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Ch
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parts of a
multicellular organism are coordinated by long-distance signals
⁃ Steps of Cell-Cell Signaling: occurs in 4 steps
⁃ signal reception
⁃ signal processing
⁃ signal response
⁃ signal deactivation
⁃ Receptor activation: signal binds to a receptor which is then activated
⁃ Signal Transduction: signal is transmitted inside the cell by signal transduction pathway
⁃ Response: cell responds (ex: activates an enzyme or turning on transcription of a gene)
⁃ Termination: response is terminated so that new signals can be received
⁃ Types of cell signaling:
⁃ Endocrine signaling: signal travels the circulatory system
⁃ Paracrine signaling: when cells are relatively close enough to signal (can diffuse
over short space)
⁃ one cell releases the signal & the other accepts
⁃ Autocrine signaling: signal is sent from the same cell it goes to
⁃ usu
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pieces
when they are activated by a signal receptor they trigger the production of messengers
inside the cell
G proteins link the receipt of an extracellular signal to the production of an intracellular
signal
they’re activated when they bind to GTP & are deactivated when they hydrolyze the
bound GTP to GDP
⁃ amplification: occurs inside membrane
⁃ activation of one signal leads to the activation of many other signals w/in
the cell
⁃ G Proteins & Signal Transduction: linking an external signal to the production of an
intracellular signal involves 3 steps
⁃ hormone binds to the membrane receptor which changes shape & activates G
proteins
⁃ G protein replaces GDP w/ GTP & splits into two parts
⁃ 1 of those 2 parts of the G protein activates an enzyme that catalyzes production
of a 2nd messenger
⁃ Second Messengers: small molecules that diffuse rapidly throughout the cell,
amplifying the hormone signal
⁃ several second messengers work by activating protein kinases, which add
a phosphate group
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Enzyme-Linked Receptors: integral proteins that bind a hormone signal & directly
catalyze a reaction inside a cell
⁃ best known group of enzyme linked receptors are the receptor tyrosine kinases
(RTKs)
Receptor Tyrosine Kinases & Signal Transduction: direct catalyzing of intracellular
reaction involves 5 steps
⁃ hormone binds to RTK
⁃ changes the confirmation of 2 individual RTKs
⁃ RTK forms a dimer (2 RTKs put together) & is phosphorylated by ATP
(phosphorylation activates RTK)
⁃ Proteins build bridge b/t RTK dimer & the Ras protein (intracellular protein)
which is subsequently activated
⁃ Ras swaps GDP w/ GTP to activate itself
⁃ Ras triggers the phosphorylation & activation of another protein
⁃ Ras phosphorylates an inactive protein
⁃ inactive proteins become activated through phosphorylation
⁃ Phosphorylation cascade, w/ each phosphorylated protein catalyzing the
phosphorylation of other proteins, it amplifies the original signal many times
over
⁃ one protein phosphorylates another, & the cycle goes on
⁃ terminates when some cellular response is triggered eventually
⁃ AMPLIFICATION: occurs in this process b/c the molecules (such as Ras)
can activate more than molecule
Results of Signaling Processing:
⁃ extracellular message is converted into an intracellular message
⁃ original message is often amplified many times over
Signal Response: the ultimate response to a cell-cell signal varies from signal to signal &
from cell to cell, but fall into two general categories
⁃ a change where genes are being expressed in the target cell
⁃ activate or deactivate a particular target protein that already exists in the cell
Signal Deactivation: cells have automatic & rapid mechanisms for signal deactivation
⁃ these mechanisms allow the cell to remain sensitive to small changes in the
concentration of hormones or in the number & activity of signal receptors
⁃ for each step of activation there’s a step of deactivation
⁃ G-protein deactivation:
⁃ phosphatase remove phosphate groups from proteins to deactivate
⁃ signal molecule detaches from the receptor
⁃ G proteins hydrolyze GTP to GDP & becomes inactive



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