IB Diploma Biology key facts

A Unity and diversity

Water

• Hydrogen bonds form between water molecules; covalent bonds hold the atoms together inside each molecule.
• Cohesion = water molecules attracting each other; it gives surface tension and lets water be pulled up the xylem under tension as an unbroken column.
• Adhesion = water attracted to polar or charged surfaces; it causes capillary action in soil and in plant cell walls.
• Hydrophilic substances (polar or charged, such as glucose and ions) dissolve in water; hydrophobic substances (non-polar, such as fats) do not.
• Compared with air, water has higher buoyancy, higher viscosity, higher thermal conductivity and a higher specific heat capacity.
• High specific heat capacity means the temperature of water changes slowly, so aquatic habitats are thermally stable.

Nucleic acids

• Nucleotide = pentose sugar + phosphate + base.
• DNA base pairs: A with T, and C with G. In RNA, U replaces T, so A pairs with U.
• In double-stranded DNA: number of A = number of T, and number of C = number of G.
• DNA: two strands, deoxyribose, bases A, T, C, G. RNA: one strand, ribose, bases A, U, C, G.
• Joining n nucleotides into one strand releases (n − 1) water molecules.
• Number of possible sequences of length n = 4ⁿ.
• The genetic code is the same in almost all organisms, which is evidence for a common ancestor.

Cell structure

• magnification = size of image ÷ actual size of specimen
• actual size = size of image ÷ magnification
• 1 mm = 1 000 µm; 1 µm = 1 000 nm. Change both sizes to the same unit before dividing.
• Cell walls: plants have cellulose, fungi have chitin, animals have none. Chloroplasts are found in plant cells only.
• Vacuoles: plants have one large permanent vacuole; animal cells have small temporary ones.
• Atypical cells: skeletal muscle fibres and aseptate fungal hyphae have many nuclei; red blood cells and phloem sieve tube elements have none.
• Freeze fracture splits membranes; cryogenic electron microscopy shows the shape of proteins; immunofluorescence uses fluorescent antibodies to locate one protein.

Diversity of organisms

• Binomial name: genus with a capital letter, species in lower case, both in italics (underlined in handwriting), e.g. Homo sapiens.
• Species with the same genus name are closely related.
• Biological species = organisms that can interbreed and produce fertile offspring.
• Diploid number (2n) is the same for all members of a species and is normally even: humans 46, chimpanzees 48.
• Karyogram: chromosomes arranged in homologous pairs by length, banding pattern and centromere position.
• Genome = all the genetic information of an organism. Genome size does not show how complex an organism is.
• Most differences between individuals of one species are single-nucleotide polymorphisms (single base differences).

Evolution and speciation

• Evolution happens to populations, not to individuals.
• Fewer differences in base or amino acid sequence = more recent common ancestor.
• Homologous structures: same structure and origin, different function (the pentadactyl limb of humans, bats and whales).
• Analogous structures: same function, different structure and origin; they arise by convergent evolution.
• Speciation needs reproductive isolation followed by differential selection.
• Geographical barriers such as rivers, mountains and seas can isolate populations.
• Speciation adds one species; extinction removes one.

Conservation of biodiversity

• Three levels of biodiversity: ecosystem, species and genetic.
• Species diversity depends on richness (number of species) and evenness (how equal their numbers are).
• Causes of the crisis: over-exploitation (hunting), habitat loss from land clearance and urbanization, pollution, invasive species and climate change.
• Case studies: giant moas in New Zealand were hunted to extinction; dipterocarp forest in Southeast Asia is cleared for oil palm.
• In situ: nature reserves, national parks, rewilding. Ex situ: zoos, botanic gardens, seed banks, frozen tissue banks.
• EDGE = Evolutionarily Distinct and Globally Endangered: priority goes to threatened species with few close relatives.
• Evidence for a crisis needs repeated surveys that use the same reliable method over a long time.

B Form and function

Carbohydrates and lipids

• Joining n monomers into one chain releases (n − 1) water molecules; fully hydrolysing it uses (n − 1).
• Glucose is soluble, stable and releases energy when oxidised.
• Starch (amylose and amylopectin) and glycogen: polymers of alpha-glucose; coiled and compact, insoluble, so they store energy with little osmotic effect. Glycogen is the most branched.
• Cellulose: beta-glucose in straight unbranched chains, linked side by side by hydrogen bonds into strong microfibrils.
• Triglyceride = 1 glycerol + 3 fatty acids, joined by 3 ester bonds. Phospholipid = 1 glycerol + 2 fatty acids + 1 phosphate group.
• Saturated fatty acids have no C=C double bonds; unsaturated ones have one or more, which lowers the melting point (oils).
• Glycoproteins on the cell surface act in cell–cell recognition, e.g. the ABO blood group antigens.

