The Fundamental Unit of Life

Chapter 1: The Fundamental Unit of Life

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1. Discovery of the Cell

The discovery of the cell marked the beginning of cell biology. In 1665, Robert Hooke observed a thin slice of cork under a microscope and found small box-like structures which he called “cells.” However, these were dead cells.

Later, Anton van Leeuwenhoek observed living cells for the first time in pond water using an improved microscope.

The Cell Theory, proposed by Schleiden and Schwann, states:

1. All living organisms are composed of cells.
2. The cell is the basic structural and functional unit of life.

Later, Rudolf Virchow added that:

• All cells arise from pre-existing cells.

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Practice Questions

1. Who discovered the cell and what did he observe?
2. Who observed living cells for the first time?
3. State the main points of the cell theory.
4. What contribution did Rudolf Virchow make to cell theory?

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2. The Cell: Structure and Functions

A cell is defined as the smallest unit of life capable of carrying out all vital functions such as respiration, nutrition, and reproduction.

Cells vary widely in shape and size depending on their function. For example, nerve cells are long and branched, while red blood cells are circular.

Organisms can be classified based on the number of cells:

• Unicellular organisms: Made up of a single cell (e.g., Amoeba, Paramecium).
• Multicellular organisms: Composed of many cells (e.g., humans, plants).

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Practice Questions

1. Define a cell.
2. What are unicellular organisms? Give examples.
3. How do cells differ in shape and size?

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3. Plasma Membrane (Cell Membrane)

The plasma membrane is the outermost covering of the cell. It separates the contents of the cell from the external environment.

It is composed mainly of lipids and proteins and is selectively permeable, meaning it allows certain substances to pass through while restricting others.

Functions:

• Protects the cell.
• Maintains the shape of the cell.
• Regulates the movement of substances in and out of the cell.

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Practice Questions

1. What is the plasma membrane?
2. Why is it called selectively permeable?
3. State the functions of the plasma membrane.

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4. Diffusion and Osmosis

Diffusion

Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration. It does not require energy.

Osmosis

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

Types of Osmotic Solutions:

• Hypotonic solution: Cell gains water and swells.
• Hypertonic solution: Cell loses water and shrinks.
• Isotonic solution: No net movement of water.

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Practice Questions

1. Define diffusion.
2. What is osmosis?
3. Differentiate between hypotonic and hypertonic solutions.

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5. Cell Wall (Plant Cells)

The cell wall is a rigid outer layer present only in plant cells. It is mainly composed of cellulose.

Functions:

• Provides structural support.
• Protects the cell.
• Maintains cell shape.

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Practice Questions

1. What is the cell wall?
2. What is it made of?
3. State its functions.

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6. Cell Organelles

Cell organelles are specialized structures within the cell that perform specific functions.

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(i) Nucleus

The nucleus is the control center of the cell. It regulates all cellular activities.

It contains genetic material (DNA) organized into chromosomes.

Structure:

• Nuclear membrane
• Nucleolus
• Chromatin (chromosomes)

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Practice Questions

1. What is the function of the nucleus?
2. What are chromosomes?
3. Where is DNA located in the cell?

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(ii) Cytoplasm

The cytoplasm is a jelly-like substance present between the nucleus and the cell membrane. It contains various organelles.

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Practice Questions

1. What is cytoplasm?
2. What does it contain?

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(iii) Endoplasmic Reticulum (ER)

The ER is a network of membranes involved in synthesis and transport of materials.

• Rough ER (RER): Contains ribosomes; synthesizes proteins.
• Smooth ER (SER): Synthesizes lipids.

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Practice Questions

1. Differentiate between RER and SER.
2. What is the function of the ER?

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(iv) Ribosomes

Ribosomes are small structures responsible for protein synthesis. They may be free in the cytoplasm or attached to the rough ER.

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Practice Questions

1. What is the function of ribosomes?
2. Where are ribosomes found?

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(v) Golgi Apparatus

The Golgi apparatus modifies, packages, and transports proteins and lipids within the cell or outside it.

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Practice Questions

1. What is the function of the Golgi apparatus?
2. Name its discoverer.

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(vi) Mitochondria

Mitochondria are known as the “powerhouse of the cell” because they produce energy in the form of ATP through respiration.

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Practice Questions

1. Why are mitochondria called the powerhouse of the cell?
2. What is ATP?

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(vii) Lysosomes

Lysosomes contain digestive enzymes that break down waste materials and cellular debris. They are called “suicide bags” because they can digest the cell itself if they burst.

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Practice Questions

1. What are lysosomes?
2. Why are they called suicide bags?

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(viii) Vacuoles

Vacuoles are storage sacs. In plant cells, they are large and help maintain turgor pressure.

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Practice Questions

1. What is the function of vacuoles?
2. Why are they larger in plant cells?

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(ix) Plastids (Plant Cells Only)

Plastids are present only in plant cells and are of three types:

• Chloroplasts: Carry out photosynthesis.
• Chromoplasts: Provide color to fruits and flowers.
• Leucoplasts: Store food.

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Practice Questions

1. Name the types of plastids.
2. What is the function of chloroplasts?

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7. Differences Between Plant Cells and Animal Cells

Feature	Plant Cell	Animal Cell
Cell wall	Present	Absent
Vacuole	Large	Small or absent
Plastids	Present	Absent
Shape	Regular	Irregular

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Practice Questions

1. List three differences between plant and animal cells.
2. Which type of cell contains plastids?

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8. Prokaryotic and Eukaryotic Cells

Prokaryotic Cells:

• No true nucleus
• Smaller in size
• Lack membrane-bound organelles
• Example: Bacteria

Eukaryotic Cells:

• True nucleus present
• Larger and more complex
• Contain membrane-bound organelles
• Example: Plant and animal cells

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Practice Questions

1. Define prokaryotic cells.
2. Give examples of prokaryotic organisms.
3. State two differences between prokaryotic and eukaryotic cells.

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Final Examination Questions

Very Short Answer

1. What is a cell?
2. Name the control center of the cell.
3. What is diffusion?

Short Answer

1. Explain osmosis.
2. Write functions of mitochondria.
3. Describe the structure of the nucleus.

Long Answer

1. Explain the structure and functions of the plasma membrane.
2. Describe all cell organelles with their functions.
3. Compare plant and animal cells in detail.