2. Passive Transport: Going with the Flow
Passive Transport is the movement of molecules across the plasma membrane down a concentration gradient—that is, from an area of higher concentration to an area of lower concentration. Because this movement is “downhill,” it does not require the cell to expend any energy. Think of it like a ball effortlessly rolling down a hill.
2.1. Simple Diffusion: The Easy Pass
Simple diffusion is the most straightforward form of passive transport, where substances move directly across the lipid bilayer without any help. This “easy pass” is reserved for molecules with specific properties.
Molecules that use simple diffusion include:
- Small nonpolar molecules (e.g., oxygen (O₂) and nitrogen (N₂))
- Small, uncharged polar molecules (e.g., water (H₂O), carbon dioxide (CO₂), and glycerol)
These molecules can slip through the membrane’s lipid core because they are either nonpolar (and thus compatible with the fatty acid tails) or small and uncharged enough to pass through without being repelled.
2.2. Facilitated Diffusion: Getting a Helping Hand
What about molecules that can’t easily cross the lipid bilayer, like charged ions and large polar molecules? They need assistance. Facilitated diffusion provides this help through specialized membrane proteins. While it still requires no energy and follows the concentration gradient, this process is much faster and more specific than simple diffusion. Two main types of proteins act as helpers:
| Protein Type | Mechanism of Action | Examples of Molecules Transported |
| Ion Channel Proteins | Form small, specific aqueous pores that allow certain molecules to pass through. Think of it as a dedicated tunnel. | Specific ions (e.g., K⁺ through leak channels) and water (through specialized channels called aquaporins). |
| Carrier Proteins | Bind to a specific molecule and undergo a reversible shape change to move it across the membrane. Think of it as a revolving door. | Ions and large polar molecules. Crucially, carrier proteins are also the key players in active transport. |
Among the ion channels, K⁺ leak channels are the most common. These ungated channels are always open, allowing K⁺ ions to “leak” out of the cell down their concentration gradient. This leakage is the single most important factor in establishing the electrical potential difference, or voltage, across the plasma membrane.
To exert even finer control, many other ion channels are gated. These gates open or close only in response to specific signals, including:
- Voltage-gated: A change in the electrical potential difference across the membrane.
- Mechanically gated: A mechanical stimulus, such as touch or pressure.
- Ligand-gated: The binding of a specific signaling molecule or ion.
While passive transport elegantly manages downhill movement, cells often need to move substances in the opposite direction, which requires an entirely different, energy-intensive approach.