FSc Part 1 Biology key facts

Introduction

Branches of biology and levels of organisation

• Order of levels: subatomic and atomic → molecular → organelle and cell → tissue → organ and system → individual → population → community → biosphere.
• Six bioelements make up about 99% of the mass of protoplasm: O 65%, C 18%, H 10%, N 3%, Ca 2%, P 1%.
• Morphology = form and structure; anatomy = internal structure; histology = microscopic study of tissues; physiology = functions of the parts.
• Embryology = development from the fertilised egg; taxonomy = naming and classification; palaeontology = fossils; genetics = heredity; ecology = organisms and their environment.
• Population = members of one species living in one place at one time; community = all the populations of different species living in one place.
• Biosphere = the part of the earth where life exists.
• Hydroponics = growing plants in an aerated nutrient solution without soil; biological control = using a living organism to control a pest; cloning = making genetically identical copies.

The biological method

• Steps in order: observation → hypothesis → deduction → experiment → theory → law.
• Inductive reasoning goes from specific cases to a general statement; deductive reasoning goes from a general statement to a specific case ("if ... then").
• Laveran found Plasmodium in the blood of malaria patients; this led to the hypothesis that Plasmodium causes malaria.
• A.F.A. King listed observations linking malaria with mosquitoes, for example that it is commoner in low, marshy places.
• Ronald Ross found Plasmodium in female Anopheles mosquitoes that had bitten patients, and showed with sparrows that mosquitoes pass it on.
• Control group = the group not given the factor under test; everything else is kept the same.
• Large samples and repeated experiments reduce the effect of chance; data are organised in tables and graphs.

Biological molecules

Water, carbohydrates and lipids

• Monosaccharides have 3 to 7 carbons: glyceraldehyde (triose), ribose (pentose), glucose, fructose and galactose (hexoses, C₆H₁₂O₆).
• Disaccharides: maltose = glucose + glucose; sucrose = glucose + fructose; lactose = glucose + galactose. The link is a glycosidic bond and one H₂O is released for each bond.
• Starch: food store of plants, made of amylose (unbranched) and amylopectin (branched); blue with iodine. Glycogen: food store of animals (liver and muscles), highly branched; red with iodine.
• Cellulose: unbranched chains of glucose in plant cell walls; no colour with iodine. Chitin: polymer of N-acetyl glucosamine in arthropod exoskeletons and fungal cell walls.
• Triacylglycerol (triglyceride) = 1 glycerol + 3 fatty acids, with 3 ester bonds and 3 H₂O released.
• Saturated fatty acids have no C=C double bond (fats, solid at room temperature); unsaturated fatty acids have one or more C=C (oils, liquid).
• Phospholipid = glycerol + 2 fatty acids + phosphate group; waxes = long-chain fatty acids + long-chain alcohols; terpenoids are built of isoprenoid units (carotenoids, steroids, rubber).

Proteins, nucleic acids and conjugated molecules

• Peptide bonds in one chain = number of amino acids − 1.
• Primary structure = number and sequence of amino acids; secondary = α-helix or β-pleated sheet held by hydrogen bonds; tertiary = folding of one chain into a 3D shape; quaternary = two or more chains held together.
• Fibrous proteins are long strands, insoluble in water, structural (keratin, silk fibre, myosin); globular proteins are spherical and soluble (enzymes, antibodies, hormones, haemoglobin).
• Insulin: 51 amino acids in two chains (21 + 30), sequence found by Sanger. Haemoglobin: 574 amino acids in four chains (two α of 141, two β of 146).
• Nucleotide = pentose sugar + nitrogenous base + phosphoric acid. Purines (double ring): adenine, guanine. Pyrimidines (single ring): cytosine, thymine, uracil.
• DNA (Watson and Crick): double helix; A pairs with T by 2 hydrogen bonds, G pairs with C by 3; so A = T and G = C. RNA is single stranded, has ribose, and uracil in place of thymine; rRNA is about 80% of cell RNA.
• Glycolipid = carbohydrate + lipid; glycoprotein = carbohydrate + protein; lipoprotein = lipid + protein; nucleoprotein = nucleic acid + protein.

