MDCAT Biology key facts

Biology · Acellular life

Viruses: structure and classification

• Virion = one complete infective virus particle; nucleocapsid = capsid + the nucleic acid inside it
• Size: about 20 nm to 250 nm, so viruses pass through filters that hold back bacteria
• Capsid symmetry: helical (tobacco mosaic virus), polyhedral or icosahedral with 20 faces (poliovirus, adenovirus), complex (T₄ bacteriophage: polyhedral head + tail)
• By host: bacteriophages infect bacteria, plant viruses infect plants, animal viruses infect animals
• DNA viruses: smallpox, herpes, hepatitis B, T₄ phage. RNA viruses: polio, influenza, measles, mumps, HIV, tobacco mosaic virus
• Enveloped viruses: influenza, HIV, herpes. Naked (no envelope): poliovirus, tobacco mosaic virus, T₄ phage
• T₄ phage: head holds DNA; tail fibres attach to receptor sites on the bacterial wall

Viral replication and disease

• Lytic cycle order: attachment (adsorption) → penetration → replication of phage DNA and proteins → assembly → lysis
• Only the phage DNA enters the bacterium; the protein coat stays outside. Lysozyme from the phage tail makes the hole in the wall
• Virulent phage = lytic cycle only; temperate phage (for example lambda) = can follow either cycle
• Prophage = phage DNA inserted in the bacterial chromosome; the bacterium carrying it is lysogenic
• HIV in a helper T cell: attachment to the CD4 receptor → reverse transcription (RNA → DNA) → integration as a provirus → transcription and translation → assembly → budding
• HIV spreads by unprotected sexual contact, infected blood or shared needles, and from mother to child (placenta, birth, breast milk). It does not spread by touch, food, coughing or mosquitoes
• Prevention: screened blood, sterile needles, safe sexual behaviour, treatment of infected mothers. There is no cure and no vaccine; antiretroviral drugs slow the virus

Biology · Bioenergetics

Photosynthesis

• 6CO₂ + 12H₂O + light → C₆H₁₂O₆ + 6O₂ + 6H₂O; the oxygen released comes from water
• Chlorophyll absorbs mainly violet-blue and orange-red light and reflects green. Absorption spectrum = light absorbed at each wavelength; action spectrum = rate of photosynthesis at each wavelength
• Photolysis: 2H₂O → 4H⁺ + 4e⁻ + O₂, at photosystem II (P680)
• Non-cyclic photophosphorylation: both photosystems; electrons go water → PS II → PS I → NADP⁺; products ATP, NADPH and O₂
• Cyclic photophosphorylation: photosystem I (P700) only; electrons return to P700; product ATP only
• Calvin cycle: carbon fixation (CO₂ + RuBP → two 3-carbon PGA, enzyme RuBisCO) → reduction (PGA → G3P, uses ATP and NADPH) → regeneration of RuBP (uses ATP)
• Each CO₂ fixed uses 3 ATP and 2 NADPH. Limiting factors: light intensity, CO₂ concentration, temperature, water

Cellular respiration

• C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP)
• Glycolysis: glucose (6C) → 2 pyruvic acid (3C); 2 ATP used, 4 made, net 2 ATP; 2 NADH; no CO₂; no oxygen needed
• Link reaction (per pyruvic acid): pyruvic acid → acetyl CoA (2C) + CO₂ + NADH
• Krebs cycle (per turn): 2 CO₂, 3 NADH, 1 FADH₂, 1 ATP; oxaloacetic acid (4C) is regenerated; two turns per glucose
• Electron transport chain: each NADH gives 3 ATP, each FADH₂ gives 2 ATP; H⁺ is pumped into the intermembrane space and flows back through ATP synthase; oxygen is the final electron acceptor
• Per glucose: 10 NADH (30 ATP) + 2 FADH₂ (4 ATP) + 4 ATP made directly = 38 ATP; 36 ATP in most eukaryotic cells, because 2 ATP are spent moving the NADH of glycolysis into the mitochondrion
• Anaerobic: yeast, pyruvic acid → acetaldehyde + CO₂ → ethanol; muscle, pyruvic acid → lactic acid. Fats enter as acetyl CoA; amino acids are deaminated first

