June S. answered • 09/14/19
Professor Physiology
All cell membranes are made up of proteins (boats, escape hatch boats, and submarines, which are not discussed in this paper) floating in a bilayer sea of phospholipids (water above and below an oil slick), where the hydrophilic portions of the phosphate heads face the watery environments found both outside and inside the cell. The oil soluble lipid tails of the phospholipids are sandwiched in between the polar phosphate heads. This arrangement allows large nonpolar molecules, like cholesterol and small gaseous molecules like oxygen and carbon dioxide to pass through the phospholipid portion of the membrane by diffusion (diffusion is like perfume, once you open the bottle the odor goes from the higher concentration of the bottle to the lower concentration of the room eventually). All cells also contain transport proteins (escape hatch boats), and if energy is expended (the boat trawler is turned on), and active transport occurs (the trawler net can bring fish to the boat, from low concentration of fish through the boat where there is a high concentration of fish, and to the port); the sodium ion and potassium ions can go against their concentration gradients to establish sodium/potassium extracellular/intracellular normalcy (which means the boat also has an energy powered net that can catch a flying fish, concentrate them through the boat and throw them through the water covered oil slick to the water below the surface, where there are few flying fish). Glycoproteins and glycolipids are also found in cellular membranes for cell recognition and cell attachment (boats need to recognize friendly flags and sometimes need to tie up to each other).
Some cells, like the cells found in the collecting ducts of kidney nephrons (tiny filtering units of the kidney), contain aquaporins (tiny straws that go through water on top of the oil slick to water below the oil slick), which open under antidiuretic hormone (the captain's command) to keep the body from dehydrating in a process of facilitative diffusion (big straws). Other cells like the beta cells of the pancreas exercise exocytosis by coalescing insulin filled lipid vesicles with the plasma membrane to release insulin into the blood when blood sugar is too high (bulk transport of big kelp bags from the boat's underwater door through the oil slick to the water above). Endocytosis is the same mechanism but in the opposite direction, both pinocytosis (bags of pure water from the dock go through the oil slick via the boat's underwater door to the water below and eventually to where it is needed) and phagocytosis (big bag of food from dock through the oil slick door to the water underneath and eventually to the fish) are types of endocytosis. Phagocytosis is exercise by white blood cells, especially neutrophils when engulfing bacteria. Osmosis usually occurs, when water follows sodium ions. In the kidney nephron, mostly at the proximal convoluted tubules (pct), 70% of water is reabsorbed by the body in between pct cells from the filtrate when sodium ions undergo active transport by ATP transport proteins in the pct cells, taking sodium ions from the nephron filtrate to the capillaries around the the pct cells (the captain turns the engine on the boat to lower straws through the oil slick). Osmosis itself is a type of passive transport (perfume example). When red blood cells (RBCs) are put in hypotonic solution (like distilled water), they will burst due to osmosis . When RBCs are put into hypertonic solutions, they will crenate (shrink) due to osmosis (the movement of water from a higher concentration of water in the cell to a lower concentration of water out side the cell). The red blood cells will retain their normal size when RBCs are put in isotonic solution (blood plasma). In the illustration of the red blood cells in plasma, osmosis occurs when active transport is not operative in order to reach isotonic equilibrium (salt/sugar/solute balance) if hypertonic (too salty or sugary of a solution) and hypotonic (too watery of a solution) conditions exist.
