The Fundamental Unit of Life
The cell is the basic structural and functional unit of all living things. This Class 9 chapter covers the discovery of the cell, the difference between prokaryotic and eukaryotic cells, the structure of plant and animal cells, the functions of the various organelles, and how substances move across the cell membrane by diffusion and osmosis.
Learning objectives
- State that the cell is the basic unit of life.
- Distinguish prokaryotic and eukaryotic cells.
- Compare plant and animal cells.
- Describe the main cell organelles and their functions.
- Explain diffusion and osmosis across the cell membrane.
Key concepts
The cell and its discovery
Robert Hooke first observed cells in cork. All living organisms are made of cells, which may be unicellular (one cell) or multicellular (many cells). The cell is the smallest unit that can carry out all life processes.
Prokaryotic and eukaryotic cells
Prokaryotic cells (e.g. bacteria) lack a true nucleus and membrane-bound organelles. Eukaryotic cells (e.g. plant and animal cells) have a well-defined nucleus enclosed by a nuclear membrane and possess membrane-bound organelles.
Plant vs animal cells
Both have a cell membrane, cytoplasm and nucleus. A plant cell additionally has a rigid cell wall, plastids (including chloroplasts) and a large central vacuole, which are absent or small in animal cells.
Membrane transport: diffusion and osmosis
The cell membrane is selectively permeable. Diffusion is the movement of substances from a higher to a lower concentration. Osmosis is the movement of water across a semi-permeable membrane from a region of higher water concentration to lower.
Key definitions
- Cell
- The basic structural and functional unit of life.
- Prokaryote
- An organism whose cells lack a true (membrane-bound) nucleus.
- Osmosis
- The movement of water across a semi-permeable membrane from higher to lower water concentration.
- Organelle
- A specialised structure within a cell that performs a specific function.
Solved examples
Q1. Who discovered the cell and how?
Solution: Robert Hooke discovered cells in 1665 while observing a thin slice of cork under a microscope; the box-like compartments he saw reminded him of small rooms, which he named 'cells'.
Q2. Why is the mitochondrion called the powerhouse of the cell?
Solution: Because it releases energy in the form of ATP during respiration, powering the cell's activities.
Q3. What happens to a plant cell placed in pure water?
Solution: Water enters the cell by osmosis (endosmosis), making it swell and become turgid; the rigid cell wall prevents it from bursting.
Common mistakes to avoid
- Saying animal cells have a cell wall — only plant cells do.
- Confusing diffusion (any substance) with osmosis (water only).
- Thinking prokaryotes have membrane-bound organelles.
- Mixing up the functions of mitochondria and chloroplasts.
The Fundamental Unit of Life — MCQ Quiz
13 questions with instant feedback. Use number keys 1–4 to answer.
The basic structural and functional unit of life is the:
Practice questions
Short answer
What is the function of the cell membrane?
It controls the movement of substances into and out of the cell (selectively permeable).
Name the organelle that contains chlorophyll.
The chloroplast.
What is the difference between diffusion and osmosis?
Diffusion is the movement of any particles from high to low concentration; osmosis is specifically the movement of water across a semi-permeable membrane.
Long answer
Compare a plant cell and an animal cell.
Both have a cell membrane, cytoplasm and nucleus. A plant cell additionally has a rigid cell wall (of cellulose), plastids (including chloroplasts for photosynthesis) and a large central vacuole. An animal cell lacks a cell wall and chloroplasts and has small or no vacuoles, but often has centrioles, which plant cells usually lack.
Describe the structure and function of the nucleus.
The nucleus is a large organelle bounded by a nuclear membrane with pores. It contains chromatin (DNA), which carries genetic information, and a nucleolus. It controls all the cell's activities and is responsible for inheritance and cell division.
HOTS (Higher Order Thinking)
Why does a dried raisin swell when soaked in water?
Water moves into the raisin by osmosis because the concentration of water is higher outside than inside, making the raisin swell.
Why are lysosomes called 'suicidal bags'?
They contain powerful digestive enzymes; if the cell is damaged or dies, the lysosomes burst and their enzymes digest the cell itself.
Quick revision
Revision notes
- Cell = basic unit of life; discovered by Robert Hooke.
- Prokaryote (no true nucleus) vs eukaryote (true nucleus, organelles).
- Plant cell has wall, plastids, large vacuole; animal cell does not.
- Diffusion = any substance; osmosis = water across a membrane.
Key takeaways
- Match each organelle to its function (mitochondria, chloroplast, nucleus, lysosome).
- Only plant cells have a cell wall and chloroplasts.
- Osmosis explains swelling and shrinking of cells.
Frequently asked questions
Who discovered the cell?
Robert Hooke, who observed cork cells under a microscope in 1665.
What is the difference between a plant cell and an animal cell?
A plant cell has a cell wall, chloroplasts and a large vacuole, which an animal cell lacks.
What is osmosis?
The movement of water across a semi-permeable membrane from a region of higher water concentration to lower.