Transport Across Membranes: Overcoming the Permeability Barrier

Chapter Overview

• Focus on transport mechanisms across cellular membranes that overcome the permeability barrier.

• Key concepts include:

  • Differences and similarities among transport types:

  • Simple diffusion vs. facilitated diffusion

  • Passive transport vs. active transport

  • Direct active transport vs. indirect active transport

  • The process of osmosis.

Transport Mechanisms and the Permeability Barrier

• Overcoming the permeability barrier of cell membranes is essential for cellular function.

• Specific molecules and ions must be selectively transported into and out of the cell or organelles.

• Biological membranes exhibit selective permeability (also known as semipermeability).

Types of Membrane Transport

Solutes and Mechanisms of Transport

• Solutes include dissolved gases, ions, and small organic molecules.

• Three transport mechanisms are involved:

  1. Simple Diffusion:

  • The direct movement across a membrane driven by concentration gradients.

  • Limited to a few small molecules; most solutes cannot cross this way.

  1. Facilitated Diffusion (Passive Transport):

  • Involves transport proteins (integral membrane proteins) that assist in moving solutes down their concentration gradient without energy expenditure.

  1. Active Transport:

  • Involves energy expenditure to move solutes against their concentration gradient.

  • Typically driven by ATP hydrolysis or simultaneous transport of another solute down its gradient.

Example: Erythrocyte Plasma Membrane

• Oxygen gas readily diffuses across the lipid bilayer into erythrocytes (red blood cells) where it is consumed, and diffuses out in tissues where the concentration is lower.

• Recognize specific molecules for each type of transport mechanism.

Transport Proteins

• Transport Proteins:

  • Large integral membrane proteins with multiple transmembrane segments.

  • Carrier Proteins (Transporters or Permeases):

  • Bind solutes on one side, change conformation, and release solutes on the other side.

  • Channel Proteins:

  • Create hydrophilic channels through the membrane, allowing solute passage.

Facilitated Diffusion Mechanisms

• Carrier proteins alternate between two conformational states to facilitate transport.

Categories of Carrier Transport

• Uniport:

  • A carrier protein transports a single solute across the membrane.

• Coupled Transport:

  • Involves carrying two solutes:

  • Symport (Cotransport): Both solutes move in the same direction.

  • Antiport (Countertransport): Solutes move in opposite directions.

• Transporters involved in these processes are classified as symporters and antiporters.

Active Transport Mechanisms

• Direct Active Transport (Primary Active Transport):

  • The accumulation of solute on one side of the membrane is directly coupled to an exergonic reaction, typically ATP hydrolysis.

  • Transport proteins driven by ATP hydrolysis are termed ATPases or ATPase pumps.

• Indirect Active Transport:

  • Depends on the movement of one solute down its gradient to drive another solute against its gradient.

  • Can be symport or antiport depending on the direction of the solutes.

Examples of Active Transport

• Transport of H extsuperscript{+} requires ATP directly.

• Transport of S requires ATP indirectly.

Osmosis

• Definition:

  • Osmosis is the diffusion of water across a selectively permeable membrane.

• Mechanism:

  • Water moves toward regions of higher solute concentration through a membrane permeable to water but impermeable to solutes.

• Osmotic movement is related to osmolarity, defined as the total solute concentration inside versus outside the cell:

  • Hypertonic: Higher solute concentration outside the cell.

  • Hypotonic: Lower solute concentration outside the cell.

• Osmosis differs from other methods as it specifically involves solvent movement when solutes cannot cross.

• The movement of water can also be viewed as passive transport since it moves from areas of higher concentration (more water) to lower (less water).

Illustrations of Diffusion and Osmosis

• Simple Diffusion:

  • Occurs when the membrane is permeable to the solute.

  • Net movement is from high to low solute concentration until equilibrium is reached.

• Osmosis:

  • Occurs when the solute cannot cross the membrane, leading to water movement from areas of high water concentration (low solute) to low water concentration (high solute).

Effects of Osmosis on Cells

• Cells utilize active transport to maintain size.

• Human red blood cells maintain an isotonic intra-cellular environment with respect to the extracellular environment (blood).