Membrane Transport

Understand membrane permeability, diffusion, osmosis, osmolarity and tonicity

• Membrane permeability- A measure of how readily a substance can cross the cell membrane.
• Diffusion- Movement of substances from an area of high concentration to an area of low concentration.
• Osmosis- Movement of water from an area with few solutes to an area containing lots of solutes.
• Osmolarity- Essentially just the concentration of everything that isn't water.
• Tonicity- The effect of osmolarity on the cell. A hypertonic solution will make water diffuse out of the cell, while a hypotonic solution will make water diffuse into the cell.

Note that osmolarity and tonicity, while often treated as being the same thing, are not exactly the same. Osmolarity refers to the concentrations of different substances whereas tonicity refers to the effect that these concentrations have on the movement of water.

Know the osmolarity of ICF and ECF

The osmolarity of both ICF and ECF is roughly 300mOsm/L. (mOsm = milliosmoles)

The ICF and ECF also differ in what kinds of substances they contain. The ICF has higher concentrations of potassium ions and phosphate ions than the ECF, and the ECF has higher concentrations of sodium ions and chloride ions than the ECF. The differing concentrations of ions will become important later as we learn about cell signalling.

Understand passive diffusion, carrier mediated transport, primary and secondary active transport. Understand how coupled transport enables the movement of solutes against an electrochemical gradient

• Passive diffusion- Substances simply diffuse through a membrane or through some kind of pore (always open) or channel (open when encountering a certain stimulus, such as a change in voltage or ligand binding). No additional energy is required as substances simply diffuse down their concentration gradients.
• Carrier-mediated transport- Also known as facilitated diffusion, substances in carrier-mediated transport travel down a concentration gradient, so no energy is required. However, in carrier-mediated transport, substances are transported across via carrier proteins, which bind a substance on one side and then change shape in order to release that substance on the other side. Carriers can be uniporters (carry one substance only), or they can be co-transporters (carry multiple solutes across simultaneously). Co-transporters can be symporters (carry all substances in the same direction) or antiporters (carry substances in opposite directions).
• Primary active transport- Primary active transport uses pumps to transport substances up a concentration gradient, and so energy (often in the form of ATP) is required. The most well-known pump is the Na⁺/K⁺ pump, which maintains ion concentrations all over our body. Another common type of primary active transporter is the ATP-Binding Cassette (ABC) transporter that pumps out small molecules.
• Secondary active transport- Secondary active transport simply uses a concentration gradient that was generated by a primary active transporter in order to drive the action of a carrier-mediated transporter. For instance, the Na⁺/K⁺ pump keeps intracellular sodium concentrations low, so a Na⁺/(insert X molecule here) symport can bring Na⁺ and molecule X into the cell down Na⁺'s concentration gradient.

Understand exo- and endo-cytosis

Exo- and endocytosis are methods of allowing larger molecules to move into or out of the cell. In exocytosis, large molecules are packaged into vesicles in order to be released at the cell surface. Endocytosis is the opposite: it is a process in which large molecules at the cell surface can become packaged into a vesicle and moved into the cell. There are several types of endocytosis, including phagocytosis (moves larger molecules) and pinocytosis (moves smaller molecules and liquids).

Exocytosis can be constitutive, meaning that it happens all the time, or it can be regulated, meaning that certain signals have to occur for it to work. There are many molecules that are needed for exocytosis to be regulated.

After a protein is packaged in the Golgi apparatus, it is released in a vesicle that is coated in several different proteins. These proteins tell the cell where the protein needs to go. For example, clathrin is normally used to move vesicles between the Golgi and plasma membrane. Rabs are another common type of protein that can bind to Rabs effectors on target membranes. Once vesicles reach their target site, v-SNAREs on the vesicles can bind to t-SNAREs on the target membrane, allowing for fusion of the vesicle with the membrane and release of the contents of the vesicle on the other side.