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Chapter 8: Cell: The Unit of Life

Grade 11 Science | Chapter 8

Cell: The Unit of Life

The cell is the basic unit of every living thing. This chapter develops the cell theory, the membrane and wall, the nucleus, the organelles, and how plant and animal cells differ.

Core Concepts

Key Principles

Worked Examples

Practice Sets

Contents

Introduction: The Cell Theory

The Cell Membrane and Wall

The Nucleus

The Cytoplasm and Its Organelles

Plant and Animal Cells

Prokaryotic and Eukaryotic Cells

Key Reasoning (Principles)

Worked Examples (10)

Practice Sets A to D

10.

Summary and Exam Quick-Check

1. Introduction: The Cell Theory

Every living thing, from a single bacterium to a blue whale, is made of cells. The cell theory states three things: all living organisms are made of cells, the cell is the basic unit of life, and all cells arise from existing cells. This chapter looks inside the cell to see the parts that carry out the work of life.

Core idea

The cell is the basic unit of life. It is bounded by a membrane, controlled by a nucleus, and filled with organelles that each carry out a special task.

2. The Cell Membrane and Wall

Every cell is surrounded by a thin cell membrane that controls what enters and leaves, keeping the cell separate from its surroundings while letting needed materials pass. Plant cells, fungi and bacteria have an extra cell wall outside the membrane that gives shape and support. Animal cells have no cell wall, which lets them take many shapes.

3. The Nucleus

The nucleus is the control centre of the cell. It holds the genetic material, DNA, which carries the instructions for building and running the cell, and it directs the cell’s activities. The nucleus is wrapped in its own membrane, the nuclear envelope, and contains a dense spot, the nucleolus. A cell with a true nucleus is a eukaryote.

4. The Cytoplasm and Its Organelles

Inside the membrane is the cytoplasm, a jelly like fluid holding the organelles, the little organs of the cell. The mitochondria release energy from food, the endoplasmic reticulum and ribosomes build and transport proteins, the Golgi body packages materials, and lysosomes break down waste. Each organelle has a structure suited to its job.

Diagram 1 – The Animal Cell

A labelled animal cell with its organelles

Fig 1. An animal cell, showing the membrane, nucleus, mitochondria, endoplasmic reticulum, Golgi body and lysosome.

Two organelles handle the cell’s energy. The mitochondrion releases energy by respiration, supplying power to the whole cell, which is why it is called the powerhouse. In plant cells, the chloroplast captures light to make food by photosynthesis. Both have folded inner membranes that increase the area for these reactions.

Diagram 2 – The Energy Organelles

A mitochondrion and a chloroplast compared

Fig 2. The mitochondrion releases energy by respiration; the chloroplast makes food by photosynthesis.

5. Plant and Animal Cells

Plant and animal cells share most organelles but differ in three clear ways. A plant cell has a rigid cell wall, green chloroplasts for photosynthesis, and a large central vacuole filled with sap that keeps the cell firm. An animal cell has none of these, so it is more flexible and gets its food ready made.

Diagram 3 – The Plant Cell

A labelled plant cell with cell wall, chloroplasts and a large vacuole

Fig 3. A plant cell, with its cell wall, chloroplasts and large central vacuole, features an animal cell lacks.

6. Prokaryotic and Eukaryotic Cells

Cells fall into two great kinds. A prokaryotic cell, found only in bacteria, has no true nucleus and few organelles, carrying its DNA loose in the cytoplasm. A eukaryotic cell, found in protists, fungi, plants and animals, has a true nucleus and many membrane bound organelles. The eukaryotic cell is larger and far more complex.

7. Key Reasoning (Principles)

Principle 1: The cell is the unit of life

All organisms are made of cells, the cell is the basic unit of life, and every cell comes from a pre-existing cell, the three points of the cell theory.

Principle 2: Structure fits function in organelles

Each organelle has a shape suited to its task, such as the folded inner membrane of a mitochondrion that increases the area for releasing energy.

Principle 3: Plant and animal cells differ in three features

A plant cell has a cell wall, chloroplasts and a large vacuole, which an animal cell lacks, reflecting how each kind of cell lives.

8. Worked Examples

Example 1

Q: State the three points of the cell theory.

▶ Show Solution

All organisms are made of cells, the cell is the basic unit of life, and all cells come from existing cells.

Answer: The three points of the cell theory.

Example 2

Q: What controls what enters and leaves a cell?

▶ Show Solution

The cell membrane controls what passes in and out.

Answer: The cell membrane.

Example 3

Q: Which organelle is the control centre of the cell?

▶ Show Solution

The nucleus holds the DNA and directs the cell.

Answer: The nucleus.

Example 4

Q: Which organelle releases energy from food?

▶ Show Solution

The mitochondrion releases energy by respiration.

Answer: The mitochondrion.

Example 5

Q: Which organelle in a plant cell makes food?

▶ Show Solution

The chloroplast makes food by photosynthesis.

Answer: The chloroplast.

Example 6

Q: Name three features a plant cell has that an animal cell lacks.

▶ Show Solution

A cell wall, chloroplasts and a large central vacuole.

