Comprehensive A-level Biology Notes

Overview

This lecture provides a comprehensive summary of the A-level Biology curriculum, covering key concepts, definitions, processes, and practical skills across cellular biology, biochemistry, genetics, physiology, ecology, and biotechnology.

Cell Structure & Microscopy

• Eukaryotic cells (animal and plant) contain membrane-bound organelles: nucleus (with chromosomes and nucleolus), mitochondria (site of respiration and ATP production), Golgi apparatus (modifies and packages proteins/lipids), ribosomes (protein synthesis), and endoplasmic reticulum (rough for proteins, smooth for lipids).
• Plant cells have unique features: chloroplasts (photosynthesis), cell wall (structural support), and a large vacuole (maintains shape and pressure).
• Prokaryotic cells lack membrane-bound organelles, have circular DNA, plasmids, and may have flagella and capsules for movement and protection.
• Cell adaptations include microvilli (increase surface area for absorption), numerous mitochondria (high energy demand), and specialized structures for storage or secretion.
• Microscopes: Optical (light) microscopes use light, have lower resolution, and can view living specimens in color. Electron microscopes use electrons, have much higher resolution, but require dead, fixed samples and produce black-and-white images.
• Magnification calculations: magnification = image size ÷ actual size. Always convert units (mm, μm, nm) as needed.
• Key microscope parts: eyepiece, objective lenses, stage, coarse/fine focus, light source.
• Biological drawings should be clear, labeled, accurate, with no shading, and include a scale.

Biological Molecules

• Water is a polar molecule, forms hydrogen bonds, and is essential for metabolic reactions, temperature regulation, and as a solvent. Its properties include high specific heat capacity, high latent heat of vaporization, cohesion, and surface tension.
• Inorganic ions (e.g., Na+, PO4^3-, H+, Fe2+/Fe3+) have specific roles: sodium in co-transport, phosphate in DNA/ATP, hydrogen in pH, iron in hemoglobin.
• Monomers (amino acids, nucleotides, glucose) join to form polymers (proteins, nucleic acids, polysaccharides) via condensation reactions; hydrolysis breaks them down.
• Reducing sugars (e.g., glucose) are detected with Benedict's test (brick red precipitate); non-reducing sugars require hydrolysis first. Starch is tested with iodine (blue-black), lipids with ethanol (cloudy emulsion), and proteins with Biuret reagent (purple).
• Triglycerides consist of three fatty acids and glycerol; saturated fats have only single C–C bonds, unsaturated have double bonds. Phospholipids have hydrophilic heads and hydrophobic tails, forming cell membranes.
• Protein structure: primary (amino acid sequence), secondary (α-helix/β-sheet), tertiary (3D folding via bonds), quaternary (multiple polypeptide chains). Bonds include hydrogen, ionic, and disulfide bridges.
• Chromatography separates mixtures (e.g., amino acids, pigments) based on solubility and interactions with stationary/mobile phases.

Enzymes

• Enzymes are biological catalysts that lower activation energy and speed up reactions. They have specific active sites (induced fit model) complementary to substrates.
• Factors affecting enzyme activity: temperature (optimum, denaturation above), pH (optimum range), substrate/enzyme concentration (saturation), and inhibitors (competitive bind active site; non-competitive bind elsewhere and change active site shape).
• Coenzymes (organic, e.g., NAD, NADP) and cofactors (inorganic, e.g., metal ions) assist enzyme function; prosthetic groups are permanently attached.
• Enzyme activity can be measured by product formation, substrate use, or color change (e.g., using indicators and colorimeters).
• Enzyme inhibition: competitive inhibitors compete for active site; non-competitive inhibitors bind elsewhere and reduce maximum rate.

Membranes & Transport

• The fluid mosaic model describes plasma membranes as a flexible bilayer of phospholipids with embedded proteins, cholesterol (stability), glycoproteins, and glycolipids (cell recognition).
• Transport mechanisms:
  • Diffusion: passive movement down a concentration gradient.
  • Facilitated diffusion: passive, via channel or carrier proteins for charged or large molecules.
  • Osmosis: diffusion of water from high to low water potential across a partially permeable membrane.
  • Active transport: movement against a concentration gradient using ATP and carrier proteins.
  • Co-transport: two substances moved together, e.g., sodium-glucose co-transport in the gut.
  • Bulk transport: endocytosis (into cell), exocytosis (out of cell), both require ATP.
• Membrane permeability is affected by temperature, solvents, and detergents.

