Learning Objectives
- Understand the cell as the basic structural and functional unit of life
- Learn about cell theory and discovery of the cell
- Differentiate between prokaryotic and eukaryotic cells
- Differentiate between plant and animal cells
- Understand the structure and function of cell organelles
Key Concepts
Discovery and Cell Theory
Robert Hooke (1665) discovered cells while examining a thin slice of cork under a microscope. He saw small compartments which he called "cells."
Cell Theory (Schleiden and Schwann): All living organisms are composed of cells. The cell is the basic structural and functional unit of life. All cells arise from pre-existing cells (added by Virchow).
Prokaryotic vs Eukaryotic Cells
Prokaryotic cells: No well-defined nucleus (nucleoid region without nuclear membrane). No membrane-bound organelles. Small size (1-10 μm). Examples: bacteria, blue-green algae.
Eukaryotic cells: Well-defined nucleus with nuclear membrane. Membrane-bound organelles present. Larger size (10-100 μm). Examples: plant cells, animal cells, fungi.
Cell Membrane (Plasma Membrane)
Made of lipid bilayer with proteins. It is selectively permeable — allows only certain substances to pass through.
Osmosis: Movement of water molecules from a region of higher concentration to a region of lower concentration through a semi-permeable membrane.
Hypotonic solution: Cell swells. Hypertonic solution: Cell shrinks. Isotonic solution: No change.
Cell Wall
Present in plant cells only. Made of cellulose. Provides rigidity, shape, and protection. Freely permeable (allows all substances to pass through).
Plasmolysis: Shrinkage of the cell membrane away from the cell wall when a plant cell is placed in a hypertonic solution.
Nucleus
The control centre of the cell. Contains genetic material (DNA) organised into chromosomes. Surrounded by a double-layered nuclear membrane with nuclear pores. Contains the nucleolus (site of ribosomal RNA synthesis).
Cytoplasm and Cell Organelles
Endoplasmic Reticulum (ER): Network of membranes. Rough ER has ribosomes (protein synthesis). Smooth ER makes lipids. Also involved in detoxification.
Golgi Apparatus: Packaging, modifying, and dispatching proteins and lipids. Forms lysosomes.
Lysosomes: Contain digestive enzymes. Break down worn-out organelles and foreign material. Called "suicide bags" of the cell.
Mitochondria: Powerhouse of the cell. Site of aerobic respiration and ATP production. Have their own DNA and ribosomes. Double membrane: outer smooth, inner folded into cristae.
Plastids (in plant cells only): Chloroplasts (green — photosynthesis), Chromoplasts (coloured), Leucoplasts (colourless — store starch, oils, proteins). Have own DNA and ribosomes.
Vacuoles: Large central vacuole in plant cells (stores sap, maintains turgor pressure). Small, temporary vacuoles in animal cells.
Summary
The cell is the fundamental unit of life. Prokaryotic cells lack a defined nucleus while eukaryotic cells have one. Cell organelles like mitochondria, ER, Golgi apparatus, and plastids perform specific functions. Plant cells differ from animal cells by having a cell wall, large central vacuole, and plastids.
Important Terms
- Cell: The basic structural and functional unit of all living organisms
- Osmosis: Movement of water through a selectively permeable membrane
- Organelle: A membrane-bound structure with a specific function inside a cell
- Mitochondria: Powerhouse of the cell; produces ATP through cellular respiration
- Chromosome: Thread-like structure of DNA carrying genetic information
- Plasmolysis: Shrinkage of cell contents from the cell wall in a hypertonic solution
Quick Revision
- Cell = basic unit of life (Cell Theory by Schleiden and Schwann)
- Prokaryotes: no nuclear membrane; Eukaryotes: true nucleus
- Mitochondria = powerhouse; Lysosomes = suicide bags
- Plant cell: cell wall + chloroplasts + large vacuole
- Animal cell: no cell wall, no chloroplasts, small vacuoles
- Nucleus controls cell activities; contains DNA in chromosomes