the stimulus-response model and the role of signalling molecules, receptors and signal transduction in coordinating cellular responses, including the role of apoptosis as a regulated cellular response

What this dot point is asking

VCAA wants the stimulus-response model applied to cells, the role of signalling molecules and receptors (with the hydrophilic vs hydrophobic split), the meaning of signal transduction, and the role of apoptosis as a regulated cellular response (compared with necrosis), including examples in development and disease.

The answer

Cells coordinate behaviour through chemical signals. A signalling cell releases a signalling molecule, which binds a receptor on a target cell and triggers a response. This sequence is an application of the stimulus-response model at the cellular level.

The stimulus-response model in cells

1. Stimulus. A change in the internal or external environment of the organism (for example, blood glucose rising, an immune trigger, a developmental cue).
2. Reception. The signalling molecule binds a specific receptor on or inside the target cell.
3. Transduction. The binding event is converted, often through a cascade, into changes inside the cell.
4. Response. The cell changes its behaviour: opens or closes channels, activates or inhibits enzymes, alters gene expression, divides, or undergoes apoptosis.

Signalling molecules

Signalling molecules include hormones (insulin, adrenaline, oestrogen, testosterone), neurotransmitters (acetylcholine, dopamine), cytokines (in immune responses) and growth factors. They fall into two broad classes based on solubility.

Hydrophilic (water-soluble) signalling molecules. Examples: peptide and protein hormones (insulin, glucagon, adrenaline), most neurotransmitters. They cannot cross the phospholipid bilayer, so they bind surface receptors on the plasma membrane.

Hydrophobic (lipid-soluble) signalling molecules. Examples: steroid hormones (oestrogen, testosterone, cortisol), thyroid hormone, vitamin D. They diffuse straight through the membrane and bind intracellular receptors in the cytosol or nucleus. The activated receptor-hormone complex usually acts as a transcription factor, changing which genes are expressed.

Receptors

A receptor is a protein with a binding site specific to one signalling molecule (or a small family of related molecules). Binding is reversible and complementary in shape and chemistry, like an enzyme-substrate fit.

• Surface receptors sit in the plasma membrane and bind hydrophilic signals on the outside. Major types include G-protein coupled receptors, receptor tyrosine kinases, and ligand-gated ion channels.
• Intracellular receptors sit in the cytosol or nucleus and bind hydrophobic signals that have crossed the membrane.

Target specificity arises because only cells expressing the matching receptor can respond.

Signal transduction

Signal transduction is the chain of events between receptor binding and the cellular response. A surface receptor cannot directly change cytoplasmic enzymes or gene expression, so it triggers a cascade of intracellular messengers.

Typical features include:

• Conformational change in the receptor on binding.
• Second messengers such as cyclic AMP (cAMP), Ca2+ or inositol triphosphate (IP3) that diffuse inside the cell.
• Protein kinases that phosphorylate downstream proteins, activating or inactivating them.
• Amplification. One signalling molecule can trigger thousands of intracellular events because each step activates many molecules of the next.
• Termination. Phosphatases and second-messenger breakdown switch the signal off.

For hydrophobic signals, transduction is shorter: the receptor-hormone complex binds DNA directly and changes transcription.

Apoptosis: regulated cell death

Apoptosis is a controlled, programmed sequence of events that dismantles a cell from within. It is triggered when receptors detect signals such as DNA damage, viral infection, withdrawal of growth factors, or developmental cues.

Process.

1. Internal or external signals activate initiator caspases (proteases).
2. Initiator caspases activate executioner caspases.
3. Executioner caspases cleave key cellular targets: the cytoskeleton collapses, the nuclear lamina breaks down, endonucleases fragment DNA into regular pieces.
4. The cell shrinks and the plasma membrane forms blebs.
5. The cell breaks into apoptotic bodies, sealed membrane-bound packages.
6. Macrophages engulf the apoptotic bodies. Cell contents are not released, so there is no inflammation.

Apoptosis vs necrosis.

Role in development. Apoptosis sculpts tissues during embryonic development. The webbing between fingers and toes in the human embryo is removed by apoptosis. Tadpole tails resorb in the same way. Excess neurons that fail to make functional connections are pruned during brain development.

Role in disease and homeostasis. Apoptosis removes:

• Cells with irreparable DNA damage (preventing cancer).
• Virus-infected cells (limiting spread).
• Self-reactive immune cells (preventing autoimmunity).

When apoptosis fails, damaged cells survive and can become cancerous. When apoptosis is excessive or misfired, healthy cells are lost, contributing to neurodegenerative diseases such as Alzheimer's and Parkinson's, and to tissue damage in heart attack and stroke.

Worked example

Blood glucose rises after a meal. Beta cells in the pancreas (the signalling cell) release insulin (a hydrophilic peptide hormone). Insulin binds receptor tyrosine kinases on the surface of muscle and liver cells (reception). The receptor phosphorylates downstream proteins (signal transduction), triggering the insertion of GLUT4 glucose transporters into the plasma membrane. The target cells then take up glucose from the blood (response), restoring blood glucose to normal. The signal is terminated by insulin breakdown and dephosphorylation. Cells that do not express insulin receptors do not respond, demonstrating target specificity.

Common traps

Saying steroid hormones bind surface receptors. Steroids are hydrophobic and pass through the membrane. They bind intracellular receptors.

Forgetting transduction. Reception and response are not enough; the transduction cascade is what amplifies and shapes the response. Markers want all four steps of the stimulus-response sequence.

Calling apoptosis "the cell exploding". That is necrosis. Apoptosis is orderly: the cell shrinks, packages itself into apoptotic bodies, and is engulfed without releasing contents.

Saying apoptosis is always bad. Apoptosis is essential. Most problems come from too little (cancer) or too much (neurodegeneration) apoptosis, not from apoptosis itself.

Mixing up necrosis with apoptosis on inflammation. Necrosis causes inflammation; apoptosis does not.

In one sentence

Cell signalling works through a stimulus-response sequence in which signalling molecules bind specific surface receptors (if hydrophilic) or intracellular receptors (if hydrophobic), trigger a transduction cascade and produce a response such as changed gene expression, enzyme activity or programmed cell death by apoptosis, the regulated dismantling of a cell that shapes development and protects against disease.