NSSCAS Biology 8223 cheat sheet

Prokaryotic cells: 70S ribosomes, no membrane-bound organelles, circular DNA, peptidoglycan wall (bacteria); typical diameter 0.5–5 µm.

Eukaryotic cells: 80S ribosomes, DNA enclosed in a nuclear envelope with pores, membrane-bound organelles; typical diameter 10–100 µm.

Magnification = image size ÷ actual size; use the same units on both sides before dividing.

Resolution = minimum distance at which two points are seen as separate; limited by wavelength, so electron microscopes resolve better than light microscopes.

Light microscope resolution ≈ 200 nm; transmission electron microscope ≈ 0.1–0.5 nm, but specimens must be dead and in a vacuum.

Organelle functions: rough ER folds and transports proteins, smooth ER synthesises lipids, Golgi modifies and packages, lysosomes contain hydrolytic enzymes, mitochondria carry out aerobic respiration.

Cell fractionation order: homogenise in cold isotonic buffered solution, then centrifuge - nuclei pellet first, then mitochondria/chloroplasts, then ER, then ribosomes at highest speed.

Surface area to volume ratio falls as cell size increases, so large cells need exchange surfaces or transport systems; this limits cell size.

Condensation joins two monomers with loss of one water; hydrolysis breaks a bond using one water.

Benedict's test: reducing sugar + heat gives brick-red precipitate; non-reducing sugar needs boiling with dilute HCl, neutralisation, then Benedict's.

Starch test: iodine in potassium iodide turns blue-black. Protein test: biuret reagent turns purple/violet. Lipid test: emulsion test with ethanol and water gives a white emulsion.

Triglyceride = glycerol + three fatty acids joined by ester bonds; phospholipid replaces one fatty acid with a phosphate group, giving a hydrophilic head and hydrophobic tails.

Saturated fatty acids have no C=C double bonds and higher melting points; unsaturated chains have C=C bonds and kink, lowering melting point.

Amino acids differ only in the R group; the peptide bond forms between the carboxyl group of one amino acid and the amino group of the next.

Protein structure levels: primary (amino acid sequence), secondary (α-helix, β-pleated sheet held by hydrogen bonds), tertiary (3D folding, ionic, hydrogen, disulfide, hydrophobic interactions), quaternary (multiple polypeptide chains).

DNA: deoxyribose, phosphate, bases A-T and C-G; two antiparallel strands joined by hydrogen bonds (2 between A-T, 3 between C-G). RNA: ribose, uracil replaces thymine, usually single-stranded.

Enzymes are globular proteins that lower activation energy by providing an alternative reaction pathway; they are not used up.

Lock-and-key model: active site is complementary to the substrate. Induced-fit model: active site moulds slightly around the substrate to form an enzyme-substrate complex.

Rate of reaction rises with temperature up to the optimum, then falls steeply as denaturation breaks hydrogen and ionic bonds in the tertiary structure.

Rate rises with substrate concentration until all active sites are saturated; further increase has no effect (Vmax reached).

pH changes alter charges on R groups in the active site, changing its shape and reducing activity; each enzyme has an optimum pH.

Competitive inhibitors resemble the substrate and bind the active site; effect is reduced by raising substrate concentration. Non-competitive inhibitors bind elsewhere and change active site shape; raising substrate concentration does not overcome them.

Enzyme concentration increases rate only if substrate is in excess; it is the limiting factor when substrate is plentiful.

Immobilised enzymes are fixed to a support (e.g. alginate beads), allowing reuse, easy separation from product, and continuous flow processes.

Fluid mosaic model: phospholipid bilayer with hydrophobic tails inward, plus proteins, cholesterol, glycoproteins and glycolipids; the membrane is 7–10 nm thick.

Cholesterol reduces membrane fluidity and permeability to small water-soluble molecules at high temperatures.

Simple diffusion: passive, down a concentration gradient, for small non-polar molecules such as O2 and CO2.

Facilitated diffusion: passive, through channel or carrier proteins, for polar molecules and ions; no ATP used.

Osmosis: net movement of water from higher to lower water potential through a partially permeable membrane; water potential of pure water at standard conditions is 0 kPa.

Active transport uses ATP and carrier proteins to move substances against a concentration gradient.

