A level-readyAll subjects · Biology · WBI15 (Unit 5)
Adrenaline, hormones and gene switching
Hormones change what cells do, often by switching genes on or off. How a hormone acts depends on whether it can cross the cell membrane.
Adrenaline and fight or flight
When danger, stress or exercise is detected, the sympathetic nervous system stimulates the adrenal medulla to release adrenaline. It prepares the body for action (fight or flight):
- heart rate and stroke volume increase
- bronchioles dilate and breathing becomes faster and deeper
- blood is diverted to muscles (vasodilation) and away from the gut and skin (vasoconstriction)
- glycogen is converted to glucose in the liver, raising blood glucose
- pupils dilate
Transcription factors
Genes are switched on or off by transcription factors: proteins that move into the nucleus and bind to specific DNA sequences, such as the promoter region of a gene.
- An activator helps RNA polymerase bind, so the gene is transcribed (switched on).
- A repressor stops RNA polymerase binding, so the gene is not transcribed (switched off).
This is how cells with the same genes become specialised and how they respond to signals.
Peptide and steroid hormones
- Peptide hormones (such as insulin, glucagon and ADH) are water-soluble and cannot cross the phospholipid bilayer. They bind to receptors on the cell surface membrane. This activates an enzyme (for example adenylyl cyclase) that makes a second messenger such as cyclic AMP inside the cell, which activates a cascade of enzymes (protein kinases). These can change enzyme activity or activate transcription factors. Adrenaline, although not a peptide, acts in the same way.
- Steroid hormones (such as oestrogen and testosterone) are lipid-soluble, so they diffuse through the cell membrane. They bind to a receptor in the cytoplasm or nucleus, forming a hormone-receptor complex that acts as a transcription factor, binding to DNA and switching genes on or off.
Worked example
Explain why oestrogen can act directly as part of a transcription factor but insulin cannot.
- Oestrogen is a lipid-soluble steroid, so it diffuses through the membrane and binds a receptor inside the cell; the complex binds DNA.
- Insulin is a water-soluble peptide, so it cannot cross the membrane; it binds a surface receptor and acts through second messengers.
Answer: Only the steroid hormone enters the cell and binds DNA as a hormone-receptor complex.
Check yourself
- Which effect does adrenaline have?
- Slows the heart and stores glucose
- Constricts the bronchioles
- Increases heart rate and converts glycogen to glucose
- Increases blood flow to the gut
Show answer
Increases heart rate and converts glycogen to glucose. Adrenaline prepares the body for action.
- Which part of the adrenal gland releases adrenaline?
- Adrenal medulla
- Adrenal cortex
- Posterior pituitary
- Hypothalamus
Show answer
Adrenal medulla. The medulla (centre) releases adrenaline.
- What is a transcription factor?
- An enzyme that joins amino acids
- A hormone that is always a steroid
- A protein that binds to DNA to switch transcription of a gene on or off
- A type of mRNA
Show answer
A protein that binds to DNA to switch transcription of a gene on or off. Transcription factors control whether RNA polymerase can transcribe a gene.
- How does a steroid hormone enter its target cell?
- Through a protein channel only
- It diffuses through the phospholipid bilayer because it is lipid-soluble
- By active transport using ATP
- By binding a surface receptor and staying outside
Show answer
It diffuses through the phospholipid bilayer because it is lipid-soluble. Lipid-soluble molecules cross the bilayer.
- How do peptide hormones affect a target cell?
- They enter the nucleus and bind DNA directly
- They dissolve the membrane
- They are broken down to release energy
- They bind a surface receptor, triggering a second messenger such as cyclic AMP
Show answer
They bind a surface receptor, triggering a second messenger such as cyclic AMP. Water-soluble hormones signal from outside the cell.
- What does a repressor transcription factor do?
- Helps RNA polymerase bind
- Stops RNA polymerase binding, so the gene is not transcribed
- Translates mRNA
- Adds nucleotides to DNA
Show answer
Stops RNA polymerase binding, so the gene is not transcribed. Repressors switch genes off.
- Why does adrenaline cause vasoconstriction in the gut during fight or flight?
- To speed up digestion
- To raise skin temperature
- To lower blood pressure
- To divert more blood to the skeletal muscles
Show answer
To divert more blood to the skeletal muscles. Blood is redirected to where it is needed for action.
Sources
Last updated 7 November 2026. Spotted a mistake? Tell us.