Unit 4 - Practice Quiz

The four main categories of large biological molecules essential for life are carbohydrates, lipids, proteins, and nucleic acids.

Carbohydrates, especially glucose, are broken down during cellular respiration to provide a quick source of energy (ATP) for cells.

Proteins are polymers made up of long chains of repeating monomer units called amino acids, linked by peptide bonds.

Glucose is a simple sugar and is the most basic unit of carbohydrates, making it a monosaccharide.

Lipids are a diverse group of molecules characterized by their nonpolar nature, which makes them hydrophobic, or insoluble in water.

A nucleotide, the monomer of nucleic acids like DNA and RNA, consists of a pentose (five-carbon) sugar, a phosphate group, and a nitrogenous base.

The primary structure of a protein is the unique linear sequence of its amino acids, determined by the genetic code.

Sucrose is formed by a glycosidic bond between one molecule of glucose and one molecule of fructose.

Secondary structure refers to the local folding of the polypeptide chain into repeating patterns like the -helix and -sheet, stabilized by hydrogen bonds.

A triglyceride molecule is an ester derived from one molecule of glycerol and three molecules of fatty acids. They are the main constituents of body fat.

Starch is a polymer of glucose and is the primary way plants store energy, commonly found in potatoes, rice, and wheat.

In RNA, the base uracil (U) replaces thymine (T), which is found in DNA. Both uracil and thymine pair with adenine.

A peptide bond is a covalent chemical bond formed between two amino acid molecules.

Cellulose is a major structural component of the primary cell wall of green plants, providing rigidity and support.

The tertiary structure is the final 3D conformation of a single protein molecule, resulting from interactions between the side chains (R groups) of the amino acids.

Nucleic acids, specifically Deoxyribonucleic Acid (DNA), contain the genetic instructions for the development, functioning, and reproduction of all known organisms.

Fructose is a simple monosaccharide and is often called "fruit sugar" due to its high concentration in many fruits.

A phospholipid molecule has a hydrophilic (water-loving) head containing a phosphate group and two hydrophobic (water-fearing) tails made of fatty acids.

These terms classify carbohydrates based on the number of sugar units they contain: mono- (one), di- (two), and poly- (many).

Maltose is formed from two units of glucose joined with a glycosidic bond. It is often found in germinating seeds as they break down their starch stores for food.

The surface of a water-soluble globular protein is typically rich in hydrophilic (polar and charged) amino acids that interact favorably with water. Substituting one hydrophilic-type residue (hydrophobic valine is a mistake in the premise, it should be a polar one being replaced by another polar one, or let's assume the question meant a surface residue is replaced) for another (charged aspartic acid) on the surface is less likely to disrupt the overall tertiary structure than a similar change in the hydrophobic core. Both can interact with the aqueous environment, so the change might have minimal impact.

The key difference between starch and cellulose lies in the stereochemistry of their glycosidic linkages. Starch uses -1,4 linkages, which the human enzyme amylase can break. Cellulose uses -1,4 linkages, which require the enzyme cellulase for digestion, an enzyme that humans do not produce.

Cis-double bonds in unsaturated fatty acids create kinks in the hydrocarbon tails. These kinks prevent the phospholipids from packing tightly together, which increases the fluidity and permeability of the membrane. Trans fats are more linear and pack more tightly, reducing fluidity.

According to Chargaff's rules, in a DNA double helix, Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C). Therefore, %A = %T and %G = %C. If T = 35%, then A = 35%. The total percentage of A and T is 35% + 35% = 70%. The remaining percentage for G and C is 100% - 70% = 30%. Since %G = %C, Guanine will be half of the remainder: 30% / 2 = 15%.

The defining difference between an aldose and a ketose sugar is the location of their carbonyl group. In glucose (an aldose), the carbonyl group is at the end of the carbon chain (C1), forming an aldehyde. In fructose (a ketose), the carbonyl group is within the chain (at C2), forming a ketone.

The primary structure is the linear sequence of amino acids in a polypeptide chain. This sequence is determined by the genetic code, where the sequence of nucleotides in a gene is transcribed and translated to specify the order of amino acids.

A sugar is reducing if its anomeric carbon is free to form an open-chain structure with a free aldehyde group. In sucrose, the glycosidic bond links the anomeric carbon of glucose (C1) to the anomeric carbon of fructose (C2). Since both anomeric carbons are locked in the bond, neither can open up, making sucrose a non-reducing sugar.

The hydrophobic effect is the major thermodynamic driving force. Water molecules become more ordered around the nonpolar tails. By aggregating, the phospholipids bury their hydrophobic tails away from water, which increases the entropy (disorder) of the water molecules, a thermodynamically favorable process.

Reducing agents like -mercaptoethanol specifically cleave disulfide bonds (-S-S-), which are covalent bonds formed between the side chains of cysteine residues. These bonds are a key feature that helps stabilize the tertiary and quaternary structures of many proteins.

The presence of the 2'-hydroxyl (-OH) group on the ribose sugar in RNA makes it susceptible to alkaline hydrolysis, where the -OH group can attack the adjacent phosphodiester bond, cleaving the RNA backbone. DNA lacks this reactive group, making its sugar-phosphate backbone much more stable over long periods.

