The Fundamental Unit of Life
Introduction
All living organisms are made up of basic units called cells. The cell is the structural and functional unit of life. Just as a building is made up of bricks, all organisms are made up of cells.
Cell: The cell is the smallest structural and functional unit of life. It is capable of performing all the basic functions of life.
Discovery of the Cell
The cell was discovered by Robert Hooke in 1665. He observed thin slices of cork under a primitive microscope and saw small box-like structures which he called "cells" (Latin word 'cellula' meaning 'little rooms').
However, these were actually dead cell walls. Living cells were first discovered by Anton van Leeuwenhoek in 1674 when he observed pond water under a microscope.
Cell Theory
Cell Theory (1838-1839):
Proposed by Matthias Schleiden (botanist) and Theodor Schwann (zoologist)
- All living organisms are composed of one or more cells
- The cell is the basic unit of structure and function in organisms
- All cells arise from pre-existing cells (added by Rudolf Virchow in 1855)
Unicellular and Multicellular Organisms
Unicellular Organisms:
- Organisms made up of a single cell
- The single cell performs all life functions
- Examples: Amoeba, Paramecium, bacteria, yeast, Chlamydomonas
Multicellular Organisms:
- Organisms made up of many cells
- Different cells perform different functions (division of labor)
- Examples: Humans, plants, animals, fungi
Shape and Size of Cells
Cells vary greatly in shape and size depending on their function:
- Shape: Cells can be spherical, spindle-shaped, elongated, disc-shaped, branched, etc.
- Size: Most cells are microscopic (1-100 micrometers)
- Smallest cell: Mycoplasma (0.1-0.5 micrometers)
- Largest cell: Ostrich egg
- Longest cell: Nerve cells (neurons) can be up to a meter long
Structure of a Cell
A cell has three main components:
1. Plasma Membrane (Cell Membrane)
Plasma Membrane:
The outermost covering of the cell that separates the cell's contents from its external environment.
Structure:
- Made up of lipids and proteins (lipid bilayer)
- Flexible and living membrane
- Selectively permeable (allows certain substances to pass through)
Functions:
- Protects the cell contents
- Controls the movement of substances in and out of the cell
- Provides shape to the cell
- Allows interaction with the environment
Movement of Substances Across Membrane
1. Diffusion:
Definition: Movement of substances from a region of higher concentration to a region of lower concentration until equilibrium is reached.
Example: Perfume spreads in a room, carbon dioxide from cells diffuses out
2. Osmosis:
Definition: Movement of water molecules from a region of higher water concentration (lower solute concentration) to a region of lower water concentration (higher solute concentration) through a selectively permeable membrane.
Types of Solutions:
- Hypotonic Solution: Solution with lower solute concentration than cell - water enters cell, cell swells
- Isotonic Solution: Solution with same solute concentration as cell - no net movement of water
- Hypertonic Solution: Solution with higher solute concentration than cell - water leaves cell, cell shrinks
2. Cytoplasm
Cytoplasm: The jelly-like fluid present between the plasma membrane and the nucleus. It contains various cell organelles suspended in it.
Components:
- Contains water, salts, proteins, and other organic molecules
- Site for many chemical reactions
- Provides medium for movement of materials
3. Nucleus
Nucleus: The control center of the cell containing genetic material (DNA).
Structure:
- Nuclear Membrane: Double-layered membrane with pores that separates nucleus from cytoplasm
- Nucleoplasm: Jelly-like substance inside nucleus
- Chromatin: Thread-like structures made of DNA and proteins
- Chromosomes: Rod-shaped structures formed from chromatin during cell division
- Nucleolus: Spherical body inside nucleus that makes ribosomes
Functions:
- Controls all cell activities
- Stores genetic information (DNA)
- Transmits hereditary information to next generation
- Controls cell division
Cell Organelles
Cell organelles are specialized structures within the cell that perform specific functions:
1. Endoplasmic Reticulum (ER)
Endoplasmic Reticulum: A network of membrane-bound tubes and sheets in the cytoplasm.
Types:
- Rough ER (RER): Has ribosomes attached on surface
- Function: Protein synthesis and transport
- Smooth ER (SER): No ribosomes attached
- Function: Lipid synthesis, detoxification
Functions:
- Provides surface for biochemical reactions
- Forms skeletal framework of cell
- Transports materials within the cell
2. Golgi Apparatus (Golgi Body/Golgi Complex)
Golgi Apparatus: A stack of membrane-bound, flattened sacs (cisternae).