Co-transport in the ileum: Na+ actively pumped out of epithelial cells, creating a gradient that draws glucose or amino acids in with Na+ via a co-transporter.

Rate of diffusion is increased by steeper concentration gradient, shorter diffusion distance, larger surface area and higher temperature.

Interphase: G1 (growth, organelle synthesis), S (DNA replication), G2 (further growth, checking); DNA content doubles in S phase while chromosome number stays the same.

Mitosis stages: prophase (chromosomes condense, nuclear envelope breaks down), metaphase (chromosomes align on the equator), anaphase (centromeres divide, chromatids pulled to poles by spindle fibres), telophase (nuclei reform).

Cytokinesis divides the cytoplasm; in plant cells a cell plate forms and becomes the middle lamella.

Mitosis produces two genetically identical diploid cells for growth, repair and asexual reproduction.

Meiosis halves chromosome number: two divisions give four genetically different haploid cells; crossing over in prophase I and independent assortment in metaphase I create variation.

Homologous chromosomes pair in prophase I to form bivalents; chiasmata are the sites of crossing over.

Cancer arises from uncontrolled mitosis caused by mutation in genes controlling cell division; carcinogens and ionising radiation increase mutation rate.

In a cell with 2n = 8, mitosis produces cells with 8 chromosomes; meiosis produces gametes with 4 chromosomes.

Semi-conservative replication: DNA helicase unwinds and breaks hydrogen bonds, each strand acts as a template, DNA polymerase joins free activated nucleotides, forming two identical DNA molecules each with one old and one new strand.

Transcription: DNA helicase separates strands, RNA polymerase builds mRNA using the template strand, uracil pairs with adenine; mRNA leaves the nucleus through a nuclear pore.

Translation: mRNA binds to a ribosome, tRNA brings amino acids with complementary anticodons, peptide bonds form between adjacent amino acids; the genetic code is degenerate and non-overlapping.

Genetic code is read in triplets (codons); a sequence of three bases codes for one amino acid.

A mutation is a change in base sequence; substitution may cause a different amino acid, deletion or insertion causes a frameshift and usually a non-functional protein.

Sickle cell anaemia: single base substitution in the β-globin gene changes glutamic acid to valine, altering haemoglobin and causing red blood cells to become sickle-shaped.

DNA replication requires a template, free nucleotides, enzymes and ATP; the two new molecules are identical to the original.

Surface area to volume ratio, diffusion distance and concentration gradient determine the need for specialised exchange surfaces.

Root hair cells: long thin extensions increase surface area for absorption of water by osmosis and mineral ions by active transport.

Xylem: dead, hollow, lignified vessels with no end walls, transports water and mineral ions upward under tension (transpiration pull); cohesion-tension and adhesion support the column.

Phloem: sieve tube elements with companion cells, transports sucrose and amino acids; translocation is an active process described by the mass flow hypothesis (source to sink).

Transpiration rate rises with temperature, wind speed, light intensity and lower humidity.

Mammalian heart: right side pumps deoxygenated blood to the lungs, left side pumps oxygenated blood to the body; valves prevent backflow; cardiac output = heart rate × stroke volume.

Arteries have thick elastic walls and narrow lumens, veins have thin walls and valves, capillaries are one cell thick for efficient exchange.

Tissue fluid forms when plasma is forced out of capillaries at the arteriole end by high hydrostatic pressure; most returns at the venule end due to osmotic effects of plasma proteins, the rest drains via lymph vessels.

Gas exchange surfaces have a large surface area, thin barrier, good blood supply and ventilation to maintain steep gradients.

Insect tracheal system: spiracles lead to tracheae and tracheoles; gas moves by diffusion and mass transport, and ventilation of air sacs assists.

Fish gills: counter-current flow of water and blood maintains a steep diffusion gradient along the whole gill lamella, extracting more oxygen than parallel flow.

Mammalian alveoli: surface area ~70 m², walls one cell thick, surfactant reduces surface tension, ventilation and blood flow maintain gradients.

Aerobic respiration equation: C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP. Stages: glycolysis (cytoplasm), link reaction and Krebs cycle (mitochondrial matrix), oxidative phosphorylation (inner membrane).

