Study guide: Carbohydrates I

📋 Course Outline

1. Carbohydrate Structure and Classification
2. Monosaccharide Isomers and Roles
3. Major Dietary Monosaccharides
4. Sugar Oxidation and Reduction
5. Disaccharide Structures and Uses
6. Lactose Digestion and Intolerance
7. Carbohydrate Intake and Digestion
8. Oligosaccharides and Prebiotic Effects

📖 1. Carbohydrate Structure and Classification

★ Must-know

• Carbohydrates are classified by monosaccharide-unit count as: monosaccharides (1 unit), disaccharides (2), oligosaccharides (3–10), polysaccharides (>10), derived carbohydrates, including sugar alcohols

Further detail

• Carbohydrates contain carbon, hydrogen and oxygen, and their general formula is Cₙ(H₂O)ₙ.

📌 Simple carbohydrates contain one or two monosaccharide units, whereas complex carbohydrates contain three or more; polyols are derived from simple sugars.

• A carbonyl group occurs in:
  • aldehydes
  • ketones
  • carboxylic acids
  • esters
  • amides
  • acid anhydrides

📖 2. Monosaccharide Isomers and Roles

★ Must-know

• Monosaccharides are grouped by carbon count as:
  • trioses (3 carbons)
  • tetroses (4)
  • pentoses (5)
  • hexoses (6)

📌 Aldoses contain an aldehyde group, whereas ketoses contain a ketone group; glucose and galactose are aldoses, while fructose is a ketose.

• Glucose, galactose and fructose can form cyclic structures, and their isomeric forms affect how the body uses carbohydrates, amino acids and lipids because enzymes act on specific isomers.

• The three monosaccharide isomers identified as biologically significant in human metabolism are α-D-glucose, β-D-fructose and α-D-galactose; other carbohydrates must be hydrolyzed to these monosaccharides before absorption.

Further detail

• Ribose occurs in RNA and ATP, while deoxyribose occurs in DNA.

💡 Memory Hook

The same formula can have different arrangements: enzymes recognize specific isomers.

📖 3. Major Dietary Monosaccharides

🔑 Key Concepts & Definitions

• Glucose: A monosaccharide with formula C₆H₁₂O₆, also called grape sugar or blood sugar, and the body's primary energy source.
• Fructose: Also called levulose, the sweetest monosaccharide, which crystallizes less readily than glucose or sucrose.

★ Must-know

• The brain and mature erythrocytes use only glucose as an energy source, so severe hypoglycemia can cause serious brain damage.

Further detail

• Glucose occurs free in fruits, sweetened beverages, glucose syrup, candies, biscuits and cakes; it also occurs in lactose, starch, and oligosaccharides in legumes and vegetables.

• Fructose is metabolized in the liver and converted into energy without requiring insulin.

• Galactose is the C-4 epimer of glucose, is not found freely in nature, and occurs with glucose as a component of lactose.

• In galactosemia, deficient or defective enzymes converting galactose to glucose cause galactose and galactitol to accumulate, producing toxic effects such as liver damage, cataracts and neurological impairment.

• Mannose is the C-2 epimer of glucose and occurs in polysaccharides, glycoproteins and hemicellulose, as well as in some fruits, honeydew and legumes.

📖 4. Sugar Oxidation and Reduction

🔑 Key Concepts & Definitions

• Polyols: Sugar alcohols formed when a sugar's aldehyde or ketone group is reduced to a hydroxyl group.

📝 Essential Points

• 🔄 The oxidation proceeds as follows: a hexose has a –CH₂OH group at C-6, oxidation converts C-6 to a carboxyl group (–COOH), the product is a uronic acid, such as glucuronic acid

• The uronic acid pathway contributes to:

  • detoxification
  • glycosaminoglycan synthesis
  • vitamin C synthesis
  • interconversion of L- and D-sugars

• Humans lack the final enzyme in the uronic acid pathway, so vitamin C must come from the diet.

• 🔄 Glucuronidation proceeds as follows: bilirubin, drugs or toxic compounds combine with glucuronic acid, the resulting compounds become water-soluble, the compounds are excreted in urine

• Polyols provide 2–2.5 kcal/g compared with 4 kcal/g for carbohydrates and may cause osmotic diarrhea when intake exceeds 50 g.

💡 Memory Hook

Oxidation makes a carboxyl group; reduction makes a sugar alcohol.

📖 5. Disaccharide Structures and Uses

🔑 Key Concepts & Definitions

• Disaccharide: A molecule consisting of two monosaccharides joined by a glycosidic bond through the loss of one water molecule.

★ Must-know

📌 Invert sugar is a mixture of free glucose and fructose in a 1:1 ratio, whereas sucrose is a disaccharide in which glucose and fructose are chemically bonded.

Further detail

• Maltose contains two glucose molecules linked by an α-1,4 glycosidic bond and is an intermediate product of starch digestion.

