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Topic 3: Biological Molecules

Chapter Analysis

  • Focus: Heavily linked to the Digestion chapter.
  • Exam Weightage:
    • Usually tested in both Multiple Choice Questions (MCQ) and Paper 2 Section A.
    • Constitutes around 4% in Paper 2 in the past 5 years.
    • Enzymes have higher weightage compared to other topics.

Role of Water

In Animals:
  • Required for chemical reactions, such as hydrolysis of food molecules.
  • Acts as a key component in tissues and bodily fluids.
  • Regulates body temperature through sweating.
  • Facilitates blood transport of substances.
In Plants:
  • Acts as a reactant in photosynthesis.
  • Provides physical support to plants via turgor pressure.
  • Enables the transport of dissolved mineral salts from roots to other plant parts.
  • Allows for the transportation of sugars from leaves to other plant parts.

Key Concept: Carbohydrates

  • Definition: Carbohydrates are organic molecules composed of carbon (C), hydrogen (H), and oxygen (O).
  • General Formula: CnH2nOnC_nH_{2n}O_n
  • Types of Carbohydrates: Divided into three broad categories:
    • Monosaccharides
    • Disaccharides
    • Polysaccharides
Monosaccharides
  • Smallest unit of carbohydrates.
  • Formula: C6H12O6C_6H_{12}O_6
  • Examples:
    • Glucose (found in both plants and animals).
    • Fructose (found in plants).
    • Galactose (milk sugar).
Disaccharides
  • Formed by the condensation reaction of two monosaccharides, accompanied by the removal of a water molecule.
  • Can be broken down into monosaccharides via hydrolysis, where a water molecule is added.
  • Enzymatic Requirement: Enzymes are required for both condensation and hydrolysis reactions.
  • Formula: C12H22O11C_{12}H_{22}O_{11}
  • Common Disaccharides:
    • Maltose: Glucose + Glucose
    • Sucrose: Glucose + Fructose
    • Lactose: Glucose + Galactose
Polysaccharides

  • Formed when thousands of glucose molecules link together through condensation reactions.

  • Examples:

    • Glycogen: Storage form of carbohydrates in mammals.
    • Starch: Storage form of carbohydrates in plants.
    • Cellulose: Composes the cell wall, providing protection against bursting or damage.

  • Location:

    • Glycogen: Found in the liver and muscles of mammals.
    • Starch: Found in storage organs of plants (e.g., potato tubers).
    • Cellulose: Found in plant cells.

Food Tests

Benedict’s Test (for reducing sugars)
  1. Add 2 cm³ of Benedict’s solution to 2 cm³ of the solution being tested.
  2. Shake the mixture.
  3. Heat the test tube in a boiling water bath for 5 minutes.
  4. Observe the precipitate formation and color changes.
  5. Color Indicators:
    • Benedict’s solution is blue:
    • Remains blue: Reducing sugar is absent.
    • Green: Little amount.
    • Yellow: Moderate amount.
    • Orange: More substantial amount.
    • Brick-red: Most amount.
Iodine Test (for starch)
  1. Place the food substance on a white tile; chop solid foods into smaller pieces if necessary.
  2. Add 2-3 drops of dilute iodine solution to the substance being tested.
  3. Color Change:
    • Iodine solution is yellowish-brown:
    • Changes to blue-black: Starch is present.
    • Remains yellowish-brown: Starch is absent.

Key Concept: Fats

  • Composition: Made up of carbon, hydrogen, and oxygen with no fixed formula; the ratio of hydrogen to oxygen is higher than in carbohydrates.
  • Structure: One fat molecule comprises 1 glycerol and 3 fatty acids, joined via a condensation reaction.
  • Breakdown: Three water molecules are required to hydrolyze fat molecules into glycerol and fatty acids.
Functions of Fats:

(a) Storage molecules that provide a substantial energy source, supplying about 9 calories per gram, more than the 4 calories provided by carbohydrates and proteins.

  • Example: Hibernating animals store fats as food reserves for cold seasons.

(b) Integral components of cell membranes and myelin sheaths in nerve cells.

(c) Utilized in the synthesis of steroids and certain hormones.

(d) Stored as adipose tissue to insulate and maintain body heat.

(e) Act as solvents for fat-soluble vitamins.

(f) Protective functions by cushioning vital organs, such as kidneys.

(g) Large animals in cold seas possess thick layers of adipose tissue for buoyancy and heat insulation.

Food Test for Fats

Ethanol Emulsion Test
  1. Add 2 cm³ of ethanol to the substance in a dry test tube.
  2. Shake the mixture thoroughly.
  3. Add 2 cm³ of water to the mixture.
  4. Observation: If fats are present, a white emulsion will be observed.

Key Concept: Proteins

  • Definition: Organic molecules made up of carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur.
  • Smallest Unit: Amino acids, characterized by the general structure:
    • Contains an R group, a carboxyl group, a hydrogen atom, and an amino group.
  • Significance of R Group: There are 20 different R groups, yielding 20 distinct naturally occurring amino acids.
Protein Formation
  • Amino acids join together to form polypeptides through condensation reactions, eliminating water molecules to create peptide bonds between amino acids.
  • Proteins consist of one or more polypeptide chains twisted, folded, and coiled into unique 3-dimensional structures, stabilized by hydrogen bonds, ionic interactions, and van der Waals interactions.
Food Test for Proteins
Biuret Test
  1. Add 2 cm³ of sodium hydroxide solution to 2 cm³ of the food solution.
  2. Shake thoroughly.
  3. Gradually add 1% copper (II) sulfate solution, drop by drop, shaking after each addition.
  4. Allow the mixture to stand for 5 minutes.
  5. Indicators:
    • Copper (II) sulfate is blue:
    • Remains blue: Protein is absent.
    • Changes from blue to violet: Protein is present.

Enzymes

Definition
  • Enzymes are biological catalysts that accelerate chemical reactions by reducing the activation energy without being chemically altered.
  • Enzymes are consumed in small amounts because they remain unchanged and can be reusable.
Active Site
  • The active site of an enzyme has a specific shape that matches the substrate(s), forming a complementary fit.
Lock and Key Hypothesis
  1. The substrate is the “key”; the enzyme is the “lock”.
  2. Only a substrate that precisely fits can attach to the active site.
  3. Binding of the substrate forms an enzyme-substrate complex.
  4. This complex lowers the activation energy, allowing the reaction to take place by aligning substrate molecules optimally.
  5. Once the reaction is catalyzed, products dissociate, leaving the unchanged enzyme available for reuse.
Effect of Temperature on Enzyme Activity
  1. At low temperatures, enzymes are inactive; the reaction rate is low due to minimal kinetic energy.
  2. As temperature increases, kinetic energy rises, leading to more frequent collisions and consequently an increased reaction rate.
  • Observation: The reaction rate approximately doubles with every 10°C rise in temperature.
  1. Maximum reaction rate occurs at the enzyme's optimum temperature.
  2. Beyond the optimum temperature, enzymes can denature, losing their 3D shape and thus their ability to bind substrates.
  3. Denaturation is irreversible; extreme temperatures result in total enzyme inactivation, reducing the reaction rate to zero.
Effect of pH on Enzyme Activity
  1. Optimum pH allows for maximum enzyme activity.
  2. Deviations from optimum pH sharply decrease activity due to disruption of hydrogen and ionic bonds maintaining 3D structure, resulting in denaturation.
  3. At extreme pH levels, enzymes become completely denatured and inactivated, dropping the reaction rate to zero.
  4. Each enzyme has a specific optimum pH.