Chemical Properties
Which feature of the carbon atom allows it to form four covalent bonds and build complex macromolecules?
Carbon has four valence electrons (tetra-valency) allowing four covalent bonds and formation of chains/branches/rings
What role do hydrogen bonds play in the secondary structure of proteins and in DNA structure?
Hydrogen bonds stabilize alpha-helices and beta-sheets in proteins and hold complementary base pairs in DNA.
Why is water considered the “medium for life”? Give one metabolic and one transport example.
Water is solvent for biochemical reactions and medium for transport (e.g., blood plasma transports nutrients; cytoplasm enables metabolic enzyme activity)
List the three components of a nucleotide.
A nucleotide = phosphate group + pentose sugar (ribose or deoxyribose) + nitrogenous base.
State two major differences between DNA and RNA.
DNA: deoxyribose sugar, double-stranded, thymine; RNA: ribose sugar, single-stranded, uracil.
Name the type of bond formed between water molecules and explain why it forms.
Hydrogen bonds: weak electrostatic attractions between a hydrogen covalently bonded to an electronegative atom (O) and another electronegative atom (O); they form because of polarity in water molecules.
How does the amphipathic nature of phospholipids lead to the formation of cell membranes?
Phospholipids have hydrophilic heads and hydrophobic tails; in aqueous environments they self-assemble into bilayers with tails inward, heads outward, creating membranes.
Explain how cohesion and adhesion of water assist plant transport from roots to leaves.
Cohesion (water–water) helps maintain a continuous column in xylem; adhesion (water–cell walls) helps water climb against gravity via capillary action and transpiration pull.
What is the condensation reaction that links monomers into polymers? Give an example with monomers and product.
Condensation (dehydration) reaction: monomers join releasing water; e.g., two glucose -> maltose + water, amino acids -> peptide + water.
What evidence from Chargaff’s data supports complementary base pairing in DNA?
Chargaff: amounts of A ≈ T and G ≈ C across species, supporting complementary base pairing.
Define “polar covalent bond”
Polar covalent bond: unequal sharing of electrons between atoms with different electronegativities
Describe how glycoproteins contribute to cell–cell recognition.
Glycoproteins present carbohydrate markers on cell surfaces that are recognized by receptors on other cells, enabling recognition and immune responses.
How do the solvent properties of water support metabolic reactions in cells?
Water dissolves ionic and polar substances, allowing reactants to collide and enzymes to access substrates; it stabilizes charged intermediates.
Explain hydrolysis in digestion using an example of a polymer broken into monomers.
Hydrolysis: water molecule breaks bond in polymer, e.g., proteases hydrolyze proteins to amino acids; amylase hydrolyzes starch to maltose/glucose.
Explain the concept of directionality in nucleic acids and why it is important for replication and transcription (HL).
Directionality: nucleic acid strands have 5′ and 3′ ends due to sugar-phosphate backbone orientation; DNA replication and RNA synthesis proceed 5′→3′, enzyme specificity depends on this orientation.
Explain the difference between saturated and polyunsaturated fatty acids at the level of chemical bonding.
Saturated fatty acids have no C=C double bonds (max H), polyunsaturated have multiple C=C double bonds (fewer H); double bonds introduce kinks and affect melting point.
Explain how triglycerides function in animals for both energy storage and thermal insulation.
Triglycerides store energy as dense, reduced carbon; adipose tissue (fat) stores them for long-term energy and provides insulation by reducing heat loss.
Describe two physical properties of water that affect organisms living in aquatic habitats and explain one biological consequence for each property.
High specific heat: buffers temperature changes — stabilizes aquatic environments; high heat of vaporization: cooling via evaporation — thermoregulation. Density anomaly (ice less dense) — ice insulates aquatic life in winter
Describe the form and function of monosaccharides and give one example of a structural and one of a storage carbohydrate derived from them.
Monosaccharides (e.g., glucose) are single sugar units used for energy (glycolysis) and as building blocks; structural carbohydrate: cellulose (β-glucose polymers); storage: starch or glycogen (α-glucose polymers).
Summarize the Hershey–Chase experiment’s evidence that DNA is the genetic material (HL).
Hershey–Chase used bacteriophages labeled with radioactive P (DNA) and S (protein); progeny phages contained radioactive P, showing DNA carried genetic information.
Describe Purine-to-pyrimidine pairing and explain how it contributes to DNA helix stability (HL).
Purine (double-ring) pairs with pyrimidine (single-ring) via hydrogen bonds (A–T two bonds; G–C three bonds), preserving uniform helix width and contributing to stability via specific H-bonding and base stacking.
Describe how non-polar steroid hormones cross the phospholipid bilayer and affect target cells.
Steroids are non-polar and diffuse through the lipid bilayer, bind intracellular receptors, and regulate gene transcription.
Summarize one hypothesis for the extraplanetary origin of Earth’s water and one reason Earth has retained it.
Example hypothesis: water delivered by icy asteroids; retention due to Earth's gravity and magnetic field preventing atmosphere/water loss.
Explain how the structure of cellulose relates to its function as a structural polysaccharide in plants.
Cellulose is a straight chain of β-1,4-linked glucose with hydrogen bonding between chains forming microfibrils, giving tensile strength for plant cell walls.
Explain how the search for extraterrestrial life is linked to the presence of liquid water (HL): give two reasons why water is considered a key indicator.
Liquid water is necessary for solvent-based metabolism and stable biochemistry; its presence indicates potential for transport, thermal regulation, and complex chemistry required by life.