Chapter 7 Flashcards
What are signals
All cells process information from the environment.
the information can be chemical or a physical stimulus such as light
Signals can come from outside the organism or from neighboring cells
Signal transduction pathway
the sequence of molecular events and chemical reactions that lead to a cell’s response to a signal
involves a signal, a receptor, transduction, and a response
why multiple steps
-Different cells can respond differently to the same signal
-Signals can be amplified
-Signals can overlap or influence one another
-A signal can have multiple effects within the same cell
Autocrine
signals affect the cells that made them
Paracrine
Signals affect nearby cells
Hormones
travel to distant cells, usually via the circulatory system
Gap Junctions
Channels between adjacent cells traversed by protein channels called connexons
-Connexons of two cells come together to form a channel only 1.5 nm wide which is too small for most proteins but wide enough for signaling molecules
How do cells communicate directly?
-Gap junctions permit metabolic cooperation between cells and sharing of ATP amino acids, coenzymes, etc
-In some tissues, chemicals must be passed to cells in the interior of the tissue
-Lens cells of mammalian eyes have many gap junctions. Only cells at periphery are close to blood vessels.
cAMP
a second messenger that is able to pass through gap junctions
common second messenger
the enzyme that catalyzes formation of cAMP from ATP (adenylyl cyclase) is located on the cytoplasmic side of the plasma membrane
only a few cells would need to have signal receptors-signal could spread through entire tissue for a coordinated response
-ie: the contraction of cardiac muscle
Plasmodesmata
membrane-lined tunnels that traverse the cell walls
-lining made of fused plasma membranes from both cells
Symplast
continuous cytoplasm
Desmotubule
A tubule derived from the ER that fills the space in the plasmodesmata channel
Ligands
-Chemical signal molecule
-binding the ligand causes receptor protein to change shape and activity
-the binding is reversible and the ligand is not altered
Adenosine
-Binds to a receptor on nerve cells, initiating a signal transduction pathway that reduces brain activity
Caffeine
similar to adenosine and binds to the receptors but it “ties up” the receptor
Membrane receptors
Large or polar ligands bind to plasma membrane receptors (like insulin)
Cytoplasmic receptors
Small or nonpolar ligands can diffuse across plasma membrane (like estrogen)
-AKA intracellular receptors
Types of plasma membrane receptors
-ion channels
-protein kinase receptors (tyrosine-kinase receptors)
-G protein-linked receptors
Ion channel receptors
Channel proteins that allow ions to enter or leave a cell
-Signals can be chemical ligands such as hormones, sensory stimuli such as light, or electric charge differences
-The acetylcholine recpetor on muscle cells is a gated ion channel
Protein Kinase receptors
-Some receptors become protein kinases they catalyze phosphorylation of themselves and/or other proteins
-The insulin receptor phosphorylates itself and other insulin response substrates
-the addition of phosphate occurs at tyrosine residues AKA tyrosine kinase receptors
-Primarily responsible for powerful growth factor signaling (insulin-like growth factor) (epidermal growth factor)
-Inactive monomers form an active dimer when bound to ligand
-Frequently mutated in human cancer
G protein-linked receptors
Ligand binding changes the shape of the cytoplasmic region which binds to a G protein
-G protein-coupled receptors
G proteins
Mobile membrane proteins which three subunits. They bind GDP and GTP (guanosine diphosphate and triphosphate)
-Enormous diversity in G proteins
-can either activate or inhibit an effector
-The response can vary from tissue to tissue
-Epinephrine binds to G protein-linked receptor in heart muscle; this activates an enzyme to produce cyclic AMP (cAMP)
G protein-mediated inhibition
-can occur with the same hormone-epinephrine-in smooth muscle cells
-In this case the enzyme that produces cAMP is inhibited
-Muscle cells relax, capillaries dilate
Cytoplasmic receptors
bind ligands that can cross the plasma membrane
-binding to ligand causes receptor to change shape-allows it to enter nucleus, where it affects gene expression
-receptor may be bound to a chaperonin; binding to ligand releases the chaperonin
