Page 62 - Understanding NCERT Science 09

The shape and size of cells are related to every cell; plasma membrane, nucleus and
the specific function they perform. Some cells cytoplasm. All activities inside the cell and
like Amoeba have changing shapes. In some interactions of the cell with its environment
cases the cell shape could be more or less are possible due to these features. Let us see
fixed and peculiar for a particular type of cell; how.
for example, nerve cells have a typical shape.
Each living cell has the capacity to 5.2.1 PLASMA MEMBRANE OR CELL
perform certain basic functions that are
MEMBRANE
characteristic of all living forms. How does
a living cell perform these basic functions? This is the outermost covering of the cell that
We know that there is a division of labour in separates the contents of the cell from its
multicellular organisms such as human external environment. The plasma membrane
beings. This means that different parts of allows or permits the entry and exit of some
the human body perform different functions. materials in and out of the cell. It also
The human body has a heart to pump blood, prevents movement of some other materials.
a stomach to digest food and so on. Similarly, The cell membrane, therefore, is called a
division of labour is also seen within a single selectively permeable membrane.
cell. In fact, each such cell has got certain How does the movement of substances
specific components within it known as cell take place into the cell? How do substances
organelles. Each kind of cell organelle move out of the cell?
performs a special function, such as making Some substances like carbon dioxide or
new material in the cell, clearing up the oxygen can move across the cell membrane
waste material from the cell and so on. A by a process called diffusion. We have studied
cell is able to live and perform all its the process of diffusion in earlier chapters.
functions because of these organelles. These We saw that there is spontaneous movement
organelles together constitute the basic unit of a substance from a region of high
called the cell. It is interesting that all cells concentration to a region where its
are found to have the same organelles, no concentration is low.
matter what their function is or what Something similar to this happens in cells
organism they are found in.
when, for example, some substance like CO
2
(which is cellular waste and requires to be
uestions
Q 1. Who discovered cells, and how? concentrations inside the cell. In the cell’s
excreted out by the cell) accumulates in high
external environment, the concentration of
2. Why is the cell called the
is low as compared to that inside the
CO
structural and functional unit of
2
cell. As soon as there is a difference of
life?
concentration of CO inside and outside a cell,
2
CO moves out of the cell, from a region of
2
high concentration, to a region of low
5.2 What is a Cell Made Up of? concentration outside the cell by the process
of diffusion. Similarly, O enters the cell by
2
What is the Structural the process of diffusion when the level or
Organisation of a Cell? concentration of O inside the cell decreases.
2
Thus, diffusion plays an important role in
We saw above that the cell has special gaseous exchange between the cells as well
components called organelles. How is a cell as the cell and its external environment.
organised? Water also obeys the law of diffusion. The
If we study a cell under a microscope, we movement of water molecules through such a
would come across three features in almost selectively permeable membrane is called osmosis.

THE FUNDAMENTAL UNIT OF LIFE 51

Rationalised 2023-24