Membrane+Transport

__**Cellular Membrane Transport **__


 * Cell membranes are made of lipid bilayers that are weakly permeable or impermeable to compounds that are essential for life processes within the cell. Substances can enter or exit through specialized membrane proteins


 * **Thermodynamics: **
 * The free energy change for moving a substance across a membrane depends on the concentration on each side of the membrane. For ions, it depends on the membrane potential.
 * For a substance that cannot diffuse freely across a membrane, transport may be mediated by a protein and may require free energy input. Nonmediated transport is simply diffusion.
 * **Passive**,  or **facilitated diffusion **is when a substance moves from a high concentration to low concentration. Examples include:
 * **Ionophores ** may carry ions or form channels.
 * **Porins ** provide a passageway for ions or nonpolar solutes.
 * **Ion channels ** are highly selective and may be gated. The coordinated opening and closing of ion channels generates an action potential in nerve cells.
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 1.5;">Aquaporins **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 1.5;"> mediate the transmembrane passage of water molecules.
 * **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 1.5;">Transport ****<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 1.5;">proteins **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 1.5;"> may mediate uniport(involves the movement of a single molecule at a time.), symport (simultaneously transports two different molecules in the same direction), and antiport transport (simultaneously transports two different molecules in opposing directions).
 * **<span style="color: #2b2ee3; font-family: 'Times New Roman',Times,serif; font-size: 12pt; line-height: 1.5;">Active transport **<span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 1.5;"> is when a specific molecule is transported from low to high concentration with the use of energy (ATP).
 * <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 1.5;">Pumps use the free energy of ATP to transport ions against their gradient.
 * <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 1.5;"><span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 1.5;">**ABC** (ATP-binding cassette) transporters move amphipathic substances from one side of the membrane to the other. These pumps pump ions, sugars, amino acids, and other polar and nonpolar substances
 * <span style="font-family: 'Times New Roman',serif; font-size: 12pt;">**Secondary active transporters** use existing ion gradients to drive the unfavorable transport of a second substance.
 * <span style="font-family: 'Times New Roman',serif; font-size: 12pt;">The most important example is the **Sodium-Potassium Pump**! <span style="font-family: 'Times New Roman',serif; font-size: 12pt; line-height: 1.5;"> It pumps sodium out and potassium into the cell. This allows animals to control the proper concentration of water in their cells. Without functioning (Na+–K +)–ATPases to maintain a low internal [Na+], water would osmotically rush in to such an extent that animalcells, which lack cell walls, would swell and burst. The electrochemical gradient generated is also responsible for the electrical excitability of nerve cells. All cells expend a large fraction of the ATP they produce, which is up to **__70%__** in nerve cells, to maintain their required cytosolic Na+ and K+ concentrations. So, its s//uper duper important//.


 * **<span style="color: #1885f6; font-family: 'Times New Roman',Times,serif; font-size: 120%;">Endocytosis and Exocytosis: **
 * <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">During **endocytosis**, cells take in substances by forming vesicles. A portion of the membrane engulfs the substance.
 * <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">**Phagocytosis** is mainly in unicellular organisms. It is the engulfing of large things.
 * <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">**Pinocytosis** occurs when vesicles form around a liquid or many small particles. Blood cells, intestinal cells, kidney cells, and plant root cells all use this method.
 * <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">**Receptor-mediated endocytosis** is a form of pinocytosis that is specific because it involves a receptor that recognizes a substance that needs to be transported into the cell. Normally this means hormones, vitamins, and lipoproteins.
 * <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">**Exocytosis** : vesicles form around a substance and secretion occurs. Insulin is a good example.


 * <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">How might this relate to you and your college career (other than keeping you alive)? Are you turning 21 soon?
 * <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">20% absorbed from the stomach to the bloodstream and to the cells, 80% through the small intestine. (Carbonation speeds up the absorption, food slows it down).
 * <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">Alcohol goes to every tissue except fat tissue (Alcohol is not fat soluble), but mostly affects the brain and nerve cells.
 * <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">A note on the //hangover//: Glucose enhances Na resorption which in turn enhances water Resorption. This is why Gatorade has glucose in addition to salt and water. Drinking only water or a salt solution is ineffective since they are rapidly excreted from the gastrointestinal tract. Drinking pedialyte is another good way to rehydrate, however undesirable in taste and it is bad for your reputation since you are not a small child if you are consuming alcohol.

__<span style="font-family: 'Times New Roman',serif; font-size: 12pt;">References: __
 * <span style="font-family: Arial,sans-serif; font-size: 12pt;">• <span style="font-family: 'Times New Roman',serif; font-size: 12pt;">Voet, D.; Voet, J.G.,: Pratt, C.W. //Fundamentals of Biochemistry: Life at the Molecular Level// 3rd ed. Wiley, Hoboken, N.J. 2008
 * <span style="font-family: Arial,sans-serif; font-size: 12pt;">• <span style="font-family: 'Times New Roman',serif; font-size: 12pt;">[]