Comprehensive study guide for Biology A Level, made by an Oxford Biochemistry student with all 9s at GCSE and 3 A*s at A Level! Information arranged by spec point. Concise notes written using past papers, multiple textbooks, class notes and more.
5.1.4 HORMONAL COMMUNICATION
a. endocrine communication by hormones
Endocrine system – a communication system using hormones as signalling
molecules. It is used alongside the nervous system for communication around
the body.
- Gland – a group of cells that produces and secretes one or more substances.
- Duct – tube or vessel that carries the secretion of a gland.
Endocrine glands – organs containing secretory cells that can secrete and pass
hormones directly into the blood to act on distant sites known as target sites.
- They consist of groups of cells that manufacture and release the hormone
directly into the blood in capillaries running through the
gland.
- They have groups of cells with associated capillaries, but
no visible ducts.
- Examples of endocrine glands and the hormones they
produce:
o Pituitary gland – ADH.
o Thyroid – thyroxine.
o Adrenal glands – adrenaline.
o Islets of Langerhans – insulin, glucagon.
o Ovaries – oestrogen, progesterone.
o Testes – testosterone.
Exocrine gland – an organ that secretes substances into a duct. The duct then
carries them to a particular part of the body.
- Exocrine glands do not secrete hormones.
- They generally transport to a closer area of the body than endocrine glands.
- Examples of exocrine glands and the substances they secrete include:
o Salivary glands – saliva.
o Liver – bile.
o Stomach – gastric juice.
o Pancreas – pancreatic juice.
Hormones – molecules secreted by endocrine glands that act as messengers,
carrying a signal to a specific target organ or
tissue.
- Method of action:
o Hormones are secreted from an
endocrine gland into the blood.
o They are transported in the blood
plasma.
o They diffuse out of the blood and bind
to specific and complementary receptors on the cell surface membrane
or in the cytoplasm of target cells.
o Once bound, the hormone stimulates the target cell to produce a
response.
- They are relatively small molecules with a short life in the body.
, - Low concentrations can have large effects.
- There are two types of hormone: protein and steroid. They work in different
ways.
Steroid hormones (e.g. oestrogen, testosterone):
- Lipid-soluble; can diffuse through cell membranes.
- They diffuse across the membrane and bind to
intracellular receptors in the cytoplasm, forming a
hormone-receptor complex.
- The hormone-receptor complex diffuses into the
nucleus. It acts as a transcription factor, either
facilitating or inhibiting the transcription of a
specific gene.
Protein and peptide hormones, derivatives of amino acids (e.g. adrenaline,
insulin, glucagon):
- Insoluble in the phospholipid membrane; cannot
pass directly through cell membranes.
- They bind to an extracellular receptor on the
plasma membrane of target cells.
- Binding triggers an enzyme cascade mediated by
chemicals called secondary messengers.
- A G-protein triggers adenylyl phosphorylase to
produce cAMP from ATP.
- cAMP activates other enzymes by adding
phosphates to them.
Target cells – the cells receiving an endocrine signal.
- Target cells may be grouped together in a target tissue (e.g. epithelium of
collecting ducts) or dispersed in a number of tissues (e.g. receptors for
adrenaline in CNS, heart, smooth muscle etc.).
- For non-steroid hormones, the target cells must possess a specific receptor
on their plasma membrane that is complementary in shape to the shape of
the hormone molecule.
o The hormone binds to this receptor and initiates changes in the cell.
o Only cells that possess the correct receptor will respond to the
hormone.
- Extra info: there are two types of adrenergic receptor – alpha and beta.
o Alpha receptors are excitatory in smooth muscles and gland cells, but
cause relaxation of intestinal smooth muscles.
o Beta receptors produce an inhibitory respond, although in heart muscle
the effect is excitatory. Beta blockers inhibit the response of these
receptors to adrenalin.
First and second messengers:
- Non-steroid hormones are first messengers.
o They are signalling molecules outside the cell that bind to the plasma
membrane and initiate an effect inside the cell.
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