Biology Chapter: Cell Membrane and Permeability

Cell Membrane

• Composed of phospholipid bilayer, crucial for cell structure and function.
• Lipid-soluble molecules easily diffuse through, while water-soluble molecules require assistance to cross.
• Impermeable to organic anions such as proteins; permeability is influenced by size, lipid solubility, and charge.

Membrane Permeability

• Permeability indicates if a substance can cross the membrane freely or with assistance.
• Gases can diffuse directly, while polar molecules and ions need specific proteins or channels.

Simple Diffusion

• Occurs for small, lipid-soluble molecules like O2 and CO2, moving down their concentration gradient.
• Passive process requiring no ATP, with diffusion rate proportional to concentration gradient.

Facilitated Diffusion

• Involves carrier proteins assisting in the movement of polar molecules, such as sugars and amino acids, down their concentration gradient.
• Also passive, relying on the concentration gradient for energy.

Active Transport

• Mechanism for moving substances against their concentration gradient via a carrier protein.
• Requires energy from ATP hydrolysis, exemplified by the Na+/K+ pump.

Secondary Active Transport

• Utilizes the energy from one substance's movement down its concentration gradient to drive another substance against its gradient.
• Involves a transporter protein which undergoes conformational changes to facilitate movement.

Channels

• Membrane channels are protein structures forming pores, allowing specific ions to diffuse.
• Selectivity is determined by the size and charge of the molecules allowed through.

Gated Channels

• Channels can open or close based on protein shape changes, regulated by factors like ligand binding or voltage changes.

Ligand Gated Channels

• Respond to the binding of specific chemical ligands, triggering cellular signaling events.

Voltage Gated Channels

• Sensitive to membrane potential; conformational changes allow ion diffusion in response to depolarization.

Endo/Exocytosis

• Endocytosis involves membrane pinching to form vesicles and transport substances into the cell, usually receptor-mediated.
• Exocytosis is the fusion of vesicles with the membrane to release or transport materials outside the cell, essential for signaling.

Exocytosis Types

• Kiss and Run: vesicles fuse temporarily at docking sites, allowing partial content release and repeated use.
• Full Exocytosis: complete vesicle fusion resulting in total content release, critical for high-volume signaling.

Membrane Potential

• Generated by ion concentration gradients and a selectively permeable membrane.
• The Na+/K+ pump actively establishes these gradients, moving 3 Na+ out and 2 K+ in for each ATP consumed.

Resting Membrane Potential (RMP)

• Neurons exhibit a resting potential closer to -70 mV due to higher permeability to K+ ions.
• K+ ions diffuse out, leading to a net negative charge inside the cell, influenced by both concentration and electrical gradients.

Equilibrium Potential

• At equilibrium, the electrical force opposing K+ efflux balances the chemical force driving K+ out, resulting in a stable resting membrane potential.

Explore the essential concepts of cell membranes, including their structure and function. Understand the mechanisms of diffusion and permeability, distinguishing between simple and facilitated diffusion. This quiz covers the crucial aspects of how substances interact with the cell membrane.