Welcome to Coordination, Response, and Gene Technology!

Hello future biologists! This chapter is one of the most exciting, covering how living things react to their environment (Coordination and Response) and how we, as scientists, are learning to manipulate life itself (Gene Technology). Don't worry if these topics seem complex; we will break them down into easy, bite-sized pieces. Understanding these processes is crucial because they govern everything from how you catch a ball to how we can cure genetic diseases. Let's get started!

Part 1: Neuronal Coordination and Action Potentials

1.1 The Basics of Neurons and Resting Potential

The nervous system uses specialised cells called neurons (nerve cells) to transmit information quickly. This transmission relies on electrical signals.

When a neuron is not transmitting an impulse, it maintains a small potential difference across its membrane called the resting potential.

How the Resting Potential is Maintained:
  • The inside of the neuron is typically negative compared to the outside (usually around \(-70 \text{ mV}\)).
  • This negativity is primarily established by the Sodium-Potassium Pump, which actively transports ions against their concentration gradients.
  • Mechanism: For every 3 sodium ions (\(\text{Na}^{+}\)) pumped out of the axon, 2 potassium ions (\(\text{K}^{+}\)) are pumped in.
  • The membrane is much more permeable to \(\text{K}^{+}\) than to \(\text{Na}^{+}\), meaning \(\text{K}^{+}\) leaks out easily, contributing to the negative charge inside.

Analogy: Think of the resting potential like a stretched rubber band. It has stored energy and is ready to spring into action when released.

1.2 Generating an Action Potential

An action potential is a rapid, temporary change in the membrane potential, moving from negative (resting) to positive, and back to negative. This is the electrical signal that travels down the axon.

The All-or-Nothing Principle

An action potential only occurs if the stimulus is strong enough to reach the threshold potential (usually about \(-55 \text{ mV}\)). If it reaches the threshold, an action potential of the same magnitude will fire. If it doesn't reach the threshold, nothing happens. It’s like pulling the trigger on a gun—you must pull hard enough for it to fire.

Step-by-Step Action Potential Process:
  1. Resting State: Voltage-gated \(\text{Na}^{+}\) and \(\text{K}^{+}\) channels are closed. Resting potential is maintained by the pump (\(-70 \text{ mV}\)).
  2. Depolarisation (Rising Phase): If the threshold is reached, voltage-gated \(\text{Na}^{+}\) channels snap open. \(\text{Na}^{+}\) ions rush into the cell (down their concentration gradient). The inside becomes positive (rising to about \(\text{+40 mV}\)).
  3. Repolarisation (Falling Phase): The voltage-gated \(\text{Na}^{+}\) channels close quickly, and the voltage-gated \(\text{K}^{+}\) channels open slowly. \(\text{K}^{+}\) ions rush out of the cell, making the inside negative again.
  4. Hyperpolarisation (Refractory Period): Too many \(\text{K}^{+}\) ions leave before the channels close, making the potential briefly more negative than the resting potential (e.g., \(-80 \text{ mV}\)). This is the refractory period, ensuring the impulse only travels in one direction.
  5. Return to Resting: The Sodium-Potassium pump works to redistribute the ions and return the membrane to its stable resting potential.

Quick Review:
Depolarisation = \(\text{Na}^{+}\) In
Repolarisation = \(\text{K}^{+}\) Out

1.3 Transmission Along the Axon

Action potentials are propagated (spread) along the axon as a wave of depolarisation.

Factors Affecting Speed of Transmission:
  • Myelin Sheath: Acts as an electrical insulator, produced by Schwann cells.
  • Saltatory Conduction: In myelinated axons, depolarisation only occurs at the gaps in the myelin sheath, called the Nodes of Ranvier. The impulse effectively "jumps" from node to node, increasing speed significantly (up to 50x faster!).
  • Axon Diameter: A wider axon has less internal resistance to the flow of ions, resulting in faster transmission.

Key Takeaway (Part 1): The nervous signal is a temporary electrical switch (the action potential) generated by the controlled movement of sodium and potassium ions across the neuron membrane.

Part 2: Synaptic Transmission and Receptors

2.1 Synaptic Transmission

Nerve impulses must cross a tiny gap between neurons called the synapse. Since the electrical signal cannot jump this gap, a chemical message is used—a neurotransmitter.

The classic example is the cholinergic synapse, which uses the neurotransmitter acetylcholine (ACh).

Step-by-Step Synaptic Transmission:
  1. The action potential arrives at the presynaptic terminal (the end of the first neuron).
  2. The depolarisation causes voltage-gated calcium ion (\(\text{Ca}^{2+}\)) channels to open. \(\text{Ca}^{2+}\) rushes into the presynaptic knob.
  3. The influx of \(\text{Ca}^{2+}\) causes vesicles containing acetylcholine (ACh) to fuse with the presynaptic membrane (exocytosis).
  4. ACh diffuses across the synaptic cleft.
  5. ACh binds to complementary receptor proteins on the postsynaptic membrane (the start of the second neuron).
  6. This binding causes ligand-gated ion channels (often \(\text{Na}^{+}\) channels) to open, allowing ions to flow in and generate a postsynaptic potential (a new wave of depolarisation). If this reaches the threshold, a new action potential is fired.
  7. ACh is broken down immediately by the enzyme acetylcholinesterase (AChE) to prevent continuous stimulation. The breakdown products are recycled back into the presynaptic knob.

Common Mistake Alert: Students often forget the vital role of \(\text{Ca}^{2+}\) in triggering neurotransmitter release. Remember, Calcium controls the vesicles!

2.2 Synaptic Integration: Excitation and Inhibition

Synapses don't just pass on signals; they filter and modify them.

