The process by which carbohydrates are made available to cells in the body begins with the movement of food from the mouth through the esophagus into the stomach and lastly into the small intestine where the final steps for chemical digestion and absorption occur. Digestion is the chemical or mechanical breakdown of nutrients whereas absorption is movement of nutrients in their simplest form into the body. Absorption occurs primarily in the small intestine. Mechanical breakdown is the physical breakdown of food which involves grinding food with teeth or the muscular layers of the stomach. Chemical breakdown requires enzymes to break bonds such as the breakdown of polysaccharides into monosaccharides. Effective carbohydrate digestion allows monosaccharides such as glucose to be absorbed into intestinal epithelial cells called enterocytes. Glucose is found in many foods even those that do not taste sweet. Glucose is a large polar molecule and therefore cannot passively diffuse across a cell membrane without transport membrane proteins. Once monosaccharides like glucose have passed through enterocytes they will diffuse into the extracellular fluid (ECF) and finally into the blood via facilitated diffusion. Lastly they will travel through the blood to the cells throughout the body such as skeletal muscle cells to produce energy or energy stores called ATP and glycogen respectively. In order for glucose to enter the bloodstream it must first make its way from the lumen of the small intestine into the epithelial cells (enterocytes) and then be transported from the enterocytes into the bloodstream. The sodium glucose-linked transporter (SGLT1) is the primary transporter used for glucose absorption from the lumen into the enterocytes. As indicated by its name the SGLT1 moves both sodium and glucose at the same time making it a symporter. Like most symporters the SGLT1 relies on the work of the Na+/K+ pumps to keep the intracellular sodium levels low. Once glucose enters the enterocyte it will diffuse into the ECF and capillaries through a glucose transporter protein called GLUT2 which moves glucose by facilitated diffusion. Figure 1 details this process and also notes where Na+ and glucose concentrations are high and low. Figure 1. Glucose absorption in epithelial cells lining the lumen of the small intestine. Glucose malabsorption is a disease in which epithelial cells lining the small intestine are impaired in their ability to transport glucose from the lumen of the small intestine into the epithelial cell; a necessary first step in the use of glucose for energy in the cell. The disease typically becomes apparent in the first few weeks of life and is characterized by severe diarrhea weight loss and a decreased pH of the blood (acidosis). This disease changes the Na+/Glucose symporter protein rendering it completely or partially impaired (Figure 2). Figure 2. Glucose-galactose malabsorption disease impairs the ability of the SGLT1 glucose-galactose/Na+ symporter to transport glucose across the membrane of the epithelial cell. Fructose absorption is not impaired in this disease. Note: this figure is similar to Figure 1 rotated 180 degrees. As a result individuals with this disease must reduce or abstain completely from glucose consumption relying instead on other sources of sugar such as fructose.
Questions 1) Based off of the information provided in Figure 1 identify the transport of Glucose and Na+ as passive or active transport in the table below. Location Glucose movement Na+ movement Intestinal Lumen to Epithelial cell Epithelial cell to Bloodstream Active
2) According to Figure 1 Na+ concentrations are low inside the epithelial cell and high in the bloodstream and the cell is able to maintain this through the use of the Na+/K+ ATP pump. Using this information and Figure 1 predict where K+ concentrations are high and low. In the space below the table provide justification for your answers. Location K+ Concentration (High or Low) Epithelial cell Bloodstream
3) In the intestinal lumen glucose concentrations are lower than inside the epithelial cell yet the transport of glucose into the epithelial cell is passive and does not require ATP. Provide an explanation for how glucose is able to move into the epithelial cell without addition of energy in the form of ATP?
4) Doctors instruct individuals with glucose/galactose malabsorption disease to abstain from all lactose-containing products. Using Figure 2 explain why. Glucose uptake After digestion glucose circulates in the blood. When blood glucose levels are high insulin is secreted. Insulin is a hormone produced by the pancreas needed for the uptake of glucose by the liver fat tissue and muscle cells. In order for the liver fat tissue and muscle to move glucose into their cells there must be glucose transporter proteins embedded in the plasma membrane such as GLUT4. The liver fat and muscle cells have a signaling system to know when glucose should be allowed into their cells. When insulin binds to insulin receptors it sends a signal within the cell to embed GLUT4 in the plasma membrane. Figure 3. Liver fat and muscle cells can uptake glucose in the presence of insulin. GLUT4 transporters are embedded in vesicle membranes and translocated to the surface of the cell
5) Type II diabetes is characterized by insensitivity of body cells to insulin due to a sustained increase in blood sugar concentrations. Using Figure 1 predict the effect that high blood sugar concentrations would have on glucose absorption in the epithelial cells.
6) Does the translocation of GLUT4 require energy? Using Figure 3 explain your answer.
7) The movement of glucose through GLUT4 is what specific type of transport? Using Figure 3 explain your answer.
Requirements: answer all the question
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