Proteins

• Amino acid: amine group (–NH₂), carboxyl group (–COOH), H atom and R-group, all on the alpha carbon.
• The peptide bond joins the C of a carboxyl group to the N of an amine group.
• A chain of n amino acids has (n − 1) peptide bonds, and (n − 1) water molecules are released in making it.
• Essential amino acids cannot be made by the body and must come from the diet; non-essential ones can be made from other amino acids.
• Number of possible chains of n amino acids = 20ⁿ.
• Denaturation breaks the weak bonds that hold the folded shape; the peptide bonds and the amino acid sequence stay the same.
• High temperature breaks bonds by making the molecule vibrate more; a change in pH alters the charges on R-groups.

Membranes and membrane transport

• Simple diffusion, facilitated diffusion and osmosis are passive: down the concentration gradient, no ATP.
• Active transport: against the concentration gradient, needs ATP and pump proteins.
• Osmosis = net movement of water from a lower solute concentration to a higher solute concentration across a partially permeable membrane.
• Aquaporins are channel proteins that greatly increase the rate at which water crosses.
• Integral proteins are embedded in the bilayer; peripheral proteins are attached to its surface.
• Glycoproteins and glycolipids have their carbohydrate chains on the outside of the cell.
• percentage change in mass = (final mass − starting mass) ÷ starting mass × 100

Organelles and compartmentalization

• Organelles include the nucleus, mitochondria, chloroplasts, ribosomes, lysosomes, Golgi apparatus, endoplasmic reticulum, vesicles and the plasma membrane.
• Not organelles: the cell wall, the cytoskeleton and the cytoplasm.
• Lysosomes keep digestive (hydrolytic) enzymes inside a membrane so they do not digest the cell itself.
• A phagocytic vacuole holds an engulfed particle; lysosomes fuse with it and their enzymes digest the contents safely.
• Enzymes and substrates held together in a compartment are more concentrated, so pathways run faster.
• Cell fractionation: break open the cells, then spin in an ultracentrifuge to separate the organelles by size and density.
• Larger, denser organelles settle first: nuclei, then mitochondria, then ribosomes.

Cell specialization

• Differentiation = expressing some genes and not others; the DNA itself is not lost or changed.
• Totipotent: can form every cell type, including the placenta (cells of the very early embryo).
• Pluripotent: can form all body cell types but not the placenta (embryonic stem cells).
• Multipotent: can form a few related cell types (adult stem cells, e.g. bone marrow makes blood cells).
• Cube of side s: surface area = 6s², volume = s³, SA:V = 6 ÷ s.
• Surface area fixes the rate of exchange; volume fixes the rate at which materials are used and wastes are made.
• The human egg is one of the largest cells (about 0.1 mm); sperm and red blood cells are very small.

Gas exchange

• Alveoli: very large total area, walls one cell thick, moist lining with surfactant, dense capillary network.
• Surfactant lowers surface tension, so the alveoli do not stick shut.
• Inhalation: diaphragm contracts and flattens, external intercostal muscles raise the ribs; volume rises, pressure falls, air enters. Exhalation at rest is the reverse as these muscles relax.
• vital capacity = tidal volume + inspiratory reserve volume + expiratory reserve volume
• ventilation rate = tidal volume × number of breaths per minute
• stomatal density = mean number of stomata ÷ area of the field of view
• Transpiration is faster in warm, dry, windy and bright conditions.

Transport

• Capillaries: wall one cell thick, very narrow lumen, much branching (large total surface area); some have pores (fenestrations) for faster exchange.
• Arteries: thick wall, narrow lumen; elastic fibres stretch and recoil to keep pressure up between beats; muscle controls the diameter.
• Veins: thin wall, wide lumen, valves to stop backflow.
• pulse rate (beats per minute) = beats counted × 60 ÷ counting time in seconds.
• Coronary occlusion: fatty deposits narrow a coronary artery; a clot can block it, heart muscle gets no oxygen and dies (heart attack).
• Xylem vessels: dead, hollow, no end walls, lignin in the walls to resist collapse under tension, pits for sideways movement.
• Dicot stem: vascular bundles in a ring, xylem inside and phloem outside. Dicot root: vascular tissue in the centre, xylem in a star shape with phloem between the arms.

