Properties of Water
Why is water considered a polar molecule?
Oxygen is more electronegative than hydrogen, so it pulls shared electrons closer, creating unequal charge distribution across the molecule
Why do carbohydrates dissolve easily in water?
Carbohydrates have many polar hydroxyl (-OH) groups, which form hydrogen bonds with water molecules
What three main elements make up lipids?
C, H, O
What is the structure of deoxyribonucleotides
It's built from three parts, deoxyribose, phosphate group, nitrogenous base
Why does an amino acid's R group determine its chemical properties?
The R group is the only variable part of the amino acid structure, so its charge, polarity, or hydrophobic determines how that amino acid interacts with others
Why do hydrogen bonds form between separate water molecules?
The partial positive charge on water's hydrogen atoms is attracted to the partial negative charge on the oxygen atom of a neighboring water molecule
Why does linking monosaccharides together require removing water, rather than adding it?
Forming a glycosidic bond (dehydration synthesis) creates a covalent bond between two sugars by removing an -OH from one and an -H from the other
Why are lipids used for long-term energy storage rather than short-term use?
Lipids are nonpolar and hydrophobic, which suits them for compact, long-term storage
Why is DNA described as antiparallel, and what holds its two strands together?
Its two strands run in opposite 5' to 3' directions; hydrogen bonds between complementary bases hold the strands together, while covalent phosphodiester bonds hold each strand's backbone together
What causes a protein's secondary structure (alpha helix or beta sheet) to form?
Hydrogen bonds form along the protein's backbone, causing it to fold into a repeating coiled or pleated pattern
Name and explain the two properties of water responsible for water moving up a plant stem.
Cohesion (hydrogen bonds cause water molecules to stick to each other) and adhesion (water molecules stick to the walls of the xylem)
Compare the structural roles of starch/glycogen versus cellulose/chitin, and explain how their structures support their functions.
Starch and glycogen are branched, allowing quick glucose release for energy storage; cellulose and chitin are linear/unbranched, allowing tight packing that provides structural rigidity
Why are phospholipids described as amphipathic, and how does this shape the cell membrane?
They have a polar, hydrophilic phosphate head and nonpolar, hydrophobic fatty acid tails; in water, this causes them to spontaneously arrange into a bilayer
If a DNA sample is 40% adenine, what percentage is guanine, and why?
10% — because A pairs only with T (so A=T=40%) and G pairs only with C, leaving the remaining 20% split evenly between G and C
What happens to a protein's structure and function if temperature or pH changes significantly?
The interactions (hydrogen bonds, ionic bonds, hydrophobic interactions) holding secondary and tertiary structure can break, causing the protein to denature and lose its function
What about carbon's atomic structure makes it so useful for building macromolecules?
It has 4 valence electrons, allowing it to form 4 stable covalent bonds and build large, complex, branching molecules
Why does the branching structure of glycogen make it well-suited for energy storage in animals?
Branching creates many ends where enzymes can act simultaneously, allowing glucose to be released quickly when energy is needed
Compare the structure and function of triglycerides and phospholipids.
Triglycerides: 3 fatty acids + glycerol, fully nonpolar, used for energy storage/insulation. Phospholipids: 2 fatty acids + glycerol + phosphate group, amphipathic, form the cell membrane
Why do guanine-cytosine pairs form a stronger bond than adenine-thymine pairs?
G-C pairs form 3 hydrogen bonds while A-T pairs form only 2, making G-C bonds require more energy to break
If a multi-subunit protein with no alpha helices or beta sheets has one amino acid substituted, which levels of its structure could be affected?
Primary, tertiary, and quaternary structure — not secondary, since it has none to begin with
Why does ice float on liquid water, and why does this matter biologically?
Hydrogen bonds lock water molecules into a lattice as they freeze, spacing them further apart and lowering density; this insulates bodies of water so aquatic life can survive below the surface ice
Chitin is a polysaccharide, like cellulose, but contains nitrogen in its monomer structure. How does this feature support its function in structures like insect exoskeletons and fungal cell walls?
The nitrogen-containing modification allows extra hydrogen bonding between chains, making chitin more rigid and durable than cellulose — suited for external protective structures
How does the ratio of saturated to unsaturated fatty acids affect fluidity?
More unsaturated fatty acids increase fluidity, since their kinks prevent tight packing; more saturated fatty acids decrease fluidity, since their straight chains pack tightly
Ribose (in RNA) has an extra hydroxyl group compared to deoxyribose (in DNA). How does this structural difference relate to why DNA is more chemically stable than RNA?
The extra -OH group on ribose makes RNA more reactive and prone to breakdown, while deoxyribose's lack of that group makes DNA more stable — suited for long-term genetic storage
Why is the specific sequence of amino acids in a protein so important?
The sequence determines the R-group interactions that drive folding, which in turn determines the protein's 3D shape and therefore its function