Theme C — Interaction and interdependence

Theme C — Interaction and interdependence — IB Biology SL Revision Notes & Topical Questions

5 min read 11 parts Lesson 1 of 9 Unit quiz

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Lesson summary

Theme C — Interaction and interdependence — IB BIOLOGY SL notes: Connect enzymes, respiration, photosynthesis, neural signalling, defence and populations in Theme C.Optimum = the temperature or pH at which the enzyme rate is highestThe rate falls on either side of the optimum as …

Cell respiration
Stage locations
Respirometer
Respiration compared with breathing
Oxygen source
Action spectrum

Part 1 of 9Enzymes

Optimum = the temperature or pH at which the enzyme rate is highest
- The rate falls on either side of the optimum as bonds in the enzyme are disrupted.

Part 2 of 9Cell respiration

Cell respiration

Cell respiration the controlled release of energy from organic compounds to produce ATP

Oxidation is the loss of electrons or hydrogen and releases energy.
Respiration releases energy and does not make energy.

Stage locations

Stage locations glycolysis occurs in the cytoplasm and not in the mitochondria

ATP yield

ATP yield aerobic respiration gives far more ATP than anaerobic respiration

Anaerobic products depend on the organism

Human muscle cells make lactate.
Yeast makes ethanol and carbon dioxide.

ATP

ATP an immediate, short-term energy carrier and not a store lasting for days

Respirometer

Respirometer a control tube with dead seeds or glass beads shows that changes are caused by living tissue

Respiration compared with breathing

Respiration compared with breathing respiration is a chemical process in cells and breathing is ventilation of the lungs

Part 3 of 9Photosynthesis

Oxygen source

Oxygen source the oxygen released comes from water

Pigments

Pigments chlorophyll absorbs mostly red and blue light

Action spectrum

Action spectrum a graph of the rate of photosynthesis at each wavelength

Limiting factors

If the rate stays the same as light rises but rises with CO₂, then CO₂ is limiting.
The rate falls above 40 °C because enzymes denature.

Rf distance moved by a pigment divided by distance moved by the solvent front

Distances of 2 cm and 8 cm give an Rf of 0.25.
A pigment with a very low Rf has a strong affinity for the stationary phase.

Pondweed experiment

Pondweed experiment the water is boiled and cooled before sodium hydrogencarbonate is added
This removes dissolved CO₂ so that the concentration is known.

FACE experiments

FACE experiments costly and hard to replicate

Part 4 of 9Action potential

The stimulus raises the membrane to threshold, about -50 mV from a resting potential of -70 mV.

Voltage-gated Na⁺ channels open.

Na⁺ enters and the inside becomes positive, reaching about +40 mV.

Na⁺ channels close and K⁺ channels open.

K⁺ leaves and the membrane repolarises.

The pump restores the resting distribution of ions.

Sodium-potassium pump

Sodium-potassium pump uses ATP to move 3 Na⁺ out and 2 K⁺ in

Propagation

Propagation an action potential is propagated along the axon and is not electricity flowing

Stimulus strength

Stimulus strength a stronger stimulus gives a higher frequency of impulses and not a bigger impulse

Myelin

Myelin insulates the axon, and impulses jump between the gaps

Part 5 of 9Synapse

Cholinergic synapse transmission

The action potential reaches the presynaptic terminal.

Ca²⁺ channels open and Ca²⁺ enters.

Vesicles release acetylcholine by exocytosis.

Acetylcholine diffuses across the cleft and binds receptors.

Na⁺ enters and the postsynaptic membrane depolarises.

Acetylcholinesterase breaks down the acetylcholine.

