Concept 11.1: External signals are converted to responses within the cell
process of communication between mating yeast cells.
process of communication between mating yeast cells.

Local and long-distance cell communication in animals.
Cell communication is essential for all organisms. A common subject of cell "conversation" is sex. For example, yeasts identify their mates by using chemical signaling. The two sexes or mating types each secrete a signaling molecule that binds to each others' receptors. These mating factors cause both cells to grow toward each other and bring about cellular changes. Eventually, fusion of the two cells of opposite types occurs. The process by which a cell's surface is converted to a specific cellular response is called a signal transduction pathway. In multicellular organisms, cell communication may be close together or far apart, and signaling molecules are used to target other cells. This sort of signaling can be done by cell-cell recognition, paracrine signaling, or synaptic signaling. Local regulators travel short distances and stimulates nearby target cells to grow and divide. On the other hand, hormones are used for long-distance signaling and occurs in plants and animals.

  • Signal Transduction Pathway- A series of steps linking a mechanical or chemical stimulus to a specific cellular response
    • Ex: A signal transduction pathway is the process by which a signal is converted to a cellular response.
  • Local Regulators- Signaling cells that influence cells near where where it is secreted
    • Ex: Local regulators only travel short distances.
  • Hormones- Secreted chemicals formed in specialized cells used for long-distance signaling
    • Ex: Hormones often diffuse through the air as a gas to reach their targets.

Overview of cell signaling.
Overview of cell signaling.
The 3 stages of cell signaling include:
1.) Reception--> The target cell's detection of a signaling molecule coming from outside the cell. This occurs when a chemical signal is sensed due to the binding of a signaling molecule to a receptor protein.
2.) Transduction--> Converts the signal to a form that can bring about a cellular response.
3.) Response--> The transduced signal triggers a specific cellular response. Ex: catalysis by an enzyme, rearrangement of the cytoskeleton, or activation of specific genes

Concept 11.2: Reception: A signaling molecule binds to a receptor protein, causing it to change shape
A cell that is targeted by a particular chemical signal possesses a receptor protein that recognizes the signal molecule. This recognition takes place when the signal binds to a specific site on the receptor that is complementary in shape to the signal. Such signal molecules act as ligands and can cause the receptor protein to undergo a shape change. Ligand bindings have the ability to activate receptors so that it can interact with other molecules. When it comes to water-soluble signaling molecules, binding takes place on receptor proteins that are embedded in the plasma membrane. Three types of major membrane receptors include: G protein-coupled receptor, receptor tyrosine kinases, and ion channel receptors.
ion channel receptors
ion channel receptors

  • Ligand- A molecule that binds specifically to another molecule
    • Ex: Ligands tend to bind to larger molecules.

  • G protein-coupled receptor- Signal receptor protein that responds to the binding of a signaling molecule by activating a G protein
    • Ex: The G protein-coupled receptor is located inside the plasma membrane.
  • G protein- Protein that relays signals from a G protein-coupled receptor to other signal transduction proteins in the cell
    • Ex: G proteins are GTP-binding proteins.
  • Receptor tyrosine kinases- Receptor proteins in the plasma membrane that can catalyze the transfer of a phosphate group from ATP to a tyrosine on another protein
    • Ex: Receptor tyrosine kinases often respond to the binding of a signaling molecule by phosphorylating tyrosines on the other receptors.
  • Ligand-gated ion channel- Type of membrane receptor that can act as a "gate" when the receptor changes
    • Ex: Ligand-gated ion channels can allow or block the flow of specific ions by opening or closing.
receptor tyrosine kinases
receptor tyrosine kinases
Concept 11.3: Transduction: Cascades of molecular interactions relay signals from receptors to target molecules in the cell

A phsphorylation cascade.
A phsphorylation cascade.

Transduction is the stage in cell signaling that greatly amplifies the signal. It occurs when the binding of a specific signaling molecule to a receptor in the plasma membrane triggers the signal transduction pathway. Once one signal-activated receptor activates a protein, that protein activates another, and so on. It can be thought of as a domino effect. This protein activity is regulated by protein kinases. One single cell can contain hundreds of different protein kinases; each responsible for a different substrate protein. To turn off the signal transduction pathway, protein phosphatases come in. These enzymes deactivate the protein kinases by rapidly removing phosphate groups from proteins, a method called desphosphorylation.

  • Protein kinase- An enzyme that transfers phosphate groups from ATP to a protein
    • Ex: Protein kinases result in the phosphorylation of proteins.
cAMP as a second messenger in a G-protein-signaling pathway.
cAMP as a second messenger in a G-protein-signaling pathway.

  • Protein phosphatases- Enzymes that can rapidly remove phosphate groups from proteins
    • Ex: By inactivating protein kinases, protein phosphatases turn off the signal transduction pathway when there is no signal present.
  • Second messengers- Small, nonprotein, water-soluble molecules or ions that relay a signal to the interior of a cell
    • Ex: Second messengers spread throughout the cell by diffusion because they are small and water-soluble.
  • cyclic AMP/cAMP- A common second messenger in eukaryotic cells made from ATP
    • Ex: A cAMP is a regulator of some bacterial operons.
  • Adenylyl cyclase- Enzyme that converts ATP to cAMP in response to an extracellular signal
    • Ex: Adenylyl cyclases are embedded in the plasma membrane and can catalyze the synthesis of cAMP.
  • Inositol trisphosphate- A second messenger that functions as an intermediate between certain nonsteroid hormones and a third messenger
    • Ex: The pathways leading to calcium release involves inositol trisphosphate.
  • Diacylglycerol (DAG)- A second messenger produced by the cleavage of a certain kind of phospholipid in the plasma membrane
    • Ex: DAG is responsible for the release of calcium from the cell's ER too.

Concept 11.4: Response: Cell signaling leads to regulation of transcription or cytoplasmic activities
The specificity of cell signaling.
The specificity of cell signaling.

All-in-all, a signal transduction pathway results in the regulation of one or multiple cell activities. By turning off specific genes, they ultimately control the synthesis of proteins. On top of that, signaling pathways can regulate proteins activities too. For example, a signal can cause the opening or closing of an ion channel, resulting in a change in the cell's metabolism. Signaling pathways that involve multiple steps have some benefits. They amplify the signal and response, they provide different check-points where a cell's response can be regulated, and they allow specific reactions from cells to signaling pathways. The most crucial part in the response is the termination of the signal.

  • Scaffolding proteins- Large relay proteins to which other relay proteins are simultaneously attached
    • Ex: The efficiency of signal transduction is caused by scaffolding proteins.
Scaffolding proteins are large relay proteins where other relay proteins attach to.
Scaffolding proteins are large relay proteins where other relay proteins attach to.

Concept 11.5: Apoptosis (programmed cell death) integrates multiple cell-signaling pathways

Infected or damaged cells often reach their lifespan and enter apoptosis. During this process, the DNA is chopped up, and the cell shrinks. The cell's remaining parts are packaged up in vesicles and are consumed by specialized scavenger cells. Apoptosis protects neighboring cells from being damaged and is vital in embryonic development. One major pathway of apoptosis is to have apoptotic proteins form molecular pores in the mitochondrial outer membrane. This causes it to leak and release proteins. At a key point during this program, proteins signal from a variety of sources and can send a cell down an apoptotic pathway. Such built-in cell suicide ability is essential for animals to be developed and maintained.
  • Apoptosis- A program of controlled cell suicide
    • Ex: A cell usually enters apoptosis when it has been infected or damaged.
Final stage of apoptosis.
Final stage of apoptosis.

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