5.2 Passive Transport

Passive Transport

Passive transport is the movement of molecules across cell membranes without the cell spending any energy (ATP). Instead, molecules move down their concentration gradient, from areas of high concentration to areas of low concentration. This process is fundamental to how cells exchange gases, absorb water, and maintain their internal environment.

Concept of Passive Transport

Passive transport is driven by three types of gradients: differences in concentration, pressure, and electrical charge across a membrane. Molecules naturally move from where they're more concentrated to where they're less concentrated.

This movement continues until equilibrium is reached, meaning the concentration of a substance is equal on both sides of the membrane. At equilibrium, molecules are still moving in both directions, but there's no net movement in either direction. Think of it like a room where people walk through two doors at equal rates: people are still moving, but the number in each room stays the same.

Diffusion and Osmosis

Diffusion is the net movement of molecules from a region of high concentration to a region of low concentration. It happens because of random molecular motion: molecules are constantly bouncing around and colliding, and over time this random movement spreads them out evenly. A classic example is oxygen and carbon dioxide exchange in the lungs, where each gas diffuses down its own concentration gradient.

Osmosis is a specific type of diffusion: the movement of water across a selectively permeable membrane. Water moves from areas of high water potential (low solute concentration) to areas of low water potential (high solute concentration). For example, root hair cells absorb water from soil because the soil solution has a higher water potential than the cell's cytoplasm.

Types of Passive Transport

There are two main types:

• Simple diffusion: Small, nonpolar molecules pass directly through the phospholipid bilayer without any help. Oxygen ($O_2$) and carbon dioxide ($CO_2$) cross membranes this way.
• Facilitated diffusion: Larger or polar molecules still move down their concentration gradient, but they need help from membrane proteins to cross. No energy is required; the proteins just provide a pathway. This involves two kinds of proteins:
  • Carrier proteins bind to a specific molecule and change shape to shuttle it across the membrane. Each carrier is selective for a particular substance.
  • Channel proteins form a pore (tunnel) through the membrane that allows specific molecules or ions to flow through. Aquaporins, for example, are channel proteins that speed up the movement of water.

Tonicity and Cell Behavior

Tonicity describes the relative solute concentration outside a cell compared to inside. It determines the direction of water movement by osmosis and directly affects cell size and shape.

• Isotonic solution: Solute concentration is equal inside and outside the cell. No net water movement occurs, so the cell maintains its normal shape. Red blood cells in blood plasma are in an isotonic environment.
• Hypotonic solution: Solute concentration is lower outside than inside the cell. Water enters the cell by osmosis, causing it to swell. In animal cells, this can lead to lysis (bursting). A freshwater amoeba lives in a hypotonic environment and must actively pump out excess water to survive.
• Hypertonic solution: Solute concentration is higher outside than inside. Water leaves the cell, causing it to shrink. In animal cells this is called crenation. In plant cells, the plasma membrane pulls away from the cell wall, a process called plasmolysis.

Factors Affecting Diffusion Rate

Several factors speed up or slow down passive transport. You should understand each one and why it has its effect.

Concentration gradient steepness A steeper gradient (bigger difference between the two sides) drives faster diffusion. As molecules spread out and the gradient decreases, the rate of net movement slows. Picture a drop of dye in water: it spreads rapidly at first, then more slowly as it approaches an even distribution.

Molecular size and mass Smaller, lighter molecules diffuse faster because they have higher average speeds at a given temperature and encounter less resistance from the surrounding medium. Hydrogen gas ($H_2$) diffuses much faster than glucose ($C_6H_{12}O_6$), for instance.

Temperature Higher temperatures increase the kinetic energy of molecules, making them move faster and collide more often. This speeds up diffusion. That's why tea brews faster in hot water than in cold.

Solvent density Denser solvents create more resistance, slowing diffusion. Molecules diffuse more slowly through syrup than through water. Inside cells, dehydration can increase cytoplasmic density and impair normal diffusion, which is one reason severe dehydration disrupts cell function.

Solubility and polarity This determines whether a molecule can cross the membrane by simple diffusion. Lipid-soluble, nonpolar molecules (like steroid hormones and $O_2$) pass easily through the hydrophobic core of the phospholipid bilayer. Water-soluble, polar, or charged molecules (like glucose, amino acids, and ions) cannot cross without transport proteins.

Surface area and membrane thickness Greater surface area means more contact points for molecules to cross, increasing the overall rate of diffusion. Thinner membranes reduce the distance molecules must travel. The body exploits both of these principles:

• Lung alveoli have an enormous total surface area and extremely thin walls for rapid gas exchange.
• Microvilli in the small intestine dramatically increase the surface area available for nutrient absorption.
• Fish gills have thin membranes and large surface areas for efficient oxygen uptake from water.

Distance Longer distances slow diffusion because molecules take more time to travel from one point to another through random motion. This is a key reason cell size is limited: as a cell grows, its volume increases faster than its surface area ($\text{volume} \propto r^3$ while $\text{surface area} \propto r^2$). Eventually, diffusion alone can't supply the cell's interior fast enough. Some large cells compensate with adaptations like membrane folds or multiple nuclei, as seen in skeletal muscle fibers.

Membrane Permeability

The rate of passive transport also depends on the membrane itself. Two main features control permeability:

• Phospholipid composition: Membranes with more unsaturated fatty acid tails are more fluid and generally more permeable. Cholesterol modulates fluidity, preventing the membrane from becoming too fluid or too rigid.
• Transport proteins: The number and type of channel and carrier proteins in a membrane determine how quickly specific polar or charged molecules can cross. A cell can regulate its permeability to particular substances by adding or removing these proteins.