Even if water is polar a small amount of it can sneak through the plasma

Even if water is polar a small amount of it can sneak

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Even if water is polar, a small amount of it can sneak through the plasma membrane because of its small size. Water also moves freely and reversibly by aquaporins (AQP). Osmosis happens when the water
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concentration vary from the 2 sides of a membrane. -1mM NaCl = 2mOsmol/L -1mM glucose = 1 mOsmol/L - Hypertonic: Pull water out of swollen tissues. - Hypotonic: Rehydrate dehydrated patients. - Isotonic: Keep their normal shape, there’s no loss nor gain of water. Active transport It occurs whenever a cell use energy to move solutes across the membrane. Substance are usually unable to pass in the necessary direction by through passive transport. They may be too large to pass through channels, not able to dissolve in the lipid bilayer, or move against its concentration gradient. There are 2 different passage; Active transport (primary and secondary) and vesicular transport. - Primary active transport (antiport): The energy to do work comes directly from hydrolysis of ATP by transport proteins (pumps). It causes the protein to change its shape to pump the bound solute across the membrane. It includes calcium and hydrogen pumps. The pump protein is an enzyme called Na+ -K+ ATPase. For each molecule of ATP used, the Na+ and -K+ pump drives 3 Na+ out of the cell and pumps 2 K+ back in. As a result, the concentration of K+ inside the cell is higher than the outside, and vice versa for the Na+.
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- Secondary active transport (cotransport) (symport): Uses a cotransport protein to couple the downhill movement of one solute to the uphill movement of another solute. Its concentration gradient is created by the primary active transport. When moving sodium across the plasma membrane against its concentration gradient, the pump stores energy. A substance pumped across a membrane can do work as it goes back downhill along its concentration gradient. EX: When sodium moves back into the cell, other substances are dragged along or cotransported by the same protein. Some sugars, amino acids, and ions are cotransported into cells lining the small intestine. Vesicular transport •Fluids containing large molecules and macromolecules are transported across the cellular membrane in-side bubble-like membranous sacs (vesicles). Transports substances in and out of the cell, they are energized by ATP. - Exocytosis (out): Transport substances from the cell interior into the extracellular fluid. It is stimulated when binding of a hormone to a membrane receptor or a change in membrane voltage. It accounts for hormone secretion, neurotransmitter release, mucus secretion, and ejection of wastes. (1) substance is enclosed in vesicle moves to the plasma membrane. (2) Proteins at the vesicle surface (V- snares) bind with plasma membrane protein (T-snares). (3) The vesicle and plasma membrane combine together and a pore opens up. (4) The substance in the vesicle are released to the cell exterior.
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- Endocytosis (in): Transports bulk solids, macromolecules, and fluids into the cell. Begins with a coated pit (an unfolding of the membrane) that have a protein coating on the cytoplasmic face that deforms the membrane to produce the vesicle.
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  • Fall '08
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