WBCHSE HS Biological Science cheat sheet

Prokaryotic cells lack a membrane-bound nucleus, mitochondria and ER; they have 70S ribosomes (50S + 30S), a single circular DNA, and peptidoglycan cell walls in bacteria.

Eukaryotic ribosomes are 80S (60S + 40S); the 60S subunit contains 28S, 5.8S and 5S rRNA, and the 40S contains 18S rRNA.

Fluid mosaic model (Singer and Nicolson, 1972): lipids and proteins diffuse laterally; membrane asymmetry is maintained by glycolipids and glycoproteins on the outer leaflet.

Mitochondrial inner membrane folds are cristae; the matrix holds Krebs cycle enzymes and mitochondrial DNA; the intermembrane space accumulates protons during electron transport.

Cell cycle phases: G1 (growth), S (DNA replication, DNA doubles but chromosome number stays 2n), G2 (checkpoint), M (mitosis and cytokinesis).

Mitosis: prophase (chromosome condensation, spindle forms), metaphase (chromosomes at equatorial plate), anaphase (centromeres split, sister chromatids separate), telophase (nuclear envelope reforms).

Meiosis I is reductional (homologous chromosomes separate, 2n to n); meiosis II is equational (sister chromatids separate). Crossing over occurs at pachytene of prophase I.

Cytokinesis differs: animal cells form a cleavage furrow of actin and myosin; plant cells build a cell plate from Golgi-derived vesicles.

Apoptosis is programmed, ATP-dependent and caspase-mediated, with membrane blebbing and no inflammation; necrosis is passive and provokes inflammation.

Proteins: amino acids joined by peptide bonds; primary (sequence), secondary (alpha helix, beta sheet via H-bonds), tertiary (3D folding) and quaternary (multiple polypeptides) structure.

Reducing sugars (glucose, fructose, maltose, lactose) reduce Cu(II) in Fehling's/Benedict's reagent to brick-red Cu2O; sucrose is non-reducing until hydrolysed.

Starch gives a blue-black colour with iodine; proteins give a violet biuret test; lipids give a translucent spot and emulsify in water.

Enzymes lower activation energy but do not change the equilibrium constant or the overall free-energy change of a reaction.

Michaelis-Menten: V = Vmax[S]/(Km + [S]); Km is the substrate concentration at half Vmax and is inversely related to enzyme affinity.

Competitive inhibitors raise apparent Km without changing Vmax; non-competitive inhibitors lower Vmax without changing Km.

Temperature: activity rises to an optimum then falls sharply as the enzyme denatures; pH extremes alter ionisation and tertiary structure.

Allosteric enzymes show sigmoidal (S-shaped) kinetics and are regulated by feedback inhibition of the end product.

Mendel's laws: dominance, segregation (alleles separate at meiosis I) and independent assortment (applies to genes on different chromosomes).

Test cross: cross an unknown dominant phenotype with a homozygous recessive; a 1:1 ratio in offspring indicates heterozygosity.

Dihybrid F2 ratio is 9:3:3:1; a modified 9:7 ratio indicates complementary gene action, and 12:3:1 indicates dominant epistasis.

DNA structure (Watson and Crick, 1953): antiparallel strands, right-handed B-DNA, base pairing A-T (2 H-bonds) and G-C (3 H-bonds); one turn of helix is 3.4 nm with 10 base pairs.

Semi-conservative replication requires template, primer, dNTPs and Mg2+; DNA polymerase synthesises only 5' to 3', so the lagging strand is made in Okazaki fragments.

Transcription uses RNA polymerase and a promoter; in eukaryotes introns are spliced out and a 5' cap and poly-A tail are added before translation.

Genetic code: triplet, degenerate, non-overlapping, comma-less and nearly universal; AUG codes methionine and is a start codon; UAA, UAG and UGA are stop codons.

Human genetics: X-linked recessive traits (haemophilia, colour blindness) affect males far more; ABO blood group shows multiple alleles and codominance.

Sex determination: XX-XO in grasshoppers, XX-XY in humans, ZZ-ZW in birds; the human SRY gene on the Y chromosome triggers testis development.

Light reactions occur in thylakoid membranes: photosystem II splits water (photolysis), electrons flow through the electron transport chain, and photophosphorylation yields ATP and NADPH.

Photosystem I absorbs light at P700 and reduces NADP+ to NADPH; PSII uses P680. Non-cyclic photophosphorylation involves both photosystems; cyclic involves only PSI.

Calvin cycle (stroma): RuBP + CO2 is fixed by RuBisCO into 3-PGA; ATP and NADPH reduce it to triose phosphate, regenerating RuBP; 3 ATP and 2 NADPH are used per CO2 fixed.

C4 plants (maize, sugarcane) fix CO2 first as oxaloacetate in mesophyll and release it in bundle-sheath cells; C4 avoids photorespiration and has high water-use efficiency.

CAM plants (cacti, pineapple) open stomata at night, store malate, and decarboxylate it by day; this separates CO2 fixation in time.

