Cell Signaling and the Endocrine System
AP Biology
Cell Signaling
Cells that are close to one another or are far away from each other have to communicate
Most communication is by signal molecules called hormones
Can be water soluble or lipid based
Can bind to specific sites on cell membranes OR
Can pass through membrane through active transport and act inside cells
3 stages in cell signaling
Stage 1: Signal reception
Signal (ligand) binds to a receptor protein in the cell membrane or inside the cell
Receptor protein changes shape, becomes activated, and can interact with other molecules
Stage 2: Signal transduction
Change in the receptor starts a process of cellular response
Can activate or deactivate proteins in the cell
Can amplify the signal by creating second messengers in the cell (small molecules or ions that activate
other enzymes in the cell)
Stage 3: Cellular response
Can regulate:
the function of proteins in the cytoplasm
transcription of DNA in the nucleus by turning genes on or off
Intracellular Receptors
Small or hydrophobic chemical messengers can cross the membrane and activate receptors inside the
cell
Examples: steroid and thyroid hormones of animals
An activated hormone-receptor complex can act as a transcription factor, turning on specific genes
Receptors in the Plasma Membrane
Most water-soluble signal molecules bind to specific sites on receptor proteins in the plasma membrane
There are three main types of membrane receptors:
G-protein-linked receptors
Receptor tyrosine kinases
Ion channel receptors
A G-protein-linked receptor is a plasma membrane receptor that works with the help of a G protein (G
proteins bind with GTP)
The G-protein acts as an on/off switch:
If GDP is bound to the G protein, the G protein is inactive
if GTP is bound to the G protein the protein is active
When a signal is received the cytoplasmic side of the receptor binds an inactive G protein
GTP displaces GDP to activate the G protein
G protein binds to an enzyme, activating it which leads to the cellular response
Conversion of GTP to GDP inactivates the G protein which leaves the G protein and enzyme ready for
reuse
G protein functions
Embryonic development
Sensory reception
Involved in human diseases (cholera, pertussis, botulism interfere with G proteins)
Up to 60% of medicines influence G protein pathways
An ion channel receptor acts as a gate when the ligand binds and the receptor changes shape
the gate allows specific ions, such as Na+ or Ca2+, through a channel in the receptor
this is how neurons transmit stimulus through your nervous system
Protein Phosphorylation and Dephosphorylation
In many pathways, the signal is transmitted by a cascade of protein phosphorylations
This phosphorylation and dephosphorylation system acts as a molecular switch, turning activities on and
off
Hormones
Endocrine glands secrete hormones into the blood stream for delivery to the target organs
Hormones work at all levels of organization and can have different action on each type of target cell
Cells need the right receptors for the hormone to bind; some cells have receptors for many different
hormones, some for only a few
Types of hormones
Steroid hormones: lipid soluble molecules made from cholesterol
Produced only by the sex organs and adrenal glands
Enters cells and interacts with DNA to stimulate or inhibit protein synthesis
ex: androgens, estrogens
Peptide hormones: made from amino acids (1-200)
Water-soluble molecules that bind to receptors on the cell membrane
Never enter the cell but cause a cascade of events inside the cell
Ex: glucagon, insulin