Membrane Permeability and Transport Notes

Membrane Permeability and Transport

Selective Permeability of Membranes

• Molecules and ions move at different rates based on their characteristics.

• Hydrocarbons and Nonpolar Molecules (e.g., Carbon Dioxide Gas $(CO<em>2)$, Oxygen Gas $(O</em>2)$):

  • Move quickly through the membrane.

  • This rapid movement is attributed to the large hydrophobic fatty acid tails of the phospholipids.

• Polar Molecules (e.g., Sugar, Water) and Ions:

  • Move slowly due to their dislike for the hydrophobic fatty acid tails.

• Increasing Membrane Permeability for Polar Molecules and Ions:

  • To allow ions, sugar, and water to move more readily through the membrane, the percentage of unsaturated fatty acids in phospholipids should be increased.

  • A higher percentage of unsaturated fatty acids causes phospholipids to be separated a little more, allowing for greater permeability to sugars and water.

• Molecules and ions may or may not require energy to cross the membrane, which will be discussed further.

Passive Transport

• Definition: Movement across the membrane that does not require cellular energy.

Diffusion

• Mechanism: Driven by the thermal motion (heat) of molecules, causing them to constantly move.

• Movement Pattern: Molecules move randomly in all directions, but the overall net movement is directional, following the concentration gradient.

  • Concentration Gradient: Movement is from an area of high concentration of a solute to an area of low concentration of that solute.

• Example: Spraying perfume in a room. Initially, perfume molecules are concentrated at the front. They will move randomly, but the net movement will be from the front (high concentration) to the back (low concentration) until they are equally distributed.

• Dynamic Equilibrium: Once equally distributed, molecules continue to move in all directions, but the net movement is zero; an equal number of molecules move in each direction.

• Biological Systems: Substances move across cell membranes and organelle membranes.

  • The membrane must be permeable to the specific solute that needs to move.

• Independent Diffusion: When two different solutes are present, each solute diffuses down its own concentration gradient, independent of the other solute.

  • Example: Round solutes and square solutes; round solutes diffuse based on their concentration gradient, and square solutes diffuse based on theirs, without affecting each other.

• Rate of Diffusion: The rate is determined by the steepness of the concentration gradient. A larger difference in concentration (e.g., $12$ to $0$ versus $6$ to $0$) results in a faster rate of diffusion.

Osmosis (Diffusion of Water)

• Definition: The specific diffusion of water across a selectively permeable membrane, moving down its concentration gradient.

• Energy Requirement: No energy is required.

• Water Interaction with Solutes:

  • Water is attracted to and forms tight clusters around hydrophilic solutes (e.g., sugar) through hydrogen bonds ($HB$).

  • This