Chapter 7

Question 1

What are signals

Answer 1

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

Question 2

Signal transduction pathway

Answer 2

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

Question 3

why multiple steps

Answer 3

-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

Question 4

Autocrine

Answer 4

signals affect the cells that made them

Question 5

Paracrine

Answer 5

Signals affect nearby cells

Question 6

Hormones

Answer 6

travel to distant cells, usually via the circulatory system

Question 7

Gap Junctions

Answer 7

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

Question 8

How do cells communicate directly?

Answer 8

-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.

Question 9

cAMP

Answer 9

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

Question 10

Plasmodesmata

Answer 10

membrane-lined tunnels that traverse the cell walls
-lining made of fused plasma membranes from both cells

Question 11

Symplast

Answer 11

continuous cytoplasm

Question 12

Desmotubule

Answer 12

A tubule derived from the ER that fills the space in the plasmodesmata channel

Question 13

Ligands

Answer 13

-Chemical signal molecule
-binding the ligand causes receptor protein to change shape and activity
-the binding is reversible and the ligand is not altered

Question 14

Adenosine

Answer 14

-Binds to a receptor on nerve cells, initiating a signal transduction pathway that reduces brain activity

Question 15

Caffeine

Answer 15

similar to adenosine and binds to the receptors but it “ties up” the receptor

Question 16

Membrane receptors

Answer 16

Large or polar ligands bind to plasma membrane receptors (like insulin)

Question 17

Cytoplasmic receptors

Answer 17

Small or nonpolar ligands can diffuse across plasma membrane (like estrogen)
-AKA intracellular receptors

Question 18

Types of plasma membrane receptors

Answer 18

-ion channels
-protein kinase receptors (tyrosine-kinase receptors)
-G protein-linked receptors

Question 19

Ion channel receptors

Answer 19

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

Question 20

Protein Kinase receptors

Answer 20

-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

Question 21

G protein-linked receptors

Answer 21

Ligand binding changes the shape of the cytoplasmic region which binds to a G protein
-G protein-coupled receptors

Question 22

G proteins

Answer 22

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)

Question 23

G protein-mediated inhibition

Answer 23

-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

Question 24

Cytoplasmic receptors

Answer 24

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

Question 25

Direct signal transduction

Answer 25

occurs at plasma membrane and is a function of the receptor itself

Question 26

Indirect signal transduction

Answer 26

Another molecule-the second messenger-mediates interaction between receptor and cells response

Question 27

Protein kinase cascade

Answer 27

a pathway in which one protein kinase activates the next by phosphorylation and so on
-at each step, signal is amplified
-information that arrived at the plasma membrane is communicated to the nucleus
-Many steps provide specificity-different target proteins provide variation in response

Question 28

phosphatase

Answer 28

enzymes later remove phosphate allowing the pathways to reset

Question 29

Cell signaling and Cancer

Answer 29

Human bladder cancer cells:

Have an abnormal form of a G protein called Ras.
The protein is permanently bound to GTP,
causing continuous cell division.

If the abnormal Ras is inhibited, the cells stop
dividing—leading to Ras inhibitors for cancer
treatment.

-This one mechanism of cancer development and
treatment—there are others…

Question 30

Transduction using a second messenger

Answer 30

The signal is the first messenger.
The second messenger is released into the
cytoplasm after signal binds to receptor.
Second messengers affect many processes in
the cell.
Usually small, easily diffusible molecules
Include cAMP, Ca2+, NO, IP3 and DAG

Question 31

cAMP targets

Answer 31

-binds to ion channels in many kinds of sensory cells and opens the channel
-binds to protein kinases and/or phosphorylases in cytoplasm-this exposes the active site and starts a protein kinase cascade

Question 32

Nitric Oxide (NO)

Answer 32

-can act as a second messenger
-acetylcholein stimulates IP3/DAG pathway of endothelial cells that lone blood vessels
-The influx of Ca2+ activates an enzyme NO synthase, that catalyzes production of NO from arginine
-Diffuses to nearby smooth muscle cells where it stimulates synthesis of cGMP

Question 33

Signal transduction regulation

Answer 33

-regulated by enzymes that convert the activated form back to inactive form
-The balance between enzymes that activate transducers and enzymes that inactivate them determines the cellular response to a signal

Question 34

Effects of a signal

Answer 34

-opening ion channels
-changing enzyme activity
-differential gene transcription

Question 35

Opening ion channels

Answer 35

-is a key step in response of nervous system cells
-ie sense of smell
-odorant molecules bind to receptors in the nose and a G protein is activated
-this activates adenylyl cyclase to catalyze formation of cAMP which opens ion channels. Influx of Na+ and Ca2+ stimulates nerves to send signals to the brain

Question 36

Prokaryotes

Answer 36

The E. coli receptor for solute concentration is
EnvZ, a transmembrane protein in the
plasma membrane.

Change in solute concentration causes a
change in conformation of the receptor
protein.

The conformation change exposes an
active site, and it becomes a protein
kinase.

The EnvZ phosphorylates itself.

The phosphate group changes
conformation of the cytoplasmic domain
of EnvZ, it binds to another protein,
OmpR, and a phosphate is transferred.

Question 37

OmpR

Answer 37

Conformation of OmpR (the responder)
changes.

The effect: Phosphorylated OmpR binds
to DNA to increase the expression of
the protein OmpC.

The OmpC protein is inserted in the outer
membrane where it blocks pores and
prevents solutes from entering.