IB Biology HL Unity and Diversity Concepts

The earliest cells arose in water, where the surrounding ocean reduced exposure to ultraviolet radiation. Modern cells still contain a high proportion of water, and most cellular chemistry occurs in an aqueous phase.

In xylem, evaporation from leaves creates tension. Cohesion keeps water molecules connected, so the pull is transmitted down a continuous column and the column resists breaking.

In narrow soil pores and spaces between cellulose fibres, adhesion pulls water along the hydrophilic wall. Cohesion draws neighbouring water molecules with it, producing capillary movement and keeping leaf cell walls moist.

Specific heat capacity describes the energy needed to change temperature; thermal conductivity describes how readily heat passes through a material. Water can therefore provide a stable habitat while still removing heat quickly from a warm animal.

Delivery alone would not create oceans. As Earth cooled, water vapour could condense; Earth's gravity then limited escape to space, allowing persistent surface water.

During a transit, some starlight passes through an exoplanet's atmosphere. Molecules absorb characteristic wavelengths, so the spectrum can reveal a water-vapour signature.

In every living organism, DNA is the genetic material. Its base sequence stores information that can be copied before cell division and passed between generations.

In eukaryotic cells, most DNA is organized into chromosomes in the nucleus. Mitochondria and chloroplasts also contain smaller DNA genomes.

Some viruses use RNA as their genetic material. This does not contradict the rule for living organisms because viruses depend on host cells and are not classified as living.

The sugar is the central connector: the base and phosphate are both bonded to it. DNA nucleotides contain deoxyribose; RNA nucleotides contain ribose. A base alone is not a nucleotide.

Four bases produce 64 possible triplets: 4 × 4 × 4. Different codons can specify the same amino acid, so the code is redundant.

These molecules share the same type of backbone and bases, but differences in sequence, length and folding give them different functions.

In a structural diagram, attach every base to a sugar, alternate sugar and phosphate along each backbone, and show the two strands running in opposite directions. The helical twist is not required.

Because each base has one complementary partner, the sequence of a template strand determines the sequence that is assembled against it. This is the common molecular logic behind copying and expression.

Example: a DNA template written 3′-A C G T-5′ produces RNA 5′-U G C A-3′. The strands are complementary and antiparallel, not identical.

Genome size is the total amount of DNA, whereas gene number counts genes. Neither value alone is a simple measure of organism complexity.

Across almost all living organisms, the same codons specify the same amino acids. Genes involved in transcription, translation and ribosome function are also highly conserved.

A complex decoding system shared so widely is best explained by inheritance from a common ancestor. Later evolution changed DNA sequences, but retained the core code and molecular machinery.

The code is nearly, not absolutely, universal. Minor exceptions do not erase the much larger conserved pattern. Redundancy also means that some base substitutions are synonymous and do not change the amino acid.

Direction labels are part of the molecular structure. Reversing them changes the sequence being described and therefore changes how a template is interpreted.

The ratios supported complementary pairing and falsified Levene’s tetranucleotide hypothesis. They constrained a successful structural model, but the ratios alone did not reveal the complete double helix.

Water, simple reactants and energy from UV, heat or electrical discharges could produce carbon compounds abiotically. The exact atmosphere is uncertain, and high-energy radiation could both build and destroy molecules.

A cell is the smallest self-sustaining unit of life because these systems operate together. Viruses contain genetic material and evolve, but they depend on host-cell machinery for metabolism and replication.

All three models address real transitions, but no experiment can recreate the exact history. Evidence can support a mechanism without proving that it occurred first on Earth.

Later experiments using different gases and energy sources have produced:

• amino acids and fatty acids
• sugars and nucleotide bases
• in some conditions, simple polymers

Together, the results support abiotic synthesis under some conditions. They do not show that a cell formed or that any experimental gas mixture exactly matched early Earth.

In water, polar regions face the water while non-polar regions turn away. As concentration rises, amphipathic molecules can form layers that curve and close into vesicles without cellular machinery.

