PACKET #17CHAPTER #11

Introduction to Cell Communication

Why Communication Between Cells is Important

A typical free-living cell must be able to Sniff out nutrients Sense the difference between light and dark Detect and avoid poisons and predators

Cells must also be able to communicate to provide long range integration of metabolism.

Therefore, cells must be able to communicate with each other.

Players of Cell Signaling

The Players

The Signal Signaling Cell

Cell sending the message Target Cell

Cell receiving the message Ligand

Molecule such as a hormone or a neurotransmitter that binds to a specific site on a protein (receptor protein)

Receptor Protein* Recognizes and responds specifically to the signal molecule Performs the first step in a series of transduction processes

at the receiving end

Molecules Used As Signals

ProteinsPeptidesAmino AcidsNucleotidesSteroidsFatty Acid DerivativesDissolved Gases

Target Cell

Contain receptor proteins in the cell membrane Recognizes and responds specifically to the signal

molecule Performs the first step in a series of transduction

processes at the receiving end

General Steps of Cell Signaling

Three Steps

Three Steps Signal reception Transduction Response

Signal Reception & Types of Receptors

Reception

Cells are stimulated by an extracellular signalSignal binds to, and activates, a receptor

proteinEach receptor protein recognizes a specific

signal molecule Once signal is recognized, a new intracellular signal is

generated. This is the first step of transduction

Receptors I

There are two basic mechanisms by which chemical signals cause a biologic effect within the cell Hence there are two

types of receptors Intracellular receptors

Inside the cell Cell surface receptors

Found on the surface of the cell

Classes of Cell-Surface Receptors

Three classes Ion-channel linked receptor G-protein linked receptor Enzyme-linked receptor

Intracellular Receptors

Used for molecules that are sufficiently small and hydrophobic to diffuse across the membrane Best known hydrophobic signal molecules

Steroid hormones Cortisol Estradiol Testosterone

Thyroid hormones Thyroxine

Vitamin D Retinoic Acid

Intracellular Receptors

Receptors lie in the interior of the target cell Cytosol or nucleus When steroid hormone binds, receptor undergoes

large conformational change that enables it to bond to its corresponding sequence in the DNA Promotes or inhibits transcription of a set of genes

Generally regulatory proteins or enzymesMore details to come later.

Cell Surface Receptors

Largest class of receptorsUsed for signals that are too large or too

hydrophillic to cross the plasma membraneLie in the plasma membrane of the cellRelays message across the membrane

Signal Transduction & Transduction Pathways

Signal Transduction

Signal transduction is the continuation and/or conversion of signals from one form to the next. The signals, although

sometimes in different forms, represent the same information.

Signal Transduction II

After reception, when the original signal molecule binds on the receptor protein, the message is passed through the cell membrane through a set of intracellular signaling molecules—transduction.

Pathways

Two classifications Catabolic pathway

Degradable pathway Reactions serve to capture chemical energy {ATP} from the

degradation of energy-rich fuel molecules Reactions that break down complex molecules, such as

proteins, polysaccharides, and lipids, to a few simple molecules

Anabolic Synthetic pathway

Combine small molecules, such as amino acids to form complex molecules such as proteins

Require energy ATP is broken down to ADP + P

Introduction of Pathways

Multistep Pathways Can amplify a signal Provide more opportunities for coordination and regulation

At each step in a pathway The signal is transduced into a different form, commonly a

conformational change in a protein Many signal pathways

Include phosphorylation cascades In this process

A series of protein kinases add a phosphate to the next one in line, activating it

Phosphatase enzymes then remove the phosphates

Phosphorylation Cascades

Cascade Utilizing G-proteins

Regulation of Pathways

How does the organism regulate metabolism? Regulatory signals inform individual cells of the

metabolic state of the body as a whole Hormones Nervous system Availability of nutrients

These influence signals that are generated within the cell itself

Signals Within the Cell

Regulatory signals Substrates Allosteric activators Allosteric inhibitors Product Cause a rapid response

Signaling Cascades

The relay chain during transduction Physically transfer the point of reception to the end

point where the cell machinery will make the response Transform the signal into a form that stimulates the

response Amplify the signal received Distribute the signal so that many processes are

influenced Each step is open to interference from other factors so

that the signal of the cell can me modulated according to conditions prevailing inside or outside the cell

Questions

How do cells receive the signal?Where do the cells receive the signal?What do cells do once they have received the

signal?How do cells respond to the signals received?Among all the hundreds of signals being sent,

and received, how do cells determine which signal was sent to them?

“Types” of Communication

Private vs. Public

Private Messages Only a select few cells receive the message

Public Messages Message sent throughout entire body

Long Distance vs. Short Distances

Long Distance Messages Are messages that are sent

over a long path from the point of origin location up until the signal's final spot.**

Short Distance Messages Are messages that are sent

over a short distance from the point of origin up until the signal’s final stop.** Sometimes within the

immediate area.

ExamplePublic & Long Distance

Hormonal Signaling Hormones may be sent throughout the bodies of

plants and animals In animals, hormonal signals are secreted into the

bloodstream In plants, hormonal signals are secreted into plant sap.

Hormones, produced in animals, are produced in endocrine cells and help make up the Endocrine System.

ExamplePublic and Short Distance

Paracrine Signaling Signal molecules diffuse locally through the

extracellular medium Signal remains in the neighborhood of the cell that

secretes them Signal acts as local mediators

Regulate inflammation at sites of infection Regulate cell proliferation in wound healing

Growth factorsStimulate target cells to grow and multiply

• Part of the bigger Immune SystemUsed in the healing process

Private & Short DistanceSynaptic Signaling

Signals sent via the nervous system are sent over long distances. However, the signal must travel along multiple individual nerve cells and the communication between individual nerve cells is called synaptic signaling.

Private & Short DistanceIntimate Messaging

Most short range of all Face to face conversation

No molecules are released Different in comparison to the other three.

Cells make direct contact, through signaling molecules, in their plasma membrane Message is delivered by the binding of a signal molecule

anchored in the plasma membrane of the signaling cell to a receptor molecule embedded in the plasma membrane of the target cell Cell to cell connection

Remember from earlier packet the functions of proteins in cell membrane.

Private & Short DistanceIntimate Messaging II

In embryonic development, this type of signaling has an important role in tissues where adjacent cells that are initially similar have to become specialized in different ways Plays role in determining what type of cell a stem cell

becomes.

Types of Cell to Cell Connection

Gap JunctionsTight JunctionsDesomosomesPlasmodesmata*

CELL TO CELL CONNECTIONS

Intimate Messaging

Gap Junctions

Communicating cell to cell junction that allows ions and small molecules to pass from the cytoplasm of one cell to the cytoplasm of the next Found in animal cells

Tight Junctions

Cell to cell junction that seals adjacent epithelial cells together, preventing dissolved molecules in the extracellular medium from passing from one side of the epithelial sheet to the other

Found in animal cells.

Desmosomes

Specialized to cell to cell junction, usually formed between two epithelial cells, mediated by cadherin molecules and characterized by dense plaques of protein into which intermediate filaments in the two adjoining cells insert

Plasmodesmata

Organelle found in plant cells that allow adjacent cells to communicate, relay signals and move materials between them

Private & Short Distance

Yeast Cells

When a yeast cell is ready to mate, it secretes a small protein called a mating factor to which other yeast cells are sensitive They detect the mating

factor They put out a

protrusion toward the source of the factor

Halt the cell cycle