Cell: The Building Block of Life – CBSE Class 9 Science Notes

A cell is the basic structural and functional unit of life. All living organisms are made up of one or more cells.

Cells perform essential life processes such as respiration, nutrition, growth, excretion and reproduction. They are therefore called the building blocks of life.

Examples:

• Amoeba consists of a single cell.
• Bacteria are made up of a single cell.
• Humans are made up of trillions of cells.
• Plants are also made up of numerous cells.

A cell is called the fundamental unit of life because:

1. All living organisms are made up of cells.
2. Cells provide the basic structure of living organisms.
3. All essential life activities take place within cells.
4. New cells arise from pre-existing cells.
5. Cells contain genetic information that controls their activities.

In multicellular organisms, cells are organised into different levels to perform specific functions.

The organisation follows this sequence:

Cells → Tissues → Organs → Organ Systems → Organism

The discovery of the cell was an important milestone in the development of biology.

In 1665, Robert Hooke observed a thin slice of cork under a microscope. He noticed numerous small, box-like compartments and named them cells.

These were actually dead cells.

Most cells are extremely small and cannot be seen with our naked eyes. Therefore, microscopes are used to observe them.

A microscope is an instrument that magnifies very small objects and makes their structures visible.

1. Magnification: The ability of a microscope to make an object appear larger than its actual size.

Total Magnification = Eyepiece Magnification × Objective Magnification

Example:

Eyepiece = 10×

Objective = 40×

Total Magnification = 10 × 40 = 400×

The single cell performs all necessary life activities, including nutrition, respiration, excretion and reproduction.

Examples:

• Amoeba
• Paramecium
• Bacteria
• Yeast

Organisms made up of many cells are called multicellular organisms.

Different cells perform different specialised functions.

Examples:

• Human beings
• Dogs
• Fish
• Mango plants
• Trees

A typical cell consists of three major components:

1. Plasma Membrane
2. Cytoplasm
3. Nucleus

The cytoplasm contains several specialised structures known as cell organelles.

The plasma membrane is called selectively permeable because it allows only certain substances to pass through it.

Cytoplasm is the jelly-like substance present between the plasma membrane and the nucleus.

It contains various cell organelles and is the site of many important chemical reactions.

Functions:

• Holds the cell organelles.
• Provides a medium for biochemical reactions.
• Helps in the movement and transport of materials inside the cell.

The nucleus is generally regarded as the control centre of the cell because it regulates many cellular activities.

It contains genetic material in the form of DNA.

Functions:

• Controls various activities of the cell.
• Stores hereditary information.
• Helps regulate cell growth and division.
• Passes genetic information to daughter cells.

The nucleus is surrounded by a double-layered nuclear membrane containing pores.

It contains:

• Nuclear membrane
• Nucleoplasm
• Nucleolus
• Chromatin

The cell wall is a rigid, non-living outer covering found in plant cells. It is also present in fungi and bacteria, although its composition differs among these groups.

In plants, the cell wall is mainly made up of cellulose.

Functions of Cell Wall:

1. Provides shape and rigidity to the cell.
2. Protects the cell against mechanical damage.
3. Provides structural support.
4. Prevents the cell from bursting when excess water enters.

Materials continuously move into and out of cells. This movement is essential for maintaining life.

Two important transport processes are diffusion and osmosis.

Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration.

It is a passive process and does not require cellular energy.

Examples:

• Spreading of perfume throughout a room.
• Movement of oxygen into cells.
• Movement of carbon dioxide out of cells.

Osmosis is the movement of water through a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration.

Examples:

• Absorption of water by plant roots.
• Movement of water into and out of cells.

1. Hypotonic Solution

A solution having a lower solute concentration than the cell contents.

Water enters the cell by osmosis. Plant cells become turgid, while animal cells may swell.

2. Hypertonic Solution

A solution having a higher solute concentration than the cell contents.

Water moves out of the cell. In plant cells, the protoplasm shrinks away from the cell wall. This process is called plasmolysis.

3. Isotonic Solution

A solution having approximately the same effective solute concentration as the cell contents.

There is no net movement of water across the membrane.

According to the fluid mosaic model, the plasma membrane consists of a double layer of lipid molecules with proteins embedded in or associated with it.

The lipid molecules and many proteins can move within the membrane, making it flexible.

