Q: Q10. Give examples of:\n(a) Monosaccharide\n(b) Polysaccharide\n(c) Amino acid\n(d) Protein\n(e) Nucleic acid

Here are examples for each category of biomolecules:\n\n(a) **Monosaccharide:**\n * Glucose\n * Fructose\n * Galactose\n * Ribose\n * Deoxyribose\n\n(b) **Polysaccharide:**\n * Starch\n * Glycogen\n * Cellulose\n * Chitin\n * Inulin\n\n(c) **Amino acid:**\n * Glycine\n * Alanine\n * Valine\n * Leucine\n * Serine\n * Lysine\n\n(d) **Protein:**\n * Collagen\n * Trypsin\n * Insulin\n * Antibodies\n * Hemoglobin\n * Keratin\n\n(e) **Nucleic acid:**\n * DNA (Deoxyribonucleic acid)\n * RNA (Ribonucleic acid)

Question 1

Q1. What are macromolecules? Give examples.

Accepted Answer

Macromolecules are large, complex organic molecules that are formed by the polymerization of smaller repeating units called monomers. They typically have a molecular weight greater than 1000 Daltons (Da). These molecules are crucial for life and perform a wide variety of functions within living organisms.

Most macromolecules are polymers, meaning they are built from many identical or similar small molecular units (monomers) linked together by covalent bonds. The formation of these bonds often involves dehydration synthesis (removal of a water molecule).

**Examples of Macromolecules:**
1.  **Proteins:** Polymers of amino acids. Examples: Enzymes, antibodies, structural proteins (collagen, keratin).
2.  **Polysaccharides (Complex Carbohydrates):** Polymers of monosaccharides. Examples: Starch, glycogen, cellulose, chitin.
3.  **Nucleic Acids:** Polymers of nucleotides. Examples: DNA (deoxyribonucleic acid), RNA (ribonucleic acid).

**Note on Lipids:** While lipids (fats, oils, phospholipids, steroids) are large molecules with high molecular weights, they are generally not considered true polymers because they are not formed by the repetitive linking of identical or similar monomers in a chain. However, they are classified as biomacromolecules due to their large size and essential biological roles.

Question 2

Q2. Illustrate a glycosidic bond.

Accepted Answer

A glycosidic bond is a type of covalent bond that joins a carbohydrate (sugar) molecule to another group, which may or may not be another carbohydrate. In the context of forming disaccharides and polysaccharides, it is specifically an ether linkage formed between two monosaccharide units.

**Formation of a Glycosidic Bond:**
1.  **Reactants:** Two monosaccharide molecules (e.g., two glucose molecules).
2.  **Mechanism:** The bond is formed by a dehydration reaction (also known as condensation reaction), where a molecule of water is removed.
3.  **Bonding:** Typically, the hydroxyl group (-OH) on the anomeric carbon (carbon-1) of one monosaccharide reacts with a hydroxyl group on another carbon of a second monosaccharide (e.g., carbon-4). This results in an oxygen atom bridging the two sugar units.

**Illustration Description (for a disaccharide like Maltose):**
Imagine two alpha-D-glucose molecules.
*   **First Glucose Molecule:** Its anomeric carbon (C1) has a hydroxyl group pointing downwards (alpha configuration).
*   **Second Glucose Molecule:** Its carbon-4 (C4) also has a hydroxyl group.
*   **Reaction:** The -OH group from C1 of the first glucose and the -H from the -OH group of C4 of the second glucose are removed as a water molecule (H₂O).
*   **Result:** An oxygen atom remains, forming a covalent bond between C1 of the first glucose and C4 of the second glucose. This specific bond is called an **α-1,4-glycosidic bond**.

**Key Features:**
*   It is an ether linkage (-O-).
*   It is crucial for the formation of disaccharides (e.g., sucrose, lactose, maltose) and complex polysaccharides (e.g., starch, glycogen, cellulose).
*   The specific carbons involved (e.g., 1→4, 1→2, 1→6) and the orientation (alpha or beta) determine the type of glycosidic bond and the properties of the resulting carbohydrate.

