6.3: Transport across membranes

As we have said before (and will say again), the living cell is a historically continuous, and ancient non-equilibrium system. To maintain its living state both energy and matter have to move into and out of the cell. This leads us to consider intracellular and extracellular environments and the properties of the boundary membrane that separates them. The differences between the regions inside and outside of the plasma membrane are profound. Outside, even for cells within a multicellular organism, the environment is generally mostly water, with relatively few complex molecules. Inside the membrane-defined space is the cytoplasm, a highly concentrated (300 to 400 μg/μl) solution of proteins, nucleic acids, smaller molecules, and thousands of interconnected chemical reactions.^[237]

A lipid bilayer membrane poses a barrier to the movement of molecules. For larger molecules, particles or other organisms it also acts as a physical barrier. Typically when larger molecules, particles (viruses), and other organisms enter a cell, they are first engulfed by the membrane (process 1 known as endocytosis)(→).^[238] A superficially similar process, exocytosis, runs in “reverse” (process 3 →) and is involved in moving molecules to the cell surface and releasing them into the extracellular space. Both endocytosis and exocytosis involve membrane vesicles emerging from or fusing into the plasma membrane. These processes leave the topology of the cell unaltered; a molecule within a vesicle is still “outside” of the cell, or at least outside of the cytoplasm. These movements are driven by various protein-based molecular machines (considered in more specialized cell biology courses). There are also processes that directly move molecules through the membrane (process 2 ↑).

How does the membrane “decide” which molecules to allow into or out of the cell? If we think about it, there are three possible mechanisms (can you think of others?) Molecules could move on their own through the membrane, driven by Brownian motion. Alternatively, their diffusion-based movement could be mediated by specific “carriers” or “channel” molecules. Finally they could be moved directionally using a “pump”, an energy dependent process involving coupled reactions. In the majority of cases, these carriers, channels, and pumps are protein-based molecular machines. The types of carriers, channels, and pumps that are present determine the types of molecules that move through the cell’s membrane, as well as their net flux into or out of the cell. We can think of this molecular movement as a reaction:

\[\left[\text { Molecule }{ }_{\text {outside }}\right] \rightleftharpoons\left[\text { Molecule }_{\text {inside membrane }}\right] \rightleftharpoons\left[\text { Molecule }_{\text {inside cell }}\right]\nonumber\]

As with standard chemical reactions, movements through membranes involve an activation energy; this includes the energy needed to remove a water soluble molecule from aqueous solution and then for it to pass through the membrane. Ask yourself why does the membrane, particularly the hydrophobic region of the membrane, pose a barrier to the movement of hydrophilic molecules. The answer involves the difference in the free energy of the moving molecule within an aqueous solution, including the hydrophilic surface region of the membrane, where H-bond type electrostatic interactions are common between molecules, and the hydrophobic region of the membrane, where only LDF-mediated interactions are present. The situation is exacerbated for charged molecules, since water molecules are typically organized in a dynamic shell around ions.

The identity of the moving molecules does not change during the transport reaction. If the concentrations of the molecules are the same on both sides of the membrane, then their Gibbs free energies are also equal, the system will be in equilibrium with respect to this reaction. In this case, as in the case of chemical reactions, there will be no net flux of molecules across the membrane, but molecules will continue to move back and forth at equal rates. The rate at which they move will depend on the size of the activation energy associated with moving across the membrane as well as the concentrations of the molecules on each side of the membrane. Molecules can be concentrated within a cell, but that requires coupling molecular movement to another, energy-dependent reaction.

To think about how molecules cross lipid membranes, let us begin with water itself, which is small and uncharged, although polarized. Typically, the concentration of water outside of a cell is greater than the concentration of water inside a cell. This implies that the reaction: \[\mathrm{H}_2 \mathrm{O} \text { outside } \rightleftharpoons \mathrm{H}_2 \mathrm{O} \text { inside }\nonumber\]

will be favorable; there will be a net flux of water molecules into the cell. What is happening in this reaction? As a water molecule moves through water, H-bonds are broken and reform - there is no net energetic change. In contrast, when a water molecule begins to leave the aqueous phase the H-bonds between it and neighboring water molecules must be broken but no new H-bonds form as the molecule enters the hydrophobic (central) region of the membrane. This asymmetry in H-bonding results in water molecules being “pulled back” into the water phase (←). In this video of a water molecule is moving through a membrane; can you explain why the water molecules moves faster in the hydrophobic region than in water?

The \(\text { Water }_{\text {outside }} \rightleftharpoons \text { Water }_{\text {inside }}\) reaction's activation energy (→) involves breaking H-bonding interactions between water molecules and the membrane's hydrophilic lipid head domains. Thermal movement is generally sufficient to break these H-bonds. Once they enter the membrane's hydrophobic region, water molecules pass through more easily, since only weak LDFs are involved.
Small non-polar molecules, such as O₂ and CO₂ can readily pass through biological membranes. There is more than enough energy available through collisions with other molecules (thermal motion) to provide the energy needed to overcome the activation energy involved in leaving the aqueous phase and passing through the molecular domains of the membrane. As with water, there are often differences in the free energies of the molecules on the inside and outside of the cell. Consider animals that depend upon O₂ (obligate aerobes). The [O₂] outside of the cell (produced by plants as a waste product) is carried throughout the organism's interior by its circulatory system. After O₂ enters the cell, it takes part in the reactions of respiration (considered soon), leading to a decrease in the O₂ concentration gradient, [O₂]_outside > [O₂]_inside. The result is a net flux of O₂ into the cell.

Another perspective into membrane behavior involves considering the interactions of different types of molecules within a bilayer membrane. If a molecule is hydrophobic (non-polar) it will be more "soluble" (concentrated) in the membrane’s central hydrophobic region than in the surrounding aqueous environment (←). A totally hydrophobic molecule will accumulate within the membrane; its entropic effects on water structure will oppose its entry to the aqueous phase.

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