5.3 Passive Transmembrane Transport
KEY CONCEPTS
By the end of this section, you will be able to do the following:
- Compare and contrast the mechanisms of simple and facilitated diffusion, and which types of solutes can move across membrane in these ways.
- Evaluate the factors that affect the rates of simple and facilitated diffusion.
- Understand the difference between osmolarity and tonicity, and their related terms (e.g., hyperosmotic, hypertonic).
- Explain how osmolarity and tonicity impact osmosis in cells.
In passive transport, solutes move across a membrane via diffusion, meaning the solutes move down (with) their concentration gradient (neutral solutes) or electrochemical gradient (charged solutes). All solutes that can directly cross the membrane (without a protein channel or carrier) can engage in passive transmembrane transport – a form of transport that requires no direct energy input from the cell. In addition, many charged and polar solutes can cross membranes via passive transport if the appropriate protein channels or carriers exist in that membrane. We will discuss several types of passive transport in this section, including simple diffusion, facilitated diffusion, and osmosis.
Simple Diffusion
Simple diffusion is a type of passive transmembrane transport in which solutes pass directly through the phospholipid bilayer from an area of high solute concentration to an area of low solute concentration until dynamic equilibrium is reached. Many gases, hydrophobic molecules, and some small very small polar molecules (e.g., water) freely move across a membrane down their concentration gradients (review Figure 5.14).
Several factors affect the rate of simple diffusion. First is the extent of the concentration gradient. The greater the difference in concentration, the more rapid the diffusion. The closer the distribution of the material gets to equilibrium, the slower the diffusion rate. Second is the mass of the solutes diffusing. Heavier molecules move more slowly; therefore, they diffuse more slowly. Third is the solubility of the substance. Nonpolar or lipid-soluble materials pass through membranes more easily than polar materials, allowing a faster diffusion rate. The final factor is the surface area and thickness of the membrane. Greater surface area increases the diffusion rate, whereas a thicker membrane reduces it.
Facilitated Diffusion
In facilitated diffusion, materials diffuse across the membrane with the help of transmembrane proteins such as the channels and carrier proteins discussed in Chapter 5.2. The types of solutes that cross membranes via facilitated diffusion included polar molecules (e.g., sugars, amino acids) and charged particles (ions) that are usually repelled by the cell membrane’s hydrophobic core (review Figure 5.14). Some molecules like water (H2O) can actually cross membranes via simple or facilitated diffusion if the appropriate channels (e.g., aquaporins) are present in that membrane. Transport protei