1. The Cell — The Building Block of Life
A cell is the basic structural and functional unit of living organisms. Some organisms consist of a single cell, while multicellular organisms contain many cells that work together.
The chapter begins by connecting cell biology with the possible early history of life. It discusses evidence and scientific ideas about life originating in water and introduces hot-spring environments such as Puga Valley in Ladakh, where heat-loving microorganisms called thermophiles live today.
2. How Do We Study Cells?
Most cells are too small to be seen clearly with the unaided human eye. Microscopes allow scientists to magnify specimens and, importantly, distinguish details that are close together.
Robert Hooke and the word “cell”
In 1665, Robert Hooke examined thin slices of cork with a microscope. He observed small box-like compartments and used the word “cells” for them because they reminded him of small rooms. The cork cells he saw were dead structures, but the term became part of biology.
Magnification and resolution
| Term | Meaning |
|---|---|
| Magnification | How much larger an image appears compared with the actual object. |
| Resolution | The ability to distinguish two very close points as separate. |
| Contrast | Difference in appearance between structures that helps make details visible. |
A light microscope uses visible light and lenses. Electron microscopes use electrons and can reveal much finer structural details than a conventional light microscope.
3. Cell Membrane — The Selective Boundary
Every living cell has a cell membrane (plasma membrane). It separates the cell interior from its surroundings and controls the movement of substances into and out of the cell.
Selective permeability
The membrane is selectively permeable: it allows some substances to pass more readily than others. This helps the cell maintain suitable internal conditions.
Fluid-mosaic model
The cell membrane is commonly described using the fluid-mosaic model. It contains a lipid bilayer with proteins embedded in it. The arrangement is dynamic, allowing many membrane components to move within the layer.
4. Diffusion and Osmosis
Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration due to random molecular motion.
Osmosis
Osmosis is the movement of water through a selectively permeable membrane from a region with higher water concentration toward a region with lower water concentration.
| Process | Main feature |
|---|---|
| Diffusion | Movement of particles down a concentration gradient; a selectively permeable membrane is not necessarily required. |
| Osmosis | Movement of water across a selectively permeable membrane. |
What happens to cells in different solutions?
5. Types of Cells — Prokaryotic and Eukaryotic
Cells differ in their internal organisation. Two broad categories are prokaryotic and eukaryotic cells.
| Feature | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Nucleus | No membrane-bound true nucleus; DNA is in a nucleoid region. | Has a membrane-bound nucleus. |
| Membrane-bound organelles | Generally absent. | Present. |
| Organisation | Generally simpler in internal organisation. | More compartmentalised. |
| Examples | Bacteria and archaea. | Plants, animals, fungi and many protists. |
Plant and animal cells
| Feature | Plant cells | Animal cells |
|---|---|---|
| Cell wall | Present; mainly cellulose in typical plant cells. | Absent. |
| Chloroplasts | Present in photosynthetic plant cells. | Absent. |
| Vacuole | Typically a large central vacuole in mature plant cells. | Usually smaller vacuoles or vesicles. |
| Shape | Often more fixed because of the cell wall. | Often more flexible in shape. |
6. Cell Structure and Organelles — Division of Labour
Eukaryotic cells contain specialised structures called organelles. Each performs particular functions, allowing the cell to operate as an organised system.
| Structure / organelle | Main function |
|---|---|
| Cell wall | Provides support and protection in plant cells; mainly cellulose. |
| Cytoplasm | Cellular region between the membrane and nucleus in eukaryotic cells; contains organelles and is the site of many metabolic activities. |
| Nucleus | Contains chromosomes and helps control cellular activities through genetic information. |
| Ribosomes | Sites of protein synthesis. |
| Endoplasmic reticulum (ER) | Membrane network involved in synthesis and transport of cellular materials. |
| Rough ER | Has ribosomes attached; associated with protein synthesis and processing. |
| Smooth ER | Lacks attached ribosomes; associated with lipid synthesis and other functions. |
| Golgi apparatus | Modifies, sorts and packages materials for transport within or outside the cell. |
| Lysosomes | Contain digestive enzymes and help break down cellular waste and worn-out components. |
| Mitochondria | Site of cellular respiration and major ATP production; contain their own DNA and ribosomes. |
| Chloroplast | Photosynthetic plastid containing chlorophyll; converts light energy into chemical energy. |
| Other plastids | Chromoplasts contain pigments other than chlorophyll; leucoplasts generally function in storage. |
| Vacuole | Stores water and dissolved substances; large plant vacuoles help maintain cell pressure and support. |
Cellular organisation
7. Nucleus, DNA and Chromosomes
The nucleus is a prominent membrane-bound organelle in eukaryotic cells. It contains chromosomes, which are made mainly of DNA associated with proteins.
The genetic information in DNA helps cells produce proteins and coordinate their activities. During cell division, chromosomes are duplicated and distributed to daughter cells according to the division process.
