Biology: Glucose Transport Mechanism

10 Questions

What is the primary mechanism by which glucose enters cells?

Facilitated diffusion

What is the function of GLUT-1 in erythrocytes and brain?

To facilitate the transport of glucose into the cell

What is the effect of insulin on GLUT-4 transporters in adipose tissue and skeletal muscle?

It increases the number of GLUT-4 transporters

What is the name of the transport system that is dependent on sodium and ATP?

Sodium-glucose cotransport system

Where does the Na+- and ATP-dependent cotransport system occur?

In the epithelial cells of the intestine, renal tubules, and choroid plexus

What is the primary function of the GLUT-3 transporter?

To transport glucose into neurons

What is the name of the glucose transporter that is abundant in erythrocytes and brain?

GLUT-1

How do glucose transporters exist in the cell membrane?

In two conformational states

What happens to the glucose transporter after extracellular glucose binds to it?

It transports glucose across the cell membrane

What is the name of the family of glucose transporters that are involved in the Na+-independent facilitated diffusion transport system?

GLUT-1 to GLUT-14

Study Notes

Secondary Active Transport of Glucose

Glucose is transported against its concentration gradient, from low extracellular concentrations to higher intracellular concentrations, while Na+ is transported down its electrochemical gradient.
This process requires the concurrent uptake of Na+, making the transporter a sodium-dependent glucose cotransporter (SGLT).

Gliflozins and Hyperglycemia

Gliflozins are used to treat hyperglycemia in people with type II diabetes.

Overview of Glycolysis

Glycolysis is a metabolic pathway that breaks down glucose to provide energy (in the form of ATP) and intermediates for other metabolic pathways.
It is employed by all tissues and is at the hub of carbohydrate metabolism, as virtually all sugars can be converted to glucose.

The Fate of Glucose

The fate of glucose varies with physiological conditions, tissues, and organisms.
Exercising muscle cells, for example, use glucose for energy production.

Aerobic Glycolysis

Aerobic glycolysis occurs in cells with mitochondria and an adequate supply of oxygen.
It produces pyruvate as the end product, which is then converted to acetyl CoA for the citric acid cycle.

Reaction 1: Phosphorylation of Glucose

Phosphorylation of glucose is an irreversible process that traps the sugar in the cell, committing it to further metabolism.
Hexokinase is the enzyme that catalyzes the phosphorylation of glucose to glucose 6-phosphate.

Hexokinases

There are four isozymes of hexokinase: hexokinases I-III and hexokinase IV (also known as glucokinase).
Hexokinases I-III have low Michaelis constant (Km) and low maximal velocity (Vmax), allowing for efficient phosphorylation of glucose even at low tissue concentrations.
Hexokinase IV has a high Km and high Vmax, with low affinity for glucose, and is found in liver parenchymal cells and pancreatic β cells.

Glucose Transport into Cells

There are two ways to transport glucose into cells: a sodium (Na+) and ATP-independent transport system, and a (Na+) and ATP-dependent cotransport system.
The sodium-independent system is mediated by a family of at least fourteen glucose transporters (GLUT-1 to GLUT-14) in cell membranes.
The sodium-dependent system involves the cotransport of glucose with sodium and is found in the epithelial cells of the intestine, the renal tubules, and the choroid plexus.

Tissue Specificity of GLUT Gene Expression

Glucose transporters display a tissue-specific pattern of expression, with different isoforms found in different tissues.
For example, GLUT-3 is the primary glucose transporter in neurons, while GLUT-1 is abundant in erythrocytes and brain, but low in adult muscle.