Cell Biology and Biomolecules — NEET Biology

The cell is the unit of life, and NEET returns to it every year for a reliable 6–8 questions spanning organelles, biomolecules and cell division. This block rewards precise recall: the function of each organelle, the exact differences between prokaryotic and eukaryotic cells, the stages of mitosis and meiosis, and the structure of the four biomolecule classes. This chapter organises all of it the way NCERT presents it — with the comparison tables, diagrams-in-words and exceptions the exam quotes almost verbatim.

1. The cell theory and cell types

Cell theory (Schleiden, Schwann; extended by Virchow):

1. All living organisms are composed of cells and their products.
2. The cell is the basic structural and functional unit of life.
3. All cells arise from pre-existing cells (Virchow: Omnis cellula-e cellula).

Prokaryotic vs eukaryotic — a top NEET comparison:

Bacteria also have a mesosome (infolded membrane), plasmid (extra circular DNA), and may bear a capsule, flagella and pili.

Worked example 1.1. Why are mitochondria and chloroplasts said to be semi-autonomous organelles? They have their own circular DNA and 70S (prokaryote-type) ribosomes and can make some of their own proteins and divide independently — evidence for the endosymbiotic origin from ancient prokaryotes.

2. The plasma membrane and cell wall

The plasma membrane is described by the fluid-mosaic model (Singer & Nicolson): a phospholipid bilayer (hydrophilic heads out, hydrophobic tails in) with proteins floating in it (integral and peripheral), giving quasi-fluid behaviour. Membrane transport:

• Passive (no energy): simple diffusion (down a gradient), facilitated diffusion (via carriers), osmosis (water across a semipermeable membrane).
• Active (uses ATP): pumps solutes against the gradient (Na⁺/K⁺ pump).
• Bulk transport: endocytosis and exocytosis.

The cell wall (outside the membrane in plants, fungi, bacteria) is rigid, providing shape and protection; the plant wall is cellulose with a middle lamella (calcium pectate) cementing adjacent cells, and plasmodesmata connecting cytoplasm.

Worked example 2.1. A plant cell placed in a hypertonic solution undergoes what change? Water leaves the cell by osmosis, the protoplast shrinks away from the wall — plasmolysis. In a hypotonic solution water enters and the cell becomes turgid (but does not burst, thanks to the wall).

3. The cell organelles

Endomembrane system (functionally coordinated):

• Endoplasmic reticulum (ER): rough ER (ribosome-studded, protein synthesis) and smooth ER (lipid/steroid synthesis, detoxification).
• Golgi apparatus: stacks of cisternae; packaging, modification and secretion of proteins; forms lysosomes; cis face receives, trans face dispatches.
• Lysosomes: membrane sacs of hydrolytic (digestive) enzymes — the cell's "suicide bags"; intracellular digestion.
• Vacuoles: storage; the large central vacuole (with tonoplast) maintains turgor in plant cells.

Energy and other organelles:

• Mitochondria: double membrane, inner folded into cristae; site of aerobic respiration and ATP synthesis — the "powerhouse". Contain their own DNA and 70S ribosomes.
• Plastids (plants): chloroplasts (photosynthesis; thylakoids stacked into grana in a stroma), chromoplasts (pigments), leucoplasts (storage).
• Ribosomes: the site of protein synthesis; 80S in the cytoplasm (60S + 40S), not membrane-bound.
• Microbodies: peroxisomes and glyoxysomes (enzyme-containing).
• Cytoskeleton: microtubules, microfilaments, intermediate filaments — shape, support, movement.
• Cilia and flagella: movement; 9 + 2 microtubule arrangement (nine doublets + a central pair).
• Centrosome/centrioles: 9 + 0 arrangement; organise the spindle in animal-cell division.

Nucleus: bounded by a double nuclear envelope with pores; contains chromatin (DNA + histones) and the nucleolus (ribosome-RNA synthesis, not membrane-bound).

Worked example 3.1. Distinguish the microtubule arrangement of cilia/flagella from that of centrioles. Cilia and flagella have a 9 + 2 pattern (nine peripheral doublets around a central pair). Centrioles have a 9 + 0 pattern (nine peripheral triplets, no central tubules).

4. Biomolecules

The chemical constituents of the cell fall into four major classes.

Carbohydrates (Cₙ(H₂O)ₙ): energy and structure. Monosaccharides (glucose, fructose, ribose), disaccharides (sucrose, maltose, lactose), polysaccharides (starch, glycogen — storage; cellulose, chitin — structural).

Proteins: polymers of 20 amino acids joined by peptide bonds; four structural levels (primary sequence → secondary α-helix/β-sheet → tertiary 3-D shape → quaternary multi-chain). Functions: enzymes, transport (haemoglobin), structure (collagen), defence (antibodies), hormones. Collagen is the most abundant protein in the animal world.

Lipids: fats, oils, phospholipids, steroids; energy store, membranes, insulation. Not true polymers. Water-insoluble.

Nucleic acids: DNA and RNA, polymers of nucleotides (base + sugar + phosphate); store and express genetic information.

