Molecules of Life Practice Flashcards

The Chemical Structure and Physics of the Water Molecule

The water molecule is defined by the chemical formula $H_2O$, consisting of two hydrogen atoms joined to a single oxygen atom by single covalent bonds. Its physical geometry is described as a V-shape with a specific bond angle of $104.5^\circ$. Water is fundamentally a polar molecule, meaning that the opposite ends of the molecule carry opposite charges due to the unequal distribution of shared electrons within the covalent bonds. This disparity arises because the oxygen atom is more electronegative than the hydrogen atoms. Consequently, each hydrogen atom carries a partial positive charge, denoted as $\delta+$, while the oxygen atom carries a partial negative charge, denoted as $\delta-$.

This polar property is the foundation for the formation of hydrogen bonds. A hydrogen bond is a chemical attraction that occurs when the $\delta+$ hydrogen of one water molecule is attracted to the $\delta-$ oxygen of another water molecule. Although individual hydrogen bonds are weaker than covalent bonds, they are strong enough to hold water molecules together in a constant state of breaking and reforming. A single water molecule is capable of forming hydrogen bonds with a maximum of four other water molecules simultaneously.

Physical and Biological Properties of Water

Water serves as a universal or versatile solvent due to its inherent polarity. It has the capacity to dissolve a wide range of ionic and polar molecules, such as sodium chloride ($NaCl$). In the case of $NaCl$, the partially negative oxygen atoms of water are attracted to the positively charged sodium ions ($Na^+$), while the partially positive hydrogen atoms are attracted to the negatively charged chloride ions ($Cl^-$). Water molecules surround these ions to form a sphere known as a hydration shell, which separates the ions and prevents them from recombining. This property is biologically vital because it allows water to dissolve essential substances within cells, provide an aqueous medium for enzymatic biochemical reactions, and serve as a major transport medium, such as xylem sap in plants or blood in humans.

Water possesses a high specific heat capacity, defined as the amount of heat energy that must be absorbed or lost for $1\,g$ of a substance to change its temperature by $1^\circ C$. For water, this value is $1\,cal/g^\circ C$ or approximately $4.2\,J/g^\circ C$. A large amount of heat is required to break the hydrogen bonds between water molecules to raise the temperature, and conversely, a large amount of heat is released when these bonds reform as temperature drops. This allow bodies of water like oceans to stabilize their temperature, providing a favorable environment for aquatic life, and acts as a heat buffer to prevent large fluctuations in the body temperature of terrestrial organisms.

Additionally, water exhibits a high latent heat of vaporization, which is the amount of heat energy required to convert $1\,g$ of liquid water into water vapor; for water, this is $580\,cal/g$. This process requires significant energy to break hydrogen bonds. Biologically, this results in an evaporative cooling effect. When sweat evaporates from human skin or water transpires from plant leaves, it reduces body heat and prevents overheating. Animals like dogs also utilize this mechanism through panting to maintain an optimum body temperature.

The property of cohesion refers to the force of attraction between water molecules via hydrogen bonds. This cohesive force results in a high surface tension, which is a measure of how difficult it is to stretch or break the surface of a liquid. At the air-water interface, water molecules are bonded to those beside and below them, forming a "skin-like" layer. This allows insects like water skaters and raft spiders to walk on the water surface. Cohesion also works alongside adhesion, the attraction of water to other substances like cellulose in cell walls, to help transport water and nutrients against gravity through a plant's xylem.

Finally, water reaches its maximum density at a temperature of $4^\circ C$. As water cools from $4^\circ C$ to $0^\circ C$, it expands rather than contracting, which is unusual for most liquids. At $0^\circ C$, water molecules move too slowly to break hydrogen bonds and become locked into a crystalline lattice. In this state, each molecule is bonded to four others, keeping them further apart than in liquid form. Consequently, ice is less dense than liquid water and floats. In nature, floating ice acts as an insulator, preventing ponds, lakes, and oceans from freezing solid and allowing aquatic organisms to survive beneath the surface during winter.

Classification and Structure of Carbohydrates

Carbohydrates are organic compounds composed of Carbon, Hydrogen, and Oxygen in a ratio of $1:2:1$. Their general molecular formula is derived from $(CH_2O)_n$. They are classified into three main groups: monosaccharides, disaccharides, and polysaccharides. Monosaccharides are the simplest sugar units and serve as the monomers for all other carbohydrates. They are soluble in water, have a sweet taste, can be crystallized, and are all reducing sugars because they possess a free functional carbonyl group. Their size is usually within the range of $3 < n < 6$. Common examples include triose ($n=3$), pentose ($n=5$), and hexose ($n=6$), with glucose ($C_6H_{12}O_6$) being the most prominent. Glucose can exist in linear chains but is more stable in ring forms, categorized as $\alpha$-glucose or $\beta$-glucose based on the position of the hydroxyl group ($-OH$) on the first carbon atom ($C1$).

