Fundamentals of Biological Molecules and Processes Quiz

The physical state of a fatty acid at room temperature depends largely on the number of double bonds in its hydrocarbon chain. Unsaturated fatty acids contain one or more cis double bonds that introduce kinks, preventing tight packing and lowering melting temperature, making them liquid. Saturated fatty acids lack double bonds and pack closely, creating higher melting points and solid structures. Chain length also contributes, but double bonds are the primary factor.

UDP-glucose is the activated sugar intermediate used in multiple biosynthetic pathways, including glycogen synthesis and lactose formation. Its high-energy UDP group allows efficient transfer of glucose to growing polysaccharides. Glycogen synthase uses UDP-glucose to extend chains, while lactose synthase uses it to form the β-1,4 bond with galactose. Other intermediates like glucose-1-phosphate or fructose-6-phosphate are precursors but not the direct donor in these pathways.

Cellulose is more rigid than starch because it contains β-1,4-glycosidic linkages between glucose molecules, allowing linear chains to align and form extensive hydrogen bonding networks. This produces strong, tightly packed microfibrils. Starch, in contrast, has α-1,4 bonds that create helical, flexible structures with less interchain hydrogen bonding. The β-linkages of cellulose therefore generate a highly ordered, load-bearing structural polysaccharide not digestible by human enzymes.

Sphingolipids differ from other membrane lipids because they contain a sphingosine backbone rather than glycerol. Sphingosine is an amino alcohol with a long hydrocarbon tail, and additional fatty acids attach via amide bonds. Their rigid structure affects membrane thickness, stability, and signaling. This contrasts with glycerophospholipids, which rely on a glycerol core. The sphingosine backbone also plays key roles in forming lipid rafts and producing bioactive molecules like ceramides.

Protein secondary structure is primarily stabilized by hydrogen bonds between backbone carbonyl oxygen atoms and amide hydrogen atoms. These interactions produce α-helices and β-sheets, which are repetitive, stable structures present in nearly all proteins. Peptide bonds define the primary sequence, while disulfide and ionic bonds stabilize tertiary or quaternary structures. Hydrogen bonds allow proteins to fold into predictable patterns essential for stability and proper function.

Unsaturated fatty acids introduce cis double bonds into hydrocarbon chains, creating kinks that prevent phospholipids from packing tightly. This spacing increases membrane fluidity by reducing van der Waals interactions and lowering melting temperature. Saturated fatty acids pack neatly into ordered structures, making membranes more rigid. The presence, number, and position of double bonds are therefore critical determinants of membrane flexibility and dynamics in response to temperature.

The hydrophobic effect drives protein folding by encouraging nonpolar amino acid side chains to cluster away from water in the protein interior. This process increases overall water entropy and stabilizes the folded structure. While hydrogen bonds, ionic interactions, and van der Waals forces fine-tune protein conformation, hydrophobic collapse is the primary energetic force initiating folding. This effect ensures proteins adopt compact, energetically favorable structures necessary for proper function.

The sugar that reacts in glycosidic bond formation temporarily adopts an open-chain form, exposing the aldehyde or ketone group that acts as the reactive center. This open form interconverts with cyclic α and β anomers, but the open-chain structure is what allows nucleophilic attack to create glycosidic linkage. Without the transient open form, the anomeric carbon would remain locked in a stable ring structure and unable to form new covalent bonds.

Cholesterol reduces membrane permeability because its rigid four-ring structure inserts between phospholipid tails, restricting their movement. This decreases membrane fluidity in specific temperature ranges and prevents small molecules from diffusing across the bilayer. Cholesterol acts as a bidirectional regulator: increasing fluidity in cold temperatures but decreasing permeability when membranes become too fluid. Its structural rigidity, not size or flexibility, is the dominant factor affecting membrane properties.

DNA synthesis always proceeds in the 5′→3′ direction because new nucleotides can only be added to the 3′-OH group of the growing strand. This hydroxyl group performs a nucleophilic attack on the incoming nucleotide’s phosphate group, forming the phosphodiester bond. Without a 3′-OH, DNA polymerase cannot extend the strand. This chemical requirement ensures unidirectional synthesis and maintains accurate replication and repair processes.

Cysteine uniquely contains a thiol (–SH) group in its side chain, enabling the formation of disulfide bonds when two cysteine residues oxidize. These covalent bonds stabilize tertiary and quaternary protein structures, particularly in extracellular proteins that encounter harsh environments. Other amino acids lack thiol groups and cannot participate in disulfide bonding. This makes cysteine essential for protein folding, stability, and structural integrity.

