Unit 5: Respiration, Internal Environment, Coordination and Gene Technology

Unit 5 — Complete A Level and AS source — IAL Edexcel Modular Revision Notes & Topical Questions

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Unit 5 — Complete A Level and AS source notes — IAL EDEXCEL MODULAR notes: Summarise respiration, muscle, exercise, homeostasis, nervous and hormonal coordination, the brain and gene technology.Glycolysis — in the cytoplasm glucose forms two pyruvate, with a net 2 ATP and 2 reduc…

Link reaction
Krebs cycle
Oxidative phosphorylation
Without oxygen
Sliding filament mechanism
Negative feedback

Section 1 of 9Respiration and muscle

Glycolysis

Glycolysis in the cytoplasm glucose forms two pyruvate, with a net 2 ATP and 2 reduced NAD.

Link reaction

Link reaction in the matrix pyruvate forms acetyl CoA, CO₂ and reduced NAD.

Krebs cycle

Krebs cycle in the matrix it makes CO₂, reduced NAD, reduced FAD and ATP.

Oxidative phosphorylation

Oxidative phosphorylation on the cristae electrons pass along carriers, protons are pumped,
- and ATP synthase makes ATP with oxygen as the final electron acceptor.

Without oxygen

Without oxygen glycolysis continues, with animals making lactate and yeast making ethanol and carbon dioxide.

ATP use

ATP use respiration provides the ATP for muscle contraction, nerve impulse transmission and active transport.

Sliding filament mechanism

Sliding filament mechanism calcium ions expose binding sites on actin, myosin heads form cross-bridges, pull the actin filaments and release using ATP.
Fast-twitch fibres suit sprinting (anaerobic) and slow-twitch fibres suit endurance.
Antagonistic pairs (flexor and extensor) move joints.

Section 2 of 9Homeostasis and hormones

Negative feedback

Negative feedback receptor, coordinator and effector produce a response that reverses the change and keeps conditions within narrow limits.

Temperature control

Temperature control the hypothalamus is the thermoregulatory centre.
When body temperature rises, arterioles near the skin dilate (vasodilation) and sweating increases.
When it falls, vasoconstriction, shivering and a raised metabolic rate occur.

Hormones

Hormones peptide hormones bind surface receptors and activate second messengers,
whereas steroid hormones enter the cell and bind receptors that act as transcription factors.

ADH

ADH it increases the permeability of the collecting duct to water.

Heart

Heart it is myogenic and its rhythm is set by the sinoatrial node.

Section 3 of 9Nervous coordination

Action potential

Action potential depolarisation by sodium ion entry, then repolarisation by potassium ion exit.

Synapses

Synapses they pass the signal using neurotransmitters.

Plants

Plants redistribution of auxin (IAA) causes tropisms, and phytochrome detects light for flowering and growth responses.

Section 4 of 9The brain and gene technology

Brain regions

Brain regions the cerebral hemispheres control conscious activity and the cerebellum controls coordination.
The hypothalamus controls temperature and water balance, and the medulla oblongata controls automatic functions such as breathing and heart rate.

Habituation

Habituation stimulus-specific loss of response to unimportant stimuli.
At the synapse fewer vesicles are released because presynaptic calcium channels become less responsive.

Parkinson's disease

Parkinson's disease it involves loss of dopamine neurones and is treated with L-dopa.

Brain imaging

MRI and fMRI image the brain, and animal experiments are justified by the 3Rs.
- (replace, reduce, refine)

Gene technology

Gene technology it includes producing recombinant proteins in GMOs, PCR, gel electrophoresis and personalised medicine based on a patient's genotype.

Section 5 of 9Heart, exercise and practical skills

Heart excitation

Heart excitation the sinoatrial node initiates excitation, the atrioventricular node delays it so the atria finish contracting,
- and Purkyne tissue spreads it through the ventricles.

Cardiac output

Cardiac output stroke volume × heart rate.

