CELL AND ITS STRUCTURE

CELL AND ITS STRUCTURE

Cell History

While examining a thin slice of cork, Robert Hooke saw that the cork resembled the structure of a honeycomb consisting of many little compartments. Cork is a substance which comes from the bark of a tree. This was in the year 1665 when Hooke made this chance observation through a self-designed microscope. Robert Hooke called these boxes cells. A cell is a Latin word for ‘a little room’. 

This may seem to be a very small and insignificant incident but it is very important in the history of science. This was the very first time that someone had observed that living things appear to consist of separate units. The use of the word ‘cell’ to describe these units is being used till this day in biology.

Let us find out about cells.

Activity 5.1: ( Observing Onion Peel Cells )

*   Let us take a small piece from an onion bulb.

*   With the help of a pair of forceps, we can peel off the skin (called epidermis) from the concave side (inner layer) of the onion.

*   This layer can be put immediately in a watch-glass containing water.

*   This will prevent the peel from getting folded or getting dry. What do we do with this peel?

Let us take a glass slide, put a drop of water on it and transfer a small piece of the peel from the watch glass to the slide. Make sure that the peel is perfectly flat on the slide. A thin camel-hair paintbrush might be necessary to help transfer the peel. Now we put a drop of safranin solution on this piece followed by a coverslip. Take care to avoid air bubbles while putting the coverslip with the help of a mounting needle. Ask your teacher for help. We have prepared a temporary mount of onion peel. We can observe this slide under low power followed by high powers of a compound microscope.

Figure 5.1: Compound microscope

What do we observe as we look through the lens? Can we draw the structures that we are able to see through the microscope, on an observation sheet? Does it look like Fig. 5.2?

Figure 5.2: Cells of an onion peel

We can try preparing temporary mounts of peels of onions of different sizes. What do we observe? Do we see similar structures or different structures?

Source: This topic is taken from NCERT TEXTBOOK

Structure Of Various Cells

What are these structures?

These structures look similar to each other. Together they form a big structure like an onion bulb! We find from this activity that onion bulbs of different sizes have similar small structures visible under a microscope. The cells of the onion peel will all look the same, regardless of the size of the onion they came from.

These small structures that we see are the basic building units of the onion bulb. These structures are called cells. Not only onions, but all organisms that we observe around are made up of cells. However, there are also single cells that live on their own. The invention of magnifying lenses led to the dicovery of the microscopic world. It is known as that a single cell may constitute a whole organism as in Amoeba, Chlamydomonas, Paramoecium and bacteria. 

These organisms are called unicellular organisms (uni=single). On the other hand, many cells group together in a single body and assume different functions in it to form various body parts in multicellular organisms ( multi = many) such as some fungi, plants, and animals. Can we find out names of some more unicellular organisms?

Cells divide to produce cells of their own kind. All cells thus come from pre-existing cells

Activity 5.2:

*   We can try preparing temporary mounts of leaf peels, tip of roots of onion or even peels of onions of different sizes.

*   After performing the above activity, let us see what the answers to the following questions would be:

              1.  Do all cells look alike in terms of shape and size?

              2.  Do all cells look alike in structure?

              3.  Could we find differences among cells from different parts of a plant body?

              4.  What similarities could we find?

Different kinds of cells

Some organisms can also have cells of different kinds. Look at the following picture. It depicts some cells from the human body. 

Figure 5.3:  Various Cells from the human body

The shape and size of cells are related to the specific function they perform. Some cells like Amoeba have changing shapes. In some cases the cell shape could be more or less fixed and peculiar for a particular type of cell; for example, nerve cells have a typical shape. Each living cell has the capacity to perform certain basic functions that are characteristic of all living forms. How does a living cell perform these basic functions? We know that there is a division of labour in multicellular organisms such as human beings. This means that different parts of the human body perform different functions. The human body has a heart to pump blood, a stomach to digest food and so on. Similarly, division of labour is also seen within a single cell. In fact, each such cell has got certain specific components within it known as cell organelles. Each kind of cell organelle performs a special function, such as making new material in the cell, clearing up the waste material from the cell and so on. A cell is able to live and perform all its functions because of these organelles. These organelles together constitute the basic unit called the cell. It is interesting that all cells are found to have the same organelles, no matter what their function is or what organism they are found in.

More to know

Cells were first discovered by Robert Hooke in 1665. He observed the cells in a cork slice with the help of a primitive microscope. Leeuwenhoek (1674), with the improved microscope, discovered the free-living cells in pond water for the first time. It was Robert Brown in 1831 who discovered the nucleus in the cell. Purkinje in 1839 coined the term ‘protoplasm’ for the fluid substance of the cell. The cell theory, that all the plants and animals are composed of cells and that the cell is the basic unit of life, was presented by two biologists, Schleiden (1838) and Schwann (1839). The cell theory was further expanded by Virchow (1855) by suggesting that all cells arise from pre-existing cells. With the discovery of the electron microscope in 1940, it was possible to observe and understand the complex structure of the cell and its various organelles.

Questions

1.  Who discovered cells, and how?

2.  Why is the cell called the structural and functional unit of life?

Source: This topic is taken from NCERT TEXTBOOK