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Cell: The Unit of Life is a fact-dense chapter, and NEET picks questions from almost every paragraph of it. These notes go organelle by organelle — what each one is made of, what it does, and the one detail examiners like to ask about — along with the diagrams worth drawing until you can label them from memory.
How to study this chapter
For every organelle, learn three things: membrane (single, double or none), main job, and one NEET hook — a name, a number or an exception. The table in section 3 is built exactly that way.
1. Cell theory
- Matthias Schleiden (1838) observed that plants are made of cells. Theodore Schwann (1839) reached the same conclusion for animals and noted that the cell wall is unique to plant cells. Together they proposed the cell theory.
- Their theory did not explain how new cells form. Rudolf Virchow (1855) filled that gap: cells arise from pre-existing cells, Omnis cellula-e cellula.
- Anton von Leeuwenhoek first saw and described a live cell. Robert Brown later discovered the nucleus.
Cell theory as understood today: all living organisms are made of cells and their products, and all cells arise from pre-existing cells.
Size range: Mycoplasma, the smallest cells, are about 0.3 µm; bacteria are 3–5 µm; a human red blood cell is about 7 µm across. The largest single cell is the egg of an ostrich, and nerve cells are among the longest.
2. The prokaryotic cell
Bacteria, blue-green algae, mycoplasma and PPLO are prokaryotes. Bacteria come in four basic shapes: bacillus (rod), coccus (spherical), vibrio (comma) and spirillum (spiral).
- Cell envelope, three layers from outside in: glycocalyx (a loose slime layer or a thick, tough capsule), cell wall and plasma membrane.
- Gram staining: Gram-positive bacteria keep the stain; Gram-negative bacteria do not.
- Mesosome: an infolding of the plasma membrane that helps in cell wall formation, DNA replication and its distribution to daughter cells, respiration and secretion, and increases membrane surface area.
- Chromatophores: membranous extensions in cyanobacteria that carry pigments.
- Genetic material: a single circular DNA lying naked in the cytoplasm, with no nuclear envelope. Many bacteria also carry small circular plasmids, which can give traits such as antibiotic resistance.
- Ribosomes: 70S, made of 50S and 30S subunits. Several ribosomes on one mRNA form a polysome.
- Inclusion bodies store reserve material free in the cytoplasm: phosphate granules, cyanophycean granules and glycogen granules. Gas vacuoles occur in blue-green algae and in purple and green photosynthetic bacteria.
- Flagellum: filament, hook and basal body. Pili and fimbriae are surface structures that do not help in movement; fimbriae help bacteria attach to rocks or host tissue.
3. Eukaryotic organelles at a glance
| Organelle | Membrane | Main job | NEET hook |
|---|---|---|---|
| Plasma membrane | Lipid bilayer with proteins | Selective transport in and out of the cell | Fluid mosaic model, Singer and Nicolson (1972). The human RBC membrane is about 52% protein and 40% lipid. |
| Cell wall (plants, fungi, algae) | Non-living layer | Shape, protection, prevents bursting | Plant wall: cellulose, hemicellulose, pectins, proteins. Middle lamella: calcium pectate. Plasmodesmata connect neighbouring cells. |
| Endoplasmic reticulum | Single | RER: protein synthesis and secretion. SER: lipid synthesis | SER makes steroidal hormones in animal cells. |
| Golgi apparatus | Single | Packaging and dispatch of materials | Named after Camillo Golgi (1898). The convex cis face receives; the concave trans face releases. Site of glycoprotein and glycolipid formation. |
| Lysosome | Single | Intracellular digestion | Contains hydrolytic enzymes (lipases, proteases, carbohydrases) that work at acidic pH. Formed from Golgi vesicles. |
| Vacuole | Single (tonoplast) | Stores water, sap, excretory products | Can fill up to 90% of a plant cell. Contractile vacuole in Amoeba handles osmoregulation and excretion. |
| Mitochondrion | Double | Aerobic respiration, ATP production | Inner membrane folds into cristae. Matrix holds circular DNA and 70S ribosomes. Divides by fission. |
