Chapter Overview:
All living organisms are made up of Cells. This chapter covers the discovery of cells, their structural
organization (Plasma Membrane, Nucleus, Cytoplasm) and detailed study of Cell Organelles. We also look
at Cell Division (Mitosis and Meiosis).
1. Discovery of Cell
- Robert Hooke (1665): Discovered Cell (Cork slice) - Dead cells.
- Leeuwenhoek (1674): Discovered free living cells in pond water.
- Robert Brown (1831): Discovered Nucleus.
- Purkinje (1839): Coined term 'Protoplasm'.
- Schleiden & Schwann: Proposed Cell Theory (All plants/animals are made of cells).
- Virchow (1855): "Omnis cellula-e cellula" (All cells arise from pre-existing
cells).
2. Structural Organization of Cell
Every cell has three basic regions:
(a) Plasma Membrane (Cell Membrane)
Outermost covering. Selectively permeable (allows entry/exit of selected materials).
Transport Mechanisms:
- Diffusion: Movement of gases ($CO_2, O_2$) from high to low concentration.
- Osmosis: Movement of water from high water concentration to low water concentration
through a semi-permeable membrane.
Types of Solutions:
- Hypotonic: Higher water conc. outside cell $\rightarrow$ Cell gains water (Swells).
- Isotonic: Same water conc. $\rightarrow$ No net change.
- Hypertonic: Lower water conc. outside $\rightarrow$ Cell loses water
(Shrinks/Plasmolysis).
(b) Cell Wall
Present only in Plant cells. Rigid outer covering outside plasma membrane. Made of
Cellulose. Provides structural strength.
(c) Nucleus
The "Brain" of the cell. Has double layered nuclear membrane with pores.
- Contains Chromosomes (visible during cell division).
- Chromosomes contain DNA (Deoxyribo Nucleic Acid) and Protein.
- Functional segments of DNA are called Genes.
- Prokaryotes: Undefined nuclear region (Nucleoid), No nuclear membrane.
- Eukaryotes: Well defined nuclear region with membrane.
3. Cell Organelles
Small structures in cytoplasm that perform specific functions.
1. Endoplasmic Reticulum (ER): Network of tubes.
- RER (Rough): Has ribosomes. Synthesizes Proteins.
- SER (Smooth): No ribosomes. Synthesizes Lipids (Fats). Helps compliance in
detoxifying poisons in liver.
- Membrane Biogenesis: Using proteins/lipids to build cell membrane.
2. Golgi Apparatus: Stacks of flattened sacs (cisternae).
- Function: Storage, modification, and packaging of products in vesicles. Formation of Lysosomes.
3. Lysosomes: "Suicide Bags".
- Contain digestive enzymes (from RER). Clean up the cell. If cell is damaged, lysosome bursts and
enzymes digest their own cell.
4. Mitochondria: "Powerhouse of the Cell".
- Double membrane. Own DNA and Ribosomes. Generates energy in form of ATP (Adenosine
Triphosphate).
5. Plastids (Plants only):
- Chromoplasts: Coloured (Flowers/Fruits).
- Leucoplasts: White/Colourless (Storage of starch/oil).
- Chloroplasts: Green (Chlorophyll). Kitchen of the cell (Photosynthesis). Own DNA.
6. Vacuoles: Storage sacs.
- Small in animals, Very large in plants (50-90% volume). Provide turgidity and rigidity. Store sap.
4. Cell Division
Q1: Why is the plasma membrane called selectively permeable?
Ans: Because it allows the entry and exit of some materials in and out of the cell,
while preventing movement of other materials.
Q2: Which organelle is known as the powerhouse of the cell?
Ans: Mitochondria. It releases energy in the form of ATP molecules required for various
chemical activities.
