Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen atoms. The family of carbohydrates includes both simple and complex sugars. Glucose and fructose are examples of simple sugars, and starch, glycogen, and cellulose are all examples of complex sugars. The complex sugars are also called polysaccharides and are made of multiple monosaccharide molecules. Polysaccharides serve as energy storage (e.g., starch and glycogen).

During digestion, carbohydrates are broken down into simple, soluble sugars that can be transported across the intestinal wall into the circulatory system to be transported throughout the body.

Transport of Glucose from GIT to Cells:

Glucose is transported into cells through specialized membrane proteins called glucose transporters. Because glucose is a large and polar molecule, it cannot freely pass through the lipid bilayer of the cell membrane. Therefore, cells use specific transport mechanisms to bring glucose inside for metabolism.

There are two mechanism for transportation of Glucose to the cell.

(1) GLUT Proteins (Facilitated Diffusion) mechanism
(2) Sodium-Glucose Cotransport (Secondary Active Transport)Mechanism.

Transport Glucose by GLUT Proteins (Facilitated Diffusion)

Glucose is transported into cells through specialized membrane proteins called glucose transporters. Because glucose is a large and polar molecule, it cannot freely pass through the lipid bilayer of the cell membrane. Therefore, cells use specific transport mechanisms to bring glucose inside for metabolism.

Most cells take up glucose through a family of transport proteins known as GLUT (Glucose Transporter) proteins. These transporters move glucose from an area of higher concentration (blood) to lower concentration (inside the cell) without using ATP directly. This process is called facilitated diffusion.

Some important GLUT transporters are:

• GLUT1 – found in red blood cells and brain; provides basal glucose uptake.
• GLUT2 – present in liver and pancreatic β-cells; transports glucose in both directions.
• GLUT3 – mainly in neurons; has high affinity for glucose.
• GLUT4 – found in skeletal muscle and adipose tissue; regulated by insulin.

Sodium-Glucose Cotransport (Secondary Active Transport)

In intestinal cells and kidney tubules, glucose is transported by SGLT (Sodium-Glucose Linked Transporter) proteins.

Here, glucose enters the cell along with sodium ions. This process uses the sodium gradient, created by the Na⁺/K⁺ ATPase pump, so it is called secondary active transport.

Pathways for Glucose Metabolism:

Glycolysis:

Glycolysis is the major metabolic pathway for the breakdown of glucose and occurs in the cytoplasm (cytosol) of all cells. In this pathway, one molecule of glucose is converted into two molecules of pyruvate through a sequence of enzyme-catalyzed reactions.

During the process, a small amount of energy is released in the form of ATP and NADH. Glycolysis does not require oxygen directly, therefore it can occur under both aerobic and anaerobic conditions.

It is the first stage of cellular respiration and provides intermediates for several other metabolic pathways.

TCA Cycle:

Citric acid cycle essentially involves the oxidation of acetyl CoA. to CO₂ and H₂O. This is also called krebs cycle. The citric acid cycle is the final common oxidative pathway for carbohydrates, fats and amino acids.

This cycle not only supplies energy but also provides many intermediates required for the synthesis of amino acids, glucose, heme etc.

Gluconeogenesis:

Gluconeogenesis is the metabolic process by which glucose is synthesized from non-carbohydrate precursors such as lactate, glycerol, and glucogenic amino acids. It mainly occurs in the liver and, to a lesser extent, in the kidneys.

This pathway helps maintain blood glucose levels during fasting, starvation, or prolonged exercise. It is essentially the reverse of glycolysis, with certain bypass reactions.

Glycogenesis:

Glycogenesis is the biochemical process by which glucose molecules are converted into glycogen for storage in the body. It mainly occurs in the liver and skeletal muscles when excess glucose is available.

During this process, glucose is first converted into glucose-6-phosphate and finally polymerized into glycogen.

Glycogenesis helps maintain blood glucose levels and provides an energy reserve for future use.

HMP Shunt:

The HMP shunt (Hexose Monophosphate shunt), also called the Pentose Phosphate Pathway (PPP), is an alternative pathway of glucose metabolism that occurs in the cytoplasm of cells. It converts glucose-6-phosphate into pentose sugars and produces NADPH. NADPH is important for biosynthetic reactions and protection against oxidative damage, while pentose sugars are used for nucleotide synthesis.

Uronic Acid Pathway:

The uronic acid pathway is an alternative pathway of glucose metabolism in which glucose is converted into glucuronic acid, pentoses, and other compounds. It mainly occurs in the liver and helps in the synthesis of substances like glucuronides and vitamin C (in some animals).

This pathway is important for detoxification reactions and the formation of glycosaminoglycans.