01 · Explore
The Fundamental Unit of Life
All living organisms are composed of cells, which represent the basic level at which life exists and functions.
Scientific evidence suggests that life originated in water approximately 3.5 billion years ago. Early organic molecules may have been protected by calcium carbonate deposits in environments like hot springs, such as those in Puga Valley, Ladakh. These deposits helped form the first protective membranes, creating the barrier that defines a cell.
Organisms are classified as unicellular (single-celled like bacteria or yeast) or multicellular (made of millions of cells like plants and humans). In multicellular organisms, cells are organized into a hierarchy: similar cells form tissues, different tissues organize into organs, and several organs work together to form organ systems. For example, the nasal pores, trachea, and lungs together form the respiratory system.
Despite this complexity, the cell remains the fundamental unit of structure and function. It is often compared to a tiny living factory where different parts perform specific jobs such as energy production, waste removal, and building new materials.
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How did calcium carbonate deposits contribute to the origin of the first cells?
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What is the organizational hierarchy from a cell to an organ system?
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02 · Explore
Tools for Exploring the Microscopic World
Since most cells are smaller than the human eye's limit of resolution, specialized instruments are required to study them.
The human eye has a limit of resolution of about 0.1 mm, meaning we cannot see two points as separate if they are closer than this. Most plant and animal cells range from 10 to 100 micrometres (µm), while bacteria are even smaller (around 1 µm). Robert Hooke first observed cells in 1665 using a self-designed microscope to examine cork.
Modern laboratories use light microscopes that use visible light and lenses for magnification. For finer details at the nanometre (one-billionth of a metre) scale, scientists use electron microscopes which use beams of electrons. Total magnification is calculated by multiplying the power of the eyepiece by the power of the objective lens.
To estimate the actual size of a cell under a microscope, one can measure the diameter of the visible field in µm and divide it by the number of cells seen along that diameter in a straight line. For example, if a visible field is estimated at 5 mm (5000 µm) and 25 cells are counted across it, the estimated size of one cell would be 200 µm.
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If a microscope has a 10X eyepiece and a 40X objective lens, what is the total magnification?
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How is the actual size of a cell estimated using a microscope field of view?
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03 · Explore
The Cell Membrane and Transport
The cell membrane, or plasma membrane, is a selectively permeable boundary that controls the movement of substances.
The cell membrane is extremely thin (7-10 nm) and follows the fluid-mosaic model. It consists of a lipid bilayer (two layers of fat molecules) with proteins embedded in it. It is 'fluid' because molecules can move sideways and 'mosaic' because proteins are arranged like tiles. These proteins act as gatekeepers.
Transport occurs via diffusion and osmosis. Diffusion is the net movement of particles from higher to lower concentration. Osmosis is the specific diffusion of water across a selectively permeable membrane. In plants, roots absorb water from the soil through osmosis.
The effect of a solution on a cell depends on concentration. In an isotonic solution, there is no net water movement. In a hypotonic solution (lower solute outside), the cell gains water and swells. In a hypertonic solution (higher solute outside), the cell loses water and shrinks.
Potato Osmosis Experiment
Weight Change = Final Weight - Initial Weight
A potato in plain water (hypotonic) swells as water enters the cells, increasing its weight. A potato in 20% salt solution (hypertonic) shrinks as water leaves the cells, decreasing its weight.
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What does 'selectively permeable' mean?
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04 · Explore
The Cell Wall: Strength and Support
Plants, fungi, and bacteria possess a rigid outer layer called the cell wall, which provides mechanical strength.
The cell wall is located outside the cell membrane. In plants, it is primarily made of cellulose, a complex carbohydrate. Unlike the selectively permeable membrane, the cell wall is permeable, allowing water and dissolved minerals to pass through easily.
The cell wall helps plants withstand environmental stresses like wind and rain since they cannot move. It maintains the cell's shape and prevents it from bursting when it takes in large amounts of water. In a concentrated solution, a plant cell's contents shrink away from the wall (plasmolysis), but the wall keeps the overall shape.
Animal cells lack a cell wall, which makes them flexible. This flexibility supports the movement of animal tissues, but it also means animal cells (like human cheek cells) shrink completely when they lose water in a hypertonic environment.
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Why do plant cells not shrink in size when placed in a concentrated sugar solution?
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05 · Explore
Prokaryotic and Eukaryotic Cells
Cells are categorized into two types based on the presence or absence of a well-defined nucleus.
Prokaryotic cells (pro = primitive, karyon = nucleus) are small (1-10 µm) and lack a nuclear membrane. Their genetic material is located in an undefined region called the nucleoid. They also lack membrane-bound organelles. Bacteria are typical prokaryotes.
Eukaryotic cells (eu = true) are larger (10-100 µm) and have a well-defined nucleus enclosed by a membrane. They contain various membrane-bound organelles and a cytoskeleton (a network of fibres for support and transport). Plants and animals are eukaryotes.
While prokaryotes perform activities directly in the cytoplasm, eukaryotes use specialized organelles to carry out different life processes independently and simultaneously. This makes eukaryotic cells like highly organized factories.
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Nucleus | Absent (Nucleoid present) | Present (Well-defined) |
| Size | 1 to 10 µm | 10 to 100 µm |
| Organelles | Membrane-bound absent | Membrane-bound present |
| Organisms | Usually unicellular | Unicellular or Multicellular |
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What is a nucleoid?
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Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 7, 8.