Question 3

Q3. What is a peptide bond?

Accepted Answer

A peptide bond is a covalent chemical bond that links two amino acid molecules. It is the fundamental linkage in proteins and peptides.

**Formation of a Peptide Bond:**
1.  **Reactants:** The carboxyl group (-COOH) of one amino acid and the amino group (-NH₂) of another amino acid.
2.  **Mechanism:** The bond is formed through a dehydration reaction (condensation reaction), where a molecule of water (H₂O) is removed.
3.  **Bonding:** The carbon atom of the carboxyl group of one amino acid forms a covalent bond with the nitrogen atom of the amino group of the other amino acid. This results in an amide linkage (-CO-NH-).

**Characteristics:**
*   The peptide bond has partial double-bond character due to resonance, which makes it rigid and planar.
*   This rigidity restricts rotation around the C-N bond, influencing the overall three-dimensional structure of proteins.
*   A chain of amino acids linked by peptide bonds is called a polypeptide chain. Proteins are typically composed of one or more long polypeptide chains.

Question 4

Q4. What is meant by tertiary structure of protein?

Accepted Answer

The tertiary structure of a protein refers to the overall three-dimensional folding of a single polypeptide chain. It describes how the entire polypeptide chain, including its secondary structural elements (alpha-helices and beta-sheets) and random coils, folds into a compact, globular, and biologically active shape. This intricate folding is crucial for the protein's specific function.

**Formation and Stabilization:**
The tertiary structure is primarily stabilized by interactions between the R-groups (side chains) of the amino acids that are far apart in the primary sequence but brought close together by the folding process. These interactions include:

1.  **Hydrophobic Interactions:** Nonpolar amino acid side chains tend to cluster together in the interior of the protein, away from the aqueous environment, to minimize contact with water. This is a major driving force for protein folding.
2.  **Hydrogen Bonds:** Form between polar R-groups, or between polar R-groups and the peptide backbone, contributing to the stability of the folded structure.
3.  **Ionic Bonds (Salt Bridges):** Form between oppositely charged R-groups (e.g., between a positively charged lysine and a negatively charged aspartate).
4.  **Disulfide Bonds:** Strong covalent bonds formed between the sulfhydryl groups (-SH) of two cysteine residues. These bonds act as molecular staples, significantly stabilizing the tertiary structure, particularly in extracellular proteins.
5.  **Van der Waals Forces:** Weak, transient attractive forces between atoms that are very close to each other, contributing to the overall stability.

**Significance:**
*   The unique tertiary structure determines the protein's specific biological function (e.g., active site of an enzyme, binding site of an antibody).
*   Denaturation (loss of tertiary structure) typically leads to loss of protein function.
*   Many proteins exist and function as single polypeptide chains in their tertiary structure.

Question 5

Q5. Find and name ten interesting small molecular weight biomolecules from the given list.

Accepted Answer

Since no specific list is provided in the question, I will list ten common small molecular weight biomolecules (micromolecules) that are typically found in living organisms and discussed in the context of biomolecules. These molecules generally have a molecular weight less than 1000 Daltons.

Here are ten interesting small molecular weight biomolecules:
1.  **Glucose:** A monosaccharide, primary energy source for cells.
2.  **Amino Acids:** Building blocks of proteins (e.g., Glycine, Alanine).
3.  **Fatty Acids:** Components of lipids, used for energy storage and membrane structure (e.g., Palmitic acid).
4.  **Glycerol:** A three-carbon alcohol, forms the backbone of triglycerides and phospholipids.
5.  **Adenine:** A nitrogenous base, component of DNA, RNA, and ATP.
6.  **Ribose:** A five-carbon sugar, component of RNA and ATP.
7.  **Cholesterol:** A steroid lipid, precursor for steroid hormones and component of animal cell membranes.
8.  **Vitamins:** Organic compounds required in small quantities for normal metabolism (e.g., Vitamin C, Vitamin B1).
9.  **Hormones:** Chemical messengers (e.g., Adrenaline, Thyroxine - some are small peptides, some are steroids).
10. **Alkaloids:** Secondary metabolites, often with pharmacological effects (e.g., Morphine, Caffeine).

