Introduction to Metabolism: Videos & Practice Problems

Metabolism refers to the sum of all chemical reactions occurring within an organism. These reactions are essential for maintaining life and are organized into metabolic pathways. Each pathway consists of a series of enzyme-mediated steps that transform substrates into intermediate molecules and ultimately into a final product. Metabolism is divided into two main types: catabolic pathways, which break down molecules to release energy, and anabolic pathways, which use energy to build larger molecules. Together, these processes ensure that cells have the energy and materials needed for growth, repair, and other vital functions.

Metabolic pathways are sequences of chemical reactions within a cell, where each step is catalyzed by a specific enzyme. These pathways transform an initial substrate through various intermediates into a final product. Enzymes are crucial because they lower the activation energy required for each reaction, making the process more efficient. For example, in a pathway, substrate A might be converted to product B by enzyme 1, then B to C by enzyme 2, and so on until the final product is formed. This stepwise process allows for precise regulation and control of cellular metabolism.

Catabolic and anabolic pathways are the two main types of metabolic pathways. Catabolic pathways, or catabolism, involve breaking down larger molecules into smaller ones, releasing energy in the process. This energy is often stored as ATP and used for cellular activities. An example is the breakdown of glucose during cellular respiration. In contrast, anabolic pathways, or anabolism, require energy input to synthesize larger molecules from smaller ones, such as building proteins from amino acids or DNA from nucleotides. These processes are essential for growth, repair, and maintaining cellular structures.

Catabolic pathways are associated with exergonic processes because they release energy as they break down complex molecules into simpler ones. In an exergonic reaction, the energy of the products is lower than that of the reactants, resulting in a net release of energy. For example, during the breakdown of glucose in cellular respiration, energy is released and captured in the form of ATP, which cells use for various functions. This energy release is a hallmark of catabolic pathways.

Anabolic pathways require energy input because they involve the synthesis of complex molecules from simpler ones, which is an endergonic process. In these reactions, the energy of the products is higher than that of the reactants, necessitating an external energy source. This energy is often supplied by ATP or other high-energy molecules. For instance, during protein synthesis, ATP provides the energy needed to link amino acids together into a polypeptide chain. This energy investment is crucial for building cellular structures and macromolecules essential for life.

The metaphor of a cat breaking down a pyramid helps illustrate catabolism by emphasizing the concept of breaking down larger structures into smaller components. Just as a cat might pounce on a pyramid of blocks and cause it to collapse into individual pieces, catabolic pathways break down complex molecules, such as carbohydrates, proteins, or lipids, into their simpler building blocks. This process releases energy, which cells can use for various functions. The metaphor provides a visual and memorable way to understand the essence of catabolic reactions.