SBI4U · Exam review

Biology Exam Review

A practice exam with questions from all 5 units, then the key facts from every lesson, unit by unit. For a deeper look at one unit, open its test review.

Unit test reviews

Practice exam

35 questions, the same number from each unit. Each one shows the lesson it comes from.

Unit 1: The Chemistry of Life: Bonds, Water and Functional Groups
1
Nitrogen has an electronegativity of 3.04 and hydrogen 2.20. Calculate $Δ EN$ for an N–H bond in an amino group. Give two decimal places.
Unit 1: The Chemistry of Life: Bonds, Water and Functional Groups
2
In cellular respiration, glucose loses electrons and oxygen gains them. What is the process called when a molecule gains electrons?
Unit 1: Carbohydrates, Lipids, Proteins and Nucleic Acids
3
A protein is heated to 70 °C and loses its function. Which statement is correct?
Unit 1: Enzymes: Structure, Mechanism and Regulation
4
Match each technological application to the enzyme it uses.
Curdling milk to make cheese
Helping yeast make bread rise
Letting lactose-intolerant people digest dairy
Removing grease stains in cold-water laundry
Unit 1: Organelles, the Fluid Mosaic Model and Membrane Transport
5
A red blood cell is placed in a solution and swells until it bursts. What was the solution?
Unit 1: Viruses, Cancer and Cell-Targeted Medicine
6
Put the steps of the HIV life cycle in order.
1. 1Host enzymes and ribosomes make viral RNA and polyproteins
2. 2Integrase inserts the viral DNA into a host chromosome
3. 3New particles bud from the cell and protease matures them
4. 4gp120 binds CD4 and the CCR5 co-receptor
5. 5gp41 fuses the viral envelope with the plasma membrane
6. 6Reverse transcriptase copies the RNA genome into DNA
Unit 1: Viruses, Cancer and Cell-Targeted Medicine
7
Pembrolizumab binds PD-1 on T cells. How does this help treat melanoma?
Unit 2: Energy, Thermodynamics and ATP
8
Which statement best expresses the second law of thermodynamics as it applies to cells?
Unit 2: Energy, Thermodynamics and ATP
9
Match each energy carrier (reduced form) to its main role.
NADH
FADH₂
NADPH
ATP
Unit 2: Aerobic Cellular Respiration
10
A muscle cell makes 30 ATP per glucose. Using $30.5 kJ/mol$ per ATP and $2870 kJ/mol$ per glucose, what is the efficiency of respiration as a percentage, to 3 significant figures?
%
Unit 2: Fermentation, Other Fuels and Metabolism in Society
11
What is the essential function of the reactions that follow glycolysis in fermentation?
Unit 2: Fermentation, Other Fuels and Metabolism in Society
12
Put these steps in order to show how the carbon of a dietary protein ends up as exhaled $CO_{2}$ .
1. 1The amino group is removed by deamination
2. 2The carbons are oxidized in the Krebs cycle and released as CO₂
3. 3The carbon skeleton is converted into pyruvate, acetyl-CoA or a Krebs cycle intermediate
4. 4Protein is digested into amino acids
Unit 2: Photosynthesis
13
How many ATP molecules does the Calvin cycle use to produce one molecule of glucose?
ATP
Unit 2: Photosynthesis
14
Which statement correctly compares photosynthesis and aerobic respiration?
Unit 3: DNA: Discovery and Structure
15
Avery, MacLeod and McCarty treated extracts of heat-killed S cells with different enzymes before mixing them with live R cells. Which result identified DNA as the transforming principle?
Unit 3: DNA: Discovery and Structure
16
In a DNA extraction lab, why is ice-cold ethanol layered on top of the filtered extract?
Unit 3: DNA Replication and Repair
17
Skin cells exposed to UV light develop thymine dimers. Which system removes this damage?
Unit 3: Protein Synthesis: Transcription and Translation
