Hormone binding sites receptor proteins Proteins embedded in the membrane, which bind to specific hormones. When the hormone binds, it causes the receptor protein to undergo a conformational change, which signals the cell to perform a function.
Fructose digestion and absorption in humans[ edit ] Figure 4: Hydrolysis of sucrose to glucose and fructose by sucrase Figure 5: Intestinal sugar transport proteins Fructose exists in foods either as a monosaccharide free fructose or as a unit of a disaccharide sucrose.
Free fructose is absorbed directly by the intestine. When fructose is consumed in the form of sucrose, it is digested broken down and then absorbed as free fructose.
As sucrose comes into contact with the membrane of the small intestine, the enzyme sucrase catalyzes the cleavage of sucrose to yield one glucose unit and one fructose unit, which are then each absorbed.
After absorption, it enters the hepatic portal vein and is directed toward the liver.
The mechanism of fructose absorption in the small intestine is not completely understood. Some evidence suggests active transportbecause fructose uptake has been shown to occur against a concentration gradient. Since the concentration of fructose is higher in the lumen, fructose is able to flow down a concentration gradient into the enterocytesassisted by transport proteins.
It appears that the GLUT5 transfer rate may be saturated at low levels, and absorption is increased through joint absorption with glucose. In addition, fructose transfer activity increases with dietary fructose intake. Fructose malabsorption Several studies have measured the intestinal absorption of fructose using the hydrogen breath test.
When fructose is not absorbed in the small intestine, it is transported into the large intestine, where it is fermented by the colonic flora. Hydrogen is produced during the fermentation process and dissolves into the blood of the portal vein. This hydrogen is transported to the lungs, where it is exchanged across the lungs and is measurable by the hydrogen breath test.
The colonic flora also produces carbon dioxide, short-chain fatty acidsorganic acids, and trace gases in the presence of unabsorbed fructose. Uptake of fructose by the liver is not regulated by insulin. However, insulin is capable of increasing the abundance and functional activity of GLUT5 in skeletal muscle cells.
Fructolysis The initial catabolism of fructose is sometimes referred to as fructolysisin analogy with glycolysisthe catabolism of glucose. In fructolysis, the enzyme fructokinase initially produces fructose 1-phosphatewhich is split by aldolase B to produce the trioses dihydroxyacetone phosphate DHAP and glyceraldehyde .
Unlike glycolysisin fructolysis the triose glyceraldehyde lacks a phosphate group. A third enzyme, triokinaseis therefore required to phosphorylate glyceraldehyde, producing glyceraldehyde 3-phosphate. The resulting trioses are identical to those obtained in glycolysis and can enter the gluconeogenic pathway for glucose or glycogen synthesis, or be further catabolized through the lower glycolytic pathway to pyruvate.
Metabolism of fructose to DHAP and glyceraldehyde[ edit ] The first step in the metabolism of fructose is the phosphorylation of fructose to fructose 1-phosphate by fructokinase, thus trapping fructose for metabolism in the liver.
Fructose 1-phosphate then undergoes hydrolysis by aldolase B to form DHAP and glyceraldehydes; DHAP can either be isomerized to glyceraldehyde 3-phosphate by triosephosphate isomerase or undergo reduction to glycerol 3-phosphate by glycerol 3-phosphate dehydrogenase.
The glyceraldehyde produced may also be converted to glyceraldehyde 3-phosphate by glyceraldehyde kinase or further converted to glycerol 3-phosphate by glycerol 3-phosphate dehydrogenase.
The metabolism of fructose at this point yields intermediates in the gluconeogenic pathway leading to glycogen synthesis as well as fatty acid and triglyceride synthesis.
Synthesis of glycogen from DHAP and glyceraldehyde 3-phosphate[ edit ] The resultant glyceraldehyde formed by aldolase B then undergoes phosphorylation to glyceraldehyde 3-phosphate.
Increased concentrations of DHAP and glyceraldehyde 3-phosphate in the liver drive the gluconeogenic pathway toward glucose and subsequent glycogen synthesis. Metabolic conversion of fructose to glycogen in the liver Synthesis of triglyceride from DHAP and glyceraldehyde 3-phosphate[ edit ] Carbons from dietary fructose are found in both the free fatty acid and glycerol moieties of plasma triglycerides.
High fructose consumption can lead to excess pyruvate production, causing a buildup of Krebs cycle intermediates. Metabolic conversion of fructose to triglyceride in the liver Potential health effects[ edit ] Weight gain[ edit ] In a meta-analysis of clinical trials with controlled feeding — where test subjects were fed a fixed amount of energy rather than being allowed to choose the amount they ate — fructose was not an independent factor for weight gain; however, fructose consumption was associated with weight gain when the fructose provided excess calories.
Fructose consumed before a meal may reduce the glycemic response of the meal.AP Biology Lab: Osmosis and Potatoes What was the initial task? 1.
To determine the isotonic point of a sucrose solution and a potato 2. To determine an unknown concentration of a . Jan 27, · Potato Osmolarity Again Hi, after doing a diffusion/osmosis lab, I am being asked to determine the osmolarity of a the potato cores.
We recorded the cores' initial mass, placed them in sucrose solutions of varying molarities, and then weighed them one day later.
Osmosis Lab Introduction: Human blood, at % salt concentration, is a little less salty than seawater, which has a salt concentration of about 35 parts per thousand (%). If we take seawater as an example of a solution, the salt is called the solute (the particles that are dissolved) and the water is the solvent (the liquid that dissolves.
To study the effects of osmosis on potato's. To find which sucrose solutions will be hypotonic or hypertonic. Three Types of Solutions A hypertonic solution, the solution will have a higher solute concentration than the cell, resulting in water leaving it.
Dear Dr. Russell, I have sinusitis or allergic rhinitis and its been with me for probably more than 6 month now. Should I start using the saline rinses including the Manuka Honey in the saline recipe?
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