- Carbohydrates
For the beginning of the biochemistry chapter, let's start with carbohydrate, and glucose in particular. The term ‘carbohydrate’ is carbo + hydrate which mean watered carbon. The carbonhydrate is equal to saccharide which means sugar. All saccharides share general formula Cx(H2O)y. Three monosaccharides; glucose , D-fructose and D-galactose, for example, have molecular formula C6H12O6. They are all isomers with identical molecular formula but different spatial arrangment. Two different monosaccharides combine together to form a disaccharide. Lactose is galactose with glucose, and sucrose is fructose with glucose. Lactose and sucrose are both broken down to composing monosachharides in the GI tract by lactase and sucrase respectively. Monosaccharides can also form polymes: polysaccharides. Starch, glycogen and cellulose are three examples of polysaccharides. Starch is a polymer of alpha-glucose found in plants. Plants store energy in the form of starch. Glycogen is also a polymer of alpha-glucose but found in animals. Animals store energy in the form of glycogen. Cellulose, a major component of fibers, is a polymer of beta-glucose. Cellulose cannot be broken down in animal GI tract as animals lack digestive enzyme for cellulose. Cellulose can instead add bulk to our stool aiding bowel function for constipation patients.
- Glucose transport
As glucose is a polar molecule, it can not pass through the cellular membrane by simple diffusion. Hence glucose needs a protein to move in and out of the cell. Glucose move across the cell membrane by either facilitated diffusion or secondary active transport. In terms of facilitated diffusion, there are 14 different carrier proteins, from GLUT-1 to GLUT-14, that vary by tissues. GLUT-1 is located at brain and red blood cells. GLUT-1 is insulin independent meaning that it uptakes glucose when glucose concentration is high. GLUT-2 is also a insulin indenpenent carrier. GLUT-2 is interesting because it allows bidirectional movement of glucose, both moving into the cell and moving out of the cell. Liver and kidney have GLUT-2. Liver and kidney are sites of gluconeogensis. They can take up glucose and also put it back to the blood when the body needs it. Intestine also have GLUT-2. Glucose move out of the epithelial cell to the the portal vein. GLUT-2 is also found at pancreas. In contrary, GLUT-4 is an insulin dependent glucose carrier. Fat tissue and skeletal muscles take up glucose via GLUT-4 when insulin is secreted. They then store glucose in the form of fatty acid and glycogen respectively.
Glucose moves down the concentration gradient. This process is also known as sodium independent entry. When glucose moves against the concentration gradient from low to high concentration, co-transporters(SGLT) are used. SGLT 1 is found in intestinal epithelium and SGLT 2 are found in renal tubules. SGLT transport glucose against its concentration gradient using energy created by transporting Na+ ions down their concentration gradient. Thus, sodium and glucose move from extracellular to intracellular space during this process. Sodium potassium pump is essential in that it maintains high extracellular sodium ion concentration by moving sodium ion out of the cell and potassium ion into the cell.
- Glucose metabolism by organs
Glucose is the most universal energy source for human cells. Nearly all cells use glucose as their primary fuels. Liver can use glucose in various ways such as generating ATP through TCA cycle or synthesizing glycogen. Brain also uses glucose for TCA cycle to generate ATP. Brain has very little glycogen storage. Therefore, it is important for brain to get constant supply of glucose. Muscles including cardiac muscle also use glucose for generating ATP though TCA cycle. Glucose transport in muscles are largely influenced by insulin, and glucose can be stored in the form of glycogen. Adipose tissue also takes up glucose mediated by insulin. Adipose tissue store glucose as fatty acids. Red blood cells also uses glucose. As red blood cells lack mitochondria, they use glucose for anaerobic respiration to generate ATP, making lactate. Glucose is also used for HMP shunt in RBCs to produce NADPH.
Today we talked about carbohydrate, glucose transport and glucose metabolism in a few organs. Next is going to be respective glucose metabolism in detail beginning with glycolysis. Thank you for reading.