Adaptation to environment

• Coral reefs need: shallow, clear water (light for the algae in the coral), warm water of about 23 to 29 °C, normal sea salinity and a pH above about 7.8.
• Tropical forest: hot and wet all year. Hot desert: hot days, very low rainfall. Tundra: very cold, low precipitation.
• Taiga: cold with moderate precipitation. Temperate forest: moderate temperature and rainfall. Grassland: too dry for forest, not dry enough to be desert.
• Desert plants: thick water-storing stems, spines in place of leaves, thick waxy cuticle, deep or wide roots, stomata open at night.
• Rainforest trees: very tall to reach light, buttress roots for support, broad leaves with drip tips to shed water.
• Sand dune grasses roll their leaves with stomata inside to cut water loss; mangrove roots grow up into the air to take in oxygen.

Ecological niches

• Obligate aerobes need oxygen. Obligate anaerobes are killed or inhibited by oxygen. Facultative anaerobes use oxygen when it is there but can live without it.
• Photosynthetic: makes carbon compounds using light. Holozoic: takes in solid food, then digests it inside the body. Saprotrophic: secretes enzymes onto dead matter and absorbs the products. Mixotrophic: can do both autotrophic and heterotrophic nutrition (Euglena).
• Holozoic stages in order: ingestion, digestion, absorption, assimilation, egestion.
• Archaea show great variety: some use light, some oxidize inorganic chemicals, some oxidize carbon compounds.
• Hominid teeth: large flat molars with thick enamel and a heavy jaw suggest a tough plant diet; smaller teeth and a lighter jaw suggest a mixed diet with softer food or meat.
• Plants defend with thorns and toxins; some herbivores evolve ways to detoxify them. Predators have speed, claws or venom; prey use camouflage, warning colours or chemical defence.
• Forest plants reach light in different ways: tall trees, lianas that climb, epiphytes that grow on branches, shade-tolerant shrubs and herbs.

C Interaction and interdependence

Enzymes and metabolism

• Anabolic reactions build larger molecules from smaller ones and need energy (protein synthesis, photosynthesis). Catabolic reactions break molecules down and release energy (digestion, respiration).
• Specificity: each enzyme acts on only one substrate or a small group, because only they fit the active site.
• Temperature: rate rises as molecules move faster and collide more, reaches an optimum, then falls quickly as the enzyme denatures.
• pH: each enzyme has an optimum pH; the rate falls on both sides of it.
• Substrate concentration: rate rises, then levels off when all active sites are occupied.
• rate of reaction = amount of product formed (or substrate used) ÷ time taken.
• Enzymes lower the activation energy only. They do not change the energy of the reactants or products.

Cell respiration

• ATP is a nucleotide: adenine + ribose + three phosphate groups.
• ATP → ADP + phosphate releases energy; ADP + phosphate → ATP needs energy from respiration.
• Aerobic (humans): glucose + oxygen → carbon dioxide + water; large ATP yield; needs mitochondria; can use glucose, fatty acids and other compounds.
• Anaerobic (humans): glucose → lactate; small ATP yield; in the cytoplasm only; no carbon dioxide produced; carbohydrates only.
• Ventilation moves air, gas exchange is diffusion of gases, cell respiration makes ATP.
• rate of respiration = oxygen taken up (or carbon dioxide released) ÷ time, often also ÷ mass of the organism.
• In a respirometer an alkali absorbs carbon dioxide, so the movement of the fluid measures oxygen uptake. Temperature must be kept constant.

Photosynthesis

• carbon dioxide + water → glucose + oxygen (light and chlorophyll needed).
• All the oxygen released comes from water, not from carbon dioxide.
• R_f = distance moved by pigment ÷ distance moved by solvent front. It has no unit and is always less than 1.
• Absorption spectrum: how much light of each wavelength a pigment absorbs. Action spectrum: the rate of photosynthesis at each wavelength. Both peak in blue and red and dip in green.
• A rate graph that rises and then levels off: the factor on the x-axis is limiting on the slope and something else is limiting on the plateau.
• light intensity is proportional to 1 ÷ distance² from the lamp.
• CO₂ enrichment in greenhouses or in open fields (FACE) is used to predict future plant growth; open-field tests are more realistic.

