Cell - Structure and Function Notes, Class 8, For CBSE from NCERT

Cell – Structure and functions

Important topics covered:

Robert Hooke, in 1665 observed the slices of cork (bark of trees) under magnifying device.
He observed that there were a number of compartment (box) like shape in the cork slice.
These boxes or compartments looked like honeycomb.
There were separating wall which separated the compartments.
Hooke named them as cell.
The cell he observed were dead cells.
After 170 years of research by different biologists, the cell theory was proposed.
It was first proposed by Matthias Jacob Schleiden and Theodore Schwann in 1838.

Cells are the primary structural and functional units of life in a living organism.
As bricks are the basic unit of a building, similarly cells together make the body of living organisms.
Every living organism is made up of one or more cells.
Cell contain cell organelle, plasma membrane, nucleus etc.
Cells vary in their shape, size as well as functions.
The size of the cell varies from 1 to 100 micrometre.
The new cell is produced by cell division of the existing cell.
The cells having genetic material is carried out to the next generation through reproduction.
Cells are complex structures and carry out different substantial functions.
Growth, repair, excretion, immunity, digestion are some of the processes carried out by cells.

There are two types of organisms on the basis of number of cells present in living organisms:

Organisms containing single cells are unicellular organisms.
All the life processes like respiration, digestion, excretion is carried out by that single cell.
Amoeba and paramecium are examples of unicellular organisms.

Multicellular organisms:

Living organisms made up of more than one cell are multicellular organisms.
In these organisms, different life processes are carried out by a group of cells.
Division of labour is seen in cells of multicellular organisms each group of cells is involved in doing a specific function.
Humans, different animals, plants etc. are some example of multicellular organisms.

Ovum in females is the largest whereas sperm cell is the smallest cell inside the human body.
Brain contains nerve cells which are said to be neurons.
Liver cells are said as Hepatocytes.
Kidney cells are known to be nephrons.

There are trillions of cells present in living organisms. The cells vary in shape, size and function. Let’s discuss structures and function of cells.

Generally, cells are elongated, round shaped or spherical.
Some cells are long and pointed at its ends.
A few of them are branched. As we see in case of neuron.
Cell walls which are an additional covering of the cell give the shape to the cells of plants. It also provides rigidity.
Amoeba cell has no definite shape.

Many cells constitute tissue, many tissues come together to form organs and different organs together called organ system.
Every organ in living organism performs different functions like digestion, absorption and assimilation.
Plants too have different organs to perform particular functions like roots help in absorption of water and minerals.

The cell membrane has cytoplasm and nucleus enclosed within it.
The membrane is thin, elastic and delicate. is
It is also called the plasma membrane.
Cell membrane is living part of the cell.
It is the protective layer around the cell.
Also we call it selectively permeable membrane because selectively allows entry of some substances it prevents the entry of harmful material.
It is also involved in checking the movement of substances in and out of the cell.
In plants cell is covered with a thick layer which is called cell wall.
Cell wall covering the cell membrane in plants helps in protection of plants against temperature, high wind speed, atmospheric moisture etc.

The jelly-like substance present between the cell membrane and nucleus is cytoplasm.
Various other components, or organelles of cells i.e. mitochondria, golgi bodies, ribosomes, etc. are present in the cytoplasm.

It is located at the centre of the cell and is the most important component of the living organism.
Also, it is a dense spherical organelle which can be seen with a microscope.
Nucleus is surrounded by a porous nuclear membrane which separates cytoplasm and nucleus.
It contains spherical body called nucleolus.
Nucleus is also called storehouse of genes.
Chromosomes are thread-like structures which are present here and can only be observed when cell divides.
Genetic codes present in genes of chromosomes are responsible for the unique physical characteristics of an animal or a plant.
The living content of cell is called protoplasm.
Cells can’t survive without nucleus.
Nucleus also controls the activities inside the cell.
It also has a main role in inheritance.

We can distinguish between plant and animal cell is given below.

Plants cell	Animal cell
Cell wall is present.	No cell wall is present.
Nucleus is present inside the cell at any position.	Nucleus is present at the centre of the cell.
Plant cells contain plastids.	Plastids are absent.
At the centre of cytoplasm a larger vacuole is present.	A number of small vacuole is present in the cytoplasm.
Plant cells contain chloroplast which is used in photosynthesis.	Animal cell does not contain chloroplasts.
Plant cells are rigid due to cell wall.	Animal cells are blobby.