Respiration, Internal Environment, Coordination and Gene Technology
Unit 5 is the second major A2 Biology unit. It connects cellular respiration with movement, cardiovascular and respiratory physiology, homeostasis, nervous coordination and modern genetic technology.
Topic 8 — Coordination, Response and Gene Technology
What you need to master
- Glycolysis
- Link reaction and Krebs cycle
- Oxidative phosphorylation
- Anaerobic respiration
- Skeletal muscle structure
- Sliding filament mechanism
- Cardiac muscle and cardiac output
- Gas exchange and exercise
- Homeostasis and negative feedback
- Kidney and osmoregulation
- Neurones and action potentials
- Synapses and neurotransmitters
- The mammalian eye
- Brain structure and imaging
- Gene technology
- Plant responses
Cellular Respiration
Respiration transfers chemical energy from organic molecules into ATP, which can then power cellular processes.
Energy is released and transferred to ATP.
Stages of aerobic respiration
Cytoplasm
Matrix
Matrix
Inner membrane
Glycolysis
Glycolysis takes place in the cytoplasm and involves the phosphorylation and subsequent splitting of glucose.
- Glucose is phosphorylated.
- A six-carbon intermediate is split into two three-carbon molecules.
- The three-carbon molecules are oxidised.
- Pyruvate is produced.
- Reduced NAD is produced.
- There is a net gain of ATP.
Reduced NAD = 2 molecules per glucose
Link reaction
Pyruvate enters the mitochondrial matrix and is converted into acetyl coenzyme A.
- Pyruvate is decarboxylated.
- Hydrogen is removed.
- NAD is reduced.
- The remaining two-carbon acetyl group combines with coenzyme A.
Krebs cycle
Acetyl CoA enters the Krebs cycle. The cycle produces carbon dioxide, reduced coenzymes and ATP.
CO₂
Released during decarboxylation.
Reduced NAD / FAD
Carry hydrogen/electrons to the electron transport chain.
ATP
Produced by substrate-level phosphorylation.
Oxidative phosphorylation
Reduced NAD and reduced FAD donate electrons to electron carriers in the inner mitochondrial membrane.
- Electrons pass along the electron transport chain.
- Energy released pumps protons across the inner mitochondrial membrane.
- A proton concentration gradient forms.
- Protons flow through ATP synthase.
- ATP is produced by chemiosmosis.
- Oxygen acts as the final electron acceptor.
- Water is formed.
Anaerobic Respiration
When oxygen availability is insufficient, cells cannot maintain aerobic respiration at the required rate.
In animal cells
Lactate can accumulate during intense exercise and is later transported and metabolised when oxygen availability increases.
In yeast
Respiratory quotient
Skeletal Muscle and Movement
Organisation
Sliding filament mechanism
Muscle contraction occurs when actin filaments slide over myosin filaments, shortening the sarcomere.
- A motor neurone stimulates the muscle fibre.
- Calcium ions are released.
- Calcium binds to troponin.
- Tropomyosin moves away from actin's binding sites.
- Myosin heads attach to actin.
- Myosin heads pivot, pulling actin.
- ATP allows the myosin head to detach and reset.
• detachment of myosin heads
• re-cocking of myosin heads
• active transport of calcium ions
Antagonistic muscle pairs
Flexor
Contracts to decrease the angle at a joint.
Extensor
Contracts to increase the angle at a joint.
Cardiac Muscle and Cardiac Output
Cardiac muscle
Cardiac muscle is specialised for continuous, rhythmic contraction and has a rich blood supply to support its high energy demand.
Myogenic contraction
The heart is myogenic: the contraction rhythm originates within the cardiac muscle itself.
Cardiac cycle
Cardiac output
A person has a heart rate of 75 beats min⁻¹ and a stroke volume of 80 cm³.
= 75 × 80
= 6000 cm³ min⁻¹
= 6.0 dm³ min⁻¹
The role of valves
Valves prevent the backflow of blood and ensure that blood flows through the heart in the correct direction.
Exercise and Gas Exchange
During exercise
- Muscle ATP demand increases.
- Rate of respiration increases.
- Oxygen demand increases.
