2.5 Organic Compounds Essential to Human Functioning

Types and Roles of Organic Molecules

Organic compounds contain carbon and are central to every process in the human body. The four major classes of organic molecules are carbohydrates, lipids, proteins, and nucleic acids. Each has a distinct structure that directly determines its function, from fueling cellular work to storing your genetic code.

Carbon as the Foundation of Organic Compounds

Carbon is what makes the diversity of organic molecules possible. Each carbon atom has four valence electrons, meaning it can form up to four stable covalent bonds with other atoms like oxygen, hydrogen, nitrogen, and even other carbons.

This bonding capacity lets carbon build long chains (like those in fatty acids), branched structures, and ring shapes (like glucose). The result is an enormous variety of molecular architectures from just one element.

Carbon-based molecules also carry functional groups, which are specific clusters of atoms (such as hydroxyl $-OH$, carboxyl $-COOH$, or amino $-NH_2$ groups) attached to the carbon backbone. These functional groups determine how a molecule behaves chemically, whether it's acidic, polar, reactive, etc.

Four Main Organic Molecules

• Carbohydrates serve as quick energy sources (glucose) and energy storage (glycogen). Some also provide structural support (cellulose in plants).
• Lipids store large amounts of energy (triglycerides), form the structural basis of cell membranes (phospholipids), and act as precursors for hormones (cholesterol).
• Proteins are the most functionally diverse molecules in the body. They act as enzymes, structural components (collagen), transport molecules (hemoglobin), and signaling hormones (insulin).
• Nucleic acids store and transmit genetic information (DNA and RNA) and provide the cell's energy currency (ATP).

Simple vs. Complex Carbohydrates

Carbohydrates are classified by size, from single sugar units to long chains of sugars linked together.

Simple carbohydrates are small and quickly absorbed for energy:

• Monosaccharides are single sugar units. Glucose (the body's primary fuel) and fructose (found in fruit) are common examples.
• Disaccharides are two monosaccharides bonded together. Sucrose (table sugar) is glucose + fructose; lactose (milk sugar) is glucose + galactose.

Complex carbohydrates are polysaccharides, meaning they're long chains of monosaccharides:

• Starch is how plants store glucose. Humans can digest it and break it down for energy.
• Glycogen is how animals (including humans) store glucose, primarily in the liver and skeletal muscles. It can be quickly mobilized when blood sugar drops.
• Cellulose makes up plant cell walls. Humans can't digest it, but it serves as dietary fiber.
• Chitin forms the exoskeletons of insects and cell walls of fungi. It's not a major factor in human nutrition, but it shows up on exams as an example of a structural polysaccharide.

Lipid Roles in Body Functions

Lipids are a diverse group of hydrophobic (water-insoluble) molecules. They don't form true polymers the way carbohydrates and proteins do, but they share the common trait of being largely nonpolar.

• Triglycerides are the body's main form of long-term energy storage. Each triglyceride consists of one glycerol molecule bonded to three fatty acid chains. Gram for gram, fats store more than twice the energy of carbohydrates.
• Phospholipids are the structural foundation of cell membranes. They have a hydrophilic (water-attracting) phosphate head and two hydrophobic (water-repelling) fatty acid tails. This dual nature causes them to spontaneously arrange into a bilayer in water, creating the selectively permeable membrane around every cell.
• Cholesterol is a steroid lipid embedded in cell membranes, where it helps regulate membrane fluidity. It also serves as the precursor for steroid hormones (testosterone, estrogen, cortisol) and bile acids used in fat digestion.

Protein Structure and Function

Proteins are built from amino acids linked by peptide bonds. The human body uses 20 different amino acids, and the specific sequence in which they're arranged determines everything about a protein's shape and function. There are four levels of protein structure, and each builds on the one before it:

1. Primary structure is the unique linear sequence of amino acids in a polypeptide chain. Even a single amino acid change can alter the protein's function (as seen in sickle cell disease).
2. Secondary structure refers to local folding patterns, specifically alpha helices (coiled spirals) and beta pleated sheets (accordion-like folds). These are stabilized by hydrogen bonds between the backbone atoms of nearby amino acids.
3. Tertiary structure is the overall 3D shape of a single polypeptide chain. It results from interactions between the R groups (side chains) of amino acids, including hydrogen bonds, ionic bonds, disulfide bridges, and hydrophobic interactions.
4. Quaternary structure exists only in proteins made of more than one polypeptide subunit. Hemoglobin, for example, has four subunits that must assemble correctly to function.

Denaturation is worth knowing: when a protein loses its 3D shape due to changes in temperature, pH, or chemical environment, it loses its function. The primary structure (amino acid sequence) stays intact, but the higher-level folding unravels.

Proteins perform a wide range of functions:

• Enzymes catalyze (speed up) chemical reactions without being consumed
• Structural proteins like keratin and collagen provide physical support
• Transport proteins like hemoglobin carry molecules (hemoglobin carries $O_2$ in blood, not across membranes)
• Hormones like insulin act as chemical messengers regulating physiology
• Antibodies recognize and help neutralize foreign invaders

Nucleic Acid Components and Roles

Nucleic acids are polymers of nucleotides. Each nucleotide has three parts: a nitrogenous base, a pentose (five-carbon) sugar, and a phosphate group.

DNA (deoxyribonucleic acid):

• Double-stranded helix that stores all genetic information
• Sugar is deoxyribose
• Bases: adenine (A), thymine (T), guanine (G), cytosine (C)
• Complementary base pairing: A pairs with T, G pairs with C
• The two strands run antiparallel and are held together by hydrogen bonds between complementary bases

RNA (ribonucleic acid):

• Single-stranded molecule involved in protein synthesis
• Sugar is ribose
• Bases: adenine (A), uracil (U) instead of thymine, guanine (G), cytosine (C)
• Three main types: mRNA (carries the genetic message from DNA to the ribosome), tRNA (delivers amino acids during translation), and rRNA (forms part of the ribosome structure)

ATP (adenosine triphosphate):

• The cell's universal energy currency
• Structurally, it's a modified nucleotide: adenine base + ribose sugar + three phosphate groups
• Energy is released when the bond between the second and third phosphate groups is broken by hydrolysis, converting ATP to ADP (adenosine diphosphate) + inorganic phosphate ($P_i$)
• Cells constantly recycle ADP back into ATP to meet energy demands

Molecular Interactions and Properties

A few recurring concepts tie all of these organic molecules together:

• Hydrophilic ("water-loving") molecules or regions are polar and interact readily with water. Glucose dissolving in blood is a good example.
• Hydrophobic ("water-fearing") molecules or regions are nonpolar and avoid water. The fatty acid tails of phospholipids are hydrophobic, which is why they cluster together inside the membrane bilayer.
• Covalent bonds form when atoms share electrons. These are the strong bonds holding organic molecules together, including the C-C bonds in carbon chains and the peptide bonds linking amino acids.
• Dehydration synthesis (condensation) joins monomers together by removing a water molecule. Hydrolysis breaks polymers apart by adding water. These two reactions are how the body builds and breaks down all major organic molecules.