Unit notes

An atom has a positively charged nucleus and negatively charged electrons. A molecule consists of atoms joined by covalent bonds. A chemical reaction rearranges atoms; it does not create new kinds of atoms. Keep three scales separate: the bond within a molecule, attractions between molecules, and the property of a whole sample.

Electrons in an O–H bond are shared unequally because oxygen attracts electrons more strongly. Water therefore has partial negative charge near oxygen and partial positive charge near hydrogen. Attractions between different water molecules are hydrogen bonds; the O–H bonds within a molecule are covalent. Breaking a hydrogen bond between water molecules does not split a water molecule.

A network of hydrogen bonds explains cohesion, adhesion to suitable surfaces, relatively high specific heat, and the large energy needed for evaporation. When the fastest molecules escape during evaporation, the remaining liquid cools. In ice, the hydrogen-bonded arrangement keeps molecules farther apart than in liquid water, allowing ice to float and insulate the water beneath.

Ions and polar molecules interact favorably with water: water molecules orient around charged particles and can separate them into solution. Nonpolar molecules do not make equally favorable interactions with water. Their clustering is important in membranes and in the folding of proteins; it does not mean that water actively repels them.

The pH scale describes hydrogen-ion concentration logarithmically. A buffer limits a pH change by accepting or releasing H⁺ over an appropriate range. Buffers do not force all solutions to pH 7 and do not have unlimited capacity. Changes in pH can alter the charge of amino-acid side chains and disrupt interactions that stabilize proteins.

Carbon can form four covalent bonds, allowing chains, branches, and rings. The arrangement of atoms matters, not just the molecular formula. Hydroxyl and charged groups can promote interactions with water; nonpolar hydrocarbon regions generally do not. A change in one functional group may alter solubility, reactivity, or binding.

Biological macromolecules contain recurring building blocks, but lipids are not all polymers of a single repeating monomer. Dehydration reactions can join building blocks while releasing water. Hydrolysis uses water to cleave a covalent linkage. Cells regulate these reactions with enzymes rather than relying on spontaneous assembly.

Carbohydrates can store energy or provide structure. The arrangement of glycosidic linkages affects shape and whether a particular enzyme can hydrolyze them. Starch and cellulose both contain glucose, yet animals do not automatically have the enzymes required to digest both.

Proteins consist of amino acids joined by peptide bonds. Lipids include energy-rich triglycerides, amphipathic phospholipids, and steroids. Nucleic acids contain nucleotide chains with sugar–phosphate backbones and nitrogenous bases. DNA is a durable information store; RNA has roles in information transfer, regulation, and catalysis. Identify the particular molecule and structural feature rather than assuming every member of a family has one function.

Primary structure is the amino-acid sequence. Backbone hydrogen bonding stabilizes common secondary structures. Interactions among side chains and with the surrounding environment contribute to tertiary structure; quaternary structure involves multiple polypeptide subunits. A substitution can affect folding or binding, but some substitutions have little measurable effect.

Heat or an extreme pH may disrupt interactions and alter protein shape. Denaturation usually changes higher-order structure without hydrolyzing every peptide bond. The strongest response connects a particular change to altered interactions, an altered active or binding site, and a measured loss or change of function.

Living organisms obtain carbon, nitrogen, phosphorus, and other elements from their surroundings and rearrange them into biological molecules. Carbon forms the skeleton of many organic compounds. Nitrogen occurs in amino groups and nitrogenous bases; phosphorus occurs in nucleotides and phospholipids. A plant gains much of the carbon in its growing biomass from atmospheric carbon dioxide, not from soil. Roots still supply water and mineral nutrients needed for growth.

When a radioactive or stable isotope labels an atom, the label follows that atom through chemical transformations. It does not label every molecule that participates in the same reaction. To interpret a tracing experiment, identify which starting molecule contains the labeled element, which products contain that element, and which bonds can be rearranged. A labeled oxygen atom and a labeled carbon atom can tell different stories about the same pathway.

A nucleotide contains a sugar, a phosphate group, and a nitrogenous base. Nucleotides join through covalent bonds in the sugar–phosphate backbone. The two strands in a DNA double helix run in opposite directions, called antiparallel orientation. Complementary base pairing allows one strand to specify the other: A pairs with T in DNA and G pairs with C. RNA generally uses U instead of T.

Base sequence stores information, while the repeated backbone provides a common structure. Hydrogen bonds contribute to the pairing of bases on opposite DNA strands; they are not the covalent links along a strand. If a DNA molecule is 30% adenine overall, it is also 30% thymine; the remaining 40% is split equally between G and C. These equalities apply to double-stranded DNA as a whole, not necessarily to a single isolated strand.

Start with a structural feature that is actually relevant to the observation. For a phospholipid, a polar head and nonpolar tails help explain bilayer organization. For a storage polysaccharide, branching can provide many accessible ends for enzyme action. For a protein, the identity and position of a side chain can alter interactions or binding. Saying “its shape helps it work” is a beginning, not a complete explanation.

