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Flashcards · AP® Biology · Unit 1

Unit 1: Chemistry of Life flashcards

48 cards on topics 1.1 to 1.7. Pick a topic or study the whole unit.

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1.1 Structure of Water and Hydrogen Bonding

Why is a water molecule polar but not charged?
Oxygen is more electronegative than hydrogen, so the O–H bonds are polar covalent, and the bent shape gives the molecule a δ⁻ end (O) and δ⁺ ends (each H). The partial charges sum to zero, so the molecule is polar but not an ion.
What is a hydrogen bond between water molecules?
A weak attraction between a δ⁺ hydrogen on one water molecule and a δ⁻ oxygen on another. Each water molecule can make up to four, and they constantly break and re-form. The O–H bond inside a molecule is covalent, not a hydrogen bond.
How do cohesion and adhesion help move water up a plant?
Cohesion (hydrogen bonds between water molecules) holds the water column together; adhesion (hydrogen bonds to the polar xylem walls) makes water cling to the walls. Evaporation from leaves during transpiration provides the pull; cohesion alone does not lift water.
Why does water have a high specific heat, and why does it matter?
Added energy must first break hydrogen bonds before it can raise the temperature, so water warms and cools slowly. This keeps body and lake temperatures stable. High specific heat means water heats slowly, not quickly.
Why does sweating or transpiration cool an organism?
Water has a high heat of vaporization: an escaping molecule must break all its hydrogen bonds. The most energetic molecules leave, lowering the average energy, and so the temperature, of what remains (evaporative cooling).
Why does ice float, and why is that important for life?
Hydrogen bonds fix water molecules in an open lattice, so ice is less dense than liquid water. Floating ice insulates the water below, so it stays liquid. Trapped air is not the explanation.
Why is water a good solvent?
Its δ⁻ and δ⁺ ends surround ions and polar molecules and separate them, so ions and sugars dissolve and can be transported in cells and blood.
How do you calculate the energy needed to warm water?
q = m × c × ΔT, with q in J, m in g, c in J g⁻¹ °C⁻¹ (water: 4.18) and ΔT in °C. Example: 100 g warmed by 5.0 °C needs 100 × 4.18 × 5.0 = 2090 J. Use the change in temperature, not the final temperature.

1.2 Elements of Life

Which elements are found in each class of biological macromolecule?
Carbohydrates and fats: C, H, O. Phospholipids: C, H, O, P (often N). Proteins: C, H, O, N, S. Nucleic acids: C, H, O, N, P.
Where is nitrogen found in biological molecules?
In the amino group of every amino acid (so in proteins) and in the nitrogenous bases of nucleic acids.
Where is phosphorus found in biological molecules?
In phosphate groups: in the sugar–phosphate backbone of DNA and RNA, in the head of every phospholipid, and in ATP. It is not only in DNA.
Where is sulfur found in biological molecules?
In proteins, in the side chains of the amino acids cysteine and methionine. DNA contains no sulfur.
How do tracer experiments tell proteins from nucleic acids?
³⁵S labels proteins (sulfur is in proteins, not DNA); ³²P labels nucleic acids and phospholipids. Subtract the background count before comparing readings.
How do you calculate the percentage by mass of an element in a molecule?
(atomic mass × number of atoms) ÷ molar mass × 100. Carbon in glucose, C₆H₁₂O₆: 6 × 12.01 = 72.06; 72.06 ÷ 180.18 × 100 = 40.0%. Use mass, not the number of atoms.
Where do organisms get the atoms to build macromolecules?
From their environment. Plants take in CO₂ from the air and nitrate, phosphate and sulfate ions from soil water; animals get them from food. Plants do not build their mass mainly from soil minerals.
Why does a plant short of phosphate grow slowly?
It cannot make enough nucleotides for new DNA and RNA, or phospholipids for new membranes.

1.3 Introduction to Macromolecules

What is dehydration synthesis?
The reaction that joins two monomers with a covalent bond. One monomer loses H, the other loses –OH, and these leave as one water molecule. Building starch, glycogen, proteins or DNA uses it.
What is hydrolysis?
The reaction that breaks the covalent bond between two monomers by adding water: the H from water joins one monomer and the –OH joins the other. Digestion is hydrolysis. It is not "dissolving".
How many bonds and water molecules are involved for a chain of n monomers?
A straight or branched chain (no rings) of n monomers has n − 1 bonds. Building it releases n − 1 waters; fully hydrolysing it uses n − 1 waters. A 30-monomer chain uses 29 waters to hydrolyse.
How do you find the mass of a polymer from its monomers?
mass of polymer = n × (monomer mass) − (n − 1) × (water mass, 18.0). Two glucose (180.2 each): 2 × 180.2 − 18.0 = 342.4 g mol⁻¹. The polymer is lighter than its separate monomers.
How do you tell dehydration synthesis from hydrolysis in a diagram?
Check which side the water is on. Water as a product with a larger molecule formed: dehydration synthesis. Water as a reactant with a molecule split into two: hydrolysis.
Which macromolecules are true polymers?
Carbohydrates, proteins and nucleic acids. Lipids are not true polymers, but their parts are joined by the same dehydration reaction.