18
Match each structure or molecule to its role.
A site
P site
E site
Release factor
Spliceosome
Aminoacyl-tRNA synthetase
Unit 3: Gene Regulation and Mutations
19
What term describes an operon that is normally on and is switched off when its end product builds up?
Unit 3: Biotechnology and Its Issues
20
A linear DNA molecule has 3 recognition sites for a restriction enzyme. How many fragments form when it is cut completely?
fragments
Unit 3: Biotechnology and Its Issues
21
At three STR loci, crime-scene DNA has repeat numbers (8, 11), (14, 14) and (6, 9). Suspect A has (8, 11), (14, 15), (6, 9). Suspect B has (8, 11), (14, 14), (6, 9). Suspect C has (7, 11), (14, 14), (6, 9). Which suspect's profile matches?
Unit 4: Homeostasis, Feedback and Thermoregulation
22
Put the parts of a homeostatic feedback loop in order.
1. 1Response
2. 2Effector
3. 3Control centre
4. 4Stimulus
5. 5Receptor
Unit 4: The Nervous System
23
Put the events of an action potential in order, starting from rest.
1. 1K⁺ flows out and the membrane repolarizes
2. 2Resting potential restored
3. 3Brief hyperpolarization below −70 mV
4. 4Membrane at rest at about −70 mV
5. 5Stimulus depolarizes the membrane to threshold (about −55 mV)
6. 6Voltage-gated Na⁺ channels open and Na⁺ rushes in
7. 7Na⁺ channels inactivate and voltage-gated K⁺ channels open
Unit 4: The Brain, Spinal Cord and Reflexes
24
Suppose the choroid plexus makes 500 mL of CSF per day and the CNS holds 150 mL of CSF at any moment. How many times is the CSF replaced each day? (1 decimal place)
times per day
Unit 4: The Endocrine System
25
A person's menstrual cycle is regularly 26 days long. Assuming a luteal phase of 14 days, on which day of the cycle would ovulation most likely occur?
day
Unit 4: The Excretory System and Water Balance
26
Why do humans excrete most of their nitrogenous waste as urea rather than ammonia?
Unit 4: Supplements, Drugs and Performance
27
A supplement ad says "Clinically proven! Thousands of satisfied customers!" Which question best tests the quality of the evidence?
Unit 4: Systems Working Together
28
What is the medical term for low blood sodium caused by drinking far more plain water than is lost?
Unit 5: Population Characteristics and Sampling
29
Northern gannets nest on cliff ledges with each nest just beyond pecking distance of its neighbours. What dispersion pattern is this, and what causes it?
Unit 5: Population Characteristics and Sampling
30
What is the term for the smallest population size that is likely to persist for a set period despite random genetic, demographic and environmental events? (Give the full term or its abbreviation.)
Unit 5: Population Growth Models
31
Match each growth pattern with its description.
Exponential (J-shaped)
Sigmoid (S-shaped)
Sinusoidal
Overshoot and dieback
Unit 5: Species Interactions and Population Cycles
32
Five warbler species feed on insects in the same spruce trees but in different zones of the tree. MacArthur's explanation for their coexistence was
Unit 5: Human Population Growth and the Ecological Footprint
33
A population pyramid has a very wide base that narrows steadily toward the top. What does this predict?
Unit 5: Environmental Contaminants and Human Health
34
Order these organisms from the lowest to the highest expected concentration of methylmercury in a northern Ontario lake.
1. 1People who eat walleye as a staple food
2. 2Walleye
3. 3Small fish such as minnows
4. 4Algae (phytoplankton)
5. 5Zooplankton
Unit 5: Environmental Contaminants and Human Health
35
Bacteria in river sediments convert inorganic mercury into what more dangerous form that biomagnifies in fish?

