Life requires energy. As our discussion of biomolecules pointed out, the major functional components of the cell are mostly polymers - long chains of smaller individual molecular units. Each addition of a small link to the chain costs energy. Chemical reactions that build up complex molecules from simple ones are known as anabolic reactions. Conversely, heterotrophic organisms such as animals ingest food made up of these large polymers, which, when broken down in the digestive process, release energy for maintaining and building that organism. Such chemical reactions, in which complex molecules are broken down to simpler components, are classified as catabolic reactions. Taken as a group of reactions within a cell or even an organism, they can be referred to as the cell’s or organism’s anabolism or catabolism. The sum total of both types of reactions is the cell’s metabolism.
- 5.2: Fermentation
- Glycolysis gave us some usable energy in the form of ATP, and then there are the other products, NADH and pyruvate. If the cell is eukaryotic and oxygen is available, then those molecules can help make more ATP. If no oxygen is available or the cell is just a lowly prokaryote, it undergoes fermentation to produce either lactate or ethyl alcohol. Why does the cell need lactate or ethanol? It does not, although the lactate can contribute to overall metabolism.
- 5.3 The TCA Cycle
- Eukaryote make scads of ATP and seemingly effortlessly at that, using only the dregs left over after glycolysis has taken its pass at a glucose molecule: NADH and pyruvate. Glycolysis in eukaryotes, as be ts its prokaryotic origins, happens in the cytoplasm. The TCA cycle (also called the citric acid cycle) happens inside the matrix of the mitochondria, a double-membraned organelle.
- 5.4: Oxidative Phosphorylation
- Oxidative phosphorylation denotes the phosphorylation of ADP into ATP, utilizing the energy from successive electron transports (hence the “oxidative”). The basic concept is that oxidation of NADH, being highly exergonic, can generate the energy needed to phosphorylate ADP. Since oxidation of NADH by oxygen can potentially release 52 kCal/mol, and the energy needed to phosphorylate ATP is approximately 7.5 kCal/mol, we should be able to expect the formation of several ATP per oxidized NADH.
- 5.8 Fatty Acid Breakdown
- Hormone-sensitive lipase in adipose tissue hydrolyzes the stored fat in those cells into glycerol and fatty acids. Glycerol can enter the glycolytic cycle via conversion to dihydroxyacetone phosphate. The fatty acids are secreted from the adipose cells into the bloodstream where they bind to a carrier protein, albumin. This complex can then be brought inside of other cells by endocytosis, where they can be broken down as an energy source.
- 5.9: Amino Acid Degradation
- Proteins are broken down by a variety of proteases that hydrolyze the peptide bonds to generate smaller peptides and amino acids. Those amino acids that are not used for building new proteins may be broken down further to enter the metabolic processes discussed in this chapter.
Thumbnail: Biochemical processes that break things down from larger to smaller are called catabolic processes. Catabolic processes are often oxidative in nature and energy releasing. Some, but not all of that energy is captured as ATP. Image used with permission (CC BY-SA-NC; Kevin Ahern & Indira Rajagopal).