A closed vesicle retains monomers, polymers and catalysts, allowing a distinct internal chemistry. Simple fatty acids are plausible early membrane components because they are chemically simpler than modern phospholipids. A compartment alone still lacks dependable heredity and metabolism.

Modern cells retain several possible relics of an RNA world:

• catalytic RNA molecules called ribozymes
• rRNA catalysis of peptide-bond formation in ribosomes
• RNA primers required to begin DNA replication
• ribose formation under plausible prebiotic conditions

These observations support an RNA-world hypothesis, but they do not identify the first replicator or reconstruct the complete historical sequence.

LUCA is the inferred last ancestral population shared by all surviving cellular life. It links the earlier abiotic phase to the continuous lineages from which Bacteria, Archaea and Eukarya descended; it was not necessarily the first life or one individual cell.

Nearly universal genetic code and shared biochemistry → conserved ribosome and core gene families across distant lineages → common inheritance is more parsimonious than repeated independent origin.

Researchers compare modern genomes, identify widespread homologous genes and exclude likely horizontal transfers. The surviving genes reveal some LUCA traits, but extinct lineages and lost genes cannot be reconstructed directly.

A 3.77-billion-year-old fossil means life existed by then; it does not date the first cell. Fossils, rock dates and molecular clocks converge on very ancient life and a LUCA near 4 billion years ago, but they do not provide an exact birthday.

Vents provide H₂, CO₂, iron-rich minerals, heat, pores and chemical gradients, allowing chemosynthetic energy transfer without sunlight. Tube shape alone is not proof of life, and gene-based physiology is a reconstruction; convergence makes the setting plausible rather than certain.

A cell is the smallest membrane-bounded unit that can carry out the processes of life. A tissue is different: it is a group of cells cooperating in a shared function.

Cell theory states that:

• living organisms consist of one or more cells
• cells are the basic units of structure and function
• new cells arise from pre-existing cells

The theory is a powerful biological model, not a claim that every cell has the same appearance. Later examples such as multinucleate fibres and mature cells without nuclei test its boundaries without removing the central idea that living organization is cellular.

At one objective, 10 graticule divisions correspond to 20 µm, so each division represents 2 µm. A cell spanning 18 divisions is therefore 36 µm long. If its printed image is 54 mm long, convert 54 mm to 54,000 µm before calculating: $M = 54{,}000/36 = 1{,}500\times$.

Calibration belongs to a particular objective lens. Changing the objective changes the real distance represented by one graticule division, so the calibration must be repeated.

Magnification makes an image larger; resolution allows two close points to be seen as separate. Enlarging beyond the resolution limit only produces a larger blur.

Preparation can introduce artefacts. Confidence increases when different preparation methods reveal a consistent structure, rather than when one impressive image is accepted alone.

None of these components alone is a cell. Together they create a bounded chemical system that can maintain itself, use information and make the proteins needed for its activities.

Plasmids, capsules, pili, flagella and specialized membrane regions occur in some bacteria, not all. E. coli, Bacillus and Staphylococcus also differ in shape and wall organization, so no optional feature should be treated as the definition of a prokaryote.

For a secreted protein, information and material move through a coordinated route: nuclear gene → transcription → rough-ER ribosome → ER lumen → transport vesicle → Golgi modification and sorting → secretory vesicle → plasma membrane.

The cytoskeleton gives internal organization and shape. Microtubules guide vesicles and move chromosomes; actin microfilaments resist tension and contribute to cell movement and shape change.

A unicellular organism cannot delegate essential work to tissues. Nutrition, metabolism, homeostasis, excretion, response, movement, growth and reproduction must all be achieved within one cell.

The structures differ, but the same functional problem remains: matter, information and energy must be coordinated inside one bounded cell.

Classification should use a combination of structures. A wall alone does not prove that a cell is plant, and the absence of a chloroplast does not exclude plant tissue because many plant cells are non-photosynthetic.

These cases do not show that cell theory is useless. They show that specialization can redistribute cellular functions across shared cytoplasm or cooperating cells, so “one small compartment with one nucleus” is not the theory itself.