Enzymes are protein biocatalysts: they lower activation energy, are highly specific (lock-and-key / induced-fit at the active site), and are affected by temperature, pH and substrate concentration. Many need cofactors (inorganic ions) or coenzymes (organic, often vitamin-derived — NAD, FAD). Enzymes are classified into six groups (oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases).

Worked example 4.1. Why does enzyme activity fall sharply above an optimum temperature? Enzymes are proteins; above the optimum, heat denatures the enzyme — the tertiary structure of the active site unfolds and substrate can no longer bind — so activity drops steeply even though molecular collisions are more frequent.

5. The cell cycle

The cell cycle has two phases: interphase (growth, ~95% of the cycle) and the M phase (division).

Interphase sub-stages:

• G₁ — cell grows, metabolically active (DNA not yet replicated).
• S — DNA replication (chromosomes duplicate; content doubles, number unchanged).
• G₂ — growth and preparation for mitosis; the cell may exit to a quiescent G₀ state.

6. Mitosis

Mitosis (equational division) produces two genetically identical diploid daughter cells; it occurs in somatic cells for growth and repair. Four stages (after S phase):

• Prophase — chromatin condenses into visible chromosomes (two sister chromatids joined at a centromere); spindle begins; nuclear envelope and nucleolus disappear.
• Metaphase — chromosomes align at the metaphase plate (equator); spindle fibres attach at kinetochores.
• Anaphase — centromeres split; sister chromatids move to opposite poles.
• Telophase — chromosomes decondense; nuclear envelope reforms; nucleolus reappears.
• Cytokinesis — cytoplasm divides (a cleavage furrow in animals; a cell plate in plants).

Worked example 6.1. At which stage do sister chromatids separate, and what happens to chromosome number? Anaphase — the centromeres split and sister chromatids move to opposite poles. Each daughter nucleus receives the same diploid (2n) number as the parent (mitosis conserves chromosome number).

7. Meiosis

Meiosis (reductional division) produces four genetically different haploid cells from one diploid cell; it occurs in germ cells for gamete formation and halves the chromosome number, restoring diploidy at fertilisation. It is two successive divisions (Meiosis I and II) with a single DNA replication.

Meiosis I (reductional):

• Prophase I — long and complex; homologous chromosomes pair (synapsis, forming bivalents/tetrads) and exchange segments by crossing over at chiasmata (the source of genetic recombination). Sub-stages: leptotene, zygotene, pachytene, diplotene, diakinesis.
• Metaphase I — bivalents align at the equator.
• Anaphase I — homologous chromosomes separate (chromatids stay joined) → chromosome number halved.
• Telophase I — two haploid cells.

Meiosis II is like mitosis (sister chromatids finally separate), giving four haploid cells.

Significance: meiosis maintains constant chromosome number across generations and, through crossing over and independent assortment, generates the variation that drives evolution.

Worked example 7.1. Why is meiosis called reductional division, and where does genetic variation arise? It halves the chromosome number (2n → n) so that fertilisation restores 2n rather than doubling it each generation. Variation comes from crossing over (Prophase I) and the independent assortment of homologous chromosomes at Anaphase I.

8. Mitosis vs meiosis — the key comparison

9. Common traps NEET sets here

• 70S vs 80S ribosomes — prokaryotes and organelles (mito/chloro) 70S; eukaryotic cytoplasm 80S.
• Nucleolus and ribosomes are not membrane-bound; most other organelles are.
• Cilia/flagella 9 + 2, centriole 9 + 0 — a guaranteed recall.
• DNA replication is in S phase, not M phase.
• Anaphase (mitosis): chromatids separate; Anaphase I (meiosis): homologues separate.
• Meiosis = one replication, two divisions, four haploid cells; crossing over in Prophase I.
• Collagen is the most abundant animal protein.
• Plasmolysis in hypertonic, turgid in hypotonic — don't reverse.

10. Memory aids

• "PMAT" — Prophase, Metaphase, Anaphase, Telophase order.
• "Leptotene-Zygotene-Pachytene-Diplotene-Diakinesis" — Prophase I sub-stages (crossing over in pachytene).
• "9 + 2 moves you, 9 + 0 organises you" — cilia/flagella vs centrioles.
• "S is for Synthesis of DNA" — replication phase.
• "Meiosis makes four, mitosis makes two" — daughter-cell count.
• "Powerhouse, suicide bag, packaging plant" — mitochondria, lysosome, Golgi.

11. Exam protocol

1. Nail the prokaryote vs eukaryote and mitosis vs meiosis comparison tables.
2. Know each organelle's one-line function and which are double-membraned / not membrane-bound.
3. Fluid-mosaic membrane; classify transport as passive/active; osmosis outcomes (plasmolysis/turgor).
4. Four biomolecule classes with their monomers and bonds; enzyme specificity, denaturation and cofactors.
5. Cell cycle: DNA replicates in S phase; know G₁/S/G₂.
6. Mitosis stages (PMAT) — chromatids separate at anaphase; number conserved.
7. Meiosis: two divisions, four haploid cells, crossing over in Prophase I, homologues separate at Anaphase I.