Cell & Membrane
What type of lipid arrangement forms the basic structure of cell membranes and creates a hydrophobic interior?
The phospholipid bilayer (phospholipids with hydrophilic heads and hydrophobic tails) forms the membrane
What three elements make up carbohydrates and lipids that are relevant to plant polysaccharides like starch and cellulose?
Carbon, hydrogen, and oxygen are primary elements in carbohydrates and lipids.
What covalent bond links amino acids into a polypeptide chain?
Peptide (amide) bonds formed by dehydration synthesis join amino acids
Which feature of DNA best encodes biological information: strand orientation, hydrogen bonds, covalent backbone bonds, or the linear sequence of bases?
The linear sequence of base pairs (the order of nucleotides) best encodes biological information.
Why does ice float on liquid water? State the structural reason and one ecological benefit.
Ice forms an open lattice stabilized by hydrogen bonds causing lower density; floating ice insulates the water below, protecting aquatic life.
Name the type of membrane molecule that increases membrane rigidity and is often found in membrane microdomains.
Cholesterol (membrane steroid) is present and contributes to microdomain rigidity.
Which reaction type connects monomers to form polymers and releases a water molecule?
Dehydration synthesis (condensation reaction) links monomers and releases water.
Which level of protein structure is defined solely by the amino acid sequence?
Primary structure (the linear amino acid sequence).
To remove and replace a damaged cytosine nucleotide without altering sequence information, which covalent bond(s) in the nucleotide backbone should be broken (phosphodiester vs glycosidic)?
Break the phosphodiester bond(s) that connect the target nucleotide to the sugar-phosphate backbone (cleaving the bond(s) flanking the nucleotide) to remove and replace it without altering base pairing elsewhere. 201 (alternate detail) — Specifically, breaking the phosphodiester bond(s) between adjacent nucleotides allows excision and replacement.
Describe how polarity of water leads to hydrogen bonding between molecules (which atom attracts electrons more strongly).
Oxygen is more electronegative, pulling electron density toward itself, creating partial negative charge on O and partial positive on H; these opposite partial charges allow hydrogen bonds between molecules.
When two cells fuse, describe how the polar heads and nonpolar tails of phospholipids from each membrane interact to form a continuous bilayer.
Polar (hydrophilic) head groups from both membranes remain exposed to aqueous environments and hydrogen-bond/ionically interact with each other and water; nonpolar tails from each membrane align and interact via hydrophobic interactions, creating a continuous hydrophobic interior.
Based on the starch vs. cellulose structures, state the structural difference that makes cellulose a strong structural polymer.
Cellulose has beta-1,4 linkages that orient every other glucose rotated 180°, producing straight chains that hydrogen-bond between chains forming strong fibers; starch has alpha linkages producing branched/helical, less rigid structure.
The normal five–amino-acid sequence contains lysine; the altered one replaces it with hydrophobic residues. Explain how substituting polar/charged for hydrophobic R groups can change protein folding.
Replacing a positively charged polar residue (lysine) with hydrophobic residues eliminates favorable ionic/hydrogen interactions with water or other polar residues, causes hydrophobic side chains to cluster inward or in membrane regions, and alters tertiary folding.
Which change would most effectively convert a DNA model to an RNA model: change sugars to ribosomes, change base sequence, change base shapes, or change backbone depiction? Explain briefly.
Changing deoxyriboses to riboses (adding the 2′-OH) is most effective because sugar identity (ribose vs deoxyribose) distinguishes RNA from DNA.
Which model best shows a hydrogen bond between water molecules: an H of one water to the O of another? Explain the arrangement briefly.
The correct arrangement shows a hydrogen atom of one water forming a hydrogen bond to the oxygen atom of another water (H—O···O pattern with dashed line between H and O).
Identify a likely immediate cellular consequence if fused B-cancer hybrid cells are transferred to medium lacking nitrogen
Immediate effect: reduced synthesis of nitrogen-containing macromolecules (nucleotides and amino acids) leading to halted DNA/RNA and protein synthesis and arrest of cell division/growth.
Explain why plants lacking phosphorus show stunted leaf growth in terms of macromolecule synthesis (name two macromolecule classes requiring phosphorus).
Phosphorus is required for nucleic acids (DNA/RNA) and phospholipids; phosphorus deficiency limits synthesis of nucleic acids and membrane lipids, reducing growth.
Indicate which levels of protein structure (primary, secondary, tertiary, quaternary) are expected to be altered by a short sequence of amino acid substitutions and justify your choices.
Primary structure: altered (sequence changed). Secondary: possibly altered if local backbone interactions change (X). Tertiary: altered because side-chain interactions change (X). Quaternary: may be altered if folding changes disrupt subunit interactions (often X depending on protein). (Expected: primary and tertiary definitely; secondary and quaternary often.)
Given a double-stranded molecule that contains thymine and deoxyribose, decide whether it is DNA or RNA and cite the key distinguishing nucleotide(s).
It is DNA because thymine is present (and deoxyribose is implied), which is specific to DNA.
Explain cohesion-tension mechanism: how evaporation at leaf surfaces results in pulling water up xylem (name the bonding responsible).
Evaporation from leaf surfaces creates tension; hydrogen bonds (cohesion) between water molecules transmit that tension down the water column, pulling water up through xylem; adhesion to xylem walls helps maintain the column.
Explain why some membrane regions (lipid rafts) have different protein and lipid composition and how that affects local membrane function.
Lipid rafts are enriched in cholesterol and specific lipids (sphingolipids) and proteins; their distinct composition packs lipids more tightly, concentrating signaling proteins and affecting membrane fluidity and signaling.
Given cancer cells treated so they cannot produce adenine and guanine but fused with B cells that can, explain how nucleotide biosynthesis or uptake would determine the hybrid cell’s ability to synthesize DNA/RNA.
If cancer cells cannot make adenine/guanine, the hybrid’s ability to synthesize DNA/RNA depends on salvage or uptake of nucleotides or on the partner B cell supplying the bases; lacking nitrogen would impede nucleotide synthesis.
— Predict how the amino-acid substitutions shown (introducing hydrophobic residues in a normally polar region of a membrane receptor) are most likely to affect ligand binding and receptor function.
Likely reduction or loss of receptor function: altered shape can change or block ligand binding site or receptor conformational change required for signaling; hydrophobic substitutions in a binding/recognition region may prevent proper extracellular ligand interaction.
— Explain why complementary base pairing allows one nucleic acid strand to act as a template for synthesizing a complementary strand (mention chemical specificity of base pairing).
Complementary base pairing (A–T/U via two H-bonds, G–C via three H-bonds) is chemically specific, allowing one strand’s sequence to determine the complementary sequence for replication or transcription.
Provide a brief explanation of how water’s properties support temperature buffering in organisms and identify the molecular basis.
Water’s high specific heat and high heat of vaporization buffer temperature changes; these arise from extensive hydrogen bonding between molecules that requires/ releases energy to disrupt/form.