Enzymes

Characteristics and mechanism of enzyme action

• Enzyme + substrate → enzyme-substrate complex → enzyme + product.
• Active site = binding site (holds the substrate) + catalytic site (changes it into product).
• Cofactor = non-protein part needed by some enzymes. Activator: a detachable inorganic ion such as Mg²⁺. Prosthetic group: covalently (firmly) bonded. Coenzyme: loosely attached organic molecule, often made from a vitamin.
• Apoenzyme (protein part) + cofactor = holoenzyme (the complete, active enzyme).
• Lock and key model (Emil Fischer): the active site is rigid and the substrate fits it exactly, like a key in a lock.
• Induced fit model (Koshland): the active site is flexible and changes shape to fit the substrate when it binds.
• Enzymes lower the activation energy; they are not used up and do not change the end products.

Factors affecting enzyme action, inhibition and classification

• Enzyme concentration: with excess substrate, rate is directly proportional to enzyme concentration.
• Substrate concentration: rate rises until all active sites are occupied (saturation), then stays constant.
• Temperature: rate rises up to the optimum (37 °C for human enzymes); above it the enzyme is denatured and the rate falls sharply.
• Optimum pH: pepsin 2.0, salivary amylase 6.8, pancreatic lipase 9.0.
• Competitive inhibitor: shaped like the substrate, binds at the active site, overcome by adding more substrate. Non-competitive inhibitor: binds at another site and changes the shape of the active site; more substrate does not help.
• Irreversible inhibitors (cyanide, heavy metal ions) bind tightly and destroy the enzyme's activity permanently.
• Six classes: oxidoreductases (oxidation-reduction), transferases (move a group), hydrolases (break with water), lyases (break bonds without water), isomerases (rearrange atoms), ligases (join molecules using ATP).

The cell

Cell theory, microscopy, cell wall and plasma membrane

• Hooke (1665) saw cells in cork; Robert Brown (1831) the nucleus; Schleiden (1838) plants are made of cells; Schwann (1839) animals are made of cells; Virchow (1855) "omnis cellula e cellula".
• magnification = size of image ÷ actual size of object; 1 mm = 1000 µm.
• Cell fractionation: cells are broken open and centrifuged at increasing speeds; nuclei settle first, then mitochondria and chloroplasts, then ribosomes.
• Cell wall from outside to inside: middle lamella (calcium and magnesium pectates) → primary wall (cellulose with pectin and hemicellulose) → secondary wall (cellulose, lignin and other deposits).
• Fluid mosaic model (Singer and Nicolson, 1972): phospholipid bilayer, hydrophilic heads outwards and hydrophobic tails inwards, with proteins embedded.
• Diffusion, facilitated diffusion and osmosis go down the concentration gradient and need no energy; active transport goes against the gradient and uses ATP and carrier proteins.
• Endocytosis takes material in: phagocytosis for solids, pinocytosis for liquids; exocytosis sends material out.

Cytoplasm and membrane-bound organelles

• Rough ER has ribosomes and makes and transports proteins; smooth ER has none and deals with lipid (steroid) metabolism and detoxification of drugs.
• Ribosomes: rRNA + protein, no membrane. Eukaryotic 80S = 60S + 40S subunits; prokaryotic 70S = 50S + 30S. A polysome is a group of ribosomes on one mRNA.
• Golgi apparatus: stacks of flattened sacs (cisternae); modifies, packages and sorts proteins for secretion; forms lysosomes.
• Lysosomes: single membrane, hydrolytic enzymes; digest food particles and worn-out organelles (autophagy). Storage diseases occur when one lysosomal enzyme is missing, e.g. glycogenosis type II and Tay-Sachs disease.
• Peroxisomes: oxidative enzymes that form H₂O₂, and catalase that breaks it down. Glyoxysomes: in germinating fatty seeds, change fats into carbohydrates.
• Mitochondria: inner membrane folded into cristae, matrix inside. Chloroplasts: stroma with grana, each granum a pile of thylakoids. Plastids: chloroplasts (green), chromoplasts (other colours), leucoplasts (colourless, storage).
• Microtubules are made of tubulin, microfilaments of actin. Centriole = 9 triplets of microtubules; cilia and flagella = 9 + 2 (nine doublets around two single microtubules).

