IB BIO [HL]: Unity and Diversity

Water as the medium for life

Water is essential for life due to its solvent properties, temperature regulation via high specific heat capacity, and role in cohesion/adhesion for transport in organisms.

Hydrogen bonds in water

Form between polar water molecules due to electronegativity difference; responsible for cohesion, adhesion, high boiling point, and surface tension.

Cohesive properties of water

Water molecules stick together via hydrogen bonds, enabling transport in xylem (transpiration) and blood vessels.

Adhesive properties of water

Water sticks to hydrophilic surfaces (e.g., cellulose in xylem), aiding capillary action in plants.

Solvent properties of water

Polar water dissolves ions and polar molecules, facilitating metabolism, transport (e.g., glucose, amino acids), and hydrolysis reactions.

Nucleic acids structure

Polymers of nucleotides (phosphate, sugar, base); DNA double helix (A-T, G-C), RNA single-stranded.

DNA replication semi-conservative

Each strand acts as template; new strands synthesized 5' to 3' by DNA polymerase; results in hybrid old-new strands.

Origins of cells

All cells from pre-existing cells; endosymbiotic theory explains mitochondria/chloroplasts from engulfed prokaryotes.

Prokaryotic vs eukaryotic cells

Prokaryotes: no nucleus, smaller, circular DNA; eukaryotes: nucleus, organelles, linear DNA, larger.

Unicellular eukaryotes

E.g., Amoeba, Paramecium; perform all life functions in single cell.

Viruses structure

Acellular; nucleic acid (DNA/RNA) in protein capsid; some with envelope; obligate intracellular parasites.

Lytic cycle of viruses

Attachment > penetration > synthesis > assembly > lysis/release; destroys host cell (e.g., T4 bacteriophage).

Lysogenic cycle

Phage DNA integrates as prophage; replicates with host DNA until induced to lytic.

Retroviruses (e.g., HIV)

RNA virus; reverse transcriptase makes DNA from RNA, integrates as provirus.

Classification need

Organizes biodiversity (~8.7 million species); aids identification, evolutionary relationships.

Binomial nomenclature

Genus species (italicized, e.g., Homo sapiens); hierarchical: domain, kingdom, phylum, etc.

Clades

Monophyletic groups sharing common ancestor and all descendants.

Cladograms

Branching diagrams showing evolutionary relationships based on shared derived characters.

Molecular evidence for cladistics

DNA/protein sequences; more differences indicate longer divergence time.

Natural selection

Mechanism of evolution: variation > heritable > differential survival/reproduction.

Speciation

Allopatric (geographic isolation > genetic divergence); sympatric (without isolation, e.g., polyploidy).

Artificial selection

Selective breeding for traits (e.g., dogs, crops); evidence for natural selection.

Biodiversity

Genetic, species, ecosystem variety; hotspots have high endemism/threat.

Conservation strategies

In situ (protected areas), ex situ (zoos, seed banks); CITES for trade regulation.