Cell Division

• The cell cycle: interphase (G1—growth, S—DNA synthesis, G2—preparation), mitosis (prophase, metaphase, anaphase, telophase), and cytokinesis.
• Mitosis produces two genetically identical diploid daughter cells for growth, repair, and asexual reproduction.
• Meiosis involves two divisions, producing four non-identical haploid gametes, increasing genetic variation via crossing over and independent assortment.
• Practical skills: root tip squashes to observe mitosis, calculation of mitotic index (cells in mitosis ÷ total cells).

Specialization, Tissues & Organs

• Stem cells are undifferentiated and can become specialized cells (e.g., red blood cells—no nucleus, biconcave; neutrophils—lobed nucleus; sperm—acrosome, flagellum; palisade cells—many chloroplasts; root hair cells—large surface area).
• Animal tissues: epithelial (lining), connective (support, e.g., cartilage), muscle (movement: skeletal, cardiac, smooth), nervous (signal transmission).
• Plant tissues: epidermal (protection), vascular (xylem for water, phloem for sugars), meristematic (growth, stem cells).
• Organs are groups of tissues working together for a specific function.

Gas Exchange, Circulatory System & Blood

• Gas exchange surfaces (alveoli in lungs, gills in fish, tracheae in insects) maximize surface area, minimize diffusion distance, and maintain concentration gradients.
• Mammals have a double circulatory system (pulmonary and systemic circuits) for efficient oxygen delivery.
• Blood vessels: arteries (thick walls, high pressure, away from heart), veins (valves, low pressure, to heart), capillaries (one cell thick, exchange).
• Tissue fluid forms at capillaries by pressure filtration; excess fluid is returned by osmosis and lymphatic drainage.
• Heart structure: atria, ventricles, valves; cardiac cycle includes diastole (relaxation), atrial systole, and ventricular systole (contraction).
• Heartbeat control: SAN (pacemaker), AVN, bundle of His, Purkinje fibers; ECG traces show P wave (atrial contraction), QRS (ventricular contraction), T wave (relaxation).
• Haemoglobin binds oxygen; affinity changes with partial pressure (Bohr effect: higher CO2 lowers affinity, shifting curve right).

Plant Transport

• Xylem transports water and minerals via cohesion-tension (transpiration pull); phloem transports sugars by mass flow (source to sink).
• Transpiration is affected by light, temperature, wind, and humidity; measured with a potometer.
• Xerophyte adaptations (e.g., thick cuticle, sunken stomata, hairs) reduce water loss.
• Water moves through plants by apoplast (cell walls) and symplast (cytoplasm) pathways; Casparian strip forces selective uptake.

Immunity & Disease

• Pathogens: bacteria, viruses, fungi, protoctists; transmission is direct (contact, droplets) or indirect (vectors, water, food).
• Non-specific defenses: skin (barrier, antimicrobial), mucous membranes (trap pathogens), inflammation, blood clotting, wound repair.
• Specific immunity: phagocytosis (engulf pathogens), T cells (cell-mediated: helper, cytotoxic, memory), B cells (humoral: plasma cells produce antibodies, memory cells).
• Antibodies have variable regions for antigen specificity; can agglutinate pathogens, neutralize toxins, or act as opsonins.
• Immunity: active (infection/vaccination, long-term, memory cells), passive (antibodies from another source, short-term).
• Vaccines induce herd immunity; ethical issues include testing, side effects, and access.
• Autoimmune diseases occur when the immune system attacks self-antigens (e.g., lupus, rheumatoid arthritis).
• Antibiotic resistance arises from overuse/misuse; can be studied using bacterial cultures and antibiotic discs.

Biodiversity, Sampling & Conservation

• Biodiversity includes species richness (number of species), evenness (relative abundance), and genetic diversity (variety of alleles).
• Sampling methods:
  • Random (quadrats for plants, random coordinates).
  • Systematic (transects for gradients).
  • Mark-release-recapture for mobile animals (estimate population size).
• Simpson's Index quantifies diversity (closer to 1 = more diverse).
• Conservation:
  • In situ (protecting species in their natural habitat: reserves, marine zones).
  • Ex situ (zoos, seed banks, botanic gardens).
• International agreements (e.g., CITES, Rio Convention) and local schemes (e.g., Countryside Stewardship) support conservation.
• Human impacts: habitat loss, overexploitation, pollution, climate change, urbanization, and modern farming reduce biodiversity.
• Sustainable management balances conservation with human needs (e.g., sustainable fishing, forestry, agriculture).

