Higher Biology revision notes: the whole course, unit by unit

These are condensed Higher Biology notes written the way the exam asks about each topic, not the way a textbook explains it. Each section gives the content and then the phrasing that the SQA marking instructions accept.

01DNA structure and replication

DNA is a double stranded helix of nucleotides, each made of deoxyribose sugar, phosphate and a base. The strands are antiparallel, held together by hydrogen bonds between complementary base pairs: adenine with thymine, guanine with cytosine. The sugar phosphate backbone runs 3 prime to 5 prime on one strand and 5 prime to 3 prime on the other.

Replication needs DNA polymerase, primers, ATP and free nucleotides. DNA polymerase adds nucleotides only to the 3 prime end, which is why the leading strand is made continuously and the lagging strand in fragments joined by ligase.

Exam phrasing: the marks live in "complementary base pairing", "hydrogen bonds", "antiparallel" and "3 prime end". Vague answers about the ladder shape score nothing.

02Gene expression: transcription and translation

Transcription happens in the nucleus. RNA polymerase unwinds the DNA and synthesises a primary transcript from the template strand. Introns are spliced out and exons joined to form mature mRNA. Alternative splicing means one gene can give different mature transcripts and so different proteins.

Translation happens at the ribosome. tRNA carries a specific amino acid and has an anticodon complementary to the mRNA codon. Peptide bonds join amino acids into a polypeptide, which folds into a protein held by hydrogen bonds and other interactions.

Order matters. If your transcription answer mentions the ribosome, you have lost the mark.

03Mutations

Single gene mutations are substitution, insertion or deletion. Substitutions are missense, nonsense or splice site. Insertions and deletions cause a frame shift, which changes every codon after the mutation and so has a much greater effect.

Chromosome structure mutations are deletion, duplication, translocation and inversion.

04Evolution and genomic sequencing

Variation arises by mutation and, in sexually reproducing organisms, recombination. Natural selection acts on that variation: stabilising, directional and disruptive. Genetic drift matters most in small populations. Speciation requires an isolation barrier, then mutation and selection acting differently on each population until they can no longer interbreed.

Genomic sequencing compares sequences between individuals and species, allows phylogenetic trees to be built, and underpins personalised medicine and pharmacogenetics.

05Metabolic pathways and enzymes

Metabolic pathways are integrated and controlled, with reversible steps, irreversible steps and alternative routes. Anabolic reactions build and require energy. Catabolic reactions break down and release energy.

Enzymes lower activation energy. The active site is flexible: the substrate binding induces a conformational change, which is induced fit, and this increases the affinity of the enzyme for the substrate. Products have low affinity and are released.

Competitive inhibitors bind the active site and their effect is reduced by increasing substrate concentration. Non competitive inhibitors bind elsewhere, change the shape of the active site, and increasing substrate concentration does not help. End product inhibition is negative feedback where the final product of a pathway inhibits an earlier enzyme.

06Cellular respiration

Glycolysis in the cytoplasm: glucose to pyruvate, net gain of two ATP, requires an initial energy investment phase. Under aerobic conditions pyruvate enters the mitochondrial matrix and is converted to acetyl coenzyme A, entering the citric acid cycle, which releases carbon dioxide and generates NADH and ATP.

The electron transport chain is on the inner mitochondrial membrane. Electrons from NADH pass along carrier proteins, releasing energy used to pump hydrogen ions across the membrane. The ions flow back through ATP synthase, which drives ATP synthesis. Oxygen is the final electron acceptor and combines with hydrogen to form water.

Exam phrasing: "final electron acceptor", "hydrogen ion gradient", "ATP synthase". Without oxygen, no acceptor, the chain stops and ATP production by this route ceases.

07Energy systems in muscle

The creatine phosphate system regenerates ATP rapidly for around ten seconds. Lactic acid fermentation converts pyruvate to lactate, regenerating NAD so glycolysis can continue, and the lactate is later converted back to pyruvate or glucose in the liver. Slow twitch fibres are fatigue resistant, use aerobic respiration and have many mitochondria. Fast twitch fibres generate rapid force and rely on glycolysis.

08Metabolism in conformers and regulators

Conformers have an internal environment that follows the external one, use less energy, and are limited to narrow niches. Regulators maintain a steady internal environment by homeostasis, costing energy but widening the niche. Thermoregulation in mammals is controlled by the hypothalamus acting through negative feedback: vasodilation, vasoconstriction, sweating, shivering and hair erection.

09Surviving adverse conditions

Dormancy, predictive and consequential. Hibernation, aestivation and daily torpor lower metabolic rate. Migration is long distance avoidance, studied by tracking and ringing, and driven by innate and learned behaviour.

10Photosynthesis

Light dependent stage: photolysis splits water, giving hydrogen, electrons and oxygen. ATP is generated by ATP synthase driven by the electron transport chain. Light independent stage: the Calvin cycle, where RuBisCO fixes carbon dioxide with RuBP to make 3PG, reduced by NADPH and ATP to G3P, some used for glucose and the rest to regenerate RuBP.

11Food supply, breeding and crop protection

Plant field trials must consider selection of treatments, number of replicates and randomisation. Breeding methods: selective breeding, inbreeding and the resulting inbreeding depression, F1 hybrids and hybrid vigour, and genetic transformation. Weeds, pests and diseases are controlled culturally, chemically and biologically, each with its own risks including bioaccumulation, biomagnification and resistance.

12Animal welfare, behaviour and biodiversity

Indicators of poor welfare include stereotypy, misdirected behaviour, failure in sexual behaviour and altered activity levels. Social behaviour covers social hierarchy, co operative hunting, altruism, kin selection and social insects.

Biodiversity has three components: genetic diversity, species diversity (richness and relative abundance) and ecosystem diversity. Threats are habitat fragmentation, introduced species, overexploitation, climate change and the bottleneck effect reducing genetic diversity.

13Using these notes

Read a section, close it, and answer a past paper question on that topic from memory. If you cannot produce the exam phrasing without looking, you have read the notes rather than learned them.