Endocrine System Part 1
The need for control
Individual cell
- A single cell is a complex web of interacting biochemical reactions
- Liver cell contains around 10,000 protein types
o many of these are enzyme that catalyse/control biochemical reactions
- A cells appearance and behaviour, are the combined effects of those biochemical reactions
- How a cell looks and behaves can be changed by affecting these pathways
o mainly by affecting enzyme activity
o to modify or adapt to changes in its external environment
Multicellular Organisms
- Cells need to influence biochemistry (i.e. the behaviour) of other cells
o over small and large distances
o over short or long periods of time
- In order to maintain the extracellular environment surround the cells (ECF)
o each cell type makes a direct or indirect contribution to the maintenance of the ECF
(homeostasis)
o ...in addition to its ‘selfish’ activities (i.e. keeping itself alive)
- Also to manage growth, development and reproduction
- Almost exclusively, that control exerted via secreted chemical
Levels of control
- Autocrine
o secreted compound affects same cell
 e.g. cytokines
- Paracrine
o secreted compound affects neighbouring cells
o relies on diffusion
 limited in distance
 can be rapid response
- Endocrine
o Hormones made in endocrine cells
o Release into bloodstream - not limited by diffusion
o All cells exposed to hormones
o Only those with suitable receptors respond
- Neurohormones
o Hormones made in neurones
o Release into bloodstream - not limited by diffusion
o All cells exposed to hormones
o Only those with suitable receptors respond
- Neurotransmitters
o Released from nerve terminal into synaptic gap
 in response to action potential
o NT diffuses across gap to bind to receptors on target cell
o Target cell could be another neurone
o Highly specific, ‘targeted” control
 costly in terms of ‘wiring’
 very rapid response possible over large distances
- Pheromones
o organism to organism chemical communication
o can demonstrate extreme sensitivity
 e.g. moth antenna
 3 mile range!
Hormones: Chemical Types
- Steroid hormones
o derived from cholesterol
 e.g. testosterone, estradiol
- Tyrosine (amino acid) derivatives include the simplest peptide hormones
o thyroid hormones; epinephrine, and norepinephrine
- Protein (peptide) hormones
o Peptide hormones
 short chains
 such as oxytocin and ADH
o Protein hormones
 longer chains
 such as insulin and parathyroid hormone
o Glycoproteins
 longer chains with attached carbohydrate molecules
 such as follicle stimulating hormone (FSH) and thyroid stimulating hormone (TSH)
- Fatty acid derived hormones (eicosanoids)
o e.g. prostaglandins, prostacyclins, leukotrienes and thromboxanes
Hormone Action: the Role of Receptors
Cell surface & Intracellular Receptors
- Receptors for catecholamines, peptide hormones, eicosanoids are in the cell membrane of
target cells.
- Thyroid and steroid hormones cross the membrane and bind to receptors in the cytoplasm or
nucleus
Cell surface receptors: role of second messengers
- When hormone cannot cross cell membrane
- Binding of hormone to receptor on outside generates a signal inside
- Called the second messenger
cAMP is a second messenger
- Cyclic AMP is most common second messenger in animal cells
- Binding of hormone affects G-protein
- Membrane-bound adenylyl cyclase is activated via the G protein
- Conversion of ATP to cAMP
- cAMP activates protein kinases
- Protein kinases phosphorylate proteins (add one or more phosphate groups)
- Makes major changes in the shape and therefore activity of existing target proteins
- Amplification/cascade effect
Summary of Second Messenger Signalling
- Hormone cannot cross cell membrane to affect cell
- So it binds to cell-surface receptor
- This generates an intracellular signal
- A small molecule or ion (cAMP, IP3, DAG, Ca++)
o ‘cheap’, diffuse rapidly
- These affect activity of existing regulatory proteins
- Altered cell metabolism -> altered ‘behaviour’
- Note…
o Amplification/cascade effects
o The central roles for binding, shape changes, and diffusion
o Generally affect protein that is already there
Intracellular Receptors: e.g. Steroid Hormones
#evernote