The transport of molecules into and out of cells is essential for life; cells cannot exist in a space void of anything else. The membrane controsl the movement of substances into and out of the cell. There are two main types of membrane transport: passive transport and active transport. These distinctions are based on their respective thermodynamics. Passive tranport is a spontaneous process that does not require energy (negative ΔG), while active transport is nonspontaneous and requires energy (positive ΔG).
In passive transport, small molecules are generally being transported down their concentration gradient. That is, they diffuse from an area of high concentration to an area of low concentration. Diffusion, facilitated diffusion, and osmosis generally increase in rate as temperature increases, though active transport may or may not be affected by temperature. In simple diffusion, substrates move down their own concentration gradients, but only if they are particles that are freely permeable ot the membrane. Some of the gradien’ts energy is dissipated as the gradient is ulilized. Osmosis is a special type of diffusion that specifically concerns water, which moves from a region of lower solute concentration to one of higher solute concentration, in order to maintain a consistent solute concentration. When a solute is impermeable to a membrane, water will move to try to bring solute concetrations to equimolarity. When there is a relatively high concentration of solute inside the cell, water will move from the surroundings into the cell. This siutation is hypotonic. You can use the O present in hypotonic to image a cell swelling like a giant letter O. When there is a relatively low concentration of solutes outside the cell, water from inside the cell will move into the surroundings. This situation is hypertonic. When solutions inside and outside are perfectly balances, as all things should be, the situation is isotonic. It is important to remember that isotonicity does not prevent movement; rather, it prevents the net movement of particles. The driving force behind osmosis is osmotic pressure, which is a colligative property. The equation for osmotic pressure (Π) is Π = iMRT, where M is the molarity of solution, R is the ideal gas constant, T is the absolute temperature in Kelvins, and i is the van’t Hoff factor. The van’t Hoff factor is simply the numer of particles obtained from the molecule when in solution (1 for glucose, 2 for sodium chloride, 3 for calcium chloride, 4 for sodium phosphate Na3Po). Osmotic pressure is proportional to the molarity of solution, so, like all colligative properties, depends only on the presences and number of particles in solution, not their actual identity.
Facilitated Diffusion is just diffusion for molecules unable to pass through the membrane because they are large, polar, or charged, or a combination. Facilitated diffusion requres integral membrane proteins to serva as transporters. Carrier proteins open at one side of the cell membrane at any given point, bind a substrate, and then open on the other side, similar to a revolving door. Binding of the substrate to the carrier results in a conformational change. The occluded state is when the carrier is open to neither side. Channels are also viable transporters for facilitated diffusion and can be either open or closed. GLUT1 is a carrier transporter that facilitates the transport of glucose accros membranes. A cell can change the amount of transport proteins on its membrane by exocytosis and endocytosis of transport proteins stored vesicles inside the cell.
Ion channels are a type of transporter that allows ions to flow down their electro-chemical gradient. Patch-clamping of ion channels reveals the effect of membrane potential this flow has. The potassium channel KcsA is a famous example of an ion channel (It’s the famous potassium leak channel in neurons). It selectively allows potassium ions to pass, but not sodium ions, even though postassium ions are larger in size. X-ray structures of the channel revealed how. It has two distinct part, a selectivity filter and a tunnel through the center of the protein, which was made of vertical alpha helices. A potassium ion (K+) can enter the tunnel and travel 2/3 of the way while reamining solvated in water, but must give up it’s hydration shell when entering the selectivity filter. Sequential C=O bonds, “o-rings,” in the selectivity filter do a good job (thermodynamically) mimicking the hydration shell of K+. However, they do a poor job mimicking the hydration shell of Na+, which holds its water molecules much closer together.
Active transport requires energy and is generally used to move a solute against its concentration gradient. Primary active transport uses ATP or another energy molecule, and generally uses a transmembrane ATPase. An example is the sodium potassium pump, which couples the hydrolysis of ATP to ADP with the expulsion of 3 sodiums (Na+) and inport of 2 potassiums (K+). This creates membrane potentials, which are critical for neurons. Secondary active transport is also known as coupled transport. It involves the coupling of one particle going down its electro-chemical gradient to drive a different particle against its gradient. When both particles flow in the same direction, this is known as symport. When they flow in opposite directions, this is known as antiport. The sodium symporter is used by intestinal epithelial cells to import one glucose against its gradient by coupling it to the import of sodium.