A Unity and Diversity

• First cells originated in water, where oceans shielded early life from UV radiation • Water remains the medium for most life processes and forms much of cell mass

• O-H covalent bonds and water's V-shape make molecules polar • Hydrogen bonds form between water molecules, not within a molecule • Hydrogen bonding explains many unusual water properties

• Hydrogen bonding creates cohesion and surface tension • Cohesion prevents xylem water columns breaking under tension • Surface tension supports droplets and surface-skating insects

• Water adheres to polar or charged surfaces • Adhesion supports capillary action in soil channels and cellulose cell walls • Cohesion is the stronger force in xylem transport; adhesion helps refill vessels

• Water dissolves ions and polar molecules by hydration and hydrogen bonding • Aqueous solution enables diffusion, transport, enzyme activity, and metabolism • Non-polar molecules are hydrophobic; O₂ and N₂ have low solubility in water

• Specific heat capacity stabilizes aquatic temperatures and body temperature • Thermal conductivity, buoyancy, and viscosity shape aquatic adaptations • Compare black-throated loon and ringed seal adaptations for diving, insulation, and movement

• DNA is the genetic material of all living organisms • DNA occurs in chromosomes and also in mitochondria and chloroplasts • Some viruses use RNA, but viruses are not considered living organisms

• A nucleotide contains a phosphate group, pentose sugar, and nitrogenous base • DNA nucleotides use deoxyribose; RNA nucleotides use ribose • Bases include purines A/G and pyrimidines C/T/U

• Condensation reactions link nucleotides by sugar-phosphate bonds • The sugar-phosphate backbone is a continuous covalent chain • Bases project from the backbone and carry sequence information

• Genetic information lies in the order of nitrogenous bases • DNA uses A, T, G, C; RNA uses A, U, G, C • Triplet codons specify amino acids in protein synthesis

• RNA is a single-stranded polynucleotide formed by condensation • RNA contains ribose and the bases A, U, G, and C • mRNA, tRNA, and rRNA have different roles in protein synthesis

• DNA has two antiparallel polynucleotide strands in a double helix • Complementary bases pair by hydrogen bonding: A-T and G-C • Draw bases attached to sugars, not phosphates

• DNA is usually double-stranded; RNA is usually single-stranded • DNA uses deoxyribose and thymine; RNA uses ribose and uracil • RNA molecules are usually much shorter than DNA molecules

• A pairs with T in DNA and U in RNA; C pairs with G • Complementary base pairing enables accurate DNA replication • It also enables transcription and translation in gene expression

• DNA can vary in length and in base sequence • Genome size and gene number vary widely between organisms • Base-sequence diversity gives DNA enormous information-storage capacity

• The 64 codons have nearly the same meanings across life • Conserved genes for transcription, translation, and ribosomes support common ancestry • Synonymous mutations can preserve amino acid sequences

• Cells are the basic structural and functional units of living organisms • Cells carry out essential life processes and are usually microscopic

• Use light microscopes to prepare mounts, stain specimens, and observe cells • Measure cells with eyepiece graticules and stage micrometers • Calculate magnification, actual size, and scale bars

• Resolution, not only magnification, determines visible detail • TEM reveals internal ultrastructure; SEM reveals surface detail • Cryogenic EM, fluorescence, and immunofluorescence reveal molecules and specific structures

• All cells have a plasma membrane, cytoplasm, DNA, and ribosomes • These structures control exchange, support metabolism, store information, and make proteins

• Prokaryotes lack a nucleus and membrane-bound organelles • Structure includes cell wall, plasma membrane, cytoplasm, 70S ribosomes, naked circular DNA, and plasmids • E. coli, Bacillus, and Staphylococcus are bacterial examples

• Eukaryotes have a nucleus, 80S ribosomes, cytoskeleton, and compartmentalized cytoplasm • Organelles include mitochondria, ER, Golgi apparatus, vesicles, lysosomes, and vacuoles • Plant cells may include chloroplasts, large vacuoles, and cellulose cell walls

• Unicellular organisms carry out all life processes in one cell • Examples include Amoeba, Chlamydomonas, and Escherichia coli • Processes include nutrition, metabolism, response, excretion, homeostasis, growth, and reproduction

• Animal, fungal, and plant cells differ in cell walls, vacuoles, and storage structures • Plant cells have cellulose walls, chloroplasts, and large permanent vacuoles • Animal cells may have centrioles, cilia, flagella, lysosomes, and temporary vacuoles

• Atypical cells show limits of simple cell theory • Examples include multinucleate fungal hyphae and striated muscle fibres • Red blood cells and phloem sieve tubes lack nuclei at maturity

• Identify prokaryotic, plant, and animal cells in light and electron micrographs • Recognize organelles such as nucleus, mitochondria, chloroplasts, ER, Golgi, vacuoles, and ribosomes • Use visible structures, scale, and tissue context as evidence

