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Introduction
The simplest forms of transport across a membrane are passive. Passive transport does not require the cell to expend any energy and involves a substance diffusing down its concentration gradient across a membrane. A concentration gradient is a just a region of space over which the concentration of a substance changes, and substances will naturally move down their gradients, from an area of higher to an area of lower concentration. In cells, some molecules can move down their concentration gradients by crossing the lipid portion of the membrane directly, while others must pass through membrane proteins in a process called facilitated diffusion. Here, we’ll look in more detail at membrane permeability and different modes of passive transport.
Selective permeability
The phospholipids of plasma membranes are amphipathic : they have both hydrophilic (water-loving) and hydrophobic (water-fearing) regions. The hydrophobic core of the plasma membrane helps some materials move through the membrane, while it blocks the movement of others.
Structure of a phospholipid, showing hydrophobic fatty acid tails and a hydrophilic head. A bilayered membrane consisting of phospholipids arranged in two layers, with their heads pointing out and their tails sandwiched in the middle, is also shown. Image modified from OpenStax Biology. Polar and charged molecules have much more trouble crossing the membrane. Polar molecules can easily interact with the outer face of the membrane, where the negatively charged head groups are found, but they have difficulty passing through its hydrophobic core. Water molecules, for instance, cannot cross the membrane rapidly (although thanks to their small size and lack of a full charge, they can cross at a slow rate). Additionally, while small ions are the right size to slip through the membrane, their charge prevents them from doing so. This means that ions like sodium, potassium, calcium, and chloride cannot cross membranes to any significant degree by simple diffusion, and must instead be transported by specialized proteins (which we’ll discuss
stored (potential) energy, and this energy is used up as the concentrations equalize. Image showing the process of diffusion across the plasma membrane. Initially, the concentration of molecules is higher on the outside. There is net movement of molecules from the outside to the inside of the cell until the concentrations are equal on both sides. Image credit: OpenStax Biology, modified from original work by Mariana Ruiz Villareal. Molecules can move through the cell’s cytosol by diffusion, and some molecules also diffuse across the plasma membrane (as shown in the picture above). Each individual substance in a solution or space has its own concentration gradient, independent of the concentration gradients of other materials, and will diffuse according to that gradient. Other factors being equal, a stronger concentration gradient (larger concentration difference between regions) results in faster diffusion. Thus, in a single cell, there can be different rates and directions of diffusion for different molecules. For example, oxygen might move into the cell by diffusion, while at the same time, carbon dioxide might move out in obedience to its own concentration gradient.
Facilitated diffusion
Some molecules, such as carbon dioxide and oxygen, can diffuse across the plasma membrane directly, but others need help to cross its hydrophobic core. In facilitated diffusion , molecules diffuse across the plasma membrane with assistance from membrane proteins, such as channels and carriers. A concentration gradient exists for these molecules, so they have the potential to diffuse into (or out of) the cell by moving down it. However, because they are charged or polar, they can't cross the phospholipid part of the membrane without help. Facilitated transport proteins shield these molecules from the hydrophobic core of the membrane, providing a route by which they can cross. Two major classes of facilitated transport proteins are channels and carrier proteins.
Channels
Channel proteins span the membrane and make hydrophilic tunnels across it, allowing their target molecules to pass through by diffusion. Channels are very selective and will accept only one type of molecule (or a few closely related molecules) for transport. Passage through a channel protein allows polar and charged compounds to avoid the hydrophobic core of the plasma membrane, which would otherwise slow or block their entry into the cell.
their shape to move a target molecule from one side of the membrane to the other. Diagram showing how a carrier protein can bind a target molecule on one side of the membrane, undergo a shape change, and release the target molecule on the other side of the membrane. Image modified from "Scheme facilitated diffusion in cell membrane," by Mariana Ruiz Villareal (public domain). Like channel proteins, carrier proteins are typically selective for one or a few substances. Often, they will change shape in response to binding of their target molecule, with the shape change moving the molecule to the opposite side of the membrane. The carrier proteins involved in facilitated diffusion simply provide hydrophilic molecules with a way to move down an existing concentration gradient (rather than acting as pumps). Channel and carrier proteins transport material at different rates. In general, channel proteins transport molecules much more quickly than do carrier proteins. This is because channel proteins are simple tunnels; unlike carrier proteins, they don’t need to change shape and “reset” each time they move a molecule. A typical channel protein might facilitate diffusion at a rate of tens of millions of molecules per second, whereas a carrier protein might work at a rate of a thousand or so molecules per second.
