10.3 Membrane transport and signaling

Membrane transport and cell signaling are crucial for cellular function. These processes allow cells to move molecules across membranes and respond to external stimuli. From passive diffusion to active transport, cells use various mechanisms to maintain homeostasis and communicate.

Understanding these concepts is essential in biological chemistry. They explain how cells interact with their environment, regulate internal conditions, and coordinate activities. These processes are fundamental to many physiological functions and play key roles in drug development and disease treatments.

Membrane Transport

Passive Transport Mechanisms

• Passive transport moves molecules across the membrane without requiring energy input
• Diffusion is the movement of molecules from high concentration to low concentration driven by the concentration gradient
• Osmosis is the diffusion of water across a semipermeable membrane from a region of high water concentration to a region of low water concentration
• Facilitated diffusion uses carrier proteins to transport specific molecules across the membrane down their concentration gradient without energy input

Active Transport and Membrane Proteins

• Active transport moves molecules against their concentration gradient using energy input, typically from ATP hydrolysis
• Ion channels are membrane proteins that allow specific ions to pass through the membrane down their electrochemical gradient
  • Gated ion channels open or close in response to specific stimuli such as ligand binding, voltage changes, or mechanical stress
  • Examples of ion channels include sodium channels, potassium channels, and calcium channels
• Carrier proteins bind to specific molecules and undergo conformational changes to transport them across the membrane
  • Uniporters transport one molecule at a time (glucose transporters)
  • Symporters transport two different molecules in the same direction (sodium-glucose cotransporter)
  • Antiporters transport two different molecules in opposite directions (sodium-calcium exchanger)
• The sodium-potassium pump is an example of primary active transport that uses ATP to pump sodium ions out of the cell and potassium ions into the cell, maintaining the electrochemical gradient

Cell Signaling

Receptor Types and Signal Transduction

• Cell signaling involves the transmission of signals from the extracellular environment to the interior of the cell, leading to specific cellular responses
• G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors that transduce signals via guanine nucleotide-binding proteins (G proteins)
  • Ligand binding to GPCRs causes a conformational change that activates the associated G protein, which then activates downstream effector molecules
  • Examples of GPCRs include beta-adrenergic receptors, odorant receptors, and light-sensitive rhodopsin in the eye
• Enzyme-linked receptors are cell surface receptors with intrinsic enzymatic activity or associated with intracellular enzymes
  • Ligand binding to enzyme-linked receptors leads to the activation of the receptor's enzymatic activity or the associated enzyme, initiating intracellular signaling cascades
  • Examples of enzyme-linked receptors include receptor tyrosine kinases (insulin receptor) and receptor serine/threonine kinases (TGF-beta receptor)
• Signal transduction is the process by which the cell converts the external signal into a specific cellular response through a series of molecular events

Second Messengers and Signaling Cascades

• Second messengers are small, diffusible molecules that relay signals from receptors on the cell surface to target molecules inside the cell
• Common second messengers include cyclic AMP (cAMP), calcium ions (Ca2+), and inositol trisphosphate (IP3)
  • cAMP is produced by the enzyme adenylyl cyclase in response to GPCR activation and activates protein kinase A (PKA)
  • IP3 is produced by the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) and triggers the release of Ca2+ from the endoplasmic reticulum
• Signaling cascades amplify the initial signal through a series of sequential molecular events, often involving the activation of protein kinases
  • Mitogen-activated protein kinase (MAPK) cascades are a common example of signaling cascades that regulate cell growth, differentiation, and stress responses
  • The phosphorylation of proteins by kinases in the cascade leads to the activation or inhibition of downstream effector molecules, ultimately resulting in changes in gene expression or cellular activity