Endocrine glands secrete chemicals called hormones directly into the bloodstream.

1. 19.1.1: State that hormones are chemical messengers secreted by endocrine glands into the blood and transported to specific target cells. • Endocrine glands secrete chemicals called hormones directly into the bloodstream. • They are unlike exocrine glands, which secrete products into ducts. • The blood transports hormones around the body. • Only cells that have specific receptors will react to the presence of the hormone. These are called target cells.

2. 19.1.2: State that hormones can be steroids, proteins and tyrosine derivatives, with one example of each. • The term ‘hormone’ refers to a group of substances produced by endocrine glands and transported around the body by the bloodstream. • Hormones can be: • Steroids, like testosterone • Proteins, like insulin • Tyrosine derivatives, like thyroxine

3. 19.1.3: Distinguish between the mode of action of steroid hormones and protein hormones. • Once a hormone reaches its target cell, it must cause a change inside the cell. • Steroid hormones: • Steroid hormones are hydrophobic and can pass through the cell membrane easily. This means they don’t need a receptor on the cell surface. • Instead, steroid hormone receptors are intracellular. They will bind to receptors in the cytoplasm or nucleus. • The hormone-receptor complex switches specific genes on or off.

4. 19.1.3 cont. • Protein hormones: • Polypeptide hormones will bind to receptors on cell membrane. • The binding of polypeptide hormones onto the outside of the membrane causes a change on the inside of the membrane. • This changes the concentration of the secondary messenger, which will activate a specific enzyme. • An example of a second messenger is cyclic AMP.

5. Steroid Hormone Protein Hormone