Answer: Cell wall, chloroplasts, large vacuole.

Example 7

Q: What is the jelly like fluid inside the cell called?

▶ Show Solution

The fluid holding the organelles is the cytoplasm.

Answer: The cytoplasm.

Example 8

Q: What is the main difference between a prokaryotic and a eukaryotic cell?

▶ Show Solution

A prokaryote has no true nucleus; a eukaryote has one.

Answer: The presence of a true nucleus.

Example 9

Q: Which organelle packages materials for transport?

▶ Show Solution

The Golgi body packages materials.

Answer: The Golgi body.

Example 10

Q: Why is the mitochondrion called the powerhouse of the cell?

▶ Show Solution

Because it releases the energy that powers the cell’s activities.

Answer: It releases the cell’s energy.

9. Practice Sets A to D

Set A – Multiple Choice (Basic)

1. The basic unit of life is the: (a) atom (b) cell (c) organ (d) tissue

2. The control centre of the cell is the: (a) membrane (b) nucleus (c) vacuole (d) wall

3. Energy is released in the: (a) nucleus (b) mitochondrion (c) ribosome (d) wall

4. Chloroplasts are found in: (a) animal cells (b) plant cells (c) bacteria only (d) no cells

5. A prokaryote lacks a: (a) membrane (b) true nucleus (c) cytoplasm (d) DNA

▶ Reveal Answers

1. (b) cell.

2. (b) nucleus.

3. (b) mitochondrion.

4. (b) plant cells.

5. (b) true nucleus.

Set B – Short Answer (Understanding)

1. State the cell theory.

2. What is the role of the cell membrane?

3. What does the nucleus contain and do?

4. Name three energy related or organelle functions.

5. List the three features unique to plant cells.

▶ Reveal Answers

1. All organisms are made of cells, the cell is the basic unit of life, and cells come from existing cells.

2. It controls what enters and leaves the cell.

3. It contains the DNA and directs the cell’s activities.

4. Mitochondria release energy, ribosomes build proteins, and the Golgi body packages materials.

5. A cell wall, chloroplasts and a large central vacuole.

Set C – Application and Reasoning

1. A cell has a wall, chloroplasts and a big vacuole. Plant or animal?

2. Why can animal cells take many shapes?

3. Which organelle would be plentiful in a very active muscle cell?

4. Why is a cell with no nucleus called a prokaryote?

5. What links the structure of a mitochondrion to its function?

▶ Reveal Answers

1. A plant cell.

2. Because they have no rigid cell wall.

3. Mitochondria, because the cell needs much energy.

4. Because prokaryote means before a nucleus, and it has no true nucleus.

5. Its folded inner membrane gives a large area for the reactions that release energy.

Set D – Higher Order (Challenge)

1. Explain why the cell is called the structural and functional unit of life.

2. Compare how a plant and an animal cell obtain their energy supply.

3. Explain why a large vacuole helps keep a plant firm.

4. A cell is large, with a true nucleus and many organelles. Classify it.

5. Explain how the cell theory implies that life is continuous.

▶ Reveal Answers

1. Because every living structure is built from cells, and every life process is carried out within them.

2. An animal cell takes in ready made food and releases energy in mitochondria; a plant cell also makes its own food in chloroplasts first.

3. The vacuole fills with sap and presses outward on the wall, keeping the cell and the plant rigid.

4. It is a eukaryotic cell.

5. Because every cell arises from a pre-existing cell, so life passes unbroken from one generation to the next.

Chapter Summary

Cell Theory

All organisms are cells; the cell is the unit of life; cells come from cells.

Membrane and Wall

Membrane controls entry and exit; plant cells add a wall.

Nucleus

Holds DNA and controls the cell.

Organelles

Mitochondria, ER, ribosomes, Golgi and lysosomes each have a task.

Plant vs Animal

Plant cells add wall, chloroplasts and a large vacuole.

Prokaryote vs Eukaryote

No true nucleus versus a true nucleus.

Quantity

Unit

Symbol

Unit of life

the cell

–

Control centre

nucleus

–

Powerhouse

mitochondrion

–

8-Point Exam Quick-Check

Cell theory: all organisms are cells, the cell is the unit of life, cells come from cells.

The membrane controls what enters and leaves the cell.

Plant cells, fungi and bacteria add a cell wall; animal cells do not.

The nucleus holds DNA and controls the cell.

Mitochondria release energy; chloroplasts make food in plants.

Plant cells uniquely have a wall, chloroplasts and a large vacuole.

Prokaryotes have no true nucleus; eukaryotes do.

Each organelle has a structure suited to its function.

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Class 11 Biology Chapter 8: Cell The Unit of Life, Complete Notes and Practice

This revision guide follows the current NCERT Class 11 Biology syllabus and develops the cell, covering the cell theory, the cell membrane and wall, the nucleus, the cytoplasm and its organelles such as mitochondria, endoplasmic reticulum, ribosomes, Golgi body and lysosomes, the differences between plant and animal cells, and the prokaryotic and eukaryotic types, with three diagrams, ten worked examples and graded practice. Visit SchoolRevise.com to revise, practise and excel.