Flashcards · IB Diploma Programme Biology · Topic 1

Unity and diversity flashcards

53 cards on Topic 1, subtopics A1.1 to A4.2. Pick a subtopic or study them all.

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A1.1 Water

Hydrogen bond between water molecules: An attraction between a δ+ hydrogen on one molecule and a δ− oxygen on another. It forms because water is polar (unequal sharing of electrons in O–H bonds). It is not a covalent bond.
Cohesion and adhesion of water: Cohesion: water molecules attract each other by hydrogen bonds; gives continuous columns under tension in xylem and surface tension. Adhesion: attraction of water to polar or charged surfaces; gives capillary action in soil and cell walls.
Which substances dissolve in water, and why?: Hydrophilic substances (ions, glucose, amino acids), because water molecules cluster round ions and polar molecules. Hydrophobic substances (fats, phospholipid tails) do not dissolve.
How do the physical properties of water affect aquatic animals compared with air?: Higher density: strong buoyancy. Higher viscosity: more drag, so streamlining. Higher thermal conductivity: faster heat loss, so insulation such as blubber. Higher specific heat capacity: slow temperature change in water habitats.
Energy to heat 500 g of water by 3 K (specific heat capacity 4.18 J g⁻¹ K⁻¹)?: energy = mass × specific heat capacity × temperature change = 500 × 4.18 × 3 = 6270 J
Origin and retention of water on Earth, and the Goldilocks zone: Hypothesis to know: water was brought by asteroids. It is retained by gravity and temperatures low enough for vapour to condense. The Goldilocks zone is the orbital region where a planet's surface could hold liquid water. (HL only)

A1.2 Nucleic acids

DNA and RNA compared: DNA: two antiparallel strands in a double helix; deoxyribose; bases A, T, G, C. RNA: usually one strand; ribose; bases A, U, G, C.
How are nucleotides linked into a strand?: A nucleotide is a phosphate, a pentose sugar and a base. Condensation reactions join the sugar of one nucleotide to the phosphate of the next, forming a strong, continuous covalent sugar–phosphate backbone.
Why does complementary base pairing matter?: A pairs with T (U in RNA) and G with C, joined by hydrogen bonds. The strands can be separated, and each base sequence determines its partner, so DNA can be copied (replication) and expressed (transcription).
Write the strand complementary to 5'-TTGACA-3', reading 5' to 3'.: Complement: 3'-AACTGT-5', which is 5'-TGTCAA-3' (the strands are antiparallel, so reverse it).
Why does purine–pyrimidine pairing give a regular double helix?: Each base pair is one purine (two rings: A, G) with one pyrimidine (one ring: C, T, U), so every pair is the same length. (HL only)
What did the Hershey–Chase experiment show?: ³²P (labelling DNA) was mostly found in the bacteria; ³⁵S (labelling protein) mostly stayed outside. The data support DNA as the genetic material. (HL only)

A2.1 Origins of cells (HL only)

Conditions on the early Earth relevant to the origin of life: No free oxygen, so no ozone layer and more ultraviolet light; more carbon dioxide and methane; higher temperatures. Carbon compounds could form spontaneously by processes that do not now occur. (HL only)
Miller–Urey experiment: what it showed and one limitation: Amino acids formed from water, methane, ammonia and hydrogen with sparks, so organic molecules can form from inorganic ones. Limits: the gas mixture is thought by many to be too reducing; only small molecules formed, not polymers or cells. (HL only)
Four processes needed for the origin of cells: Catalysis, self-replication of molecules, self-assembly and compartmentalisation (for example fatty acids coalescing into spherical bilayer vesicles). (HL only)
Why is RNA thought to have come first?: RNA can be replicated by complementary base pairing and can fold and catalyse reactions, so it could act as both gene and enzyme. Ribozymes in the ribosome still catalyse peptide bond formation. (HL only)
LUCA: The last universal common ancestor: the most recent organism from which all life descends. It is not the first life; other lineages probably existed and became extinct. (HL only)
Evidence for LUCA and for where it lived: Evidence for LUCA: the universal genetic code and genes shared by all organisms. Link to hydrothermal vents: fossilised life in ancient vent precipitates and conserved sequences suggesting an anaerobic, hydrogen-using organism. (HL only)