Glycolysis yields net 2 ATP and 2 reduced NAD per glucose; Krebs cycle yields reduced NAD and reduced FAD; the electron transport chain generates most ATP via chemiosmosis.

Anaerobic respiration in mammals: glucose → lactate + 2 ATP; in yeast: glucose → ethanol + CO2 + 2 ATP; oxygen debt repays by oxidising lactate in the liver.

Respiratory quotient (RQ) = CO2 produced ÷ O2 consumed; RQ ≈ 1 for carbohydrate, ≈ 0.7 for lipid.

Equation: 6CO2 + 6H2O → C6H12O6 + 6O2, using light energy absorbed by chlorophyll in chloroplasts.

Light-dependent stage occurs in the thylakoid membranes: light excites electrons, photolysis of water gives O2, protons and electrons; reduced NADP and ATP are produced.

Light-independent stage (Calvin cycle) occurs in the stroma: CO2 combines with ribulose bisphosphate (RuBP) using rubisco, forming glycerate-3-phosphate, then triose phosphate, which regenerates RuBP or forms glucose and amino acids.

Chlorophyll absorbs mainly red and blue-violet light and reflects green; absorption spectrum plots absorbance against wavelength, action spectrum plots photosynthesis rate.

Limiting factors: light intensity, CO2 concentration and temperature; raising a non-limiting factor has no effect on rate.

Chromatography separates photosynthetic pigments; Rf value = distance moved by pigment ÷ distance moved by solvent front.

Net photosynthesis = gross photosynthesis − respiration. Increasing light intensity raises the rate until another factor becomes limiting.

Homeostasis maintains a stable internal environment; negative feedback reverses a change, positive feedback amplifies it.

Thermoregulation: hypothalamus detects blood temperature; responses include vasodilation, sweating, shivering, vasoconstriction and reduced sweating.

Blood glucose control: high glucose → β cells of the islets of Langerhans secrete insulin → cells take up glucose and glycogen synthesis rises; low glucose → α cells secrete glucagon → glycogenolysis in the liver.

Diabetes: type 1 is insulin-dependent due to β cell destruction; type 2 involves reduced insulin sensitivity and is managed with diet and exercise.

Neurone structure: dendrites, cell body, axon, myelin sheath, Schwann cells and nodes of Ranvier; saltatory conduction speeds up conduction.

Action potential: resting potential ≈ −70 mV, depolarisation due to Na+ influx, repolarisation due to K+ efflux, refractory period prevents backflow; the all-or-nothing law applies.

Synapse: an impulse triggers Ca2+ entry, vesicles release neurotransmitter into the synaptic cleft, it binds receptors on the postsynaptic membrane, causing depolarisation; acetylcholinesterase breaks it down.

Hormones are chemical messengers secreted by glands into the blood, acting on target cells with complementary receptors; they act more slowly than nerve impulses.

Population = all organisms of one species in a habitat; community = all populations in a habitat; ecosystem = community plus abiotic factors.

Random sampling uses quadrats; systematic sampling uses transects. Estimate population size with the Lincoln index: N = (number in first sample × number in second sample) ÷ number of marked individuals recaptured.

Energy transfer between trophic levels is about 10%; losses occur through respiration, excretion, uneaten parts and heat.

Pyramids of number, biomass and energy; only energy pyramids are always pyramid-shaped.

Interspecific competition occurs between species; intraspecific competition occurs within a species and can regulate population size.

Succession: pioneer species colonise bare rock, gradually altering conditions until a climax community forms; conservation may halt succession (plagioclimax).

Conservation methods include protected areas, seed banks, captive breeding, habitat corridors and sustainable management; biodiversity includes species and genetic diversity.

Carbon cycle: photosynthesis removes CO2; respiration, decomposition and combustion release it. Nitrogen cycle: nitrogen fixation, ammonification, nitrification and denitrification.

Graticule calibration: stage micrometer divisions give the actual size of one eyepiece graticule unit, which changes with magnification.

Differential staining: eosin stains cytoplasm red, haematoxylin stains nuclei blue; Gram staining distinguishes bacterial cell wall types.

Cell walls: cellulose in plants, chitin in fungi, peptidoglycan in bacteria; animal cells lack a wall.

Magnification increases with objective and eyepiece lens power; resolution is limited by the wavelength of the radiation used.