Question 6

Q6. Describe the important properties of enzymes.

Accepted Answer

Enzymes are biological catalysts, primarily proteinaceous in nature, that accelerate the rate of biochemical reactions without themselves being consumed in the process. They exhibit several important properties:

**1. Proteinaceous Nature:**
Most enzymes are proteins. They are typically globular proteins with a specific three-dimensional structure essential for their catalytic activity. Some nucleic acids (ribozymes) also act as enzymes.

**2. Catalytic Power:**
Enzymes are highly efficient catalysts, capable of increasing reaction rates by millions to trillions of times compared to uncatalyzed reactions. They achieve this by lowering the activation energy required for the reaction to proceed.

**3. Specificity:**
Enzymes are highly specific in their action. Each enzyme typically catalyzes only one type of reaction or acts on a very specific substrate or a group of structurally related substrates. This specificity arises from the unique shape of the enzyme's active site, which complements the shape of its substrate (lock and key model or induced fit model).

**4. Optimum Temperature and pH:**
Enzymes function most efficiently within a narrow range of temperature and pH, known as their optimum temperature and pH. Deviations from these optimal conditions can lead to a decrease in enzyme activity. For most human enzymes, the optimum temperature is around 37°C and optimum pH is near neutral (pH 7.0), though exceptions exist (e.g., pepsin in the stomach works best at pH 1.5-2.5).

**5. Denaturation:**
Being proteins, enzymes are sensitive to extreme changes in temperature and pH. High temperatures or extreme pH values can cause denaturation, leading to the loss of their specific three-dimensional structure and, consequently, their catalytic activity. Denaturation is often irreversible.

**6. Reversibility of Action:**
Many enzyme-catalyzed reactions are reversible. The same enzyme can catalyze both the forward and reverse reactions, depending on the relative concentrations of reactants and products, helping to maintain equilibrium.

**7. Small Quantity Required:**
Enzymes are required in very small quantities because they are not consumed in the reaction. They are regenerated after each catalytic cycle and can be used repeatedly.

**8. Presence of Active Site:**
Each enzyme possesses a specific region called the active site, which is a crevice or pocket where the substrate binds. The active site is formed by a specific arrangement of amino acid residues and is responsible for the enzyme's catalytic activity and specificity.

**9. Regulation of Activity:**
Enzyme activity can be regulated by various mechanisms, including allosteric regulation, feedback inhibition, covalent modification (e.g., phosphorylation), and genetic control (regulating enzyme synthesis). This allows cells to control metabolic pathways efficiently.

**10. Requirement of Co-factors:**
Many enzymes require non-protein components called co-factors for their activity. Co-factors can be inorganic ions (e.g., Zn²⁺, Mg²⁺, Fe²⁺) or organic molecules (coenzymes like NAD, FAD, vitamins, or prosthetic groups like heme). An enzyme without its co-factor is called an apoenzyme, and with its co-factor, it's a holoenzyme.

Question 7

Q7. Differentiate between a polynucleotide and a polypeptide.

Accepted Answer

The key differences between a polynucleotide and a polypeptide are summarized below:

| Feature           | Polynucleotide                                  | Polypeptide                                     |
|-------------------|-------------------------------------------------|-------------------------------------------------|
| **Definition**    | A polymer made up of many nucleotide units.     | A polymer made up of many amino acid units.     |
| **Monomer Unit**  | Nucleotide (composed of a nitrogenous base, a pentose sugar, and a phosphate group). | Amino acid (composed of an amino group, a carboxyl group, a hydrogen atom, and a variable 'R' group, all attached to a central alpha carbon). |
| **Type of Bond**  | Phosphodiester bond (links the 3'-hydroxyl of one sugar to the 5'-phosphate of the next sugar). | Peptide bond (formed between the carboxyl group of one amino acid and the amino group of another amino acid). |
| **Backbone**      | Sugar-phosphate backbone.                       | Polypeptide backbone (N-C-C repeating unit).    |
| **Examples**      | DNA (Deoxyribonucleic acid), RNA (Ribonucleic acid). | Proteins (e.g., enzymes, hormones, structural proteins like collagen, keratin). |
| **Primary Function** | Store and transmit genetic information (DNA), involved in protein synthesis (RNA). | Perform diverse functions like catalysis, transport, structural support, defense, regulation. |
| **Elements**      | C, H, O, N, P                                   | C, H, O, N, S (sulfur in some amino acids like methionine, cysteine) |
| **Directionality**| 5' to 3' direction.                             | N-terminus (amino end) to C-terminus (carboxyl end). |