Key facts by unit

Unit 1: Biochemistry

The Chemistry of Life: Bonds, Water and Functional Groups

$Δ EN$ < about 0.5: nonpolar covalent; about 0.5–1.7: polar covalent; > about 1.7: ionic.
A hydrogen bond is an attraction between a $δ^{+}$ H (on O or N) and a $δ^{-}$ O or N elsewhere; it is weak alone but powerful in large numbers.
$pH = - lo g [H^{+}]$ ; each pH unit is a tenfold change in $[H^{+}]$ . Blood is buffered at 7.35–7.45 by $H_{2} CO_{3} / HCO_{3}^{-}$ .
Functional groups: hydroxyl –OH, carbonyl C=O, carboxyl –COOH (acid), amino –NH₂ (base), phosphate –OPO₃²⁻, sulfhydryl –SH (disulfide bridges).
Condensation releases $H_{2} O$ to join monomers; hydrolysis uses $H_{2} O$ to split them. Joining $n$ monomers releases $n - 1$ water molecules.
Redox: OIL RIG. The molecule that loses electrons is oxidized; the one that gains them is reduced.

Carbohydrates, Lipids, Proteins and Nucleic Acids

Glycosidic bond (sugars), ester bond (fats), peptide bond (proteins), phosphodiester bond (nucleic acids): all made by condensation, all broken by hydrolysis.
Alpha glucose builds coiled storage polymers (starch, glycogen); beta glucose builds straight structural fibres (cellulose). Chitin is a beta-linked structural polysaccharide.
Saturated fats: no C=C, straight tails, solid. Unsaturated: cis C=C kinks, liquid. Trans fats: straight despite C=C, raise LDL.
Protein structure: primary (sequence, peptide bonds), secondary (α-helix, β-sheet, backbone H bonds), tertiary (R group interactions, disulfide bridges), quaternary (multiple chains). Denaturation destroys levels 2–4, not 1.
DNA: deoxyribose, A-T and G-C, double-stranded. RNA: ribose, uracil replaces thymine, single-stranded. ATP = adenine + ribose + 3 phosphates.
Food tests: Benedict's (reducing sugar, brick-red when heated), iodine (starch, blue-black), biuret (protein, violet), Sudan IV or grease spot (lipid).

Enzymes: Structure, Mechanism and Regulation

Enzymes lower activation energy ( $E_{a}$ ); they do not change $Δ G$ and are not used up.
$E + S \to ES \to E + P$ . Induced fit: binding changes the active site's shape, straining the substrate's bonds.
Rate rises with temperature up to an optimum, then falls as the enzyme denatures; each enzyme has an optimum pH.
Increasing [S] raises the rate until the enzyme is saturated at $V_{ma x}$ ; increasing [E] raises $V_{ma x}$ when substrate is in excess.
Competitive inhibitors block the active site (overcome by more substrate). Non-competitive inhibitors bind elsewhere and lower $V_{ma x}$ .
Feedback inhibition: a pathway's end product allosterically inhibits its first enzyme.

Organelles, the Fluid Mosaic Model and Membrane Transport

Endomembrane pathway for secreted proteins: rough ER ribosome $\to$ ER lumen $\to$ transport vesicle $\to$ Golgi $\to$ secretory vesicle $\to$ exocytosis.
Fluid mosaic model: phospholipid bilayer + cholesterol + integral and peripheral proteins + glycoproteins; small nonpolar molecules cross freely, ions and large polar molecules need proteins.
Passive transport (diffusion, osmosis, facilitated diffusion) goes down the concentration gradient with no ATP; active transport goes against it and uses ATP.
Water moves toward the hypertonic side. Hypotonic solution: animal cells lyse, plant cells turgid. Hypertonic: animal cells crenate, plant cells plasmolyze.
Sodium-potassium pump: 3 Na⁺ out and 2 K⁺ in per ATP.
$% change in mass = \frac{final - initial}{initial} \times 100%$

