Coordination, Response and Gene Technology

Q: What is the difference between depolarisation and repolarisation in an action potential?

Depolarisation occurs when sodium ion () channels open, allowing to move into the neurone, raising the membrane potential from -70mV. Repolarisation happens when sodium channels close and potassium ion () channels open, allowing to move out of the neurone, returning the membrane potential towards resting levels.

Edexcel International A-Level Biology· Unit 5 WBI15, statements 8.1-8.22· 45 min read

1. Mammalian Nervous System Structure and Function★★★☆☆⏱ 10 min

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The mammalian nervous system is split into the central nervous system (CNS: brain + spinal cord) and peripheral nervous system (nerves connecting CNS to effectors). It enables fast, specific responses to stimuli via electrical impulses transmitted through neurones.

📘 Definition

Neurone

Specialised cell that transmits electrical impulses; three main types: sensory (carry impulses from receptors to CNS), relay (connect sensory and motor neurones within CNS), motor (carry impulses from CNS to effectors: muscles/glands). Many neurones are myelinated, wrapped in Schwann cell layers that insulate the axon, leaving gaps called nodes of Ranvier.

📐 Worked Example

Describe the sequence of events in an action potential, including ion movements.

1
1. Resting potential: neurone membrane is polarised at -70mV, maintained by $N a^{+} / K^{+}$ pump (3 $N a^{+}$ pumped out, 2 $K^{+}$ pumped in).
2
1. Depolarisation: stimulus opens $N a^{+}$ channels, $N a^{+}$ diffuses into neurone, membrane potential rises to +40mV if threshold is reached.
3
1. Repolarisation: $N a^{+}$ channels close, $K^{+}$ channels open, $K^{+}$ diffuses out of neurone, membrane potential falls back towards resting level.
4
1. Hyperpolarisation: $K^{+}$ channels close slowly, so membrane potential temporarily falls below -70mV before returning to resting potential during the refractory period, during which another action potential cannot be fired.

Synapses are junctions between two neurones, or a neurone and effector. Transmission across synapses is unidirectional, as neurotransmitters (e.g. acetylcholine) are only released from the presynaptic neurone.

📐 Worked Example

Outline the sequence of events at a cholinergic synapse when an action potential arrives at the presynaptic knob.

1
1. Action potential depolarises presynaptic membrane, opening $C a^{2 +}$ channels, allowing $C a^{2 +}$ to diffuse into the presynaptic knob.
2
1. $C a^{2 +}$ influx causes vesicles containing acetylcholine to fuse with the presynaptic membrane, releasing neurotransmitter into the synaptic cleft via exocytosis.
3
1. Acetylcholine binds to complementary receptors on the postsynaptic membrane, opening $N a^{+}$ channels, triggering depolarisation of the postsynaptic membrane and a new action potential if threshold is reached.
4
1. Acetylcholinesterase breaks down acetylcholine in the synaptic cleft to prevent continuous stimulation, and the products are reabsorbed into the presynaptic knob to resynthesize acetylcholine.

Exam tip:

Remember synapse transmission order: $C a^{2 +}$ influx first, then vesicle fusion, then neurotransmitter release, then receptor binding. This sequence is a common multiple choice question.

2. Sensory Responses and Coordination★★☆☆☆⏱ 8 min

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Sensory receptors convert stimuli into electrical impulses that are transmitted to the CNS for processing, leading to appropriate responses. Key sensory structures include rod cells in the retina of the eye, which detect low intensity light.

📘 Definition

Spinal Reflex Arc

Rapid, involuntary, innate response to a stimulus that does not involve conscious processing in the brain, reducing reaction time to avoid damage. Structure: receptor → sensory neurone → relay neurone in spinal cord grey matter → motor neurone → effector. White matter in the spinal cord contains myelinated axons carrying impulses to and from the brain.

📐 Worked Example

Explain how light stimulation of a rod cell leads to an action potential in the optic nerve.

