT AND ANIMAL CELLS

  

  NEARILY all plants and animals have one characteristic in Common they are made up of cells, If any structures from plants or animals are examined microscopically they will be seen to consist of more or less distinct units-cells-which, Although too small to be seen with the naked eye, in their vast numbers make up the structures or organs,

Since the cells of any organ are usually specially developed in their size, shape and chemistry to carry out one particular function (e.g, muscle cells for contracting) there is, strictly speaking, no such thing as a "typical" cell of plants or animals.

Nevertheless, certain of the features common to most cells can be illustrated diagrammatically,

PARTS OF THE CELL

Cell membrane, All cells are bounded by a very thin flexible membrane which retains the cell contents and controls the substances entering and leaving the cells.

Cell walI. In plant cells only, there is a wall outside the cell membrane, it confers shape and, to some extent, rigidity on the cell, While the cell is growing the cell wall is fairly plastic and extensible, but once the cell has reached full size, the wall becomes tough and resists stretching.

Unless impregnated with chemicals, as in the cells of corky tree bark, the cell wall is freely permeable to gases and water, i.e, it allows them to pass through in either direction. The cell wall is made by the cytoplasm and is non-living, being made of a transparent substance called cellulose.

The middle lamella is the layer which first forms between cells after a plant cell has divided and may remain visible between mature cells in microscopical preparatons.

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Protoplasm is the material inside the cell which is truly alive.

There are two principal kinds of protoplasm; the protoplasm which constitutes the nucleus (see below) is called nucleoplasm.

All other forms of protoplasm are referred to as cytoplasm.

     Cytoplasm is jelly-like and transparent, fluid or semi-solid and may contain particles such as chloroplasts or starch grains. In some cells it is able to flow about. In the cytoplasm the chemical processes essential to life are carried on. The cell membrane is selectively permeable, allowing some substances to pass through and preventing others from doing so. This selection helps to maintain the best conditions for chemical reactions in the protoplasm.

    The nucleus consists of nucleoplasm bounded by a nuclear membrane. It is always embedded in the cytoplasm, is frequently ovoid in shape and lighter in colour than the cytoplasm. In diagrams it is often shaded darker because most microscopical preparations are stained with dyes to show it up clearly. It is less easily seen in the unstained cell. The nucleus is thought to be a centre of chemical activity, playing a part in determining the shape, size and function of the cell and controlling most of the physiological processes within it.

    Without the nucleus the cell is not capable of its normal functions or of division, although it may continue to live for a time. When cell division occurs, the nucleus initiates and controls the process.

    Vacuole In animal cells there may be small droplets of fluid in the cytoplasm, variable in size and position. In plant cells the vacuole is usually a large, permanent, fluid-filled cavity occupying the greater part of the cell. In plants, this fluid is called cell sap and may contain salts, sugar and pigments dissolved in water. The outward pressure of the vacuole on the cell wall makes the plant cells firm, giving strength and resilience to the tissues.

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 DIFFERENT BETWEEN PLANT AND ANIMAL CELLS

  1. Although there is a very wide range of variation, the cells of plant tissues are usually easier to demonstrate under the microscope than are the cells of animal tissues. This is partly because the plant cells are larger and their cell walls give them a distinctive outline.

2. Plants have cell walls made of cellulose. Animal cells have no definite walls and do not possess any cellulose.

3. Plant cells have only a thin lining of cytoplasm, with a large central vacuole. Animal cells consist almost entirely of cytoplasm (Plate 1). If any vacuoles are present they are usually temporary and small, concerned with excretion or secretion.

4. Animal cells never contain chloroplasts whereas these are present in a great many plant cells.

Relation of cells to the organism as a whole

Although each cell can carry on the vital chemistry of living, it is not capable of existence on its own. A muscle cell could not obtain its own food or oxygen. Other specialized cells, present in a tissue or organ, collect food or carry oxygen. Unless in- dividual cells are grouped together in large numbers and made to work together by the co-ordinating mechanisms of the body, they cannot exist for long.

    

    Tissue. A tissue such as bone, nerve or muscle in animals.

and epidermis, phloem or pith in plants, 1s made up of many hundreds of cells of one or a few types, each type having a more or less identical structure and function so that the tissue can also be said to have a specific function, e.g. nerves conduct impulses, phloem carries food.

   Organs consist of several tissues grouped together making a functional unit: for example, a muscle is an organ containing long muscle cells held together with connective tissue and permeated with blood vessels and nerve fibres. The arrival of a nerve impulse causes the muscle fibres to contract, using the food and oxygen brought by the blood vessels to provide the necessary energy.

In a plant, the roots, stems and leaves are the organs.

   System usually refers to a series of organs whose functions are co-ordinated to produce effective action n the organism: for example, the heart and blood vessels constitute the circulatory system; the brain, spinal cord and nerves make up the nervous system. 

   An organism results from the efficient co-ordination of the organs and systems to produce an individual capable of separate existence and able to perpetuate its own kind.

 

Specialization

In its structure and physiology, each cell is often adapted to a particular function in the organ of which it is a part. The notes and drawings on the opposite page illustrate this.

        PRACTICAL WORK

Plant cells, The epidermis can be stripped fairly easily from the inside of an onion scale or other bulb scale or the lower surface of certain fleshy leaves. Since the epidermis here is one cell thick, the cells can be seen in transparency if a small piece of the tissue is placed flat on a slide, covered with a drop of water, and examined under the low power of the microscope. A little iodine solution may stain the nuclei light brown, and any starch grains present will turn dark blue. Cells and chloroplasts can be seen in a moss leaf if it is mounted flat on a slide with a drop of water.

Animal cells. If a finger is run round the inside of the cheek, and the fluid so collected is smeared on a slide, examination under the low power of the microscope will show numbers of epithelial cells scraped from the mouth lining (Plate 1). The. nuclei can be seen without staining but will show up more clearly if a drop of methylene blue solution is placed on the slide for about one minute. The slide is tilted to let the stain run off. The cytoplasm and nuclei will both take up the stain but the nuclei will be darker.