Respiration, Internal Environment, Coordination and Gene Technology

Sprint for a bus and your body performs a symphony: muscles burn glucose faster, the heart doubles its output, ventilation deepens, sweat starts, and the kidney re-tunes its water recovery — all controlled without a conscious thought. This unit follows that machinery: respiration at molecular scale, muscle and heart at tissue scale, nerves and hormones as the controllers, and gene technology as the modern window onto it.

WBI15 Respiration, Internal Environment, Coordination and Gene Technology is the second IA2 unit: 1 hour 45 minutes, 90 marks. It assesses Topic 7 (Respiration, Muscles and the Internal Environment) and Topic 8 (Coordination, Response and Gene Technology), and one 20-mark question is built on a pre-released scientific article — you will have studied it in advance.

Respiration releases energy in steps, each catalysed by a specific enzyme. Aerobic respiration's summary:

Anaerobic conditions stop the chain — reduced NAD cannot be recycled — so pyruvate accepts its hydrogen and becomes lactate 乳酸 in muscle. After exercise, lactate is carried to the liver and rebuilt into glucose (needing oxygen: the oxygen debt 氧债).

Respiratory quotient 呼吸商: $\mathrm{RQ} = \mathrm{CO_2}\ \text{produced} / \mathrm{O_2}\ \text{consumed}$. Carbohydrate 1.0, protein 0.9, lipid ≈ 0.7 — an organism's RQ reveals what it is burning.

Worked check. A respirometer shows 60 cm³ CO₂ produced and 60 cm³ O₂ consumed in the same interval: RQ = 1.0 — carbohydrate. A germinating seed with RQ 0.7 is burning lipid reserves.

A tendon 肌腱 joins muscle to bone; a ligament 韧带 joins bone to bone across a joint; muscles work in antagonistic pairs. A skeletal muscle fibre is a fused, multinucleate cell packed with myofibrils 肌原纤维 of actin 肌动蛋白 and myosin 肌球蛋白.

Contraction is the sliding filament cycle: an action potential releases Ca²⁺; calcium exposes actin's binding sites; the energised myosin head (its ATP split) attaches, pulls the actin, then detaches when new ATP binds — thousands of heads per second, each stroke a few nanometres.

Fast-twitch fibres: thick, few mitochondria, quick and powerful, fatigue fast (sprinting). Slow-twitch: many mitochondria, rich blood supply, myoglobin — endurance.

Cardiac muscle is myogenic 自主性的: it beats without nerve input. The sino-atrial node 窦房结 sets the pace; the wave spreads over the atria to the atrio-ventricular node, then along Purkyne fibres to the ventricles — atria contract first, ventricles a fraction later.

The cardiovascular control centre and the ventilation centre in the medulla oblongata 延髓 adjust both systems to demand: chemical receptors sense CO₂ and pH, baroreceptors pressure.

Worked check. At rest: 70 cm³ × 72 min⁻¹ ≈ 5.0 dm³ min⁻¹. During exercise: 120 cm³ × 150 min⁻¹ = 18 dm³ min⁻¹ — the table the exam loves. Ventilation rises in step, read from spirometer traces (tidal volume × breathing rate = minute ventilation 每分钟通气量).

Homeostasis 稳态 holds the internal environment in dynamic equilibrium — not stillness, but variables oscillating inside narrow limits. Negative feedback 负反馈 corrects deviations in either direction (the thermostat pattern); positive feedback 正反馈 amplifies them (a spiral, not a loop — as in an action potential or fever runaway).

Thermoregulation runs through the hypothalamus 下丘脑: heat loss by vasodilation and sweating; heat conservation by vasoconstriction, shivering and hair erection. Adrenaline 肾上腺素 prepares the fight-or-flight response — heart rate, blood glucose and airways all respond together.

Blood enters the nephron 肾单位's glomerulus at high pressure — the afferent arteriole is wider than the efferent — and small molecules are forced through podocytes and the basement membrane into Bowman's capsule 肾小囊 (ultrafiltration: everything except cells and large proteins). The proximal convoluted tubule 近曲小管 reabsorbs all glucose and amino acids plus most salts and water (active transport then osmosis). The loop of Henle 亨勒环 builds a water-potential gradient down the medulla by countercurrent multiplication. The collecting duct 集合管 makes the final decision: ADH 抗利尿激素 from the pituitary inserts aquaporins so more water is reabsorbed — concentrated urine when dehydrated, dilute when not.

Excess amino acids are deaminated 脱氨 in the liver; the toxic ammonia becomes urea 尿素 for excretion.

Worked check. ADH present → collecting duct walls permeable → water leaves down the gradient into the medullary blood → small volume, concentrated urine. Alcohol suppresses ADH — large volume, dilute.

A transcription factor 转录因子 binds a gene's promoter and switches transcription on or off. Steroid hormones diffuse through the membrane and act as transcription factors themselves — one gene can serve several tissues. Epigenetics 表观遗传学 silences genes without changing the base sequence: methylation 甲基化 of DNA and modification of histones (acetylation) pack the DNA out of the transcription machinery's reach. Failures matter: a methylated tumour-suppressor gene cannot hold cell division in check; imprinting errors explain Prader–Willi syndrome.