Viruses, Cancer and Cell-Targeted Medicine

Viruses are obligate intracellular parasites: attach to a specific receptor, enter, copy the genome with host machinery, assemble, leave.
HIV: gp120 binds CD4, then the CCR5 (or CXCR4) co-receptor; gp41 drives fusion; reverse transcriptase makes DNA; integrase inserts it; protease matures new virions.
Antiretroviral classes match life-cycle steps: entry (CCR5 antagonists, fusion inhibitors), NRTIs and NNRTIs, integrase inhibitors, protease inhibitors. ART combines about three drugs to prevent resistance.
HIV cannot yet be cured because integrated proviruses hide in resting cells; HCV never integrates, so 8–12 weeks of direct-acting antivirals cure more than 95% of patients.
Monoclonal antibodies come from hybridomas (B cell fused with a myeloma cell); they neutralize, block, flag cells for destruction, release immune brakes or deliver drugs.
Log reduction in viral load: $lo g_{10} (\frac{initial}{final})$ ; each 1-log drop is a tenfold decrease.

Unit 2: Metabolic Processes

Energy, Thermodynamics and ATP

First law: energy is transformed, never created or destroyed. Second law: every transformation increases the entropy of the universe; some energy is lost as heat.
$Δ G = Δ H - T Δ S$ ; $Δ G < 0$ is exergonic (spontaneous), $Δ G > 0$ is endergonic (needs energy input).
$ATP + H_{2} O \to ADP + P_{i}$ , $Δ G^{\circ'} = - 30.5 kJ/mol$ .
Energy coupling: $Δ G_{total} = Δ G_{endergonic} + Δ G_{ATP hydrolysis};$ the coupled process runs if the sum is negative.
OIL RIG: oxidation is loss (of electrons, often as H atoms); reduction is gain.
Energy carriers: $NADH$ and $FADH_{2}$ in catabolism; $NADPH$ in anabolism (photosynthesis).

Aerobic Cellular Respiration

$C_{6} H_{12} O_{6} + 6 O_{2} \to 6 CO_{2} + 6 H_{2} O,$ $Δ G \approx - 2870 kJ/mol$ .
Glycolysis (cytosol): net 2 ATP, 2 NADH, 2 pyruvate. Pyruvate oxidation (matrix): 2 acetyl-CoA, 2 $CO_{2}$ , 2 NADH.
Krebs cycle (matrix), per glucose: 2 ATP, 6 NADH, 2 $FADH_{2}$ , 4 $CO_{2}$ (per turn: 1 ATP, 3 NADH, 1 $FADH_{2}$ , 2 $CO_{2}$ ).
ETC pumps $H^{+}$ into the intermembrane space; $H^{+}$ returns through ATP synthase (chemiosmosis); $O_{2}$ is the final electron acceptor, forming $H_{2} O$ .
Yield: about 2.5 ATP per NADH and 1.5 per $FADH_{2}$ , so about 30–32 ATP per glucose (older value 36–38).
Efficiency $\approx \frac{32 \times 30.5}{2870} \approx 34%$ .

Fermentation, Other Fuels and Metabolism in Society

Fermentation = glycolysis + regeneration of $NAD^{+}$ ; net 2 ATP per glucose (about 2% efficient).
Alcoholic: pyruvate $\to$ acetaldehyde $+ CO_{2}$ , then acetaldehyde $+$ NADH $\to$ ethanol $+ NAD^{+}$ .
Lactic acid: pyruvate $+$ NADH $\to$ lactate $+ NAD^{+}$ (no $CO_{2}$ ).
Fats: glycerol $\to$ glycolysis; fatty acids $\to$ acetyl-CoA by beta-oxidation. Proteins: deamination $\to$ urea; carbon skeletons enter glycolysis or the Krebs cycle.
PFK is the main control point: inhibited by ATP and citrate, activated by AMP/ADP (allosteric feedback).
Cyanide blocks cytochrome c oxidase (ETC stops); DNP uncouples (ETC runs, energy lost as heat, little ATP).