Nucleus and prokaryotic and eukaryotic cells

• Nucleolus: no membrane; makes rRNA and assembles the subunits of ribosomes.
• Euchromatin is loosely coiled, lightly stained and active; heterochromatin is tightly coiled, darkly stained and inactive.
• A chromosome at the start of division has two chromatids joined at one centromere. Centromere in the middle = metacentric; slightly off the middle = sub-metacentric; near one end = acrocentric; at the end = telocentric.
• Chromosome number is fixed for a species: human 46, chimpanzee 48, frog 26, fruit fly 8, onion 16, garden pea 14.
• Prokaryotes: DNA in a nucleoid, circular and without histones; 70S ribosomes; wall of peptidoglycan (murein); divide by binary fission.
• Eukaryotes: nuclear envelope; linear DNA with histones; 80S ribosomes in the cytoplasm; membrane-bound organelles; divide by mitosis or meiosis.
• Plant cell: cellulose cell wall, chloroplasts, large central vacuole, no centrioles. Animal cell: no cell wall, no chloroplasts, small vacuoles, centrioles present.

Variety of life

Classification, nomenclature and the kingdom systems

• Hierarchy from largest to smallest: kingdom → phylum (division in plants) → class → order → family → genus → species.
• Binomial nomenclature was introduced by Carolus Linnaeus: first word = genus, second word = species.
• Writing rules: genus starts with a capital letter, species with a small letter; printed in italics or underlined when handwritten, e.g. Homo sapiens.
• Two-kingdom system: Plantae and Animalia. Ernst Haeckel (1866) added a third kingdom, Protista, for organisms such as Euglena that are neither plants nor animals.
• Robert Whittaker (1969), five kingdoms: Monera, Protista, Fungi, Plantae, Animalia, based on cell organisation and mode of nutrition (photosynthesis, absorption, ingestion).
• Margulis and Schwartz later modified it: Prokaryotae (Monera), Protoctista, Fungi, Plantae, Animalia.
• Viruses are acellular and are not placed in any of the five kingdoms.

Viruses

• Iwanowsky (1892) showed that the agent of tobacco mosaic disease passes through filters that hold back bacteria; Stanley (1935) crystallised the virus.
• Capsid = protein coat made of subunits called capsomeres; some viruses also have an outer envelope.
• T₄ phage: head containing double-stranded DNA, tail with a contractile sheath, base plate and six tail fibres.
• Lytic cycle: attachment → injection of DNA → replication → assembly → lysis (bursting) of the bacterium.
• Lysogenic cycle: phage DNA becomes a prophage in the host chromosome; induction (for example by ultraviolet light) switches it to the lytic cycle.
• HIV is a retrovirus: reverse transcriptase makes DNA from its RNA; it destroys helper T lymphocytes and causes AIDS.
• Hepatitis A and E spread by contaminated food and water; B, C and D spread by blood and body fluids; D infects only people who already have B.

Kingdom Prokaryotae

Structure, forms and nutrition of bacteria

• Gram-positive: thick peptidoglycan wall, keeps crystal violet, looks purple. Gram-negative: thin wall plus an outer membrane, loses the dye in alcohol, looks pink.
• Shapes: cocci (spheres), bacilli (rods), spiral forms (vibrio = comma, spirillum, spirochete).
• Flagella: atrichous (none), monotrichous (one at one end), lophotrichous (tuft at one end), amphitrichous (at both ends), peritrichous (all over).
• Pili: attachment and conjugation. Mesosomes: infoldings of the cell membrane that help in DNA replication, cell division and respiration.
• Plasmids: small circular DNA outside the chromosome, often with antibiotic resistance genes. Endospores: resistant resting bodies, e.g. Bacillus and Clostridium.
• Nutrition: heterotrophs (saprotrophs and parasites); autotrophs (photosynthetic and chemosynthetic). Photosynthetic bacteria use H₂S, not water, so they release sulphur and no oxygen.
• Respiration: obligate aerobes, obligate anaerobes, facultative anaerobes and microaerophilic bacteria (need low oxygen).