Water is a polar molecule; hydrogen bonding gives high specific heat capacity, high latent heat of vaporisation and cohesion, supporting temperature stability and transport.

Inorganic ions: nitrate for amino acids and DNA, magnesium for chlorophyll, calcium for bones and clotting, phosphate for ATP and nucleic acids, iron for haemoglobin.

Maltose = glucose + glucose; sucrose = glucose + fructose; lactose = glucose + galactose.

Cellulose is a polymer of β-glucose with straight chains cross-linked by hydrogen bonds, giving strength; starch is α-glucose, coiled and branched for storage.

Water potential of plant tissue is estimated by plotting percentage change in mass against sucrose concentration; the concentration giving zero change has the same water potential as the tissue.

Plasmolysis occurs when a plant cell loses water in a hypertonic solution; the vacuole shrinks and the membrane pulls away from the wall.

Animal cells in pure water take up water and may lyse; plant cells become turgid because the cell wall resists further expansion.

Diffusion rate can be measured using agar blocks with indicator; smaller blocks decolourise faster because of the larger surface area to volume ratio.

Number of possible gametes from independent assortment = 2^n, where n is the number of homologous pairs.

Number of cells produced by mitosis after k divisions = 2^k.

Number of DNA molecules in a cell doubles during S phase but chromosome number is unchanged until anaphase.

Mitotic index = number of cells in mitosis ÷ total number of cells observed.

In double-stranded DNA, %A = %T and %C = %G; if %A is known, %T equals it and %C + %G = 100 − 2(%A).

Number of hydrogen bonds between complementary bases: A-T has 2 and C-G has 3.

A triplet codon of 3 bases codes for one amino acid; a polypeptide of n amino acids needs at least 3n bases in the coding strand.

DNA polymerase and RNA polymerase both build nucleic acids in the 5' to 3' direction.

Rate of transpiration can be estimated by the movement of an air bubble in a potometer over a measured distance and time.

Potometer readings estimate water uptake, not water loss, because some water is used in photosynthesis and stored in cells.

Translocation is bidirectional (source to sink); xylem transport is unidirectional (roots to shoots).

Stomata close when water is scarce due to abscisic acid, reducing transpiration but also reducing CO2 uptake.

Pulmonary ventilation rate = tidal volume × breathing rate.

Oxygen dissociation curve shifts right with higher CO2 (Bohr shift), higher temperature and lower pH, increasing oxygen unloading to respiring tissues.

Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin, allowing uptake across the placenta.

Myoglobin has a higher affinity for oxygen than haemoglobin and stores oxygen in muscle.

Rate of photosynthesis can be measured by oxygen production (bubbles), CO2 uptake or biomass increase.

Photosynthesis rate is often measured as volume of oxygen released per unit time per unit area of leaf.

Photolysis of water produces electrons, protons and oxygen; the oxygen released comes from water, not carbon dioxide.

Triose phosphate is used to synthesise glucose, amino acids and lipids, and to regenerate RuBP in the Calvin cycle.

Plant growth substances: auxins promote cell elongation and are used in rooting powders and weedkillers; gibberellins promote stem elongation and seed germination.

Phototropism is caused by unequal distribution of auxin, causing greater elongation on the shaded side.

Negative feedback loop components: receptor, coordinator, effector, response.

Osmoregulation: the hypothalamus detects changes in blood water potential and ADH release alters water reabsorption in the kidney collecting ducts.

A quadrat gives an estimate of percentage cover or frequency; sampling must be random to avoid bias.

Mark-release-recapture assumes no births, deaths, immigration, emigration or marking loss between samples.

Productivity = rate of production of biomass per unit area per unit time; gross primary productivity minus respiration equals net primary productivity.

Deforestation and burning fossil fuels increase atmospheric CO2; eutrophication reduces oxygen levels and kills aquatic organisms.

Rate of reaction = change in amount of product or substrate ÷ change in time; initial rate is taken from the steepest part of the graph.

Vmax is the maximum rate when all active sites are occupied; Km is the substrate concentration at which rate is half of Vmax.

Enzymes are specific because the active site is complementary in shape and charge to the substrate.

Denaturation is irreversible for most enzymes because the tertiary structure is permanently altered.