Class 7 Science Cell Structure and Function Chapter Notes

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Cell - Structure & Function

Living organisms take in and digest food, respire and excrete waste products. They also reproduce their own kind. These functions are carried out by different organs of the organisms. The body parts of both animals and plants are called organs, and they perform different functions. For example, leaves help in synthesis of food, roots anchor the plant, the mouth in animals helps in taking food, other organs help in its digestion, and the lungs help in respiration.

In spite of great variation in the 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 living organisms are complex structures, unlike bricks in a building. Cells, like the bricks of a building, are the 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 have been present in living organisms since the origin of life.

Fig. 1 (a) Building blocks of a wall; (b) onion peel. (Photograph not reproduced: the wall is made of bricks and the onion peel is made of cells.)

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

Variety of shape and size in the cells

Cells found in living organisms show variety in number, shape and size. An individual organism can either have one cell, where all the functions are carried out by a single cell, 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 a large number of cells are termed multicellular organisms. A multicellular organism, in fact, has millions of cells.

Cells in multicellular plants and animals have a variety of shapes. Cells are generally round or spherical. However, there is a lot of variation in the shape of cells. These can be cuboidal or columnar. Some animal cells are long and branched, as in a nerve cell.

Fig. 2 Various types of cells showing different shapes: diatom, sperm, fat cell, leucocyte, muscle cell, nerve cell, goblet cell, fibroblast and amoeba. (Illustration not reproduced.)

Most cells are very small in size and not visible to the unaided eye. Even long cells running into a few centimetres can be seen only with a microscope. The smallest cell, of size 0.1 micrometre (µm), i.e. one ten-thousandth part of a millimetre, has been observed in bacteria (Mycoplasma). Muscle cells in animals and fibres in plants like jute and hemp are a few centimetres long. In the egg, the central yellow part called the yolk is surrounded by the white albumen. The yellow yolk represents a single cell. An ostrich egg with a 170 mm diameter represents the largest cell observable with the unaided eye. Table 1 provides information about the different sizes of cells.

Table 1: Average Size of Different Cells in the Living Organisms

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 of further smaller parts. These parts are called cell organelles. Some of these organelles are common to all cells.

Fig. 3: A plant cell

Fig. 4: An animal cell

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 cell. 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 exit 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:
   1. 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.
   2. Plastids: These are exclusively present in plant cells in large numbers, and are absent in animal cells. These are of three types, namely:
      1. 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.
      2. Leucoplasts: These are colourless plastids. They are mostly present in the roots and underground modified stems.
      3. Chromoplasts: These are coloured plastids. They are present in abundance mainly in flowers, petals and fruits.
   3. 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.
   4. Ribosomes: These are granules attached to the endoplasmic reticulum or found freely in the cytoplasm. They help in the synthesis of proteins.
   5. Golgi bodies: These are small hollow plate-like or cup shaped bodies found in animal cells. They synthesise, store and secrete substances. The Golgi apparatus in plants is known as dictyosome.
   6. Centrosome: It is a star-like structure found mostly in animal cells. It consists of centrioles. It helps in cell division.
   7. Lysosome: They are present in animal cells only. They contain enzymes for cellular digestion. If they burst, the cell may get damaged or destroyed. Hence, they are called the suicidal bags of the cell.
   8. 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: The 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 nucleoprotein RNA (ribonucleic acid). It is responsible for protein synthesis.

The nucleus also contains chromosomes, the thread-like structures composed of the 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.

Fig. 5: Structure of nucleus

The summary of cellular structures and their functions is given in Table 2.

Table 2: Summary of Cellular Structures and their Functions

Table 3: Differences between a Plant Cell and an Animal Cell

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 and Paramecium, which are made up of only one cell. Such organisms are called unicellular organisms. These organisms show the cellular level of organization.

Fig. 6: Some unicellular organisms: Amoeba, Paramecium, bacteria and yeast. (Illustration not reproduced.)

Organisms which consist 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 a group of cells is called a tissue. When cells are organized to form tissues, it is called the tissue level of organization. It is higher than the cellular level.

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