In essence, polynucleotides are the building blocks of genetic material, while polypeptides are the building blocks of proteins, each with distinct chemical structures and biological roles.

Question 8

Q8. Give any three examples of metabolic transformations.

Accepted Answer

Metabolic transformations refer to the chemical reactions that occur within a living organism to maintain life. These reactions involve the breakdown (catabolism) and synthesis (anabolism) of molecules. Here are three examples:

1.  **Glucose to Lactic Acid (Glycolysis under anaerobic conditions):**
    This is a catabolic transformation. In the absence of oxygen, glucose (a 6-carbon sugar) is broken down through glycolysis into two molecules of pyruvate, which is then converted to two molecules of lactic acid. This process generates a small amount of ATP and occurs in muscle cells during intense exercise.

2.  **Amino Acids to Proteins (Protein Synthesis):**
    This is an anabolic transformation. Individual amino acids are linked together by peptide bonds to form long polypeptide chains, which then fold into functional proteins. This complex process involves ribosomes, mRNA, and tRNA, and requires energy (ATP and GTP).

3.  **Fatty Acids and Glycerol to Triglycerides (Lipogenesis):**
    This is an anabolic transformation. Excess dietary fatty acids and glycerol are synthesized into triglycerides (fats) for energy storage. This process primarily occurs in the liver and adipose tissue.

Other examples include: Photosynthesis (CO₂ and H₂O to glucose), Cellular Respiration (glucose to CO₂ and H₂O), Glycogenolysis (glycogen to glucose), Gluconeogenesis (non-carbohydrate precursors to glucose).

Question 9

Q9. Differentiate between primary and secondary metabolites.

Accepted Answer

The key differences between primary and secondary metabolites are outlined below:

| Feature           | Primary Metabolites                               | Secondary Metabolites                               |
|-------------------|---------------------------------------------------|-----------------------------------------------------|
| **Definition**    | Organic compounds directly involved in the normal growth, development, and reproduction of an organism. | Organic compounds not directly involved in the normal growth, development, or reproduction of an organism, but often have ecological roles. |
| **Essentiality**  | Essential for survival and fundamental metabolic processes. | Not essential for the immediate survival of the organism, but may provide advantages (e.g., defense, signaling). |
| **Occurrence**    | Found in almost all organisms (plants, animals, microbes). | Typically found in specific organisms or groups of organisms (e.g., certain plants, fungi, bacteria). |
| **Function**      | Direct physiological functions: energy production, structural components, genetic information, basic building blocks. | Ecological functions: defense against predators/pathogens, attractants for pollinators, signaling, competition. | | **Examples**      | Amino acids, proteins, carbohydrates (glucose, starch), lipids (fatty acids, glycerol), nucleic acids (DNA, RNA), vitamins, ATP. | Alkaloids (e.g., morphine, codeine), Flavonoids, Rubber, Essential oils (e.g., lemon grass oil), Antibiotics (e.g., penicillin), Pigments (e.g., carotenoids, anthocyanins), Toxins (e.g., abrin, ricin), Lectins (e.g., concanavalin A), Drugs (e.g., vinblastin, curcumin). |
| **Detection**     | Relatively easy to detect and quantify due to their ubiquitous nature and high concentrations. | Often present in small quantities, requiring specialized analytical techniques for detection and isolation. |

In summary, primary metabolites are the universal building blocks and energy sources for life, while secondary metabolites are specialized compounds that provide specific adaptive advantages, particularly in plants and microbes.