Biology · Biological molecules

Water and carbohydrates

• High specific heat capacity: keeps body and lake temperatures steady. High heat of vaporisation: sweating and transpiration cool. Ice is less dense than water, so it floats
• Monosaccharides by carbon number: triose 3C (glyceraldehyde), pentose 5C (ribose, deoxyribose, ribulose), hexose 6C (glucose, fructose, galactose). Aldose = aldehyde group (glucose); ketose = ketone group (fructose)
• Maltose = glucose + glucose; sucrose = glucose + fructose; lactose = glucose + galactose
• Starch = amylose (unbranched, α 1→4) + amylopectin (branched, α 1→4 and α 1→6): plant store
• Glycogen: α-glucose, more branched than amylopectin: animal store in liver and muscle
• Cellulose: β-glucose, β 1→4, straight unbranched chains: plant cell walls
• Iodine test: starch blue-black, glycogen red-brown, cellulose no colour. n monosaccharides in a chain have (n − 1) glycosidic bonds

Proteins and lipids

• About 20 kinds of amino acid occur in proteins; glycine is the simplest (R = H). A chain of n amino acids has (n − 1) peptide bonds
• Primary structure = sequence of amino acids. Secondary = α-helix or β-pleated sheet held by hydrogen bonds
• Tertiary = 3-D folding of one chain (ionic bonds, hydrogen bonds, disulphide bridges between cysteines, hydrophobic interactions). Quaternary = two or more chains together (haemoglobin has four)
• Fibrous proteins: long, insoluble, structural (keratin, collagen, silk). Globular proteins: compact, soluble (enzymes, antibodies, haemoglobin)
• Triglyceride = 1 glycerol + 3 fatty acids, 3 ester bonds, 3 water molecules released
• Saturated fatty acid: no C=C, higher melting point (fats). Unsaturated: one or more C=C, lower melting point (oils)
• Other lipids: waxes (waterproofing), steroids such as cholesterol (four fused rings), terpenoids such as carotenoids (isoprene units). Fat stores about twice as much energy per gram as carbohydrate

Nucleic acids

• Purines (two rings): adenine, guanine. Pyrimidines (one ring): cytosine, thymine, uracil. Nucleoside = sugar + base; nucleotide = sugar + base + phosphate
• Base pairing: A=T with 2 hydrogen bonds, G≡C with 3. So A = T, G = C and (A + G) = (T + C)
• Double helix: 10 base pairs per turn, 3.4 nm per turn, 0.34 nm between base pairs, 2 nm wide; nucleotides in one strand joined by phosphodiester bonds
• Replication: helicase unwinds; primase makes an RNA primer; DNA polymerase III adds nucleotides 5′ → 3′; leading strand continuous, lagging strand in Okazaki fragments joined by DNA ligase
• Transcription: RNA polymerase copies the template strand into mRNA; mRNA has the same sequence as the coding strand with U in place of T
• Genetic code: 64 triplet codons; 61 code for amino acids, 3 are stop codons (UAA, UAG, UGA); AUG is the start codon (methionine)
• mRNA carries the code, tRNA carries amino acids and has the anticodon, rRNA builds ribosomes

Biology · Cell structure and function

Cell membrane and transport

• Diffusion: net movement from higher to lower concentration; passive; no protein needed
• Facilitated diffusion: down the gradient through a specific channel or carrier protein; passive; rate levels off when all carriers are in use
• Osmosis: net movement of water through a selectively permeable membrane from higher water potential (dilute solution) to lower water potential (concentrated solution)
• Water potential (ψ): pure water = 0; any solution is negative; more solute = more negative
• Active transport: against the gradient, carrier protein + ATP; sodium–potassium pump moves 3 Na⁺ out and 2 K⁺ in per ATP
• Endocytosis (into the cell): phagocytosis = solid particles, pinocytosis = fluid droplets. Exocytosis: vesicles fuse with the membrane and release their contents. Both need ATP
• Animal cell: bursts in a hypotonic solution, shrinks in a hypertonic one. Plant cell: turgid in hypotonic, plasmolysed in hypertonic

