Lipids move laterally across the membrane with ease. For example, lipid rafts. One method for measuring this lateral diffusion is Fluorescence Recovery After Photobleaching (FRAP).
- Label the surface of membrane with fluorescence
- Expose a small region of the membrane to a laser that bleaches the fluorescence, creating a dark spot surrounded by a sea of fluorescence.
- Measure the time for that dark spot to re-fluoresce because of lateral diffusion
- Graph this time to determine the diffusion coefficient D.
The diffusion equation: s= 2√Dt, where s is the average distance traveled
A lipid, for example, can travel across a bacterium in about one second.
The diffusion rate for proteins is much more variable. A photoreceptor protein diffuses at the roughly the same rate as a lipid, but in a red blood cell the chloride bicarbonate exchanger diffuses very slowly. This is because it interacts with a dense network of proteins just below the membrane surface called a membrane skeleton.
Functions of membrane proteins
The concentration of protein in a membrane varies cell-to-cell, organelle-to-organelle. For example, the inner membrane of a mitochondrial is about 70% protein due to its function: the transportation of electrons during oxidative phosphorylation from one complex to another. It also contains the proteins needed for the synthesis for ATP. Schwann cells (attached to neurons), on the other hand, have very little protein since their main function is that of an insulator—the job of lipids, not proteins.
The composition of proteins varies too. In RBCs, there’s a wide range of proteins serving various functions; whereas, in rod cells there’s only one protein, the photoreceptor.
How proteins associate with membranes
Integral Membrane Proteins: embedded in the membrane. The portion of the receptor that passes through the membrane is hydrophobic. On the cytoplasmic side, it’s hydrophilic.
Peripheral Protein: not embedded. Found both inside and outside of the membrane. Recall the G Trimeric Protein, which was attached to the membrane via non-covalent bonds to other membrane proteins and via covalent bonds to membrane lipids.
In addition to acting as transductors of signals from one side of a membrane to the other, proteins are also responsible for the permeability of membranes.
Membranes are semi-permeable. In particular, they’re impermeable to ions and other polar molecules, molecules that can only traverse the membrane via membrane proteins called pumps and channels.
The control of membrane permeability drives function
Example: the inner membrane of the mitochondria needs to be impermeable to protons in order to create the gradient of protons needed for the synthesis of ATP. These channels are also critical to neurons as they maintain their electrical balance via passage of Na and K ions.
Water is the exception. First, there is a high concentration of water outside the cell putting considerable pressure on the membrane. Then, because water is so small compared to the lipids, it simply sneaks through the membrane despite the membrane’s hydrophobic core.
There’s also a special channel protein for water called an aquaporin. Water molecules travel across this channel by interacting with the protein surface inside the channel. There are variations on these aquaporin that allow other small polar molecules through, such as glycerol.
Since water is critical for cellular function, none of this should be surprising.