Cell Structure and Function
Class 8 Science Notes - Willer Academy
Introduction to Cells
The cell is the basic structural and functional unit of life. All living organisms are made up of cells. Some organisms are made of a single cell (unicellular) while others contain many cells (multicellular).
Discovery of Cells: The cell was first discovered by Robert Hooke in 1665. He observed a thin slice of cork under his self-designed microscope and noticed small compartments that reminded him of small rooms in a monastery, which he called "cells".
Cell Theory
The cell theory, developed by Matthias Schleiden and Theodor Schwann in 1839, states that:
- All living organisms are composed of one or more cells
- The cell is the basic unit of structure and organization in organisms
- All cells come from pre-existing cells
Types of Cells
Prokaryotic Cells
- Simpler and smaller than eukaryotic cells
- No membrane-bound nucleus
- Genetic material is in the nucleoid region
- No membrane-bound organelles
- Example: Bacteria
Eukaryotic Cells
- More complex and larger than prokaryotic cells
- Have a true nucleus enclosed in a membrane
- Contain membrane-bound organelles
- Example: Plant and animal cells
Cell Structure and Organelles
Cells contain various structures called organelles that perform specific functions.
Cell Structure Diagram
[Image showing labeled plant and animal cells]
| Cell Part | Description | Function |
|---|---|---|
| Cell Membrane | Thin, flexible barrier | Controls what enters and leaves the cell |
| Cell Wall | Rigid outer layer (plant cells only) | Provides structural support and protection |
| Nucleus | Control center of the cell | Contains DNA and directs cell activities |
| Cytoplasm | Jelly-like substance | Holds organelles in place and site of many chemical reactions |
| Mitochondria | Powerhouse of the cell | Produces energy (ATP) through cellular respiration |
| Chloroplasts | Green organelles (plant cells only) | Site of photosynthesis |
| Endoplasmic Reticulum | Network of membranes | Transport system and site of protein/lipid synthesis |
| Golgi Apparatus | Stack of flattened membranes | Modifies, sorts, and packages proteins |
| Vacuoles | Storage sacs | Store water, nutrients, and waste products |
| Lysosomes | Contain digestive enzymes | Break down waste materials and cellular debris |
| Ribosomes | Small particles | Site of protein synthesis |
Plant Cell vs Animal Cell
Plant Cell
- Has a cell wall made of cellulose
- Contains chloroplasts for photosynthesis
- Has a large central vacuole
- Usually rectangular or fixed shape
- Stores energy as starch
Animal Cell
- No cell wall
- No chloroplasts
- Has many small vacuoles
- Usually round or irregular shape
- Stores energy as glycogen
Question & Answers
1. What is a cell?
Answer: A cell is the basic structural and functional unit of life. It is the smallest unit that can carry out all life processes.
2. Who discovered cells and when?
Answer: Robert Hooke discovered cells in 1665 while observing a thin slice of cork under his microscope.
3. State the cell theory.
Answer: The cell theory states that:
- All living organisms are composed of one or more cells
- The cell is the basic unit of structure and organization in organisms
- All cells come from pre-existing cells
4. What is the function of the nucleus?
Answer: The nucleus is the control center of the cell. It contains the cell's genetic material (DNA) and directs all cellular activities.
5. List three differences between plant and animal cells.
Answer:
- Plant cells have a cell wall, while animal cells do not
- Plant cells contain chloroplasts for photosynthesis, while animal cells do not
- Plant cells have a large central vacuole, while animal cells have many small vacuoles
6. What is the function of mitochondria?
Answer: Mitochondria are the powerhouses of the cell. They produce energy (ATP) through the process of cellular respiration.
7. Why is the cell membrane important?
Answer: The cell membrane is important because it controls what enters and leaves the cell, maintaining the cell's internal environment and allowing it to interact with its surroundings.
Multiple Choice Questions
Test your knowledge with these 10 multiple choice questions:
1. Who discovered cells?
2. Which of these is known as the powerhouse of the cell?
3. Which organelle is found only in plant cells?
4. The control center of the cell is the:
5. Which of these is NOT a part of the cell theory?
6. The jelly-like substance that fills the cell is called:
7. Which organelle is responsible for protein synthesis?
8. The rigid outer layer in plant cells is called:
9. Which of these cells would contain chloroplasts?
10. The process by which plants make food using sunlight is called:
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