8. Cell Growth and Cell Division
Cells grow and divide as part of the life of an organism. Cell division is essential for growth, repair, replacement of cells and reproduction.
Mitosis
Mitosis produces daughter cells with the same chromosome number as the parent cell and, under normal conditions, genetically very similar to the parent and to each other.
Meiosis
Meiosis is a specialised type of cell division involved in the formation of reproductive cells. It reduces the chromosome number by half and contributes to genetic variation.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | One | Two successive divisions |
| Typical outcome | Two daughter cells | Four daughter cells |
| Chromosome number | Maintained | Reduced by half |
| Main role | Growth, repair, replacement and asexual reproduction in some organisms. | Formation of reproductive cells and generation of variation. |
9. Cell Theory
Cell theory is one of the central unifying ideas of biology. It developed through the work of several scientists and continued to be refined as evidence accumulated.
| Scientist | Contribution |
|---|---|
| Matthias Schleiden | Proposed that plants are made of cells. |
| Theodor Schwann | Extended the cell-based view to animals and helped formulate cell theory. |
| Rudolf Virchow | Emphasised that new cells arise from pre-existing cells. |
Core statements
- Living organisms are composed of one or more cells.
- The cell is the basic structural and functional unit of life.
- New cells arise from pre-existing cells through cell division.
10. Cell Growth, Control and Cancer
Normal multicellular organisms regulate cell growth and division. Cells also respond to signals from neighbouring cells and their environment.
Contact inhibition
Many normal cells reduce or stop proliferation when they come into close contact with neighbouring cells. This phenomenon is commonly called contact inhibition.
Tumour cells
In cancer, cells can lose normal controls over growth and division. A group of abnormally multiplying cells may form a tumour. Some tumours remain localised, while malignant tumours can invade nearby tissues and spread to other parts of the body.
11. Cell Biology in Life, Agriculture and Technology
Understanding cells has practical importance. Knowledge of cellular processes supports research and applications in medicine, agriculture, biotechnology, food science and environmental studies.
12. Quick Revision Capsule
Frequently confused pairs
| Pair | Key difference |
|---|---|
| Magnification vs resolution | Magnification enlarges an image; resolution determines how clearly close details can be distinguished. |
| Cell membrane vs cell wall | Membrane is selectively permeable; plant cell wall is a rigid supporting outer layer. |
| Diffusion vs osmosis | Diffusion concerns movement of particles; osmosis specifically concerns water across a selectively permeable membrane. |
| Prokaryotic vs eukaryotic | Prokaryotes lack a membrane-bound true nucleus; eukaryotes possess one. |
| RER vs SER | RER has ribosomes attached; SER lacks attached ribosomes. |
| Mitosis vs meiosis | Mitosis maintains chromosome number; meiosis halves it and contributes to variation. |
| Chloroplast vs chromoplast vs leucoplast | Chloroplasts are photosynthetic; chromoplasts contain pigments; leucoplasts are generally involved in storage. |
13. Practice Questions
Short Answer
- Why is the cell called the basic unit of life?
- What did Robert Hooke observe in cork?
- Differentiate between magnification and resolution.
- Why is the cell membrane called selectively permeable?
- Differentiate between diffusion and osmosis.
- What happens to an animal cell placed in a concentrated salt solution?
- Give two differences between prokaryotic and eukaryotic cells.
- What is the function of the nucleus?
- Write the functions of mitochondria, ribosomes and Golgi apparatus.
- Differentiate between rough ER and smooth ER.
- Why are chloroplasts important in plant cells?
- What is the difference between mitosis and meiosis?
- State the main ideas of cell theory.
- What is contact inhibition?
- How can abnormal control of cell division lead to cancer?
Answer Hints
2. He observed box-like compartments in cork and named them cells.
3. Magnification enlarges; resolution distinguishes close details.
4. It permits some substances to cross more readily than others.
5. Diffusion is net movement of particles down a concentration gradient; osmosis is water movement across a selectively permeable membrane.
6. Water tends to move out, so the cell shrinks.
7. Prokaryotes lack a membrane-bound true nucleus and membrane-bound organelles; eukaryotes have them.
8. It contains chromosomes and coordinates cellular activities through genetic information.
9. Mitochondria: ATP production through respiration; ribosomes: protein synthesis; Golgi: modification, sorting and packaging.
10. RER has ribosomes; SER does not.
11. They contain chlorophyll and carry out photosynthesis.
12. Mitosis produces two cells while maintaining chromosome number; meiosis produces reproductive cells with half the chromosome number.
13. Organisms are made of cells; cell is the basic unit; cells arise from pre-existing cells.
14. Normal cells often reduce proliferation when they contact neighbouring cells.
15. Loss of normal growth and division controls can produce uncontrolled cell multiplication.
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