1
1. In the dark, rod cells are depolarised, as cation channels are open, allowing constant $N a^{+}$ influx, so they release inhibitory neurotransmitter that prevents action potentials in the optic nerve neurones.
2
1. When light hits the rod cell, the light-sensitive pigment rhodopsin (made of opsin and retinal) bleaches, changing shape.
3
1. This causes cation channels in the rod cell membrane to close, stopping $N a^{+}$ influx, so the rod cell hyperpolarises and stops releasing inhibitory neurotransmitter.
4
1. The absence of inhibitory neurotransmitter allows an action potential to be generated in the optic nerve neurone, which is transmitted to the brain for processing.

A range of drugs interfere with synapse function: nicotine mimics acetylcholine binding to postsynaptic receptors, lidocaine blocks $N a^{+}$ channels preventing action potentials, cobra alpha toxin blocks acetylcholine receptors preventing postsynaptic depolarisation, L-DOPA is converted to dopamine to treat Parkinson's disease, MDMA increases serotonin levels in synapses to affect mood.

✓ Quick check

Which of the following drugs blocks acetylcholine receptors at synapses?
- Nicotine
- Lidocaine
- Cobra alpha toxin
- MDMA
Reveal answer
Cobra alpha toxin —
Cobra alpha toxin binds to acetylcholine receptors, preventing acetylcholine from binding, so no postsynaptic action potential is generated, leading to paralysis.

Habituation is a learned response where an organism stops responding to a repeated harmless stimulus, as reduced $C a^{2 +}$ influx at presynaptic membranes leads to less neurotransmitter release, so no action potential is triggered in postsynaptic neurones.

3. Plant Responses to the Environment★★☆☆☆⏱ 8 min

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Plants coordinate responses to external stimuli (light, gravity, water) using plant growth regulators (hormones), as they do not have a nervous system. Key hormones include auxin (IAA) and gibberellins, plus the light-sensitive pigment phytochrome that detects red and far-red light to control germination and flowering.

📘 Definition

Gibberellin

Plant hormone that regulates stem elongation, seed germination and flowering. In germinating cereal grains, gibberellin is released by the embryo, stimulating the aleurone layer to produce amylase enzyme, which breaks down starch in the endosperm into maltose for the growing embryo to use as an energy source.

📐 Worked Example

Outline the method for Core Practical 18: investigating the effect of gibberellin on amylase production in germinating cereal grains.

1
1. Sterilise seeds to kill any microorganisms on the surface that could produce amylase and affect results.
2
1. Cut seeds in half to separate the embryo-containing half from the endosperm-only half, to ensure any gibberellin produced by the embryo does not affect results.
3
1. Incubate endosperm halves in different concentrations of gibberellin solution for a set time period, alongside a control group incubated in water only.
4
1. Place endosperm halves on agar plates containing starch, incubate for 24 hours, then flood plates with iodine solution. Measure the diameter of the clear zone around each endosperm half (clear zone = starch broken down by amylase).
5
1. Repeat with multiple samples for each concentration, calculate mean clear zone diameter, and compare results across gibberellin concentrations and the control group.

Auxin (IAA) controls phototropic responses in shoots: it is produced in the apical tip of the shoot, diffuses down the shaded side of the shoot when exposed to unidirectional light, causing cells on the shaded side to elongate more than cells on the light side, so the shoot bends towards the light source.

Exam tip:

When answering questions on Core Practical 18, always mention sterilising seeds and removing the embryo as control variables, as these are standard mark scheme points.

4. Brain Structure and Medical Imaging★★☆☆☆⏱ 7 min

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The mammalian brain has distinct regions with specialised functions, and chemical imbalances in brain neurotransmitters can lead to neurological and psychiatric conditions, which are treated with targeted drugs. Medical imaging techniques are used to diagnose brain abnormalities and study brain function.

📘 Definition

Key Brain Regions

Cerebral hemispheres: higher functions (conscious thought, memory, language, voluntary movement). 2. Hypothalamus: homeostatic control (temperature regulation, osmoregulation, hormone production for pituitary gland). 3. Pituitary gland: releases hormones regulating growth, reproduction and metabolism. 4. Cerebellum: coordination of movement and balance. 5. Medulla oblongata: controls autonomic functions (heart rate, breathing rate, peristalsis).

Example:

Damage to the cerebellum leads to uncoordinated movement and loss of balance.