Glycolysis in cytoplasm yields 2 pyruvate, 2 ATP net and 2 NADH per glucose; Krebs cycle and oxidative phosphorylation in mitochondria yield about 36-38 ATP total.

RQ = CO2 released / O2 consumed: about 1.0 for carbohydrate, about 0.7 for fat, about 0.8 for protein.

Transpiration pull, cohesion-tension and root pressure move water through xylem; phloem transport is explained by the pressure-flow (mass flow) hypothesis.

Plant growth regulators: auxin (apical dominance, phototropism), gibberellin (stem elongation, seed germination), cytokinin (cell division), ABA (stomatal closure, stress), ethylene (fruit ripening).

Human dental formula: 2/1/2/3 per half jaw (incisors, canines, premolars, molars), giving 32 permanent teeth; enamel is the hardest tissue.

Enzymes: salivary amylase (starch to maltose), pepsin (protein, acid pH), trypsin (protein, alkaline pH), lipase (fat); bile salts emulsify fat without being enzymes.

Lung volumes: tidal volume about 500 mL; vital capacity = IRV + TV + ERV; residual volume cannot be exhaled; FRC = ERV + RV.

Human heart: SA node is the pacemaker; cardiac cycle has atrial systole, ventricular systole and joint diastole; normal rate 72 beats per minute.

Blood pressure: systolic about 120 mm Hg, diastolic about 80 mm Hg; the QRS complex of the ECG marks ventricular depolarisation.

Nephron: filtration in the glomerulus, selective reabsorption of glucose and amino acids in the proximal tubule, and water reabsorption regulated by ADH in the collecting duct.

Counter-current mechanism in the loop of Henle and vasa recta creates a hyperosmotic medullary gradient, concentrating urine.

Resting potential (about -70 mV) is maintained by the Na+/K+ pump; depolarisation opens Na+ channels; the refractory period prevents backward conduction.

Hormones: insulin lowers blood glucose, glucagon raises it, thyroxine regulates basal metabolic rate, and adrenaline produces the fight-or-flight response.

Spermatogenesis: spermatogonia to primary spermatocytes (meiosis I) to secondary spermatocytes to spermatids to spermatozoa; one primary spermatocyte yields four sperm.

Oogenesis produces one ovum and three polar bodies; meiosis II completes only after fertilisation; the ovum is released at metaphase II.

Human menstrual cycle: FSH stimulates follicle growth, LH surge triggers ovulation (about day 14), and the corpus luteum secretes progesterone.

Fertilisation occurs in the ampulla of the fallopian tube; the acrosome reaction releases enzymes to penetrate the zona pellucida.

Cleavage produces a morula, then a blastocyst with trophoblast and inner cell mass; implantation occurs in the endometrium about 6-7 days after fertilisation.

Extraembryonic membranes: amnion (fluid, protection), chorion (placenta formation), allantois (vessels, waste) and yolk sac (early blood cells).

Placenta secretes hCG, progesterone and oestrogen; hCG maintains the corpus luteum and is the basis of pregnancy tests.

Double fertilisation in angiosperms: one male gamete fuses with the egg (2n zygote) and the other with two polar nuclei (3n endosperm).

Apomixis is asexual seed formation; parthenocarpy is fruit development without fertilisation, induced by auxin or gibberellin.

Lamarck proposed use and disuse with inheritance of acquired characters; Darwin and Wallace proposed natural selection acting on heritable variation.

Homologous organs share common ancestry but may differ in function (forelimbs of vertebrates); analogous organs differ in ancestry but share function (wings of insects and birds).

Hardy-Weinberg equilibrium: p + q = 1 and p2 + 2pq + q2 = 1; it holds only with no mutation, no migration, random mating, large population and no selection.

Directional selection shifts a trait mean; stabilising selection favours intermediates; disruptive selection favours both extremes and can split a population.

Evidence for evolution: fossil record, comparative anatomy, embryology, molecular homology (cytochrome c, DNA sequences) and biogeography.

Industrial melanism in the peppered moth (Biston betularia) is a classic example of directional selection caused by pollution.

Adaptation types: xerophytic (thick cuticle, sunken stomata), hydrophytic (aerenchyma), and behavioural (migration, hibernation, aestivation).

Speciation: allopatric (geographic isolation) versus sympatric (polyploidy, habitat or behavioural isolation); reproductive isolation is the key criterion.

Human evolution sequence: Australopithecus, Homo habilis, Homo erectus, Homo neanderthalensis, Homo sapiens; bipedalism precedes large brain size.

Population growth: exponential dN/dt = rN; logistic dN/dt = rN(K - N)/K, where K is carrying capacity and gives a sigmoid curve.

Population density measures: quadrat for plants, mark-recapture (Lincoln index) for animals; N = (M x C)/R.

Species interactions: mutualism (+,+), commensalism (+,0), parasitism (+,−), predation (+,−) and competition (−,−).

Ecological pyramid types: numbers, biomass and energy; the energy pyramid is always upright because about 10% of energy passes to the next trophic level.

