Explain the roles of amino acids in peptide hormones and neurotransmitters, and describe the fluid mosaic model of the plasma membrane.

Here are the answers to the remaining questions:

1. Functions of Amino Acids in Peptide Hormones and Neurotransmitters
Amino acids are fundamental to the structure and function of both peptide hormones and neurotransmitters.

• Peptide hormones are short chains of amino acids linked by peptide bonds. They act as signaling molecules, regulating diverse physiological processes such as growth, metabolism, and reproduction. For example, insulin (a peptide hormone) regulates blood glucose levels, and oxytocin plays a role in social bonding and childbirth.
• Neurotransmitters are chemical messengers that transmit signals between neurons. Some amino acids directly function as neurotransmitters, while others serve as precursors. For instance, glutamate is a major excitatory neurotransmitter, and GABA (gamma-aminobutyric acid), derived from glutamate, is a major inhibitory neurotransmitter. Tyrosine is a precursor for catecholamine neurotransmitters like dopamine and norepinephrine.

3. (a) Fluid Mosaic Model of Plasma Membrane
The Fluid Mosaic Model describes the plasma membrane as a dynamic, fluid structure composed of a lipid bilayer with proteins embedded within it or associated with its surfaces. The lipid bilayer consists of phospholipids arranged with their hydrophilic heads facing outwards and hydrophobic tails facing inwards, forming a barrier. Proteins can be integral (spanning the membrane) or peripheral (loosely attached to the surface). Both lipids and proteins can move laterally within the membrane, giving it a "fluid" character, while the arrangement of components forms a "mosaic."

3. (b) Composition of Biological Membrane and Factors Contributing to Membrane Fluidity

• Composition: Biological membranes are primarily composed of lipids (phospholipids, cholesterol, glycolipids) and proteins (integral and peripheral). Carbohydrates are also present, typically attached to lipids (glycolipids) or proteins (glycoproteins) on the outer surface.
• Factors contributing to fluidity:
  • Unsaturated fatty acids: The presence of double bonds in fatty acid tails creates kinks, preventing tight packing and increasing fluidity.
  • Cholesterol: At moderate temperatures, cholesterol reduces membrane fluidity by restricting phospholipid movement. At low temperatures, it prevents tight packing, thus increasing fluidity.
  • Temperature: Higher temperatures increase the kinetic energy of lipids, leading to greater fluidity.
  • Length of fatty acid tails: Shorter fatty acid tails reduce the extent of hydrophobic interactions, increasing fluidity.

3. (c) Functions of Glycoproteins in the Cell Membrane
Glycoproteins, which are proteins with attached carbohydrate chains, are found on the outer surface of the cell membrane and perform several crucial functions:

• Cell-cell recognition: They act as markers that allow cells to recognize each other, which is vital for immune responses and tissue formation.
• Cell adhesion: They help cells bind to each other to form tissues and organs.
• Receptors: They can serve as receptors for hormones, neurotransmitters, and other signaling molecules, initiating cellular responses.
• Structural support: They contribute to the structural integrity of the cell surface.

4. (a) Michaelis-Menten Equation
The Michaelis-Menten equation describes the rate of enzymatic reactions, relating reaction velocity to substrate concentration:

$$V_0 = \frac{V_{max}[S]}{K_m + [S]}$$

Where:

• $V_0$ is the initial reaction velocity.
• $V_{max}$ is the maximum reaction velocity when the enzyme is saturated with substrate.
• $[S]$ is the substrate concentration.
• $K_m$ (Michaelis constant) is the substrate concentration at which the reaction velocity is half of $V_{max}$.

4. (b) Enzyme Related Terms

• (i) Enzyme Active Site: The active site is a specific region on an enzyme molecule where substrate molecules bind and undergo a chemical reaction. It is typically a three-dimensional cleft or pocket formed by amino acid residues that are brought together by the enzyme's specific folding. The active site's unique shape and chemical properties facilitate substrate binding and catalysis.
• (ii) Enzyme Inhibition: Enzyme inhibition is a process where a molecule (an inhibitor) binds to an enzyme and decreases its activity. This can occur reversibly or irreversibly. Inhibitors can compete with the substrate for the active site (competitive inhibition), bind to a different site on the enzyme (non-competitive inhibition), or bind only to the enzyme-substrate complex (uncompetitive inhibition).
• (iii) Enzyme Activation: Enzyme activation refers to any process that increases the catalytic activity of an enzyme. This can involve various mechanisms, such as the binding of an allosteric activator to a site distinct from the active site, covalent modification (e.g., phosphorylation), or proteolytic cleavage of a zymogen (inactive precursor) to yield an active enzyme.