Organelles

• Nucleus: double membrane (envelope) with pores; chromatin = DNA + histone proteins; nucleolus makes rRNA and ribosome subunits
• Rough ER: ribosomes attached, makes and transports proteins. Smooth ER: no ribosomes, makes lipids and steroids, detoxifies drugs
• Ribosomes: no membrane; 80S (60S + 40S) in eukaryotic cytoplasm, 70S (50S + 30S) in prokaryotes, mitochondria and chloroplasts
• Golgi apparatus: receives vesicles at the forming (cis) face, adds carbohydrate to make glycoproteins, releases vesicles and lysosomes from the maturing (trans) face
• Lysosomes: single membrane, hydrolytic enzymes; digest food particles and worn-out organelles (autophagy)
• Mitochondrion: inner membrane folded into cristae, matrix inside. Chloroplast: thylakoids stacked into grana, stroma around them
• Microtubules (tubulin): spindle, cilia, centrioles (9 triplets). Microfilaments (actin): cyclosis, amoeboid movement. Plant vacuole is bounded by the tonoplast; plant cell wall is cellulose and fully permeable

Prokaryotic and eukaryotic cells

• Prokaryote: 70S ribosomes, wall of peptidoglycan (murein), circular DNA without histones, no mitochondria (respiratory enzymes on the plasma membrane), divides by binary fission
• Eukaryote: 80S ribosomes in the cytoplasm, linear DNA with histones in a nucleus, membrane-bound organelles, divides by mitosis or meiosis
• Typical size: prokaryotic cell about 1 to 10 μm; eukaryotic cell about 10 to 100 μm
• Plant cell only: cellulose wall, chloroplasts and other plastids, large central vacuole. Animal cell only: centrioles; stores glycogen, not starch
• magnification = image size ÷ actual size; 1 mm = 1000 μm; 1 μm = 1000 nm
• Light microscope: lower resolution, can view living cells. Electron microscope: resolution to below 1 nm, specimen dead and in a vacuum; TEM shows internal detail, SEM shows 3-D surfaces
• Structure fits function: sperm has many mitochondria for movement; red blood cell has no nucleus, more room for haemoglobin; root hair has a large surface area

Biology · Coordination and control

Neurons and the nerve impulse

• Resting potential: sodium–potassium pump moves 3 Na⁺ out and 2 K⁺ in; the membrane leaks K⁺ out far more easily than Na⁺ in; large negative ions stay inside
• Depolarisation = Na⁺ diffuses in. Repolarisation = K⁺ diffuses out. The pump then restores the original ion balance
• All-or-none: below threshold there is no impulse; above it every impulse has the same size. A stronger stimulus gives more impulses per second, not bigger ones
• Myelin sheath (Schwann cells) insulates the axon; the impulse jumps between nodes of Ranvier (saltatory conduction), up to about 100 m s⁻¹
• Synapse: impulse arrives → Ca²⁺ enters the synaptic knob → vesicles release neurotransmitter → it diffuses across the cleft → binds receptors on the postsynaptic membrane → new impulse
• Acetylcholine is broken down by acetylcholinesterase. A synapse passes impulses in one direction only
• Sensory neuron cell bodies lie in the dorsal root ganglion; Nissl's granules are groups of ribosomes in the cell body

The nervous system

• Cerebrum (cerebral cortex): thinking, memory, intelligence, voluntary movement, senses. Each hemisphere controls the opposite side of the body; the corpus callosum joins the two
• Thalamus: relays sensory impulses to the cortex. Hypothalamus: temperature, hunger, thirst, controls the pituitary. Amygdala and hippocampus: emotion and memory
• Cerebellum: balance, posture, coordination of movement. Medulla oblongata: breathing, heartbeat, blood pressure. Pons: links cerebellum, medulla and cerebrum
• Spinal cord: grey matter (cell bodies) inside in an H shape, white matter (myelinated axons) outside; the cerebrum has grey matter outside
• Reflex arc: receptor → sensory neuron (dorsal root) → interneuron in the spinal cord → motor neuron (ventral root) → effector
• Sympathetic system: prepares for emergency (faster heart, dilated pupils, slower digestion). Parasympathetic: rest (slower heart, constricted pupils, active digestion)
• Receptors: chemoreceptors (taste, smell), mechanoreceptors (touch, pressure, hearing), photoreceptors (light), thermoreceptors (temperature), nociceptors (pain). Disorders: Parkinson's (low dopamine, tremor), epilepsy (seizures), Alzheimer's (loss of memory)