Biological polymers are built from specific monomeric subunits. Proteins are polymers of amino acids. Nucleic acids (like DNA and RNA) are polymers of nucleotides. Polysaccharides (like starch and cellulose) are polymers of monosaccharides.

Both the -helix and the -pleated sheet are regular, repeating structures that arise from hydrogen bonding between the carbonyl oxygen (C=O) and amide hydrogen (N-H) groups of the peptide backbone, not the side chains (R-groups).

Enzymes that mobilize glucose from glycogen act on the non-reducing ends of the polymer chains. The numerous branches provide many such ends, allowing for the simultaneous and rapid release of glucose molecules when the body requires a quick source of energy.

This is the fundamental definition. Monosaccharides are the simplest form of carbohydrates (e.g., glucose). Disaccharides are formed when two monosaccharides are linked together via a dehydration reaction, creating a glycosidic bond (e.g., sucrose = glucose + fructose).

A triglyceride consists of a glycerol molecule bonded to three fatty acid tails. Unlike phospholipids, they lack a polar phosphate head group. This makes the entire molecule nonpolar and hydrophobic, ideal for packing tightly into lipid droplets for efficient, water-free energy storage, but unsuitable for forming a bilayer membrane which requires an amphipathic nature.

The two strands of the DNA double helix are held together by hydrogen bonds between the complementary nitrogenous bases. Adenine (A) and Thymine (T) form two hydrogen bonds, while Guanine (G) and Cytosine (C) form three hydrogen bonds. These bonds are individually weak but collectively strong enough to stabilize the helix.

The sequence of amino acids (primary structure) determines how a protein folds into a unique three-dimensional shape. The diversity in the side chains (R-groups)—which can be polar, nonpolar, acidic, or basic—allows for an enormous variety of structures and chemical surfaces, enabling proteins to perform a vast array of specific functions.

The linear form of glucose cyclizes when the hydroxyl group on carbon 5 (C5) acts as a nucleophile, attacking the electrophilic carbon of the aldehyde group on carbon 1 (C1). This intramolecular reaction forms a stable six-membered ring called a hemiacetal.

Hydrolysis is the process of breaking a bond by adding a water molecule. Since maltose is formed from two -glucose units, its complete hydrolysis will break the glycosidic bond and yield its constituent monomers, which are two molecules of -glucose.

Quaternary structure refers to the arrangement and interaction of multiple, separate polypeptide subunits to form a single, functional protein complex. Since hemoglobin is composed of four such subunits, its overall structure is described as quaternary.

The primary structure is changed (Cys -> Ser), but the direct structural disruption is at the quaternary level because the inter-subunit disulfide bridge is lost. This covalent bond specifically stabilizes the assembly of multiple subunits. While the subunit's tertiary structure might be slightly altered, the primary impact is on the multimer's stability and assembly.

The hydrogen bonds between the C=O and N-H groups of the peptide backbone stabilize the -helix. In a nonpolar solvent with a low dielectric constant, electrostatic interactions (like H-bonds) are much stronger because there is less charge screening from the solvent. Therefore, the internal backbone H-bonds are strengthened, increasing the helix's stability.

The key is the stereochemistry of the glycosidic bond. The -1,4 linkage in cellulose creates a linear, flat chain. These chains can stack efficiently, forming a massive network of hydrogen bonds between chains, resulting in a strong, insoluble fiber. The -1,4 linkage in amylose forces the chain into a helical coil, which is more accessible to water and prevents the tight, crystalline packing seen in cellulose.

While increasing salt concentration (Na⁺) also increases by shielding phosphate repulsion, the most significant intrinsic factor is base composition. A G-C pair is stabilized by three hydrogen bonds, whereas an A-T pair is stabilized by two. Replacing an A-T pair with a G-C pair adds one hydrogen bond and improves base-stacking interactions, significantly increasing the energy required to separate the strands, thus raising the .

For a rigid, low-permeability membrane, lipid packing must be maximized. Long, saturated fatty acid tails lack kinks and can pack tightly via van der Waals forces. Cholesterol acts as a 'fluidity buffer'; at physiological temperatures (like 37°C), it inserts between phospholipids, restricting their movement and thus increasing rigidity and decreasing permeability.

While Proline is known as an '-helix breaker' because its cyclic side chain restricts the backbone dihedral angle , this same property makes it conformationally ideal for the tight turns that connect strands in a -sheet. Glycine is also common in turns due to its flexibility. Replacing Glycine with Proline in a turn is often a stabilizing substitution, not a destabilizing one, contrary to its effect within a helix.

Epimers differ at a single chiral center. The mild oxidation of the C1 aldehyde to a carboxylic acid does not affect the stereochemistry of any of the other chiral centers (C2, C3, C4, C5). Therefore, the resulting aldonic acids will still differ only at the C2 carbon and will remain epimers of each other.