4. (c) Therapeutic Drugs and Michaelis-Menten Kinetics
Many therapeutic drugs function by interacting with enzymes, and their metabolism often follows Michaelis-Menten kinetics.

• Drugs following Michaelis-Menten kinetics: These drugs are metabolized by enzymes that can become saturated at higher drug concentrations. For example, ethanol metabolism by alcohol dehydrogenase and aldehyde dehydrogenase follows Michaelis-Menten kinetics. At low concentrations, the metabolism is first-order (rate proportional to concentration), but at high concentrations, the enzymes become saturated, and the metabolism approaches zero-order (constant rate, independent of concentration). This can lead to drug accumulation and toxicity if the dose is too high.
• Drugs not following Michaelis-Menten kinetics (e.g., first-order kinetics): Some drugs are metabolized by enzymes that are not easily saturated within the therapeutic concentration range. Their elimination rate is directly proportional to their concentration. For example, many drugs are eliminated via glomerular filtration in the kidneys, which often follows first-order kinetics. The rate of elimination decreases as the drug concentration decreases, without enzyme saturation being a limiting factor.

5. (a) Watson and Crick Structure of DNA
The Watson and Crick model describes DNA as a double helix structure.

• It consists of two polynucleotide strands coiled around a central axis, forming a right-handed helix.
• The sugar-phosphate backbone (composed of deoxyribose sugars and phosphate groups) forms the outer part of the helix, while the nitrogenous bases (adenine, guanine, cytosine, thymine) project inwards.
• The two strands are antiparallel, meaning they run in opposite 5' to 3' directions.
• The bases on opposite strands are paired specifically: Adenine (A) always pairs with Thymine (T) via two hydrogen bonds, and Guanine (G) always pairs with Cytosine (C) via three hydrogen bonds. This specific pairing is known as complementary base pairing.
• The base pairs are stacked perpendicular to the helical axis, contributing to the stability of the structure.

5. (b) Major Differences Between DNA and RNA

Feature	DNA (Deoxyribonucleic Acid)	RNA (Ribonucleic Acid)
Sugar	Deoxyribose (lacks an oxygen at the 2' carbon)	Ribose (has a hydroxyl group at the 2' carbon)
Nitrogenous Bases	Adenine (A), Guanine (G), Cytosine (C), Thymine (T)	Adenine (A), Guanine (G), Cytosine (C), Uracil (U)
Structure	Typically double-stranded helix	Typically single-stranded (can fold into complex 3D structures)
Primary Function	Stores and transmits genetic information	Involved in gene expression (mRNA, tRNA, rRNA)
Stability	More stable due to deoxyribose and double-stranded nature	Less stable due to ribose and single-stranded nature

5. (c) Structure of the Following

• (i) Nucleoside: A nucleoside consists of a nitrogenous base (purine or pyrimidine) covalently linked to a pentose sugar (ribose in RNA, deoxyribose in DNA) via a $\beta$-N-glycosidic bond, typically at the 1' carbon of the sugar. It lacks a phosphate group.
  • Example: Adenosine (adenine + ribose), Deoxyguanosine (guanine + deoxyribose).
• (ii) Nucleotide: A nucleotide is a nucleoside with one or more phosphate groups attached to the sugar, usually at the 5' carbon. Nucleotides are the monomeric units of DNA and RNA.
  • Example: Adenosine monophosphate (AMP), Deoxycytidine triphosphate (dCTP).
• (iii) Phosphodiester bond between two deoxyribonucleotides: A phosphodiester bond is a covalent bond that links two deoxyribonucleotides in a 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 linkage forms the sugar-phosphate backbone of DNA.

