1 Chemistry Of Life

Water contains polar covalent bonds because oxygen is more electronegative than hydrogen, creating partial charges within each molecule. The partially positive hydrogen of one water molecule is attracted to the partially negative oxygen of a neighboring water molecule, forming a hydrogen bond.

The polarity of water enables hydrogen bonding not only between water molecules but also between and within biological molecules. For example, polar regions of proteins and nucleic acids can form hydrogen bonds with water or with other polar regions within the same molecule, which influences molecular structure and function.

Water's high specific heat capacity means that a large amount of heat energy must be absorbed or released to change its temperature. Because organisms contain a high proportion of water, their internal temperatures remain relatively stable despite fluctuations in the external environment, supporting homeostasis.

If water's heat of vaporization were significantly lower, each gram of sweat that evaporated would carry away less heat energy from the body. The organism would need to produce much more sweat to achieve the same cooling effect, making thermoregulation less efficient and potentially leading to overheating in warm environments.

Water molecules in the liquid phase are connected by an extensive network of hydrogen bonds. To vaporize, individual molecules must break free from these bonds, which requires substantial energy input. Therefore, the strength and number of hydrogen bonds directly account for water's high heat of vaporization.

Adhesion pulls water molecules along the polar walls of xylem vessels, while cohesion holds the water molecules together as a continuous column. Together, these properties allow water to move upward from roots to leaves against the force of gravity during transpiration.

At the surface of a body of water, molecules lack hydrogen-bonding partners above them and therefore form stronger hydrogen bonds with neighboring molecules beside and below them. This creates a net inward pull that makes the surface behave like an elastic film, resisting disruption.

A substance that disrupts hydrogen bonding would reduce surface tension. Surface tension depends on the net inward hydrogen bonding among surface water molecules; weakening these bonds would decrease the cohesive force at the surface, making it easier for objects to break through the water's surface layer.

Carbon, hydrogen, and oxygen are the three most prevalent elements used to build carbohydrates, proteins, lipids, and nucleic acids.

The plant would be unable to synthesize adequate phospholipids and nucleic acids because both require phosphorus — phospholipids need phosphate in their head group, and nucleic acids need phosphate in their sugar-phosphate backbone. Carbohydrate and most other lipid synthesis would remain unaffected because those molecules do not contain phosphorus.

Sulfur is incorporated into certain amino acids that form proteins, so a lack of sulfur would prevent proper protein assembly. Carbohydrates are composed only of carbon, hydrogen, and oxygen, so their synthesis does not depend on sulfur availability.

During dehydration synthesis, a hydrogen ion is removed from one monomer and a hydroxyl group is removed from the other, resulting in the release of a water molecule. A covalent bond forms between the two monomers, linking them together. Repeating this process produces a polymer through polymerization.

Hydrolysis breaks the covalent bond between monomers by adding water across the bond. The hydrogen ion from the water molecule attaches to one monomer, and the hydroxyl group attaches to the adjacent monomer, separating them and restoring each to its individual form.

The cell would be unable to form new polymers because dehydration synthesis is the reaction that creates the covalent bonds linking monomers together. Without this reaction, monomers would remain unlinked, and essential macromolecules such as proteins and polysaccharides could not be assembled, impairing growth and repair.

Monosaccharides are linked by covalent bonds through dehydration synthesis, which removes a water molecule each time a new monomer is added, producing a polysaccharide chain. Polysaccharides are broken down by hydrolysis, which adds a water molecule across each covalent bond to release individual monosaccharides.

Glycogen is highly branched, providing many free ends where enzymes can simultaneously cleave glucose monomers, allowing rapid energy release. Cellulose has a rigid, linear structure suited for structural support rather than quick enzymatic breakdown, so it cannot be mobilized as an energy source efficiently.

The degree of branching in a polysaccharide determines whether it functions primarily in energy storage or structural support. Starch and glycogen are branched to varying degrees, enabling enzymes to access glucose monomers for energy, with glycogen's greater branching allowing faster mobilization. Cellulose is linear and unbranched, forming rigid fibers that provide structural integrity to plant cell walls rather than serving as an energy reserve.

Double bonds cause kinks in the hydrocarbon chain, which prevent unsaturated fatty acid molecules from packing tightly together. The weaker intermolecular interactions mean less energy is needed to separate the molecules, so unsaturated lipids tend to be liquid at room temperature rather than solid.

The melting point would decrease. The polyunsaturated tail introduces multiple kinks, which disrupts the tight packing of the fatty acid chains. With less orderly packing and weaker intermolecular interactions, the fat transitions from solid to liquid at a lower temperature.

