Study "Carbohydrate Digestion", a flashcard deck on FORKSAI.
How does salivary alpha-amylase contribute to carbohydrate digestion in the mouth, and what are its limitations?
Salivary alpha-amylase, secreted by salivary glands, initiates carbohydrate digestion by cleaving internal alpha-1,4 glycosidic bonds in starch and glycogen. This produces maltose, maltotriose, and limit dextrins. However, it cannot cleave alpha-1,6 branch points and is activated by chloride ions.
Why is there no specific enzyme for carbohydrate digestion in the stomach?
There are no specific enzymes for carbohydrate digestion in the stomach because salivary alpha-amylase, which begins the process in the mouth, is inactivated by the stomach's low pH, which is around 2. This acidic environment denatures the enzyme, halting its activity.
How are glucose and galactose absorbed from the intestinal lumen into intestinal cells, and what type of transport mechanism is involved?
Glucose and galactose are absorbed from the intestinal lumen into intestinal cells via SGLT1 (sodium-glucose linked transporter 1) on the apical side and then transported out by GLUT2 on the basolateral side. This process is secondary active transport, as it co-transports with Na+ and relies on the Na+ gradient maintained by the Na+/K+ ATPase.
Explain the enzymatic basis of lactose intolerance and its physiological consequences.
Lactose intolerance results from a deficiency or reduced activity of the brush border enzyme lactase (lactase-phlorizin hydrolase). Without sufficient lactase, undigested lactose passes into the large intestine, where colonic bacteria ferment it. This fermentation produces gas (CO2, H2, methane) and short-chain fatty acids, leading to symptoms like bloating, flatulence, diarrhea, and abdominal cramping due to gas production and osmotic water retention.
Define a glycoside and provide examples of both O-glycosides and N-glycosides.
A glycoside is a compound where a monosaccharide is linked via a glycosidic bond to another molecule, known as an aglycone. O-glycosides feature a glycosidic bond through an oxygen atom, with examples including lactose, sucrose, maltose, and cardiac glycosides. N-glycosides have a glycosidic bond through a nitrogen atom, such as nucleosides (in DNA/RNA) and coenzymes like NAD+ and FAD.
How do starch (amylose and amylopectin) and glycogen differ in terms of their structural characteristics and branching?
Amylose is a linear helical coil with only alpha-1,4 glycosidic bonds, while amylopectin and glycogen both have alpha-1,4 and alpha-1,6 bonds, forming branched structures. Glycogen is more highly branched than amylopectin, with branches every 8-12 glucose residues compared to every 25-30 residues in amylopectin. This higher branching makes glycogen more compact and provides more non-reducing ends for faster mobilization.
Why are humans unable to digest cellulose, despite it being composed of glucose units?
Humans cannot digest cellulose because it consists of glucose units linked by beta-1,4 glycosidic bonds. Humans lack the specific enzyme, beta-glucosidase (cellulase), required to cleave these bonds. Consequently, cellulose passes through the human digestive system as dietary fiber, undigested.
What is the role of pancreatic alpha-amylase in carbohydrate digestion, and how does its specificity compare to salivary alpha-amylase?
Pancreatic alpha-amylase, secreted by the pancreas into the small intestine, continues the digestion of starch oligosaccharides. It cleaves internal alpha-1,4 glycosidic bonds, producing maltose, isomaltose, and limit dextrins. Its specificity is the same as salivary alpha-amylase, meaning it also cannot cleave alpha-1,6 branch points.
Describe the function of maltase, sucrase, and lactase enzymes at the brush border of the jejunum.
Maltase hydrolyzes maltose into two glucose molecules by cleaving an alpha-1,4 bond. Sucrase, part of the sucrase-isomaltase complex, digests sucrose into glucose and fructose. Lactase (lactase-phlorizin hydrolase) breaks down lactose into glucose and galactose.
How is fructose absorbed by intestinal cells, and what type of transport mechanism is employed?
Fructose is absorbed by intestinal cells via GLUT5 on the apical (luminal) side and then transported out by GLUT2 on the basolateral side. This process is facilitated diffusion, which is a passive mechanism that does not require direct energy input.