Biochemistry Chapter 15 – Complete Study Guide

Biochemistry Chapter 15

Complete Study Guide with MCQs & Short Questions

Chapter Progress
Carbohydrates
Proteins
Enzymes
Lipids
Minerals
Vitamins
Water
Carbohydrates

Organic macromolecules composed of C, H, and O. Primary energy source and structural components.

Proteins

Nitrogen-containing macromolecules made of amino acids. Essential for structure and function.

Enzymes

Biological catalysts that speed up biochemical reactions without being consumed.

Lipids

Hydrophobic molecules including fats, oils, and steroids. Energy storage and membrane components.

15.1 – Carbohydrates

Key Notes

Definition: Carbohydrates are organic macromolecules composed of C, H, and O (ratio ≈ 1:2:1). They contain aldehyde or keto groups and multiple hydroxyl groups.

General Formula: Cn(H2O)n — Often called hydrates of carbon.

Functions: Energy source, energy storage, structural support, protein sparing, metabolic role, digestive health, joint lubrication.

Classification of Carbohydrates

Type Definition Examples Properties
Monosaccharides Simple sugars (3-6 C atoms), can’t be hydrolyzed Glucose, Fructose, Ribose Sweet, soluble, reducing
Oligosaccharides Yield 2-10 monosaccharides on hydrolysis Sucrose, Maltose, Lactose Sweet, soluble
Polysaccharides Yield hundreds of monosaccharides on hydrolysis Starch, Cellulose, Glycogen Tasteless, insoluble, non-reducing

Multiple Choice Questions

1. Which element combination forms all carbohydrates?
  • C, H, O
  • C, H, N
  • C, O, S
  • C, H, P
Answer: a) C, H, O
Explanation: All carbohydrates consist of carbon, hydrogen, and oxygen in nearly 1:2:1 ratio.
2. Which carbohydrate is a storage polysaccharide in animals?
  • Starch
  • Cellulose
  • Glycogen
  • Maltose
Answer: c) Glycogen
Explanation: Glycogen is the animal storage form of glucose, stored in liver and muscles.
3. Glucose and fructose have the same molecular formula but differ in:
  • Number of carbon atoms
  • Functional group
  • Taste
  • Solubility
Answer: b) Functional group
Explanation: Glucose is an aldehyde sugar; fructose is a ketone sugar (structural isomers).
4. Which carbohydrate cannot be hydrolyzed?
  • Starch
  • Maltose
  • Glucose
  • Sucrose
Answer: c) Glucose
Explanation: Monosaccharides like glucose cannot be further hydrolyzed.

Short Questions

1. What are carbohydrates?
Answer: Carbohydrates are organic macromolecules made of carbon, hydrogen, and oxygen. They serve as energy sources and structural materials. Examples: glucose, starch, cellulose.
2. How are carbohydrates classified?
Answer: Based on hydrolysis:
  • Monosaccharides: Simple sugars (e.g. glucose)
  • Oligosaccharides: 2-10 monosaccharides (e.g. sucrose)
  • Polysaccharides: Hundreds of monosaccharides (e.g. starch, cellulose)
3. What is the role of glycogen in the human body?
Answer: Glycogen is the stored form of glucose in liver and muscles. When energy is needed, glycogen breaks down into glucose to provide ATP, especially during exercise.
4. How does fiber benefit the human body?
Answer: Fiber adds bulk to stool, prevents constipation, lowers cholesterol, and reduces heart disease risk. It also supports beneficial gut bacteria.

15.2 – Proteins

Key Notes

Definition: Proteins are nitrogen-containing macromolecules made up of amino acids linked by peptide bonds.

Composition: Elements: C, H, O, N, and sometimes S or P.

Classification: Based on structure (primary, secondary, tertiary, quaternary), constitution (simple, conjugated, derived), shape (fibrous, globular), and function (enzymatic, structural, transport, etc.).

Protein Structure Levels

Level Description Example
Primary Linear sequence of amino acids linked by peptide bonds Insulin
Secondary Coiling (α-helix) or folding (β-pleated) due to H-bonds Collagen
Tertiary 3D folding due to disulfide bridges, ionic and H-bonds Myoglobin
Quaternary Complex of multiple polypeptide chains Hemoglobin

Multiple Choice Questions

1. Which structural level of a protein determines its overall function?
  • Primary
  • Secondary
  • Tertiary
  • Quaternary
Answer: c) Tertiary
Explanation: The tertiary structure determines the 3D folding and shape, which directly affects enzyme activity, binding, and function.
2. Hemoglobin has multiple subunits working together. It is an example of:
  • Simple protein
  • Fibrous protein
  • Quaternary protein
  • Derived protein
Answer: c) Quaternary protein
Explanation: Hemoglobin has four polypeptide chains held by non-covalent interactions, a hallmark of quaternary structure.
3. Albumin in egg white is:
  • Simple protein
  • Conjugated protein
  • Derived protein
  • Fibrous protein
Answer: a) Simple protein
Explanation: Albumin yields only amino acids on hydrolysis — no prosthetic group, hence a simple protein.
4. A mutation changing one amino acid in the primary sequence could:
  • Increase sweetness
  • Alter tertiary structure and function
  • Have no effect
  • Make it insoluble only
Answer: b) Alter tertiary structure and function
Explanation: Primary sequence determines folding; even one amino acid change can alter bonding and 3D shape (as in sickle-cell anemia).