Growth, reproduction, importance and control of bacteria; cyanobacteria

• Lag: cells adjust, no increase. Log: fastest division. Stationary: rate of division = rate of death. Death: cells die faster than they divide.
• Number of cells = starting number × 2ⁿ, where n = number of generations = time ÷ generation time.
• Conjugation: DNA passes through a pilus between two touching cells. Transformation: a cell takes up free DNA (Griffith). Transduction: a bacteriophage carries the DNA.
• Sterilisation kills all microbes and endospores, e.g. steam under pressure in an autoclave at 121 °C. Pasteurisation uses moderate heat (about 71 °C for 15 seconds or about 62 °C for 30 minutes) and does not kill endospores.
• Antibiotics are substances made by microbes that kill or stop the growth of other microbes; penicillin was discovered by Alexander Fleming.
• Cyanobacteria have chlorophyll a and phycocyanin and release oxygen.
• Nostoc: a filament (trichome) in a jelly sheath; heterocysts fix nitrogen; akinetes are resting spores; hormogonia are short pieces for reproduction.

Kingdom Protista

Protozoa: animal-like protists

• Amoebas: move and feed with pseudopodia. Entamoeba histolytica causes amoebic dysentery.
• Zooflagellates: Trypanosoma causes African sleeping sickness, spread by the tsetse fly. Trichonympha lives in the gut of termites and digests cellulose.
• Ciliates (Paramecium, Vorticella, Stentor): cilia, a macronucleus for everyday metabolism and growth, a micronucleus for conjugation.
• Contractile vacuoles of freshwater protozoa pump out the water that enters by osmosis.
• Foraminiferans: shells (tests) of calcium carbonate with pores; their remains form chalk and limestone.
• Actinopods (e.g. radiolarians): glassy silica shells and thin axopodia.
• Apicomplexans: all parasites, form spores, no locomotory organelles in adults. Plasmodium causes malaria and is spread by the female Anopheles mosquito.

Algae and fungus-like protists

• Euglenoids (Euglena): flexible protein pellicle, no cell wall, eyespot, chlorophylls a and b; can also absorb organic food in the dark.
• Dinoflagellates: cellulose plates, two flagella in grooves; blooms cause red tides.
• Diatoms: silica shell in two overlapping halves; chlorophylls a and c; important producers; remains form diatomaceous earth.
• Green algae (Chlorella, Spirogyra, Ulva): chlorophylls a and b, starch, cellulose walls.
• Brown algae (kelps, Fucus, Laminaria): fucoxanthin, food stored as laminarin, algin in walls.
• Red algae (Polysiphonia): phycoerythrin absorbs the blue and green light of deep water; source of agar.
• Plasmodial slime mould: a multinucleate mass of cytoplasm with no walls. Water moulds: cellulose walls and flagellated spores; Phytophthora infestans causes late blight of potato.

Kingdom Fungi

Body, nutrition and reproduction of fungi

• Septate hyphae have cross-walls (septa) with pores; non-septate (coenocytic) hyphae have none and hold many nuclei in one cytoplasm.
• Saprotrophs anchor with rhizoids; parasites absorb from living host cells through haustoria; Arthrobotrys is a predator that traps nematodes.
• Lichen = fungus + alga or cyanobacterium. The alga makes food; the fungus gives protection, water and minerals. Lichens are indicators of air pollution.
• Mycorrhiza = fungus + plant roots. The fungus supplies minerals such as phosphate; the plant supplies sugars.
• Asexual reproduction: spores in sporangia, conidia cut off from the tips of conidiophores, budding in yeast, fragmentation.
• Sexual reproduction: plasmogamy (fusion of cytoplasm) → dikaryotic stage (n + n) → karyogamy (fusion of nuclei, 2n) → meiosis → haploid spores.

Classification and importance of fungi

• Zygomycota: non-septate hyphae, zygospores; Rhizopus (black bread mould).
• Ascomycota: usually 8 ascospores in each ascus; yeasts, morels, truffles, powdery mildews.
• Basidiomycota: 4 basidiospores on each basidium; mushrooms, puffballs, rusts and smuts.
• Deuteromycota: reproduce by conidia only; Penicillium, Aspergillus.
• Yeast (Saccharomyces cerevisiae) ferments sugar: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. The carbon dioxide raises bread dough.
• Useful products: penicillin from Penicillium, cyclosporine for organ transplants. Harmful: aflatoxins from Aspergillus flavus; ergotism from Claviceps purpurea on rye.
• Diseases: rusts (Puccinia) and smuts (Ustilago) in wheat; ringworm and athlete's foot of the skin; candidiasis caused by Candida.