A2.2 Cell structure

Cell theory: Organisms are made of cells; the cell is the smallest unit of life; cells arise from existing cells.
Prokaryote (Gram-positive bacterium) and eukaryote compared: Prokaryote: naked DNA loop in the nucleoid; no nuclear envelope; 70S ribosomes; no membrane-bound organelles; cell wall present. Eukaryote: chromosomes of DNA with histones in a nucleus; double membrane with pores; 80S ribosomes in the cytoplasm; membrane-bound organelles.
Name atypical cells and their number of nuclei: Aseptate fungal hypha: many nuclei in one cytoplasm. Skeletal muscle fibre: many (formed by fusion). Mature mammalian red blood cell: none. Mature phloem sieve tube element: none.
A 15 mm scale bar is labelled 0.5 µm. A mitochondrion is 24 mm wide on the image. Find the magnification and actual width.: 15 mm = 15 000 µm; 15 000 ÷ 0.5 = ×30 000. Actual width = 24 mm ÷ 30 000 = 0.0008 mm = 0.8 µm.
Animal, fungal and plant cells: wall, vacuole and plastids: Animal: no wall, small vacuoles, no plastids, centrioles present. Fungus: chitin wall, no plastids. Plant: cellulose wall, large central vacuole, plastids (such as chloroplasts).
Evidence that mitochondria and chloroplasts arose by endosymbiosis: They have 70S ribosomes and naked circular DNA, and they replicate themselves, like the prokaryotes they are thought to have come from. (HL only)

A2.3 Viruses (HL only)

Features shared by viruses: Small, fixed size; genetic material of DNA or RNA; a protein capsid; no cytoplasm; few or no enzymes. They have no metabolism of their own and reproduce only inside a host cell. (HL only)
Capsid and envelope: Capsid: the protein coat that every virus has. Envelope: a layer of host membrane that only some viruses have. (HL only)
Lytic cycle of bacteriophage lambda in order: Attach to the host cell, inject DNA, synthesise viral components, assemble new phages, lyse (burst) the host cell to release them. (HL only)
Lysogenic cycle and prophage: The injected phage DNA is integrated into the host chromosome as a prophage. The host survives and divides, copying the prophage to daughter cells. Stress can trigger induction and a switch to the lytic cycle. (HL only)
Genetic material and envelope: lambda phage, coronaviruses, HIV: Bacteriophage lambda: double-stranded DNA, not enveloped. Coronaviruses: single-stranded RNA, enveloped. HIV: single-stranded RNA, enveloped. (HL only)
Why do some viruses evolve rapidly?: Short generation times, very large numbers, high mutation rates and no proofreading in many RNA viruses (for example error-prone reverse transcriptase in HIV). Influenza also changes suddenly when genome segments mix between strains. (HL only)

A3.1 Diversity of organisms

Biological species concept, and when it is hard to apply: A species is a group that can breed together and produce fertile offspring. Hard to apply to asexual organisms, fossils, populations that never meet, and fertile hybrids between clearly different forms.
Karyogram, and how chromosomes are paired in one: The chromosomes of one cell arranged in homologous pairs. Pair them by length, centromere position and banding pattern, longest first, with the sex chromosomes last.
Evidence that human chromosome 2 formed by fusion of two ape chromosomes: Its banding matches two ape chromosomes end to end; it has telomere-type sequence in the middle; and it has a remnant of a second centromere. Humans 2n = 46, chimpanzees 2n = 48.
SNP: Single-nucleotide polymorphism: a base that differs between individuals at one position in the genome. SNPs are the main source of genome diversity within a species.
Is genome size a good guide to an organism's complexity?: No. Genome size (total DNA, often in Mb) does not closely match complexity: some plants have very large genomes and some complex animals small ones.
Environmental DNA (eDNA) barcoding: method and limits: Sample water, soil or air; extract DNA; amplify a short standard barcode region by PCR; sequence it; match to a reference database. Fast and non-invasive; limited by incomplete databases and DNA that has drifted from where the organism lives. (HL only)