Short Questions

1. What are proteins and why are they vital for life?
Answer: Proteins are complex nitrogenous polymers made from amino acids. They are the building blocks of cells, forming muscles, enzymes, hormones, and membranes. Without proteins, cells cannot perform metabolism, no tissue repair or growth occurs, and the immune system collapses.
2. Explain the four structural levels of proteins with examples.
Answer:
  1. Primary Structure: Linear chain of amino acids (e.g., insulin).
  2. Secondary Structure: Coiling or folding due to H-bonds forming α-helix or β-sheet (e.g., collagen).
  3. Tertiary Structure: Further folding into a 3D shape stabilized by disulfide and ionic bonds (e.g., myoglobin).
  4. Quaternary Structure: Two or more polypeptide chains assemble (e.g., hemoglobin).
3. Differentiate between fibrous and globular proteins with biological significance.
Answer:
Property Fibrous Globular
Shape Long and thread-like Spherical
Solubility Insoluble in water Soluble
Function Structural (support, protection) Functional (enzymes, hormones)
Example Keratin, Collagen Enzymes, Insulin, Hemoglobin

Analytical Insight: Fibrous proteins give strength (e.g. skin elasticity), while globular proteins drive life reactions (e.g. catalysis and regulation).

4. What are conjugated proteins? Give two examples.
Answer: Conjugated proteins are made of a protein part (apoprotein) and a non-protein part (prosthetic group). Examples:
  • Hemoglobin: Protein + heme group (iron-containing)
  • Lipoprotein: Protein + lipid for fat transport
These combinations expand protein function beyond structure — enabling oxygen transport, signal recognition, or membrane stability.

15.4 – Enzymes

Key Notes

Definition: Enzymes are biocatalysts — complex protein molecules that speed up biochemical reactions without being consumed.

Characteristics: Protein nature, catalytic power, specificity, reversibility, activity conditions, saturation, inhibition.

Mechanism: Enzyme + Substrate → Enzyme-Substrate Complex → Enzyme + Product(s)

Models of Enzyme Action

Model Description Key Point
Lock and Key (Fischer, 1894) Active site fits substrate exactly like a key in a lock Explains specificity but not flexibility
Induced Fit (Koshland, 1958) Active site changes shape slightly to fit the substrate More accurate, explains enzyme flexibility and efficiency

Multiple Choice Questions

1. Enzymes are called biocatalysts because they:
  • Get consumed during reaction
  • Slow down reactions
  • Speed up reactions without changing themselves
  • Change equilibrium position
Answer: c) Speed up reactions without changing themselves
Explanation: Enzymes lower activation energy, thus increasing reaction rate but remain unchanged after reaction.
2. The lock and key model of enzyme action shows:
  • Flexibility of enzyme shape
  • Exact structural fit between enzyme and substrate
  • Induced active site change
  • Non-specific action
Answer: b) Exact structural fit between enzyme and substrate
Explanation: Proposed by Fischer — enzyme’s active site is complementary to substrate shape, ensuring specificity.
3. Which model of enzyme action explains flexibility of the active site?
  • Lock and key
  • Induced fit
  • Substrate saturation
  • Allosteric model
Answer: b) Induced fit
Explanation: According to Koshland, enzyme’s active site molds around the substrate for better catalysis.
4. Pepsin works best in the stomach at:
  • pH 2
  • pH 4
  • pH 7
  • pH 9
Answer: a) pH 2
Explanation: Pepsin is a gastric enzyme adapted to highly acidic conditions.

Short Questions

1. What are enzymes and why are they called biocatalysts?
Answer: Enzymes are proteins that catalyze biochemical reactions inside cells. They speed up reactions by lowering the activation energy. They are called biocatalysts because they operate under mild biological conditions, increase rate of reaction without being consumed, and maintain reaction control and specificity.
2. Explain the mechanism of enzyme action.
Answer: The enzyme first binds with its substrate forming an enzyme-substrate complex (ES) at the active site. This complex stabilizes the transition state and lowers activation energy. Finally, the complex breaks, releasing products and free enzyme.
E + S ↔ ES → E + P
3. Differentiate between the lock-and-key and induced-fit models.
Answer:
Model Description Key Concept Example
Lock and Key (Fischer) Substrate fits perfectly into enzyme’s pre-shaped active site Explains specificity Sucrase-sucrose
Induced Fit (Koshland) Active site changes shape slightly to mold around substrate Explains flexibility Hexokinase-glucose

Analytical Note: Induced-fit model is more realistic — it shows that enzymes are dynamic, not rigid structures.

4. Describe the effects of temperature on enzyme activity.
Answer:
  • Low Temp: Reaction rate is slow due to reduced kinetic energy.
  • Optimum Temp (≈37°C): Maximum enzyme activity.
  • High Temp (>50°C): Hydrogen bonds break → loss of 3D structure → denaturation.

Analytical Insight: Denaturation is irreversible; once shape is lost, the active site no longer binds the substrate — explaining enzyme sensitivity to fever or heat sterilization.