Cell Membrane Signaling Systems Quiz

Cell Membrane Signaling Systems

• Cells use chemical signals to communicate.
• A sending cell secretes a ligand that binds to a receptor on a target cell.
• Nontarget cells lack the specific receptor for the ligand.
• The binding of a ligand to its receptor triggers a signaling cascade inside the target cell.
• This cascade ultimately leads to a cellular response, such as gene activation or cell division.
• The original intercellular signal is transformed into an intracellular signal.
• Endocrine signaling involves a cell targeting a distant cell through the bloodstream.

Signaling Through Cell-Cell Contact

• Gap junctions (in animals) and plasmodesmata (in plants) directly connect neighboring cells.
• These channels allow small signaling molecules (intracellular mediators) to pass between cells.
• This allows groups of cells to coordinate their response to a signal.
• Cells can also bind to each other through complementary proteins on their surfaces.
• This interaction can change the shape of the proteins, transmitting a signal.

Signal Transduction

• Signal transduction is the cellular response to an external stimulus.
• Ligand binding to receptors on the plasma membrane often activates kinase domains within the cell.
• Kinase cascades occur when kinases activate a series of reactions similar to a waterfall.
• A well-known example is the MAPK (mitogen-activated protein kinase) pathway, which is activated by growth factor signaling.
• Growth factor receptors act as kinases, triggering a cascade that ultimately promotes cell growth and division.
• The MAPK cascade involves Raf, MEK, and ERK kinases.
• Proto-oncogenes are genes encoding proteins involved in cell division, and their overactivation can lead to cancer.

Second Messengers

• Second messengers are small, non-protein molecules that relay signals initiated by ligand binding.
• Examples include Ca2+ ions, cyclic AMP (cAMP), and inositol phosphates.

Calcium Ions

• The concentration of Ca2+ in the cytosol is low, and it is actively pumped out of the cell.
• Ca2+ is often stored in compartments like the endoplasmic reticulum.
• In some signaling pathways, ligand binding opens ligand-gated calcium ion channels, allowing Ca2+ to flow into the cytoplasm, increasing its concentration.
• Ca2+ binds to specific proteins, altering their shape and activity.
• This can lead to different cellular responses depending on the cell type, such as insulin release or muscle contraction.

Cyclic AMP (cAMP)

• Adenylyl cyclase converts ATP to cAMP, removing two phosphates and forming a ring structure.
• cAMP activates protein kinase A (PKA), which phosphorylates target proteins to pass along the signal.
• PKA has different target proteins in different cell types, leading to diverse cellular responses.

IP3

• Inositol triphosphate (IP3) is produced from phospholipids.
• IP3 can rapidly diffuse into the cytosol and bind to IP3 receptors, which open to release Ca2+ from the endoplasmic reticulum.
• This allows for signal transduction from the plasma membrane to the intracellular calcium stores.
• IP3 signaling, along with Ca2+, plays a role in diverse cellular processes, including smooth muscle contraction and enzyme secretion.

Diacylglycerol (DAG)

• Diacylglycerol (DAG) stays in the plasma membrane and activates protein kinase C (PKC).
• PKC then phosphorylates its own targets.
• DAG signaling is involved in the activation of enzymes, ultimately leading to processes like DNA transcription.