Exercise

Exercise breathing rate and tidal volume rise.

Respiration practicals

Respiration practicals a respirometer with soda lime absorbs CO₂, and movement of coloured fluid shows oxygen uptake.

Exercise investigations

Exercise investigations use enough participants, standardise the activity and use a statistical test such as a t-test.

Section 6 of 9More from the specification

Mitochondrial inner membrane electron transport

Mitochondrial inner membrane electron transport electron transfer through inner membrane complexes pumps hydrogen ions, generating electrochemical gradients that synthesize ATP molecules.

Substrate-level phosphorylation in living cells

Substrate-level phosphorylation in living cells direct enzymatic transfer of phosphate groups from phosphorylated substrate intermediates yields ATP energy molecules during glycolysis.

Aerobic respiration stoichiometry equation

Aerobic respiration stoichiometry equation balancing cellular respiration equations demonstrates complete oxidation of glucose molecules into water, carbon dioxide,
and metabolic energy.

Cellular Respiration and Overall Chemical Equation

Respiration is the universal biochemical pathway in which living cells release energy from organic substrate molecules;
- the balanced equation for aerobic respiration oxidises glucose: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O, synthesising ATP.

Glycolysis and Substrate-Level Phosphorylation

In the cell cytoplasm, a six-carbon glucose molecule is broken down into two three-carbon pyruvate molecules during glycolysis, yielding a net gain of 2 ATP via substrate-level phosphorylation and reducing NAD⁺ by accepting high-energy hydrogen atoms.

Mitochondrial Electron Transport Chain and Chemiosmosis

In eukaryotic cells, reduced coenzymes donate hydrogen electrons to the electron transport chain on the inner mitochondrial membrane;
- the energy released drives proton pumping into the intermembrane space, powering ATP synthase before oxygen acts as the terminal electron and hydrogen acceptor.

Concentration gradient

A difference in concentration between two regions.

Molecule

Two or more atoms bonded together.

Hydrogen

A flammable gas that burns with a squeaky pop.

Substrate

The substance an enzyme acts on.

Mitochondria

Structures where respiration releases energy.

Concentration

The number of particles in a volume.

Respiratory quotient

CO~2~ produced divided by O~2~ consumed.

CO₂ ÷ O₂

What the symbols mean: RQ = respiratory quotient (no unit) · about 1.0 for carbohydrate, 0.7 for lipid, 0.9 for protein

Section 7 of 9More on this topic

Respirometer

Respirometer temperature and organism mass are controlled when comparing oxygen uptake.

Supplying active muscle

Supplying active muscle cardiac output = heart rate × stroke volume, and it rises in exercise to deliver oxygen and nutrients and remove CO₂.

Exercise investigations

Exercise investigations they need measured baselines, consistent activity and recovery times, and repeated observations.

Term	Meaning
Glycolysis	Splitting of glucose to pyruvate in the cytoplasm.
Respiratory quotient	CO~2~ produced divided by O~2~ consumed.
Cardiac output	Heart rate multiplied by stroke volume.
Negative feedback	A process that reverses a change to restore a set point.
ADH	Antidiuretic hormone, which makes the collecting duct more permeable to water.
Synapse	A junction where neurotransmitter passes between cells.

Section 8 of 9Key terms

Term	Meaning
Negative feedback	Process that reverses a change to return a condition to its norm.
Glycolysis	Splitting of glucose to pyruvate in the cytoplasm.
Habituation	Loss of response to a repeated, harmless stimulus.
Second messenger	Intracellular molecule that relays a signal from a surface receptor.

Section 9 of 9Exam technique

Locating glycolysis in the mitochondrion — it occurs in the cytoplasm.

Stating that oxygen is used in the Krebs cycle directly — oxygen acts at the end of the electron transport chain.

Writing that peptide hormones enter the cell — they bind surface receptors.

Confusing vasoconstriction and vasodilation.

Last updated: 18 October 2026