Chapter-Cell: The Building Block of Life Science Exploration class 9 in english Medium CBSE Notes

Chapter 2. Cell: The Building Block of Life

This section explains the structure of a cell, the plasma membrane, movement of substances across the membrane, the cell wall and the differences between plant and animal cells. These structures help cells perform various life processes efficiently.

7. Structure of a Cell

A cell appears very simple under a microscope, but it is highly organised. Every cell contains different parts, and each part performs a specific function to keep the cell alive.

Definition

The structure of a cell refers to the arrangement of its different components that work together to perform life processes.

Main Parts of a Cell

• Cell Membrane – Forms the outer boundary of the cell and controls the movement of substances.
• Cytoplasm – Jelly-like material where various cell organelles are present.
• Nucleus – Controls all cellular activities and contains hereditary material.

Importance

• Protection – Protects the internal components of the cell.
• Coordination – Different cell parts work together to perform life functions.
• Growth and Survival – Helps the cell grow, reproduce and respond to its surroundings.

8. Cell Membrane (Plasma Membrane)

The cell membrane is the outer living covering of the cell. It separates the contents of the cell from the external environment and regulates the movement of materials.

Definition

The cell membrane or plasma membrane is a thin, flexible and selectively permeable membrane that surrounds the cell.

Characteristics

• Thin and Flexible – Allows the cell to change its shape.
• Living Membrane – Actively participates in cellular activities.
• Selectively Permeable – Allows only certain substances to enter or leave the cell.
• Protective Covering – Protects the internal contents of the cell.

Functions

• Protection – Protects the cell from external damage.
• Transport – Controls the movement of nutrients, gases and waste materials.
• Maintains Cell Shape – Helps maintain the structure of the cell.
• Communication – Helps cells interact with neighbouring cells.

9. Transport Across the Cell Membrane

Living cells continuously exchange materials with their surroundings. Nutrients enter the cell, while waste materials move out through the plasma membrane.

Types of Transport

• Diffusion – Movement of substances from a region of higher concentration to a region of lower concentration.
• Osmosis – Movement of water through a selectively permeable membrane from a dilute solution to a concentrated solution.

Difference between Diffusion and Osmosis

Diffusion	Osmosis
Movement of particles.	Movement of water molecules only.
May occur with or without a membrane.	Occurs only through a selectively permeable membrane.
Occurs in gases and liquids.	Occurs only in liquid solutions.
Equalises the concentration of particles.	Maintains the water balance of cells.

Importance

• Exchange of Materials – Helps cells obtain oxygen, water and nutrients.
• Removal of Wastes – Carbon dioxide and waste products leave the cell.
• Maintains Internal Balance – Keeps the cell functioning normally.

10. Osmosis Activity

NCERT explains osmosis through a potato experiment. The activity demonstrates how water moves through a selectively permeable membrane depending on the concentration of the surrounding solution.

Observation

• Fresh Water – Water enters the potato cells, making them firm and swollen.
• Salt Solution – Water moves out of the potato cells, causing them to shrink.

Conclusion

Water always moves through a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration.

11. Cell Wall

Plant cells have an additional outer covering called the cell wall. Unlike the plasma membrane, the cell wall is non-living and provides strength and protection.

Definition

The cell wall is a rigid, non-living outer covering present outside the plasma membrane in plant cells.

Characteristics

• Rigid – Gives strength and support.
• Non-Living – Made mainly of cellulose.
• Freely Permeable – Allows most substances to pass through easily.
• Present in Plant Cells – Absent in animal cells.

Functions

• Protection – Protects the cell from mechanical injury.
• Support – Maintains the shape of plant cells.
• Prevents Bursting – Stops the cell from bursting when excess water enters.
• Provides Rigidity – Helps plants remain upright.

12. Plant Cell and Animal Cell

Although plant and animal cells share many common features, they differ in several structural aspects because they perform different functions.

Similarities

• Cell Membrane – Present in both.
• Cytoplasm – Present in both.
• Nucleus – Present in both.
• Mitochondria – Present in both.
• Ribosomes – Present in both.

Differences

Plant Cell	Animal Cell
Cell wall present.	Cell wall absent.
Chloroplast present.	Chloroplast absent.
Large central vacuole.	Small or temporary vacuoles.
Usually rectangular in shape.	Usually round or irregular in shape.
Stores food mainly as starch.	Stores food mainly as glycogen.

ATP Education Tip

Remember these three important CBSE points: Cell membrane is selectively permeable, Cell wall is freely permeable, and Osmosis always involves the movement of water through a selectively permeable membrane. These concepts are frequently asked in objective, competency-based and case-study questions.

Class 9, all subjects CBSE Notes in english medium, cbse class 9 Science Exploration notes, class 9 Science Exploration notes english medium, cbse 9 Science Exploration cbse notes, class 9 Science Exploration revision notes, cbse class 9 Science Exploration study material, ncert class 9 science notes pdf, class 9 science exam preparation, cbse class 9 physics chemistry biology notes