2.(a)(ii) Differentiate between facilitated and passive diffusion across the membrane with examples.

Understanding Membrane Transport

Membrane transport refers to the collection of mechanisms that regulate the passage of solutes, such as ions and small molecules, through biological membranes. This regulation is a critical aspect of cell survival and function.

Passive Diffusion (Simple Diffusion)

Passive diffusion, also known as simple diffusion, is the simplest form of membrane transport. It involves the spontaneous movement of substances directly across the lipid bilayer of the cell membrane from an area of higher concentration to an area of lower concentration. This movement occurs down the concentration gradient and does not require the assistance of any membrane transport proteins or the expenditure of cellular energy (ATP).

• Characteristics:
  • Non-selective: Any molecule capable of dissolving in the lipid bilayer can cross.
  • No protein involvement: Molecules pass directly through the phospholipid bilayer.
  • No energy required: Driven solely by the kinetic energy of molecules and the concentration gradient.
  • Rate dependent on: Concentration gradient, molecule size, lipid solubility, and temperature.
• Examples:
  • Gases: Oxygen (O₂) and Carbon Dioxide (CO₂) easily diffuse across the cell membrane in and out of cells, respectively, driven by their partial pressure gradients (concentration gradients) during gas exchange in the lungs and tissues [9, 11].
  • Small, uncharged polar molecules: Water (H₂O) and ethanol can also diffuse across the plasma membrane, though water's movement is often more specifically referred to as osmosis [21].
  • Hydrophobic molecules: Benzene and other lipid-soluble substances readily pass through the lipid bilayer [21].

Facilitated Diffusion

Facilitated diffusion is a type of passive transport where molecules move across a biological membrane down their concentration gradient with the assistance of specific transmembrane integral proteins (channel proteins or carrier proteins). While it still follows the concentration gradient and does not directly consume ATP, the presence of these proteins facilitates the movement of molecules that are too large, polar, or charged to pass directly through the lipid bilayer.

• Characteristics:
  • Selective: Transport proteins are specific for the molecules they transport.
  • Protein involvement: Requires channel or carrier proteins embedded in the membrane [1, 7].
  • No direct energy required: Still driven by the concentration gradient; however, the proteins act as a bypass mechanism [1].
  • Saturation kinetics: The rate of transport is limited by the number of available transport proteins and can become saturated at high substrate concentrations [1].
  • Can be regulated: The activity of transport proteins can be modulated (e.g., by hormones or electrical signals), allowing the cell to control the rate of transport.
• Examples:
  • Glucose transport: Glucose, a large polar molecule, cannot directly cross the lipid bilayer. It is transported into cells (e.g., red blood cells, muscle cells, brain cells) via glucose transporter proteins (GLUTs) down its concentration gradient [1, 5].
  • Amino acid transport: Similar to glucose, amino acids are transported into cells by specific carrier proteins (amino acid permeases) [5, 13].
  • Ion channels: Ions such as Na⁺, K⁺, Ca²⁺, and Cl⁻, being charged, cannot cross the hydrophobic lipid bilayer. They move through specific ion channels embedded in the membrane, for example, in nerve impulse transmission [1, 13].
  • Water channels (Aquaporins): While water can undergo some simple diffusion, its rapid movement across certain membranes (e.g., kidney tubules) is greatly enhanced by aquaporin channels [4].

Key Differences Between Facilitated and Passive Diffusion

The following table summarizes the main distinctions between facilitated and passive diffusion:

Feature	Passive Diffusion (Simple Diffusion)	Facilitated Diffusion
Energy Requirement	No metabolic energy (ATP) required.	No metabolic energy (ATP) directly required.
Protein Involvement	No membrane proteins involved. Molecules pass directly through the lipid bilayer.	Requires specific transmembrane proteins (carrier proteins or channel proteins).
Specificity	Non-specific; depends on molecule size and lipid solubility.	Highly specific; proteins bind to or allow passage of only certain molecules or ions.
Saturation	Does not exhibit saturation; rate increases linearly with concentration gradient.	Exhibits saturation; rate becomes maximal when all transport proteins are occupied.
Rate of Transport	Generally slower for larger, polar, or charged molecules.	Faster for specific molecules, especially those that cannot cross by simple diffusion.
Direction of Movement	Always down the concentration gradient.	Always down the concentration gradient.
Regulation	Generally not regulated by the cell directly (except indirectly by influencing gradients).	Can be regulated by the cell (e.g., by controlling protein synthesis, activity, or localization).
Typical Molecules	Small, nonpolar molecules (O₂, CO₂, N₂), lipid-soluble substances, very small uncharged polar molecules (e.g., ethanol).	Large polar molecules (glucose, amino acids), charged ions (Na⁺, K⁺, Ca²⁺, Cl⁻), water (via aquaporins).