Genetics & Inheritance

• DNA: double helix, complementary base pairing (A-T, C-G); RNA is single-stranded, uses uracil instead of thymine.
• DNA replication is semiconservative (each new molecule has one old and one new strand).
• Protein synthesis: transcription (DNA to mRNA), splicing (removal of introns), translation (mRNA to protein at ribosome).
• Key genetic terms: gene (codes for polypeptide), allele (version of a gene), locus (gene location), intron/exon (non-coding/coding), homologous chromosomes (matching pairs).
• Inheritance patterns:
  • Monohybrid (one gene, two alleles).
  • Dihybrid (two genes, four possible allele combinations).
  • Codominance (both alleles expressed), multiple alleles (more than two forms), sex linkage (genes on X/Y chromosomes), epistasis (one gene affects another).
• Hardy-Weinberg equations predict allele and genotype frequencies in populations (assumes no evolution).
• Evolution: natural selection (advantageous alleles increase), genetic drift (random changes, especially in small populations).
• Speciation: allopatric (geographical isolation), sympatric (behavioral or ecological isolation).

Genetic Technologies & Biotechnology

• DNA sequencing determines base order; high-throughput methods allow rapid, large-scale sequencing.
• PCR (polymerase chain reaction) amplifies DNA fragments; gel electrophoresis separates DNA by size for analysis (e.g., genetic fingerprinting).
• Genetic engineering uses restriction enzymes (cut DNA), ligases (join DNA), and vectors (plasmids, viruses) to insert genes into organisms.
• Gene therapy replaces faulty alleles; can be somatic (body cells, not inherited) or germ-line (gametes/zygotes, inherited—currently not allowed in humans).
• Genetically modified organisms (GMOs) are used in agriculture (pest resistance, higher yield), medicine (insulin production), and research; benefits and risks must be considered.
• Cloning: natural (identical twins, asexual reproduction), artificial (embryo splitting, somatic cell nuclear transfer); used for conservation, agriculture, and biotechnology.

Ecology & Energy Transfer

• Ecosystems consist of producers (autotrophs), consumers (primary, secondary, tertiary), and decomposers (break down dead matter).
• Energy transfer between trophic levels is inefficient (~10% per level); measured as biomass or energy pyramids.
• Productivity: gross primary productivity (GPP, total energy fixed by producers), net primary productivity (NPP, GPP minus respiratory losses).
• Nutrient cycles:
  • Nitrogen cycle: ammonification, nitrification, denitrification, nitrogen fixation; involves bacteria and decomposers.
  • Carbon cycle: photosynthesis, respiration, decomposition, combustion, fossilization.
• Succession: changes in species composition over time, from pioneer species to climax community; can be primary (bare rock) or secondary (after disturbance).
• Human activities (deforestation, pollution, overfishing, agriculture) reduce biodiversity and disrupt cycles; sustainable management is essential.

Key Terms & Definitions

• Osmosis: Diffusion of water from high to low water potential across a partially permeable membrane.
• Enzyme: Biological catalyst that speeds up reactions by lowering activation energy.
• Genotype: Genetic makeup of an organism.
• Phenotype: Observable characteristics resulting from genotype and environment.
• Allele: Different version of a gene.
• Transpiration: Evaporation of water from plant leaves.
• Homeostasis: Maintenance of a stable internal environment.
• Pathogen: Organism that causes disease.
• Hardy-Weinberg principle: Predicts allele/genotype frequencies in a non-evolving population.

Action Items / Next Steps

• Review practical skills and methods (e.g., microscope calibration, Benedict's test, quadrat sampling, colorimetry, mitotic index).
• Practice past paper questions, focusing on data interpretation and application.
• Revise and memorize definitions for all key terms to improve exam performance.
• Practice calculations for Hardy-Weinberg, genetic crosses, and energy transfer.
• Ensure understanding of all required practicals and their evaluation, including sources of error and improvements.