Chapter 11 Cell Communication

Question?

• How do cells communicate?

• By “cellular” phones.

• But seriously, cells do need to

communicate for many reasons.

Why do cells communicate?

• Regulation - cells need to control

cellular processes.

• Environmental Stimuli - cells need

to be able to respond to signals

from their environment.

Cell Communication

Cell Signaling (C.S.)

• Is a relatively “new” topic in

Biology and AP Biology.

• Appears to answer many

questions in medicine.

• Is a topic you’ll be hearing more

about in your future.

Stages of C.S.

1. Reception - receiving the signal.

2. Transduction - passing on the

signal.

3. Response - cellular changes

because of the signal.

Reception

Transduction

Response

Reception

• The target cell’s detection of a

signal coming from outside the

cell.

• May occur by: – Direct Contact

– Through signal molecules

Direct Contact

• When molecules can flow directly

from cell to cell without crossing

membranes.

• Plants - plasmodesmata

• Animals - gap junctions

Direct Contact

• May also occur by cell surface

molecules that project from the

surface and “touch” another cell.

Signal Molecules

• The actual chemical signal that travels from cell to cell.

• Often water soluble.

• Usually too large to travel through membranes.

• Double reason why they can’t cross cell membranes.

Signal Molecules

• Behave as “ligands”: a smaller

molecule that binds to a larger

one.

Receptor Molecules

• Usually made of protein.

• Change shape when bind to a

signal molecule.

• Transmits information from the

exterior to the interior of a cell.

Receptor Mechanisms

1. G-Protein linked

2. Tyrosine-Kinase

3. Ion channels

4. Intracellular

G-protein linked

• Plasma membrane receptor.

• Works with “G-protein”, an

intracellular protein with GDP or

GTP.

G-protein

• GDP and GTP acts as a switch.

• If GDP - inactive

• If GTP - active

G-protein

• When active (GTP), the protein

binds to another protein (enzyme)

and alters its activation.

• Active state is only temporary.

G-protein linked receptors

• Very widespread and diverse in

functions.

• Ex - vision, smell, blood vessel

development.

G-protein linked receptors

• Many diseases work by affecting

g-protein linked receptors.

• Ex - whooping cough, botulism,

cholera, some cancers

G-protein linked receptors

• Up to 60% of all medicines exert

their effects through G-protein

linked receptors.

Tyrosine-Kinase Receptors

• Extends through the cell

membrane.

• Intracellular part functions as a

“kinase”, which transfers Pi from

ATP to tyrosine on a substrate

protein.

Mechanism

1. Ligand binding - causes two

receptor molecules to aggregate.

Ex - growth hormone

2. Activation of Tyrosine-kinase parts

in cytoplasm.

3. Phosphorylation of tyrosines by

ATP.

Intracellular Proteins

• Become activated & cause the

cellular response.

Tyrosine-Kinase Receptors

• Often activate several different

pathways at once, helping

regulate complicated functions

such as cell division.

Ion-channel Receptors

• Protein pores in the membrane

that open or close in response to

chemical signals.

• Allow or block the flow of ions

such as Na+ or Ca2+.

Ion-channel Receptors

• Activated by a ligand on the

extracellular side.

• Causes a change in ion

concentration inside the cell.

• Ex - nervous system signals.

Intracellular Signals

• Proteins located in the cytoplasm

or nucleus that receive a signal

that CAN pass through the cell

membrane.

• Ex - steroids (hormones), NO or

nitric oxide

Intracellular Signals

• Activated protein turns on genes

in nucleus.

Comment

• Most signals never enter a cell.

The signal is received at the

membrane and passed on.

• Exception - intracellular receptors

Signal-Transduction Pathways

• The further amplification and

movement of a signal in the

cytoplasm.

• Often has multiple steps using

relay proteins such as Protein

Kinases.

Protein Kinase

• General name for any enzyme

that transfers Pi from ATP to a

protein.

• About 1% of our genes are for

Protein Kinases.

Protein Phosphorylation

• The addition of Pi to a protein,

which activates the protein.

• Usually adds Pi to Serine or

Threonine.

Amplification

• Protein Kinases often work in a

cascade with each being able to

activate several molecules.

• Result - from one signal, many

molecules can be activated.

Secondary Messengers

• Small water soluble non-protein

molecules or ions that pass on a

signal.

• Spread rapidly by diffusion.

• Activates relay proteins.

Secondary Messengers

• Examples - cAMP, Ca2+, inositol

trisphosphate (IP3)

cAMP

• A form of AMP made directly from

ATP by Adenylyl cyclase.

• Short lived - converted back to

AMP.

• Activates a number of Protein

Kinases.

Calcium Ions

• More widely used than cAMP.

• Used as a secondary messenger

in both G-protein pathways and

tyrosine-kinase receptor

pathways.

Calcium Ions

• Works because of differences in

concentration between

extracellular and intracellular

environments. (10,000X)

• Used in plants, muscles and other

places.

Homework

•Read Chapter 11

•Chapter 11 – Mon. Nov. 5

•Ps Lab – Wed. Nov. 7

Inositol Trisphosphate (IP3)

• Secondary messenger attached to

phospholipids of cell membrane.

• Sent to Ca channel on the ER.

• Allows flood of Ca2+ into the

cytoplasm from the ER.

Start here

Or Start here

Cellular Responses

• Cytoplasmic Regulation

• Transcription Regulation in the

nucleus (DNA --> RNA).

Cytoplasmic Regulation

• Rearrangement of the

cytoskeleton.

• Opening or closing of an ion

channel.

• Alteration of cell metabolism.

Transcription Regulation

• Activating protein synthesis for

new enzymes.

• Transcription control factors are

often activated by a Protein

Kinase.

Question

• If liver and heart cells both are

exposed to ligands, why does one

respond and the other not?

• Different cells have different

collections of receptors.

Alternate explanation

Comment

• Chapter focused only on

activating signals. There are also

inactivation mechanisms to stop

signals.

Signaling Efficiency

• Often increased by the use of

scaffolding proteins.

• Scaffolding proteins – a protein

that holds or groups signal

pathway proteins together.

Apoptosis

• Programmed cell death

• Uses cell signaling pathways

• DNA is chopped up

• Cell shrinks and becomes lobed (blebbing)

• Pieces are digested by specialized scavenger cells

WBC before and after

Apoptosis

• Balance between signals for “live” or “die”

• Triggered by mitochondria damage, neighbor cells, internal signals

• Involved with Parkinson’s Alzheimer’s, Cancer

Apoptosis video

• http://www.youtube.com/watch?v=

DR80Huxp4y8

Summary

• Don’t get bogged down in details in this chapter. Use the KISS principle.

• Know - 3 stages of cell signaling.

• Know - At least one example of a receptor and how it works (in detail).

Summary

• Know - protein kinases and

cascades (amplification)

• Know – example of a secondary

signal

• Apoptosis