OCR A2 Biology F214: Hormones

Terms

• Endocrine gland - A ductless gland that secretes hormones directly into the blood, via exocytosis.
• Exocrine gland - A gland that secretes substances into a duct which carries the molecules to a particular part of the body.
• Hormones - Molecules that act as chemical messengers. They are transported in blood plasma and lymph fluid, and cause a specific response in target cells.
• Target tissue/cells - Cells that contain specific receptors on their plasma membranes. The receptors are complementary in shape to the hormone molecule.

Steroid hormones

• Are lipid soluble and diffuse through the phospholipids plasma membrane, and bind to receptors in the nucleus.
• The hormone-receptor complex then binds to DNA, regulating transcription.
• e.g. Oestrogen, cortisol.

Protein hormones

• Bind to receptors in the plasma membrane of the target cell and bring about response without entering the cell.
• These are called first messengers.
• A second messenger is a molecule affected by the binding of a first messenger to a receptor.
• e.g. adrenaline, insulin.

Mode of action of adrenaline

• Adrenaline is a first messenger and is unable to enter cells.
• Target cells have complementary shaped receptor proteins.
• Binding of adrenaline activates the enzyme Adenyl Cyclase, which converts ATP to cyclic AMP (cAMP), which is the second messenger inside the cell.
• cAMP starts a cascade of enzyme actions within the cell.

The Adrenal Glands

• Are located above the kidneys. There are 2 regions within the glands.
• Medulla region - Adrenaline is synthesised from amino acids and is secreted by the medulla, in response to danger, excitement or stress. This is also part of the fight/flight response.
• Cortex region - Produces cholesterol to produce steroid hormones such as mineralocorticoids (e.g. aldosterone) which help control the concentration of sodium and potassium ions in the blood. The glucocorticoids control synthesis of glycogen, carbohydrates and proteins in the liver.

The Pancreas

• The pancreas functions as both an endocrine and exocrine organ.
• The Islets of Langerhans have an endocrine function, secreting insulin from Beta (B) cells and glucagon from Alpha (a) cells.
• The rest of the pancreas is exocrine, secreting digestive enzymes e.g. amylase and trypsinogen via the pancreatic duct to the duodenum.

Control of blood glucose

• Is regulated by negative feedback. (see comm&homeo notes)
• Blood glucose levels vary on food intake and energy requirements, however needs to be homeostatically maintained at around 90mg in every 100cm3 of blood.
• Is brought about by insulin and glucagon.

If blood glucose rises too high

• Following a carbohydrate meal (high in sucrose, starch, etc) when glucose is absorbed by the gut, the plasma glucose levels rise (hyperglycaemia).

1. B cells detect the increase in plasma glucose.

2. B cells start to release insulin, and a cell action is inhibited.

3. Insulin lowers glucose levels by binding to receptors on the plasma membrane of hepatocytes (liver cells), fat and muscle cells.

4. This activates Adenyl Cyclase to…