Chapter 6 Objective
Describe the sequence of events that take place in cell signaling.
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Cells communicate by cell signaling, which involves:
o Synthesis, release, and transport of signaling molecules, which can be
neurotransmitters, hormones, and other regulatory molecules (a signaling
molecule that binds to a specific receptor is called a ligand)
o Reception of information by target cells
o Signal transduction, the process in which a receptor converts an extracellular
signal into an intracellular signal that causes some change in the cell (signal
transduction typically involves a series of molecules that relay information)
o Response by the cell
Describe three types of signaling molecules and give examples of each: local regulators,
neurotransmitters, hormones.
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Local regulators act by paracrine regulation, that is, they diffuse through the interstitial
fluid and act on nearby cells. Local regulators include histamine, growth factors,
substances called prostaglandins, and nitric oxide, a gaseous signaling molecule that
passes into target cells
Most neurons (nerve cells) signal one another by releasing chemical compounds called
neurotransmitters.
Hormones are chemical messengers in plants and animals. In animals, they are secreted
by endocrine glands, glands that have no ducts. Most hormones diffuse into capillaries
and are transported by the blood to target cells
Compare and contrast the location, structure, and action of ion channel-linked receptors,
G proteins-linked receptors, enzyme-linked receptors, and intracellular receptors.
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When a signaling molecule binds to an ion channel-linked receptor, the ion channel
opens, or in some cases, closes.
G protein-linked receptors are transmembrane proteins composed of seven alpha
helices connected by loops that extend into the cytosol or outside the cell. These
receptors couple certain signaling molecules to signal transduction pathways inside the
cell. The tail of the receptor that extends into the cytosol has a binding site for a specific
G protein, a regulatory protein that binds to GTP
Enzyme-linked receptors are transmembrane proteins with a binding site for an enzyme
inside the cell. Some of these receptors function directly as enzymes. Many enzymelinked receptors are tyrosine kinases in which they enzyme is part of the receptor.
Intracellular receptors are located in the cytosol or in the nucleus. Their ligands are
small, hydrophobic molecules that diffuse across the plasma membrane.
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Ion channel-linked receptors convert chemical signals into electrical signals. The gates
of many ion channels remain closed until ligands bind to them.
G protein-linked receptors activate G proteins. A G protein consists of three subunits. It
is linked to a molecule of guanosine diphosphate (GDP), a molecule similar to ADP but
containing the base guanine instead of adenine. When a ligand binds with the receptor,
the GDP is release and is replaced by guanosine triphosphate (GTP). Then one subunit
of the G protein separates from the other two subunits. When activated, G protein
initiates signal transduction by binding with a specific protein in the cell. Some G
proteins directly activate enzymes that catalyze changes in certain proteins, leading to
changes in cell function.
Most enzyme-linked
Trace and describe the sequence of events for at least two second messenger pathways
involving G proteins.
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In many cases, the signaling molecule serves as the first messenger. Information is
relayed by the G protein to a second messenger, an intracellular signaling agent.
When certain G proteins undergo a conformational change, they bind with and activate
adenylyl cyclase, an enzyme on the cytoplasmic side of the plasma membrane. Adenylyl
cyclase catalyzes the formation of cyclic AMP (cAMP) from ATP. Cyclic AMP
activates protein kinase A, an enzyme that phosphorylates certain proteins. The
phosphorylated protein triggers a chain of reactions that lead to some response in the cell.
Certain G proteins activate the membrane-bound enzyme phospholipase C. This enzyme
splits a phopholipid, PIPs (phophotidylinositol-4,5-bisphosphate), into two products,
inositol triphossphate (IP3) and diacylglycerol (DAG). IP3 is a second messenger that
can donate phosphate groups to proteins. IP3 binds to calcium channels in the
endoplasmic reticulum, which causes the channels to open and release calcium ions into
the cytosol. DAG is a second messenger that activates protein kinase C enzymes.
These enzymes phosphorylate a variety of target proteins.
Calcium ions also act as second messengers. They typically combine with the protein
calmodulin, which then affects the activity of protein kinases and protein phosphatases.
Describe signal amplification.
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Signal amplification is the process of enhancing signal strength as a signal is relayed
through a signal transduction pathway.
Signal termination is the process of inactivation the receptor and each component of the
signal transduction
Describe the structure of scaffolding proteins.
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They hold the kinases and phosphates near the proteins they regulate and so decrease the
chance that they might act on the wrong proteins. Scaffolding proteins ensure that
signals are relayed accurately, rapidly, and more efficiently.