LC Biology cheat sheet

Hypothesis: a testable, falsifiable statement predicting a relationship between variables; a theory is a well-supported explanatory framework, a law is a descriptive regularity.

Variable types: independent variable is manipulated by the investigator; dependent variable is measured; controlled variables are held constant to prevent confounding.

A control group receives no treatment (or a placebo) and is used to establish a baseline against which the experimental group is compared.

Validity = the experiment measures what it claims to measure; reliability = repeatability of results; accuracy = closeness to true value; precision = closeness of repeated measurements to each other.

Sample size and random selection reduce sampling error; a large representative sample increases the reliability of conclusions drawn.

The biological method sequence: observation → hypothesis → prediction → experiment → data collection → analysis → conclusion → communication/publication.

Scientific names use binomial nomenclature: genus capitalised, species lowercase, both italicised or underlined (e.g. Homo sapiens).

Taxonomic hierarchy: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.

SI units and prefixes: kilo (10³), milli (10⁻³), micro (10⁻⁶), nano (10⁻⁹); length in metres, mass in kilograms, time in seconds.

Bunsen burner use: light with striker, keep flame on yellow safety flame when not heating, use blue flame for heating, never leave unattended.

Qualitative tests: starch + iodine → blue-black; reducing sugar + Benedict's reagent, heat 5 min → brick-red precipitate; protein + Biuret → purple/violet; lipid + Sudan III → red-stained layer.

Benedict's test is semi-quantitative: colour change green → yellow → orange → brick-red indicates increasing reducing sugar concentration.

Use a control in food tests: distilled water instead of sample, to confirm the reagent itself does not change colour.

Microscopy: magnification = eyepiece lens × objective lens; total magnification of 10× eyepiece and 40× objective = 400×.

Specimen size = measured image size ÷ magnification; calibrate the eyepiece graticule with a stage micrometer before measuring.

When drawing a specimen: use a sharp pencil, no shading, label lines straight and not crossing, include title, magnification and scale.

Chromatography: Rf value = distance travelled by pigment ÷ distance travelled by solvent front; Rf is constant for a pigment under fixed conditions.

Record raw data in a table with units in the header, repeat measurements, and calculate a mean; report anomalies and their treatment.

Biomolecule elements: carbohydrates C, H, O; proteins C, H, O, N (sometimes S); lipids C, H, O (less O than carbohydrate); nucleic acids C, H, O, N, P.

Building blocks: monosaccharides (glucose, fructose) → starch/glycogen/cellulose; amino acids → polypeptides; glycerol + 3 fatty acids → triglyceride.

Structural polysaccharides: cellulose in plant cell walls; chitin in fungal walls and arthropod exoskeletons; glycogen is the animal storage carbohydrate.

Enzymes are protein catalysts that lower activation energy; they are specific to substrate at the active site (induced fit).

Factors affecting enzyme rate: temperature (denaturation above optimum, typically ~37 °C in humans), pH (pepsin ~pH 2, trypsin ~pH 8), substrate and enzyme concentration.

Human digestive sequence: mouth (amylase, mechanical breakdown) → stomach (pepsin, HCl) → small intestine (pancreatic amylase, lipase, protease; bile emulsifies fats) → large intestine (water absorption).

Bile is produced in the liver, stored in the gall bladder, and emulsifies lipids into smaller droplets, increasing surface area for lipase.

Villi and microvilli increase surface area for absorption; lacteals absorb fatty acids and glycerol; glucose and amino acids enter blood capillaries.

Deficiency diseases: vitamin C → scurvy; vitamin D/calcium → rickets; iron → anaemia; protein-energy malnutrition → kwashiorkor (oedema) and marasmus.

Eukaryotic organelles: nucleus (DNA, control), mitochondria (aerobic respiration, ATP), ribosomes (protein synthesis), ER (transport/synthesis), Golgi (modification and packaging), lysosomes (hydrolytic enzymes), chloroplasts (photosynthesis).

Prokaryotes lack membrane-bound organelles and a true nucleus; they have a nucleoid, 70S ribosomes, a cell wall and often plasmids and a capsule.