In a phosphodiester linkage, the phosphate is bonded to two sugars, leaving one acidic -OH group. This structure is a phosphate monoester derivative. The pKa of this remaining proton is significantly lower (around 1) than the first pKa of free phosphoric acid (~2.1) because the electron-donating effect of an -OH group is replaced by the less-donating -OR group of the sugar. This makes the remaining proton much more acidic and ensures it is deprotonated at pH 7.

Saponification breaks the three ester bonds in a triglyceride. This process releases the glycerol backbone (one mole) and the three fatty acid tails. In the presence of a strong base like NaOH, the carboxylic acid groups of the fatty acids are deprotonated, forming their corresponding carboxylate salts (soaps). Since the starting triglyceride was heterogeneous, three different types of fatty acid salts will be produced.

The native tertiary structure is required to bring the correct cysteine residues into close proximity for proper disulfide bond formation. Urea disrupts the non-covalent interactions (like the hydrophobic effect) that drive folding. If oxidation occurs while the protein is still denatured in urea, the chain is a random coil, and disulfide bonds will form between whichever cysteines are spatially close by chance, leading to incorrect, non-native pairings and a misfolded, inactive protein.

A sugar is 'reducing' if its anomeric carbon is part of a hemiacetal (or hemiketal) group, which exists in equilibrium with an open-chain aldehyde (or ketone) form. In maltose (glucose--1,4-glucose), the anomeric carbon of the second glucose is a free hemiacetal. In sucrose (glucose--1,-2-fructose), the glycosidic bond links the anomeric carbon of glucose (C1) to the anomeric carbon of fructose (C2). Both anomeric carbons are tied up in the bond, so neither ring can open, and the sugar is non-reducing.

This statement correctly links monomer properties to polymer function for both classes. For sugars, the vs linkage creates vastly different polymers (helical starch for energy vs. fibrous cellulose for structure). For proteins, the planar nature of the peptide bond constrains the polypeptide backbone, allowing for stable secondary structures like -helices and -sheets, which are fundamental to the protein's final 3D shape and function.

The Ramachandran plot is determined by steric clashes. Proline is unique because its side chain forms a cyclic structure with its own backbone nitrogen atom. This locks the N-C bond, restricting the angle to a narrow range around -60°. This makes its allowed regions on the Ramachandran plot the most restricted of all amino acids. Glycine is the opposite; its lack of a bulky side chain makes it the most flexible.

Under alkaline conditions (high pH), the 2'-hydroxyl group on the ribose sugar of RNA can be deprotonated to form a reactive alkoxide ion. This ion then performs an intramolecular nucleophilic attack on the adjacent phosphorus atom of the phosphodiester backbone, leading to cleavage. DNA lacks this 2'-OH group and is therefore stable in alkaline solutions.

Human amylase is an enzyme that specifically hydrolyzes the -1,4 glycosidic bonds found in starch and glycogen. Therefore, a hydrogel made from these polysaccharides would be degradable by this endogenous enzyme, allowing for controlled release. Cellulose and chitin contain -1,4 linkages, which humans cannot digest. Hyaluronic acid and alginate are not substrates for amylase.

Cholesterol has a dual, temperature-dependent role. Its rigid steroid ring structure restricts the motion of phospholipid tails at physiological/high temperatures, thus decreasing fluidity. However, at low temperatures, this same rigid structure prevents the saturated fatty acid tails from packing tightly and crystallizing (freezing), thereby increasing fluidity and maintaining the membrane's functional state.

The pI is the pH where a protein has no net charge. Replacing basic residues (Lys, Arg; positive charge) with acidic residues (Asp, Glu; negative charge) makes the protein more negative overall. A lower pH (more H⁺) is now needed to achieve neutrality, so the pI decreases. A protein's solubility is lowest at its pI. The new, lower pI will be closer to pH 5.0 than the original pI was. As the solution pH approaches the protein's pI, its net charge approaches zero, reducing electrostatic repulsion and causing aggregation, thus decreasing solubility.

While the pyranose form is more stable for free fructose in solution, its incorporation into sucrose involves a specific glycosidic bond between the anomeric C1 of glucose and the anomeric C2 of fructose. This -1,-2 linkage thermodynamically traps the fructose molecule in its five-membered furanose ring configuration.

There are two key chemical reasons for DNA's stability. First, the 2'-OH in RNA can act as an internal nucleophile, making the phosphodiester backbone susceptible to hydrolysis. DNA's lack of this group makes it far more stable. Second, cytosine can spontaneously deaminate to form uracil. In DNA, repair enzymes recognize uracil as an error and replace it. If RNA used uracil as a standard base, the cell could not distinguish a correct uracil from a mutated cytosine. Using thymine (5-methyluracil) solves this information-integrity problem.

The peptide is overwhelmingly composed of amino acids with nonpolar, hydrophobic side chains (Ala, Val, Leu, Ile, Trp, Phe, Met). In water, minimizing the surface area of these groups exposed to the solvent is entropically favorable. This drives the peptide to fold and bury the hydrophobic side chains in a central core, a phenomenon known as the hydrophobic effect. This is the dominant force driving the initial collapse of a polypeptide chain into a compact structure.