Signal transfer
- a chemical neurotransmitter crosses the synapse and the impulse itself does not jump across

Part 6 of 9Neural and defence terms

Dendrite

Dendrite a short branch of a neurone that receives signals

Interneurone

Interneurone a relay neurone inside the CNS

Neuromuscular junction

Neuromuscular junction the synapse between a motor neurone and a muscle fibre

Feedback

Feedback involves a receptor, a control centre and an effector, and signals are nerve impulses

Phototropism

Phototropism involves efflux carriers and cell elongation

Phagocyte

Phagocyte a white blood cell that engulfs and digests pathogens by endocytosis

Signs and symptoms

Signs and symptoms signs are measurable features of disease and symptoms are what the patient feels

Part 7 of 9Sampling populations

Transect

Transect a line across a habitat along which samples are taken
It is better than random quadrats when an environmental gradient exists, such as from shore to dunes.
Systematic sampling shows how species change along the gradient.

Lincoln index

Lincoln index population size = first catch × second catch / marked recaptures
Round estimates to whole organisms.

Saprotroph

Saprotroph a decomposer that secretes enzymes externally and absorbs the digested products

Omnivore

Omnivore hard to place in a pyramid because it feeds at several trophic levels and its energy must be split between them

Part 8 of 9Population growth and relationships

Sigmoid growth

Sigmoid growth has three phases, with limiting factors setting the carrying capacity
At carrying capacity births and deaths fluctuate around a balance.

Competition

Competition interspecific between species and intraspecific within a species, for a named resource

Parasites

Parasites they harm hosts, usually without killing them quickly

Chi-squared test

Chi-squared test state the null hypothesis, give the degrees of freedom and the critical value
A significant result shows an association and does not prove that species depend on each other.

Part 9 of 9More from the specification

1Catalytic reaction rates and amino acids

Catalytic reaction rates and amino acids enzyme catalytic rates depend on correct three-dimensional folding of constituent amino acids forming active binding sites.

2Enzyme Active Site Specificity

An enzyme molecule features a distinct three-dimensional active site whose catalytic shape is complementary to specific substrate molecules.

3Enzyme-Substrate Complex Formation

Catalysis begins when substrate molecules collide with and bind to the active site,
forming a temporary enzyme-substrate complex that lowers the activation energy of the chemical reaction.

4Induced Fit Model

Upon substrate binding, the active site undergoes conformational changes that strain chemical bonds in the substrate molecule, accelerating metabolic conversion into products.

5Enzymatic Digestion of Starch

The enzyme amylase breaks down complex starch polysaccharides into maltose disaccharide molecules through specific enzyme-substrate interactions.

6Amylase

Enzyme that breaks starch into maltose.

7Enzyme-substrate complex

The temporary structure formed when a substrate binds to an enzyme's active site.

8Activation energy

The energy needed for a substrate to become unstable enough to react.

9Anabolic reaction

Builds large molecules from small ones using energy.

10Catabolic reaction

A reaction breaking larger molecules into smaller ones.

11Amino acid

The unit from which proteins are made; the sequence decides the protein's shape and function.

12Immobilised enzyme

An enzyme fixed to a surface or trapped in a matrix so it can be reused.

13Substrate

The molecule that an enzyme acts on and which binds to the active site.

14Complex

Two or more molecules joined, such as an antibody and antigen.

15Starch

An insoluble polysaccharide of glucose used by plants for energy storage.

16Metabolism

The chemical reactions in a cell that release energy and build molecules.

Key terms

Term	Meaning
Optimum	The condition giving the highest enzyme rate.
Rf value	Distance moved by the pigment divided by distance moved by the solvent front.
Sodium-potassium pump	A pump that uses ATP to move 3 Na⁺ out and 2 K⁺ in.
Phagocyte	A white blood cell that engulfs and digests pathogens.
Lincoln index	A mark-recapture estimate of population size.
Saprotroph	A decomposer that secretes enzymes externally and absorbs the products.

Avoid these mistakes

Saying respiration makes energy — it releases energy from organic compounds to make ATP

Saying that the impulse jumps across the synapse — a neurotransmitter diffuses across the cleft

Saying that the chi-squared test proves dependence — it shows a significant association only

Saying glycolysis happens in the mitochondria — it happens in the cytoplasm

Last updated: 26 October 2026