NUMBER OF CELLS

NUMBER OF CELLS

How do scientists observe and study living cells? They use microscopes that magnify objects. Stains (dyes) are used to color parts of the cell to study the detailed structure.

There are millions of living organisms. They are of different shapes and sizes. Their organs also vary in shape, size, and number of cells. Let us study some of them.

Can you guess the number of cells in a tall tree or in a huge animal like an elephant? The number runs into billions and trillions. The human body has trillions of cells that vary in shape and size. Different groups of cells perform a variety of functions.

Organisms made of more than one cell are called multicellular (multi: many; cellular: cell) organisms. The number of cells is less in smaller organisms does not, in any way, affect the functioning of the organisms. You will be surprised to know that an organism with billions of cells begins life as a single cell which is the fertilized egg. The fertilized egg cell multiplies and the number of cells increases as development proceeds.

Look at Fig 8.3 (a) and (b). Both organisms are made up of a single cell. The single-celled organisms are called Unicellular (uni: one; cellular: cell) organisms. A single-celled organism performs all the necessary functions that multicellular organisms perform.

Figure 8.3: (a) Amoeba

Figure 8.3: (b) Paramecium

A single-celled organism, like the amoeba, captures and digests food, respires, excretes, grows, and reproduces. Similar functions in multicellular organisms are carried out by groups of specialized cells forming different tissues. Tissues, in turn, form organs.

Activity 8.1:

The teacher may show a permanent slide of amoeba and paramecium under a microscope. Alternatively, the teacher can collect pond water and show these organisms by preparing the slides.

Source: This topic is taken from NCERT TEXTBOOK

SHAPE OF CELLS

SHAPE OF CELLS

Refer to Fig. 8.3 (a). How do you define the shape of the amoeba in the figure? You may say that the shape appears irregular. In fact, the amoeba has no definite shape, unlike other organisms. It keeps on changing its shape. Observe the projections of varying lengths protruding out of its body. These are called pseudopodia (pseudo: false; podia: feet), as you learned in Class VII. These projections appear and disappear as the amoeba moves or feeds.

The change in shape is due to the formation of pseudopodia which facilitates movement and helps in capturing food.

Figure 8.3(a): Amoeba

A white blood cell (WBC) in human blood is another example of a single cell that can change its shape. But while WBC is a cell, the amoeba is a full-fledged organism capable of independent existence.

What shape would you expect in organisms with millions of cells? Fig. 8.4(a, b, c) shows different cells such as blood, muscle, and nerve of human beings. The different shapes are related to their specific functions.

Figure 8.4(a): Spherical red blood cells of humans

Figure 8.4(b): spindle-shaped muscle cells

Figure 8.4(c): Long branched nerve cell

Generally, cells are round, spherical, or elongated [Fig. 8.4(a)]. Some cells are long and pointed at both ends. They exhibit a spindle shape [Fig. 8.4(b)]. Cells sometimes are quite long. Some are branched like the nerve cell or a neuron [Fig. 8.4(c)]. The nerve cell receives and transfers messages, thereby helping to control and coordinate the working of different parts of the body.

Can you guess, which part of the cell gives it shape? Components of the cell are enclosed in a membrane. This membrane provides shape to the cells of plants and animals. The cell wall is an additional covering over the cell membrane in plant cells. It gives shape and rigidity to these cells (Fig. 8.7). The bacterial cell also has a cell wall.

Questions

What advantage does amoeba derive by changing shape?

Source: This topic is taken from NCERT TEXTBOOK

SIZE OF CELLS

SIZE OF CELLS

The size of cells in living organisms may be as small as a millionth of a metre (micrometre or micron) or maybe as large as a few centimetres. However, most of the cells are microscopic in size and are not visible to the unaided eye. They need to be enlarged or magnified by a microscope. The smallest cell is 0.1 to 0.5 micrometre in bacteria. The largest cell measuring 170 mm ×130 mm, is the egg of an ostrich.

The size of the cells has no relation with the size of the body of the animal or plant. It is not necessary that the cells in the elephant be much bigger than those in a rat. The size of the cell is related to its function. For example, nerve cells, both in the elephant and rat, are long and branched. They perform the same function, that of transferring messages.

Activity 8.2:

Boil a hen’s egg. Remove the shell. What do you observe? A white material surrounds the yellow part. The white material is albumin which solidifies on boiling. The yellow part is the yolk. It is part of a single cell. You can observe this single cell without any magnifying device.

Questions

Are the cells in an elephant larger than the cells in a rat?

Source: This topic is taken from NCERT TEXTBOOK