Question 10

Q10. Give examples of:
(a) Monosaccharide
(b) Polysaccharide
(c) Amino acid
(d) Protein
(e) Nucleic acid

Accepted Answer

Here are examples for each category of biomolecules:

(a) **Monosaccharide:**
    *   Glucose
    *   Fructose
    *   Galactose
    *   Ribose
    *   Deoxyribose

(b) **Polysaccharide:**
    *   Starch
    *   Glycogen
    *   Cellulose
    *   Chitin
    *   Inulin

(c) **Amino acid:**
    *   Glycine
    *   Alanine
    *   Valine
    *   Leucine
    *   Serine
    *   Lysine

(d) **Protein:**
    *   Collagen
    *   Trypsin
    *   Insulin
    *   Antibodies
    *   Hemoglobin
    *   Keratin

(e) **Nucleic acid:**
    *   DNA (Deoxyribonucleic acid)
    *   RNA (Ribonucleic acid)

Question 11

Q11. Explain the composition of a triglyceride.

Accepted Answer

A triglyceride, also known as triacylglycerol, is a type of lipid that serves as a major form of energy storage in animals and plants. Its composition involves two main types of molecules:

1.  **Glycerol:** This is a three-carbon alcohol with three hydroxyl (-OH) groups. It forms the backbone of the triglyceride molecule.

2.  **Fatty Acids:** These are long hydrocarbon chains with a carboxyl (-COOH) group at one end. They can be saturated (no double bonds between carbon atoms) or unsaturated (one or more double bonds between carbon atoms). The length of the hydrocarbon chain and the presence/absence of double bonds determine the specific properties of the fatty acid.

**Formation:** A triglyceride molecule is formed when one molecule of glycerol undergoes an esterification reaction with three molecules of fatty acids. Each hydroxyl group of glycerol reacts with the carboxyl group of a fatty acid, forming an **ester bond** and releasing a molecule of water. This process is a dehydration synthesis reaction.

Therefore, a triglyceride molecule consists of a glycerol backbone to which three fatty acid chains are attached via ester bonds.

Question 12

Q12. Can you describe some properties of enzymes?

Accepted Answer

Enzymes are biological catalysts, primarily proteins, that accelerate the rate of biochemical reactions without being consumed in the process. They are essential for virtually all metabolic processes in living organisms. Here are some key properties of enzymes:

### 1. Catalytic Power
Enzymes are highly efficient catalysts, capable of increasing reaction rates by factors of 10^6 to 10^12 or more compared to uncatalyzed reactions. They achieve this by lowering the activation energy of the reaction.

### 2. Specificity
Enzymes are highly specific in their action. Each enzyme typically catalyzes only one type of reaction or acts on a very limited range of substrates. This specificity arises from the unique three-dimensional structure of the enzyme's active site, which is complementary to the shape of its specific substrate(s). This is often explained by the 'lock and key' or 'induced fit' model.

### 3. Reversibility
Most enzyme-catalyzed reactions are reversible. The enzyme can catalyze the reaction in both forward and reverse directions, depending on the relative concentrations of substrates and products, helping to maintain equilibrium.

### 4. Sensitivity to Temperature
Enzymes have an optimal temperature at which they exhibit maximum activity. Below this temperature, enzyme activity decreases due to reduced kinetic energy of molecules. Above the optimal temperature, the enzyme's protein structure begins to denature (unfold), leading to a permanent loss of its catalytic activity. The optimal temperature for most human enzymes is around 37°C.

### 5. Sensitivity to pH
Similar to temperature, enzymes have an optimal pH range for maximum activity. Deviations from this optimal pH (either too acidic or too alkaline) can alter the ionization state of amino acid residues in the active site, disrupting the enzyme's three-dimensional structure and leading to denaturation and loss of activity. For example, pepsin works best in acidic conditions (pH 1.5-2.5), while trypsin works best in alkaline conditions (pH 8-9).