Chemical coordination

• Anterior pituitary: growth hormone, TSH (thyroid), ACTH (adrenal cortex), FSH, LH, prolactin (milk production). Posterior pituitary releases ADH (water reabsorption in the kidney) and oxytocin (labour, milk ejection), both made in the hypothalamus
• Thyroid: thyroxine raises metabolic rate and needs iodine; calcitonin lowers blood Ca²⁺. Parathyroid: parathormone raises blood Ca²⁺
• Pancreas (islets of Langerhans): β cells, insulin lowers blood glucose (glucose → glycogen); α cells, glucagon raises it (glycogen → glucose)
• Adrenal medulla: adrenaline and noradrenaline (emergency). Adrenal cortex: cortisol (raises blood glucose in stress), aldosterone (Na⁺ reabsorption)
• Gonads: testosterone (testes); oestrogen and progesterone (ovaries)
• Growth hormone: too little in childhood = dwarfism; too much in childhood = gigantism; too much in adults = acromegaly
• Thyroxine: too little = cretinism (child), myxoedema (adult); too much = Graves' disease. Insulin lack = diabetes mellitus; ADH lack = diabetes insipidus; cortex hormones low = Addison's, high = Cushing's

Biology · Enzymes

Enzyme action

• Activation energy = energy of the peak − energy of the reactants. An enzyme lowers the peak only.
• Lock-and-key model (Fischer): the active site is rigid and the substrate (key) fits it exactly.
• Induced-fit model (Koshland): the active site changes shape slightly as the substrate binds, to fit it closely.
• Holoenzyme (active) = apoenzyme (protein part) + cofactor (non-protein part).
• Activator: a detachable inorganic ion, e.g. Mg²⁺, Cl⁻.
• Coenzyme: an organic cofactor that is loosely attached, e.g. NAD, FAD; many are made from vitamins.
• Prosthetic group: a non-protein part bound tightly (covalently) to the protein, e.g. haem.

Factors and inhibition

• Enzyme concentration: with excess substrate, rate is directly proportional to the amount of enzyme.
• Substrate concentration: rate rises, then levels off at a maximum when every active site is occupied (saturation).
• Optimum pH: pepsin about 2, salivary amylase about 6.8, pancreatic lipase about 9.
• Competitive inhibitor: resembles the substrate and binds at the active site; more substrate overcomes it, so the maximum rate is still reached.
• Non-competitive inhibitor: binds at another site and changes the shape of the active site; more substrate does not help, so the maximum rate is lower.
• Irreversible inhibitors (e.g. cyanide) bind by strong covalent bonds; the enzyme stays inactive.
• Feedback inhibition: the end product of a pathway slows an early enzyme of that pathway.

Biology · Evolution

Theories of evolution

• Lamarck: use and disuse of organs, and inheritance of acquired characters.
• Darwin: overproduction → struggle for existence → variation → survival of the fittest → inheritance of the favoured variations.
• Darwin published 'On the Origin of Species' in 1859; Malthus gave him the idea that populations outgrow their food supply.
• Fitness means leaving the most offspring that survive to breed, not being the strongest.
• Mutation is the only source of new alleles; crossing over and independent assortment only make new combinations.
• Only mutations in germ cells (which form gametes) can be inherited.
• Hardy-Weinberg: allele frequencies stay constant if the population is large, mates at random and has no mutation, migration or selection.

Evidence and speciation

• Fossils: in undisturbed sedimentary rock, deeper layers are older. Archaeopteryx links reptiles and birds.
• Homologous organs: same basic structure and origin, different function (human arm, bat wing, whale flipper); show divergent evolution from a common ancestor.
• Analogous organs: same function, different structure and origin (bird wing, insect wing); show convergent evolution.
• Vestigial organs: reduced and no longer used for their original function, e.g. coccyx, appendix.
• Embryology: early vertebrate embryos all have gill pouches and a tail.
• Biochemistry: the fewer the differences in a protein or in DNA, the more recent the common ancestor.
• Adaptive radiation: one ancestor gives many species suited to different ways of life, e.g. Galápagos finches.

