How do cells detect and respond to signals from other cells?
Communication and signalling: extracellular signalling molecules and receptors, hydrophobic and hydrophilic signals, intracellular and transmembrane receptors, G-protein-coupled receptors and second messengers, phosphorylation cascades and signal amplification.
An SQA Advanced Higher Biology answer on communication and signalling, covering extracellular signals and specific receptors, hydrophobic signals binding intracellular receptors, hydrophilic signals binding transmembrane receptors, G-protein-coupled receptors and second messengers, phosphorylation cascades, and how signals are amplified.
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What this key area is asking
The SQA wants you to explain how cells signal to one another: how a signalling molecule is detected by a specific receptor, why hydrophobic and hydrophilic signals bind different receptors, how G-protein-coupled receptors produce second messengers, and how phosphorylation cascades amplify a signal into a large cellular response.
Signalling molecules and receptors
Hydrophobic versus hydrophilic signals
G-protein-coupled receptors and second messengers
The second messenger spreads the signal through the cytoplasm and amplifies it, because one activated effector enzyme makes many second messenger molecules. The second messenger then activates downstream proteins, often kinases.
Phosphorylation cascades and amplification
Examples in context
Example 1. Adrenaline and glucose release. Adrenaline is hydrophilic and binds a G-protein-coupled receptor on liver cells, raising cAMP, which triggers a phosphorylation cascade that activates the enzymes breaking down glycogen. A small amount of hormone rapidly mobilises a large amount of glucose, showing amplification in action.
Example 2. A steroid changing gene expression. Oestrogen is hydrophobic, crosses the membrane and binds an intracellular receptor. The complex enters the nucleus and acts as a transcription factor, switching on target genes. The example contrasts the slower, gene-level action of a hydrophobic signal with the fast cascade of a hydrophilic one.
Try this
Q1. State where the receptor for a hydrophobic signalling molecule is found. [1 mark]
- Cue. Inside the cell, in the cytoplasm or nucleus (an intracellular receptor).
Q2. Explain the role of a second messenger in cell signalling. [2 marks]
- Cue. It spreads and amplifies the signal inside the cell after the surface receptor is activated.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA AH style4 marksExplain why a hydrophobic signalling molecule and a hydrophilic signalling molecule bind different types of receptor.Show worked answer →
A 4-mark answer needs the property of each signal and where its receptor is.
A hydrophobic signalling molecule, such as a steroid hormone, can dissolve through the phospholipid bilayer and enter the cell. It therefore binds an intracellular receptor in the cytoplasm or nucleus, and the hormone-receptor complex often acts as a transcription factor that changes gene expression.
A hydrophilic signalling molecule, such as a peptide hormone, cannot cross the hydrophobic bilayer. It therefore binds a transmembrane receptor on the cell surface, which passes the signal into the cell without the molecule itself entering.
Markers reward (1) hydrophobic signals cross the membrane, (2) and bind intracellular receptors affecting transcription, (3) hydrophilic signals cannot cross, and (4) so bind transmembrane surface receptors.
SQA AH style3 marksExplain how a phosphorylation cascade amplifies a signal inside a cell.Show worked answer →
A 3-mark answer needs the cascade, the multiplication and the outcome.
In a phosphorylation cascade, one activated kinase phosphorylates and activates many molecules of the next kinase, each of which activates many more of the following kinase. The signal is multiplied at every step.
Because each enzyme acts on many targets, a small number of signal molecules at the surface leads to a very large intracellular response, such as the activation of many transcription factors or metabolic enzymes.
Markers reward (1) each kinase activates many of the next, (2) the effect multiplies down the cascade, and (3) a small signal gives a large response.
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Sources & how we know this
- SQA Advanced Higher Biology Course Specification — SQA (2019)