The resting neurone pumps Na⁺ out and K⁺ in (the sodium-potassium pump 钠钾泵), holding an interior at about −70 mV. A stimulus past threshold 阈值 (about −55 mV) opens sodium channels: an all-or-nothing 全或无 action potential spikes to +30 mV, then potassium channels repolarise; the refractory period 不应期 prevents backward travel and sets a maximum frequency.

In myelinated 有髓 neurones the impulse jumps node to node (saltatory conduction 跳跃传导) — far faster. At the synapse 突触, calcium entry triggers vesicles of neurotransmitter to fuse with the presynaptic membrane; the transmitter diffuses and binds postsynaptic receptors; enzymes then clear the gap. Drugs act here: SSRIs block serotonin re-uptake, so the signal persists; lidocaine blocks sodium channels; ecstasy (MDMA) floods the cleft with serotonin then depletes it.

The nervous system divides: CNS (brain, spinal cord) and PNS — somatic (voluntary) and autonomic (involuntary; sympathetic accelerates, parasympathetic calms). Receptors are transducers: rods (dim light, many per bipolar cell — sensitivity), cones (colour, one-to-one — acuity), plus thermoreceptors, chemoreceptors, baroreceptors, nociceptors 伤害感受器 (pain). The spinal reflex arc 反射弧 passes sensory → relay → motor neurone in the cord's grey matter — fast, involuntary, protective. Habituation 习惯化 is a decreasing response to a repeated harmless stimulus.

Phytochrome 光敏色素 flips between Pr (absorbs red) and Pfr (absorbs far-red); sunlight's red bias leaves Pfr dominant, controlling germination and flowering by day length. Auxin 生长素 elongates cells by acidifying walls; gibberellins 赤霉素 induce enzymes (amylase in the aleurone layer) that mobilise starch in germination.

Landmarks and jobs: cerebral hemispheres 大脑半球 (voluntary, thought), cerebellum 小脑 (coordination, balance), hypothalamus and pituitary 垂体 below them (homeostasis, hormones), medulla oblongata (heart, breathing). MRI shows structure; fMRI shows activity via oxygenated-blood flow; CT fast structural scans; PET metabolism with radioactive tracers.

Chemistry first: Parkinson's is a dopamine 多巴胺 deficit — treated with L-Dopa, which crosses the blood-brain barrier and is converted to dopamine in the brain. Low serotonin links to depression — the target of SSRIs.

Recombinant DNA 重组DNA: a restriction enzyme 限制性内切酶 cuts DNA at its recognition sequence, leaving sticky ends 黏性末端; the human gene (made without introns from mRNA) is spliced into a plasmid 质粒 with DNA ligase 连接酶; the plasmid transforms host bacteria; marker genes (antibiotic resistance or fluorescence) select the cells that took it up. Scale up in a fermenter and the bacteria secrete human insulin 胰岛素 — cleaner and safer than extracting from pigs. GM animals make human proteins in milk; risks and benefits of GM crops (resistance, yields versus gene flow to wild relatives) are the standard evaluation.

Microarrays 微阵列 identify active genes: sample mRNA hybridises to complementary probes on the chip; spots that fluoresce mark genes being transcribed. Bioinformatics 生物信息学 compares sequences across species and individuals — how relatedness is now measured.

1. Name the stage of respiration occurring in the mitochondrial matrix and one of its inputs.
2. A runner's muscles produce lactate. Explain why, and what happens to the lactate afterwards.
3. Myosin heads cannot detach without ATP. Explain what this means for muscles after death.
4. Calculate cardiac output for stroke volume 85 cm³ and heart rate 140 min⁻¹.
5. Explain why the afferent arteriole is wider than the efferent arteriole.
6. State two differences between negative and positive feedback, with one example of each.
7. Why does myelination increase conduction speed?
8. An axon's threshold is −55 mV. A stimulus moves it to −60 mV. What happens, and why?
9. Red light converts phytochrome to which form?
10. Explain why L-Dopa is given instead of dopamine itself.
11. A microarray spot fluoresces strongly. What does that show?
12. Why must the human insulin gene be made from mRNA rather than from genomic DNA when inserting into bacteria?

Answers: 1 the link reaction (or Krebs cycle) — input pyruvate (or acetyl); 2 oxygen is short so reduced NAD is recycled by reducing pyruvate to lactate; afterwards lactate goes to the liver and is converted back to glucose, using oxygen; 3 without ATP the myosin stays bound to actin — rigor mortis; 4 $85 \times 140 = 11\,900\ \text{cm}^3\,\text{min}^{-1} = 11.9\ \text{dm}^3\,\text{min}^{-1}$; 5 the pressure difference drives ultrafiltration of plasma into Bowman's capsule; 6 negative returns a variable toward its set point (thermoregulation); positive pushes it further away (fever rising); 7 the impulse jumps between nodes of Ranvier instead of depolarising the whole membrane — fewer, bigger steps; 8 nothing — threshold is not reached, so no action potential (all-or-nothing); 9 Pfr; 10 dopamine cannot cross the blood-brain barrier but L-Dopa can, and is converted to dopamine inside the brain; 11 that gene was being actively transcribed — its mRNA was present and hybridised; 12 bacteria cannot splice introns, so the gene must come from mature mRNA that already lacks them.