Homeostasis; the framework
- Definition
- Maintenance of stable internal conditions within narrow limits despite external change.
- Components
- Receptor (detects stimulus) → Control centre (compares to set point; usually hypothalamus) → Effector (restores set point).
- Negative feedback
- Response opposes the change. The default mechanism.
- Positive feedback
- Response amplifies the change. Examples: childbirth (oxytocin), blood clotting.
Three feedback loops you must know
| System | Receptor | Control | Effector → Effect |
|---|
| Thermoregulation | Skin & hypothalamus thermoreceptors | Hypothalamus (~37 °C set point) | Sweat glands, blood vessels, muscle (shiver) → restore temp |
| Blood glucose | Pancreatic α/β cells | Pancreas + liver | Insulin (β) lowers BG; glucagon (α) raises BG |
| Osmoregulation | Hypothalamic osmoreceptors | Posterior pituitary (ADH) | Kidney tubules → ↑ water reabsorption |
Mnemonic for thermoregulation: "Cool down = Sweat, Vasodilation, Slow metabolism. Warm up = Shiver, Vasoconstriction, Pilo-erection, ↑ thyroxin."
Immunity; the three lines of defence
- 1st line (non-specific)
- Skin, mucous, cilia, stomach acid, lysozyme in tears, normal flora.
- 2nd line (innate)
- Inflammation, fever, phagocytosis (macrophages, neutrophils), NK cells, complement, interferons.
- 3rd line (adaptive)
- B-cells (humoral) + T-cells (cell-mediated). Specific. Slower. Has memory.
| B-cells | T-cells |
|---|
| Mature in | Bone marrow | Thymus |
| Acts on | Free antigen | Infected/abnormal cells |
| Effectors | Plasma cells → antibodies | Helper (CD4), Cytotoxic (CD8) |
| Memory | Yes | Yes |
- Antibody actions
- Neutralisation, agglutination, opsonisation, complement activation.
- Active immunity
- Body makes own antibodies. From infection (natural) or vaccination (artificial). Has memory.
- Passive immunity
- Antibodies received from elsewhere. From placenta/breastmilk (natural) or injection (artificial). No memory.
Disease; infectious vs non-infectious
| Pathogen | Examples | Treatment |
|---|
| Bacteria | TB, Strep, Salmonella | Antibiotics |
| Virus | HIV, influenza, COVID-19 | Antivirals (limited) |
| Fungi | Tinea, Candida | Antifungals |
| Protozoa | Malaria, Giardia | Antiprotozoals |
| Prion | CJD | None |
- Transmission
- Droplet, fomite, food/water, vector (mosquito), sexual, vertical (mother→fetus).
- Endemic / epidemic / pandemic
- Constant in a population / sudden ↑ in a region / across multiple countries.
- Non-infectious diseases
- Cancer (mutation → uncontrolled cell division), cardiovascular (atherosclerosis → MI/stroke), Type 2 diabetes (insulin resistance), asthma, autoimmune.
DNA, gene expression & mutation
- DNA structure
- Double helix; sugar-phosphate backbone; bases A-T (2 H-bonds), G-C (3 H-bonds); antiparallel 5'→3' / 3'→5'.
- Replication
- Semi-conservative. Helicase unwinds → DNA polymerase adds free nucleotides → two identical daughter strands. Site: nucleus, S-phase.
- Transcription
- Nucleus. DNA → mRNA. RNA polymerase reads template (3'→5'). U replaces T. Introns spliced out.
- Translation
- Ribosome (cytoplasm or RER). mRNA codons read 3 at a time. tRNA brings amino acids. Peptide bonds form polypeptide. Start: AUG (Met). Stop: UAA, UAG, UGA.
- Point mutations
- Substitution (silent, missense, nonsense), insertion, deletion (frameshift).
- Chromosomal mutations
- Deletion, duplication, inversion, translocation. Aneuploidy: trisomy 21 (Down), XXY (Klinefelter), XO (Turner).
- Germline vs somatic
- Germline = heritable. Somatic = not passed on. Causes: mutagens (UV, radiation, chemicals), replication errors, viruses.
