Uneven sharing of electrons: oxygen is slightly negative (δ−), hydrogens slightly positive (δ+).
Hydrogen bond
A weak attraction between a δ+ hydrogen of one molecule and a δ− atom of another.
Cohesion
Water molecules sticking to each other via hydrogen bonds.
Adhesion
Water sticking to other polar surfaces, e.g. cellulose in xylem walls.
Specific heat capacity
Energy needed to raise 1 kg by 1 °C; very high for water.
Water is polar, so molecules form many hydrogen bonds. Almost every property below comes from this.
Cohesion creates surface tension (pond skaters) and lets water be pulled up xylem as a continuous column under tension.
Adhesion to cellulose causes capillary action in xylem walls and in soil.
Solvent: hydrophilic substances (ions, polar molecules such as glucose and amino acids) dissolve; hydrophobic ones (lipids, non-polar molecules) do not. This is why blood plasma carries glucose and NaCl dissolved, but fats travel in lipoproteins and oxygen mostly bound to haemoglobin.
Thermal properties: high specific heat capacity keeps aquatic habitats and bodies thermally stable; high latent heat of vaporisation makes sweating and transpiration effective cooling.
Water vs air as a medium: water gives more buoyancy, more viscosity (drag), higher thermal conductivity and heat capacity. Compare a seal (blubber insulates in cold water) with a diving bird.
HL: hypotheses for the origin of Earth's water include delivery by asteroids and comets; water stayed liquid because of Earth's gravity and distance from the Sun. Liquid water is a key requirement in the search for extraterrestrial life (the “Goldilocks zone”).
Exam tip: When explaining any property, follow the chain: polarity → hydrogen bonds → property → biological consequence with an example.
A with T (U in RNA) and C with G, held by hydrogen bonds.
Antiparallel
The two DNA strands run in opposite directions (5′→3′ and 3′→5′).
Purine / pyrimidine
Purines (A, G) have two rings; pyrimidines (C, T, U) have one.
Nucleosome
DNA wrapped around eight histone proteins (HL).
DNA is the genetic material of all living organisms; some viruses use RNA instead.
Nucleotides join by covalent bonds between the phosphate of one and the sugar of the next, forming a sugar–phosphate backbone.
DNA: deoxyribose, bases A, T, C, G, double helix of two antiparallel strands. RNA: ribose, bases A, U, C, G, usually a single strand.
The order of bases stores information; the number of possible sequences is enormous (4ⁿ for a sequence of n bases).
The genetic code is universal, which is evidence for common ancestry.
HL: a purine always pairs with a pyrimidine, keeping the helix width constant. DNA in eukaryotes is packed into nucleosomes. The Hershey–Chase experiment (radioactive ³⁵S protein vs ³²P DNA in bacteriophages) showed DNA is the genetic material. Chargaff's data showed A = T and C = G in many species.
Worked example
A DNA sample is 22% adenine. Find the percentage of cytosine.
A = T, so T = 22% and A + T = 44%.
C + G = 100 − 44 = 56%.
C = G, so C = 28%.
A2.1Origins of cells HL
Key terms
Prebiotic Earth
Early Earth: little or no free oxygen, high temperatures, lightning, UV radiation, volcanic gases.
Miller–Urey experiment
Electric sparks in a mixture of simple gases produced amino acids, supporting abiotic synthesis of organic molecules.
Ribozyme
RNA that acts as a catalyst.
LUCA
Last universal common ancestor of all living organisms.
Cells are the smallest units of self-sustaining life; viruses are not cells.
Problems that had to be solved: making carbon compounds without life, assembling them into polymers, forming membranes, and self-replication.
Fatty acids can spontaneously form vesicles in water, giving a possible first compartment.
RNA world: RNA can store information and catalyse reactions (ribozymes), so it may have been the first genetic material.
LUCA likely lived near deep-sea hydrothermal vents; genes shared by all life today give clues about it.
Evidence for early life: stromatolites and microfossils, dated by radioisotopes, suggest cells existed over 3.5 billion years ago.
A2.2Cell structure
Key terms
Cell theory
All living things are made of cells; cells are the basic unit of life; cells come from pre-existing cells.
Resolution
The ability to distinguish two close points as separate.
Prokaryote
Cell without a nucleus: 70S ribosomes, naked circular DNA, often plasmids, cell wall.
Eukaryote
Cell with a nucleus and membrane-bound organelles; 80S ribosomes.
Atypical cells
Cells that break normal rules, e.g. red blood cells (no nucleus), skeletal muscle fibres (many nuclei).
Microscopy: light microscopes show living cells in colour; electron microscopes give much higher resolution. Also: fluorescent stains, immunofluorescence, freeze-fracture and cryogenic electron microscopy.
All cells have DNA, cytoplasm and a plasma membrane.
Prokaryotes: cell wall, plasma membrane, cytoplasm, nucleoid (naked circular DNA), 70S ribosomes; some have plasmids, pili, flagella.
Plants vs animals vs fungi: plants have cellulose walls, chloroplasts and a large permanent vacuole; animals have no wall, centrioles, small vacuoles; fungi have chitin walls.
Functions of life in unicellular organisms: homeostasis, metabolism, nutrition, movement, excretion, growth, response to stimuli, reproduction.