### 6. Proteinaceous Nature
Most enzymes are proteins, composed of one or more polypeptide chains. Their catalytic activity is dependent on their specific three-dimensional conformation. However, some RNA molecules (ribozymes) also exhibit catalytic activity.

### 7. Presence of Active Site
Each enzyme possesses a specific region called the active site, which is a three-dimensional cleft or pocket formed by the folding of the polypeptide chain. The substrate binds to this active site, forming an enzyme-substrate complex, where the catalytic reaction takes place.

### 8. Requirement for Cofactors (for some enzymes)
Many enzymes require non-protein components called cofactors for their activity. Cofactors can be inorganic ions (e.g., Mg²⁺, Zn²⁺, Fe²⁺) or organic molecules (coenzymes like NAD⁺, FAD, vitamins). If the coenzyme is tightly bound to the enzyme, it is called a prosthetic group.

### 9. Regulation
Enzyme activity can be regulated by various mechanisms, including allosteric regulation, feedback inhibition, covalent modification (e.g., phosphorylation), and gene expression. This allows cells to control metabolic pathways efficiently.

In summary, enzymes are highly efficient, specific, and regulated biological catalysts crucial for life processes, whose activity is sensitive to environmental factors like temperature and pH.

Question 13

Q13. Briefly describe the structure of an amino acid.

Accepted Answer

Amino acids are the fundamental building blocks of proteins. All amino acids (except proline, which is an imino acid) share a common basic structure, which can be described as follows:

At the center of an amino acid molecule is a carbon atom, known as the **alpha (α) carbon**. This alpha carbon is covalently bonded to four different groups:

1.  **An Amino Group (-NH₂):** This is a basic group, typically protonated to -NH₃⁺ at physiological pH.
2.  **A Carboxyl Group (-COOH):** This is an acidic group, typically deprotonated to -COO⁻ at physiological pH.
3.  **A Hydrogen Atom (-H):** A single hydrogen atom.
4.  **A Variable Side Chain (R-group):** This is the unique part of each amino acid that distinguishes it from others. The R-group can vary greatly in size, shape, charge, and chemical properties (e.g., nonpolar, polar uncharged, acidic, basic). It is the R-group that determines the specific characteristics and functions of an amino acid within a protein.

Due to the presence of both an amino group (basic) and a carboxyl group (acidic), amino acids are amphoteric and can exist as **zwitterions** (dipolar ions) at physiological pH, where the amino group is protonated and the carboxyl group is deprotonated, resulting in a molecule with both positive and negative charges but a net neutral charge.

Question 14

Q14. Describe the structure of a nucleotide. Compare it with a nucleoside.

Accepted Answer

### Structure of a Nucleotide
A nucleotide is the fundamental monomeric unit of nucleic acids (DNA and RNA). Each nucleotide is composed of three distinct components covalently linked together:

1.  **A Nitrogenous Base:** These are heterocyclic compounds containing nitrogen. They are broadly classified into two types:
    *   **Purines:** Double-ring structures (adenine - A, guanine - G).
    *   **Pyrimidines:** Single-ring structures (cytosine - C, thymine - T in DNA, uracil - U in RNA).
    The nitrogenous base is attached to the 1' carbon of the pentose sugar by an N-glycosidic bond.

2.  **A Pentose Sugar:** This is a five-carbon sugar. The type of sugar determines whether the nucleotide is part of DNA or RNA:
    *   **Deoxyribose:** Found in DNA, it lacks an oxygen atom at the 2' carbon position.
    *   **Ribose:** Found in RNA, it has a hydroxyl (-OH) group at the 2' carbon position.

3.  **One or More Phosphate Groups:** These are acidic groups derived from phosphoric acid. In a mononucleotide, one phosphate group is attached to the 5' carbon of the pentose sugar via a phosphoester bond. Nucleotides can have one (monophosphate), two (diphosphate), or three (triphosphate) phosphate groups, such as ATP (adenosine triphosphate).