Formation and Breakdown of Disaccharides and Polysaccharides

Disaccharides consist of two monosaccharides joined by a glycosidic bond. This bond is formed through a condensation reaction, where one water molecule is removed. Conversely, the bond is broken through hydrolysis, a process requiring the addition of one water molecule. Important disaccharides include maltose ($\alpha$-glucose + $\alpha$-glucose with an $\alpha$-1,4 glycosidic linkage), sucrose ($\alpha$-glucose + fructose with an $\alpha$-1,2 glycosidic linkage), and lactose ($\beta$-galactose + $\alpha$-glucose with a $\beta$-1,4 glycosidic linkage). While maltose and lactose are reducing sugars, sucrose is a non-reducing sugar.

Polysaccharides are large macromolecules formed by many monosaccharides linked by glycosidic bonds. Unlike simpler sugars, they do not taste sweet, are insoluble in water, cannot be crystallized, and are non-reducing sugars. Starch is the primary storage carbohydrate in plants, stored in plastids. It is ideal for storage because its large size makes it compact and high in energy, and its insolubility prevents it from affecting the cell's water potential. Starch comprises two types of molecules: amylose (long, unbranched, helical chains with $\alpha$-1,4 glycosidic linkages) and amylopectin (long, branched, helical chains with $\alpha$-1,4 linkages in the main chain and $\alpha$-1,6 linkages at branch points occurring every 25 to 30 units).

Glycogen serves as the main carbohydrate storage in animals, concentrated in liver and muscle cells. Its structure is similar to amylopectin but is more highly branched, with branching occurring every 8 to 10 units of $\alpha$-glucose. Cellulose is a structural polysaccharide that is a major component of plant cell walls. It is made of $\beta$-glucose monomers joined by $\beta$-1,4 glycosidic linkages, forming long, straight, unbranched chains. These chains are arranged parallel to one another and held together by hydrogen bonds to form microfibrils, which provide significant structural strength.

General Features and Types of Lipids

Lipids are organic compounds containing carbon, hydrogen, and oxygen, characterized by their insolubility in water and solubility in organic solvents. They consist mostly of hydrocarbon regions with non-polar $C-H$ bonds, which store high amounts of energy. Unlike carbohydrates or proteins, lipids are not considered polymers because they are not built from repeating monomers. The three main types of lipids are triglycerides, phospholipids, and steroids.

Steroids are characterized by a structure consisting of four fused hydrocarbon rings with various functional side chains. Cholesterol is a key steroid that regulates the fluidity of cell membranes and serves as a precursor for steroid hormones like estrogen and testosterone, as well as bile salts. Phospholipids are amphipathic molecules, meaning they have both a hydrophilic head (composed of glycerol, a phosphate group, and an R group) and two hydrophobic tails (fatty acid hydrocarbon chains). Lecithin is a primary phospholipid in cell membranes, typically featuring one saturated and one unsaturated fatty acid.

Triglycerides: Structure, Formation, and Importance

Triglycerides, commonly known as fats (solid at room temperature) or oils (liquid at room temperature), consist of one glycerol molecule and three fatty acids. Glycerol is an alcohol with three carbons and three hydroxyl groups ($C_3H_8O_3$). Fatty acids have the general formula $RCOOH$, consisting of a long, hydrophobic hydrocarbon chain and a hydrophilic carboxyl group. Saturated fatty acids contain no double bonds between carbon atoms and are typically found in animal fats like butter. Unsaturated fatty acids contain at least one double bond, which creates kinks in the chain, preventing tight packing and making them liquid at room temperature; these are found in plants and fish (e.g., olive oil).

The formation of a triglyceride occurs through condensation or esterification. In this process, three fatty acid molecules join with one glycerol by removing three water molecules, resulting in three ester linkages. The intermediate stages include monoacylglycerol and diacylglycerol before reaching the final triacylglycerol state. To break down a triglyceride, hydrolysis is required, where three water molecules are added back to reconstitute the glycerol and fatty acids. Triglycerides are essential for energy storage (storing more energy per gram than carbohydrates due to more $C-H$ bonds), acting as heat insulators in adipose tissue, protecting internal organs, and providing buoyancy for aquatic organisms.