HL: eukaryotes arose by endosymbiosis: mitochondria and chloroplasts were once free-living prokaryotes (evidence: double membranes, own circular DNA, 70S ribosomes, binary fission). Multicellularity evolved several times; it allows cell specialisation and larger size.
Worked example
A mitochondrion is 3 µm long. In an image it measures 45 mm. Find the magnification.
Same units: 45 mm = 45 000 µm.
Magnification = 45 000 ÷ 3.
×15 000.
Exam tip: Always convert to the same units before using the magnification formula, and give magnification with a × sign and no units.
A2.3Viruses HL
Key terms
Capsid
Protein coat surrounding the viral genetic material.
Envelope
Membrane taken from the host cell surrounding some viruses.
Lytic cycle
Virus replicates and bursts the host cell.
Lysogenic cycle
Viral DNA joins the host genome and is copied with it until triggered.
Shared features: small, fixed size, nucleic acid (DNA or RNA) inside a capsid, no cytoplasm, few or no enzymes. They can only reproduce inside a host cell.
Diversity: DNA or RNA, single- or double-stranded, enveloped (e.g. influenza, HIV) or not (e.g. bacteriophage λ).
Lytic cycle (e.g. λ phage): attach → inject DNA → host makes viral parts → assembly → lysis releases new viruses.
Lysogenic cycle: viral DNA integrates as a prophage and is copied in each division; stress can switch it to the lytic cycle.
Viruses probably have several origins; their similarities may be convergent evolution.
They evolve fast (high mutation rates, short generation times), e.g. influenza needs a new vaccine each year; HIV evolves within a single patient.
A3.1Diversity of organisms
Key terms
Species
A group of organisms that can interbreed to produce fertile offspring (biological species concept).
Binomial nomenclature
Two-part Latin name: genus then species, e.g. <i>Homo sapiens</i>.
Karyotype
The number and appearance of chromosomes in a cell, arranged in pairs.
Genome
All the genetic information of an organism.
Dichotomous key
A series of paired choices used to identify an organism.
Variation exists between individuals of all species; the species concept groups individuals with shared traits.
The morphological species concept uses appearance; the biological concept uses interbreeding. Both have problems: populations diverging gradually, asexual organisms, fossils, hybrids.
Chromosome number is a characteristic of a species (humans 46, chimpanzees 48). Karyotypes are used to diagnose conditions such as Down syndrome.
Genome size varies hugely and is not linked simply to complexity. Whole-genome sequencing is now fast and cheap, helping identify species and relationships.
HL: the biological species concept struggles with bacteria, which share genes by horizontal gene transfer. Chromosome fusion can explain differences in chromosome number (e.g. human chromosome 2).
A3.2Classification and cladistics HL
Key terms
Clade
A group containing a common ancestor and all its descendants.
Cladogram
A tree diagram showing the most likely sequence of divergence in clades.
Molecular clock
Using the number of sequence differences to estimate time since divergence.
Three domains
Archaea, Bacteria and Eukarya.
Traditional hierarchy: domain, kingdom, phylum, class, order, family, genus, species.
Good classification should reflect evolutionary relationships, so all members of a group share a common ancestor.
Cladograms are built from base or amino acid sequences: nodes = common ancestors, branches = lineages. The fewer differences, the more recent the split.
Cladistics has led to reclassification, e.g. the figwort family was split when DNA showed some genera were not closely related.
rRNA evidence split prokaryotes into two domains, Archaea and Bacteria; Archaea are more closely related to Eukarya.
A4.1Evolution and speciation
Key terms
Evolution
Change in the heritable characteristics of a population over time.
Homologous structures
Same basic structure from a common ancestor but different functions, e.g. the pentadactyl limb.
Analogous structures
Same function but different origins (convergent evolution), e.g. wings of birds and insects.
Speciation
Formation of a new species by splitting of an existing one.
Reproductive isolation
Populations can no longer interbreed.
Evidence for evolution: DNA and protein sequence comparisons, selective breeding of crops and livestock, homologous structures, the fossil record.
Convergent evolution produces analogous structures in unrelated groups facing similar pressures.
Speciation needs reproductive isolation plus differential selection, so populations diverge.
HL: allopatric speciation (geographical separation) vs sympatric speciation (same area, e.g. behavioural or temporal isolation). Adaptive radiation: one ancestor diversifies into many species filling different niches (e.g. Galápagos finches). Hybrids are often sterile (mule), a barrier between species. Polyploidy can cause abrupt speciation, common in plants.
A4.2Conservation of biodiversity
Key terms
Biodiversity
Variety of life at ecosystem, species and genetic levels.
In situ conservation
Protecting species in their natural habitat, e.g. nature reserves, rewilding.
Ex situ conservation
Protecting species outside their habitat, e.g. zoos, botanic gardens, seed banks.
EDGE species
Evolutionarily Distinct and Globally Endangered species, prioritised for conservation.
Biodiversity today is far higher than at most points in Earth's history, but extinction rates are now unusually high: evidence of a biodiversity crisis.
Evidence comes from reliable surveys over time, often with citizen science (e.g. bird counts).
Conservation approaches: protected areas and rewilding (in situ); captive breeding, seed banks (ex situ); prioritising EDGE species to preserve unique evolutionary history.
Examples of extinctions: the dodo (overhunting and invasive species), the Caribbean monk seal.