Imaging Technique	Use Case
MRI	Produces high resolution 3D images of soft tissue, used to diagnose brain tumours, stroke damage and structural abnormalities
fMRI	Measures blood flow/oxygenation in the brain to identify active brain regions during specific tasks, used to study brain function and map regions before surgery
PET	Uses radioactive tracers to measure metabolic activity in brain tissue, used to diagnose Alzheimer's disease and locate cancerous tumours
CT	Uses X-rays to produce cross-sectional images of the brain, used for quick diagnosis of bleeding in the brain after head injury

Dopamine deficiency in the brain causes Parkinson's disease, leading to tremors and movement difficulties, treated with L-DOPA which is converted to dopamine in the brain. Low serotonin levels are linked to depression, treated with SSRI drugs that prevent reuptake of serotonin in synapses, increasing serotonin levels.

✓ Quick check

Which brain region is responsible for controlling breathing rate?
- Cerebellum
- Medulla oblongata
- Hypothalamus
- Cerebral hemisphere
Reveal answer
Medulla oblongata —
The medulla oblongata controls autonomic, involuntary functions including breathing rate, heart rate and peristalsis in the digestive system.

5. Gene Technology and Genetically Modified Organisms★★★☆☆⏱ 10 min

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Gene technology involves modifying the genome of organisms to produce desired traits, including producing recombinant proteins for medical use, creating crop plants resistant to pests or drought, and studying gene function.

📘 Definition

Recombinant DNA Technology

Process of combining DNA from two different organisms to produce transgenic (genetically modified) organisms. Key tools: restriction endonucleases (cut DNA at specific sequences to produce sticky ends), DNA ligase (seals gaps in DNA backbone to join target DNA to vector DNA), vectors (usually plasmids, used to carry target DNA into host cells).

📐 Worked Example

Describe the steps to produce a genetically modified bacterium that produces human insulin.

1
1. Isolate the human insulin gene from human pancreatic cell DNA, or synthesise the gene from mRNA using reverse transcriptase.
2
1. Cut the insulin gene and a bacterial plasmid using the same restriction endonuclease, so they have complementary sticky ends.
3
1. Mix the cut insulin gene and cut plasmid together, add DNA ligase enzyme which seals the sugar-phosphate backbone of the DNA, joining the insulin gene to the plasmid to form recombinant DNA.
4
1. Insert the recombinant plasmid into a host bacterial cell, using techniques such as heat shock to make the bacterial membrane permeable to the plasmid.
5
1. Grow the transgenic bacteria in large fermenters, where they express the insulin gene and produce human insulin, which is then extracted, purified and used to treat diabetes.

DNA microarrays are used to identify active genes in cells: mRNA is extracted from cells, converted to fluorescently labelled cDNA, which binds to complementary gene probes on the microarray. Fluorescent spots on the microarray indicate which genes are being actively transcribed in the cell. Bioinformatics uses computer software to analyse and store large biological datasets, including DNA sequences, protein structures and gene expression data.

Benefits of GMOs include: production of medical drugs (insulin, human growth hormone) cheaply and in large quantities, crop plants with higher yield, pest resistance or higher nutritional value, reduced use of pesticides. Risks include: potential transfer of antibiotic resistance genes to bacteria in the gut, potential cross-pollination of GMO crops with wild plants leading to herbicide resistant weeds, unknown long term health effects of consuming GMO food, corporate control of seed supplies.

6. Common Pitfalls

Wrong move:

Stating that rod cells depolarise when exposed to light

Why:

Rod cells are depolarised in the dark, and hyperpolarise when exposed to light as cation channels close, stopping Na+ influx. This is a common mark scheme discriminator and incorrect answers get zero marks.

Correct move:

Always state that rod cells hyperpolarise in response to light, leading to cessation of inhibitory neurotransmitter release and generation of an action potential in the optic nerve.

Wrong move:

Describing saltatory conduction as impulses moving through the myelin sheath

Why:

Myelin is an insulator, so impulses cannot pass through it. Saltatory conduction involves impulses jumping between the nodes of Ranvier, the gaps in the myelin sheath, which speeds up transmission significantly.

Correct move:

Define saltatory conduction as the propagation of action potentials along myelinated axons by jumping from one node of Ranvier to the next, increasing conduction speed.

