Chemical control in mammalsEdexcel A-Level Biology B: Revision notes
Section 1
Hormones and endocrine glands
Endocrine glands are ductless. Their cells make hormones, which are chemical messengers, and secrete them directly into the blood in response to a stimulus. Hormones are carried in the blood plasma to all parts of the body, but they affect only target cells.
Examples of endocrine glands are the adrenal glands (adrenaline), the pancreas (insulin and glucagon) and the ovaries (oestrogen). Hormonal communication is slower than nervous communication, and the effects often last longer.
Section 2
Receptors and target cells
A target cell has receptors with a shape that is complementary to the hormone. When the hormone binds, it triggers a response. Cells without the receptor do not respond, even though the hormone reaches them.
There are two main modes of action, depending on whether the hormone can cross the cell surface membrane.
- Hormones that are not lipid-soluble, such as adrenaline, bind to receptors on the cell surface membrane and use a second messenger.
- Lipid-soluble hormones, such as oestrogen, enter the cell and bind to intracellular receptors.
Do not say that a hormone only travels to its target cells. It reaches all cells in the blood, but only target cells have the receptor.
Section 3
Adrenaline and the second messenger
Adrenaline is not lipid-soluble, so it cannot pass through the phospholipid bilayer. In a liver cell:
- Adrenaline (the first messenger) binds to a receptor on the cell surface membrane.
- This activates the enzyme adenylyl cyclase inside the membrane.
- Adenylyl cyclase converts ATP to cyclic AMP (cAMP), the second messenger.
- cAMP activates other enzymes (such as protein kinase), which activate the enzyme that breaks down glycogen.
- Glycogen is broken down to glucose, which is released into the blood.
The cascade amplifies the signal, as each enzyme activates many more, so a small amount of hormone has a large effect. The response is fast because the enzymes are already in the cell.
Section 4
Oestrogen and transcription factors
Oestrogen is a steroid, so it is lipid-soluble. It diffuses through the cell surface membrane and binds to a receptor in the cytoplasm or nucleus. The hormone-receptor complex acts as a transcription factor: it binds to a specific DNA sequence and stimulates (or inhibits) transcription of particular genes.
The mRNA is made and translated into new proteins, which change how the cell behaves. No second messenger is needed. Since new protein has to be made, the response is slower than for adrenaline but usually lasts longer.
In an answer about oestrogen, link each step: lipid-soluble, enters the cell, binds receptor, complex binds DNA, transcription, new protein.
Section 5
Comparing the two modes of action
- Adrenaline: not lipid-soluble, receptor on the cell surface, second messenger, activates existing enzymes, fast response.
- Oestrogen: lipid-soluble, intracellular receptor, no second messenger, acts on transcription, slower response.
Both work only on target cells with the complementary receptor.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Chemical control in mammals
- The adrenal glands, the thyroid gland and the pancreas are endocrine glands. A hormone released from an adrenal gland is carried in the blood to every tissue in the body, but only some tissues respond to it.Explain why adrenaline causes a response in liver cells but not in every cell it reaches.2 marks
- Adrenaline is secreted by the adrenal glands during a stress response. In a liver cell, adrenaline binds to a receptor on the cell surface membrane. This activates an enzyme, adenylyl cyclase, which converts ATP to cyclic AMP (cAMP). The cAMP then activates other enzymes in the cell, which break down glycogen to glucose.Explain how the use of a second messenger allows a small amount of adrenaline to cause a large response.2 marks
- Oestrogen is a steroid hormone secreted by the ovaries. It is lipid-soluble. It stimulates cells in the lining of the uterus to grow and to produce new proteins.Describe how oestrogen causes a target cell to produce a new protein.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).