Neural signalling

• Sodium–potassium pump: uses ATP to pump 3 Na⁺ out and 2 K⁺ in, so more positive charge leaves than enters.
• Depolarization: Na⁺ channels open and Na⁺ diffuses in; the inside rises to about +30 mV.
• Repolarization: K⁺ channels open and K⁺ diffuses out; the inside becomes negative again.
• Speed is higher in myelinated fibres and in fibres of larger diameter.
• speed of conduction = distance ÷ time (m s⁻¹); 1 ms = 0.001 s.
• At a synapse: impulse arrives, Ca²⁺ diffuses into the presynaptic neuron, vesicles fuse with the membrane and release neurotransmitter by exocytosis.
• Acetylcholine diffuses across the cleft and binds to receptors; Na⁺ channels open and the postsynaptic membrane depolarizes (excitatory postsynaptic potential).

Integration of body systems

• Sensory neurons carry impulses from receptors to the central nervous system (brain and spinal cord); motor neurons carry impulses from it to muscles and glands. A nerve is a bundle of nerve fibres in a protective sheath.
• Pain reflex arc: pain receptor → sensory neuron → interneuron in the spinal cord → motor neuron → muscle (effector). The brain is not needed, so it is fast. The cerebellum coordinates skeletal muscle contraction and balance.
• Melatonin: from the pineal gland, secreted in darkness, promotes sleep and sets the daily (circadian) rhythm.
• Epinephrine (adrenaline): from the adrenal glands; raises heart rate and ventilation rate, widens the airways, sends more blood to muscles and releases glucose from glycogen.
• The hypothalamus links the nervous and hormonal systems and controls the pituitary gland, which releases hormones that control other glands.
• Heart rate: baroreceptors (pressure) and chemoreceptors (pH, CO₂, O₂) signal the medulla. Ventilation: a rise in blood CO₂ lowers pH and the brainstem makes breathing faster and deeper.
• Peristalsis: swallowing and egestion are voluntary (central nervous system); movement in between is involuntary (enteric nervous system).

Defence against disease

• Clotting: platelets release clotting factors → thrombin is formed → thrombin changes soluble fibrinogen into insoluble fibrin → the fibrin mesh traps blood cells.
• Antibody production: antigen activates a helper T-cell → the helper T-cell activates the matching B-cell → the B-cell divides to form plasma cells (secrete antibodies) and memory cells.
• HIV is passed on in body fluids (unprotected sex, shared needles, blood, mother to child). It destroys helper T-cells, so antibodies cannot be made and AIDS develops.
• Antibiotics block processes of prokaryotic cells only, so they do not harm human cells and do not work on viruses.
• Resistance: a few bacteria are resistant by chance; the antibiotic kills the rest; the resistant ones multiply (natural selection).
• Zoonosis: a disease that passes from other animals to humans. Herd immunity: so many people are immune that the pathogen cannot spread to those who are not.
• percentage change = (new value − old value) ÷ old value × 100.

Populations and communities

• Quadrats (organisms that do not move): population = mean number per quadrat × total area ÷ area of one quadrat.
• Lincoln index (moving animals): population = M × N ÷ R. M = number marked in the first sample, N = total caught in the second sample, R = number in the second sample that are marked.
• The growth curve is sigmoid (S-shaped). On a logarithmic scale the exponential phase is a straight line.
• Mutualism: both species benefit. Intraspecific = within one species; interspecific = between species.
• Invasive species often succeed because they have no natural predators or diseases in the new area and outcompete endemic species.
• Chi-squared test: if χ² is greater than the critical value at p = 0.05, the two species are associated (reject the null hypothesis).
• Top-down control: numbers limited by predators or grazers. Bottom-up control: numbers limited by resources such as nutrients. Allelopathy: releasing chemicals that harm competing plants.