| Plastids | Double | Photosynthesis and storage | Chloroplasts (green), chromoplasts (carotenoids), leucoplasts (colourless storage: amyloplasts store starch, elaioplasts oils, aleuroplasts proteins). |
| Ribosome | None | Protein synthesis | Discovered by George Palade (1953). Eukaryotic 80S = 60S + 40S. S is the Svedberg unit, a measure of sedimentation. |
| Cytoskeleton | None | Support, motility, cell shape | Microtubules, microfilaments and intermediate filaments. |
| Cilia and flagella | Covered by plasma membrane | Movement | Axoneme with 9 peripheral doublets and 2 central microtubules: the 9 + 2 arrangement. Arise from a basal body. |
| Centrosome | None | Forms basal bodies; forms spindle fibres in animal cells | Two centrioles at right angles. Each centriole has 9 peripheral triplet fibrils in a cartwheel pattern. |
| Nucleus | Double (nuclear envelope with pores) | Controls the cell; houses DNA | Nucleolus is the site of active rRNA synthesis. The term chromatin was given by Flemming. |
| Microbodies | Single | Various enzymatic reactions | Found in both plant and animal cells. |
Endomembrane system — a favourite trap
The endomembrane system is the ER, Golgi apparatus, lysosomes and vacuoles, whose functions are coordinated. Mitochondria, chloroplasts and peroxisomes are not part of it. Mitochondria and chloroplasts are called semi-autonomous because they have their own DNA and 70S ribosomes.
4. Plant cell versus animal cell
| Feature | Plant cell | Animal cell |
|---|---|---|
| Cell wall | Present | Absent |
| Plastids | Present | Absent |
| Vacuole | One large central vacuole | Small or absent |
| Centrioles | Absent in most higher plants | Present |
| Shape | Usually fixed, often rectangular | Usually irregular or rounded |
5. Chromosomes by centromere position
| Type | Centromere position | Arms |
|---|---|---|
| Metacentric | In the middle | Two equal arms |
| Sub-metacentric | Slightly away from the middle | One shorter and one longer arm |
| Acrocentric | Close to one end | One extremely short and one very long arm |
| Telocentric | At the very end | A single arm |
Some chromosomes have a non-staining secondary constriction at a constant position, which cuts off a small fragment called the satellite.
6. Diagrams worth drawing
NEET often shows a labelled diagram and asks you to match the labels. Draw each of these at least once, from memory:
- A bacterial cell, with glycocalyx, cell wall, mesosome, nucleoid, plasmid, ribosomes and flagellum
- A mitochondrion, with outer membrane, inner membrane, cristae, matrix and inter-membrane space
- A chloroplast, with grana, thylakoids, stroma lamellae and stroma
- The Golgi apparatus, with its cis and trans faces
- A cross-section of a cilium, showing the 9 + 2 arrangement
- A centriole, showing the cartwheel of nine triplets
- The four chromosome types by centromere position
Quick check
1. Which organelle has no membrane at all?
Answer: the ribosome.
2. A bacterial ribosome is 70S. What are its subunits?
Answer: 50S and 30S. Svedberg values do not simply add up.
3. Name the plastid that stores oils and fats.
Answer: elaioplast.
4. Which face of the Golgi apparatus releases vesicles?
Answer: the trans, or maturing, face.
Mistakes that cost marks
- Counting mitochondria or chloroplasts as part of the endomembrane system.
- Adding Svedberg units: 60S + 40S gives an 80S ribosome, not 100S.
- Calling ribosomes membrane-bound.
- Confusing the mesosome, a membrane infolding in bacteria, with the mitochondrion.
- Saying pili help bacteria move. Only flagella do.
- Writing that lysosomal enzymes work at alkaline pH. They need acidic pH.
- Assuming all plant cells have centrioles. Most higher plants lack them.
60-second recap
- Cell theory: Schleiden and Schwann; Virchow added that cells come from pre-existing cells.
- Prokaryotes: no nuclear envelope, 70S ribosomes, mesosomes, plasmids.
- Endomembrane system: ER, Golgi, lysosomes, vacuoles only.
- Double membrane: nucleus, mitochondria, plastids. No membrane: ribosomes, centrioles, cytoskeleton.
- Cilia and flagella are 9 + 2; centrioles are 9 triplets in a cartwheel.
- Nucleolus is the site of rRNA synthesis.