A3.2 Classification and cladistics (HL only)

Clade: A common ancestor and all of its descendants. A group that leaves out any descendant is not a clade. (HL only)
Two species that split 40 million years ago differ at 16 amino acids. Another pair differs at 12. Estimate when that pair diverged.: Rate = 16 ÷ 40 = 0.4 differences per million years. Time = 12 ÷ 0.4 = 30 million years ago. It is an estimate, because mutation rates vary. (HL only)
Parsimony (in building cladograms): Choosing the explanation, or cladogram, that needs the smallest number of changes. Only sites where a derived base is shared by some but not all species help to choose a tree. (HL only)
Reading a cladogram: what is a node, and how is relatedness judged?: A node is a fork standing for a hypothetical common ancestor. The closest relatives share the most recent node. Tip order and position on the page do not show closeness. (HL only)
The three domains: evidence and names: Proposed in 1977 from base sequences of ribosomal RNA, found in all organisms. Bacteria, Archaea and Eukarya; the domain is a level above kingdom. (HL only)
Why can morphology mislead classification?: Convergent evolution gives similar features in unrelated lineages. Only features shared because they were inherited from a common ancestor support grouping; this is why base and amino acid sequences are the stronger evidence. (HL only)

A4.1 Evolution and speciation

Evolution: A genetic (heritable) change in the features of a population over generations. Acquired traits that are not genetic do not count.
Homologous and analogous structures: Homologous: same basic structure from a shared ancestor, possibly different function (pentadactyl limb of human, bat, whale, horse). Analogous: same function, different origin, from convergent evolution (bat and insect wings).
How do base sequences and selective breeding provide evidence for evolution?: Fewer differences in DNA, RNA or protein sequences mean a more recent common ancestor. Selective breeding (dog breeds from wolves; varieties of Brassica oleracea) shows variation is heritable and populations change quickly under selection.
Explain speciation in steps: A population is split; gene flow stops (reproductive isolation); different conditions give differential selection and mutations arise independently; allele frequencies diverge; the populations can no longer breed successfully together. Example: bonobos and chimpanzees either side of the Congo River.
Allopatric and sympatric speciation: Allopatric: populations in separate areas, first barrier geographical. Sympatric: same area, first barrier behavioural or temporal. Both need reproductive isolation, variation and divergence under selection. (HL only)
How can polyploidy cause abrupt speciation in plants?: Hybridisation gives a sterile hybrid whose chromosomes cannot pair. Doubling of the chromosomes (polyploidy) lets them pair, so the plant is fertile and is a new species in one generation. Example: Persicaria. (HL only)

A4.2 Conservation of biodiversity

Biodiversity: The variety of life at ecosystem, species and genetic levels.
Causes of the current biodiversity crisis: Human population growth overall; specifically over-exploitation, urbanisation, deforestation and land clearance for agriculture, pollution, and pests, diseases and invasive alien species spread by global transport.
In situ and ex situ conservation: In situ: in the natural habitat (managed nature reserves, rewilding, repairing degraded ecosystems). Ex situ: outside it (zoos, botanic gardens, seed or tissue banks).
What does the EDGE of Existence programme prioritise, and why?: Species that are both evolutionarily distinct and globally endangered, because losing them loses a large amount of evolutionary history. Prioritisation is a matter for debate.
Name two species driven extinct by humans and the cause: Giant moa (North Island, New Zealand): terrestrial megafauna hunted by early settlers. Caribbean monk seal: killed for oil and meat and affected by overfishing; declared extinct in 2008.