• Draw clear biological diagrams from electron micrographs without shading • Label visible organelles and include magnification or scale information • Annotate structures with functions where required

• Variation occurs between individuals within every species • Continuous variation often results from polygenes and environmental effects • Darwin's observations of variation supported natural selection

• Species can be grouped by shared morphology and structural traits • Morphological species concept is useful in fieldwork, fossils, and many plants • Sexual dimorphism and within-species variation can make morphology difficult

• Binomial nomenclature gives each species a universal two-part Latin name • Genus is capitalized; species is lowercase; both are italicized or underlined • Species in the same genus share similar traits

• Biological species concept defines species by interbreeding and fertile offspring • It emphasizes gene flow between members of the same species • Limits include extinct organisms, asexual species, and cases with gene flow between distinct forms

• Speciation is gradual as isolated populations diverge • Populations may still be theoretically able to interbreed but cannot meet • Species boundaries can therefore be partly arbitrary

• Chromosome number is usually constant within a species but varies between species • Diploid chromosome numbers are even because chromosomes occur in homologous pairs • Examples include fruit fly 8, rice 24, human 46, chimpanzee 48, and dog 78

• Karyotypes show chromosome number and structure in a cell • Karyograms arrange chromosomes by length, banding pattern, and centromere position • Human chromosome 2 supports fusion from two ancestral ape chromosomes

• A genome is all genetic information in an organism • Members of a species share most of their genome but differ at variants • SNPs are common single-base differences used as biological markers

• Eukaryote genomes vary in size and base sequence • Between-species variation is much larger than within-species variation • Not all DNA codes for proteins

• Genome size is the DNA amount in one chromosome set • Larger genome size does not necessarily mean greater organism complexity • Polyploidy can greatly increase genome size, especially in plants

• Whole genome sequencing began at scale with the Human Genome Project • Current uses include evolutionary relationships, gene discovery, and pathogen research • Future uses include personalized medicine, diagnosis, disease prevention, and vaccine targets

• Evolution is cumulative change in heritable characteristics of a population • Acquired characteristics are not inherited in the Darwinian sense

• Universal DNA, genetic code, ATP use, and core metabolism support common ancestry • DNA, RNA, and protein sequence differences indicate relatedness and divergence

• Selective breeding shows traits can change rapidly under selection • Examples include pigeons, domesticated animals, Brassica crops, wheat, and maize

• Homologous structures share ancestry even when functions differ • Vertebrate pentadactyl limbs support divergent evolution and adaptive radiation

• Analogous structures have similar functions but different evolutionary origins • Similar selection pressures can produce convergence, such as bat and insect wings

• Speciation forms new species by splitting a pre-existing species • Isolated gene pools diverge through selection, mutation, and genetic drift

• Reproductive isolation reduces gene flow so populations can diverge • Geographic separation and different selection pressures can drive speciation • Chimpanzees and bonobos illustrate river-linked isolation and divergence

A4.2.1Biodiversity definition

• Biodiversity is the variety of life in its forms, levels, and combinations • It includes ecosystem, species, and genetic diversity • Species diversity depends on richness and evenness; Simpson's reciprocal index compares communities

A4.2.2Current vs. past biodiversity

• About 9 million species are estimated, with many still undescribed • Fossils and radiometric dating give incomplete evidence of past biodiversity • Current species diversity may be higher than in the past, but fossil gaps make this uncertain

A4.2.3Anthropogenic species extinction

• Human activity is driving a sixth mass extinction • Hunting, habitat change, and resource use caused extinctions such as North Island giant moa • Caribbean monk seal loss shows how removing a top predator can alter food webs

A4.2.4Ecosystem loss causes

• Ecosystem loss is mainly caused by anthropogenic disturbance and habitat conversion • Mixed dipterocarp forests in Borneo are lost through logging and oil palm plantations • Great Barrier Reef loss is linked to climate, pollution, fishing, and other human pressures

A4.2.5Evidence for biodiversity crisis

• IPBES reports and repeated biodiversity surveys provide evidence of crisis • Richness, evenness, and Simpson's reciprocal index can compare disturbed and undisturbed habitats • Peer-reviewed studies and citizen science both contribute data, with reliability checks needed

A4.2.6Causes of biodiversity crisis

• Human population growth increases demand for food, housing, energy, and land • Major drivers include overexploitation, urbanization, deforestation, agriculture, mining, and pollution • Invasive species such as lionfish and water hyacinth can outcompete or disrupt native species

A4.2.7Conservation approaches

• Conservation needs multiple approaches because species and threats differ • In situ conservation uses protected areas, reserve design, corridors, active management, and rewilding • Ex situ conservation uses zoos, captive breeding, botanic gardens, seed banks, and tissue banks

A4.2.8EDGE of Existence programme

• EDGE prioritizes evolutionarily distinct and globally endangered species • It combines phylogenetic uniqueness with IUCN extinction risk • The goal is to prevent disproportionate loss of evolutionary history