Biology · Reproduction

Human reproductive systems

• Spermatogonium (2n) → primary spermatocyte (2n) → 2 secondary spermatocytes (n) → 4 spermatids (n) → 4 sperm.
• Oogonium (2n) → primary oocyte (2n) → secondary oocyte (n) + first polar body → ovum (n) + second polar body.
• One primary spermatocyte gives 4 sperm; one primary oocyte gives 1 ovum.
• Primary oocytes are held at prophase I from before birth; meiosis II is completed only after a sperm enters.
• Spermatogenesis begins at puberty and is continuous; oogenesis begins before birth and ends at menopause.
• In the male, FSH promotes spermatogenesis; LH stimulates the interstitial cells to secrete testosterone.
• Seminal vesicles secrete fructose, the energy source of sperm.

Reproductive cycle and development

• Order of events: menstruation (days 1 to 5) → follicle growth and oestrogen → LH surge and ovulation (about day 14) → corpus luteum and progesterone.
• Oestrogen and progesterone inhibit FSH and LH (negative feedback); this is how the contraceptive pill prevents ovulation.
• hCG from the embryo keeps the corpus luteum alive in early pregnancy; later the placenta makes the progesterone.
• Cleavage is mitosis with little growth: zygote → morula → blastocyst, which implants.
• Germ layers: ectoderm → nervous system and epidermis; mesoderm → muscle, skeleton, blood; endoderm → lining of gut and lungs.
• Placenta: maternal and foetal blood do not mix. The umbilical cord has two arteries (deoxygenated blood to the placenta) and one vein.
• STDs: gonorrhoea (Neisseria gonorrhoeae), syphilis (Treponema pallidum), AIDS (HIV), genital herpes (herpes simplex virus).

Biology · Support and movement

Skeleton and joints

• Vertebrae: 7 cervical, 12 thoracic, 5 lumbar, then the sacrum (5 fused) and coccyx (4 fused). Ribs: 12 pairs.
• Bone cells: osteoblasts build bone, osteocytes maintain it, osteoclasts dissolve it and release calcium.
• Cartilage: hyaline (ends of bones, nose, trachea), elastic (pinna, epiglottis), fibrocartilage (intervertebral discs, pubic symphysis).
• Joints: fibrous, immovable (skull sutures); cartilaginous, slightly movable (between vertebrae); synovial, freely movable.
• Synovial types: hinge (elbow, knee), ball and socket (shoulder, hip), pivot (atlas and axis).
• Osteoarthritis: articular cartilage wears away with age. Rheumatoid arthritis: the immune system attacks the joint lining.
• Osteoporosis: loss of bone mass, so bones become porous and break easily; common after menopause as oestrogen falls.

Muscles and contraction

• A band = full length of the myosin filaments (dark); I band = actin only (light); H zone = myosin only, in the middle of the A band.
• On contraction the A band stays the same; the I band and H zone get shorter; Z lines move closer.
• Ca²⁺ is released from the sarcoplasmic reticulum, binds troponin, and tropomyosin moves off the binding sites on actin.
• Myosin heads are ATPases. ATP is needed for the head to detach from actin; without ATP the muscle stays stiff (rigor mortis).
• Skeletal: striated, voluntary, many nuclei, unbranched. Cardiac: striated, involuntary, branched, with intercalated discs.
• Smooth: unstriated, involuntary, spindle-shaped cells with one nucleus; found in the gut and blood vessels.
• Tendon joins muscle to bone; ligament joins bone to bone. Origin is the fixed end, insertion the moving end.

Biology · Inheritance

Mendelian inheritance

• Monohybrid cross Aa × Aa: genotypes 1 AA : 2 Aa : 1 aa; phenotypes 3 dominant : 1 recessive.
• Test cross: unknown dominant phenotype × homozygous recessive. All dominant offspring → parent is AA; 1 : 1 → parent is Aa.
• Dihybrid cross AaBb × AaBb: phenotypes 9 : 3 : 3 : 1 (out of 16).
• Dihybrid test cross AaBb × aabb: 1 : 1 : 1 : 1.
• Number of kinds of gamete = 2ⁿ, where n is the number of heterozygous gene pairs.
• For independent genes, multiply the separate probabilities (product rule).

