02.13 Chapter Summary

Purpose:

• Detect the presence of reducing sugars (e.g., glucose, fructose).

Principle:

• Reducing sugars can reduce Cu²⁺ ions in Benedict’s reagent to Cu⁺, forming a brick-red precipitate of copper(I) oxide.

Procedure:

1. Reagents Needed:
   • Benedict’s reagent (a blue solution containing copper(II) sulfate, sodium carbonate, and sodium citrate).
2. Steps:
   • Add a few drops of the sample solution to a test tube.
   • Add an equal volume of Benedict’s reagent.
   • Heat the mixture in a boiling water bath for 5 minutes.
3. Observation:
   • Color change from blue to green, yellow, orange, or brick-red precipitate indicates the presence of reducing sugars.

Interpretation:

• Blue: No reducing sugars.
• Green to Brick-Red: Increasing concentrations of reducing sugars.

Purpose:

• Identify the presence of starch.

Principle:

• Iodine interacts with the helical structure of starch (amylose) to form a blue-black complex.

Procedure:

1. Reagents Needed:
   • Iodine solution (iodine-potassium iodide).
2. Steps:
   • Add a few drops of iodine solution to the sample.
3. Observation:
   • A color change to blue-black indicates the presence of starch.

Interpretation:

• Yellow/Brown: No starch.
• Blue-Black: Presence of starch.

Purpose:

• Detect the presence of lipids (fats and oils).

Principle:

• Lipids are insoluble in water but form a cloudy emulsion when mixed with ethanol and then water is added.

Procedure:

1. Reagents Needed:
   • Ethanol.
2. Steps:
   • Add a small amount of the sample to a test tube.
   • Add ethanol and shake to dissolve any lipids.
   • Carefully add water without shaking; formation of a cloudy emulsion indicates lipids.
3. Observation:
   • Cloudiness signifies lipid presence.

Interpretation:

• Clear Solution: No lipids.
• Cloudy Emulsion: Presence of lipids.

Purpose:

• Detect the presence of proteins.

Principle:

• Proteins contain peptide bonds that react with copper(II) ions in an alkaline solution to form a violet-colored complex.

Procedure:

1. Reagents Needed:
   • Biuret reagent (a solution of copper(II) sulfate and sodium hydroxide).
2. Steps:
   • Add a few drops of Biuret reagent to the sample.
   • Mix gently.
3. Observation:
   • Formation of a violet or purple color indicates the presence of proteins.

Interpretation:

• No Color Change: No proteins.
• Violet/Purple: Presence of proteins.

Purpose:

• Estimate the concentration of reducing sugars in a solution.

Principle:

• The rate of color change or the intensity of the precipitate correlates with sugar concentration.

Procedure:

1. Standardization:
   • Prepare standard solutions with known concentrations of a reducing sugar (e.g., glucose).
2. Testing Unknowns:
   • Perform Benedict’s test on both standard and unknown samples under identical conditions.
3. Observation:
   • Note the time taken for the first color change or compare the color intensity to a standard color chart.
4. Estimation:
   • Determine the concentration of reducing sugars in the unknown based on the comparison with standards.

Tips for Accuracy:

• Ensure all solutions are fresh.
• Compare colors against a standardized color chart for precise estimation.
• Use consistent heating times.

Purpose:

• Identify non-reducing sugars (e.g., sucrose).

Principle:

• Non-reducing sugars can be hydrolyzed into reducing sugars using acid before performing Benedict’s test.

Procedure:

1. Reagents Needed:
   • Dilute acid (e.g., dilute hydrochloric acid).
   • Benedict’s reagent.
2. Steps:
   • Add a few drops of the sample to a test tube.
   • Add dilute acid and gently heat to hydrolyze non-reducing sugars into reducing sugars.
   • Neutralize the acid if necessary.
   • Perform Benedict’s test on the hydrolyzed sample.
3. Observation:
   • A positive Benedict’s test after hydrolysis indicates the presence of non-reducing sugars.

Interpretation:

• No Color Change Initially, Positive After Hydrolysis: Presence of non-reducing sugars.
• Positive Without Hydrolysis: Presence of reducing sugars (may contain both types).

Structure:

• Glucose: A six-carbon monosaccharide with the molecular formula C₆H₁₂O₆.
• Ring Formation: Glucose forms a cyclic hemiacetal structure, creating a six-membered ring (pyranose form).

