Membrane Transport and Ion Concentrations
Membrane Transport: Principles and Mechanisms
Fundamental Concepts of Membrane Transport
Most Important Ions: The ions with the most significant physiological roles in membrane transport are Na^+ (Sodium), K^+ (Potassium), and Ca^{2+} (Calcium). Ca^{2+} typically has a much stronger concentration gradient compared to Na^+ and K^+ which translates to a larger free energy difference for its movement.
Passive Transport:
This type of transport is thermodynamically favorable, meaning it proceeds spontaneously.
It does not require the cell to expend any energy (ATP).
Substances move down their electrochemical gradients (from high to low concentration or down an electrical potential).
Active Transport:
This type of transport is thermodynamically unfavorable, meaning it requires external energy input to proceed.
It requires the cell to expend energy, typically in the form of ATP hydrolysis or by coupling to another favorable transport process.
Substances move against their electrochemical gradients (from low to high concentration).
Transporters/Carrier Proteins:
These are proteins that physically bind to specific molecules (solutes) and undergo conformational changes to move them across the membrane.
They are slower than channel proteins due to the conformational changes involved.
Channel Proteins:
These are transmembrane proteins that form a pore through which specific molecules can pass.
They move molecules from high to low concentrations without binding to them or using cellular energy.
They are much faster than carrier proteins because they do not require significant conformational changes to transport solutes.
Channels are highly selective for their specific solutes.
Simple Diffusion:
The passive movement of small, nonpolar molecules (e.g., O2, CO2, steroid hormones) directly across the lipid bilayer.
They move down their concentration gradients without the help of membrane proteins.
Plasma Membrane Structure and Permeability
Maintaining Cellular Environment: The cell's plasma membrane is crucial for maintaining a highly specific and distinct internal environment, separate from the extracellular space.
Hydrophobic Interior: The lipid bilayer has a hydrophobic core, which significantly restricts the passage of polar molecules, charged ions, and large uncharged polar molecules.
Permeability Scale (from most to least permeable):
Hydrophobic things: Easily pass through.
Small, uncharged polar molecules (e.g., water, urea, glycerol): Can pass through, but much less frequently than hydrophobic molecules.
Large, uncharged polar molecules (e.g., glucose, sucrose): Rarely pass through.
Ions (any charge): Almost completely impermeable to the lipid bilayer due to their charge and associated water shells; they are essentially