Absence is weaker than presence: a thin section may miss an organelle that exists elsewhere in the cell. Identification should therefore rest on visible positive evidence whenever possible.

Before adding an annotation, ask whether the feature can be pointed to in the drawing and whether the claimed function follows from that observed structure. Biological knowledge may guide interpretation, but it must not fabricate visual evidence.

The engulfing host was not already a modern eukaryotic cell. Endosymbiosis is part of the explanation for how eukaryotic complexity arose.

No single observation proves the full history. The theory is strong because structural, biochemical and sequence evidence converge on the same ancestry and make successful predictions about organelle genomes.

The evidence strongly supports bacterial origins of mitochondria and chloroplasts; it does not imply that every eukaryotic organelle arose by endosymbiosis.

Most cells in a multicellular organism descend from one original cell and therefore contain essentially the same genome. They differ because developmental signals and position cause different genes to be expressed or suppressed.

different gene expression → different RNAs and proteins → different proteomes → different structures and reactions → specialized cell functions

A muscle fibre produces abundant contractile proteins and organizes them into myofibrils. A protein-secreting cell develops extensive rough ER and Golgi apparatus. Their specialization reflects different use of shared genetic information, not a different complete genome.

Division of labour increases efficiency but also dependence: a highly specialized cell often cannot perform every process needed for independent life.

Multicellularity evolved independently in animals, plants, fungi and several algal lineages. Its repeated origin shows that there is no single compulsory route, but stable multicellular organisms must solve a common set of coordination problems.

Larger size can reduce predation and broaden resource use; specialization can make tissues more efficient. These gains are selected only when they outweigh the energy cost of coordination and the loss of cellular independence.

Viruses lack the systems needed for independent life:

• no cytoplasm in which to maintain metabolism
• no ribosomes for protein synthesis
• few or no metabolic enzymes
• no independent supply of energy or raw materials

A virus is therefore an obligate intracellular parasite: its genome can be copied and expressed only after it enters a compatible host and redirects the host's enzymes, ribosomes, energy and materials. Outside a host, the virion does not grow, metabolize or reproduce.

• Tobacco mosaic virus: single-stranded RNA, helical capsid, no envelope
• HIV: two copies of single-stranded RNA, conical capsid, envelope present
• Coronavirus: single-stranded RNA, helical nucleocapsid, envelope present
• Lambda phage: linear double-stranded DNA, icosahedral head with a complex tail, no envelope

Genome may be DNA or RNA, single- or double-stranded, linear, circular or segmented. Capsids may be helical, polyhedral, conical or complex. An envelope is an additional lipid membrane, not a replacement for the capsid.

Receptor binding creates host and tissue specificity and initiates entry. The envelope can aid membrane fusion, but non-enveloped viruses use other entry mechanisms, so an envelope is useful rather than universal.

If the DNA remains separate and is expressed immediately, infection enters the lytic route. If it integrates into the bacterial chromosome, it begins the lysogenic route.

The decisive distinction is not whether viral DNA entered the cell—it does in both routes. The distinction is the state of that DNA after entry, which determines immediate virion production or inherited latency.

Lambda is a temperate phage because the same virus can use either route. Lysogeny postpones virion production; it does not remove the possibility of later lysis.

Cellular life shares deeply conserved machinery and genes, supporting common ancestry. Viruses show a different pattern: no gene is shared by every viral group, and their genomes, replication strategies and particle structures are exceptionally diverse.

This pattern supports polyphyly—different viral groups probably arose independently, at different times and by more than one route. It does not identify a separate origin for every virus; it rejects the assumption that one known ancestor explains them all.

All viruses nevertheless share extreme host dependence. That similarity may be convergent evolution: unrelated replicating systems exposed to the same selective problem independently evolved compact transmissible particles that exploit cells.

• Virus-first: self-replicating molecules existed before or alongside early cells and later became dependent parasites.
• Escaped genes (progressive): mobile genetic elements left cells and gained structures for transmission between hosts.
• Regressive (reduction): cellular parasites lost genes and cellular structures as host dependence increased.