Kingdom Plantae

Bryophytes and seedless vascular plants

• Bryophytes: liverworts (Marchantia, Riccia), hornworts (Anthoceros), mosses (Funaria, Polytrichum).
• Moss: spore → protonema → leafy shoot (all n); sporophyte = foot, seta and capsule (2n), attached to the gametophyte.
• Sex organs: antheridia make sperm (antherozoids); archegonia each hold one egg.
• Microphyll: small leaf with one vein, evolved as an enation (outgrowth) of the stem. Megaphyll: large leaf with many veins, evolved by overtopping → planation → webbing (fusion).
• Tracheophytes: Psilopsida (Psilotum, Rhynia: no true roots or leaves), Lycopsida (club mosses), Sphenopsida (Equisetum), Pteropsida (ferns and seed plants).
• Adiantum: sori lie under the folded margin of the leaflet (false indusium); spores grow into a heart-shaped prothallus (n) bearing antheridia and archegonia.

Seed plants: gymnosperms and angiosperms

• Pinus: needle leaves, male and female cones on the same tree, winged pollen carried by wind, winged seeds.
• Pinus endosperm is the female gametophyte, formed before fertilisation: haploid (n).
• Double fertilisation: male gamete + egg → zygote (2n); second male gamete + two polar nuclei → endosperm nucleus (3n).
• Rosaceae: 5 free petals, many stamens; rose, apple, pear, peach, strawberry.
• Solanaceae: 5 fused petals, 5 stamens on the corolla, 2 carpels, fruit a berry or capsule; potato, tomato, brinjal, tobacco.
• Fabaceae (Papilionaceae): standard, two wings and keel, fruit a legume, root nodules with Rhizobium; pea, gram, bean.
• Poaceae: flowers in spikelets, fruit a caryopsis (grain); wheat, rice, maize, sugarcane.

Kingdom Animalia

Bases of classification and non-coelomate phyla

• Protostomes: blastopore becomes the mouth. Deuterostomes (echinoderms and chordates): blastopore becomes the anus.
• Porifera: water enters by ostia, passes the spongocoel and leaves by the osculum; choanocytes (collar cells) make the current and trap food; skeleton of spicules or spongin. Examples: Sycon, Spongilla.
• Cnidaria: gastrovascular cavity with one opening, cnidocytes (stinging cells), polyp and medusa forms. Examples: Hydra, Obelia, jellyfish (Aurelia), sea anemone, corals.
• Metagenesis: an asexual polyp generation alternates with a sexual medusa generation, as in Obelia.
• Platyhelminthes: flat, bilateral, gut with a mouth but no anus, flame cells for excretion. Examples: Planaria, liver fluke (Fasciola), tapeworm (Taenia).
• Nematoda: unsegmented round body pointed at both ends, gut with mouth and anus, separate sexes. Examples: Ascaris, hookworm (Ancylostoma), pinworm (Enterobius).

Coelomate phyla and Chordata

• Annelida: metameric segmentation, closed blood system, nephridia. Polychaeta (Nereis), Oligochaeta (earthworm, Pheretima), Hirudinea (leech: no setae, makes hirudin).
• Arthropoda: jointed appendages, chitin exoskeleton, open blood system (haemocoel). Insects have 3 pairs of legs, arachnids 4 pairs and no antennae; centipedes 1 pair of legs per segment, millipedes 2 pairs.
• Mollusca: soft unsegmented body, mantle, shell, muscular foot, radula. Gastropoda (snail), Pelecypoda or Bivalvia (mussel), Cephalopoda (Sepia, octopus).
• Echinodermata: spiny skin, water vascular system with tube feet, radial adults, bilateral larvae, all marine.
• Hemichordata: Balanoglossus (acorn worm), body of proboscis, collar and trunk.
• Chordate subphyla: Urochordata (notochord only in the larval tail), Cephalochordata (Amphioxus, notochord for life, along the whole body), Vertebrata.
• Mammals: Prototheria lay eggs (platypus), Metatheria have a pouch (kangaroo), Eutheria have a true placenta. Cartilaginous fishes have placoid scales and no operculum.