Therefore, a nucleotide can be represented as: **Nitrogenous Base – Pentose Sugar – Phosphate Group(s)**.

### Structure of a Nucleoside
A nucleoside is a simpler structure compared to a nucleotide. It consists of only two components:

1.  **A Nitrogenous Base:** Same as described for a nucleotide (purine or pyrimidine).
2.  **A Pentose Sugar:** Same as described for a nucleotide (deoxyribose or ribose).

The nitrogenous base is attached to the 1' carbon of the pentose sugar by an N-glycosidic bond. A nucleoside lacks the phosphate group(s).

Therefore, a nucleoside can be represented as: **Nitrogenous Base – Pentose Sugar**.

### Comparison between Nucleotide and Nucleoside

In essence, a nucleoside is a nucleotide without its phosphate group(s). The addition of a phosphate group to a nucleoside converts it into a nucleotide.

Question 15

Q15. Describe the types of bonds holding together a DNA strand.

Accepted Answer

A single DNA strand (polynucleotide chain) is a polymer of deoxyribonucleotides. The bonds that hold together the components within a single DNA strand are primarily:

1.  **N-glycosidic Bond:** This bond links the nitrogenous base (purine or pyrimidine) to the 1' carbon of the deoxyribose sugar. Specifically, it's formed between the N-9 of a purine or N-1 of a pyrimidine and the 1' carbon of the sugar.

2.  **Phosphoester Bond:** This bond links the phosphate group to the 5' carbon of the deoxyribose sugar. This forms the sugar-phosphate backbone of the DNA strand.

3.  **Phosphodiester Bond:** This is the crucial bond that links successive nucleotides within a single DNA strand. It is formed between the phosphate group attached to the 5' carbon of one deoxyribonucleotide and the hydroxyl group at the 3' carbon of the adjacent deoxyribonucleotide. This creates a strong, covalent sugar-phosphate backbone, giving the DNA strand its directionality (5' to 3').

While the question specifically asks about 'a DNA strand', it's important to note that in a double-stranded DNA molecule, two such polynucleotide strands are held together by **hydrogen bonds** formed between complementary nitrogenous bases (A with T, G with C). These hydrogen bonds are crucial for maintaining the double helix structure but are intermolecular bonds between strands, not intramolecular bonds within a single strand.

Question 16

Q16. Explain the composition of a triglyceride.

Accepted Answer

Triglycerides are the most common type of lipid found in the body and are the main form of fat storage in animals and plants. They are esters formed from one molecule of glycerol and three molecules of fatty acids.

### Composition of a Triglyceride:

1.  **Glycerol Backbone:**
    *   Glycerol (also known as propane-1,2,3-triol) is a simple three-carbon alcohol. Its chemical formula is C₃H₈O₃. It has three hydroxyl (-OH) groups, each capable of forming an ester bond.
    *   It serves as the structural backbone to which fatty acids are attached.

2.  **Fatty Acids:**
    *   Fatty acids are long hydrocarbon chains with a carboxyl group (-COOH) at one end. They can be saturated (no double bonds between carbon atoms) or unsaturated (one or more double bonds).
    *   In a triglyceride, three molecules of fatty acids are attached to the glycerol backbone.
    *   The fatty acids can be the same or different in terms of chain length and degree of saturation.

### Formation of a Triglyceride:

Triglycerides are formed through a process called **esterification** or **dehydration synthesis**. Each hydroxyl group of the glycerol molecule reacts with the carboxyl group of a fatty acid molecule. During this reaction, a molecule of water is removed, and an **ester bond** (C-O-C=O) is formed between the glycerol and the fatty acid. Since there are three hydroxyl groups on glycerol, three fatty acid molecules are attached, resulting in the formation of three ester bonds and the release of three water molecules.

**General Reaction:**

Glycerol + 3 Fatty Acids → Triglyceride + 3 H₂O

### Example:

If the three fatty acids are palmitic acid, stearic acid, and oleic acid, the resulting triglyceride would be a mixed triglyceride. If all three fatty acids are the same (e.g., three palmitic acids), it's a simple triglyceride (e.g., tripalmitin).

