Glucose is a six-carbon sugar that is directly metabolized by cells to provide energy. The cells along your small intestine absorb glucose along with other nutrients from the food you eat. A glucose molecule is too large to pass through a cell membrane via simple diffusion. Instead, cells assist glucose diffusion through facilitated diffusion and two types of active transport.
A cell membrane is composed of two phospholipid layers in which each molecule contains a single phosphate head and two lipid, or fatty acid, tails. The heads align along the inner and outer boundaries of the cell membrane, while the tails occupy the space in between. Only small, nonpolar molecules can pass through the membrane through simple diffusion. The lipid tails reject polar, or partially charged, molecules, which include many water-soluble substances such as glucose. However, the cell membrane is peppered with transmembrane proteins that provide passage to molecules that the tails would otherwise block.
Facilitated diffusion is a passive transport mechanism in which carrier proteins shuttle molecules across the cell membrane without using the cell’s energy supplies. Instead, the energy is provide by the concentration gradient, which means that molecules are transported from higher to lower concentrations, into or out of the cell. The carrier proteins bind to glucose, which causes them to change shape and translocate the glucose from one side of the membrane to the other. Red blood cells use facilitated diffusion to absorb glucose.
Primary Active Transport
The cells along the small intestine use primary active transport to ensure that glucose only flows one way: from digested food to the inside of cells. Active transport proteins use adenosine triphosphate (ATP), the cell’s energy storage molecule, to pump glucose into the cell, either with or against the concentration gradient. The transport proteins are known as ATPase enzymes because they can free a phosphate group from ATP and utilize the resulting energy to do work. Active transport ensures that glucose won’t leak out of small intestine cells during periods of glucose starvation.
Secondary Active Transport
Secondary active transport is another method by which cells import glucose. In this method, a transmembrane protein known as a symporter imports two sodium ions for every glucose molecule it imports. The method doesn't use ATP, but instead relies on the higher concentration gradient of sodium outside the cell relative to the cell interior. The positively charged sodium ions provide electrochemical energy to import glucose with or against the glucose concentration gradient. Secondary active transport is used by cells in the small intestine, heart, brain, kidneys and certain other organs.
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