Which one of the following is not one of the four major classes of organic compounds? A. Nucleic acids B. Water C. Proteins D. Carbohydrates E. Lipids

Nucleic acids are essential biomolecules found in all living organisms. They store and transmit genetic information. The two main types of nucleic acids are DNA (Deoxyribonucleic Acid) and RNA (Ribonucleic Acid). DNA holds the blueprint for the development, functioning, growth, and reproduction of organisms. It is composed of two strands that coil around each other to form a double helix. RNA, on the other hand, is single-stranded and is crucial for protein synthesis.
Nucleotides are the building blocks of nucleic acids. Each nucleotide consists of a phosphate group, a sugar molecule (deoxyribose in DNA and ribose in RNA), and a nitrogenous base. There are four types of nitrogenous bases in DNA: adenine (A), thymine (T), guanine (G), and cytosine (C). In RNA, uracil (U) replaces thymine.
Functions of nucleic acids include:

• Storing genetic information
• Transmitting genetic information during reproduction
• Serving as templates for protein synthesis

Proteins are large, complex molecules that play many critical roles in the body. They are made up of long chains of amino acids, which are organic compounds composed of carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur. There are 20 different amino acids that can combine to form proteins.
Proteins have various functions:

• Enzymatic activity: Enzymes are proteins that act as catalysts in biochemical reactions, speeding up the processes significantly.
• Structural support: Proteins like collagen provide structural support to cells and tissues.
• Transport and storage: Hemoglobin in blood transports oxygen, and ferritin stores iron.
• Signaling: Hormones, like insulin, are proteins that communicate between cells.

Proteins have four levels of structure: primary (sequence of amino acids), secondary (α-helix and β-sheet), tertiary (3D shape), and quaternary (arrangement of multiple protein subunits).

Carbohydrates are a major source of energy for the body. They are composed of carbon, hydrogen, and oxygen atoms, typically with a hydrogen:oxygen atom ratio of 2:1. Carbohydrates are categorized into three types based on their structure:

• Monosaccharides: Simple sugars like glucose and fructose
• Disaccharides: Two monosaccharides linked together, like sucrose (table sugar)
• Polysaccharides: Long chains of monosaccharides, like starch and cellulose

Functions of carbohydrates include:

• Providing energy: Glucose is a primary energy source for cells.
• Storing energy: Glycogen is stored in the liver and muscle cells for later use.
• Structural components: Cellulose in plant cell walls and chitin in the exoskeleton of insects.

Carbohydrates are broken down into glucose, which is then used in cellular respiration to produce ATP, the energy currency of the cell.

Lipids are a diverse group of hydrophobic molecules, which means they do not mix well with water. They are primarily composed of carbon, hydrogen, and oxygen, but in different ratios than carbohydrates. The main types of lipids include:

• Triglycerides: Made up of one glycerol molecule bonded to three fatty acids. They are a primary form of energy storage in the body.
• Phospholipids: Composed of two fatty acids, a glycerol unit, and a phosphate group. They are a key component of cell membranes, forming a bilayer.
• Steroids: Have a structure of four fused carbon rings. Cholesterol, a type of steroid, is a component of cell membranes and a precursor for steroid hormones.

Functions of lipids include:

• Energy storage: Lipids store more energy per gram than carbohydrates.
• Cell membrane structure: Phospholipids create barriers that protect cellular contents.
• Insulation and protection: Adipose tissue, which is largely made of triglycerides, insulates the body and cushions organs.

Lipids also play a role in signaling pathways and are precursors to important bioactive molecules like hormones.