The Importance Of Membrane Transport
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The lipid bilayer makes up the plasma membrane in cells. It consists of a hydrophilic head and a lipophilic tail. This creates a selectively permeable membrane. Only molecules that are non-polar and are small enough to pass through the membrane can travel through the membrane without the help of a transport protein. Anything polar or too large must travel through the membrane through transport proteins. There are two types of transport across the plasma membrane: active and passive. Passive transport includes simple diffusion which is movement of non-polar molecules across the cell membrane across a concentration gradient and it also includes assisted diffusion which is a molecule traveling across a concentration gradient through a protein channel. Active transport is a protein using ATP to move certain ions and molecules across the membrane against its concentration gradient.
Water is a special molecule. It cannot diffuse across the plasma membrane, so it travels through protein channels called aquaporins. Water travels across the membrane depending on a factor called water potential. Water potential is a measure of solute potential and pressure potential combined. Water travels from high water potential to low water potential. In regular cells there is no pressure potential, so water moves depending on solute potential. More solutes create lower solute potential. Water therefore travels from the side of the cell membrane with less solute to the side with more.
Because life has not yet evolved a way to actively transport water, a different system is used in the kidneys of animals to reabsorb the water out of urine. Blood enters the kidney through the renal artery and then enters into the nephron system. The blood enters the glomerulus and Bowmans capsule where the filtrate is forced out of the blood and into the proximal tubule. Here reabsorption occurs and much of the filtrate is reabsorbed back into the blood. The body wants to hold onto the bicarbonate, a valuable buffer, salt, water, and nutrients so all of these things are transported in someway (active or passive transport) back into the blood. However not all of the salt and water is taken out of the filtrate at this point.
Therefore the filtrate travels down the loop of Henle where sodium and chloride ions are actively pumped out of the nephron creating a high water potential inside the nephron and a low water potential outside the nephron. Since water travels from low water potential to high water potential it crosses out of the nephron and into the blood vessels surrounding the nephron.
The rest of the filtrate is then transported around the rest of the nephron where it continues to be reabsorbed and secreted into. By the end most of the water and salt has been removed from the urine and all that is left is metabolic waste in the form of urea, hydrogen ions, and small amounts of other ions.
Roots of plants use a very similar method