Answer: Fructose is transported by facilitated diffusion.
Explanation:
Glucose is a monosaccharide with the molecular formula C6H12O6.3 It is a hexose. It is a form of sugar that is found freely in fruits and in honey. Its energy yield is 3.75 Kcal/g under standard conditions.
Fructose is a type of carbohydrate found in vegetables, fruits, and honey. It is a monosaccharide with the same molecular formula as glucose, C6H12O6, but with a different structure, that is, it is an isomer of glucose. Its energetic power is the same as that of glucose, 4 kilocalories per gram, and it is a reducing carbohydrate.
Glucose is the main energy substance of a cell and for its entry it requires a transport protein in the cell membrane, called transporter. The transport of glucose through the cell membrane is carried out by two families of membrane proteins:
- Sodium-coupled glucose transporters (SGLT sodium-glucose-transporters), which involves the co-transport of Na+ by the SGLT-1 by maintaining the Na+ gradient thanks to the Na+/K+ pump
- Glucose transport facilitation proteins (GLUT)
On the other hand, fructose is absorbed by another type of transporter, called Glut-5, a protein that crosses the membrane 12 times. Finally, the passage of both glucose and fructose into the blood takes place through the Glut-2, a transporter with low affinity and high transport capacity.
If any Glut is considered within the context of a large family of proteins, it can be immediately noticed that they all possess common characteristics that in biochemical terms are called "molecular signature of glucose transporters" and that it is no more than a set of extremely conserved primary amino acidic sequences that determine secondary and tertiary structures (domains or motifs) that are responsible for the functional characteristics of the protein.
<u>Facilitated diffusion is a type of cellular transport where the presence of a carrier or transporter (integral protein, Glut is this example</u>)<u> is necessary for substances to cross the membrane.</u> It happens because the molecules are larger or insoluble in lipids and need to be transported with the help of membrane proteins. So, in the first step, fructose binds to the transport protein, and this changes shape, allowing the passage of this sugar. In this way, fructose concentrations inside the cell are always very low, and the external and internal concentration gradient favours diffusion.
Summarizing, fructose is transported by facilitated diffusion. So if we replaced glucose with fructose we would have seen no change in each conditions. And since fructose is transported by this type of mechanism, there is not a concentration gradient.
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