Class 7 Science Cell Structure and Function Chapter Notes

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Cell - Structure & Function

Living organisms take and digest food, respire and excrete waste products. They reproduce their own kind. These functions are carried out by different organs of the organisms. The body parts both in animals and plants are called organs. They perform different functions. For example, leaves help in synthesis of food, roots in anchoring the plants. Mouth in animals helps in taking food other organs help in its digestion lungs help in respiration.
In spite of great variation in structure of various organs, all organisms have basic similarities among them. In
fact, each one of them is made up of cells.

Cell - The Unit of Life
Cells in the living organisms are complex structures unlike bricks in a building.. Cells like bricks of a building, are basic structural units of living organisms. (Fig.1). Buildings usually differ from one another, organisms also differ from one another. Both have basic units. Cells are there in living organisms since the origin of life.

Majority of the cells are too small to be visible to the unaided eye. However, some cells like hen’s egg are visible to eye, without any aid. The word ‘ cell’ was coined in seventeenth century by Robert Hooke while observing a slice of cork (plant cells) under the microscope. His microscope, in fact, was primitive. Earlier, lenses were used directly as an aid to see the smaller things / objects. Later on, these lenses were put in a device called microscope to magnify the objects. With passage of time, microscopes, in the last four centuries have been improved by increasing their magnifying ability. Nowadays, a microscope can enable to observe the objects as small as one thousandth of a millimetre (10–6m). This has helped the scientists to study the minute detail of cells. Sometimes, the materials to be observed with microscopes are stained with dyes to identify the different components of a cell.

Variety of shape and size in the cells
Cells found in the living organisms show variety in number, shape and size. An individual organisms can either have one cell where all the functions are carried out by a single cells or have many cells which together perform various functions. Single celled organisms like Amoeba, Paramecium and bacteria are called unicellular organisms. On the other hand, organisms which have large number of cells are termed as multicellular organisms. A multicellular organisms in fact, has millions of cells.

Structure of the Cell
The microscopic structure of a typical plant cell and that of an animal cell is shown in Fig. 3 and 4. Each cell has a definite structure or further smaller parts. These parts are called cell organelles. Some of these organelles are common to all cells.

1. Cell Wall: This is the outermost part of a plant cell. It is non-living and is made up of cellulose. It is protective in function and mainly determines the shape of the cells. It is absent in animal cells.

2. Cell Membrane: This is also called the plasma membrane. In an animal cell, it is the outermost layer whereas in a plant cell it is protected by the cell wall. It is a living structure and controls the entry and exist of some substances in and out of the cell. It also protects the internal components of the cell.

3. Protoplasm: The material comprising the living contents of a cell i.e. all the substances in a cell except large vacuoles and material recently ingested or to be excreted.

4. Cytoplasm: It is a transparent, jelly - like living substance which fills the interior of an animal and a plant cell. It contains a number of minute living structures known as the cell organelles and many non-living substances known as cell inclusions.

5. Cell Organelles: These are active, living, permanent extremely small structures present in the cytoplasm and are concerned with cell function. The various cell organelles present in a typical cell are:

i) Endoplasmic reticulum: A system of membranes within the cytoplasm of plant and animal cells. It forms a link between the cell and nuclear membranes and is the site of protein synthesis. It helps in the transport of substances within the cell.

ii) Plastids: Plastids are exclusively present in plant cells in large numbers, and are absence in animal cells. These are of three types, namely:

a) Chloroplasts: These are green coloured plastids, containing the green pigment chlorophyll. They are present in the leaves and stems of most of the plants. They are regarded as the kitchen of the cell.

b) Leucoplasts: The are colourless plastids. They are mostly present in the roots and underground modified stems.

c) Chromoplasts: They are coloured plastids. These are present in abundance mainly in flowers, petals and fruits.

iii) Mitochondria: These are rod or oval shaped structures, bounded by two membranes and concerned with the release of energy from food during respiration. Hence they are often referred to as the power house of the cell.

iv) Ribosomes: These are granules attached to the endoplasmic reticulum or found freely in the cytoplasm. They help in the synthesis of proteins.

v) Golgi bodies: These are small hollow plate - like or cup shaped bodies found in animal cells. They synthesis, store and secrete substances the golgi apparatus in plants is known as dictyosome.

vi) Centrosome: It is a star like structure found mostly in animal cells. It consists of centrioles. It helps in cell division.

vii) Lysosome: They are present in animal cells only. They contain enzymes of cellular digestion. If they burst, the cell may get damaged or destroyed. Hence, they are called the suicidal bags of the cell.

viii) Vacuoles: These are fluid filled spaces containing minerals dissolved in water. The solution that fills the vacuoles of plant cells is called cell sap.
Vacuoles are usually large and many in plant cells but small and few in animal cells.

6. Cell Inclusions: The non-living constituents of a cell are known as cell inclusions. These include water, minerals, salts, sugar, proteins etc.

7. Nucleus: Nucleus is the most important part of a cell. It is usually spherical or oval in shape. It controls all the vital functions of the cell. It is made up of the nuclear membrane, nucleoplasm, nucleolus and chromosomes.

The nuclear membrane surrounds the nucleus and separates it from the cytoplasm. It is permeable and controls the passage of materials through and from the nucleus. The
nucleoplasm or nuclear sap makes up the body of the nucleus. It is denser than the cytoplasm.

The nucleolus is a spherical body in the nucleus. It is composed of the nucleooprotein RNA (Ribonucleic acid). It is responsible for protein synthesis.
Nucleus also contains chromosomes, the thread like structures and composed of nucleoprotein DNA (deoxyribonucleic acid). The hereditary units of chromosomes are the genes. They are responsible for the transmission of characters from the parents to the offspring. It is an account of these genes that a child resembles its parents. The summary of cellular structures and their functions is given in Table 2.

Cell Division

One of the most important characteristics of the living cell is its power to grow and divide. When a cell reaches the limit of the size peculiar to its species, it divides into two parts. These parts grow to their full size and then divide again. The new cells are always formed by the division of pre-existing cells.
The process by which a cell multiplies in number is called cell division. Cell division is of two types, namely mitosis and meiosis.

Various levels of Organization

There are certain organisms like Amoeba, Paramecium, which are made up of only one cell. Such organisms are called unicellular organisms. These organisms show cellular level of organization.

Organisms which consists of two or more cells are called multicellular organisms, e.g. man. In these organisms, a group of similar cells performs a particular function. Such group of cells are called tissue. When cells are organized to form tissues, it is called a tissue level of organization. It is higher than the cellular level.

In higher organisms, group of tissues further orgnaise to form organs and this is called an organ level of organization. Several organs form a system which is responsible for performing specific functions. This is called an organ system level of organization. So, the various levels of organization in a living body are:

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