• Sucrose contains one glucose and one fructose molecule linked by an α-1,2 glycosidic bond and is the table sugar found in sugar cane and sugar beet.

• Invert sugar is sweeter than sucrose at the same concentration, has smaller crystals and is used in delicate confectionery and creams; honey contains natural invert sugar formed by bee enzymes.

• Trehalose consists of two glucose molecules linked by an α-1,1 glycosidic bond, is common in fungi and is used to stabilize texture and flavor in processed foods.

📖 6. Lactose Digestion and Intolerance

🔑 Key Concepts & Definitions

• Lactose: The main carbohydrate of milk, consisting of glucose and galactose joined by a β-1,4 glycosidic bond.
• Lactose Intolerance: The inability to digest lactose because lactase activity in the small intestine is insufficient.

★ Must-know

• 🔄 The mechanism proceeds as follows: insufficient lactase leaves lactose undigested, undigested lactose passes into the large intestine, colonic bacteria ferment lactose and produce gas and organic acids, bloating, abdominal pain, diarrhea and flatulence may result

Further detail

• Lactose makes up 7–8% of human milk and 4–5% of cow's milk.

• Lactose intolerance may be: primary, due to a genetically programmed decline in lactase after weaning, secondary, due to intestinal mucosal injury, congenital, due to complete lactase absence from birth

• Lactose intolerance can be managed by limiting lactose, tolerating small amounts through adaptation or lactase supplements, and choosing lactose-free or fermented dairy products.

💡 Memory Hook

Low lactase → lactose reaches the colon → bacterial fermentation → symptoms.

📖 7. Carbohydrate Intake and Digestion

★ Must-know

• Carbohydrates should provide 45–60% of total daily energy, and each gram of carbohydrate provides 4 kcal.

📐 Formula - The daily carbohydrate requirement in grams is calculated as $\frac{\text{energy from carbohydrate (kcal)}}{4\text{ kcal/g}}$.

• 🔄 Carbohydrate digestion and absorption proceed as follows: brush-border enzymes hydrolyze disaccharides at specific bonds, glucose and galactose are absorbed by active transport, fructose is absorbed by facilitated diffusion, all monosaccharides enter the blood and travel to the liver

Further detail

• At 2500 kcal/day, 45–60% of energy from carbohydrates corresponds to 281–375 g/day; at 2000 kcal/day, it corresponds to 225–300 g/day.

• Carbohydrates are structural components of cell membranes and can determine cell specificity, including erythrocyte blood-group specificity.

• Cellulose provides rigidity and protection in plant cell walls, while lipopolysaccharides are structural and antigenic components of Gram-negative bacterial membranes.

📖 8. Oligosaccharides and Prebiotic Effects

🔑 Key Concepts & Definitions

• Oligosaccharides: Carbohydrates formed from 3–10 monosaccharide units joined by glycosidic bonds.

★ Must-know

• Except for maltooligosaccharides, most oligosaccharides cannot be completely digested in the small intestine; partly or fully indigestible forms act as prebiotics supporting beneficial gut bacteria.

Further detail

• The main oligosaccharide groups are: galactooligosaccharides (GOS), containing galactose units, fructooligosaccharides (FOS), containing fructose and glucose units, maltooligosaccharides (MOS), containing glucose units

• Galactooligosaccharides occur in small amounts in milk and legumes and promote Bifidobacteria and Lactobacilli as prebiotics; the stated recommended intake is about 0.3–0.4 g/kg body weight per day.

• Fructooligosaccharides occur naturally in fruits and vegetables; short-chain FOS are found mainly in wheat and rye, while long-chain FOS occur in onion, garlic, banana, artichoke and Jerusalem artichoke.

• FOS may increase colonic Bifidobacteria growth up to 10-fold, have a recommended upper daily intake of 20 g, and excessive consumption may cause gas, nausea or diarrhea; FOS intake is significantly restricted in irritable bowel syndrome.

• Raffinose, stachyose and verbascose are raffinose-family oligosaccharides that humans do not fully digest because they lack α-galactosidase, and these sugars may cause gas and flatulence.

• In the raffinose family, chain length increases by one galactose at each step: raffinose is a trisaccharide, stachyose a tetrasaccharide and verbascose a pentasaccharide.

• Maltooligosaccharides are digestible glucose chains linked by α-1,4 glycosidic bonds, occur in starch, syrups, infant formulas, sports drinks and beer, and have low osmotic pressure.

📊 Synthesis Tables

Disaccharide Composition and Linkage

Disaccharide	Monosaccharides	Bond	Key feature
Maltose	Glucose + glucose	α-1,4	Intermediate in starch digestion
Sucrose	Glucose + fructose	α-1,2	Table sugar
Lactose	Glucose + galactose	β-1,4	Main carbohydrate of milk
Trehalose	Glucose + glucose	α-1,1	Common in fungi