Transfers of energy and matter

• Arrows in a food chain point in the direction of energy flow, from the organism eaten to the eater.
• Trophic levels: producer (1), primary consumer (2), secondary consumer (3), tertiary consumer (4).
• Photoautotrophs use light as their energy source; chemoautotrophs use the oxidation of inorganic chemicals; heterotrophs take carbon compounds from other organisms.
• efficiency of transfer (%) = energy in the higher level ÷ energy in the level below × 100.
• Energy is lost through respiration (heat), parts not eaten and faeces (egestion).
• Primary production: biomass made by autotrophs. Secondary production: biomass made by heterotrophs; it is always lower. Units such as g m⁻² yr⁻¹.
• Carbon sink: takes in more carbon than it releases. Carbon source: releases more than it takes in. The Keeling Curve shows CO₂ rising year by year, with a small fall each northern summer when photosynthesis is high.

D Continuity and change

DNA replication

• Helicase unwinds the double helix and separates the strands by breaking the hydrogen bonds between bases.
• DNA polymerase joins nucleotides into a new strand, using the old strand as a template.
• PCR cycle: about 95 °C, strands separate; about 55 °C, primers bind to the ends of the target sequence; about 72 °C, Taq polymerase builds the new strands.
• Taq polymerase comes from a hot-spring bacterium, so it is not denatured at 95 °C.
• Each PCR cycle doubles the DNA: copies = starting copies × 2ⁿ after n cycles.
• Gel electrophoresis: DNA is negatively charged and moves towards the positive electrode; small fragments move faster and further.
• DNA profiling: every band in a child's profile must match a band in the mother or the father. It is used in paternity tests and forensic work.

Protein synthesis

• Transcription pairing (DNA template → mRNA): A → U, T → A, C → G, G → C. RNA has uracil (U), not thymine.
• The anticodon on tRNA is complementary to the mRNA codon, for example codon AUG pairs with anticodon UAC.
• The genetic code is a triplet code: 4³ = 64 codons for 20 amino acids. Two bases would give only 4² = 16.
• The code is degenerate (several codons for one amino acid) and universal (the same in almost all organisms).
• AUG is the start codon (Met). A stop codon codes for no amino acid.
• Number of amino acids = (number of coding bases ÷ 3) − 1 if the stop codon is included in the count.
• Sickle cell anaemia: mRNA codon GAG becomes GUG, so glutamic acid is replaced by valine in haemoglobin.

Mutations and gene editing

• Substitution: one codon changes. Result: the same amino acid, a different amino acid, or a stop codon.
• Insertion or deletion of 1 or 2 bases: frameshift, all later codons change.
• Insertion or deletion of 3 bases: one amino acid is added or lost, and the reading frame is kept.
• Causes: errors in DNA replication or repair, and mutagens (UV, X-rays, gamma rays, some chemicals such as those in tobacco smoke).
• Germ cell mutation: can pass to offspring. Somatic cell mutation: affects that individual only and may cause cancer.
• Mutation is the original source of all new alleles, so it is the source of genetic variation.

Cell and nuclear division

• Phases of mitosis in order: prophase, metaphase (chromosomes at the equator), anaphase (chromatids pulled to the poles), telophase.
• Mitosis: 1 division, 2 nuclei, diploid (2n), identical. Meiosis: 2 divisions, 4 nuclei, haploid (n), different.
• After replication the chromosome number is unchanged, but the number of DNA molecules is doubled.
• Cytokinesis: animal cells are pinched in by a ring of actin and myosin. Plant cells build a new membrane and wall from vesicles.
• Unequal cytokinesis: oogenesis in humans and budding in yeast.
• Variation from meiosis: crossing over and random orientation of bivalents. Combinations from random orientation = 2ⁿ, where n is the haploid number.
• Non-disjunction: chromosomes fail to separate. Three copies of chromosome 21 (47 chromosomes in total) cause Down syndrome.

Water potential

• Net water movement: from hypotonic to hypertonic.
• Solvation: the δ− oxygen of water faces positive ions, and the δ+ hydrogens face negative ions.
• Percentage change = (final − initial) ÷ initial × 100.
• Isotonic concentration of a tissue: the concentration where the graph line crosses 0% change in mass or length.
• Animal cell: bursts in hypotonic, shrinks (crenation) in hypertonic.
• Plant cell: turgid in hypotonic (the wall stops it bursting), plasmolysed in hypertonic (the membrane pulls away from the wall).
• Isotonic solutions are used for intravenous drips and for storing organs for transplant.

