1.6.1: Catabolic and Anabolic Reactions

Catabolic reactions involve the disintegration of substantial molecules like glucose into smaller components—carbon dioxide and water—resulting in energy liberation. This energy release, however, is not wholly efficient, as the released energy is slightly less than the original content of a molecule. Roughly 40 percent of this liberated energy is promptly transferred to adenosine triphosphate (ATP), a high-energy compound acting as a vital energy currency within the body. This ATP can be promptly harnessed to support cellular functions or, if in excess, saved to meet future energy demands. The residual 60 percent of released energy from catabolic reactions is dissipated as heat, a form that tissues and body fluids absorb, contributing to preserving normal body temperature.

The structure of an ATP molecule is composed of adenine, a nitrogenous base, ribose—a monosaccharide with five carbon atoms—and three phosphate groups. The chemical linkage between the second and third phosphate groups forms a high-energy bond that harbors the most significant energy reservoir within the ATP molecule. Catabolic enzymes frequently break the initial bond during energy-demanding cellular activities, a process known as dephosphorylation, which leads to the generation of adenosine diphosphate (ADP) and a phosphate group (P). ATP, ADP, and P are in a perpetual cycle of interchange between reactions. The construction of ATP through a reaction is called phosphorylation, a method employed in energy storage, whereas reactions dismantling ATP release energy (Figure 31). The energy furnished by ATP propels all bodily functions such as muscle contraction, respiration, and nutrient absorption in the gastrointestinal tract.

Anabolic processes involve the synthesis of complex molecules. For instance, the combination of multiple glucose molecules results in the formation of polysaccharides. Similarly, the attachment of three fatty acids to glycerol forms triglycerides, the bonding of amino acids gives rise to proteins, and the assembly of nucleotides constructs nucleic acids. As a result, anabolic reactions play a crucial role in the formation of larger molecules that serve as essential structural components within cells.

Four primary macromolecules constitute the foundation of biological molecules: carbohydrates, lipids, proteins, and nucleic acids. Carbohydrates, the predominant energy source for fueling the body, are divided into monosaccharides, disaccharides, and polysaccharides. Monosaccharides encompass simple sugars such as glucose (Figure 31), galactose, and fructose—where glucose is the most common fuel for cellular ATP production (Figure 34). Galactose and fructose undergo conversion into glucose within the liver. These three monosaccharides are hexoses consisting of six carbons. Monosaccharide ribose in RNA and deoxyribose in DNA are examples of pentoses consisting of five carbons. Disaccharides include sucrose (table sugar, formed from glucose and fructose), lactose (milk sugar, comprising glucose and galactose), and maltose (malt sugar, consisting of two glucose molecules). Polysaccharides include glycogen (Figure 33) and starch—lengthy chains of glucose units. Cellulose is another polysaccharide; however, humans lack the enzymes required to digest it. While indigestible polysaccharides lack nutritional value, they provide dietary fiber that aids in moving food through the digestive tract. Several hormones regulate glucose levels in the bloodstream. Excess glucose is stored as glycogen—an energy reserve in the liver and muscles—or transformed into triglycerides in the liver or adipose tissue.

Dietary lipids, commonly referred to as fats, manifest in two distinct categories: saturated and unsaturated. Saturated fats are set apart from unsaturated fats by the presence of solely single covalent bonds within their fatty acid hydrocarbon chains. In contrast, unsaturated fats can encompass one or more double covalent bonds. Additionally, saturated fats remain solid at room temperature, whereas unsaturated fats maintain a liquid state. Sources rich in saturated fats include red meat, egg yolk, butter, and certain plant products like coconut and palm oil. In contrast, unsaturated fats are prevalent in sunflower, corn, canola, soybean, and flaxseed oils, and foods like walnuts and fish.

Figure 34 | Structure of Different Types of Lipids | ⁵⁰ This figure depicts molecular structures characterizing different lipid types, each playing a pivotal role in cellular functions. a) Cholesterol: A steroid consisting of four interconnected hydrocarbon rings. b) Fatty Acid: The basic structure of a fatty acid is displayed here, showcasing its hydrocarbon chain with a carboxyl group at one end. The varying lengths and degrees of saturation in these chains contribute to the diversity of fatty acids in biological systems. The fatty acid shown here contains one double bond, which is called monounsaturated fatty acid. c) Triglyceride: The structure of a triglyceride consists of three fatty acid chains attached to a glycerol backbone. d) Phospholipid: The phospholipid structure features a hydrophilic phosphate head and hydrophobic fatty acid tails.

Lipids can be categorized into three distinct groups based on their structures. Triglycerides are the most prevalent lipid type. They consist of three fatty acids attached to a glycerol backbone (Figure 34). They are stored within adipose tissue, serving as a significant energy reservoir (Figure 35). Roughly half of the excess fat accumulates in adipocytes beneath the skin in subcutaneous tissue, while the remainder lines internal organs. Phospholipids, on the other hand, play a vital role in membrane composition, constructing lipid bilayers and regulating transport of substances across membranes. The cell membrane serves as an excellent example (see Figure 1). Similar arrangements can be found in other membranous structures such as the nuclear envelope and endoplasmic reticulum. Steroids represent another lipid class important in hormone and bile salt synthesis. Cholesterol is a member of the steroid family (Figure 34). It is made of four interconnected hydrocarbon rings. It is essential for the synthesis of steroid hormones, bile acids, and vitamin D. Cholesterol molecules are amphipathic, allowing them to integrate into cell membranes and influence membrane fluidity. While excess cholesterol can contribute to health issues, maintaining a balance is vital.

Proteins are formed through the linkage of amino acids by peptide bonds. Protein digestion releases amino acids that can serve as the basic units for constructing new proteins or be further broken down to generate ATP (Figure 35). Roughly 14 percent of the body's composition comprises proteins, manifesting in diverse roles like forming collagen fibers, serving as clotting agents, facilitating muscular contraction, catalyzing enzymatic reactions, and functioning as receptors. In prolonged starvation, when resources are scarce, amino acids are utilized for energy production through protein breakdown, potentially leading to the deterioration of bodily structures.

Nucleic acids, DNA and RNA, are obtained through the diet as they are present in most of the foods. Through digestion, nucleic acids like DNA and RNA are disassembled into their elemental constituents known as nucleotides. Each nucleotide comprises a nitrogenous base such as adenine, guanine, cytosine, thymine, or uracil, a monosaccharide, ribose or deoxyribose, and a phosphate group. These nucleotides are readily assimilated and transported throughout the body for utilization by individual cells.

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