Then connect that feature to an interaction and a consequence. A nonpolar tail interacts favorably with other nonpolar regions; clustering shelters it from water; a bilayer can form a selective boundary. To test a causal claim, compare molecules differing in the proposed feature while controlling concentration, temperature, and other conditions. A correlation between structure and function supports a hypothesis but may not isolate the mechanism without additional controls.

An atom retains its elemental identity during an ordinary chemical reaction. Carbon atoms in a sugar must come from a carbon-containing source; sunlight can supply energy but is not a source of carbon atoms. Separate the source of matter from the process that makes its assembly possible. Plants obtain much of the carbon in their organic molecules from atmospheric CO₂.

Carbon, hydrogen and oxygen occur in many biological molecules. Nitrogen is found in amino groups and nitrogenous bases. Phosphorus occurs in nucleic-acid backbones and many phospholipid heads. Sulfur occurs in some amino-acid side chains. Composition narrows possibilities, but rarely identifies a molecule uniquely: a phosphorus-containing sample could include several molecular classes.

Ask which atoms could enter a product and which observations actually track them. Labeled carbon can reveal the movement of carbon-containing matter. It does not show that the same energy stays attached to that atom throughout metabolism. Tracers provide evidence about the labeled entity, not a universal map of every process.

Oxygen and hydrogen share electrons in polar covalent O–H bonds. Oxygen attracts these electrons more strongly, producing partial negative charge near oxygen and partial positive charges near the hydrogens. Partial charges do not mean complete electron transfer has created separate ions. A water molecule is neutral overall but has an uneven charge distribution.

A partially positive hydrogen of one molecule can be attracted to a partially negative oxygen of another. This hydrogen bond is an intermolecular interaction in this example. Many such interactions form and break as molecules move. A dashed line between molecules represents a different interaction from an O–H line within a molecule.

Evaporation separates water molecules from neighbors; it does not normally break their covalent O–H bonds. Water’s polarity and hydrogen bonding help explain its collective behavior. Start an explanation with the charges and interactions, then connect them to the observation. Naming hydrogen bonding alone often leaves the causal link unfinished.

Cohesion is attraction among water molecules; adhesion is attraction between water and other suitable materials. Polar or charged groups on a wettable surface can interact with water. In plants, cohesion helps maintain a connected water column while adhesion contributes to interactions with conducting surfaces. These are distinct interactions; neither means the molecules stop moving.

Water has high specific heat: substantial energy transfer is needed for a given temperature change per unit mass. Added energy can disrupt intermolecular hydrogen-bond interactions as well as alter molecular motion. This helps aqueous systems resist sudden temperature swings. It does not mean water cannot warm or that equal heat inputs warm all substances equally.

Molecules with sufficient energy can escape a liquid. Their departure can lower the average kinetic energy of those remaining, producing cooling. High specific heat concerns temperature change with added heat; evaporative cooling concerns energy carried away by escaping molecules. Identify which mechanism the observation supports rather than treating these properties as interchangeable.

Carbon can form four covalent bonds, allowing chains, branches and rings. A molecular formula counts atoms but does not show arrangement. Structural differences change shape, the positions of polar regions, and access to a binding site. Biological recognition depends on these details rather than simply on total carbon content.

Hydroxyl groups participate in hydrogen bonding; carboxyl and amino groups can carry charge under suitable conditions; phosphate-containing groups often contribute negative charge. A hydrocarbon-rich region interacts differently with water than a region with multiple polar groups. Predict interactions in the stated environment rather than labeling all carbon-containing compounds hydrophobic.

Adding a charged group to a nonpolar binding surface may alter interactions with water or a partner. It is reasonable to predict a possible binding change, but not an exact rate or certain loss of function without evidence. Molecular reasoning should identify a mechanism and suggest a measurement that could test it.

Polymers contain covalently linked subunits. A simplified dehydration or condensation model joins subunits while releasing water. Real biosynthetic pathways can use activated intermediates, but the structural point is that a new covalent linkage connects building blocks. Sequence and linkage type influence the polymer’s resulting behavior.

Hydrolysis uses water to cleave a covalent linkage, incorporating the elements of water into the products. One internal cut in a linear polymer yields two shorter fragments, not automatically every free monomer. Complete digestion needs enough cleavage events and enzymes that recognize the relevant linkages.

One linear chain with n subunits has n−1 adjacent links. Complete separation requires cleavage of those links. This assumes a simple unbranched chain and one linkage between each neighboring pair. For branches or rings, inspect the actual structure instead of automatically applying a memorized count.

Starch and glycogen serve storage roles; cellulose supports plant cell walls. These glucose polymers differ in glycosidic linkage orientation and branching. Their common monomer does not imply equal digestibility or mechanical properties. Enzymes interact with particular arrangements of bonds and surfaces.

Human enzymes that hydrolyze common starch linkages do not efficiently cleave cellulose’s β-1,4 linkages. This reflects specificity, not a lack of chemical energy in cellulose. Some organisms use appropriate enzymes or microbial partners. Indigestible for one organism does not mean indigestible for every organism.

