Chapter 11 (Notes) Flashcards by Amy Horton

Cell-to-cell communication is essential for both

multicellular and unicellular organisms

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Biologists have discovered some universal mechanisms of

cellular regulation

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Cells most often communicate with each other via

chemical signals.

For example, the fight-or-flight response is triggered by a signaling molecule called epinephrine

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External signals are converted to

responses within the cell

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Microbes provide a glimpse of the role of

cell signaling in the evolution of life

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A signal transduction pathway is a

series of steps by which a signal on a cell’s surface is converted into a specific cellular response

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Signal transduction pathways convert signals on a cell’s surface into

cellular responses

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Pathway similarities suggest that

ancestral signaling molecules evolved in prokaryotes and were modified later in eukaryotes

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Cells in a multicellular organisms communicate by

chemical messengers

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Animal and plant cells have cell junctions that

directly connect the cytoplasm of adjacent cells

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In local signaling, animal cells may communicate by

direct contact, or cell-cell recognition

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In many other cases, animal cells communicate using

local regulators, messenger molecules that travel only short distances

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paracrine signaling-

local regulator

grown factors from one cell stimulate numerous neighboring cells

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synaptic signaling-

local regulator

neurotransmitters

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In long-distance signaling, plants and animals use

chemicals called hormones

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Endocrine signaling-

long-distance signaling. hormones

hormones are released into the circulatory system to reach distance regions of the body

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The ability of a cell to respond to a signal depends on whether or not it has a

receptor specific to that signal

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Earl W. Sutherland discovered how the hormone

epinephrine acts on cells

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Sutherland suggested that cells receiving signals went through three processess:

Reception
Transduction
Response

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Reception: a signaling molecules binds to a

receptor protein, causing it to change shape

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The binding between a signal molecule (ligand) and receptor is

highly specific

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Ligand-

a molecule that specifically binds to a larger molecule

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A shape change in a receptor is often the

initial transduction of the signal

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Most signal receptors are

plasma membrane proteins

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Most water-soluble signal molecules bind to

specific sites on receptor proteins that span the plasma membrane

There are three main types of membrane receptors

1. G protein-coupled receptors 2. Receptor tyrosine kinases 3. Ion channel receptors Also have intracellular receptors

G protein-coupled receptors (GPCRs) are

the largest family of cell-surface receptors

A G protein-coupled receptor (GPCR) is a

plasma membrane receptor that works with the help of a G protein

The G protein acts as an

on/off switch: If GDP is bound to the G protein, the G protein is inactive

Receptor tyrosine kinases (RTKs) are

membrane receptors that attach phosphates to tyrosines

A kinase is an

enzyme that catalyzes the transfer of phosphate groups

A receptor tyrosine kinase can trigger

multiple signal transduction pathways at once

Abnormal functioning of receptor tyrosine kinases (RTKs) is associated with

many types of cancers

A ligand-gated ion channel receptor acts as a

gate when the receptor changes shape

When a signal molecule binds as a ligand to the receptor, the

gate allows specific ions, such as NA+ or CA^2+, through a channel in the receptor

Important in the nervous system

-neurotransmitter release

Intracellular receptor proteins are found in the

cytosol or nucleus of target cells

Small or hydrophobic chemical messengers can

readily cross the membrane and activate receptors

Examples of hydrophobic messengers are

the steroid and thyroid hormones of animals

An activated hormone-receptor complex can act as a

transcription factor, turning on specific genes

Transduction:

cascades of molecular interactions relay signals from receptors to target molecules in the cell

Signal transduction usually involves

multiple steps

Multistep pathways can amplify

a signal: A few molecules can produce a large cellular response

Multistep pathways provide more opportunities for

coordination and regulation of cellular response

(Signal transduction pathways) The molecules that relay a signal from receptor to

response are mostly proteins

(Signal transduction pathways) Like falling dominoes, the receptor activates another protein, which

activates another, and so on, until the protein producing the response is activated

(Signal transduction pathways) At each step, the signal is transduced into a different form, usually a

shape change in a protein | -usually phosphorylation

((Protein phosphorylation and dephosphorylation)) In many pathways, the signal is transmitted by

a cascade of protein phosphorylations

((Protein phosphorylation and dephosphorylation)) Protein kinases transfer phosphates from