Wrong move:

Forgetting to mention that the same restriction endonuclease is used to cut both target DNA and plasmid DNA in recombinant DNA technology

Why:

Using the same restriction enzyme produces complementary sticky ends on both the target gene and plasmid, allowing them to anneal together before DNA ligase seals the backbone. This is a required mark point in most gene technology questions.

Correct move:

Explicitly state that the same restriction endonuclease is used to cut both the target gene and the vector plasmid, producing complementary sticky ends.

Wrong move:

Mixing up the functions of the cerebellum and medulla oblongata

Why:

Cerebellum controls balance and coordinated movement, while the medulla oblongata controls involuntary autonomic functions like breathing rate and heart rate. Incorrect matching of brain regions to functions loses easy marks in multiple choice and short answer questions.

Correct move:

Use the mnemonic 'C for Cerebellum = C for Coordination' and 'M for Medulla = M for Motor (autonomic)' to remember their functions.

Wrong move:

Including PCR or gel electrophoresis in descriptions of recombinant DNA technology for this topic

Why:

PCR and gel electrophoresis are out of scope for Unit 5 Topic 8, as they are covered in Unit 4 Topic 6, and mentioning them will not gain you marks, and may lead to you missing required mark points.

Correct move:

Stick to the required tools: restriction endonucleases, DNA ligase, plasmid vectors, when describing recombinant DNA technology for this topic.

7. Quick Reference Cheatsheet

Concept	Key Mark Scheme Points
Action Potential	Resting (-70mV, Na+/K+ pump: 3 Na+ out, 2 K+ in) → Depolarisation (Na+ in) → Repolarisation (K+ out) → Hyperpolarisation → Refractory period
Synapse Transmission	Ca2+ influx presynaptic → vesicle fusion → neurotransmitter release → binds postsynaptic receptors → acetylcholinesterase breaks down ACh
Rod Cell Response	Light → rhodopsin bleaches → cation channels close → Na+ influx stops → rod hyperpolarises → no inhibitory neurotransmitter → action potential in optic nerve
Core Practical 18	Sterilise seeds → remove embryo → incubate endosperms in gibberellin concentrations → starch agar + iodine → measure clear zone diameter
Brain Regions	Cerebrum: conscious thought, Hypothalamus: homeostasis, Pituitary: hormone release, Cerebellum: balance/coordination, Medulla: autonomic functions
Recombinant DNA	Same restriction enzyme cuts target + plasmid → complementary sticky ends → DNA ligase joins → recombinant plasmid inserted into host cell

8. Frequently Asked

What is the difference between depolarisation and repolarisation in an action potential?

Depolarisation occurs when sodium ion ( $N a^{+}$ ) channels open, allowing $N a^{+}$ to move into the neurone, raising the membrane potential from -70mV. Repolarisation happens when sodium channels close and potassium ion ( $K^{+}$ ) channels open, allowing $K^{+}$ to move out of the neurone, returning the membrane potential towards resting levels.

How do restriction endonucleases work in recombinant DNA technology?

Restriction endonucleases are enzymes that cut DNA at specific palindromic recognition sequences, producing either blunt or sticky ends. Complementary sticky ends allow target DNA to be annealed to a plasmid vector before DNA ligase seals the phosphodiester backbone to form recombinant DNA.

What is the function of rhodopsin in rod cells?

Rhodopsin is a light-sensitive pigment in rod cells made of opsin and retinal. When exposed to light, rhodopsin bleaches, closing cation channels in the rod cell membrane, leading to hyperpolarisation of the cell, which triggers an action potential to the brain via the optic nerve.

Going deeper

past_paperEdexcel IAL Biology WBI15 Past Papers 2018-2025Official past papers for Unit 5 practice
core_practical_guideEdexcel IAL Biology Core Practical 18 Guide

What's Next

Now that you have mastered the content for Coordination, Response and Gene Technology, move on to practising exam-style questions for this topic to reinforce your knowledge and identify gaps. Pay close attention to mark scheme wording, as many questions require specific terminology to gain full marks. Review all Unit 5 core practicals and work through WBI15 past papers to familiarise yourself with the exam format, ensuring you do not mix up content from Unit 4 and Unit 5 when answering questions, as mark schemes strictly follow specification boundaries.