Biomolecules

• Definition and significance of biomolecules:

  • Organic molecules that occur naturally in living organisms.

  • Composed primarily of carbon and hydrogen, and may also include nitrogen, oxygen, sulfur, and phosphorus.

  • Fundamental building blocks of all life forms.

Classes of Biomolecules

• Major classes identified:

  • Carbohydrates

  • Proteins

  • Lipids

  • Nucleic Acids

Learning Objectives

• By the end of the lecture, students should be able to:

  • Define biomolecules and enumerate types of biomolecules.

  • Define carbohydrates and their classes.

  • Classify monosaccharides by:

  • Number of carbon atoms.

  • Type of carbonyl group.

  • Differentiate between reducing sugars and non-reducing sugars.

  • Draw Fischer projections of various monosaccharides.

  • Explain Benedict's test for detecting reducing and non-reducing sugars.

  • Distinguish between D- and L-sugars.

  • Define disaccharides and polysaccharides with examples.

Diagnostic Questions

1. How many classes of biomolecules exist?

2. Enumerate known classes of biomolecules.

3. Identify elements that constitute biomolecules.

4. What is the branch of chemistry focusing on biomolecules?

5. Reiterate the classes of biomolecules and their biological importance.

6. Identify energy-storing carbohydrates:

   • In plants.

   • In animals.

7. Name disaccharides obtained from hydrolysis of:

   • Starch.

   • Cellulose.

Carbohydrates

• Definition of carbohydrates:

  • Organic compounds that can be represented as hydrates of carbon.

  • Example: Glucose (C6H12O6) can be represented as C6(H2O)6.

  • Structurally defined as polyhydroxy aldehydes, polyhydroxy ketones, or substances that yield these compounds upon hydrolysis.

• Importance of carbohydrates:

  • Serves as a primary fuel source and energy for physiological functions.

  • Critical for the proper functioning of the brain, heart, nervous, digestive, and immune systems.

  • Plays a role in disease pathogenesis.

  • Influences blood group determination.

  • Deficiency leads to fatigue and cognitive impairment.

• Sources of carbohydrates:

  • Found in all plants and animals; essential for life.

• Synthesis of carbohydrates:

  • Plants synthesize carbohydrates through photosynthesis:

  • Converts CO2 and water using solar energy into sugars like cellulose, starch, and sugars.

  • Carbohydrates are oxidized in respiration to produce energy, CO2, and H2O.

  • Photosynthesis equation to be noted.

Classification of Carbohydrates

• Three classes of carbohydrates:

  • Monosaccharides

  • Disaccharides

  • Polysaccharides

• Monosaccharides:

  • Defined as simple sugars; cannot be hydrolyzed further.

  • Classification based on:

  • Number of carbon atoms (triose, tetrose, pentose, hexose).

  • Type of carbonyl group (aldose or ketose).

  • Examples:

  • Triose: Glyceraldehyde, Dihydroxyacetone.

  • Tetrose: Erythrose, Erythrulose.

  • Pentose: Ribose, Xylose.

  • Hexose: Glucose, Galactose, Fructose.

  • Heptose: Glucoheptose, Sedoheptulose.

Fischer Projections

• Definition:

  • Two-dimensional representation of three-dimensional organic molecules.

• D-sugars:

  • Hydroxyl group on next-to-last carbon points right in Fischer projection.

  • Common in natural sugars, most notably glucose.

• L-sugars:

  • Hydroxyl group points left in Fischer projection.

Cyclic Structures of Sugars

• Cyclic forms of monosaccharides formed through:

  • Alcohol functional groups combining with a carbonyl group forming hemiacetals/hemiketals.

  • Stable structures favoring five-membered or six-membered rings.

• Haworth Projections:

  • Representation of cyclic sugars.

  • Lower horizontal bond projected towards the viewer; upper horizontal bond projected away.

• Anomers:

  • Defined by the positioning of the hydroxyl group on the anomeric carbon.

Important Monosaccharides

Glucose

• Most abundant monosaccharide.

• Also known as dextrose, blood sugar, grape sugar.

• Key role in energy metabolism and building block for complex carbohydrates:

  • Found in disaccharides (sucrose, maltose, lactose).

  • Found in polysaccharides (starch, cellulose, glycogen).

• Monitoring in diabetes is crucial:

  • Insulin promotes glycogenesis; glucagon promotes glycogenolysis and gluconeogenesis.

Galactose

• Component of lactose; important in brain cellular membranes.

• C-4 epimer of glucose.

• Galactosemia: a metabolic disorder affecting galactose metabolism.

Fructose

• Sweetest carbohydrate, found in fruits, vegetables, and honey.

• Component of sucrose.

Glycosidic Bonds

• Definition:

  • Covalent linkage between carbohydrate molecules.

• Types of glycosidic bonds:

  • Alpha and beta types (applicable to digestion).

Disaccharides

• Comprised of two monosaccharides joined by glycosidic bonds.

• Examples:

  • Maltose (malt sugar, hydrolysable from starch).

  • Lactose (milk sugar, composed of galactose and glucose).

  • Sucrose (table sugar, composed of glucose and fructose).

• Importance of lactase enzyme in lactose intolerance.

Polysaccharides

• Composed of multiple monosaccharide units; vary in chain length and molecular weight.

• Types:

  • Homopolysaccharides: one type of monomer (glycogen, starch, cellulose).

  • Heteropolysaccharides: multiple monomer types (peptidoglycans, glycosaminoglycans).

• Storage polysaccharides contain only alpha glucose (starch, glycogen); structural polysaccharides only beta glucose (cellulose, chitin).

Reducing and Non-Reducing Sugars

• Definition of reducing sugars:

  • Possess free aldehyde or a hemiacetal linkage, capable of reducing agents like Fehling’s solution, Benedict’s solution, Tollen’s reagent.

• Fehling's Test:

  • Positive results indicate reducing sugar presence (red precipitate formation).

  • Sucrose is a non-reducing sugar, maltose, and lactose are reducing sugars.

• Benedict’s Test compares reducing and non-reducing sugars, utilizing copper (II) salts and yielding reddish-brown precipitate for reducing sugars.

Sample Questions

1. Identify the polymer of β-glucose:

   • A. Starch

   • B. Cellulose

   • C. Amylose

2. Optical family of amino acids:

   • A. L

   • B. D

   • C. Mixed

3. Carbohydrates hydrolyzed into simpler forms:

   • A. Complex carbohydrates

   • B. Polymers

   • C. Polysaccharides

4. Composition of sucrose:

   • A. Two glucoses

   • B. Maltose and lactose

   • C. Glucose and fructose

5. Solubility in organic solvents indicates the class of biomolecule:

   • A. Lipids

   • B. Carbohydrates

   • C. Nucleic acids

6. Natural occurrence of sugars:

   • A. L-sugars

   • B. D-sugars

   • C. Sweetness comparison to glucose

7. Poly alpha glucose common name:

   • A. Starch

   • B. Sucrose

   • C. Cellulose