α-Glucose vs. β-Glucose:

• Anomeric Carbon: Carbon 1 (C1) in glucose.
• α-Glucose: –OH group on C1 is trans (below the plane) relative to the CH₂OH group on C5.
• β-Glucose: –OH group on C1 is cis (above the plane) relative to the CH₂OH group on C5.

Diagram:

• Draw the Haworth projections showing the orientation of the –OH group on the anomeric carbon for both α and β forms.

• Monomer: The basic building block of a polymer (e.g., glucose for carbohydrates).
• Polymer: A large molecule composed of repeating monomer units (e.g., starch, glycogen).
• Macromolecule: A very large molecule, typically a polymer (e.g., proteins, nucleic acids).
• Monosaccharide: The simplest form of carbohydrate; single sugar unit (e.g., glucose, fructose).
• Disaccharide: A carbohydrate formed by two monosaccharides joined by a glycosidic bond (e.g., sucrose, maltose).
• Polysaccharide: A carbohydrate composed of many monosaccharide units (e.g., starch, glycogen, cellulose).

• Covalent Bonds: Strong chemical bonds where electrons are shared between atoms.
• Polymerization: Smaller molecules (monomers) are linked together by covalent bonds to form polymers.
• Example: In carbohydrates, glycosidic bonds (a type of covalent bond) link monosaccharides to form disaccharides and polysaccharides.

• Reducing Sugars: Can donate electrons or reduce other molecules; have a free anomeric carbon.
• Examples: Glucose, Fructose, Maltose.

• Non-Reducing Sugars: Cannot donate electrons; have their anomeric carbons involved in glycosidic bonds.
• Example: Sucrose.

Test: Benedict’s solution can distinguish between reducing and non-reducing sugars.

• Condensation Reaction: Removal of a water molecule to form a bond between two molecules.
• Glycosidic Bond: A covalent bond joining a carbohydrate molecule to another group, which may also be a carbohydrate.

Formation Process:

• Disaccharides: Two monosaccharides join via a glycosidic bond (e.g., glucose + fructose → sucrose).
• Polysaccharides: Multiple monosaccharides linked together (e.g., glucose units in starch).
• Diagram: Show the condensation reaction forming a glycosidic bond between two glucose molecules.

Hydrolysis of Glycosidic Bonds :

• Hydrolysis: The chemical breakdown of a bond through the addition of water.
• Polysaccharides/Disaccharides: Glycosidic bonds are broken down into monosaccharides.

Non-Reducing Sugar Test:

• Sucrose: Does not react with Fehling’s solution unless hydrolyzed into glucose and fructose.
• Procedure: Hydrolyze the disaccharide, then perform the reducing sugar test.

Starch:

• Amylose: Linear chains of α-glucose units connected by α-1,4-glycosidic bonds. Forms helical structures.
• Amylopectin: Branched chains with α-1,6-glycosidic bonds at branch points.

Glycogen:

• Highly branched structure with more α-1,6-glycosidic bonds than amylopectin, allowing rapid release of glucose.

Function:

• Starch: Energy storage in plants.
• Glycogen: Energy storage in animals.

Cellulose:

• Linear chains of β-glucose units linked by β-1,4-glycosidic bonds.
• Hydrogen Bonding: Forms strong hydrogen bonds between adjacent chains, creating rigid, insoluble fibers.

Function:

• Plant Cell Walls: Provides structural support and rigidity to plants.
• Diagram: Show the linear structure and hydrogen bonding in cellulose.

Triglycerides:

• Non-Polar, Hydrophobic Molecules: Do not mix with water.

Molecular Structure:

• Glycerol: A three-carbon alcohol.
• Fatty Acids: Long hydrocarbon chains (saturated: no double bonds; unsaturated: one or more double bonds).
• Ester Bonds: Formed between the hydroxyl groups of glycerol and the carboxyl groups of fatty acids.
• Diagram: Illustrate glycerol linked to three fatty acids via ester bonds.

• Energy Storage: High-energy bonds store significant energy, making triglycerides efficient for long-term energy storage.
• Insulation and Protection: Provide thermal insulation and protect vital organs due to their hydrophobic nature and storage in adipose tissue.

Phospholipids:

Structure:

• Hydrophilic (Polar) Heads: Contain phosphate groups, which interact with water.
• Hydrophobic (Non-Polar) Tails: Composed of fatty acid chains.
• Amphipathic Nature: Possess both hydrophilic and hydrophobic regions.