Plant cells uniquely have cellulose cell walls, chloroplasts, a large permanent vacuole and plasmodesmata; both plant and animal cells have mitochondria and ribosomes.

Fluid mosaic model: phospholipid bilayer with embedded proteins, cholesterol, glycolipids and glycoproteins; the membrane is selectively permeable.

Passive transport: diffusion (high → low concentration) and osmosis (water across a semi-permeable membrane from high to low water potential); no ATP required.

Active transport uses carrier proteins and ATP to move substances against a concentration gradient (e.g. sodium-potassium pump, root hair ion uptake).

Osmosis in cells: animal cells lyse in hypotonic solution and shrivel in hypertonic; plant cells become turgid in hypotonic and plasmolysed in hypertonic solutions.

Mitosis produces two genetically identical diploid daughter cells; stages are prophase, metaphase, anaphase, telophase; function is growth, repair and asexual reproduction.

Meiosis produces four genetically varied haploid gametes through two divisions; crossing over in prophase I and independent assortment in metaphase I generate variation.

DNA is a double helix of nucleotides (deoxyribose sugar, phosphate, base A/T/C/G); A pairs with T by two hydrogen bonds, C pairs with G by three.

DNA replication is semi-conservative: helicase unwinds, each strand acts as a template, DNA polymerase adds complementary nucleotides.

Transcription copies DNA into mRNA in the nucleus; translation at the ribosome uses codons and tRNA to assemble amino acids into protein.

Genotype is the allele combination; phenotype is the observable trait; homozygous = identical alleles; heterozygous = different alleles.

Monohybrid cross of two heterozygotes (Tt × Tt) gives a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio.

Mendel's laws: segregation of alleles at meiosis and independent assortment of genes on different chromosomes.

Sex-linked traits are carried on sex chromosomes (e.g. X-linked colour blindness); males express X-linked recessive traits more often because they have one X.

Genetic engineering steps: isolate gene (restriction enzyme), insert into a vector (plasmid), transform host cell, select and clone; examples include human insulin production.

Biotechnology applications: PCR amplifies DNA; gel electrophoresis separates DNA fragments by size; gene therapy, selective breeding and cloning are related technologies.

Aerobic respiration equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP); occurs in cytoplasm (glycolysis) and mitochondria (Krebs cycle, electron transport).

Anaerobic respiration in muscle: glucose → lactic acid + small energy release; in yeast: glucose → ethanol + CO₂ + energy (fermentation).

ATP is the universal energy currency; it releases energy when the terminal phosphate bond is hydrolysed to ADP + Pi.

Photosynthesis equation: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂; occurs in chloroplasts, light-dependent stage in thylakoids, light-independent (Calvin) stage in stroma.

Limiting factors of photosynthesis: light intensity, CO₂ concentration and temperature; the rate plateaus when another factor becomes limiting.

Chlorophyll absorbs mainly red and blue light and reflects green; use of a photosynthometer or pondweed bubble count measures rate.

Respirometer measures oxygen uptake; limewater turns milky with CO₂; these are standard demonstrations of respiration.

Energy transfer between trophic levels is inefficient (about 10% passed on); energy is lost as heat, in movement and in undigested material.

Photolysis of water in the light stage supplies electrons and releases oxygen as a by-product.

Ecosystem = community of organisms plus their physical environment; habitat is where an organism lives; niche is its role including feeding relationships.

Food chain terminology: producer (autotroph), primary/secondary consumer, decomposer; arrows show direction of energy flow.

Population estimation: quadrats for plants/sessile organisms, capture-recapture (Lincoln index) for mobile animals: N = (M × C) ÷ R.

Carbon cycle: photosynthesis removes CO₂; respiration, combustion and decomposition release it; fossil fuel burning increases atmospheric CO₂.

Nitrogen cycle: nitrogen fixation (Rhizobium, lightning), nitrification (Nitrosomonas, Nitrobacter), assimilation, ammonification and denitrification.

Human impact: eutrophication from fertiliser runoff causes algal blooms, oxygen depletion and fish kills; acid rain from SO₂ and NOₓ damages ecosystems.