Natural selection & evidence for evolution
- Natural selection
- Variation exists → heritable via genes → selection pressure → differential survival & reproduction → allele frequency shifts. Types: directional, stabilising, disruptive.
- Other mechanisms
- Gene flow (migration), genetic drift (founder effect, bottleneck), mutation.
- Speciation
- Allopatric (geographic isolation) or sympatric (within range).
- Fossils
- Transitional forms; relative + radiometric dating.
- Comparative anatomy
- Homologous (common ancestor) vs analogous (convergent) vs vestigial structures.
- Molecular evidence
- DNA/protein similarity → molecular clocks; cytochrome c, mtDNA.
- Biogeography
- Species distribution reflects continental drift & isolation.
Hardy-Weinberg: p² + 2pq + q² = 1, where p + q = 1. Population in equilibrium when no selection, mutation, drift, gene flow, random mating.
Hominin evolution timeline
| Species | Date | Key feature | Brain size |
|---|
| A. afarensis ("Lucy") | ~4 mya | Bipedal, small canines | ~400 cc |
| H. habilis | ~2.5 mya | First stone tools (Oldowan) | ~600 cc |
| H. erectus | ~1.8 mya | Fire, hand axes, migration out of Africa | ~900 cc |
| H. heidelbergensis | ~700 kya | Common ancestor of Sapiens & Neanderthals | ~1200 cc |
| H. neanderthalensis | ~400 kya | Cold-adapted, burial rituals | ~1500 cc |
| H. sapiens | ~300 kya | Symbolic art, language, agriculture | ~1350 cc |
Trends: bipedalism · ↑ cranial capacity · ↓ prognathism · smaller teeth · longer juvenile period · increasing tool sophistication.
Biotechnology
- PCR (polymerase chain reaction)
- Amplifies DNA. Cycle: denaturation (~95 °C, separates strands) → annealing (~55 °C, primers attach) → extension (72 °C, Taq polymerase adds nucleotides). Repeat 25-30x.
- Gel electrophoresis
- Separates DNA fragments by size. Negatively charged DNA moves toward + electrode. Smaller fragments travel further.
- Restriction enzymes
- Cut DNA at specific recognition sequences. Create sticky or blunt ends.
- DNA profiling
- STR (short tandem repeat) analysis. Used in forensics, paternity, ancestry.
- Gene therapy
- Replacing or supplementing a defective gene with a functional copy. Vector usually a virus.
- Ethics
- Privacy of genetic data; consent for testing; GMO concerns; designer traits; gene-editing equity.
Common mistakes that cost marks
Negative vs positive feedback
Negative feedback opposes the change (the default for homeostasis). Positive feedback amplifies it (childbirth, clotting). Calling thermoregulation "positive feedback" is the #1 error every year.
B-cells vs T-cells confusion
B-cells make antibodies; they're factory workers. T-cells attack infected cells directly; they're soldiers. Mixing the two in long-response questions costs marks instantly.
Forgetting "semi-conservative"
DNA replication produces two daughter strands, each with one original strand and one new strand. Saying "two new strands" or "two original strands" is wrong, every time.
Genotype vs phenotype ratios
Monohybrid cross gives 1:2:1 genotype but 3:1 phenotype. Students quote one when the question asks the other. Read the prompt twice.
Missing specific examples
"Natural selection happens when..." gets fewer marks than "Natural selection: peppered moths went from light-coloured (camouflage on lichen) to dark-coloured during the Industrial Revolution as soot darkened tree bark." Examples convert.
Innate vs adaptive immunity
Innate = fast, non-specific, no memory (skin, phagocytes, fever). Adaptive = slow, specific, has memory (B-cells, T-cells). Vaccination triggers adaptive only.
Mutation isn't always bad
Many mutations are silent (no protein change). Some are beneficial (e.g. lactase persistence, sickle-cell heterozygote advantage in malaria zones). Saying "mutations are harmful" loses marks in evolution questions.
Hominin dates
Sapiens ~300 kya, not "millions of years ago". Australopithecines ~4 mya. Mixing these is common; make a timeline and revise it weekly.
Print and pin this
Print this page double-sided. Pin it above your desk. Cover it and try to write each section from memory weekly through Term 3. Where you stumble is your study map for that week.
