Theme A — Unity and diversity

Theme A — Unity and diversity — IB Biology HL Revision Notes & Topical Questions

5 min read 12 parts Lesson 1 of 14 Unit quiz

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Theme A — Unity and diversity — IB BIOLOGY HL notes: Link the Theme A topics (water, nucleic acids, origins of cells, cell structure, viruses, diversity, classification and evolution) and apply them in short calculations.Hydrogen bonds — water is polar, so molecules form hydrogen…

Latent heat of vaporisation
Calculations use E
DNA and RNA
Possible sequences
Length of a DNA molecule
Primordial soup

Part 1 of 10Water and its thermal properties

Water and its thermal properties

Hydrogen bonds

Hydrogen bonds water is polar, so molecules form hydrogen bonds
This gives cohesion, so a continuous water column can be pulled up the xylem under tension.
It makes water a good solvent and gives it a high specific heat capacity.

Latent heat of vaporisation

Latent heat of vaporisation the energy absorbed when a liquid becomes a gas without a change in temperature

It is high, about 2257 kJ kg^-1, because hydrogen bonds must be broken.
Evaporation of sweat or transpiration therefore cools organisms.

Calculations use E

Calculations use E mcΔT and E = mL

Warming 2.0 kg of water from 15 °C to 37 °C with c = 4.18 kJ kg^-1 °C^-1 needs 2.0 × 4.18 × 22 ≈ 184 kJ.
A runner evaporating 0.50 kg of sweat with L = 2.4 × 10³ kJ kg^-1 loses 0.50 × 2.4 × 10³ = 1200 kJ.

Part 2 of 10Nucleic acids

1DNA and RNA

DNA and RNA polymers of nucleotides
A purine has a double ring, adenine or guanine.
A coding sequence carries instructions for a protein and other regions are non-coding.

2Possible sequences

Possible sequences with four bases a sequence of 10 nucleotides can be arranged in 4¹⁰ = 1 048 576 ways

3Length of a DNA molecule

Length of a DNA molecule one turn of 10 base pairs is 3.4 nm, so each base pair is 0.34 nm
A DNA molecule 6.8 µm long has 6800 nm / 0.34 nm = 2.0 × 10⁴ base pairs.

Part 3 of 10Origins of cells

Primordial soup

Primordial soup the idea that early oceans held a rich mix of simple organic compounds

Miller-Urey experiment

Miller-Urey experiment a sealed apparatus with water, methane, ammonia and hydrogen
An electrical discharge was the energy source instead of UV light.
It showed that amino acids could form without living things.
The early atmosphere may have been less reducing than assumed.

Cell

Cell the smallest unit that carries out all the functions of life

The functions are metabolism, growth, response, homeostasis, reproduction, excretion and nutrition.

Part 4 of 10Cell structure

Cell structure

Prokaryotic cells

Prokaryotic cells a cell wall, plasma membrane, cytoplasm, 70S ribosomes and naked DNA in a nucleoid
Some have plasmids, pili and flagella.

Atypical cells

Atypical cells they challenge the simple idea of one nucleus per independent unit
Skeletal muscle fibres form by fusion of many cells.
Aseptate hyphae have continuous multinucleate cytoplasm.
Sieve tube elements depend on companion cells.
Mature red blood cells lack a nucleus.

Part 5 of 10Viruses: structure

Viruses: structure

Virus

Virus a non-cellular infectious particle, not a cell because it lacks metabolism
It is 20-300 nm across and can only be seen with an electron microscope.
It sits outside the three-domain system of classification.

Common features

A small, fixed size, as viruses do not grow.
A nucleic acid core of DNA or RNA, single or double stranded, linear or circular.
A protein coat called a capsid with attachment proteins.
No cytoplasm and very few or no enzymes.

Lipid envelope

Lipid envelope some viruses have one, formed from the membrane of the cell they were made in

Host specificity

Host specificity attachment proteins decide which host cell is infected
HIV infects white blood cells and hepatitis infects liver cells.

Examples

Bacteriophage lambda has double-stranded DNA in its head and injects it through a contractile tail.
Coronaviruses have single-stranded RNA, an envelope and surface glycoproteins that give a corona.
HIV has two RNA strands, reverse transcriptase, a capsid and an envelope from the host helper T cell.

Part 6 of 10Viruses: replication and evolution

Lytic pathway

The virus attaches to the host cell using attachment proteins.

It injects its nucleic acid into the cytoplasm.

The host machinery makes viral proteins and nucleic acid by biosynthesis.

New particles are assembled and matured into virions.

The host cell lyses, releasing the virions to infect more cells.

1Lysogenic pathway

Lysogenic pathway viral nucleic acid joins the host DNA and stays dormant
A repressor protein prevents its transcription.
It is copied every time the host cell divides.
UV rays or certain chemicals can trigger the lytic pathway.

2Rapid evolution

Rapid evolution influenza and HIV have high mutation rates, large populations and short generation times
RNA genomes mutate readily during copying.

3Antigenic drift

Antigenic drift the gradual accumulation of small changes in viral surface antigens, which lets influenza evade immunity

4Antigenic shift

Antigenic shift a major change when two virus types infect one cell and combine their genetic material

5Origin

Origin endogenous retroviruses make up about 8% of the human genome
The escape, regressive and virus-first theories are still debated.

Part 7 of 10Viruses: calculations

Burst size

Burst size a phage with a burst size of 100 gives 100³ = 1.0 × 10⁶ virions after three cycles

Image size

Image size a virus 12 mm wide in an image at ×100 000 has an actual width of 12 / 100 000 = 1.2 × 10^-4 mm = 120 nm

Part 8 of 10Diversity and classification

Diversity and classification

Binomial name

Binomial name humans are Homo sapiens, with the genus capitalised, the species in lower case and both italicised

Species

Species a group that interbreeds to give fertile offspring

A horse and a donkey produce infertile mules, showing they are separate species.
In HL the biological species concept is hard to apply to bacteria, which reproduce asexually and exchange genes by horizontal transfer.

Karyogram

Karyogram chromosomes in homologous pairs by decreasing length, with the sex chromosomes last
Three copies of chromosome 21 (trisomy 21) indicate Down syndrome.

Genome size

Genome size it varies greatly mainly because of non-coding and repetitive DNA, not because of complexity

Part 9 of 10Evolution and biodiversity

Evidence for evolution

DNA and protein sequence similarities, homologous structures and selective breeding

Speciation

Speciation it requires barriers to gene flow
The barriers can be geographical, behavioural or temporal.

Biodiversity loss

Biodiversity loss it is caused by habitat destruction, introduced species, pollution and overharvesting

Part 10 of 10More from the specification

Replication
- Exact copying of chromosomes before division.

Key terms

Term	Meaning
Latent heat of vaporisation	The energy absorbed when a liquid becomes a gas without a change in temperature.
Primordial soup	The idea that early oceans held a rich mix of simple organic compounds.
Karyogram	An image of chromosomes arranged in pairs by size and centromere position.
Antigenic drift	The gradual change in viral surface antigens caused by mutations.
Purine	A double-ring base, adenine or guanine.
Capsid	The protein coat that surrounds the nucleic acid of a virus.
Coding sequence	The part of the genome that carries instructions for a protein.

Avoid these mistakes

Writing species names with a capital letter on the species word — only the genus is capitalised

Thinking genome size reflects organism complexity — non-coding DNA accounts for much of the variation

Using the biological species concept for bacteria — they reproduce asexually

Forgetting to convert units in magnification calculations — convert to the same unit first

Last updated: 26 October 2026