Bioenergetics

Photosynthesis: pigments and light-dependent reactions

• Overall equation: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O (light and chlorophyll needed).
• Chlorophyll a has a –CH₃ group where chlorophyll b has –CHO. Both have a porphyrin head with magnesium at the centre and a phytol tail.
• Chlorophylls absorb mainly violet-blue and orange-red light and reflect green. Carotenoids absorb other wavelengths and protect chlorophyll.
• Absorption spectrum: light absorbed by a pigment at each wavelength. Action spectrum: rate of photosynthesis at each wavelength.
• Photosystem I has reaction centre P700. Photosystem II has P680 and is linked to the photolysis of water.
• Non-cyclic phosphorylation: both photosystems; makes ATP, NADPH and O₂.
• Cyclic phosphorylation: photosystem I only; makes ATP only, no NADPH and no O₂.

Photosynthesis: light-independent reactions and photorespiration

• Fixation: 3CO₂ + 3RuBP → 6PGA (first stable product, 3 carbons, so these are C₃ plants).
• Reduction: 6PGA → 6G3P, using 6 ATP and 6 NADPH.
• Regeneration: 5 of the 6 G3P remake 3RuBP, using 3 ATP; 1 G3P is the net product.
• Per CO₂ fixed: 3 ATP and 2 NADPH.
• One glucose needs 6CO₂: 18 ATP and 12 NADPH.
• Photorespiration: RuBP + O₂ gives one PGA and one 2-carbon glycolate; CO₂ is later released. Organelles involved: chloroplast, peroxisome, mitochondrion.

Respiration

• Aerobic: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy.
• Alcoholic fermentation (yeast): pyruvic acid → ethyl alcohol + CO₂. Lactic acid fermentation (muscle): pyruvic acid → lactic acid, no CO₂.
• Glycolysis per glucose: net 2 ATP (4 made, 2 used), 2 NADH, 2 pyruvic acid.
• Oxidation of each pyruvic acid: 1 acetyl CoA + 1 CO₂ + 1 NADH.
• Each turn of the Krebs cycle: 2 CO₂, 3 NADH, 1 FADH₂, 1 ATP. Two turns per glucose.
• In the respiratory chain, 1 NADH gives 3 ATP and 1 FADH₂ gives 2 ATP. Oxygen is the final electron acceptor.
• Total per glucose with these values: 4 ATP made directly + 30 from 10 NADH + 4 from 2 FADH₂ = 38 ATP at most; 36 ATP net where 2 ATP are spent carrying the glycolysis NADH into the mitochondrion. Newer books give a lower yield, about 30 to 32.

Nutrition

Nutrition in plants and non-human animals

• Nitrogen: part of proteins, nucleic acids and chlorophyll. Deficiency gives stunted growth and chlorosis, seen first in older leaves.
• Magnesium: central atom of chlorophyll. Deficiency gives chlorosis.
• Parasitic plant: Cuscuta (dodder) absorbs food through haustoria.
• Insectivorous plants: Nepenthes (pitcher plant), Dionaea (Venus flytrap), Drosera (sundew).
• Amoeba: food taken in by pseudopodia; digestion intracellular in a food vacuole.
• Hydra and Planaria: sac-like gut with a single opening; digestion extracellular first, then intracellular. Planaria has a branched intestine.
• Cockroach: tube-like gut. Crop stores food, gizzard grinds it with chitinous teeth, hepatic (gastric) caeca secrete enzymes.

Digestion in humans and its disorders

• Saliva: amylase (ptyalin) turns starch into maltose; pH about 7.
• Gastric glands: mucous cells make mucus, parietal (oxyntic) cells make HCl, chief (zymogen) cells make pepsinogen. HCl changes pepsinogen into pepsin; pepsin turns proteins into peptides at about pH 2.
• Bile (made in the liver, stored in the gall bladder) has no enzymes; bile salts emulsify fats.
• Pancreatic juice: amylase, lipase, trypsinogen, and bicarbonate to neutralise acid chyme. Enterokinase changes trypsinogen into trypsin.
• Hormones: gastrin (stomach) → gastric juice; secretin (duodenum) → bicarbonate-rich pancreatic juice and bile; cholecystokinin → enzyme-rich pancreatic juice and emptying of the gall bladder.
• Diarrhoea: too little water absorbed. Constipation: too much water absorbed. Peptic ulcer: gut wall damaged by acid and pepsin, often with Helicobacter pylori.
• Anorexia nervosa: loss of appetite from fear of becoming fat. Bulimia nervosa: overeating followed by forced vomiting.