Primary productivity is the rate of organic matter fixation by producers; secondary productivity is the rate of new biomass formation by consumers.

Nutrient cycles: gaseous (carbon, nitrogen) and sedimentary (phosphorus, sulphur); nitrogen fixation is by Rhizobium, Azotobacter and cyanobacteria.

Ecological succession: primary succession starts on bare rock (lithosere, hydrosere); climax community is stable and self-perpetuating.

Gause's competitive exclusion principle: two species competing for identical resources cannot coexist indefinitely in the same niche.

Biodiversity hotspots in India: Western Ghats, Eastern Himalaya and Indo-Burma; hotspots have high endemism and severe habitat loss.

In-situ conservation: national parks, sanctuaries, biosphere reserves, sacred groves; ex-situ: zoos, botanical gardens, seed and gene banks, cryopreservation.

IUCN Red List categories: Extinct, Extinct in the Wild, Critically Endangered, Endangered, Vulnerable, Near Threatened, Least Concern.

Pollution indicators: BOD measures organic load in water; high BOD means high organic pollution; dissolved oxygen falls as BOD rises.

Greenhouse gases: CO2, CH4, N2O and CFCs; the greenhouse effect warms the lower atmosphere by trapping outgoing infrared radiation.

Ozone depletion is caused by CFCs releasing chlorine atoms in the stratosphere; the Montreal Protocol phases out ozone-depleting substances.

Eutrophication: nutrient enrichment causes algal blooms, oxygen depletion and fish kills; biomagnification concentrates persistent toxins up the food chain.

Joint Forest Management and the Chipko movement are community-based conservation examples; the Wildlife (Protection) Act 1972 protects Indian species.

Pathogens: Plasmodium (malaria, vector female Anopheles), Wuchereria (filariasis), Ascaris (ascariasis), Salmonella typhi (typhoid), HIV (AIDS, retrovirus).

Immunity: innate (non-specific barriers, phagocytes, inflammation) versus acquired (B and T lymphocytes); primary response is slow, secondary response is fast and large.

Antibodies are immunoglobulins with two heavy and two light chains; IgA, IgD, IgE, IgG and IgM differ in role; IgG crosses the placenta.

Vaccination induces memory cells; passive immunity is transferred ready-made antibody (colostrum, antiserum) and is short-lived.

Antibiotics such as penicillin inhibit bacterial cell wall synthesis and are ineffective against viruses; resistance arises by mutation and selection.

Biotechnology: restriction endonucleases cut DNA at palindromic sites; DNA ligase joins fragments; plasmids are cloning vectors with an origin of replication and selectable marker.

PCR uses Taq polymerase and thermal cycling (denaturation, annealing, extension) to amplify DNA; gel electrophoresis separates fragments by size.

Applications: human insulin from recombinant E. coli, Bt cotton (Bt toxin gene from Bacillus thuringiensis), gene therapy for ADA deficiency, ELISA and DNA fingerprinting.

Microscopy: magnification = eyepiece x objective; resolution depends on wavelength and numerical aperture, not magnification alone.

Staining: safranin stains lignin red; iodine detects starch (blue-black); acetocarmine stains chromosomes; methylene blue stains animal cells.

Onion root tip squash: fix in Carnoy's fluid, hydrolyse in 1 N HCl, stain in acetocarmine, and score mitotic stages to calculate the mitotic index.

Chromatography: Rf = distance travelled by solute / distance travelled by solvent front; Rf is characteristic of a pigment under fixed conditions.

Colorimetry: use a standard curve of known concentrations versus absorbance to read unknown concentrations; absorbance is proportional to concentration (Beer-Lambert law).

Rate calculations: rate = change in quantity / time; always report units and use the straight-line portion of a progress curve.

Controls: a control lacks the independent variable; replicates reduce random error; a blank zeroes the instrument.

Graph reading: identify axes and units, describe trends in words, and quote values with the precision of the scale; distinguish correlation from causation.

Microscopy: magnification = eyepiece x objective; resolution depends on wavelength and numerical aperture, not magnification alone.

Staining: safranin stains lignin red; iodine detects starch (blue-black); acetocarmine stains chromosomes; methylene blue stains animal cells.

Onion root tip squash: fix in Carnoy's fluid, hydrolyse in 1 N HCl, stain in acetocarmine, and score mitotic stages to calculate the mitotic index.

Chromatography: Rf = distance travelled by solute / distance travelled by solvent front; Rf is characteristic of a pigment under fixed conditions.

Colorimetry: use a standard curve of known concentrations versus absorbance to read unknown concentrations; absorbance is proportional to concentration (Beer-Lambert law).

Rate calculations: rate = change in quantity / time; always report units and use the straight-line portion of a progress curve.

Controls: a control lacks the independent variable; replicates reduce random error; a blank zeroes the instrument.

Graph reading: identify axes and units, describe trends in words, and quote values with the precision of the scale; distinguish correlation from causation.