This arrangement helps the membrane control the movement of substances into and out of the cell.

Prokaryotic cells are simple cells that do not have a well-defined nucleus surrounded by a nuclear membrane.

Their genetic material is present in a region called the nucleoid.

Membrane-bound organelles are absent.

Examples:

• Bacteria
• Cyanobacteria

Eukaryotic cells have a well-defined nucleus surrounded by a nuclear membrane.

They contain membrane-bound organelles such as mitochondria, endoplasmic reticulum and Golgi apparatus.

Examples:

• Plant cells
• Animal cells
• Fungal cells

The endoplasmic reticulum is a network of membrane-bound tubes and sacs present in the cytoplasm.

It helps in the synthesis and transport of materials within the cell.

There are two types:

Rough Endoplasmic Reticulum (RER)

• Ribosomes are attached to its surface.
• Helps in protein synthesis and processing.
• Particularly important in cells that produce proteins for secretion.

Smooth Endoplasmic Reticulum (SER)

• Ribosomes are absent on its surface.
• Helps in the synthesis of lipids.
• Participates in detoxification in certain cells.

The Golgi apparatus consists of stacks of flattened membrane-bound sacs.

It acts as the packaging and distribution centre of the cell.

Functions:

• Modifies proteins and lipids.
• Sorts and packages materials.
• Helps transport materials to different parts of the cell.
• Plays an important role in the formation of lysosomes.

Lysosomes are membrane-bound sacs containing digestive enzymes.

They help break down waste materials, foreign particles and damaged cell organelles.

Functions:

• Digest unwanted materials.
• Remove damaged organelles.
• Help keep the cell clean.

Lysosomes are sometimes called the suicide bags of the cell because their digestive enzymes may cause self-digestion if released uncontrollably inside a severely damaged cell.

Mitochondria are known as the powerhouses of the cell because they play a major role in releasing energy from food during cellular respiration.

The energy is stored in the form of ATP (Adenosine Triphosphate).

Functions:

• Help in cellular respiration.
• Produce ATP.
• Supply energy for cellular activities.

Important features:

• Have a double membrane.
• Inner membrane is folded into structures called cristae.
• Contain their own DNA and ribosomes.

Plastids are membrane-bound organelles found in plant cells and algae.

They are mainly responsible for food production, storage and pigmentation.

There are three major types:

A. Chloroplasts

• Contain the green pigment chlorophyll.
• Help in photosynthesis.
• Absorb sunlight for the preparation of food.

B. Chromoplasts

• Contain pigments responsible for yellow, orange and red colours.
• Give colour to many flowers and fruits.

C. Leucoplasts

• Colourless plastids.
• Help store food materials such as starch, oils and proteins.

Vacuoles are membrane-bound storage sacs present in cells.

They contain a fluid called cell sap.

Functions:

• Store water and nutrients.
• Store waste materials.
• Maintain the internal pressure of plant cells.
• Help maintain the rigidity of plants.

In plant cells, a large central vacuole is generally present, while animal cells usually have smaller vacuoles.

Cell division is the process by which a parent cell divides to produce new daughter cells.

It is essential for:

• Growth of organisms.
• Repair of damaged tissues.
• Replacement of old and dead cells.
• Reproduction.

There are two major types of cell division:

A. Mitosis

Mitosis is a type of cell division in which one parent cell produces two genetically identical daughter cells, generally having the same chromosome number as the parent cell.

Importance of Mitosis:

• Helps in growth.
• Repairs damaged tissues.
• Replaces worn-out cells.
• Maintains chromosome number in body cells.

B. Meiosis

Meiosis is a special type of cell division in which one parent cell undergoes two successive divisions to produce four daughter cells with half the chromosome number of the parent cell.

It is important for the formation of gametes in sexually reproducing organisms.

Importance of Meiosis:

• Produces reproductive cells.
• Reduces the chromosome number by half.
• Helps maintain the chromosome number across generations after fertilisation.
• Contributes to genetic variation.

Cell theory is one of the fundamental principles of biology.

The classical cell theory states:

1. All living organisms are made up of one or more cells.
2. The cell is the basic structural and functional unit of life.
3. All cells arise from pre-existing cells.

This theory explains the importance of cells in the structure, functioning and continuity of life.