6. (a) Steps Involved in Protein Isolation, Separation, and Purification
Protein isolation, separation, and purification typically involve several steps:

1. Cell Lysis/Homogenization: Breaking open cells or tissues to release intracellular proteins using mechanical (e.g., sonication, grinding), chemical (e.g., detergents), or enzymatic methods.
2. Centrifugation: Separating cellular debris and organelles from the soluble protein fraction based on size and density.
3. Salting Out (e.g., Ammonium Sulfate Precipitation): Adding a high concentration of salt to selectively precipitate proteins based on their solubility, which can then be removed by centrifugation.
4. Dialysis/Desalting: Removing small molecules (like salts) from the protein solution using a semi-permeable membrane or gel filtration chromatography.
5. Chromatography: A series of techniques used for high-resolution separation. Common types include:
   • Ion-exchange chromatography: Separates proteins based on their net charge.
   • Gel filtration (size exclusion) chromatography: Separates proteins based on their size and shape.
   • Affinity chromatography: Separates proteins based on specific binding interactions (e.g., antibody-antigen, enzyme-substrate analog).
   • Hydrophobic interaction chromatography: Separates proteins based on their hydrophobicity.
6. Electrophoresis (e.g., SDS-PAGE): Used to analyze protein purity and molecular weight, often as a final assessment step.

6. (b) Fibrous Proteins
Fibrous proteins are long, insoluble protein molecules that typically have an elongated, rod-like or filamentous shape. They are characterized by their structural roles, providing strength, support, and protection to tissues and cells. Their polypeptide chains are usually arranged in parallel along a single axis, forming fibers or sheets.
Examples: Collagen (provides tensile strength to connective tissues), keratin (forms hair, nails, and skin), elastin (provides elasticity to tissues), and myosin (involved in muscle contraction).

6. (c) 5 Factors That Can Denature Proteins
Protein denaturation is the process by which a protein loses its native three-dimensional structure (secondary, tertiary, and quaternary) due to the disruption of non-covalent bonds and disulfide bridges, without breaking peptide bonds. This often leads to loss of biological function.

1. Heat: Increased temperature provides kinetic energy that disrupts weak interactions (hydrogen bonds, hydrophobic interactions) holding the protein's structure.
2. Extreme pH: Changes in pH alter the ionization state of amino acid side chains, disrupting ionic bonds and hydrogen bonds, and altering the protein's overall charge.
3. Organic Solvents: Nonpolar solvents (e.g., ethanol, acetone) can disrupt hydrophobic interactions within the protein's core and interfere with hydrogen bonding.
4. Heavy Metal Ions: Ions like $\text{Pb}^{2+}$, $\text{Hg}^{2+}$, $\text{Cd}^{2+}$ can bind to sulfhydryl groups ($\text{-SH}$) or carboxylate groups, disrupting disulfide bonds and ionic interactions.
5. Detergents: Amphipathic molecules (e.g., SDS) can disrupt hydrophobic interactions and unfold proteins, especially membrane proteins, by forming micelles around them.

7. (a) Define Vitamin and Outline the Various Types
A vitamin is an organic compound required in small quantities in the diet for normal metabolic function, growth, and maintenance of an organism. Vitamins cannot be synthesized by the body (or not in sufficient amounts) and must be obtained from external sources.
Vitamins are broadly classified into two main types:

1. Fat-soluble vitamins: These are absorbed with dietary fats and stored in the body's fatty tissues and liver. They include Vitamins A, D, E, and K.
2. Water-soluble vitamins: These are not stored in the body to a significant extent and are readily excreted in urine. They include the B-complex vitamins (B1, B2, B3, B5, B6, B7, B9, B12) and Vitamin C.

7. (b) State 2 Sources, Functions, and Deficiency Symptoms from Each of the Types of Vitamins Named Above

Fat-Soluble Vitamins:

• Vitamin A (Retinol)

  • Sources: Carrots, sweet potatoes, liver, dairy products.
  • Functions: Vision (component of rhodopsin), cell growth and differentiation, immune function.
  • Deficiency Symptoms: Night blindness, xerophthalmia (dry eyes), impaired immune function.

• Vitamin D (Calciferol)

  • Sources: Sunlight exposure (skin synthesis), fatty fish (salmon), fortified milk.
  • Functions: Calcium and phosphate absorption, bone mineralization, immune regulation.
  • Deficiency Symptoms: Rickets in children (soft, weak bones), osteomalacia in adults (bone softening).

• Vitamin E (Tocopherols)

  • Sources: Vegetable oils (sunflower, corn), nuts (almonds), seeds.
  • Functions: Antioxidant (protect