CBSE Class 9 Science The Fundamental Unit of Life Notes

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CELL

Cell is called the fundamental unit of life.

A cell is capable of independent existence and can carry out all the functions which are necessary for a living being. A cell carries out nutrition, respiration, excretion, transportation and reproduction; the way an individual organism does. Unicellular organisms are capable of independent existence which shows a cell’s capability to exist independently. Due to this, a cell is called the fundamental and structural unit of life. All living beings are composed of the basic unit of life, i.e. cell.

CELL THEORY (Schleiden, Schwann and Virchow):

• All living organisms are composed of one or more cells.

• The cell is the basic unit of structure, function, and organization in all organisms.

• All cells come from preexisting, living cells.

CELL SIZE, SHAPE AND NUMBER

There is much variation in size, shape and number of cells in different organisms, and also in various parts of the body. Most of the cells are only a few micrometres in diameter and are visible only with the help of a microscope. Cells may be spherical, spindle shaped, elongated, polyhedral or irregular in shape. The shape of the cells is determined by the specific function they perform. The number of cells is related to the size of the organ or body. Thus, small organisms have limited number of cells, while the larger ones such as elephant, whale or banyan tree have a countless number of cells. Some organisms can also have cells of different kinds. Look at the following picture. It depicts some cells from the human body.

Functions of Plasma Membrane

• Plasma membrane selectively regulates the entry and exit of the substances into and out of the cell. Therefore, it is called a selectively permeable membrane or semipermeable membrane.

• It provides an outer boundary to the cell and protects the cell from injury.

• It allows the flow of materials and information between different organelles of the same cell, as well as between the adjacent cells.

• It provides some organic connections between the adjacent cells.

 

INTEXT QUESTIONS PAGE NO. 61

Q1. How do substances like CO2 and water move in and out of the cell? Discuss.

Answer: The cell membrane is selectively permeable and regulates themovement of substances in and out of the cell.

Movement of CO₂: CO2 is produced during cellular respiration. Therefore, it is present in high concentrations inside the cell. This CO₂ must be excreted out of the cell. In the cell’s external environment, the concentration of CO₂ is low as compared to that inside the cell.

Therefore, according to the principle of diffusion, CO₂ moves from a region of higher concentration (inside the cell) towards a region of lower concentration (outside the cell).

Similarly, O₂ enters the cell by the process of diffusion when the concentration of O₂ inside the cell is low as compared to its surroundings.

Movement of water: Water moves from a region of high concentration to a region of low concentration through the plasma membrane. The plasma membrane acts as a semi-permeable membrane, and this movement of water is known as osmosis. However, the movement of water across the plasma membrane of the cell is affected by the amount of substance dissolved in water.

 

Q2. Why is the plasma membrane called a selectively permeable membrane?

Answer: The cell membraneor the plasma membraneis known as a selectively permeable membrane because it regulates the movement of substances in and out of the cell. This means that the plasma membrane allows the entry of only some substances and prevents the movement of some other materials.

 

CELL WALL

Cell wall is made of cellulose. Cell wall is present only in plant cells. It is a rigid protective covering outside the plasma membrane. Presence of cell wall in plant cells distinguishes them from animal cells. Most of the plant cell walls are made of cellulose.

The cell wall consists of three layers namely, middle lamella, primary wall and secondary wall.

The middle lamella is a thin amorphous cement like layer between two adjacent cells. Primary wall is the first formed wall of the cell and is produced inner to the middle lamella. The secondary wall is a thick layer found inner to the primary wall.

 

 

 

The nucleolus is a spherical body rich in protein and RNA. It is the site of ribosome formation.

There may be one or more nucleoli in the nucleoplasm. The chromatin is a network of fine threads composed of genetic material DNA (Deoxyribo nucleic acid) and proteins. During cell division chromatin is condensed into thick cord like structures called Chromosomes. The chromosomes contain genes and each gene is responsible for one hereditary character of the organism. Genes contain information for inheritance of features from parents to next generation in the form of DNA molecule.

Functions of Nucleus:

i) Nucleus controls all the metabolic activities of the cell.

ii) It controls the inheritance of characters from parents to off-springs.

iii) It controls cell division.

 

PROKARYOTES AND EUKARYOTES

Based on the complexity of organization, especially nuclear organization, the cells are classified into two types.

i) Prokaryotic cells.

ii) Eukaryotic cells.

 

PROKARYOTIC CELLS

The cells of Bacteria and Cyano Bacteria (blue green algae) lack a well organised nucleus and are called prokaryotic cells. Their DNA (Deoxyribo Nucleic Acid) is not enclosed by a nuclear membrane. They also lack membrane bound organelles. The organisms which possess prokaryotic cells are called prokaryotic organisms or prokaryotes. They are considered to be primitive organisms.

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