  • Excitatory Synapse: Causes depolarisation in the postsynaptic neuron, making it more likely to fire an action potential. (e.g., ACh often has an excitatory effect).
  • Inhibitory Synapse: Causes hyperpolarisation (more negative) in the postsynaptic neuron, making it less likely to fire an action potential.

The postsynaptic neuron sums up all the excitatory and inhibitory signals it receives—this is called summation.

  • Temporal Summation: A single presynaptic neuron releases neurotransmitter repeatedly and rapidly.
  • Spatial Summation: Multiple different presynaptic neurons release neurotransmitter simultaneously onto the same postsynaptic neuron.

2.3 Response: The Pacinian Corpuscle

Before coordination begins, the body needs information from the environment. Receptors convert energy from a stimulus (like pressure or light) into an electrical signal (an action potential). This process is called transduction.

The Pacinian Corpuscle is a specialised receptor found deep in the skin, responsible for detecting pressure and vibration.

Mechanism of the Pacinian Corpuscle:
  1. Pressure deforms the connective tissue layers (lamellae) surrounding the nerve ending.
  2. The deformation stretches the membrane of the sensory neuron.
  3. This stretching opens specific stretch-mediated sodium channels.
  4. \(\text{Na}^{+}\) ions rush into the sensory neuron, causing a small depolarisation called the generator potential.
  5. If the pressure is strong enough, the generator potential reaches the threshold, triggering an action potential that travels towards the central nervous system.

Did You Know? Pacinian corpuscles are rapidly adapting. If you put on a watch, you feel it immediately, but the sensation quickly fades because the corpuscle stops firing action potentials unless the pressure changes.

Key Takeaway (Part 2): Synapses allow neurons to communicate using chemical signals (neurotransmitters). Receptors like the Pacinian corpuscle convert physical stimuli into electrical energy (transduction).



Part 3: Gene Technology (Recombinant DNA)

Now we shift gears completely. Gene technology uses biological methods to modify the genetic material of living organisms for practical purposes. This is known as genetic engineering.

3.1 The Tools of Genetic Engineering

To insert a desired gene into a recipient cell (often a bacterium), we need specific molecular tools:

a) Restriction Endonucleases (Molecular Scissors)
  • These enzymes cut DNA at specific base sequences, known as recognition sequences (or restriction sites).
  • If the cut is staggered, it leaves unpaired bases called sticky ends. These are crucial because they allow the cut gene and the cut vector DNA to easily join together using complementary base pairing.
b) DNA Ligase (Molecular Glue)

This enzyme rejoins the sugar-phosphate backbone, permanently sealing the desired gene into the carrier DNA (the vector).

c) Vectors

A vector is a DNA molecule used to carry the desired gene into the host cell. The most common vectors are plasmids (small, circular DNA found in bacteria) and bacteriophages (viruses that infect bacteria).

3.2 The Process of Creating Recombinant DNA

The goal is to create recombinant DNA—a molecule made by combining genetic material from two different sources.

Step-by-Step Production (e.g., Human Insulin):
  1. Isolation: The gene for human insulin is isolated.
  2. Cutting the Gene: The gene is cut out using a specific restriction endonuclease. This enzyme creates sticky ends.
  3. Cutting the Vector: The same restriction endonuclease is used to cut the plasmid vector. This ensures the sticky ends of the gene and the plasmid are complementary.
  4. Ligation (Joining): The isolated gene and the open plasmid are mixed together. Due to complementary sticky ends, they anneal (join). DNA Ligase then forms phosphodiester bonds, making the recombinant plasmid permanent.
  5. Transformation: The recombinant plasmid is inserted into a host cell (usually E. coli bacteria). This is called transformation.
  6. Cloning/Growth: The bacteria are grown in large fermenters. As the bacteria divide, they replicate the recombinant plasmid, creating millions of copies of the gene and producing large amounts of the desired protein (insulin).

3.3 Identifying Transformed Organisms (Markers)

Inserting the plasmid into the bacteria isn't 100% efficient. We need a way to distinguish:
A) Bacteria that took up the plasmid (Transformed).
B) Bacteria that did not take up the plasmid (Non-Transformed).
C) Bacteria that took up the plasmid, but it failed to incorporate the desired gene (Reformed, non-recombinant).

To do this, plasmids are engineered to contain marker genes, often genes for antibiotic resistance (e.g., ampicillin resistance).

  • If the bacteria grow on a medium containing the antibiotic, we know they must have taken up the plasmid (because they survived the antibiotic).

3.4 Ethical and Social Implications

Genetic engineering offers huge benefits (e.g., insulin production, disease cure), but raises serious ethical questions that A-Level students must consider.

Benefits:
  • Mass production of vital substances (e.g., drugs, hormones) quickly and cheaply.
  • Potential to cure genetic diseases (e.g., gene therapy).
  • Improved crop yields (e.g., resistance to pests).
Concerns and Ethical Issues:
  • Safety: The possibility of transferring antibiotic resistance to harmful bacteria.
  • Eugenics: Concerns that genetic technology could be used to create 'designer babies' or enforce specific genetic traits.
  • Manipulation: Some people have moral objections to manipulating the natural processes of life, often citing religious or philosophical reasons.
  • Ecology: Potential unforeseen long-term effects of releasing genetically modified organisms (GMOs) into the wild ecosystem.

Key Takeaway (Part 3): Recombinant DNA technology relies on enzymes (Restriction Endonucleases and Ligase) to cut and paste genes into vectors (like plasmids), allowing host cells to express useful foreign proteins.

You've successfully covered Coordination, Response, and the fundamentals of Gene Technology. Keep practicing those step-by-step processes—they are key exam targets! Good luck!