Enzymes lower activation energy by stabilizing the transition state—the high-energy intermediate between reactants and products. They do this through precise substrate orientation, induced fit interactions, microenvironment changes, and strain on specific bonds. Enzymes do not add heat or break all bonds; instead, they reduce the energy barrier required for the reaction to proceed. This dramatically increases reaction rate without altering overall thermodynamics.

Phospholipids spontaneously assemble into bilayers because the hydrophobic effect drives fatty acid tails away from water while hydrophilic heads face outward. This arrangement minimizes free energy by reducing ordered water structures around nonpolar tails. No ATP or protein scaffolding is needed—the bilayer forms because amphipathic molecules naturally organize into stable structures. This is a fundamental principle governing membrane formation in all living cells.

RNA is more reactive than DNA because it contains a 2′-OH group on ribose. This hydroxyl can participate in nucleophilic attacks, increasing susceptibility to hydrolysis and enabling catalytic activity in ribozymes. DNA lacks this group, making it more chemically stable for long-term information storage. The 2′-OH also influences RNA folding and structural diversity, giving it functional versatility beyond that of DNA.

Triacylglycerols are exceptionally efficient energy storage molecules because their long hydrocarbon chains contain densely packed carbon-hydrogen bonds that release large amounts of ATP when oxidized. Unlike glycogen, they store energy without water, making them far more energy-dense by weight. This hydrophobic, anhydrous nature allows organisms to store vast fuel reserves in small volumes, supporting prolonged energy needs such as hibernation or long-distance migration.

Glyceraldehyde is reduced to glycerol, a three-carbon molecule that forms the backbone of both triacylglycerides and phospholipids. Triacylglycerides act as major storage lipids, while phospholipids are essential components of cell membranes. This conversion is fundamental in lipid metabolism because glycerol connects fatty acids in storage lipids and links phosphate-containing head groups in membrane lipids. Other options incorrectly describe molecules not derived from glyceraldehyde.

Phosphorus is a defining component of phospholipids, where it appears as a phosphate group attached to carbon-3 of glycerol. This phosphate forms the hydrophilic head of the lipid, enabling membrane formation and polarity. Carbohydrates, proteins, and typical fats do not contain phosphorus unless chemically modified, making phospholipids the primary non-nucleotide biomolecule containing phosphorus. Their phosphate group is crucial for membrane stability and signaling.

Mammals cannot hydrolyze cellulose because they lack cellulase, the enzyme required to break β-1,4 bonds in cellulose. Herbivores such as cows, sheep, and horses rely on symbiotic bacteria in their digestive systems. These microbes produce cellulases that convert cellulose into glucose, which the host can absorb and use for energy. This mutualistic relationship allows mammals to benefit nutritionally from plant fibers despite lacking the necessary enzyme.

Oil bodies contain triacylglycerols, giving them a fully hydrophobic interior. A typical bilayer would position hydrophilic heads inward, which is energetically unfavorable. Instead, oil bodies form a monolayer in which phospholipid hydrophobic tails face the hydrophobic core, while the hydrophilic heads face the cytosol. This arrangement stabilizes the structure without requiring a second layer. Alternative explanations like mutations or protein replacement do not account for this lipid-based organization.

The correct structures of galactose and lactose must include proper stereochemistry at carbon-4 distinguishing galactose from glucose. Lactose consists of a β-1→4 glycosidic bond linking galactose to glucose. Incorrect structures often show wrong hydroxyl orientations or linkage configurations. Accurate drawings reveal the proper anomeric carbon and linkage needed for enzyme recognition and biological function, making option C the only correct representation.

Protein molecular weight can be estimated by multiplying the number of amino acids by the average residue mass of 110 Daltons. A protein containing 550 amino acids is approximately 550 × 110 = 60,500 Daltons, or 60.5 kDa. Other estimates using 90, 100, or 120 Daltons produce unrealistic values because they do not reflect average amino acid mass. This approximation is widely used in molecular biology and protein biochemistry.

Improving ethanol production from corn stover requires breaking cellulose into fermentable glucose. While starch is easily hydrolyzed, cellulose is resistant due to its β-1,4 bonds. Identifying cellulose-degrading enzymes from termites, cows, and sheep allows scientists to mass-produce cellulases. These enzymes convert cellulose to glucose, which can then undergo fermentation. Without efficient cellulose hydrolysis, corn stover remains largely unusable for ethanol production, making enzymatic treatment the key step.