- Carbon dioxide production increases.
- Heart rate increases.
- Ventilation increases.
Respiratory minute ventilation
Breathing rate = 20 breaths min⁻¹
Tidal volume = 2.5 dm³
= 20 × 2.5
= 50 dm³ min⁻¹
Oxygen debt
After intense exercise, oxygen consumption can remain elevated while the body restores normal physiological conditions and processes accumulated metabolic products.
Homeostasis and Negative Feedback
Negative feedback
Negative feedback reverses a change and returns a variable towards its set point or normal range.
Blood glucose regulation
High blood glucose
Pancreatic β cells release insulin. Cells increase glucose uptake and the liver converts glucose into glycogen.
Low blood glucose
Pancreatic α cells release glucagon. Glycogen is broken down and glucose is released into the blood.
Glucagon → raises blood glucose
Liver, Kidney and Internal Environment
Deamination
Excess amino acids cannot simply be stored. Their amino groups are removed in the liver.
Urea is transported in the blood to the kidneys for excretion.
Nephron structure
Glomerulus
Network of capillaries where ultrafiltration occurs.
Bowman's capsule
Collects the filtrate produced by ultrafiltration.
Loop of Henle
Important in creating the conditions needed for water reabsorption.
Ultrafiltration
High hydrostatic pressure forces small molecules from the blood through the filtration barrier into Bowman's capsule.
Selective reabsorption
Useful substances such as glucose and required ions are reabsorbed from the filtrate.
ADH
Antidiuretic hormone increases the permeability of collecting ducts to water, allowing more water to be reabsorbed.
Nervous Coordination
Central and peripheral nervous systems
Central nervous system
Brain and spinal cord.
Peripheral nervous system
Nerves connecting the CNS with receptors and effectors.
Neurones
Sensory neurone
Carries impulses from receptors towards the CNS.
Relay neurone
Connects neurones within the CNS.
Motor neurone
Carries impulses from the CNS towards effectors.
Resting potential
A neurone at rest maintains a potential difference across its membrane through ion movements and selective permeability.
Action potential
- A stimulus causes depolarisation if threshold is reached.
- Sodium ions enter the axon through voltage-gated channels.
- The membrane potential becomes positive.
- Sodium channels close and potassium channels open.
- Potassium ions leave the axon.
- The membrane repolarises.
Synaptic Transmission
Sequence of transmission
- Action potential reaches the presynaptic membrane.
- Calcium ion channels open.
- Calcium ions enter the synaptic knob.
- Vesicles fuse with the presynaptic membrane.
- Neurotransmitter is released by exocytosis.
- Neurotransmitter diffuses across the synaptic cleft.
- It binds to receptors on the postsynaptic membrane.
- Ion channels open and a response is generated.
Why are synapses important?
- Ensure one-way transmission.
- Allow integration of signals.
- Permit excitation or inhibition.
- Allow modification of nervous responses.
The Mammalian Eye
Cornea
Refracts incoming light strongly.
Iris
Controls pupil diameter.
Lens
Fine-focuses light onto the retina.
Retina
Contains photoreceptors.
Optic nerve
Carries impulses from the retina to the brain.
Ciliary muscles
Change lens shape during accommodation.
Accommodation
| Viewing | Ciliary muscles | Suspensory ligaments | Lens |
|---|---|---|---|
| Near object | Contract | Slack | More rounded |
| Distant object | Relax | Tight | Flatter |
Rods and cones
Rods
Highly sensitive to low light intensity and do not provide colour vision.
Cones
Function best in brighter conditions and provide colour vision.
The Brain
Cerebral hemispheres
Responsible for conscious processing, voluntary movement and higher cognitive functions.
Hypothalamus
Important in homeostasis and control of the endocrine system.
Pituitary gland
Releases hormones involved in coordination of many physiological processes.
Cerebellum
Important in coordination and balance.
Medulla oblongata
Controls important involuntary processes.
Brain imaging
MRI
Produces detailed images of brain structures using magnetic fields and radio waves.
fMRI
Can identify changes in brain activity associated with changes in blood flow.
PET
Uses radioactive tracers to investigate metabolic activity.
