Phospholipid bilayer hydrophilic and hydrophobic

Phospholipid bilayer – hydrophilic and
hydrophobic portions
 Integral proteins/transmembrane – span the
bilayer
 Peripheral proteins – found on the outer or
inner surfaces
 Glycoproteins – act mainly as cell identifiers.
 Binding of hormones can activate different
types of proteins
 Channel proteins
 Act as channels for water soluble solutes
 Some channels are powered by ATP
 Membrane protein enzymes
 Act as an enzyme when stimulated
 Adenylate cyclase is one example
 Receptor proteins
 1 st messenger is the hormone (ligand)
 binding to receptor triggers activation of 2 nd messengers within the cell
 direct phosphorylation as a result
 protein based attracted to hydrophilic portions of cell membrane- rapid response rate
 steroids, lipids, cholesterol derived, attracted to hydrophobic slide between heads of
phospholipids and pass freely through lipid bilayer- slower response rate
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cAMP messenger system (an example of a second messenger system)
Ca++
calmodulin complex (another type of 2 nd messenger system)
(DAG and IP3 are second messengers, work together to increase cascade effect)
Neuro hormones (secreted by hypothalamus and adrenal medulla) use this pathway
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E. Hormone-Target Specificity
1. Hormone Receptors
 Are 3D shaped protein or glycoprotein receptors on the surface of the cells of the target
tissues .
 Each receptor is specific to its own type of hormone.
e.g. - square block in square hole
e.g. – thyroid hormone will not bind to insulin receptors.
 Hormone-receptor interaction depends on
1. blood levels of hormone
2. relative number of receptors on target cells
3. Affinity of the bond between the hormone and receptor
 High vs. low affinity
 Hormone-receptor complexes are very specific
 Only hormones with a corresponding shape will activate them
2. Down Regulation and Up Regulation of receptors
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 Down regulation Occurs when a target cell has been exposed to a hormone for a prolonged
period of time, causes fatigue of receptors on target tissue.
 Makes target cell less responsive to hormone over time bc of less receptors