1.4 Carbohydrates

What are monosaccharides?
The monomers of carbohydrates (simple sugars). Glucose, fructose and galactose are all C₆H₁₂O₆ with different arrangements of atoms. Ribose and deoxyribose are five-carbon sugars in RNA and DNA.
Which monomers make maltose, sucrose and lactose?
Maltose: glucose + glucose. Sucrose: glucose + fructose. Lactose: glucose + galactose. Each forms by dehydration synthesis with one water released.
How do starch, glycogen and cellulose differ?
All are glucose polymers. Starch (plants): linear and branched chains, energy storage. Glycogen (animals, fungi): highly branched, storage with fast release. Cellulose (plant cell walls): linear chains hydrogen-bonded side by side, structural support.
Why does glycogen release glucose faster than an unbranched chain of the same size?
Branching adds chain ends (not monomers), and enzymes remove glucose from many ends at once.
Why does cellulose form strong fibres?
Its straight, unbranched glucose chains lie side by side and form many hydrogen bonds between chains. The links within each chain are covalent.
How do you find the molar mass of a glucose chain?
molar mass = n × 180.16 − (n − 1) × 18.02 g mol⁻¹. Maltose: 2 × 180.16 − 18.02 = 342.3 g mol⁻¹.

1.5 Lipids

Why are most lipids hydrophobic?
They are mostly carbon and hydrogen, and C–H bonds are nonpolar. Lipids are grouped by this property, not by a shared monomer, so they are not polymers.
What is the difference between saturated and unsaturated fatty acids?
Saturated: only C–C single bonds, straight tails that pack tightly, so solid at room temperature (butter). Unsaturated: at least one C=C double bond, which kinks the tail, so tails pack loosely and are liquid (plant oils).
What is a triglyceride (fat) made of?
Glycerol + 3 fatty acids, joined by dehydration synthesis with 3 water molecules released. Hydrolysis reverses it.
Why do phospholipids form bilayers but fats do not?
A phospholipid (glycerol + 2 fatty acids + phosphate) is amphipathic: hydrophilic head, hydrophobic tails. In water, heads face outward and tails hide inside. Fats are hydrophobic all over, so they clump into droplets.
What are steroids, and what do they do?
Lipids with four fused carbon rings. Steroid hormones control growth, development, energy metabolism and homeostasis. Cholesterol gives animal cell membranes structural stability.
Why is fat a compact energy store?
Many C–H bonds and little oxygen give about 37 kJ g⁻¹, against about 17 kJ g⁻¹ for carbohydrate. Storing 740 kJ needs 740 ÷ 37 = 20 g of fat but about 44 g of carbohydrate.
Why do double bonds lower a fat's melting point?
Each double bond kinks the tail, so tails pack loosely with fewer attractions to overcome. The covalent bonds are not weaker; the packing changes.

1.6 Nucleic Acids

What are the parts of a nucleotide?
A five-carbon sugar, a phosphate group on carbon 5′, and a nitrogenous base on carbon 1′.
How are nucleotides joined in a strand?
Covalent bonds between the 3′ –OH of one nucleotide and the phosphate of the next form the sugar–phosphate backbone. The strand has a 5′ end (free phosphate) and a 3′ end (free –OH); new nucleotides are added only to the 3′ end.
How does DNA differ from RNA?
DNA: deoxyribose (–H on 2′), bases A, G, C, T, typically two strands. RNA: ribose (–OH on 2′), bases A, G, C, U, typically one strand.
How do the bases pair in DNA, and what holds the strands together?
A–T with two hydrogen bonds; C–G with three. In RNA, A pairs with U. The two strands are antiparallel and held by weak hydrogen bonds, so they can separate without breaking either backbone.
How do you use base-pairing rules to find base percentages?
In double-stranded DNA, %A = %T and %G = %C, totalling 100. If C = 40%, then G = 40%, A + T = 20%, so A = 10%. This does not apply to single strands.
How do you write the complement of a DNA strand 5′ to 3′?
Pair each base, then reverse the order. Complement of 5′-GGATC-3′ is 5′-GATCC-3′. Forgetting to reverse gives wrong 5′/3′ labels.

1.7 Proteins

What is the structure of an amino acid?
A central carbon bonded to an H, an amine group (−NH₂), a carboxyl group (−COOH) and a variable R group. R groups can be nonpolar (hydrophobic), polar (hydrophilic) or ionic (charged).
How is a peptide bond formed?
By dehydration synthesis between the carboxyl group of one amino acid and the amine group of the next. It is a covalent bond, not a hydrogen bond, and amino acids do not join through their R groups.
What are the four levels of protein structure?
Primary: amino acid sequence (peptide bonds). Secondary: α-helices and β-sheets (H bonds between backbone atoms). Tertiary: 3D shape of one chain (R-group interactions). Quaternary: two or more chains together.
What interactions hold tertiary structure together?
R-group interactions: hydrogen bonds, hydrophobic interactions, ionic interactions and covalent disulfide bridges (S–S).
How many peptide bonds are in a protein?
n − 1 per chain, or total amino acids − number of chains. Three chains of 40 amino acids: 3 × 39 = 117 bonds, and 117 water molecules released.
What happens if a buried nonpolar R group is replaced by an ionic one?
The charge is trapped away from water and the hydrophobic core is disrupted, so the tertiary structure is likely to change and the protein may lose function. Chain: R group → interaction → shape → function.
Does denaturation break peptide bonds?
No. Denaturation disrupts the weaker interactions that hold the higher-level shape; the primary sequence of peptide bonds stays intact.

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