Beyond simple dominance

• Incomplete dominance, heterozygote × heterozygote: 1 : 2 : 1 for both phenotype and genotype.
• ABO: I^A and I^B are codominant and both are dominant to i. Group A = I^AI^A or I^Ai; B = I^BI^B or I^Bi; AB = I^AI^B; O = ii.
• Three alleles give 6 genotypes and 4 phenotypes.
• Group O has no A or B antigens (universal donor of red cells); group AB has no anti-A or anti-B antibodies (universal recipient).
• Epistasis changes 9 : 3 : 3 : 1 to 9 : 3 : 4 (recessive epistasis) or 12 : 3 : 1 (dominant epistasis).
• Rh factor: Rh-positive (D) is dominant to Rh-negative (d).
• Haemolytic disease of the newborn: Rh-negative mother, Rh-positive foetus, usually in a second or later such pregnancy.

Sex linkage and chromosomes

• A father gives his X to all his daughters and his Y to all his sons; a son's X always comes from his mother.
• Carrier mother × normal father: half the sons affected, half the daughters carriers, no daughter affected.
• Affected father × normal (non-carrier) mother: all daughters carriers, all sons normal.
• Haemophilia A is a lack of clotting factor VIII.
• Down syndrome: trisomy 21, 47 chromosomes. Turner syndrome: 45, XO, a sterile female. Klinefelter syndrome: 47, XXY, a sterile male.
• In humans the presence of a Y chromosome gives a male.
• Other systems: XO in grasshoppers (male has one X only); ZW in birds (the female is ZW and decides the sex).

Biology · Circulation

The heart

• Path: venae cavae → right atrium → tricuspid valve → right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium → bicuspid (mitral) valve → left ventricle → aorta.
• The pulmonary artery carries deoxygenated blood; the pulmonary veins carry oxygenated blood.
• Impulse: SA node → AV node (short delay) → bundle of His → Purkinje fibres → ventricles contract from the apex upwards.
• Cardiac cycle at 75 beats per minute = 60 ÷ 75 = 0.8 s: atrial systole 0.1 s, ventricular systole 0.3 s, diastole of the whole heart 0.4 s.
• First sound 'lubb': the AV (tricuspid and bicuspid) valves close as the ventricles start to contract.
• Second sound 'dupp': the semilunar valves close as the ventricles start to relax.
• Sympathetic nerves speed up the SA node; the vagus (parasympathetic) nerve slows it.

Blood and blood vessels

• Normal resting blood pressure is about 120/80 mm Hg (systolic/diastolic).
• Blood flows slowest in the capillaries, because their total cross-sectional area is the greatest.
• Blood returns up the leg veins by the squeezing of skeletal muscles, with valves stopping backflow.
• Red blood cells: biconcave, no nucleus, carry oxygen on haemoglobin; made in red bone marrow; live about 120 days.
• White blood cells (about 7 000 to 8 000 per mm³): neutrophils (most common, phagocytes), lymphocytes (immunity), monocytes (largest, become macrophages), eosinophils (allergy, worms), basophils (heparin and histamine).
• Platelets are cell fragments that start clotting: prothrombin → thrombin (needs Ca²⁺); thrombin turns fibrinogen → fibrin.
• Plasma proteins: albumin (osmotic pressure), globulins (antibodies), fibrinogen (clotting).

Lymphatic system and disorders

• Net filtration pressure = blood (hydrostatic) pressure − osmotic pressure of the plasma
• Arterial end: hydrostatic pressure is greater, so fluid leaves. Venous end: osmotic pressure is greater, so fluid returns
• Lymph nodes filter lymph and hold lymphocytes and macrophages; lacteals in the villi absorb digested fats
• Oedema = swelling from excess tissue fluid (low plasma protein, blocked lymph vessels or high blood pressure)
• Atherosclerosis = cholesterol-rich plaque (atheroma) in the artery wall; arteriosclerosis = hardening of the wall, with calcium deposits
• Thrombus = a clot fixed where it formed; embolus = a clot or fragment carried in the blood
• Hypertension = blood pressure that stays at about 140/90 mm Hg or above; normal is about 120/80 mm Hg