Photosynthesis

$6 CO_{2} + 6 H_{2} O + light \to C_{6} H_{12} O_{6} + 6 O_{2}$ (endergonic, $Δ G \approx + 2870 kJ/mol$ ).
Light reactions (thylakoids): PSII (P680) then PSI (P700); photolysis $2 H_{2} O \to 4 H^{+} + 4 e^{-} + O_{2}$ ; products ATP, NADPH, $O_{2}$ .
Calvin cycle (stroma): rubisco fixes $CO_{2}$ + RuBP $\to$ 2 PGA; PGA reduced to G3P; RuBP regenerated.
Per G3P: 3 $CO_{2}$ , 9 ATP, 6 NADPH. Per glucose: 6 $CO_{2}$ , 18 ATP, 12 NADPH.
Noncyclic flow: ATP + NADPH + $O_{2}$ . Cyclic flow (PSI only): ATP only.
Photorespiration: rubisco fixes $O_{2}$ when hot and dry. C4 separates fixation in space; CAM separates it in time.

Unit 3: Molecular Genetics

DNA: Discovery and Structure

Griffith (1928): transformation. Avery–MacLeod–McCarty (1944): the transforming principle is DNA. Hershey–Chase (1952): phage DNA ( $^{32} P$ ), not protein ( $^{35} S$ ), enters the cell.
Chargaff's rules for double-stranded DNA: $% A = % T$ and $% G = % C$ , so purines = pyrimidines = 50%.
A–T pairs have 2 hydrogen bonds and G–C pairs have 3. A purine (A, G) always pairs with a pyrimidine (C, T).
The strands are antiparallel: 5'→3' pairs with 3'→5'. The 5' end has a free phosphate; the 3' end has a free –OH.
Double helix: about 2 nm wide, 0.34 nm per base pair, about 10 base pairs (3.4 nm) per turn.
Eukaryotic DNA wraps around histone octamers to form nucleosomes, then chromatin; prokaryotes usually have one circular chromosome without nucleosomes.

DNA Replication and Repair

Replication is semiconservative: each daughter helix has one parental strand and one new strand (Meselson and Stahl, 1958, using $^{15} N$ and $^{14} N$ ).
DNA polymerase adds nucleotides only to a free 3'-OH, so new strands grow 5'→3', the template is read 3'→5', and a primer is required.
At the fork: helicase unwinds, SSBs stabilize, topoisomerase relieves twisting, primase primes, pol III extends, pol I replaces primers, ligase seals.
Leading strand: continuous, toward the fork. Lagging strand: discontinuous Okazaki fragments, built away from the fork.
Error control: proofreading (during), mismatch repair (just after), excision repair (damage at any time); final error rate about 1 in $1 0^{9}$ .
Telomerase extends telomere repeats (TTAGGG in humans) at the ends of linear chromosomes.

Protein Synthesis: Transcription and Translation

Central dogma: DNA $\to$ RNA $\to$ protein (transcription, then translation).
RNA polymerase reads the template strand 3'→5' and builds mRNA 5'→3'; the coding strand matches the mRNA with T in place of U.
Eukaryotic pre-mRNA processing: 5' cap, 3' poly-A tail, and splicing (introns removed, exons joined).
Genetic code: triplet, $4^{3} = 64$ codons (61 for amino acids, 3 stops), redundant, unambiguous, nearly universal. Start AUG (Met); stop UAA, UAG, UGA.
Anticodons pair antiparallel with codons: codon 5'-AUG-3' pairs with anticodon 3'-UAC-5'.
Ribosome sites: A (new aminoacyl-tRNA enters), P (holds the growing chain), E (empty tRNA exits); the ribosome moves 5'→3' along the mRNA.