These are hypotheses, not three stages of one history. Escaped-gene models may fit some small mobile-element-derived viruses, while reduction may fit some large DNA viruses. More than one hypothesis can therefore be correct for different viral lineages.

Mutation creates new alleles during genome replication; recombination or segment reassortment can create new combinations. These processes generate heritable variation before selection acts.

Viruses can evolve rapidly because:

• replication enzymes—especially many RNA-dependent polymerases and HIV reverse transcriptase—make frequent uncorrected errors
• short generation times expose variants to selection quickly
• enormous populations generate many mutations in every infection
• repeated transmission exposes variants to changing hosts, immunity and treatments

Selection does not create a needed mutation. A variant already present increases in frequency when it leaves more descendants—for example by replicating faster, transmitting more effectively, evading immunity or surviving an antiviral drug.

High mutation rate alone is not evolution. Population evolution is the change in variant frequencies across generations after mutation, inheritance and differential reproductive success interact.

Antibodies bind particular antigen shapes. Drift can gradually reduce recognition and helps explain repeated vaccine updates. Shift can expose a population to a much less familiar antigen combination, so its epidemiological effect may be larger and more sudden.

Shift depends on a segmented genome and co-infection of one host cell; it is not simply a larger point mutation. Drift and shift both generate variation, after which transmission and immunity determine which variants spread.

HIV carries RNA and reverse transcriptase, which copies viral RNA into DNA before that DNA integrates into the host genome. Reverse transcriptase lacks effective proofreading, so copying creates frequent mutations.

Within one person, rapid replication and a large viral population produce many related HIV variants. Most mutations are neutral or harmful, but some alter a protein targeted by an antiviral drug.

A single drug creates strong selection: susceptible variants are suppressed while a resistant variant continues reproducing and becomes more common. The drug selects resistance from existing variation; it does not direct HIV to make the useful mutation.

Combination therapy attacks different viral targets at once. A virion would need several compatible resistance changes to reproduce successfully, greatly reducing the probability that an already resistant variant survives treatment.

Variation occurs among individuals in every species. Darwin recognized that, when some inherited variants affect survival or reproduction, their frequencies can change across generations by natural selection.

Appearance varies within species. Sexual dimorphism can make males and females look unlike, while unrelated species can converge on similar forms. A boundary should therefore use several traits and, where possible, independent evidence.

A scientific species name has two parts: the genus followed by the specific epithet. In Panthera leo, Panthera names the genus and leo distinguishes the species within that genus.

• Capitalize the genus; write the specific epithet in lowercase.
• Italicize both words when typed; underline each word separately when handwritten.
• After the first full use, the genus may be abbreviated: P. leo.

The shared genus indicates a group of similar, closely related species. The binomial prevents ambiguity between local common names, but the name records a classification—it is not itself evidence that the species boundary is correct.

Under the biological species concept, members of one species can interbreed and produce viable, fertile offspring. Successful reproduction lets alleles move through a shared gene pool.

This concept cannot be tested directly for fossils, and it does not fit organisms that reproduce only asexually. Distinct forms may also retain limited gene flow, so interbreeding evidence must be interpreted with ecology and evolutionary history.

In a typical diploid cell, chromosomes occur as homologous pairs—one member of each pair inherited from each parent—so the diploid number is usually even and normally constant within a species.

Chromosome number varies widely between species but does not rank organism size or complexity. Closely related species can differ by a chromosome fusion or split while retaining many homologous genes.

A karyotype is the chromosome number and structural features of a cell. A karyogram is an arranged image in which homologous chromosomes are paired using length, banding pattern and centromere position.

A genome is all genetic information in an organism: nuclear DNA plus mitochondrial DNA and, in plants, chloroplast DNA. Messenger RNA and transfer RNA are products of genes, not additional parts of the genome.

Most SNPs are in non-coding DNA and have no known phenotypic effect. Some lie in genes or regulatory regions and can affect function, drug response or disease risk; association alone does not prove causation.

