Glycerol + three fatty acids joined by ester bonds.
Phospholipid
Glycerol + two fatty acids + a phosphate group; amphipathic.
Glycoprotein
Protein with carbohydrate chains, used in cell recognition (e.g. ABO blood groups).
Carbon forms four covalent bonds, allowing chains, rings and huge diversity of molecules.
The full notes, worked example and check questions for this subtopic are in the app.
B1.2Proteins
Key terms
Amino acid
Monomer with an amine group, a carboxyl group, a hydrogen and a variable R group on a central carbon.
Peptide bond
Bond between amino acids formed by condensation.
Essential amino acid
One the body cannot make, so it must come from the diet.
Denaturation
Change to a protein's 3D shape, usually permanent, caused by heat or extreme pH.
Conjugated protein
Contains a non-polypeptide part (prosthetic group), e.g. haem in haemoglobin.
20 amino acids are coded for in the genetic code; their sequence gives almost unlimited protein diversity.
The full notes, worked example and check questions for this subtopic are in the app.
B2.1Membranes and membrane transport
Key terms
Simple diffusion
Passive net movement of particles down a concentration gradient through the bilayer.
Facilitated diffusion
Passive movement through channel proteins.
Osmosis
Passive movement of water across a membrane, often via aquaporins.
Active transport
Movement against a concentration gradient using pump proteins and ATP.
Fluid mosaic model
Membrane with phospholipids and proteins that move laterally.
Phospholipid bilayers are a barrier to ions and polar molecules because of the hydrophobic core.
The full notes, worked example and check questions for this subtopic are in the app.
B2.2Organelles and compartmentalisation
Key terms
Organelle
A discrete subunit of a cell adapted for a specific function.
Compartmentalisation
Separating reactions into membrane-bound areas.
Cristae
Folds of the inner mitochondrial membrane.
Thylakoid
Flattened membrane sacs in chloroplasts where light-dependent reactions occur.
Separating the nucleus from the cytoplasm lets eukaryotes process mRNA (splicing) before translation and control gene expression.
The full notes, worked example and check questions for this subtopic are in the app.
B2.3Cell specialisation
Key terms
Differentiation
Cells becoming specialised by expressing different genes.
Stem cell
Unspecialised cell that can divide and differentiate.
Totipotent / pluripotent / multipotent
Can form any cell incl. placenta / any body cell / a limited range of cells.
Stem cell niche
A location where stem cells are kept, e.g. bone marrow, hair follicles.
SA:V ratio
Surface area divided by volume; falls as cells get bigger.
After fertilisation, the zygote divides to make unspecialised cells. Signals, e.g. morphogen gradients, trigger differentiation.
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
B3.1Gas exchange
Key terms
Ventilation
Moving air in and out of the lungs to keep concentration gradients high.
Alveolus
Tiny air sac where gas exchange happens.
Surfactant
Fluid that reduces surface tension and stops alveoli collapsing.
Tidal volume
Volume of air breathed in or out in one normal breath.
Vital capacity
Maximum volume exhaled after a maximum inhalation.
Bohr shift (HL)
Higher CO₂ lowers haemoglobin's affinity for O₂, releasing more O₂ to respiring tissues.
Good gas exchange surfaces are permeable, thin, moist and large.
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
B3.2Transport
Key terms
Artery
Carries blood away from the heart at high pressure; thick muscular and elastic walls.
Vein
Returns blood to the heart at low pressure; thin walls, wide lumen, valves.
Capillary
One-cell-thick wall, sometimes fenestrated, for exchange with tissues.
Transpiration stream
Water pulled up xylem by tension from evaporation in leaves.
Translocation (HL)
Movement of sugars in phloem from sources to sinks.
Capillaries: narrow, thin, permeable, large total surface area.
The full notes, worked example and check questions for this subtopic are in the app.
B3.3Muscle and motility HL
Key terms
Sarcomere
Repeating contractile unit of a myofibril, between two Z lines.
Sliding filament model
Myosin heads pull actin filaments towards the sarcomere centre using ATP.
Antagonistic pair
Muscles that produce opposite movements, e.g. biceps and triceps.
Synovial joint
Freely moving joint with cartilage, synovial fluid and a capsule, e.g. the hip.
Contraction: nerve impulse → Ca²⁺ released from the sarcoplasmic reticulum → binds troponin → tropomyosin moves → myosin binds actin (cross-bridge) → power stroke → ATP breaks the cross-bridge and resets the head.
The full notes, worked example and check questions for this subtopic are in the app.
B4.1Adaptation to environment
Key terms
Habitat
The place where an organism lives.
Adaptation
A heritable trait that improves survival and reproduction in an environment.
Xerophyte
Plant adapted to dry conditions.
Halophyte
Plant adapted to salty conditions.
Biome
A large group of ecosystems with similar climate and communities.
Abiotic factors (temperature, water, light, salinity, soil) determine where species can live; each species has a range of tolerance.
The full notes, worked example and check questions for this subtopic are in the app.
B4.2Ecological niches
Key terms
Niche
The role of a species in its ecosystem, including its habitat, food and interactions.
Fundamental niche (HL)
Where a species could live without competition.
Realised niche (HL)
Where a species actually lives because of competition.
Saprotroph
Feeds on dead matter by external digestion, e.g. fungi.
Mixotroph
Can be autotrophic or heterotrophic, e.g. Euglena.
Respiration types: obligate anaerobes (killed by O₂), facultative anaerobes (use O₂ if available), obligate aerobes (need O₂).
The full notes, worked example and check questions for this subtopic are in the app.