Cell: Structure And Function, Notes, Class 8, Science PDF Download

Cell: Structure And Function

Discovery of the Cell:

In 1655, the English scientist Robert Hooke made an observation that would change basic biological theory and research forever. While examining a dried section of cork tree with a crude light microscope, he observed small chambers and named them cells.

Over the next 175 years, research led to the formation of the cell theory, first proposed by the German botanist Matthias Jacob Schleiden and the German physiologist Theodore Schwann in 1838 and formalized by the German researcher Rudolf Virchow in 1858. In its modern form, this theorem has four basic parts:

1. The cell is the basic structural and functional unit of life; all organisms are composed of cells.

2. All cells are produced by the division of preexisting cells (in other words, through reproduction). Each cell contains genetic material that is passed down during this process.

3. All basic chemical and physiological functions - for example, repair, growth, movement, immunity, communication, and digestion - are carried out inside of cells.

4. The activities of cells depends on the activities of subcellular structures within the cell (these subcellular structures include organelles, the plasma membrane, and, if present, the nucleus)

Variety in Cell Number, Shape and Size:

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

Number of Cells

Human body has trillions of cells which vary in shapes and sizes. Different groups of cells perform a variety of functions. Organisms made of more than one cell are called multicellular organisms. An organism with billions of cells begins life as a single cell which is the fertilized egg. The fertilised egg cell multiplies and the number of cells increase as development proceeds.

The single-celled organisms are called unicellular organisms. A single celled organism performs all the necessary functions that multicellular organisms perform.

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

Shape of Cells

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

Components of the cell are enclosed in a membrane. This membrane provides shape to the cells of plants and animals. Cell wall is an additional covering over the cell membrane in plant cells. It gives shape and rigidity to these cells.

Cell Structure and Function:

Each organ in the system performs different functions such as digestion, assimilation and absorption. Similarly, different organs of a plant perform specific/specialized functions. For example, roots help in the absorption of water and minerals.

Each organ is further made up of smaller parts called tissues. A tissue is a group of similar cells performing a specific function.

Parts of the Cell

Cell Membrane: -

The cell membrane functions as a semi-permeable barrier, allowing a very few molecules across it while fencing the majority of organically produced chemicals inside the cell.

The basic components of a cell are cell membrane, cytoplasm and nucleus. The cytoplasm and nucleus are enclosed within the cell membrane, also called the plasma membrane. The membrane separates cells from one another and also the cell from the surrounding medium. The plasma membrane is porous and allows the movement of substances or materials both inward and outward.

The cell membrane gives shape to the cell. In addition to the cell membrane, there is an outer thick layer in cells of plants called cell wall. This additional layer surrounding the cell membrane is required by the plants for protection. Plant cells need protection against variations in temperature, high wind speed, atmospheric moisture, etc. They are exposed to these variations because they cannot move. Cells can be observed in the leaf peel of Tradescantia, Elodea or Rhoeo. You can prepare a slide as in the case of onion.

Cytoplasm

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

Nucleus

It is an important component of the living cell. It is generally spherical and located in the centre of the cell. It can be stained and seen easily with the help of a microscope. Nucleus is separated from the cytoplasm by a membrane called the nuclear membrane. This membrane is also porous and allows the movement of materials between the cytoplasm and the inside of the nucleus.

With a microscope of higher magnification, we can see a smaller spherical body in the nucleus. It is called the nucleolus. In addition, nucleus contains thread-like structures called chromosomes. These carry genes and help in inheritance or transfer of characters from the parents to the offspring. The chromosomes can be seen only when the cell divides.

You can visualize genes as memory stick or CD which we use to carry data from one computer to another or we can carry music as well. Similarly genes carry genetic codes which are responsible for the unique physical character of an animal or a plant.

Nucleus acts as control centre of the activities of the cell. The entire content of a living cell is known as protoplasm. It includes the cytoplasm and the nucleus. Protoplasm is called the living substance of the cell.

Comparison of Plant and Animal Cells

There are a few differences between a plant and animal cell.

• Plant cells have a cell wall surrounding the cell membrane, and animal cells only have a membrane.

• Plant cells contain chloroplasts which are used for photosynthesis.

• Plant cells have a large vacuole (it's like a fluid sack), compared to an animal cell.

• Animal cells are 'blobby', but cells in plants are more structured due to the cell wall, and form a lattice like structure which helps with rigid ness.

Summary

• All organisms are made of smaller parts called organs.

• Organs are made of still smaller parts. The smallest living part of an organism is a ‘cell’.

• Cells were first observed in cork by Robert Hooke in 1665.

• Cells exhibit variety of shapes and sizes.

• Number of cells also varies from organism to organism.

• Some cells are big enough to be seen with the unaided eye. Hen’s egg is an example.

• Some organisms are single-celled, while others contain large number of cells.

• The single cell of unicellular organisms performs all the basic functions performed by a variety of cells in multicellular organisms.

• The cell has three main parts, (i) the cell membrane, (ii) cytoplasm which contains smaller components called organelles, and (iii) the nucleus.

• Nucleus is separated from cytoplasm by a nuclear membrane.

• Cells without well organised nucleus, i.e. lacking nuclear membrane, are called prokaryotic cells.

• Plant cells differ from animal cells in having an additional layer around the cell membrane termed cell wall.

• Coloured bodies called plastids are found in the plant cells only. Green plastids containing chlorophyll are called chloroplasts.

• Plant cell has a big central vacuole unlike a number of small vacuoles in animal cells.