Eukaryote genomes vary in both sequence and total DNA. Sequence variation between species is much larger than variation within one species, while repeated and other non-coding DNA can contribute greatly to genome size.

Whole-genome duplication produces polyploid organisms with several chromosome sets. This can greatly enlarge plant genomes without making the organism proportionally more complex, so a genome-size comparison is not a complexity ranking.

Whole-genome sequencing determines the order of bases across an organism’s complete DNA. The Human Genome Project established large-scale reference sequencing and accelerated bioinformatics for storing, comparing and interpreting sequence data.

Current and developing uses include:

• comparing evolutionary relationships
• locating genes and variants associated with disease
• identifying and tracking pathogens
• finding possible diagnostic, treatment or vaccine targets
• guiding prevention or treatment using an individual’s genetic profile

A sequence match or statistical association generates a hypothesis. Clinical effect, causation and safety require independent evidence; sequencing alone does not diagnose every disease or guarantee that a proposed target will work.

Bacteria reproduce asexually, so ‘interbreed and produce fertile offspring’ cannot define their species. Mutation creates new variants within lineages, while horizontal gene transfer moves DNA between cells and sometimes across established taxonomic boundaries.

Because ancestry is partly tree-like and partly network-like, no mating test encloses a single bacterial gene pool. Bacterial groups are therefore distinguished using combinations of genome sequence, evolutionary history, physiology and ecology.

Meiosis separates homologous pairs to produce haploid gametes; fertilization combines two haploid sets and restores the characteristic diploid number. This cycle prevents chromosome number doubling in every generation.

Each row asks about one observable contrast, and every answer either gives an identification or points to exactly one next couplet. The key identifies only organisms represented when it was designed; it does not prove that the resulting names are evolutionarily closest.

A key can fail when the specimen is outside its scope, damaged or at a different life stage; when a trait varies with sex or environment; or when terminology and hidden physiological features cannot be judged reliably in the field.

A detection shows that matching DNA was present in the sample. It does not by itself prove population size, current survival or ecological importance; missing reference sequences, degradation and contamination can also change the result.

These independent features agree with the rRNA evidence. Archaea share a prokaryotic cell organization with Bacteria but differ in fundamental molecular structures; some information-processing features resemble those of Eukarya.

Evolution is the cumulative change in the heritable characteristics of a population across generations. The population evolves when inherited variants change in frequency; an individual does not evolve during its lifetime.

Mutation and recombination generate heritable variation. Selection, genetic drift and gene flow alter which variants are passed on, so the distribution of characteristics in the next generations can differ from the starting population.

Sequence similarity is evidence, not a complete history by itself. The sequences must be homologous, sampling must be adequate, and different genes or evolutionary models can yield different estimates.

Artificial and natural selection act on inherited variation, but the selecting agent differs: human preference in breeding, and differential survival or reproduction in the environment.

Similarity of function does not establish close ancestry. Classify evolutionary relationships using homologous features and independent molecular evidence; use analogous features to explain how similar niches can produce similar solutions.

A present-day barrier and a modern distribution support a historical hypothesis but do not by themselves date the split. Fossil, geological and molecular evidence are needed to test when and how isolation occurred.

Geography is not the definition of reproductive isolation. Some gene flow can persist if divergent selection is strong enough to overcome its homogenizing effect, and intrinsic barriers may evolve after an earlier period of geographic separation.

Adaptive radiation is therefore rapid diversification from one ancestor into multiple species adapted to different ecological roles; it is not simply variation within one population.

Prezygotic barriers prevent mating or fertilization. They avoid the cost of producing low-fitness hybrids and can therefore be strengthened by selection where related populations meet, a process called reinforcement.

Species boundaries can be partly permeable. Occasional hybrids do not prove that two gene pools are fully connected: measure how often hybrids form, whether they survive and reproduce, and how much gene flow actually occurs.

The plant genus Persicaria contains multiple allopolyploid species; widespread P. lapathifolia has contributed to several hybrid lineages. The named case illustrates the mechanism—it does not imply that every hybrid automatically becomes a species.