Theme B — Form and function

Theme B — Form and function — IB Biology HL Revision Notes & Topical Questions

5 min read 9 parts Lesson 1 of 14 Unit quiz

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Theme B — Form and function — IB BIOLOGY HL notes: Connect the Theme B topics from molecules to organs, using short facts on proteins, membranes, specialisation, gas exchange, transport and muscle.Condensation — it releases water and forms a covalent bond when monomers join to bu…

Hydrolysis
Polypeptide
Essential amino acids
Immobilised enzymes
Mitochondrion
Early embryo

Part 1 of 7Condensation, hydrolysis and polypeptides

Condensation

Condensation it releases water and forms a covalent bond when monomers join to build carbohydrates and lipids.

Hydrolysis

Hydrolysis it breaks a covalent bond by adding water.

Polypeptide

Polypeptide a chain of amino acids joined by peptide bonds, with a sequence coded by a gene.

Protein

Protein it consists of one or more polypeptides.

Essential amino acids

Essential amino acids they must come from food, and lacking one limits protein synthesis.
Vegan diets combine sources such as legumes and grains to supply all of them.

Part 2 of 7Globular and fibrous proteins

Haemoglobin is a compact globular protein with four interacting subunits.

Globular haemoglobin and fibrous collagen illustrate contrasting protein forms.

Feature	Globular proteins	Fibrous proteins
Shape	Compact, roughly spherical	Long strands with cross-links (hydrogen bonds)
R groups	Hydrophobic inside, hydrophilic outside	Many hydrophobic R groups
Solubility	Soluble, so easily transported	Insoluble
Examples	Enzymes, immunoglobulins, haemoglobin (with a haem prosthetic group), insulin	Proteins with structural roles

Insulin
- it was the first protein sequenced.
- Polypeptide A has 21 amino acid residues and polypeptide B has 30, joined by three disulfide bridges.
- It is made in the pancreas and controls blood glucose.

Part 3 of 7Membrane glycoproteins and mitochondria

Glycoproteins and glycolipids in the cell-surface membrane have several roles.

They act as receptors for hormones and neurotransmitters.
They take part in endocytosis.
They help cells stick together.
They act as antigens, letting the immune system tell self from non-self.

Immobilised enzymes

Immobilised enzymes some integral proteins are enzymes, such as those on intestinal microvilli that digest disaccharides.

Mitochondrion

Mitochondrion it has a double membrane, 70S ribosomes and circular DNA.
The link reaction and Krebs cycle occur in the matrix.
Oxidative phosphorylation occurs on the inner membrane cristae.

Part 4 of 7Stem cells and cell specialisation

Stem cells and cell specialisation

Early embryo

Early embryo the zygote divides by mitosis, and early cells are unspecialised and totipotent.

Blastocyst

Blastocyst by this stage the inner cell mass is pluripotent.

Stem cell

Stem cell a cell that can divide repeatedly (self-renewal) and differentiate.

Differential gene expression

Differential gene expression cells specialise in response to their position.
Morphogens diffuse from source cells to form concentration gradients, such as Sonic hedgehog in limb development.
Cells at high and low concentrations express different genes.

Silencing

Silencing genes are switched off by DNA methylation or histone modification, and the DNA sequence is unchanged.

Part 5 of 7Ventilation and the heart

Inhalation

The diaphragm and external intercostal muscles contract.

Thoracic volume increases.

Pressure falls below atmospheric pressure (Boyle's law).

Air flows in.

Forced exhalation

Forced exhalation it uses the internal intercostal and abdominal muscles.

Spirometer

Spirometer it measures tidal volume, vital capacity and reserve volumes, which increase with exercise.

Alveolar epithelium

Alveolar epithelium it contains type I and type II pneumocytes.

Heart pressure graphs

Heart pressure graphs the crossing points of the lines mark valve events.
Ventricle pressure rising above atrium pressure closes the bicuspid valve.
Ventricle pressure rising above aortic pressure opens the aortic valve.
Ventricle pressure falling below aortic pressure closes the aortic valve.
Atrium pressure rising above ventricle pressure reopens the bicuspid valve.

Heart rate

60 ÷ the length of one cardiac cycle in seconds.

Part 6 of 7Water transport and the lymphatic system

Water transport and the lymphatic system

Xylem

Xylem water rises because of cohesion, the attraction between water molecules from hydrogen bonding.

Tissue fluid

Tissue fluid it forms at capillaries, and some re-enters the capillaries while the rest enters closed-ended, large-pored lymph capillaries.

Lymph

It moves by body movements and valves.
It passes through lymph nodes, which contain macrophages.
It carries lipids from the intestine and returns plasma proteins to the blood.

Protein return

Protein return plasma proteins left in tissue fluid would raise its solute concentration and prevent water reabsorption.

Sedentary travellers can get swollen legs because lymph accumulates.

Part 7 of 7Muscle tissue

Skeletal muscle

Skeletal muscle it is striated and made of fibres.
Each fibre has a sarcolemma and many nuclei.
Contractile proteins are organised in the fibre.
The sarcoplasmic reticulum stores Ca²⁺.

Cardiac muscle is found in the heart.

Smooth muscle is found in blood vessels and organs.

Key terms

Term	Meaning
Hydrolysis	breaking a covalent bond by adding water.
Polypeptide	a chain of amino acids linked by peptide bonds.
Stem cell	an unspecialised cell that can self-renew and differentiate.
Morphogen	a signalling molecule forming a gradient that determines cell fate.
Cohesion	attraction between water molecules due to hydrogen bonding.
Lymph	fluid in lymph vessels that carries lipids and returns proteins to the blood.

Avoid these mistakes

Specialised cells have lost genes — their DNA is unchanged and different genes are expressed.

Totipotent and pluripotent are the same — totipotent cells can form any cell including extra-embryonic tissue, and pluripotent cells can form any body cell.

Lymph vessels are part of the blood circulation — they form a separate system that drains into veins.

Air is drawn in by the lungs actively expanding — muscles enlarge the thorax and the lower pressure draws air in.

Last updated: 26 October 2026