Cell – The Structural and Functional Unit of Life (Detailed Notes)
1. Introduction to Cell
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A cell is the smallest unit of life capable of performing all vital functions of living organisms.
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All plants and animals are made up of cells.
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Cells carry out important processes like respiration, digestion, excretion, growth and reproduction.
Because cells perform all life processes, they are called the structural and functional unit of life.
2. Discovery of the Cell
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The cell was first discovered in 1665 by Robert Hooke.
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He observed a thin slice of cork under a microscope and saw small box-like compartments which he called cells.
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Later, Antonie van Leeuwenhoek observed living cells in pond water using a powerful microscope.
3. Cell Theory
The Cell Theory states:
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All living organisms are made of one or more cells.
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The cell is the basic unit of life.
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All cells arise from pre-existing cells.
4. Size and Shape of Cells
Size
Cells vary greatly in size.
Examples:
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Smallest cells: Bacteria
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Largest cell: Ostrich egg
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Longest cell: Nerve cell (Neuron)
Most cells are microscopic and can only be seen with a microscope.
Shape
Cells have different shapes depending on their function.
Examples:
5. Types of Organisms Based on Cells
1. Unicellular Organisms
Organisms made up of only one cell.
Examples:
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Amoeba
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Paramecium
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Bacteria
Characteristics:
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One cell performs all life functions.
2. Multicellular Organisms
Organisms made up of many cells.
Examples:
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Humans
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Plants
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Animals
Characteristics:
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Different cells perform different functions.
6. Basic Structure of a Cell
A typical cell has three main parts:
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Cell Membrane (Plasma Membrane)
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Cytoplasm
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Nucleus
1. Cell Membrane (Plasma Membrane)
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Thin protective covering around the cell.
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Controls entry and exit of substances.
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Maintains the shape of the cell.
Functions:
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Protects the cell
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Regulates movement of materials
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Allows nutrients to enter and wastes to leave
2. Cytoplasm
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Jelly-like substance present between the cell membrane and nucleus.
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Contains many cell organelles.
Functions:
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Site of many chemical reactions
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Supports organelles
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Helps in transport of substances
3. Nucleus
The control center of the cell.
Structure:
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Surrounded by nuclear membrane
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Contains nucleolus
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Contains chromosomes
Functions:
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Controls all activities of the cell
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Contains genetic material (DNA)
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Responsible for growth and reproduction
7. Cell Organelles
Cell organelles are specialized structures inside the cell that perform specific functions.
1. Mitochondria
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Called the powerhouse of the cell.
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Produces energy (ATP) through respiration.
2. Endoplasmic Reticulum (ER)
Network of membranes inside cytoplasm.
Types:
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Rough ER – has ribosomes, helps in protein synthesis
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Smooth ER – helps in lipid synthesis
3. Ribosomes
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Small particles found in cytoplasm.
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Known as protein factories of the cell.
4. Golgi Apparatus
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Packages and transports proteins and lipids.
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Acts like the post office of the cell.
5. Lysosomes
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Known as suicide bags of the cell.
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Contain digestive enzymes.
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Destroy damaged organelles and waste materials.
6. Vacuole
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Storage sac inside the cell.
Functions:
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Stores water, food and wastes.
In plants:
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Vacuole is large and central.
In animals:
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Vacuoles are small or temporary.
8. Plant Cell vs Animal Cell
9. Cell Wall (Plant Cells Only)
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Rigid outer covering present outside the cell membrane.
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Made of cellulose.
Functions:
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Provides strength and protection
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Maintains shape of the plant cell
10. Plastids (Plant Cells)
Types of plastids:
1. Chloroplast
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Contains chlorophyll.
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Site of photosynthesis.
2. Chromoplast
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Gives color to fruits and flowers.
3. Leucoplast
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Stores food like starch, oils and proteins.
11. Differences Between Prokaryotic and Eukaryotic Cells
12. Importance of Cells
Cells are important because they:
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Form the body of all living organisms
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Carry out metabolism
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Help in growth and development
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Store genetic information
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Enable reproduction
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