Biology · Immunity

Innate and adaptive immunity

• Phagocytosis: attachment to the microbe → engulfing into a vacuole (phagosome) → lysosomes fuse with it → enzymes digest the microbe
• Inflammation: mast cells release histamine → arterioles dilate and capillaries become leaky → redness, heat, swelling and pain
• Interferon is released by virus-infected cells and makes neighbouring cells resist viruses
• B cells mature in the bone marrow; T cells are made in the bone marrow and mature in the thymus
• An activated B cell divides to form plasma cells (which secrete antibodies) and memory cells
• Helper T cells activate B cells and killer T cells; killer (cytotoxic) T cells destroy infected body cells
• Antibody = 2 heavy chains + 2 light chains joined by disulphide bonds; the variable regions at the tips form 2 antigen-binding sites

Types of immunity and vaccination

• Natural active: recovering from an infection. Artificial active: vaccination
• Natural passive: antibodies across the placenta or in breast milk (colostrum). Artificial passive: an injection of antiserum or antitoxin
• A vaccine may be a live weakened (attenuated) microbe, a killed microbe, a toxoid (harmless toxin) or a single antigen
• Primary response: slow start, small amount of antibody, short-lived. Secondary response: quick, much more antibody, long-lasting
• Allergy = an excessive response to a harmless antigen (allergen); mast cells release histamine
• Autoimmune disease = the immune system attacks the body's own tissues, e.g. rheumatoid arthritis, type 1 diabetes, multiple sclerosis
• Edward Jenner introduced the first vaccine, against smallpox, in 1796

Biology · Respiration

Respiratory system and breathing

• Inspiration (active): diaphragm contracts and flattens; external intercostal muscles contract, ribs move up and out; volume rises, pressure falls, air enters
• Quiet expiration (passive): diaphragm and external intercostals relax; elastic recoil of the lungs; volume falls, pressure rises, air leaves
• Forced expiration is active: internal intercostal muscles and abdominal muscles contract
• Trachea and bronchi have cartilage; bronchioles have none, only smooth muscle. Goblet cells make mucus and cilia sweep it up to the pharynx
• Tidal volume = air breathed in or out in one quiet breath, about 500 cm³ (0.5 dm³)
• Vital capacity = tidal volume + inspiratory reserve volume + expiratory reserve volume
• Total lung capacity = vital capacity + residual volume; minute volume = tidal volume × breathing rate

Gas transport and disorders

• Hb + 4O₂ ⇌ Hb(O₂)₄ (oxyhaemoglobin); the oxygen dissociation curve is S-shaped (sigmoid)
• Bohr effect: more CO₂ or lower pH shifts the curve to the right (higher temperature does the same), so haemoglobin gives up more oxygen
• CO₂ transport: about 70% as hydrogencarbonate ions, most of the rest as carbaminohaemoglobin, a small amount dissolved in plasma
• In red cells: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻, speeded up by carbonic anhydrase
• Chloride shift: HCO₃⁻ diffuses out of the red cell and Cl⁻ moves in to keep the charges balanced
• Smoke: tar causes cancer and damages cilia; nicotine is addictive and raises heart rate and blood pressure; carbon monoxide cuts oxygen carriage
• Emphysema = alveolar walls break down, less surface; pneumonia = alveoli fill with fluid; tuberculosis = Mycobacterium tuberculosis, prevented by BCG vaccine

Biology · Digestion

Alimentary canal and enzymes

• Salivary amylase (salivary glands) and pancreatic amylase (pancreas): starch → maltose
• Pepsin (gastric glands; secreted as pepsinogen and activated by HCl): proteins → shorter polypeptides
• Trypsin (pancreas; secreted as trypsinogen and activated by enterokinase): proteins and polypeptides → smaller peptides
• Lipase (mainly pancreas): fats → fatty acids + glycerol
• Intestinal wall: maltase gives glucose; sucrase gives glucose + fructose; lactase gives glucose + galactose; peptidases give amino acids
• HCl kills microbes and activates pepsinogen; mucus protects the stomach wall; gastrin stimulates the release of gastric juice
• Bile is made in the liver and stored in the gall bladder; the colon absorbs water and its bacteria make vitamin K