ATP to protein, a process called phosphorylation

Protein phosphatases remove the phosphates from

proteins, a process called dephosphorylation

((Protein phosphorylation and dephosphorylation)) This phosphorylation and dephosphorylation system acts as a

molecular switch, turning activities on and off or up or down, as required

Small molecules and ions as

second messengers

The extracellular signal molecule (ligand) that binds to the receptor is a

pathway's "first messenger"

Second messengers are

small, nonprotein, water-soluble molecules or ions that spread throughout a cell by diffusion

Second messengers participate in pathways initiated by

GPCRs and RTKs

Cyclic AMP and calcium ions are

common second messengers

Cyclic AMP (cAMP) is one of the

most widely used second messengers

Adenylyl cyclase, an enzyme in the plasma membrane, converts

ATP to cAMP in response to an extracellular signal

Many signal molecules trigger formation of

cAMP

Other components of cAMP pathways are

G proteins, G protein-coupled receptors, and protein kinases

cAMP usually activates

protein kinase A, which phosphorylates various other proteins

Further regulation of cell metabolism is provided by G-protein systems that inhibit

adenylyl cyclase

Calcium ions (Ca^2+) act as a

second messenger in many pathways

Calcium is an important second messenger because

cells can regulate its concentration

A signal relayed by a signal transduction pathway may trigger an increase in

calcium in the cytosol

Pathways leading to the release of calcium involved

inositol triphosphate (IP3) and diaclyglycerol (DAG) as additional second messengers

Response:

Cell signaling leads to regulation of transcription or cytoplasmic activities

The cell's responses to an extracellular signal is sometimes called the

"output response"

Ultimately, a signal transduction pathway leads to a

regulation of one or more cellular activities

The response may occur in the

cytoplasm or in the nucleus

Many signaling pathways regulate the synthesis of

enzymes or other proteins, usually by turning genes on or off in the nucleus

The final activated molecule in the signaling pathway may function as

a transcription factor

Other pathways regulate the activity of

enzymes rather than their synthesis

There are four aspects of fine-tuning to consider

1. Amplification of the signal (and thus the response) 2. Specificity of the response 3. Overall efficiency of response, enhanced by scaffolding proteins 4. Termination of the signal

Enzyme cascades amplify the

cell's response

(((signal amplification))) At each step, the number of activated products is much greater than in the

preceding step

Amplification stems from the fact that these proteins persist in the active form long enough to

process numerous molecules of substrate before they become inactive again

Few numbers of signal can affect

hundreds of millions of end molecules

Different kinds of cells have

different collections of proteins

These different proteins allow cells to

detect and respond to different signals. - A liver cell in the presence of epinephrine breaks down glycogen - A heart cell in the presence of epinephrine contracts

Even the same signal can have different effects in cells with

different proteins and pathways

Pathway branching and "cross-talk" further help

coordinate incoming signals

Scaffolding proteins are

large relay proteins to which other relay proteins are attached

Scaffolding proteins can

increase the signal transduction efficiency by grouping together different proteins involved in the same pathway

In some cases, scaffolding proteins may also help

activate some of the relay proteins

Inactivation mechanisms are an

essential aspect of cell signaling

If ligand concentration falls,

fewer receptors will be bound

Unbound receptors revert to an

inactive state - signal molecules are reversible (on/off) - allows the cell to be ready for any new signals

Apoptosis integrates

multiple cell-signaling pathways

Apoptosis is

programmed or controlled cell suicide

Components of the cell are

chopped up and packaged into vesicles that are digested by scavenger cells

The cell shrinks and becomes

lobed (blebbing)

Apoptosis prevents enzymes from

leaking out of a dying cell and damaging neighboring cells

Caspases are the main

proteases (enzymes that cut up proteins) that carry out apoptosis

Apoptosis can be triggered by

- an extracellular death-signaling ligand - DNA damage in the nucleus - protein misfolding in the endoplasmic reticulum

Apoptosis evolved early in animal evolution and is essential for

the development and maintenance of all animals

Apoptosis may be involved in some

diseases (for example, parkinson's and alzheimer's); interference with apoptosis may contribute to some cancers