Reproduction

• FSH: stimulates follicles to develop and to secrete oestradiol.
• Oestradiol: thickens the endometrium. A high level before ovulation stimulates the LH surge (positive feedback).
• LH surge at about day 14: causes ovulation. The empty follicle becomes the corpus luteum.
• Progesterone from the corpus luteum: maintains the endometrium and inhibits FSH and LH (negative feedback). When it falls, menstruation begins.
• Fertilization: the sperm nucleus enters the egg. The tail and mitochondria of the sperm are destroyed.
• IVF: normal hormone secretion is suspended, then FSH is given to cause superovulation. Eggs are collected and fertilized outside the body.
• Plants: anther makes pollen, stigma receives it. Self-incompatibility and different ripening times promote cross-pollination. Germination needs water, oxygen and warmth.

Inheritance

• Aa × Aa gives 1 AA : 2 Aa : 1 aa, which is 3 : 1 in phenotypes. Aa × aa gives 1 : 1.
• ABO blood groups: I^A and I^B are codominant, and both are dominant to i. Group O is ii and group AB is I^AI^B.
• Sex: XX is female, XY is male. A father gives his X to every daughter and his Y to every son.
• Haemophilia: recessive allele on the X chromosome (X^h). X^HX^h is a carrier female and X^hY is an affected male.
• Pedigree: two unaffected parents with an affected child means the allele is recessive and both parents are carriers.
• Phenylketonuria (PKU): an autosomal recessive condition. Phenylalanine cannot be converted to tyrosine.
• Box-and-whisker plot: interquartile range (IQR) = Q3 − Q1. An outlier lies more than 1.5 × IQR above Q3 or below Q1.

Homeostasis

• Blood glucose too high: beta cells secrete insulin. Liver and muscle cells take up glucose and store it as glycogen.
• Blood glucose too low: alpha cells secrete glucagon. The liver breaks down glycogen and releases glucose.
• Type 1 diabetes: beta cells are destroyed by the immune system, so little or no insulin is made. Treated with insulin.
• Type 2 diabetes: target cells respond less to insulin. Risk factors include obesity, diet and lack of exercise.
• Too cold: vasoconstriction, shivering, hair erection, uncoupled respiration in brown adipose tissue, more thyroxin (via the pituitary and thyroid).
• Too hot: vasodilation of skin arterioles and sweating. Evaporation of sweat takes heat from the skin.

Natural selection

• Sequence: variation → overproduction → competition → differential survival and reproduction → heritable traits passed on → allele frequencies change.
• Mutation gives new alleles. Sexual reproduction gives new combinations of alleles.
• Fitness means reproductive success: the number of offspring that survive to breed, not how long an individual lives.
• Selection pressures can be biotic (predators, competition, disease) or abiotic (temperature, drought). Abiotic factors are usually density-independent.
• Resistance alleles exist before the antibiotic or insecticide is used. The chemical selects them. It does not create them.
• Guppies (Endler): female choice favours bright males, predators favour dull males. Colour depends on the balance between the two.

Stability and change

• Sustainable harvesting: rate of removal ≤ rate of replacement.
• Keystone species: its effect on the community is much greater than expected from its numbers.
• Rewilding: reintroduce apex predators and other keystone species, reconnect habitats, reduce human management.
• Agriculture is less sustainable because of soil erosion, leaching of nutrients, fertilizer use, agrochemical pollution and its carbon footprint.
• Eutrophication: nutrients → algal bloom → algae die → decomposition raises biochemical oxygen demand (BOD) → oxygen falls → fish die.
• Biomagnification: a persistent toxin (DDT, mercury) becomes more concentrated at each trophic level.
• Plastics are non-biodegradable. They break into microplastics but are not decomposed.

Climate change

• Carbon dioxide sources: burning fossil fuels and deforestation. Methane sources: livestock, rice paddies, landfill.
• Positive feedbacks: melting ice reflects less sunlight; thawing permafrost releases CO₂ and methane; drought and fires release stored carbon.
• Boreal forest: warming, drought and fires may change it from a carbon sink into a carbon source.
• Polar habitats: emperor penguins breed on landfast ice and walruses use sea ice, so early ice loss threatens both.
• Warmer surface water reduces nutrient upwelling, so phytoplankton production and marine food chains decline.
• Coral reefs: dissolved CO₂ lowers ocean pH and reduces calcification. Warming causes bleaching (loss of zooxanthellae).
• Sequestration: afforestation, forest regeneration, and restoring peat wetlands, where waterlogging slows decomposition.