Biochemistry: Digestion and Absorption of Carbohydrates (Lecture 2)

These flashcards cover key definitions, biochemical pathways, digestion steps, metabolic disorders, and classifications of carbohydrates from Lecture 2 on Biochemistry: Digestion and Absorption of Carbohydrates.

What are the two major dietary forms of carbohydrates mentioned in the lecture?

Sugar (with glucose being the most abundant) and starch.

From which sources can the body obtain glucose?

Breakdown of dietary carbohydrates, glycogen stores, and endogenous synthesis from protein or glycerol of triglycerides.

What is the general chemical formula for carbohydrates as ‘hydrates of carbon’?

Cn(H2O)n.

Chemically, how are carbohydrates defined?

Aldehyde or ketone compounds that possess multiple hydroxyl (-OH) groups.

Which three hormones primarily maintain blood glucose concentration within a narrow range?

Insulin, glucagon, and epinephrine.

Name the two major disorders of carbohydrate metabolism discussed.

Diabetes mellitus (high blood glucose) and hypoglycemia (low blood glucose).

What is the primary biological function of glucose?

To be oxidized via glycolysis and the tricarboxylic acid (TCA) cycle to generate ATP, the cell’s main energy currency.

When energy intake exceeds needs, into which two major storage forms is excess carbohydrate converted?

Fat (triglycerides) and glycogen.

What four properties are used to classify carbohydrates chemically?

Number of carbons in the chain, number of sugar units, location of the carbonyl (C=O) group, and stereochemistry.

What term is used for a 6-carbon monosaccharide and give two examples.

Hexose; examples include glucose, galactose, and fructose.

If the carbonyl group of a monosaccharide is on carbon 1, what type of sugar is it called?

An aldose.

Which three common disaccharides were highlighted and what monosaccharides compose each?

Maltose = glucose + glucose; Lactose = glucose + galactose; Sucrose = glucose + fructose.

Which two polysaccharides serve as the major storage forms of glucose in plants and animals, respectively?

Starch in plants and glycogen in animals.

What structural polysaccharide in plants was mentioned that humans cannot digest?

Cellulose.

In stereochemistry, what distinguishes D- from L-isomers of sugars?

The position of the hydroxyl group (-OH) on the chiral carbon next to the terminal CH2OH group—right side for D, left for L.

Which form of glucose (α or β) does the enzyme glucose oxidase specifically react with?

β-D-glucose.

Why must freshly prepared glucose calibration solutions stand for 2–3 hours at room temperature before use?

To allow mutarotation so that α- and β-glucose reach equilibrium, ensuring accurate measurement by glucose oxidase assays.

What does the term ‘reducing sugar’ refer to?

A sugar capable of reducing other compounds, such as converting cupric (Cu2+) ions to cuprous (Cu+) ions in alkaline solution.

Which classical clinical test exploits the reducing power of glucose in hot alkaline solution?

The Clinitest (copper reduction test).

Where does carbohydrate digestion begin, and which enzyme initiates it?

In the mouth, initiated by salivary amylase.

Why does carbohydrate digestion temporarily halt in the stomach?

Gastric acidity inactivates salivary amylase, and no other stomach enzyme acts on carbohydrates.

Which enzyme continues starch digestion in the small intestine after gastric neutralization?

Pancreatic amylase.

Name the three disaccharidases produced by the jejunal mucosa and the bonds they hydrolyze.

Maltase (maltose → glucose + glucose), Sucrase (sucrose → glucose + fructose), Lactase (lactose → glucose + galactose).

What condition results from an inability to produce lactase, and what symptom is common?

Lactose intolerance, causing gastrointestinal irritation and diarrhea.

After absorption, monosaccharides enter which blood supply and which organ first?

They enter the portal circulation and first reach the liver.

List four fates of glucose once it reaches the liver.

Energy production, storage as liver glycogen, conversion to triglycerides for adipose storage, or conversion to keto acids/amino acids/protein.

Define metabolism in the context of cellular reactions.

A sequence of enzyme-catalyzed chemical reactions occurring within living cells.

Differentiate between anabolic and catabolic pathways regarding electrons and energy.

Anabolic pathways synthesize molecules and require electrons and energy; catabolic pathways break down molecules, donate electrons (via NAD/FAD), and produce energy.

What are the three stages of catabolism outlined in the lecture?

Stage 1: Breakdown of macromolecules into small units; Stage 2: Production of Acetyl-CoA; Stage 3: Oxidation of Acetyl-CoA in the citric acid cycle with electron transfer in oxidative phosphorylation.

Define glycogenesis.

The conversion of glucose to glycogen.

Define glycogenolysis.

The breakdown of glycogen to form glucose and intermediary products.

Define gluconeogenesis.

The formation of glucose from non-carbohydrate sources such as lipids and proteins.

Define glycolysis.

The conversion of glucose or other hexoses to pyruvate or lactate.

Define lipogenesis.

The metabolic pathway that synthesizes fat (triglycerides).

Define lipolysis.

The breakdown of fat into glycerol and fatty acids.