CT
Uses X-rays to produce cross-sectional images of the body.
Neurotransmitters and Ill Health
Dopamine
Dopamine is an important neurotransmitter involved in several brain pathways. Changes in dopamine signalling are associated with conditions including Parkinson's disease.
Serotonin
Serotonin is involved in mood and other physiological processes. Altered serotonin signalling has been associated with depression.
Understanding neurotransmitter pathways can help scientists identify potential targets for therapeutic drugs.
Recombinant DNA and Genetic Engineering
Restriction enzymes
Restriction enzymes recognise specific DNA sequences and cut DNA at or near those sequences.
DNA ligase
DNA ligase joins DNA fragments by forming phosphodiester bonds in the sugar-phosphate backbone.
Vectors
A vector can carry a gene into a host cell. Plasmids are commonly used as vectors in genetic engineering.
Recombinant DNA
PCR
Polymerase chain reaction is used to amplify a specific DNA sequence.
DNA Sequencing and Bioinformatics
DNA sequencing
DNA sequencing determines the order of bases in a DNA molecule.
Bioinformatics
Bioinformatics uses computational methods to store, analyse and compare biological data.
Sequence comparison
Compare DNA sequences between organisms or individuals.
Gene identification
Use sequence information to identify possible genes and coding regions.
Evolutionary analysis
Compare molecular sequences to investigate evolutionary relationships.
Sensitivity and Responses in Plants
Auxin
Auxin is a plant hormone involved in growth responses.
Phototropism
Shoots generally grow towards light.
- Light is detected.
- Auxin distribution becomes uneven.
- Cells on the shaded side elongate more.
- The shoot bends towards the light.
Gravitropism
Plants also respond directionally to gravity. Roots and shoots show different growth responses.
Unit 5 Practical Skills
Respiration investigation
Investigate respiration using suitable organisms or tissues and measure changes associated with respiration.
Exercise investigation
Investigate how exercise affects variables such as breathing rate, tidal volume, ventilation and oxygen consumption.
Muscle investigation
Interpret evidence relating muscle structure, contraction and force production.
Biological data
Analyse physiological datasets using appropriate calculations, graphs and statistical reasoning.
Independent variable • dependent variable • control variables • sample size • repeats • accuracy • precision • uncertainty • anomalies • conclusion • evaluation
Important Unit 5 Calculations
Cardiac output
Minute ventilation
Respiratory quotient
Percentage change
Unit awareness
Always check units before calculating.
- cm³ and dm³
- seconds and minutes
- concentrations
- rates
- percentages
Unit 5 Practice
Show answer
Cytoplasm.
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Oxygen acts as the final electron acceptor at the end of the electron transport chain. It combines with electrons and hydrogen ions to form water.
Show answer
Cardiac output = 80 × 75
= 6000 cm³ min⁻¹
= 6.0 dm³ min⁻¹
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ATP provides energy for the cycling of myosin heads and is also required for processes such as calcium ion transport.
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A change in an internal variable is detected, producing a response that opposes the original change and returns the variable towards its normal range.
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High pressure in the glomerulus forces small molecules from the blood through the filtration barrier into Bowman's capsule. Blood cells and large plasma proteins remain in the blood.
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An action potential causes calcium ions to enter the synaptic knob. Vesicles fuse with the presynaptic membrane and release neurotransmitter. The neurotransmitter diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane.
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DNA ligase joins DNA fragments by forming phosphodiester bonds in the sugar-phosphate backbone.
Show answer
Denaturation separates the DNA strands. During annealing, primers bind to complementary sequences. During extension, DNA polymerase adds nucleotides to form new DNA strands.
How to Approach Unit 5 Questions
For mechanisms, write the events in the correct order rather than listing disconnected facts.
For muscles, kidneys, neurones and eyes, explain why the structure allows the function.
For example:
"protons move down an electrochemical gradient
through ATP synthase"
is stronger than simply saying
"protons make ATP."
Write the equation, substitute values, calculate and give the correct unit.
Restriction enzyme → cuts DNA
DNA ligase → joins DNA
DNA polymerase → synthesises DNA
Primers → provide starting points for DNA synthesis