Fats store energy in their many carbon–hydrogen bonds, providing a concentrated fuel reserve for the organism. In some mammals, subcutaneous fat also provides insulation that helps maintain body temperature in cold environments.

Phospholipids have hydrophilic phosphate heads and hydrophobic fatty acid tails. In water, the hydrophobic tails are repelled by the aqueous surroundings, so they orient inward, away from water, while the hydrophilic heads face outward toward the water on both sides. This spontaneous arrangement produces a bilayer that acts as a selectively permeable barrier.

Without cholesterol, the membrane would lose its ability to moderate fluidity. At higher temperatures, the membrane would become excessively fluid because nothing would restrict phospholipid movement. At lower temperatures, the membrane could become too rigid because cholesterol normally prevents tight packing of phospholipid tails. Overall, the membrane would be less stable and less able to maintain consistent function across a range of temperatures.

A nucleotide consists of a five-carbon sugar (deoxyribose or ribose), a phosphate group, and a nitrogenous base.

Biological information is encoded in the specific linear sequence of nucleotide monomers. Different arrangements of the four nitrogenous bases along the strand create different genetic messages, so the order of bases determines what information is stored and transmitted.

The 5' end has an exposed phosphate group attached to the fifth carbon of the sugar, while the 3' end has an exposed hydroxyl group on the third carbon of the sugar. These chemically distinct ends give the strand directionality.

During synthesis, each new nucleotide is added to the 3' hydroxyl end of the existing strand through a covalent bond. Because addition always occurs at the 3' end, the strand extends in the 5' to 3' direction.

The two strands of the DNA double helix run in opposite directions — one oriented 5' to 3' and the other oriented 3' to 5'. This antiparallel arrangement allows complementary base pairing between the two strands.

Complementary base pairing rules dictate that adenine always pairs with thymine and cytosine always pairs with guanine. Therefore, each base on one strand specifies the identity of the base across from it on the complementary strand.

Using RNA base pairing rules (A pairs with U, T pairs with A, C pairs with G, G pairs with C), the mRNA sequence produced would be 5'-AUGCCU-3'. Thymine in DNA is replaced by uracil in RNA, so the adenine on the template pairs with uracil in the mRNA.

DNA contains deoxyribose sugar while RNA contains ribose, DNA uses thymine while RNA uses uracil, and DNA is typically double stranded while RNA is typically single stranded.

The double-stranded structure provides stability because hydrogen bonds between complementary base pairs hold the two strands together, protecting the encoded genetic information. Additionally, having two complementary strands means that if one strand is damaged, the other can serve as a template for repair, making DNA a reliable long-term storage molecule.

A peptide bond forms through a dehydration synthesis reaction in which the carboxyl group (−COOH) of one amino acid reacts with the amine group (−NH₂) of the next amino acid. This covalent bond links the two amino acids, releasing one molecule of water and extending the growing polypeptide chain.

Hydrophobic (nonpolar) R groups avoid water and tend to cluster in the interior of the protein, away from the aqueous environment. Hydrophilic (polar) and ionic (charged) R groups interact favorably with water and are typically found on the protein's surface. These tendencies drive the protein to fold into a specific three-dimensional shape, because each R group settles into the position that is most thermodynamically stable given its chemical properties.

The primary structure is the specific amino acid sequence of a polypeptide. This sequence determines how the chain folds, because the identity and order of R groups dictate which interactions form at each position. Therefore, the primary structure ultimately determines the secondary, tertiary, and quaternary structures — and thus the protein's function.

Secondary structures form because hydrogen bonds develop at regular intervals between atoms of the polypeptide backbone. In an alpha-helix, backbone hydrogen bonds create a coiled shape, while in a beta-pleated sheet, hydrogen bonds between adjacent stretches of the backbone produce a flat, folded arrangement. These local folding patterns arise spontaneously from the repeating chemistry of the backbone.

The four interactions are hydrogen bonds between polar R groups, hydrophobic interactions among nonpolar R groups, ionic interactions between charged R groups, and disulfide bridges (covalent bonds between sulfur-containing cysteine R groups). Together, these interactions fold the polypeptide into its three-dimensional shape.

A change in pH alters the ionization state of charged and polar R groups, which disrupts the ionic interactions and hydrogen bonds that stabilize the protein's tertiary (and potentially quaternary) structure. Without the correct three-dimensional shape, the protein can no longer bind its substrate or perform its biological role.

The hydrophilic amino acid would no longer participate in the hydrophobic interactions that hold the two subunits together. This would weaken the quaternary structure, potentially causing the subunits to dissociate or assemble incorrectly. Because hemoglobin requires all four subunits in the correct arrangement to carry oxygen efficiently, the mutation would likely reduce or eliminate hemoglobin's oxygen-carrying function.