Plasma Membrane Transport Mechanisms: Passive Transport and Diffusion

The Plasma Membrane: Primary Function is Transport Across the Membrane

Overview of Membrane Transport

The plasma membrane regulates the movement of substances into and out of the cell, maintaining homeostasis. Transport across the membrane can be classified as either passive or active based on energy requirements and direction relative to concentration gradients.

• Passive Transport: Movement of substances down their concentration gradient without the use of cellular energy (ATP).

• Active Transport: Movement of substances against their concentration gradient, requiring energy input from the cell (usually ATP).

Key Point: The permeability of the plasma membrane to a substance determines whether and how it can cross the membrane.

Semi-Permeable vs. Selectively Permeable Membranes

• Semi-permeable: Allows certain molecules or ions to pass through by diffusion but not others.

• Selectively permeable: The membrane "selects" which substances can cross, often based on size, charge, or solubility.

Section 3.4: Passive Membrane Transport

Diffusion of Molecules Down Their Concentration Gradient

Passive membrane transport involves the movement of molecules from areas of higher concentration to areas of lower concentration, a process known as diffusion. This process does not require cellular energy.

Main Types of Passive Transport

There are three main types of passive transport across the plasma membrane:

1. Simple Diffusion

   • Definition: The movement of solute molecules from an area of higher solute concentration to an area of lower solute concentration, as long as the solute is permeable to the membrane.

   • Hydrophobic substances (e.g., O2, CO2, steroid hormones) are generally permeable and can cross the phospholipid bilayer easily.

   • Hydrophilic substances (e.g., ions, glucose) are not automatically permeable and may require channels or carriers to cross the membrane. Permeability depends on the size and polarity of the substance.

   • Concentration Gradient: The difference in concentration across the membrane is the driving force for simple diffusion.

   Example: Oxygen diffusing from the lungs into the blood.

2. Osmosis

   • Definition: The movement of water across a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration.

   • Water can move through the lipid bilayer (slowly) and more efficiently through specialized protein channels called aquaporins.

   • Osmosis is essential for maintaining cell volume and fluid balance.

   Example: Water moving into plant root cells from the soil.

Key Terms and Concepts

• Concentration Gradient: The difference in the concentration of a substance between two areas. Substances naturally move "down" their concentration gradient (from high to low concentration).

• Permeability: The property of a membrane that determines which substances can pass through it.

• Hydrophobic: "Water-fearing"; substances that are nonpolar and can dissolve in lipids, thus crossing the membrane easily.

• Hydrophilic: "Water-loving"; substances that are polar or charged and do not cross the lipid bilayer easily without assistance.

Summary Table: Types of Passive Transport

Type	Substance Moved	Energy Required?	Direction	Membrane Proteins Needed?
Simple Diffusion	Small, nonpolar molecules (e.g., O2, CO2)	No	Down gradient	No
Osmosis	Water	No	Down water gradient (toward higher solute)	Yes (aquaporins) or no (slowly through bilayer)

Key Equations

• Fick's Law of Diffusion: The rate of diffusion ($J$) is proportional to the concentration gradient ($\Delta C$), the surface area ($A$), and the permeability coefficient ($P$): $J = P \times A \times \Delta C$

Additional info: Facilitated diffusion, another type of passive transport, involves membrane proteins that help specific molecules cross the membrane without energy input. It is not explicitly mentioned in the provided notes but is commonly covered in this context.