Function:

• Cell Membranes: Form bilayers with hydrophobic tails facing inward and hydrophilic heads facing outward, creating a semi-permeable membrane.
• Diagram: Depict a phospholipid molecule with distinct head and tail regions, and illustrate the bilayer structure.

Amino Acid Structure

• General Structure:
  • Central Carbon (α-Carbon): Connected to four groups:
    • Amino Group (—NH₂)
    • Carboxyl Group (—COOH)
    • Hydrogen Atom (—H)
    • R Group (Side Chain): Varies among different amino acids, determining their properties.
• Diagram: Draw a central carbon with the four groups attached, labeling each part.

Peptide Bond Formation

Formation:

• Condensation Reaction: Carboxyl group of one amino acid reacts with amino group of another, releasing a molecule of water (H₂O).
• Peptide Bond: The bond formed is a covalent bond between the carbon of the carboxyl group and the nitrogen of the amino group (—CO—NH—).

Peptide Bond Breakage:

• Hydrolysis Reaction: Addition of water breaks the peptide bond, separating the amino acids.

Primary Structure

• Definition: The linear sequence of amino acids in a polypeptide chain.
• Importance: Determines all subsequent levels of structure.

Secondary Structure

• Types:
  • Alpha (α) Helix: Spiral structure stabilized by hydrogen bonds between backbone atoms.
  • Beta (β) Pleated Sheet: Sheet-like structure formed by hydrogen bonds between adjacent polypeptide chains.

Tertiary Structure

• Definition: The three-dimensional folding of a single polypeptide chain.
• Stabilized by: Hydrophobic interactions, hydrogen bonds, ionic bonds, and disulfide bridges.

Quaternary Structure

• Definition: The arrangement of multiple polypeptide chains (subunits) into a functional protein.
• Example: Hemoglobin consists of four subunits.

Hydrophobic Interactions

• Description: Nonpolar side chains avoid water, clustering together inside the protein.
• Role: Stabilize the protein’s core.

Hydrogen Bonding

• Description: Attraction between hydrogen and electronegative atoms (e.g., O, N).
• Role: Stabilizes secondary and tertiary structures.

Ionic Bonding

• Description: Electrostatic attractions between positively and negatively charged side chains.
• Role: Contributes to tertiary and quaternary structures.

Covalent Bonding (Including Disulfide Bonds)

• Description: Strong bonds formed between atoms sharing electrons.
• Disulfide Bonds: Covalent bonds between sulfur atoms of cysteine residues.
• Role: Provide significant stability, especially in tertiary and quaternary structures.

Globular Proteins

• Characteristics:
  • Solubility: Generally soluble in water.
  • Function: Physiological roles (e.g., enzymes, hormones, transport proteins).
  • Structure: Compact, spherical shapes.

Fibrous Proteins

Characteristics:

• Solubility: Generally insoluble in water.
• Function: Structural roles (e.g., collagen, keratin).
• Structure: Long, fibrous, and elongated shapes.

Structure of Haemoglobin

• Subunits:
  • Two Alpha (α) Chains (α–globin)
  • Two Beta (β) Chains (β–globin)
• Haem Groups:
  • Each Subunit Contains One Haem Group
  • Haem Group: Contains an iron (Fe) atom that binds oxygen.

Quaternary Structure

• Assembly: Four polypeptide chains (2 α and 2 β) form the functional protein.
• Functionality: Each subunit can bind one oxygen molecule, allowing haemoglobin to carry four oxygen molecules.

Function of Haemoglobin

• Oxygen Transport: Binds oxygen in the lungs and releases it in tissues.

Importance of Iron

• Role of Iron in Haem Group:
  • Binding Site: Iron atom binds reversibly to oxygen.
  • Essential for Oxygen Transport: Without iron, haemoglobin cannot carry oxygen.

Structural Features Facilitating Function

• Quaternary Structure: Allows cooperative binding of oxygen (binding of one O₂ increases affinity for the next).
• Globular Shape: Enhances solubility and mobility in blood.

Structure of Collagen

• Triple Helix:
  • Three Polypeptide Chains: Wound together in a triple helix formation.
  • Amino Acid Sequence: Repeating Glycine-Proline-X or Glycine-X-Hydroxyproline, where X is any amino acid.

