The cell is the fundamental unit of life. Every living organism, from microscopic bacteria to large plants and animals, is made up of cells. Cells perform essential functions that sustain life, such as growth, reproduction, and metabolism. Understanding the structure and function of cells helps us learn how organisms function.
In this chapter, we will explore cell structure, types, organelles, and their functions.
1. Discovery of Cells
- Robert Hooke (1665): Discovered cells while examining a thin slice of cork under a microscope.
- Anton van Leeuwenhoek: First observed living cells using a simple microscope.
- Matthias Schleiden and Theodor Schwann: Proposed the Cell Theory.
Cell Theory
- All living organisms are made up of cells.
- Cells are the basic unit of structure and function in living beings.
- All cells arise from pre-existing cells (Rudolf Virchow).
2. Types of Cells
Cells can be classified based on their structure and complexity.
2.1 Prokaryotic Cells
- Lack a well-defined nucleus (DNA is freely floating in the cytoplasm).
- No membrane-bound organelles.
- Found in bacteria and blue-green algae (cyanobacteria).
2.2 Eukaryotic Cells
- Have a well-defined nucleus enclosed by a nuclear membrane.
- Contain membrane-bound organelles.
- Found in plants, animals, fungi, and protists.
3. Structure of a Cell
A typical cell consists of three main parts:
- Cell Membrane (Plasma Membrane)
- Cytoplasm
- Nucleus
3.1 Cell Membrane (Plasma Membrane)
- The outermost covering of an animal cell.
- Semi-permeable: Allows only selected substances to enter or exit.
- Made up of lipids and proteins.
- Function: Protects the cell and regulates material exchange.
3.2 Cytoplasm
- A jelly-like fluid inside the cell.
- Contains cell organelles that perform various functions.
- Function: Site for chemical reactions inside the cell.
3.3 Nucleus (Control Center)
- A spherical structure that contains genetic material (DNA).
- Enclosed by a nuclear membrane.
- Contains:
- Nucleolus (produces ribosomes).
- Chromatin (thread-like structures that form chromosomes).
- Function: Controls all cell activities and stores genetic information.
4. Cell Organelles and Their Functions
Organelles are specialized structures inside a cell that perform specific functions.
| Organelle | Structure | Function | Present in |
|---|---|---|---|
| Mitochondria | Oval-shaped, double membrane | Powerhouse of the cell – produces energy (ATP) | Plant & Animal |
| Ribosomes | Small granules | Protein synthesis | Plant & Animal |
| Endoplasmic Reticulum (ER) | Network of membranes | Smooth ER: Fat synthesis Rough ER: Protein synthesis | Plant & Animal |
| Golgi Apparatus | Stacked pouches | Packaging and transport of proteins | Plant & Animal |
| Lysosomes | Sac-like structures | "Suicide bags" – digest waste materials | Mostly in Animals |
| Vacuole | Large in plants, small in animals | Stores food, water, and waste | Plant & Animal |
| Chloroplasts | Green, contains chlorophyll | Site of photosynthesis | Only in Plants |
| Cell Wall | Rigid outer layer | Provides shape and protection | Only in Plants |
| Centrioles | Cylinder-shaped | Helps in cell division | Only in Animals |
5. Difference Between Plant and Animal Cells
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Shape | Rectangular | Round |
| Cell Wall | Present (Made of Cellulose) | Absent |
| Chloroplasts | Present (for Photosynthesis) | Absent |
| Vacuole | Large and Central | Small and Scattered |
| Lysosomes | Rarely Present | Present |
| Centrioles | Absent | Present |
6. Function of Cells
Cells perform several important functions in living organisms:
- Provide Structure & Support – Maintain body shape.
- Carry Out Metabolism – Perform chemical reactions for survival.
- Enable Growth & Repair – Produce new cells for growth.
- Reproduction – Pass genetic material to the next generation.
- Transport Nutrients & Waste – Control material exchange.
- Synthesize Proteins – Build enzymes and hormones.
7. Types of Specialized Cells
In multicellular organisms, different cells perform different 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 |
8. Cell Division: Mitosis and Meiosis
Cells reproduce through cell division, ensuring growth and repair.
- Mitosis – Produces two identical cells (used for growth and healing).
- Meiosis – Produces four non-identical cells (for reproduction).
Conclusion
Cells are the basic building blocks of life, performing essential functions for survival. The structure and function of cells vary between plants and animals, but all cells work together to sustain life. Understanding cells helps us explore biology at a deeper level.
FAQs on Cell Structure and Function (Class 8 Science)
1. What is the structure of a cell?
A cell consists of a cell membrane, cytoplasm, and nucleus. It also contains organelles like mitochondria, ribosomes, and vacuoles.
2. Who discovered the cell?
Robert Hooke discovered the cell in 1665 using a microscope.
3. What is the function of the nucleus?
The nucleus controls all cell activities and contains DNA.
4. What is the difference between prokaryotic and eukaryotic cells?
- Prokaryotic cells have no nucleus (e.g., bacteria).
- Eukaryotic cells have a well-defined nucleus (e.g., plant and animal cells).
5. What is the function of mitochondria?
Mitochondria are called the powerhouse of the cell because they generate energy (ATP).
6. How do plant and animal cells differ?
- 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 required for cell function.
8. What is the function of the plasma membrane?
The plasma membrane is semi-permeable, controlling what enters and exits the cell.
9. What are lysosomes known as "suicide bags"?
Lysosomes contain digestive enzymes that break down waste and damaged cells.
10. How do cells divide?
Cells divide by mitosis (for growth and repair) and meiosis (for reproduction).