Gene Regulation and Mutations

Operon = promoter + operator + structural genes transcribed as one mRNA; a repressor bound to the operator blocks RNA polymerase.
lac operon (inducible): allolactose inactivates the repressor; CAP–cAMP (glucose low) boosts transcription. Fully on only when lactose is present and glucose is low.
trp operon (repressible): tryptophan is a corepressor that activates the repressor and switches the operon off.
Eukaryotic control points: chromatin (acetylation opens, methylation closes), transcription (transcription factors, enhancers), RNA processing, translation, and after translation.
Substitutions: silent (same amino acid), missense (different amino acid), nonsense (stop codon). Insertions or deletions that are not a multiple of 3 cause frameshifts.
Mutagens: UV (thymine dimers), ionizing radiation (strand breaks), chemicals (tobacco smoke, base analogues, intercalating agents).

Biotechnology and Its Issues

EcoRI cuts 5'-GAATTC-3' between G and A on each strand, leaving complementary AATT sticky ends; DNA ligase seals the sugar-phosphate backbone.
Recombinant DNA: cut gene and plasmid with the same enzyme, ligate, transform bacteria, and select on antibiotic agar.
Gel electrophoresis: DNA (negative) moves toward the positive electrode; smaller fragments travel farther.
PCR: denature about 95 °C, anneal about 55–65 °C, extend about 72 °C with Taq polymerase; copies $N = N_{0} \times 2^{n}$ .
Cutting a linear molecule at $k$ sites gives $k + 1$ fragments; cutting a circular plasmid at $k$ sites gives $k$ fragments.
Canada: higher life forms not patentable (Harvard mouse, 2002) but patented genes in plants are enforceable (Schmeiser, 2004); human cloning banned (AHRA, 2004); genetic discrimination banned (GNDA, 2017); GM labelling voluntary (EU: mandatory above 0.9%).

Unit 4: Homeostasis

Homeostasis, Feedback and Thermoregulation

Feedback loop: stimulus $\to$ receptor $\to$ control centre $\to$ effector $\to$ response, which is sensed again.
Negative feedback reverses a change (stabilizing); positive feedback amplifies it and needs an end point (childbirth, clotting).
Hypothalamus = thermostat. Hot: vasodilation, sweating. Cold: vasoconstriction, shivering, piloerection, more thyroxine.
Bicarbonate buffer: $CO_{2} + H_{2} O ⇌ H_{2} CO_{3} ⇌ H^{+} + HCO_{3}^{-};$ normal blood pH is 7.35–7.45.
pH defence speeds: buffer (seconds), lungs exhale $CO_{2}$ (minutes), kidneys secrete $H^{+}$ and reabsorb $HCO_{3}^{-}$ (hours to days).
Choice chamber: change one factor, control all others, run a no-difference control trial, replicate and rotate.

The Nervous System

Resting potential ≈ −70 mV: the Na⁺/K⁺ pump moves 3 Na⁺ out and 2 K⁺ in per ATP, and K⁺ leaks out through open channels.
Threshold ≈ −55 mV. Depolarization: voltage-gated Na⁺ channels open, Na⁺ in, peak ≈ +35 mV. Repolarization: K⁺ channels open, K⁺ out.
All-or-none: stimulus strength is coded by the frequency of action potentials, not their size.
Saltatory conduction: in myelinated axons the impulse jumps between nodes of Ranvier, which is faster and uses less ATP.
Synapse: action potential → Ca²⁺ enters → vesicles release neurotransmitter → binds receptors → removed by enzyme (acetylcholinesterase for ACh) or reuptake.
Reflex arc: receptor → sensory neuron → interneuron → motor neuron → effector.

The Brain, Spinal Cord and Reflexes

Meninges from outside in: dura mater, arachnoid mater, (CSF in the subarachnoid space), pia mater. CSF is made by the choroid plexus and cushions, floats and cleans the CNS.
Lobes: frontal (planning, motor cortex, Broca's speech), parietal (touch, body position), temporal (hearing, Wernicke's comprehension, hippocampus memory), occipital (vision). Cerebellum: coordination and balance. Medulla: breathing and heart rate.
Contralateral control: each hemisphere controls and senses the opposite side of the body.
Spinal cord: grey matter inside (synapses, interneurons), white matter outside (tracts). Sensory in through the dorsal root, motor out through the ventral root.
Reflex arc: receptor → sensory neuron → (interneuron) → motor neuron → effector. Stretch reflexes are monosynaptic; withdrawal reflexes are polysynaptic with reciprocal inhibition.
EEG measures electrical activity (good timing); fMRI measures blood oxygenation (good location).