Collagen Fibres

• Arrangement:
  • Fibrils: Individual collagen molecules align in a staggered pattern.
  • Fibres: Multiple fibrils bundle together to form strong, insoluble fibres.

Stabilizing Features

• Hydrogen Bonds: Stabilize the triple helix.
• Cross-linking: Covalent bonds between collagen molecules enhance strength.

Function of Collagen

• Structural Support: Provides strength and elasticity to connective tissues (e.g., skin, bones, tendons).

Structural Features Facilitating Function

• Triple Helix: Offers high tensile strength, resisting stretching.
• Fibrous Structure: Insolubility and robust fibre formation support structural integrity.
• Arrangement in Fibres: Efficient distribution of mechanical stress across tissues.

a. Structure of Water (H₂O):

• Molecular Composition: Each water molecule consists of two hydrogen atoms covalently bonded to one oxygen atom.
• Polarity: Oxygen is more electronegative than hydrogen, creating a polar molecule with a partial negative charge (δ⁻) on the oxygen atom and partial positive charges (δ⁺) on the hydrogen atoms.

b. Formation of Hydrogen Bonds:

• Definition: A hydrogen bond is a weak electrostatic attraction between the partially positive hydrogen atom of one water molecule and the partially negative oxygen atom of another.
• Mechanism:
  • Orientation: Due to the polarity, water molecules arrange themselves so that hydrogen atoms are near oxygen atoms of neighboring molecules.
  • Network Formation: Each water molecule can form up to four hydrogen bonds (two through its hydrogen atoms and two through lone pairs on the oxygen atom), creating a cohesive network.

c. Significance of Hydrogen Bonds:

• Physical Properties: Contribute to high boiling and melting points relative to other similar-sized molecules.
• Cohesion and Adhesion: Enable water to exhibit strong cohesion (molecules sticking together) and adhesion (molecules sticking to other surfaces).
• Surface Tension: High surface tension due to hydrogen bonding allows insects like water striders to walk on water.

Diagram:

a. Solvent Action

Definition:

• Water is often called the “universal solvent” because it can dissolve a wide variety of substances.

Mechanism:

• Polarity: The polar nature of water molecules allows them to surround and interact with charged ions and polar molecules, effectively separating and dispersing them in solution.
• Hydration Shells: Ions in solution are surrounded by water molecules, stabilizing them and preventing them from recombining.

Biological Significance:

• Biochemical Reactions: Facilitates enzyme-substrate interactions and metabolic pathways by dissolving reactants.
• Transport Medium: Enables the movement of nutrients, gases (like oxygen and carbon dioxide), and waste products within organisms (e.g., blood plasma).
• Cellular Environment: Maintains the proper environment for cellular processes by dissolving necessary ions and molecules.

Examples:

• Dissolution of glucose in blood plasma for energy transport.
• Transport of oxygen in blood bound to hemoglobin.

b. High Specific Heat Capacity

Definition:

• Specific heat capacity is the amount of heat required to raise the temperature of one gram of a substance by one degree Celsius.

Water’s Specific Heat:

• Approximately 4.18 J/g°C, which is significantly higher than most other common substances.

Biological Significance:

• Temperature Regulation: Helps organisms maintain stable internal temperatures despite external temperature fluctuations.
• Climate Moderation: Large bodies of water (oceans, lakes) absorb and store heat, mitigating extreme temperature changes in the environment.
• Homeostasis: In humans, high water content in tissues and blood helps buffer against temperature changes, supporting metabolic functions.

Examples:

• Human bodies use water to regulate temperature through sweating.
• Coastal regions experience milder climates due to the high specific heat of ocean water.

c. Latent Heat of Vaporisation

Definition:

• The latent heat of vaporisation is the amount of heat required to convert one gram of liquid into vapor without changing its temperature.

Water’s Latent Heat:

• Approximately 2260 J/g at 100°C.

Biological Significance:

• Evaporative Cooling: Critical for temperature regulation in organisms.
  • Humans: Sweating releases heat as water evaporates from the skin, cooling the body.
  • Plants: Transpiration releases water vapor from leaves, cooling the plant and enabling nutrient uptake.
• Energy Transfer: The high latent heat of vaporisation allows water to carry large amounts of energy during phase changes, aiding in thermal regulation.

Examples:

• Evaporation of sweat from human skin during exercise.
• Transpiration in plants facilitating nutrient transport from roots to leaves.