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Cell: The Building Block of Life – Class 9 Science notes, important questions and MCQs

NCERT Class 9 Science, chapter 2. Notes, key terms, important questions with answers and MCQs.

Chapter overview

The cell is the basic structural and functional unit of life. Cells have a membrane, cytoplasm and genetic material, and plant and animal cells differ in a few key parts.

Discovery and types

Robert Hooke saw cells in cork (1665); Leeuwenhoek saw living cells.
Prokaryotic cells (bacteria) have no true nucleus; eukaryotic cells have a nucleus with a membrane.
Unicellular (Amoeba) and multicellular (humans) organisms.

Cell parts

Cell membrane: selectively permeable; cell wall (plants, of cellulose) gives shape.
Nucleus: contains chromosomes (DNA), controls the cell.
Mitochondria: powerhouse (ATP); plastids: chloroplasts make food; ribosomes make proteins; lysosomes: "suicide bags"; Golgi packs materials; ER transports.

Movement across membranes

Diffusion: movement from high to low concentration.
Osmosis: water moves through a selectively permeable membrane from dilute to concentrated solution.
Endosmosis makes cells swell; exosmosis makes them shrink (plasmolysis in plants).

Key terms

Cell: smallest unit that can carry out life processes
Organelle: a structure inside a cell with a special job
Osmosis: movement of water across a selectively permeable membrane
Plasmolysis: shrinking of the cytoplasm of a plant cell in a concentrated solution

Formulas

Plant cell = animal cell + cell wall + large vacuole + plastids

Important questions with answers

Why are lysosomes called "suicide bags"?2 marks

Lysosomes contain powerful digestive enzymes. When a cell is damaged, they may burst and digest the cell itself.

What is osmosis? What happens when a plant cell is placed in a concentrated salt solution?3 marks

Osmosis is the movement of water through a selectively permeable membrane from a dilute to a concentrated solution. In a concentrated salt solution the plant cell loses water, the cytoplasm shrinks away from the cell wall, and the cell becomes plasmolysed.

Give five differences between plant cells and animal cells, and state the functions of the cell wall, chloroplast and vacuole.5 marks

Plant cells have a cell wall, chloroplasts and one large central vacuole, are usually rectangular, and lack centrioles; animal cells have no cell wall or chloroplasts, have small or no vacuoles, are usually rounded, and have centrioles. Cell wall: gives shape, strength and protection, and lets the cell survive in dilute surroundings. Chloroplast: carries out photosynthesis using chlorophyll. Vacuole: stores water, sap and wastes and keeps the cell turgid.

Quick revision

Hooke discovered cells in cork.
Cell membrane is selectively permeable.
Ribosomes make proteins.

Exam tips

Draw a labelled plant cell and animal cell — differences are often asked.
Remember: lysosomes = suicide bags; mitochondria = powerhouse.

Practice MCQs

Who first observed cells?
1. Darwin
2. Newton
3. Mendel
4. Robert Hooke
Show answer
Answer: (d) Robert Hooke. In cork, 1665.
The powerhouse of the cell is
1. nucleus
2. ribosome
3. mitochondrion
4. vacuole
Show answer
Answer: (c) mitochondrion. Produces ATP.
The cell wall of plants is made of
1. protein
2. cellulose
3. fat
4. chitin
Show answer
Answer: (b) cellulose. Cellulose.
Which organelle is known as the suicide bag?
1. plastid
2. Golgi body
3. lysosome
4. ribosome
Show answer
Answer: (c) lysosome. Its enzymes can digest the cell.
Bacteria are
1. eukaryotic
2. multicellular plants
3. prokaryotic
4. viruses
Show answer
Answer: (c) prokaryotic. No membrane-bound nucleus.
Raisins swell in water because of
1. exosmosis
2. endosmosis
3. diffusion of sugar out
4. evaporation
Show answer
Answer: (b) endosmosis. Water moves in.
A plant cell placed in a concentrated salt solution shows
1. turgidity
2. plasmolysis
3. cell division
4. photosynthesis
Show answer
Answer: (b) plasmolysis. Water leaves the cell.
Which organelles have their own DNA?
1. ribosomes and lysosomes
2. mitochondria and plastids
3. Golgi and ER
4. vacuoles and nucleolus
Show answer
Answer: (b) mitochondria and plastids. They can make some of their own proteins.

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Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.