Cell Structure & Function — NEET Class 11 revision
The full NEET Cell Structure & Function unit in one place: concept notes on the cell, its organelles, prokaryotic vs eukaryotic cells and the cell cycle, differentiate tables, the doubts students get wrong most, and quick-recall facts.
Key concepts
Unit 3 covers prokaryotic and eukaryotic cells, plant and animal cells, cell envelope and organelles, biomolecules and enzymes, and the cell cycle with mitosis, meiosis and their biological significance.
Cell theory
Schleiden and Schwann proposed that organisms are made of cells; Virchow added that new cells arise from pre-existing cells.
- Cell = basic structural and functional unit
- Omnis cellula e cellula
Prokaryotic vs eukaryotic cells
Prokaryotes lack a membrane-bound nucleus and organelles; eukaryotes possess them.
- Prokaryote: nucleoid, 70S ribosomes
- Eukaryote: true nucleus, membrane-bound organelles
Plant vs animal cells
Plant cells have a cellulose wall, plastids and usually a large central vacuole; animal cells lack a cell wall and plastids and commonly have centrioles.
- Plant: cell wall + plastids + large vacuole
- Animal: centrioles common, no cellulose wall
Cell membrane
The plasma membrane is a selectively permeable lipid-protein membrane described by the fluid-mosaic model.
- Phospholipid bilayer + proteins
- Controls movement into and out of the cell
Cell wall & cell envelope
Plant walls contain cellulose and provide shape and protection. Bacterial cell envelopes include glycocalyx, cell wall and plasma membrane.
- Plant middle lamella rich in calcium pectate
- Bacterial envelope: glycocalyx + wall + membrane
Endomembrane system
The ER, Golgi apparatus, lysosomes and vacuoles work together in synthesis, modification, packaging, digestion and storage.
- RER: proteins · SER: lipids/detox
- Golgi: modify/sort/package
- Lysosome: hydrolytic enzymes
Mitochondria
Mitochondria are double-membraned, semiautonomous organelles; the inner membrane forms cristae and the matrix contains enzymes, DNA and ribosomes.
- Aerobic ATP production
- Own DNA and 70S-like ribosomes
Plastids
Chloroplasts contain thylakoids arranged into grana within the stroma. Chromoplasts contain coloured pigments; leucoplasts store food.
- Chloroplast: photosynthesis
- Leucoplasts: amyloplast/elaioplast/aleuroplast
Ribosomes
Ribosomes are non-membranous sites of protein synthesis. Cytosolic eukaryotic ribosomes are 80S; prokaryotic, mitochondrial and chloroplast ribosomes are 70S.
- 80S = 60S + 40S
- 70S = 50S + 30S
Cytoskeleton, cilia & flagella
Microtubules, microfilaments and intermediate filaments support cell shape and movement. Eukaryotic cilia and flagella have a 9+2 axoneme.
- Cilia/flagella: 9 peripheral doublets + 2 central microtubules
Centrosome & centrioles
A centrosome usually contains two perpendicular centrioles. Each centriole has nine peripheral triplet microtubules and helps organise spindle fibres.
- Centriole: 9 triplets, no central pair
Nucleus
The nucleus has a double nuclear envelope with pores, chromatin and a nucleolus. Chromatin condenses into chromosomes during division.
- Nucleolus: rRNA synthesis/ribosome assembly
- Chromatin = DNA + proteins
Carbohydrates
Carbohydrates include monosaccharides, oligosaccharides and polysaccharides; they serve as energy sources and structural materials.
- Glucose: monosaccharide
- Starch/glycogen: storage · cellulose: structural
Proteins
Proteins are polymers of amino acids joined by peptide bonds. Their function depends on primary, secondary, tertiary and sometimes quaternary structure.
- Peptide bond: –CO–NH–
- Structure determines function
Lipids
Lipids are water-insoluble molecules including fats, phospholipids and steroids. Phospholipids are major membrane components.
- Triglyceride = glycerol + 3 fatty acids
- Phospholipids form membrane bilayers
Nucleic acids
DNA and RNA are polymers of nucleotides. Each nucleotide contains a sugar, nitrogenous base and phosphate.
- DNA: deoxyribose + A,T,G,C
- RNA: ribose + A,U,G,C
Enzymes
Most enzymes are proteins that lower activation energy without changing equilibrium. Their activity depends on temperature, pH and substrate concentration.
