Signal Transduction Quiz

Cell Communication Overview

• Cell-to-cell communication is vital for both multicellular and unicellular organisms.
• Scientists have identified common mechanisms for cellular regulation.
• Cells often communicate chemically.
• For instance, the "fight-or-flight" response is triggered by the signaling molecule epinephrine.
• External signals are processed inside the cell.
• Microbes are a resource for understanding the role of cell signaling in evolution.

Evolution of Cell Signaling

• The yeast Saccharomyces cerevisiae has two mating types: a and α.
• Cells of different mating types find each other through specific secreted factors.
• A signal transduction pathway involves a series of steps converting a surface signal into a cellular response.
• Signal transduction pathways translate surface signals into cellular responses.

Local and Long-Distance Signaling

• Cells in multicellular organisms communicate using chemical messengers.
• Animal and plant cells have specialized junctions to connect their cytoplasms.
• Local signaling in animal cells can be triggered by direct contact or recognition.
• In many instances, animal cells use local regulators for communication.
• Plant and animal cells use hormones for long-distance signaling.
• A cell's response depends on whether it has specific receptors for the signal.

Types of Signaling

• Paracrine signaling: A cell releases a signal to influence nearby cells, changing their behaviour.
• Endocrine signaling: Hormones travel through the bloodstream to target cells, causing a slower, sustained response.
• Synaptic signaling: A specialized structure called a synapse connects the signaling cell and receiving cell. This type of signaling occurs only between cells with synapses, such as a neuron and muscle cell.

Three Stages of Cell Signaling

• Earl W. Sutherland discovered how the hormone epinephrine affects cells.
• Cellular responses to signals involve three stages: reception, transduction, and response.

Reception

• Binding between a signal molecule and receptor is highly specific.
• A shape change in the receptor often initiates signal transduction.
• Most signal receptors are plasma membrane proteins.

Receptors in the Plasma Membrane

• Most water-soluble signaling molecules bind to specific sites on receptor proteins that span the plasma membrane.
• There are three main types of membrane receptors: G protein-coupled receptors, receptor tyrosine kinases, and ion channel receptors.

G Protein-Coupled Receptors (GPCRs)

• GPCRs are the largest family of cell-surface receptors.
• A GPCR is a plasma membrane receptor that interacts with a G protein.
• The G protein acts as a molecular switch. If GDP is bound, the G protein is inactive.

Receptor Tyrosine Kinases (RTKs)

• RTKs are membrane receptors that attach phosphates to tyrosines.
• A single RTK can trigger multiple signal transduction pathways.
• Abnormal RTK function is linked to many cancers.

Ligand-Gated Ion Channel Receptors

• These receptors act as gates when a signal molecule (ligand) changes their shape.
• The gate allows specific ions (e.g., Na⁺ or Ca²⁺) through a channel in the receptor.

Intracellular Receptors

• Intracellular receptor proteins are located in the cytosol or nucleus of target cells.
• Small or hydrophobic chemical messengers can readily cross the membrane and activate receptors.
• Steroid and thyroid hormones are examples of hydrophobic messengers.
• An activated hormone-receptor complex acts as a transcription factor to turn on specific genes.

Signal Transduction: Cascades

• Signal transduction usually involves multiple steps.
• Multistep pathways amplify the signal, allowing a few molecules to trigger a large cellular response.
• Multistep pathways offer opportunities for coordination and control of cellular responses.
• Signal transduction relies primarily on protein molecules like dominoes; one triggers another, until the final protein is activated.
• At each cascade step, the signal changes form, typically as a protein shape change.

Protein Phosphorylation and Dephosphorylation

• Many pathways utilize protein phosphorylation cascades.
• Protein kinases transfer phosphates from ATP to a protein, a process called phosphorylation.
• Protein phosphatases remove phosphates from proteins, dephosphorylating them; this system works like a molecular switch, turning activities on and off as required .

Small Molecules and Ions as Second Messengers

• The extracellular signaling molecule (ligand) is the "first messenger".
• Second messengers are small, non-protein water-soluble molecules or ions that spread through a cell by diffusion.
• Second messengers participate in pathways started by GPCRs and RTKs.
• Cyclic AMP (cAMP) and calcium ions are common second messengers.

Cyclic AMP (cAMP)

• cAMP is a frequently used second messenger.
• Adenylyl cyclase, a plasma membrane enzyme, converts ATP to cAMP in response to extracellular signals.

Calcium Ions and Inositol Triphosphate (IP₃)

• Calcium ions (Ca²⁺) are a second messenger in many pathways.
• Cells regulate Ca²⁺ concentration, making it an important second messenger.
• Pathways releasing Ca²⁺ involve IP₃ and diacylglycerol (DAG) as additional second messengers.

Response

• A cell's output response is its reaction to the extracellular signal.
• Signal transduction pathways often regulate the synthesis or activity of enzymes or other proteins, usually by controlling gene expression in the nucleus.
• The final activated molecule in the pathway often acts as a transcription factor.
• Other pathways control enzyme activity.

Apoptosis

• Apoptosis is programmed cell death.
• During apoptosis, cellular components are broken down and packaged in vesicles for digestion by scavenger cells.
• Apoptosis prevents enzymes from leaking out, preventing damage to surrounding cells.
• Apoptosis is important during embryonic development and maintenance of animals.
• Apoptosis can be involved in diseases like Parkinson's and Alzheimer's, and interference with apoptosis can contribute to cancers.

Fine-Tuning of the Response

• Four key aspects fine-tune responses: signal amplification, specificity, efficiency, and termination of the signal.

Signal Amplification

• Enzyme cascades amplify a cellular response.
• Activated product numbers increase significantly at each step in the cascade.

Specificity of Cell Signaling

• Different cells have different protein collections, allowing them to respond differently to signals.
• The same signal may have different effects in cells with different proteins and pathways.
• Pathway branching and cross-talk help cells coordinate incoming signals.

Signaling Efficiency: Scaffolding Proteins

• Scaffolding proteins are large relay proteins to which other relay proteins attach.
• They increase signaling pathway efficiency by bringing together proteins involved in the same pathway.
• They may also activate some of the relay proteins.

Termination of the Signal

• Signal termination mechanisms are essential for cell signaling.
• If ligand concentration decreases, fewer receptors bind.
• Signalling molecules are degraded to terminate the signal.

Test your understanding of signal transduction pathways and their components. This quiz covers the fundamental concepts of cell signaling, including the roles of signaling molecules, receptors, and the various pathways involved in regulating cellular responses. Assess your knowledge and readiness in the field of cellular biology.