Absorption, liver and disorders

• Glucose and amino acids: absorbed by diffusion and active transport (cells have many mitochondria) → blood capillaries → hepatic portal vein
• Fatty acids and glycerol → reformed into fats → lacteal → lymph; fat-soluble vitamins A, D, E and K are absorbed with fats and need bile
• Liver: makes bile, stores glucose as glycogen, deaminates excess amino acids and makes urea, detoxifies poisons, makes plasma proteins (albumin, fibrinogen, prothrombin), stores iron and vitamins
• Pancreas: exocrine part sends pancreatic juice into a duct; endocrine part (islets of Langerhans) sends insulin and glucagon into the blood
• Peptic ulcer = a sore where acid and pepsin erode the wall of the stomach or duodenum; often linked with Helicobacter pylori
• Dyspepsia = indigestion, with discomfort in the upper abdomen
• Food poisoning = illness from food carrying bacteria or their toxins, e.g. Salmonella, Campylobacter

Biology · Homeostasis

Osmoregulation and the kidney

• The afferent arteriole is wider than the efferent arteriole, so the blood pressure in the glomerulus is high
• Filtrate contains water, glucose, amino acids, salts and urea; blood cells and plasma proteins are too large to pass
• Proximal convoluted tubule: reabsorbs all the glucose and amino acids and most of the water and salts
• Descending limb of the loop is permeable to water; ascending limb moves salts out and is impermeable to water
• Less water in the blood → more ADH from the posterior pituitary → collecting ducts more permeable to water → small volume of concentrated urine
• Too little ADH = diabetes insipidus: large volumes of dilute urine with no glucose
• Kidney stones are mostly calcium oxalate (lithotripsy breaks them with shock waves); in kidney failure, dialysis removes urea by diffusion across a partially permeable membrane

Thermoregulation and excretion

• Too hot: vasodilation of skin arterioles, sweating (cooling by evaporation), hairs lie flat, lower metabolic rate
• Too cold: vasoconstriction of skin arterioles, shivering (heat from skeletal muscles), hairs raised to trap air, more thyroxine and adrenaline raise the metabolic rate
• Fever: pyrogens raise the set point of the hypothalamus
• Ammonia: very toxic, very soluble, needs most water; aquatic animals such as bony fishes (ammonotelic)
• Urea: less toxic, soluble, needs moderate water; mammals and adult amphibians (ureotelic)
• Uric acid: least toxic, almost insoluble, needs least water; birds, reptiles and insects (uricotelic)
• Urea cycle: ornithine → citrulline → arginine; arginase splits arginine into urea + ornithine. Overall: 2NH₃ + CO₂ → CO(NH₂)₂ + H₂O

Biology · Biotechnology

Recombinant DNA techniques

• Restriction enzymes (restriction endonucleases) cut DNA at specific palindromic sites, e.g. EcoRI at 5′-GAATTC-3′, often leaving sticky ends
• DNA ligase seals the sugar-phosphate backbone and joins the gene to the vector
• Cloning steps: isolate the gene → cut gene and plasmid with the same enzyme → join with ligase → transform host cells → select (antibiotic resistance marker) → grow
• One PCR cycle: denaturation at about 94 °C → primer annealing at about 50 to 60 °C → extension by Taq polymerase at about 72 °C
• Number of DNA molecules after n PCR cycles = starting number × 2ⁿ
• Electrophoresis: DNA is negatively charged and moves towards the positive electrode; smaller fragments travel further
• Sanger sequencing uses dideoxynucleotides, which lack a 3′-OH group and so stop the growing chain

Applications of biotechnology

• Human insulin from recombinant bacteria treats diabetes mellitus; recombinant growth hormone treats pituitary dwarfism
• Hepatitis B vaccine is a viral surface protein made in yeast; it has no whole virus, so it cannot cause the disease
• Gene therapy of body (somatic) cells is not inherited, because the eggs and sperm still carry the faulty allele
• Bt crops carry a gene from Bacillus thuringiensis for a protein that kills insect larvae; golden rice makes beta-carotene, which the body turns into vitamin A
• DNA fingerprinting compares repeated non-coding sequences (VNTRs or STRs); only identical twins share a profile, and every band in a child comes from the mother or the father
• Reproductive cloning by nuclear transfer (Dolly, 1996): the clone is genetically identical to the animal that gave the nucleus
• Ethical issues include gene flow to wild plants, long-term safety of GM food, privacy of genetic data and the cloning of humans