- Active site binds substrate
- Enzymes are specific and reusable
Enzyme classes
Enzymes are named and classified by the reactions they catalyse. The classical major classes include oxidoreductases, transferases, hydrolases, lyases, isomerases and ligases.
- Enzyme names commonly end in -ase
Cell cycle
The cell cycle includes interphase (G1, S, G2) and M phase. DNA replication occurs in S phase, not during mitosis itself.
- G1 → S → G2 → M
- DNA doubles in S phase
Mitosis
Mitosis is an equational division producing two genetically similar daughter cells and maintaining chromosome number; it supports growth and repair.
- Prophase → metaphase → anaphase → telophase
- 2 daughter cells · same ploidy
Meiosis
Meiosis has one DNA replication followed by two divisions. Meiosis I is reductional; meiosis II is equational, producing four haploid cells.
- Prophase I: synapsis + crossing over
- Meiosis I: homologues separate
- Meiosis II: sister chromatids separate
Significance of meiosis
Meiosis halves chromosome number in gametes and creates variation through crossing over and independent assortment, allowing chromosome number to remain constant across generations.
- Variation + haploid gametes
Differentiate cell structures and divisions
Focus on location, structure and consequence.
Ribosomes
| 70S | Prokaryotic cytoplasm; mitochondria; chloroplasts |
| 80S | Eukaryotic cytoplasm |
Cell cycle
| G1 | Cell growth and normal metabolism |
| S | DNA replication |
| G2 | Preparation for division |
| M | Nuclear division + cytokinesis |
High-yield distinctions
Keep the defining difference in mind.
Prokaryote vs eukaryote ★
Prokaryote
- No true nucleus
- No membrane-bound organelles
- 70S ribosomes
Eukaryote
- True nucleus
- Membrane-bound organelles
- 80S cytosolic ribosomes
RER vs SER
Rough ER
- Ribosomes attached
- Protein synthesis
- Often continuous with nuclear envelope
Smooth ER
- No ribosomes
- Lipid synthesis
- Detoxification
Mitosis vs meiosis ★
Mitosis
- One division
- 2 similar daughter cells
- Chromosome number maintained
Meiosis
- Two divisions
- 4 varied haploid cells
- Chromosome number halved
Cilia/flagella vs centriole ★
Cilium/flagellum axoneme
- 9+2 arrangement
- Movement
- Doublet microtubules
Centriole
- 9+0 arrangement
- Spindle organisation
- Triplet microtubules
Common NEET doubts
Does a prokaryotic cell have no DNA because it has no nucleus?
No. Its DNA lies in the nucleoid; it simply lacks a nuclear membrane.
Are mitochondria and chloroplasts completely dependent on nuclear DNA?
No. They are semiautonomous and contain their own DNA and ribosomes, although many of their proteins are nuclear-encoded.
Are lysosomes part of the endomembrane system?
Yes. ER, Golgi, lysosomes and vacuoles function as an integrated endomembrane system.
Are all lipids true polymers?
No. Lipids are important biomolecules but are not polymers in the same repeating-monomer sense as proteins or nucleic acids.
Does an enzyme make an unfavourable equilibrium favourable?
No. It lowers activation energy and speeds attainment of equilibrium; it does not change ΔG or the equilibrium position.
Is DNA replicated during prophase of mitosis?
No. DNA replication occurs in S phase before mitosis.
Do sister chromatids separate in anaphase I?
No. Homologous chromosomes separate in anaphase I; sister chromatids separate in anaphase II.
Does meiosis always produce genetically identical cells?
No. Crossing over and independent assortment generate genetic variation.
Quick recall
Q. State the cell theory in one line.
Organisms are made of cells; the cell is the basic unit of life; cells arise from pre-existing cells.
Q. Which organelles form the endomembrane system?
ER, Golgi apparatus, lysosomes and vacuoles.
Q. What is the ribosome size in prokaryotes?
70S; eukaryotic cytosolic ribosomes are 80S.
Q. What is the 9+2 arrangement?
The axonemal microtubule arrangement of eukaryotic cilia and flagella.
Q. What is a peptide bond?
The –CO–NH– linkage joining amino acids in proteins.
Q. What are the three parts of a nucleotide?
Pentose sugar, nitrogenous base and phosphate.
Q. During which cell-cycle phase is DNA replicated?
S phase.
Q. What separates in meiosis I?
Homologous chromosomes.
Q. What separates in meiosis II?
Sister chromatids.
Q. Why is meiosis biologically important?
It produces haploid gametes and genetic variation while preserving chromosome number across generations.
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