5.1 Passive transport: diffusion and osmosis

Passive Transport Mechanisms

Passive transport moves molecules across cell membranes without any energy input from the cell. Instead, these processes rely on concentration gradients to drive movement. Simple diffusion and osmosis are the two main types, and together they allow cells to exchange gases, water, nutrients, and waste with their surroundings.

Passive Transport Mechanisms

Process of Simple Diffusion

Simple diffusion is the net movement of molecules from a region of high concentration to a region of low concentration. This movement is driven by the concentration gradient, and it requires no energy from the cell.

At the molecular level, molecules are always moving randomly. But when there's a higher concentration on one side, more molecules happen to move toward the lower side than the other way around. The result is net movement down the gradient until concentrations equalize.

What can cross by simple diffusion?

• Small, nonpolar molecules pass directly through the phospholipid bilayer. This includes $O_2$ and $CO_2$, which is how gas exchange works between cells and their environment.
• Larger or polar molecules like glucose and amino acids cannot cross by simple diffusion. They need help from transport proteins (that's facilitated diffusion, a related but distinct process).

Concept of Osmosis

Osmosis is the movement of water across a selectively permeable membrane. Water moves from a region of high water potential (low solute concentration) to a region of low water potential (high solute concentration). The driving force is the difference in solute concentration on each side of the membrane.

Think of it this way: water moves toward wherever there's more solute, as if trying to dilute the more concentrated side.

Osmosis has two major effects on cells:

• Cell volume changes. In a hypotonic solution, water enters the cell and it swells. In a hypertonic solution, water leaves and the cell shrinks.
• Solute concentration changes. As water enters a cell, it dilutes the internal solutes. As water leaves, the remaining solutes become more concentrated.

Types of Osmotic Solutions

Understanding these three solution types is essential for predicting what happens to a cell in a given environment.

• Hypotonic solution — Lower solute concentration than the cell's interior. Water moves into the cell by osmosis, causing swelling. If the cell can't regulate its volume, it may undergo lysis (bursting). A classic example: red blood cells placed in pure water will lyse.
• Hypertonic solution — Higher solute concentration than the cell's interior. Water moves out of the cell, causing shrinkage. In animal cells, this shriveling is called crenation. In plant cells placed in a salt solution, the plasma membrane pulls away from the cell wall in a process called plasmolysis.
• Isotonic solution — Same solute concentration as the cell. There's no net movement of water, so the cell maintains its normal volume and shape. Blood cells in 0.9% saline (normal saline) are in an isotonic environment.

Factors Affecting Diffusion Rate

Five main factors determine how fast diffusion occurs:

1. Concentration gradient — A steeper gradient (bigger difference between the two sides) means a faster rate of diffusion. As the gradient decreases, diffusion slows.

2. Membrane permeability — The more permeable the membrane is to a given molecule, the faster that molecule diffuses across. Permeability depends on the molecule's size, charge, and polarity, as well as whether specific transport proteins (like ion channels) are present. Lipid-soluble molecules cross more easily than charged or polar ones.

3. Temperature — Higher temperatures increase the kinetic energy of molecules, so they move faster and diffusion rates go up. This relationship is sometimes described by the $Q_{10}$ coefficient, which measures how much a rate changes with a 10°C increase.

4. Molecular size and shape — Smaller molecules diffuse faster than larger ones. For example, $H_2O$ diffuses much more rapidly than glucose. More compact (spherical) shapes also diffuse faster than elongated ones.

5. Pressure — Higher pressure on one side of a membrane can push molecules toward the lower-pressure side, increasing the diffusion rate. This is particularly relevant in gas exchange in the lungs, where pressure differences help drive $O_2$ into the blood.

Cellular Osmotic Regulation

Cells don't just passively accept whatever osmotic conditions they're in. They have mechanisms to actively control their volume and internal composition.

Osmotic Regulation Mechanisms

Aquaporins are channel proteins embedded in the membrane that are specifically designed for water transport. They're selectively permeable to water molecules (excluding solutes based on size), and they allow cells to move large amounts of water very quickly. Cells in the kidney's renal collecting duct, for example, use aquaporins to rapidly adjust water reabsorption in response to changing conditions.

Ion pumps and transporters maintain the cell's osmotic balance by controlling ion concentrations on each side of the membrane.

• The $Na^+/K^+$ ATPase is a key example. It pumps 3 sodium ions out of the cell and 2 potassium ions in per cycle, using one ATP. This pump maintains the resting membrane potential (about $-70$ mV in nerve cells) and helps regulate cell volume by keeping intracellular ion concentrations in check. Note that this pump itself uses energy, so it's an active transport mechanism, but it supports osmotic regulation by setting up the ion gradients that influence water movement.
• Other transporters also contribute, including $Cl^-$ channels and $H^+/K^+$ antiporters, each fine-tuning the ionic environment inside the cell.