Every living thing — from a single bacterium to the human body — is made of cells. This module covers everything about cells that appears in CDS, NDA & AFCAT General Science, explained from scratch.
In this module you will learn: what a cell is, the difference between plant and animal cells, key organelles and their functions, prokaryotes vs eukaryotes, and cell division basics. All of these appear directly in CDS/NDA/AFCAT every year.
A cell is the smallest structural and functional unit of all living organisms. It is the basic building block of life — everything a living thing does (breathe, digest, grow, reproduce) happens inside or because of cells.
If a living body is like a building, then cells are the bricks. Each brick is self-contained, does its job, and together they make the whole structure work.
Robert Hooke observed cells in dead cork — not living cells. The first to see living cells was Leeuwenhoek. This distinction is frequently tested.
All cells in the world fall into just two categories based on whether they have a proper nucleus or not.
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| Nucleus | No true nucleus — DNA floats freely in cytoplasm (nucleoid region) | True nucleus enclosed in nuclear membrane |
| Size | Very small (1–10 µm) | Larger (10–100 µm) |
| Membrane-bound organelles | Absent (no mitochondria, ER, Golgi) | Present (mitochondria, ER, Golgi, etc.) |
| DNA shape | Circular, single chromosome | Linear, multiple chromosomes |
| Cell wall | Present (made of peptidoglycan) | Present in plants (cellulose), fungi (chitin); absent in animals |
| Ribosomes | 70S (smaller) | 80S (larger); 70S in mitochondria/chloroplast |
| Examples | Bacteria, Cyanobacteria (blue-green algae), Mycoplasma | Plants, Animals, Fungi, Protists |
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell wall | Present (cellulose) | Absent |
| Chloroplasts | Present (for photosynthesis) | Absent |
| Central vacuole | Large, single central vacuole | Small, many or absent |
| Centrioles | Absent (except lower plants) | Present (for cell division) |
| Lysosomes | Rare | Common ("suicide bags") |
| Plastids | Present (chloro-, leuco-, chromo-) | Absent |
| Shape | Fixed, rectangular (due to cell wall) | Irregular, flexible |
| Glycogen storage | Absent (stores starch) | Present |
Every cell is surrounded by a cell membrane (also called plasma membrane). In plant cells, there is also a stiff cell wall outside the membrane.
| Cell Membrane | Cell Wall | |
|---|---|---|
| Present in | ALL cells (plant + animal) | Plants, Fungi, Bacteria (not animals) |
| Made of | Phospholipid bilayer + proteins (Fluid Mosaic Model) | Cellulose (plants) · Chitin (fungi) · Peptidoglycan (bacteria) |
| Function | Controls what enters and exits the cell (selectively permeable) | Provides shape, rigidity, protection |
The cell membrane is described as a fluid mosaic — "fluid" because the phospholipid molecules can move around, and "mosaic" because proteins are scattered throughout like tiles in a mosaic. This is the accepted model for all cell membranes.
The cell membrane is selectively permeable (lets some things in, blocks others). It is NOT fully permeable or fully impermeable. This exact phrase — "selectively permeable" — is tested directly.
Mitochondria are the sites of cellular respiration — they convert food (glucose) into energy in the form of ATP (Adenosine Triphosphate).
Mitochondria are like the power plant of a city. The city (cell) needs electricity (ATP) to function. The power plant burns fuel (glucose) and generates the electricity.
Human Red Blood Cells (RBCs) have NO mitochondria and no nucleus. They rely entirely on anaerobic respiration (glycolysis) for energy. This is a very common question in CDS/NDA.
Chloroplasts are found only in plant cells and some algae. They contain chlorophyll — the green pigment that traps sunlight and uses it to make food through photosynthesis.
Brain of the cell. Contains DNA (genetic material). Surrounded by nuclear envelope with pores. Contains nucleolus (makes ribosomes). Controls all cell activities.
Site of protein synthesis. No membrane. Two types: 70S (prokaryotes, mitochondria, chloroplasts) and 80S (eukaryote cytoplasm). Discovered by George Palade.
Rough ER (has ribosomes) — makes proteins. Smooth ER (no ribosomes) — makes lipids, detoxifies. Forms a network of channels throughout the cell.
Packages and ships proteins made by ribosomes. Modifies, sorts, and sends materials to correct destinations inside or outside the cell. Also makes lysosomes.
Contains digestive enzymes. Breaks down worn-out organelles, bacteria, and dead cells (autophagy). If lysosome bursts — the cell self-destructs. Found mainly in animal cells.
Stores water, sap, waste. Plant cells have ONE large central vacuole (maintains cell turgidity). Animal cells have several small vacuoles or none.
Contains centrioles. Forms the spindle fibres during cell division. Present in animal cells. Absent in higher plant cells. Made of microtubules arranged in 9+0 pattern.
Breaks down fatty acids and amino acids. Neutralises toxic hydrogen peroxide (H₂O₂) using catalase enzyme. Important in liver and kidney cells.
Cells reproduce by dividing. There are two types of division, and they serve completely different purposes.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Also called | Equational division | Reductional division |
| Divisions | One division | Two divisions (Meiosis I + II) |
| Daughter cells produced | 2 daughter cells | 4 daughter cells |
| Chromosome number | Same as parent (diploid → diploid) | Half of parent (diploid → haploid) |
| Where it occurs | Body (somatic) cells — skin, liver, bones | Reproductive cells — testes, ovary |
| Purpose | Growth, repair, replacement of cells | Formation of gametes (sperm and eggs) |
| Genetic variation | No — daughter cells identical to parent | Yes — crossing over creates variation |
All the high-yield facts from this module in one place. Scan this page before your exam.