Structure:
- Consists of flattened, stacked pouches
- Has two faces: cis face (receiving) and trans face (shipping)
Functions:
- Modifies and packages proteins from ER
- Stores and dispatches proteins
- Forms lysosomes
- Synthesis of complex sugars from simple sugars
3. Lysosomes
Lysosomes: Membrane-bound sacs containing digestive enzymes.
Features:
- Called "suicide bags" of the cell
- Contain powerful digestive enzymes
- More common in animal cells
Functions:
- Digest foreign materials entering the cell
- Break down worn-out cell organelles
- Help in cell death when needed (autolysis)
- Digest food particles
4. Mitochondria
Mitochondria: Rod-shaped or spherical organelles, known as the "powerhouse of the cell."
Structure:
- Double membrane structure
- Outer membrane is smooth
- Inner membrane is folded to form cristae (increases surface area)
- Contains its own DNA and ribosomes
- Can divide independently
Functions:
- Site of cellular respiration
- Produces energy in the form of ATP (Adenosine Triphosphate)
- Called "powerhouse" or "power plant" of the cell
5. Plastids
Plastids: Organelles found only in plant cells, containing pigments.
Types:
- Chloroplasts: Green plastids containing chlorophyll
- Function: Photosynthesis (making food)
- Have their own DNA and ribosomes
- Double membrane structure
- Internal system of membrane sacs called thylakoids
- Chromoplasts: Colored plastids (yellow, red, orange)
- Function: Provide color to flowers and fruits
- Leucoplasts: Colorless plastids
- Function: Store food (starch, proteins, oils)
6. Vacuoles
Vacuoles: Membrane-bound spaces filled with liquid (cell sap).
In Plant Cells:
- Large central vacuole (occupies 50-90% of cell volume)
- Surrounded by tonoplast membrane
- Contains cell sap (water, minerals, sugars, amino acids, waste products)
In Animal Cells:
- Small and numerous
- Much smaller than plant cell vacuoles
Functions:
- Storage of food, water, minerals, waste products
- Provides turgidity and rigidity to plant cells
- Helps in osmoregulation
- In some organisms, food vacuoles digest food
- Contractile vacuoles remove excess water (in Amoeba)
Cell Wall
Cell Wall: A rigid, non-living protective layer outside the plasma membrane in plant cells, bacteria, and fungi.
Composition:
- Made up of cellulose in plants
- Made up of chitin in fungi
- Made up of peptidoglycan in bacteria
Functions:
- Provides shape and rigidity to the cell
- Protects cell from mechanical damage
- Protects from infection
- Allows high pressure to develop inside cell (turgor pressure)
- Freely permeable (allows all substances to pass through)
Note: Animal cells do not have cell walls.
Differences Between Plant and Animal Cells
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell Wall | Present (made of cellulose) | Absent |
| Plastids | Present (chloroplasts, etc.) | Absent |
| Vacuoles | Large, single central vacuole | Small, many vacuoles |
| Centrioles | Absent | Present |
| Shape | Fixed (due to cell wall) | Variable |
| Nucleus | Usually peripheral | Usually central |
| Lysosomes | Rare or absent | Common |
Prokaryotic vs Eukaryotic Cells
| Feature | Prokaryotic Cells | Eukaryotic Cells |
|---|---|---|
| Nucleus | No true nucleus (nucleoid region) | Well-defined nucleus with membrane |
| Nuclear Membrane | Absent | Present |
| Nucleolus | Absent | Present |
| Cell Organelles | Few organelles, no membrane-bound organelles | Many membrane-bound organelles |
| DNA | Circular DNA, not associated with histones | Linear DNA, associated with histones |
| Ribosomes | 70S type | 80S type |
| Size | Generally smaller (1-10 Ī¼m) | Generally larger (10-100 Ī¼m) |
| Examples | Bacteria, blue-green algae | Plants, animals, fungi, protists |
Important Points to Remember
- Cell is the basic structural and functional unit of life
- All cells arise from pre-existing cells
- Plasma membrane is selectively permeable
- Nucleus controls all cellular activities
- Mitochondria produce energy (ATP)
- Chloroplasts perform photosynthesis (only in plants)
- Lysosomes are "suicide bags" of the cell
- Plant cells have cell wall, plastids, and large vacuoles
- Animal cells lack cell wall, plastids, and have small vacuoles
- Prokaryotic cells lack true nucleus and membrane-bound organelles
Multiple Choice Questions (MCQ)
Subjective Questions
Practice these questions to strengthen your understanding. Write your answers in the space provided.