Protein Structure and Amino Acids

Proteins are complex molecules composed of carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur. They consist of one or more polypeptide chains made of amino acid monomers. An amino acid features a central alpha ($\alpha$) carbon attached to four components: an amino group ($-NH_2$), a carboxyl group ($-COOH$), a hydrogen atom, and a variable R group or side chain. At a cellular $pH$ of approximately $7.4$, amino acids exist as zwitterions (bipolar ions with both positive and negative charges). This allows them to act as buffers, resisting $pH$ changes by accepting or releasing protons.

Amino acids are grouped based on the properties of their R groups: non-polar (hydrophobic), polar (hydrophilic), acidic (containing a carboxyl group), or basic (containing an amino group). They link together via peptide bonds formed through condensation between the carboxyl group of one amino acid and the amino group of another. A dipeptide consists of two linked amino acids, while a polypeptide contains three or more.

The Four Levels of Protein Organization

Protein structure is organized into four levels. The primary ($1^\circ$) structure is the unique linear sequence of amino acids determined by genetic information, joined by peptide bonds. This sequence dictates the protein's eventual shape and function. The secondary ($2^\circ$) structure involves regions of the polypeptide chain stabilized by hydrogen bonds within the protein backbone. Common forms include the $\alpha$-helix (a helical coil like keratin) and the $\beta$-pleated sheet (parallel regions like fibroin in silk).

The tertiary ($3^\circ$) structure is the overall three-dimensional shape formed by interactions between R groups. These interactions include weak hydrophobic interactions (van der Waals forces), hydrogen bonds, ionic bonds between charged side chains, and strong covalent disulfide bridges between cysteine amino acids. Myoglobin is a classic example of a tertiary-structured globular protein. The quaternary ($4^\circ$) structure occurs when two or more polypeptide chains (subunits) associate to form a functional protein, such as hemoglobin (four subunits) or collagen (three subunits).

Protein Denaturation and Classification

Protein structure is highly sensitive to environmental conditions. High temperatures (typically above $40^\circ C$) or extreme $pH$ levels can cause denaturation, where the chemical bonds and interactions (hydrogen, ionic, disulfide, and hydrophobic) break. This causes the protein to lose its original conformation and become biologically inactive. A common example of denaturation is egg white (albumin) turning solid and opaque during cooking. In some cases, renaturation can occur if the protein returns to its functional shape once normal conditions are restored.

Proteins are classified by structure into fibrous and globular types. Fibrous proteins, like keratin and collagen, are long, coiled, stable, and insoluble in water, making them ideal for structural support. Globular proteins, like enzymes and hemoglobin, are coiled into spherical shapes, are generally soluble in water, and act as biological catalysts or agents. They are also classified by composition: simple proteins consist only of amino acids (e.g., insulin), while conjugated proteins contain non-protein prosthetic groups. Examples of conjugated proteins include glycoproteins (with polysaccharides), lipoproteins (with lipids), chromoproteins like hemoglobin (with heme containing iron), and nucleoproteins (with nucleic acids in ribosomes or chromosomes).

Nucleic Acids: DNA and RNA

Nucleic acids are polymers called polynucleotides, composed of monomers known as nucleotides. Each nucleotide consists of a pentose sugar, a nitrogenous base, and a phosphate group, all joined by condensation. In DNA, the sugar is deoxyribose (which lacks an oxygen atom on the second carbon), whereas in RNA, the sugar is ribose. The nitrogenous bases are divided into purines (double-ring structures: Adenine and Guanine) and pyrimidines (single-ring structures: Cytosine, Thymine, and Uracil). DNA contains A, G, C, and T, while RNA contains A, G, C, and U.

The structure of DNA, according to the Watson and Crick Model, consists of two polynucleotide chains twisted into a double helix. The two strands are antiparallel, meaning they run in opposite directions; one ends in a $5'$ phosphate group and the other in a $3'$ hydroxyl group. The sugar-phosphate backbones are on the outside, and the nitrogenous bases are paired on the inside. Bases are held together by hydrogen bonds following specific base-pairing rules: Adenine pairs with Thymine (2 hydrogen bonds), and Cytosine pairs with Guanine (3 hydrogen bonds). Each full turn of the helix contains 10 base pairs.

RNA is generally shorter than DNA and consists of a single polynucleotide strand. There are three main types of RNA: Messenger RNA (mRNA), which carries genetic information from DNA to serve as a template for protein synthesis; Ribosomal RNA (rRNA), which combines with proteins to form ribosomal subunits; and Transfer RNA (tRNA), which transfers specific amino acids to the ribosome during the process of protein synthesis.