QCAA Biology EA cheat sheet

Prokaryotic cells lack membrane-bound organelles and a true nucleus; their DNA is a single circular chromosome in the nucleoid region, and they may also carry plasmids.

Eukaryotic cells contain membrane-bound organelles including a nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes and, in plants, chloroplasts and a large central vacuole.

The fluid mosaic model describes the plasma membrane as a phospholipid bilayer with embedded proteins, cholesterol and glycolipids; the hydrophobic tails face inward and hydrophilic heads face outward.

Surface area to volume ratio decreases as cell size increases; efficient exchange of materials requires a high SA:V ratio, which is why cells remain small or develop folded surfaces.

Mitochondria are the site of aerobic cellular respiration: glycolysis occurs in the cytosol, the Krebs cycle in the mitochondrial matrix, and the electron transport chain on the inner membrane.

Chloroplasts carry out photosynthesis: light-dependent reactions occur on the thylakoid membranes and the Calvin cycle occurs in the stroma.

The endomembrane system (nuclear envelope, ER, Golgi, vesicles, lysosomes) synthesises, modifies, packages and transports proteins and lipids; ribosomes on rough ER synthesise proteins destined for secretion.

Cell division by mitosis produces two genetically identical diploid daughter cells and is used for growth, repair and asexual reproduction; meiosis produces four genetically varied haploid gametes.

In multicellular organisms, cell signalling and differential gene expression allow cells with identical genomes to specialise into distinct tissues, organs and organ systems.

Homeostasis is the maintenance of a relatively constant internal environment despite external fluctuations; it relies on negative feedback loops that detect and reverse deviations from a set point.

A negative feedback loop contains a stimulus, receptor, afferent pathway, control centre, efferent pathway and effector; the response opposes the initial change.

Positive feedback amplifies a change away from the set point and is less common; examples include oxytocin release during childbirth and blood clotting cascades.

Thermoregulation in endotherms involves physiological responses such as sweating, vasodilation, shivering and vasoconstriction, plus behavioural responses such as seeking shade or shelter.

Osmoregulation is controlled by the hypothalamus detecting plasma osmolarity and releasing ADH from the posterior pituitary; ADH increases water reabsorption in the kidney collecting ducts.

Blood glucose is regulated by insulin (released from pancreatic beta cells when glucose is high, promoting uptake and glycogenesis) and glucagon (from alpha cells when glucose is low, promoting glycogenolysis and gluconeogenesis).

The nephron is the functional unit of the kidney; filtration occurs at the glomerulus, reabsorption mainly in the proximal tubule, and fine-tuning of water and salt in the loop of Henle and collecting duct.

Gas exchange surfaces have thin walls, a moist surface, a large surface area and a concentration gradient maintained by ventilation and blood flow; in mammals this occurs across alveoli.

Plant stomata regulate water loss and gas exchange; guard cells open when turgid and close when flaccid, responding to light, water stress and abscisic acid.

DNA is a double helix of nucleotides (sugar, phosphate, nitrogenous base); adenine pairs with thymine by two hydrogen bonds, and guanine pairs with cytosine by three.

DNA replication is semi-conservative: helicase unwinds the double helix, DNA polymerase adds complementary nucleotides in the 5' to 3' direction, and each new molecule contains one original and one new strand.

Transcription produces mRNA from a DNA template in the nucleus; translation occurs on ribosomes where tRNA delivers amino acids according to the codon sequence.

The genetic code is degenerate (more than one codon can specify the same amino acid), universal and non-overlapping; a codon is three bases and there are 64 possible codons.

Mendel's laws: segregation states that alleles separate during gamete formation; independent assortment states that alleles of different genes assort independently when genes are on different chromosomes.

Genotype describes the alleles present and phenotype describes the observable trait; homozygous means two identical alleles and heterozygous means two different alleles.

A monohybrid cross of two heterozygotes (Aa × Aa) gives a 3:1 phenotypic ratio for a completely dominant trait and a 1:2:1 genotypic ratio.

A test cross between an individual of unknown genotype and a homozygous recessive individual reveals whether the unknown is homozygous dominant or heterozygous.

Mutations are changes in DNA sequence; point mutations include substitutions, insertions and deletions, and frameshift mutations alter the reading frame downstream of the mutation.

Pedigree analysis uses symbols (squares for males, circles for females, shaded for affected) and inheritance patterns to infer genotypes; autosomal recessive traits can skip generations.

Biodiversity has three components: genetic diversity within species, species diversity (richness and evenness) and ecosystem diversity.

Species richness is the number of different species in a community; species evenness describes how similar the abundances of those species are.

Simpson's diversity index D = 1 − Σ(n/N)², where n is the number of individuals of each species and N is the total number of individuals; a higher D indicates greater diversity.

A trophic level is a feeding position in a food chain: producers (autotrophs) at level 1, primary consumers at level 2, secondary consumers at level 3, and so on.

Energy transfer between trophic levels is inefficient (typically about 10%); energy is lost as heat through respiration, in uneaten parts and in excretory wastes.

Ecological pyramids can represent numbers, biomass or energy; energy pyramids are always upright because energy decreases at each successive trophic level.

Symbiotic relationships include mutualism (both species benefit), commensalism (one benefits, the other is unaffected) and parasitism (one benefits, the other is harmed).

Natural selection acts on heritable variation: individuals with traits better suited to the environment survive and reproduce more successfully, increasing the frequency of those alleles in the population.

Human impacts on biodiversity include habitat destruction, introduced species, pollution, overexploitation and climate change; conservation strategies include protected areas, captive breeding and habitat restoration.