Biodiversity loss drivers: habitat destruction, pollution, invasive species, overexploitation and climate change; conservation includes protected areas and captive breeding.

Pyramids of numbers can be inverted (e.g. one tree supporting many insects); pyramids of energy are always upright.

Abiotic factors include light, temperature, pH, water and soil minerals; biotic factors include predation, competition and disease.

Mammalian heart: right side pumps deoxygenated blood to lungs (pulmonary circuit), left side pumps oxygenated blood to body (systemic); left ventricle wall is thickest.

Blood vessel structure: arteries thick muscular walls, narrow lumen, high pressure; veins thin walls, valves, low pressure; capillaries one-cell-thick walls for exchange.

Blood components: red cells (haemoglobin, O₂ transport), white cells (immunity), platelets (clotting), plasma (transport of nutrients, wastes, hormones).

Gas exchange surfaces: alveoli have large surface area, thin walls, moist lining and rich blood supply; ventilation maintains a steep diffusion gradient.

Excretion: kidneys filter blood in the nephron (glomerulus, Bowman's capsule, tubules); selective reabsorption returns glucose, amino acids and water; urine contains urea.

Osmoregulation by the kidney is controlled by ADH from the pituitary; ADH increases water reabsorption, producing concentrated urine.

Nervous coordination: stimulus → receptor → sensory neurone → synapse → relay neurone → motor neurone → effector → response; reflex arcs are rapid and involuntary.

Hormones are chemical messengers carried in blood; insulin lowers blood glucose, glucagon raises it; adrenaline prepares the body for fight or flight.

Comparative gas exchange: fish use gills with counter-current flow; insects use tracheae and spiracles; plants use stomata and lenticels.

Asexual reproduction produces genetically identical offspring by mitosis (e.g. binary fission, budding, runners); sexual reproduction involves gametes and meiosis, generating variation.

Human male system: testes produce sperm and testosterone; sperm duct, prostate and urethra deliver semen. Female system: ovaries produce ova and oestrogen; oviduct, uterus, cervix and vagina.

Menstrual cycle: FSH stimulates follicle development, oestrogen builds the uterine lining, LH triggers ovulation (~day 14), progesterone maintains the lining.

Fertilisation occurs in the oviduct; the zygote divides by mitosis to form a blastocyst, which implants in the uterus; the placenta exchanges materials between mother and fetus.

Pregnancy hormones: hCG maintains the corpus luteum early; progesterone and oestrogen are produced by the placenta later.

Growth is increase in size/mass; development is differentiation into specialised tissues and organs; plants show indeterminate growth via meristems, animals determinate growth.

Contraception methods include barrier, hormonal (pill), intrauterine devices and sterilisation; each acts at a different stage of the reproductive process.

In vitro fertilisation (IVF) involves collecting ova, fertilising outside the body, and transferring embryos to the uterus.

Seed formation involves double fertilisation in flowering plants; the zygote forms the embryo and the triploid endosperm provides stored food.

Leaf structure: waxy cuticle, upper epidermis, palisade mesophyll (main photosynthesis), spongy mesophyll (air spaces), vascular bundles (xylem and phloem), stomata with guard cells.

Xylem transports water and minerals upward; vessels are dead, lignified and continuous; transpiration pull and cohesion-tension drive the flow.

Phloem transports sucrose and amino acids bidirectionally; sieve tubes are living cells with companion cells supplying ATP.

Transpiration rate increases with higher temperature, wind, low humidity and high light intensity; stomata close to reduce water loss.

Root hair cells increase surface area for water and mineral uptake by osmosis and active transport.

Tropisms: phototropism (shoot grows toward light, auxin accumulates on shaded side), geotropism (root grows downward); auxins are plant growth regulators.

Plant reproduction: flowers contain stamens (anther, filament) and carpels (stigma, style, ovary); pollination precedes fertilisation; seeds disperse by wind, water or animals.

Vegetative propagation produces clones (e.g. runners, tubers, bulbs); artificial propagation includes cuttings and grafting.

Adaptations: xerophytes have thick cuticles and sunken stomata; hydrophytes have large air spaces and reduced cuticle.