The Endocrine System

Steroid hormones are lipid-soluble, bind intracellular receptors and change gene expression; peptide hormones are water-soluble, bind membrane receptors and act through second messengers such as cAMP.
Hypothalamus releasing hormones → anterior pituitary (TSH, ACTH, FSH, LH, GH, prolactin). Posterior pituitary stores and releases ADH and oxytocin made in the hypothalamus.
TRH → TSH → thyroxine; thyroxine inhibits the hypothalamus and pituitary (negative feedback). Iodine deficiency → high TSH → goitre.
High glucose → beta cells → insulin → uptake and glycogen storage. Low glucose → alpha cells → glucagon → liver releases glucose.
Adrenal medulla: adrenaline (short-term stress). Adrenal cortex: cortisol and aldosterone (long-term).
Menstrual cycle: FSH → follicle → estrogen → LH surge (positive feedback) → ovulation → corpus luteum → progesterone → negative feedback.

The Excretory System and Water Balance

Nitrogenous wastes: ammonia (toxic, needs lots of water), urea (humans; moderate water and energy), uric acid (least water, most energy).
Nephron path: glomerulus → Bowman's capsule → proximal tubule → loop of Henle → distal tubule → collecting duct.
Filtration (by size, pressure-driven) → reabsorption (useful substances back to blood) → secretion (blood to filtrate: H⁺, K⁺, drugs).
Loop of Henle: ascending limb pumps out salt (impermeable to water); descending limb loses water. This builds a salty medulla.
ADH (high blood solute concentration) → aquaporins in the collecting duct → more water reabsorbed. Aldosterone (low blood pressure, via renin–angiotensin) → more Na⁺ and water reabsorbed.
Dialysis: blood and dialysate separated by a selectively permeable membrane; urea diffuses out because dialysate has none.

Supplements, Drugs and Performance

Excess of a substance is excreted (extra liver and kidney load), stored (toxic build-up) or causes negative feedback that switches off the body's own production.
Fat-soluble vitamins (A, D, E, K) are stored in the liver and fat and can reach toxic levels; water-soluble vitamins (B, C) are mostly excreted in urine.
Excess amino acids are deaminated in the liver; the nitrogen becomes urea, which the kidneys must excrete with extra water.
EPO raises red blood cell production; doping with EPO or transfusions raises hematocrit and blood viscosity, increasing clot, stroke and heart attack risk.
Anabolic steroids suppress GnRH, LH and FSH by negative feedback; the axis is slow to recover after stopping.
Alcohol inhibits ADH (more urine, dehydration); stimulants mimic the sympathetic response and mask fatigue and heat signals.

Systems Working Together

Real disturbances trigger several loops at once; when loops compete for the same resource (such as blood volume), the body protects blood pressure and brain supply first.
Exercise in heat: sweating and skin vasodilation (temperature), ADH and thirst (concentration), renin–angiotensin–aldosterone (blood pressure), glucagon and cortisol (glucose).
DKA: no insulin → hyperglycemia and ketone production → acidic blood; glucose in urine causes water loss; deep rapid breathing removes CO₂ to raise pH.
Fight or flight has three tiers: sympathetic nerves (seconds), adrenaline (seconds to minutes), cortisol via CRH and ACTH (minutes to hours).
A feedback flowchart: stimulus → receptor → control centre → messenger → effector → response, with a minus-sign arrow back to the stimulus.
pH is logarithmic: a fall of 0.3 pH units doubles $[H^{+}]$ , since $1 0^{0.3} \approx 2$ .

