Cell Membranes and Signalinggilsonscience.weebly.com/uploads/2/1/1/4/21140528/... · Concept 5.5 The Membrane Plays a Key Role in a Cell’s Response to Environmental Signals Cells

Cell Membranes and

Signaling

Concept 5.5 The Membrane Plays a Key Role in a Cell’s

Response to Environmental Signals

Cells can respond to many signals if they

have a specific receptor for that signal.

A signal transduction pathway is a

sequence of molecular events and

chemical reactions that lead to a cellular

response, following the receptor’s

activation by a signal.



Cells are exposed to many signals and may have different responses:

•Autocrine signals affect the same cells that release them.

• Paracrine signals diffuse to and affect nearby cells.

•Hormones travel to distant cells.

Figure 5.10 Chemical Signaling Concepts



Only cells with the necessary receptors

can respond to a signal—the target cell

must be able to sense it and respond to

it.

A signal transduction pathway involves a

signal, a receptor, and a response.

Figure 5.11 Signal Transduction Concepts



A common mechanism of signal

transduction is allosteric regulation.

This involves an alteration in a protein’s

shape as a result of a molecule binding

to it.

A signal transduction pathway may

produce short or long term responses.



A signal molecule, or ligand, fits into a

three-dimensional site on the receptor

protein.

Binding of the ligand causes the receptor

to change its three-dimensional shape.

The change in shape initiates a cellular

response.

Figure 5.12 A Signal Binds to Its Receptor



Ligands are generally not metabolized

further, but their binding may expose an

active site on the receptor.

Binding is reversible and the ligand can

be released, to end stimulation.

An inhibitor, or antagonist, can bind in

place of the normal ligand.



Receptors can be classified by their

location in the cell.

This is determined by whether or not their

ligand can diffuse through the

membrane.



Cytoplasmic receptors have ligands, such

as estrogen, that are small or nonpolar

and can diffuse across the membrane.

Membrane receptors have large or polar

ligands, such as insulin, that cannot

diffuse and must bind to a

transmembrane receptor at an

extracellular site.



Receptors are also classified by their

activity:

• Ion channel receptors

• Protein kinase receptors

• G protein–linked receptors



Ion channel receptors, or gated ion

channels, change their three-

dimensional shape when a ligand binds.

The acetylcholine receptor, a ligand-

gated sodium channel, binds

acetylcholine to open the channel and

allow Na+ to diffuse into the cell.



Protein kinase receptors change their

shape when a ligand binds.

The new shape exposes or activates a

cytoplasmic domain that has catalytic

(protein kinase) activity.

Figure 5.13 A Protein Kinase Receptor



Protein kinases catalyze the following

reaction:

ATP + protein → ADP + phosphorylated

protein

Each protein kinase has a specific target

protein, whose activity is changed when

it is phosphorylated.



Ligands binding to G protein–linked

receptors expose a site that can bind to

a membrane protein, a G protein.

The G protein is partially inserted in the

lipid bilayer, and partially exposed on

the cytoplasmic surface.



Many G proteins have three subunits and

can bind three molecules:

• The receptor

• GDP and GTP, used for energy transfer

• An effector protein to cause an effect in

the cell



The activated G protein–linked receptor

exchanges a GDP nucleotide bound to

the G protein for a higher energy GTP.

The activated G protein activates the

effector protein, leading to signal

amplification.

Figure 5.14 A G Protein–Linked Receptor

Concept 5.6 Signal Transduction Allows the Cell to Respond to Its

Environment

Signal activation of a specific receptor

leads to a cellular response, which is

mediated by a signal transduction

pathway.

Signaling can initiate a cascade of protein

interactions—the signal can then be

amplified and distributed to cause

different responses.


Environment

A second messenger is an intermediary between the receptor and the cascade of responses.

In the fight-or-flight response, epinephrine (adrenaline) activates the liver enzyme glycogen phosphorylase.

The enzyme catalyzes the breakdown of glycogen to provide quick energy.


Environment

Researchers found that the cytoplasmic

enzyme could be activated by the

membrane-bound epinephrine in broken

cells, as long as all parts were present.

They discovered that another molecule

delivered the message from the “first

messenger,” epinephrine, to the

enzyme.

Figure 5.15 The Discovery of a Second Messenger (Part 1)

Figure 5.15 The Discovery of a Second Messenger (Part 2)


Environment

The second messenger was later discovered to be cyclic AMP (cAMP).

Second messengers allow the cell to respond to a single membrane event with many events inside the cell—they distribute the signal.

They amplify the signal by activating more than one enzyme target.

Figure 5.16 The Formation of Cyclic AMP


Environment

Signal transduction pathways involve

multiple steps—enzymes may be either

activated or inhibited by other enzymes.

In liver cells, a signal cascade begins

when epinephrine stimulates a G

protein–mediated protein kinase

pathway.


Environment

Epinephrine binds to its receptor and activates a G protein.

cAMP is produced and activates protein kinase A—it phosphorylates two other enzymes, with opposite effects:

• Inhibition

• Activation

Figure 5.17 A Cascade of Reactions Leads to Altered Enzyme Activity (Part 1)

Figure 5.17 A Cascade of Reactions Leads to Altered Enzyme Activity (Part 2)


Environment

• Inhibition—protein kinase A inactivates

glycogen synthase through

phosphorylation, and prevents glucose

storage.

• Activation—Phosphorylase kinase is

activated when phosphorylated and is

part of a cascade that results in the

liberation of glucose molecules.


Environment

Signal transduction ends after the cell

responds—enzymes convert each

transducer back to its inactive precursor.

The balance between the regulating

enzymes and the signal enzymes

determines the cell’s response.

Figure 5.18 Signal Transduction Regulatory Mechanisms


Environment

Cells can alter the balance of enzymes in

two ways:

• Synthesis or breakdown of the enzyme

• Activation or inhibition of the enzymes

by other molecules


Environment

Cell functions change in response to

environmental signals:

• Opening of ion channels

• Alterations in gene expression

• Alteration of enzyme activities

Answer to Opening Question

Caffeine is a large, polar molecule that

binds to receptors on nerve cells in the

brain.

Its structure is similar to adenosine, which

binds to receptors after activity or stress

and results in drowsiness.

Caffeine binds to the same receptor, but

does not activate it—the result is that

the person remains alert.

Figure 5.19 Caffeine and the Cell Membrane (Part 1)

Figure 5.19 Caffeine and the Cell Membrane (Part 2)

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