AS Biology – Condensed Revision Notes

All eukaryotic cells: cell surface membrane, cytoplasm, nucleus, mitochondria, rough & smooth ER, Golgi, vesicles/lysosomes, ribosomes, microtubules
Plant-only: cellulose cell wall, large permanent vacuole (+ tonoplast), chloroplasts, plasmodesmata
Animal-only: centrioles, microvilli
Prokaryotes: 70S ribosomes, circular DNA, cell wall (peptidoglycan), sometimes plasmids/flagella; size $500\,\text{–}\,5000\,\text{nm}$
Eukaryotes up to $10^5\,\text{nm}$ ; viruses $20\,\text{–}\,300\,\text{nm}$ , non-cellular (DNA/RNA core, capsid, sometimes envelope)

Nucleus: stores DNA, nucleolus makes ribosomes, nuclear pores allow $\text{mRNA}$ /ribosomes out, enzymes in
RER: protein folding/transport; SER: lipid & steroid synthesis
Golgi: modifies & packages proteins into vesicles
Mitochondrion: double membrane, cristae ↑SA; matrix enzymes + DNA + 70S ribosomes → aerobic respiration/ATP
Ribosomes: 80S (cytosol/RER), 70S (prokaryotes, mito, chloro); site of translation
Vesicles: transport; lysosomes contain hydrolytic enzymes (autolysis, phagocytosis, apoptosis)
Cytoskeleton: microtubules (tubulin) + microfilaments → shape & transport; centrioles organise spindle
Cilia (microtubule 9+2) move fluid; microvilli ↑SA absorption
Chloroplast: double membrane, thylakoid → grana, lamellae; chlorophyll; own DNA & ribosomes → photosynthesis

Fluid mosaic: phospholipid bilayer (hydrophilic heads, hydrophobic tails) + proteins, cholesterol, glyco-lipids/proteins
Cholesterol: buffers fluidity; glycolipids/glycoproteins: recognition, receptors, adhesion
Transport:
- Simple diffusion: non-polar/small molecules down gradient
- Facilitated diffusion: channel/carrier proteins, passive
- Osmosis: water from high to low $\Psi$ (water potential)
- Active transport: carrier + $\text{ATP}$ (e.g. $\text{Na}^+!/!\text{K}^+$ pump)
- Bulk: endocytosis (phago/pino), exocytosis (secretion)
Membrane fluidity ↓ with saturated FA & low T; ↑ with unsaturated FA & high T

Monomers ↔ polymers via condensation (removes $\text{H}2\text{O}$ ) / hydrolysis (adds $\text{H}2\text{O}$ )
Carbohydrates:
- Monosaccharides (tri/pent/hexose), disaccharides, polysaccharides
- Glycosidic bond; Benedict’s test for reducing sugars
- Starch (amylose $\alpha$ -1,4 helix; amylopectin $\alpha$ -1,4 & 1,6 branched) – plant storage, insoluble
- Glycogen: highly branched $\alpha$ -1,4 & 1,6 – animal storage
- Cellulose: $\beta$ -1,4, straight chains → microfibrils, cell wall strength
Lipids:
- Triglyceride = glycerol + 3 FA via esterification; energy dense ( $37\,\text{kJ g}^{-1}$ ), insulation, protection
- Phospholipid = glycerol + 2 FA + phosphate; amphipathic → bilayers
Proteins:
- Amino acid: NH$_2$–CHR–COOH; peptide bond
- Structure levels: primary (sequence), secondary ( $\alpha$ -helix/ $\beta$ -sheet H-bonds), tertiary (R-group bonds: H, ionic, disulfide, hydrophobic), quaternary (≥2 chains)
- Globular (soluble, e.g. enzymes, haemoglobin with Fe $^{2+}$ haem) vs fibrous (insoluble, e.g. collagen triple helix)
Water: polar, H-bonding → solvent, high $c_p$ , high latent heat, cohesion, turgor support
ATP: adenine + ribose + 3P; hydrolysis releases energy for cellular work

Biological catalysts (globular proteins) with specific active site
Models: lock-and-key, induced fit (active site moulds)
Lower activation energy by stabilising transition state
Factors: T (optimum ≈ $37^{\circ}\text{C}$ ; high T denatures), pH, $[E]$ , $[S]$
Kinetics: $V{max}$ (saturation); $Km$ = $[S]$ at $0.5V_{max}$ (affinity)
Inhibition: competitive (↑Km, same $V{max}$ ), non-competitive (↓ $V{max}$ , Km unchanged); end-product inhibition = feedback control

DNA: double helix, antiparallel (5'→3'/3'→5'); sugar-phosphate backbone (phosphodiester); A=T (2 H-bonds), G≡C (3)
Semi-conservative replication (S phase): DNA helicase unzips; DNA polymerase adds activated nucleotides 5'→3'; leading vs lagging (Okazaki fragments joined by ligase)
Genetic code: triplet, non-overlapping, degenerate, universal; start $\text{AUG}$ , stop $\text{UAA/UAG/UGA}$ (in mRNA)
Transcription: RNA polymerase makes pre-mRNA from template strand; in eukaryotes introns removed (splicing) → mature mRNA leaves nucleus
Translation: ribosome reads mRNA; tRNA anticodon + amino acid; peptide bonds form → polypeptide folds
Mutations: insertion/deletion (frameshift), substitution → silent, missense, nonsense; degeneracy buffers effect

Cell cycle: Interphase (G1 growth, S DNA replication, G2 prep) → Mitosis (PMAT) → Cytokinesis
Mitosis role: growth, repair, asexual reproduction; produces 2 genetically identical diploid cells
Telomeres (repetitive non-coding ends) protect genes; shorten each division unless telomerase active
Stem cells: totipotent, pluripotent, multipotent; self-renew or differentiate; basis of development & therapy
Cancer: mutations → oncogenes → uncontrolled mitosis; benign vs malignant (metastasis); carcinogens (UV, tar, X-rays, oncoviruses)

Need bulk flow for water, minerals, sucrose beyond diffusion limits
Xylem: dead vessels + tracheids; lignified walls, no end walls; upward transpiration stream; pits allow sideways flow
Phloem: living sieve tube elements (sieve plates) + companion cells (ATP); translocates assimilates source→sink (bidirectional)
Vascular bundle layout (dicot):
- Root: xylem central star, phloem between arms
- Stem: bundles in ring; xylem inside, phloem outside, cambium between
- Leaf: midrib/veins; xylem upper, phloem lower