Unit 5: Population Dynamics

Population Characteristics and Sampling

Density: $D = \frac{N}{A}$ (or $\frac{N}{V}$ in water); quadrat estimate $N \approx \frac{total count}{total area sampled} \times total area .$
Lincoln–Petersen mark–recapture: $N = \frac{M n}{m}$ , assuming a closed population, no lost marks, random mixing and equal catchability.
Dispersion: clumped (patchy resources, social groups), uniform (territoriality, competition, allelopathy), random (uniform resources, no interaction).
Survivorship: Type I dies mostly old (humans), Type II constant rate (songbirds), Type III dies mostly young (oysters, cod).
Fecundity is potential reproductive output; natality is the actual birth rate; mortality is the death rate.
Minimum viable population: the smallest size that can survive demographic, environmental and genetic chance events over a set time.

Population Growth Models

$Δ N = (B + I) - (D + E)$ ; per-capita growth rate $cgr = \frac{Δ N}{N}$ . Closed populations have no $I$ or $E$ .
Geometric (discrete breeding): $N_{t} = N_{0} λ^{t}$ with $λ = \frac{N _{t + 1}}{N _{t}}$ . Exponential (continuous): $N_{t} = N_{0} e^{r t}$ .
Doubling time: $t_{d} = \frac{ln 2}{r} \approx \frac{70}{growth rate in %}$ (rule of 70).
Logistic: $\frac{d N}{d t} = r_{ma x} N \frac{K - N}{K}$ ; growth is fastest at $N = \frac{K}{2}$ and zero at $N = K$ .
Density-dependent factors (competition, predation, disease, waste) regulate near $K$ ; density-independent factors (weather, fire, pollution) do not.
r-selected: many offspring, little care, unstable habitats. K-selected: few offspring, much care, stable habitats near $K$ .

Species Interactions and Population Cycles

Competition is $- / -$ ; predation and parasitism are $+ / -$ ; mutualism is $+ / +$ ; commensalism is $+ /0$ .
Competitive exclusion principle (Gause): two species cannot coexist indefinitely on exactly the same limited resources; resource partitioning (MacArthur's warblers) allows coexistence.
In predator–prey cycles the predator peak lags behind the prey peak because predators need time to reproduce in response to abundant prey.
Batesian mimicry: a harmless species copies a harmful one. Müllerian mimicry: harmful species resemble each other.
A keystone species (sea otter) has an effect out of proportion to its numbers; removing it triggers a trophic cascade.

Human Population Growth and the Ecological Footprint

Demographic transition: death rate falls first (stage 2, rapid growth), birth rate follows (stage 3), then both are low (stage 4); stage 5 is natural decrease.
Pyramids: expanding (wide base, rapid growth), stable (straight sides, little growth), contracting (narrow base, decline).
About 10% of energy passes to the next trophic level, so eating plants directly feeds roughly ten times as many people as feeding the plants to livestock first.
Ecological footprint is measured in global hectares; overshoot occurs when footprint exceeds biocapacity.
Doubling time $\approx \frac{70}{growth rate in %}$ ; world growth fell from about 2% per year (late 1960s) to under 1% per year.

Environmental Contaminants and Human Health

Endocrine disruptors (EE2, BPA, phthalates, PFAS) mimic, block or interfere with hormones and can act at nanogram-per-litre levels.
Experimental Lakes Area, Lake 260: about 5–6 ng/L of EE2 feminized male fathead minnows and collapsed the population; it recovered after additions stopped.
Bioaccumulation: build-up within one organism. Biomagnification: concentration rises at each trophic level, so top predators carry the most.
Biomagnifying substances are persistent, retained in fat or bound to protein, and slowly excreted. Biomagnification factor $= \frac{C _{predator}}{C _{food}}$ .
Grassy Narrows: mercury released at Dryden (1962–1970) became methylmercury, biomagnified in fish and caused Minamata disease symptoms.
Dose-response: threshold, NOAEL and LD₅₀; tolerable intakes are set by dividing the NOAEL by safety factors.