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.

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

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.

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.

Steroid Hormone Protein Hormone