In-Depth Notes on Membranes and Transport Mechanisms

Membrane Structure and Function

Selective Permeability

• Selective permeability allows certain substances to pass through while preventing others.

• Cell membranes are crucial for maintaining the internal environment of the cell.

Phospholipid Bilayer

• Composed of phospholipids that have:

  • Hydrophilic (polar) heads facing the outside (outer face) and inside (inner face) of the cell.

  • Hydrophobic (nonpolar) fatty acid tails that face each other, forming a bilayer.

• Provides structural integrity and flexibility to the membrane.

Membrane Components

• Integral (Intrinsic) Proteins: Embedded within the lipid bilayer, these proteins can span across the membrane.

• Peripheral (Extrinsic) Proteins: Loosely attached to the outer or inner membrane surfaces.

• Cholesterol: Interspersed within the bilayer, it helps to stabilize membrane fluidity.

• Carbohydrate Chains: Attached to proteins and lipids on the extracellular face, assist in cell recognition and signaling.

Types of Transport Mechanisms

Passive Transport

• Facilitated Diffusion:

  • No energy required. Movement is from areas of high concentration to low concentration through transport proteins (tunnels).

• Simple Diffusion:

  • Small nonpolar molecules (O2, CO2, N2) can pass directly through the lipid bilayer.

• Water (H2O): Moves through special channels called aquaporins.

Active Transport

• Requires Energy (ATP): Movement from low to high concentration, which is contrary to natural diffusion.

• Sodium-Potassium Pump (Na-K ATPase): Moves sodium ions (Na+) out and potassium ions (K+) into the cell against their concentration gradients.

• Transport Proteins: Work as pumps to move ions or molecules across membranes.

Glucose Transport

• The human genome contains 14 glucose transporter (GLUT) genes, crucial for glucose uptake in cells.

• Glucose cannot diffuse through the membrane quickly enough, which is why active transporters are necessary.

Special Case: Gore-Tex Membrane

• Gore-Tex: A semi-permeable fabric that acts similarly to biological membranes, allowing water vapor (transpiration) to escape while preventing liquid water from entering.

Summary of Transport Types

• Facilitated Transport: No energy, high to low concentration.

• Active Transport: Energy required, can move substances against concentration gradient.

Examples of Membrane Proteins

• Glucose Transporters (SLC2A family): Facilitate the transport of glucose across cell membranes.

• Sodium and Potassium Channels: Composed of multiple proteins that form a channel for ions.

Importance of Membrane Proteins

• Serve specific functions and pathways; they do not typically perform multiple roles (moonlighting).

• They facilitate communication and substance exchange which is vital for cellular metabolism and function.