Branches can provide multiple ends for enzymes that remove glucose from appropriate chain ends. More accessible ends can support rapid mobilization under suitable conditions. The prediction depends on enzyme action and accessibility; it does not establish that every branched molecule is always processed faster than every unbranched one.

An amino acid has a central carbon associated with an amino group, a carboxyl group, a hydrogen and a variable side chain. Peptide bonds connect amino acids into polypeptides. Side chains differ: some are nonpolar, some polar, and some charged under cellular conditions. A substitution changes a particular local chemical environment, not necessarily every part of the protein.

The ordered amino-acid sequence is primary structure. The same collection of amino acids in a different order can fold differently because different residues become neighbors or meet within the folded structure. Sequence is more than a parts list: it constrains where particular interactions can occur.

A charged residue placed in a nonpolar core can disrupt favorable organization. The same change on an exposed surface may have a different effect. Avoid claiming that every substitution destroys the protein or that chemically similar residues never matter. Predict using properties and location, then test the relevant function.

Secondary structures include α helices and β sheets, stabilized by hydrogen bonds involving peptide-backbone groups. They do not require identical side chains throughout. A common error is to attribute all secondary-structure hydrogen bonding to variable R groups rather than the backbone.

Tertiary structure is the three-dimensional arrangement of a polypeptide. Hydrophobic interactions, hydrogen bonds, ionic interactions, van der Waals interactions and sometimes disulfide bridges contribute. In an aqueous environment, nonpolar groups often cluster away from water, while compatible polar groups are often exposed. Specific environments can modify this general pattern.

Quaternary structure describes multiple polypeptide subunits in an assembly. A single-chain protein does not require quaternary structure to work. Disrupting an interface may separate subunits without cleaving peptide bonds within them. Name the level supported by the evidence rather than assuming every level was destroyed.

Binding and catalytic regions depend on three-dimensional structure. Heat, pH or other conditions can disrupt stabilizing interactions, causing denaturation. Under many denaturing conditions, primary sequence remains intact. Hydrolysis of peptide bonds is a different process even though both can reduce activity.

Some proteins regain functional structure when favorable conditions return; others aggregate or remain misfolded. Recovery supports reversibility in that experiment. Failure to recover does not identify exactly which bonds changed. An activity assay measures function; size or structural measurements help distinguish unfolding, aggregation and fragmentation.

Changing pH can alter protonation and charge of particular groups, affecting ionic interactions and binding. Effects depend on the protein and conditions. Do not assume every protein works best at neutral pH. Compare controlled pH conditions while holding protein amount, temperature and other relevant factors constant.

A nucleotide contains a sugar, phosphate and nitrogenous base. DNA generally contains deoxyribose and A, T, G and C; RNA generally contains ribose and A, U, G and C. Bases vary along the strand while covalent sugar–phosphate linkages connect nucleotides. A nucleotide is not an amino acid, and a DNA strand is not a protein chain.

Strands have chemically distinct 5′ and 3′ ends. In double-stranded DNA, the strands run in opposite directions. Direction labels matter when writing complementary sequences; letters alone can hide an orientation error. Here the focus is structural information storage; detailed copying mechanisms are developed in the gene-expression unit.

Hydrogen bonds between complementary bases help stabilize strand association. Covalent phosphodiester linkages maintain each individual backbone. Separating strands need not cut them into nucleotides. Keep between-strand interactions distinct from along-strand linkages when interpreting heating or separation experiments.

In ordinary double-stranded DNA, A pairs with T and G with C. Opposite 5′-AGTC-3′, the aligned strand is 3′-TCAG-5′. Written in its own 5′→3′ direction, that partner is 5′-GACT-3′. Pairing determines the letters; antiparallel organization determines the direction.

Across both strands of an ordinary DNA molecule, total A equals total T and total G equals total C. This does not require A to equal G. It also does not guarantee equal A and T within one isolated strand. Determine whether counts refer to one strand or the complete double-stranded molecule.

If A accounts for 30% of bases in a double-stranded sample, T is also 30%. The remaining 40% divides equally into G and C, giving 20% each. Applying this automatically to arbitrary single-stranded RNA is unjustified. The calculation works because of the stated pairing constraints, not because all nucleic acids share equal base percentages.

Different classes share elements. Nitrogen occurs in proteins and nucleic acids; phosphorus occurs in nucleic acids and many phospholipids. Combine composition with hydrolysis products, selective enzyme sensitivity or structural observations. A single positive result can narrow possibilities without establishing a unique identity.

A positive control shows a test can detect a known target under the chosen conditions. A negative control helps identify background or contamination. A negative unknown is difficult to interpret if the positive control also fails. Controls test the measurement procedure; they are not simply examples that agree with expectations.

Amino acids released by appropriate hydrolysis support a peptide-containing component. They do not establish that lipids and nucleic acids are absent. Presence is not purity. A useful follow-up tests an alternative explanation rather than merely repeating the same ambiguous measurement many times.