The cell is the basic structural and functional unit of life. All living organisms, from the tiniest bacteria to the largest animals and plants, are made up of cells. Cells carry out all necessary life processes and form the building blocks of all living beings.
In this chapter, we will explore the structure, types, and functions of cells, along with the differences between plant and animal cells.
1. Discovery of the Cell
- The cell was first discovered by Robert Hooke in 1665 when he observed a thin slice of cork under a self-designed microscope.
- He found tiny box-like structures, which he called "cells" (Latin: cellula, meaning "small room").
- Later, Anton van Leeuwenhoek observed living cells under a microscope.
2. Cell Theory
The Cell Theory was proposed by Matthias Schleiden and Theodor Schwann, and later expanded by Rudolf Virchow.
Key Points of the Cell Theory:
- All living organisms are made up of cells.
- The cell is the basic unit of life.
- All cells arise from pre-existing cells. (Omnis cellula e cellula - Virchow)
3. Types of Organisms Based on Cell Number
3.1 Unicellular Organisms
- Organisms made up of a single cell.
- All life functions (movement, respiration, digestion) are performed by this one cell.
- Examples: Amoeba, Paramecium, Bacteria.
3.2 Multicellular Organisms
- Organisms made up of many cells.
- Cells specialize to perform different functions.
- Examples: Humans, Trees, Animals.
4. Types of Cells
Cells are categorized based on their structure and complexity.
4.1 Prokaryotic Cells (Primitive Cells)
- No well-defined nucleus (nuclear material is scattered in the cytoplasm).
- No membrane-bound organelles.
- Simple in structure and smaller in size.
- Examples: Bacteria, Blue-green algae (Cyanobacteria).
4.2 Eukaryotic Cells (Advanced Cells)
- Have a well-defined nucleus enclosed in a nuclear membrane.
- Contain membrane-bound organelles.
- Larger and more complex.
- Examples: Plants, Animals, Fungi, Protists.
5. Structure of a Cell
A typical cell consists of three main parts:
- Cell membrane (Plasma membrane)
- Cytoplasm
- Nucleus
5.1 Cell Membrane (Plasma Membrane)
- Outer covering of the cell.
- Semi-permeable: Allows selective substances to enter or leave the cell.
- Made of lipids and proteins.
- Function: Protects the cell and regulates material exchange.
5.2 Cytoplasm
- Jelly-like substance present inside the cell.
- Contains cell organelles (mitochondria, ribosomes, etc.).
- Function: Site for chemical reactions.
5.3 Nucleus (Control Center)
- Spherical structure usually located at the center.
- Contains genetic material (DNA/Chromosomes).
- Surrounded by a nuclear membrane.
- Function: Controls all cell activities.
Inside the nucleus:
- Nucleolus – Produces ribosomes.
- Chromatin – Contains DNA (hereditary material).
6. Cell Organelles and Their Functions
Cell organelles are tiny structures inside a cell, each having a specific function.
| Organelle | Function | Found in |
|---|---|---|
| Mitochondria | Powerhouse of the cell – produces energy (ATP) | Plant & Animal |
| Ribosomes | Protein synthesis | Plant & Animal |
| Endoplasmic Reticulum (ER) | Transport of materials (Rough ER - has ribosomes; Smooth ER - no ribosomes) | Plant & Animal |
| Golgi Apparatus | Packaging and transport of proteins | Plant & Animal |
| Lysosomes | "Suicide bags" – digest waste materials | Mostly in Animals |
| Vacuole | Stores food, water, and waste (Larger in plants) | Plant & Animal |
| Chloroplasts | Contain chlorophyll for photosynthesis | Only in Plants |
| Cell Wall | Provides shape and protection (Made of cellulose) | Only in Plants |
7. Difference Between Plant and Animal Cells
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Shape | Rectangular | Round |
| Cell Wall | Present | Absent |
| Chloroplasts | Present (for photosynthesis) | Absent |
| Vacuole | Large and central | Small and scattered |
| Centrioles | Absent | Present |
| Lysosomes | Rarely present | Present |
8. Functions of a Cell
- Provides structure and support to the body.
- Performs metabolic activities (e.g., digestion, respiration).
- Reproduces to form new cells.
- Carries genetic information to the next generation.
- Synthesizes proteins and enzymes for the body.
9. Specialized Cells in the Human Body
In multicellular organisms, cells perform specialized functions.
| Cell Type | Function |
|---|---|
| Muscle Cells | Help in movement |
| Nerve Cells (Neurons) | Transmit messages |
| Blood Cells (RBCs, WBCs, Platelets) | Transport oxygen, fight infections, clot blood |
| Bone Cells | Provide structural support |
10. Growth and Division of Cells
Cells grow and divide to form new cells. The process of cell division occurs in two ways:
- Mitosis – Produces two identical cells, used for growth and repair.
- Meiosis – Produces gametes (sperm and egg cells), used for reproduction.
Conclusion
Cells are the fundamental units of life that perform all necessary functions in an organism. Understanding the structure and function of cells helps us learn how living beings grow, develop, and sustain life.
FAQs on Cells (Class 8 Science)
1. What is a cell?
A cell is the basic structural and functional unit of all living organisms.
2. Who discovered the cell?
Robert Hooke discovered the cell in 1665 using a microscope.
3. What is the difference between prokaryotic and eukaryotic cells?
- Prokaryotic cells lack a nucleus and membrane-bound organelles (e.g., bacteria).
- Eukaryotic cells have a well-defined nucleus and organelles (e.g., plants, animals).
4. What is the function of mitochondria?
Mitochondria are the powerhouse of the cell, producing energy (ATP).
5. What is the role of the nucleus?
The nucleus controls all cell activities and contains genetic material (DNA).
6. How are plant cells different from animal cells?
- Plant cells have a cell wall, chloroplasts, and a large vacuole.
- Animal cells have centrioles and lysosomes.
7. What is the function of ribosomes?
Ribosomes synthesize proteins in a cell.
8. What is the plasma membrane?
The plasma membrane is the outer covering of the cell that controls what enters and exits.