Question 17

Q17. Can you describe some properties of enzymes?

Accepted Answer

Enzymes are biological catalysts that speed up the rate of biochemical reactions without being consumed in the process. They are crucial for almost all metabolic processes in living organisms. Here are some key properties of enzymes:

### Properties of Enzymes:

1.  **Proteinaceous Nature:** Almost all enzymes are proteins. They have complex three-dimensional structures, including primary, secondary, tertiary, and sometimes quaternary structures. This specific 3D conformation is essential for their catalytic activity.

2.  **Catalytic Activity:** Enzymes are highly efficient catalysts. They increase the rate of reactions by lowering the activation energy required for the reaction to proceed. A small amount of enzyme can catalyze the conversion of a large amount of substrate into products.

3.  **Specificity:** Enzymes are highly specific in their action. Each enzyme typically catalyzes only one type of reaction or acts on a very specific substrate or a group of structurally related substrates. This specificity is due to the unique shape of their active site, which fits only specific substrate molecules (lock and key model or induced fit model).

4.  **Optimum Temperature:** Enzymes exhibit maximum activity at a specific temperature, known as the optimum temperature (typically around 37°C for human enzymes). Below this temperature, enzyme activity decreases due to reduced kinetic energy. Above the optimum temperature, enzymes start to denature, losing their 3D structure and catalytic activity irreversibly.

5.  **Optimum pH:** Similar to temperature, enzymes function best within a narrow pH range, known as the optimum pH. Deviations from the optimum pH (either too acidic or too alkaline) can alter the ionization state of amino acid residues in the active site, leading to changes in enzyme structure and activity, eventually causing denaturation.

6.  **Sensitivity to Inhibitors:** Enzyme activity can be regulated by various molecules called inhibitors. Inhibitors can decrease or completely block enzyme activity by binding to the active site (competitive inhibition) or to another site on the enzyme (non-competitive inhibition), altering its conformation.

7.  **Reversibility of Action:** Many enzyme-catalyzed reactions are reversible. The same enzyme can catalyze both the forward and reverse reactions, depending on the relative concentrations of substrates and products.

8.  **Efficiency:** Enzymes are incredibly efficient. They can increase reaction rates by factors of 10⁶ to 10¹² compared to uncatalyzed reactions.

9.  **Denaturation:** Enzymes are sensitive to harsh conditions like extreme temperatures, pH, or strong chemicals. These conditions can disrupt their delicate 3D structure, leading to loss of function, a process called denaturation.

10. **Cofactors:** Many enzymes require non-protein components called cofactors for their activity. Cofactors can be inorganic ions (e.g., Mg²⁺, Zn²⁺) or organic molecules (coenzymes like NAD⁺, FAD, vitamins). If the cofactor is tightly bound to the enzyme, it's called a prosthetic group.

Question 18

Q18. Briefly describe the structure of an amino acid.

Accepted Answer

Amino acids are the fundamental building blocks (monomers) of proteins. Their general structure is characterized by a central carbon atom, known as the **alpha (α) carbon**, to which four different groups are covalently bonded:

1.  **Amino Group (-NH₂):** A basic group, typically protonated to -NH₃⁺ at physiological pH.
2.  **Carboxyl Group (-COOH):** An acidic group, typically deprotonated to -COO⁻ at physiological pH.
3.  **Hydrogen Atom (-H):** A single hydrogen atom.
4.  **R-group (Side Chain):** This is a variable group, also known as the side chain, which differs among the 20 common amino acids. The R-group determines the specific chemical properties (e.g., polarity, charge, size, shape) of each amino acid and, consequently, the protein it forms.

Due to the presence of both an acidic carboxyl group and a basic amino group, amino acids can exist as **zwitterions** (dipolar ions) at physiological pH, where both groups are ionized (i.e., -NH₃⁺ and -COO⁻), resulting in a net neutral charge for the molecule as a whole (though it contains both positive and negative charges).

Chapter 9: Biomolecules