Gaseous exchange

Gaseous exchange in plants and animals

• Respiratory surface: large area, thin, moist, permeable; rich blood supply and ventilation keep the diffusion gradient steep.
• Plants: stomata in leaves and young stems, lenticels in the bark of woody stems, root hairs in roots.
• Hydra and Planaria: no respiratory organs; diffusion over the whole body surface.
• Earthworm: moist skin with many blood capillaries; blood contains haemoglobin dissolved in plasma.
• Cockroach: 10 pairs of spiracles → tracheae → tracheoles ending at the cells. Blood does not carry the gases.
• Fish: gills with lamellae; blood flows opposite to the water (countercurrent), so blood gains oxygen along the whole lamella.
• Birds: air sacs act as bellows and give one-way flow of air through the lungs (parabronchi); no gas exchange happens in the air sacs.

Human respiratory system

• Inspiration (active): diaphragm contracts and flattens, external intercostals raise the ribs, chest volume rises, lung pressure falls, air enters.
• Expiration at rest (passive): muscles relax, volume falls, pressure rises, air leaves.
• One haemoglobin molecule carries up to 4 O₂. The dissociation curve is S-shaped because binding one O₂ makes the next easier.
• Bohr effect: more CO₂ (lower pH) makes haemoglobin release more oxygen.
• CO₂ transport: about 70% as bicarbonate ions, roughly 20 to 23% joined to haemoglobin (carbaminohaemoglobin), and the small remainder dissolved in plasma.
• Vital capacity = tidal volume + inspiratory reserve volume + expiratory reserve volume. Total lung capacity = vital capacity + residual volume. Tidal volume at rest is about 500 mL.
• Tuberculosis: Mycobacterium tuberculosis. Asthma: bronchioles narrow. Emphysema: alveolar walls break down. Lung cancer: mainly caused by smoking.

Transport

Transport in plants

• water potential = solute potential + pressure potential (ψ_w = ψ_s + ψ_p); unit MPa. ψ_s is zero or negative; ψ_p is usually positive in a turgid cell.
• Routes across the root: apoplast (cell walls), symplast (cytoplasm through plasmodesmata), vacuolar. The Casparian strip of the endodermis blocks the apoplast.
• Ions are taken up mainly by active transport; water by osmosis.
• Cohesion-tension theory: transpiration pull + cohesion between water molecules + adhesion to xylem walls.
• Root pressure causes guttation (liquid water lost through hydathodes).
• Stomata open when K⁺ enters the guard cells, water follows and they become turgid. They close when K⁺ leaves.
• Transpiration rises with light, temperature and wind; it falls with high humidity. Phloem carries sugar mainly as sucrose.

Transport in humans

• White cells: granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (monocytes, lymphocytes). Neutrophils are the most numerous and are phagocytic.
• Clotting: prothrombin → thrombin; thrombin changes soluble fibrinogen into insoluble fibrin.
• Leukaemia: cancer with uncontrolled production of white cells. Thalassaemia: inherited defect in making haemoglobin chains.
• Cardiac cycle about 0.8 s: atrial systole 0.1 s, ventricular systole 0.3 s, diastole 0.4 s. "Lubb" = atrioventricular valves close; "dubb" = semilunar valves close.
• Impulse path: SA node → AV node → bundle of His → Purkinje fibres. cardiac output = stroke volume × heart rate.
• Arteries: thick muscular elastic walls, narrow lumen, high pressure. Veins: thin walls, wide lumen, valves. Capillaries: wall one cell thick. Normal blood pressure is about 120/80 mm Hg.
• Active immunity: the body makes its own antibodies (infection or vaccine). Passive immunity: ready-made antibodies are received (placenta, milk or antiserum).