Nearly all animals depend on three main classes of molecule as sources of energy and building blocks for growth and development: carbohydrates, fats, and proteins. These molecules are broken down, and in the process the energy in their chemical bonds is transferred to biologically useful forms, typically adenosine triphosphate (ATP). While other nucleic acid triphosphates also serve as usable forms of energy, ATP is by far the one most commonly used by animals. The breakdown of carbohydrates, fats, and proteins to produce ATP takes place through a series of chemical reactions. Reactions that break down food sources to fuel the energy needs of a cell are termed catabolic. In contrast, reactions that result in net energy storage within cells and the organism are anabolic.

Let’s first review the breakdown of carbohydrates, including sugars (such as glucose) and starches, food sources that are chemically similar but clearly different in taste and texture. Glucose can be partially broken down in the absence of oxygen by glycolysis (Fig. 40.1). Glycolysis occurs in the cytosol of the cell, and the breakdown of each glucose molecule results in the production of 2 molecules of pyruvate, 2 molecules of ATP, and 2 molecules of NADH. In animals, if oxygen is not present, or is present only in small amounts, pyruvate is converted by fermentation into lactic acid. The production of ATP by anaerobic (“absence of oxygen”) metabolism provides rapid but short-term energy to the cell and organism.

In contrast, aerobic (“of oxygen”) metabolism (specifically, cellular respiration), carried out within the mitochondria of eukaryotic animals, provides a steady supply of ATP for longer-term, sustainable activity. When enough oxygen diffuses into the mitochondria, pyruvate can be further broken down by the citric acid cycle rather than converted to lactic acid (Fig. 40.1). In a series of oxidation–reduction reactions, energy-rich NADH and FADH₂ molecules are produced. These molecules then enter the electron transport chain in the inner mitochondrial membrane. The electron transport chain couples the transfer of electrons to the pumping of protons across the inner mitochondrial membrane, resulting in a proton electrochemical gradient that drives the synthesis of ATP by oxidative phosphorylation. Because oxygen is reduced to water in the process, oxygen is consumed and water is produced as a by-product.

Overall, aerobic respiration results in the formation of a total of 32 molecules of ATP when 1 molecule of glucose is broken down, much more than the 2 molecules of ATP from 1 molecule of glucose produced anaerobically. We can define metabolic efficiency as the amount of energy captured in a usable form (at least 234 kcal of useful ATP energy) per amount of energy in the starting molecule (686 kcal in 1 mol of glucose). The metabolic efficiency of aerobic respiration is therefore at least 34% (or 234/686 × 100). The remaining 66% of this energy is converted to heat and either lost to the environment or used to warm the animal. Glucose that is not metabolized is stored as glycogen, primarily in the liver and muscles.

853

Lipids are another important energy source for most animals. Lipids, such as triacylglycerol, consumed in the diet are broken down to glycerol and free fatty acids, which enter glycolysis or the citric acid cycle to yield ATP by oxidative phosphorylation (Chapter 7). Alternatively, they can be stored as fat (adipose tissue). Storing energy as fat is a particularly efficient way to store energy because it yields more than twice the energy supply per unit weight (9.4 kcal/g) as glycogen (4.2 kcal/g). Like lipids consumed in the diet, stored fats are also broken down into glycerol and free fatty acids, which yield ATP by the same pathways as lipids from the diet.

Proteins consumed in the diet can also be a useful energy source for animals. Carnivores such as cats get most of their energy from the protein in the meat they eat. Proteins, which are needed for building and maintaining the body, constitute the enzymes and structural elements of cells and tissues. Only following prolonged food deprivation, when fat and carbohydrate reserves are depleted, do animals break down protein reserves (mainly from their muscles) to form ATP. Animals preferentially rely on fats for long-term energy supply. Well-nourished humans have enough fat reserves to support their metabolic needs for 5–6 days before they start to use protein reserves. Because nerve cells depend primarily on glucose to produce ATP, carbohydrates are spared for the brain and nervous system when animals face starvation.

854

Quick Check 1 Muscle protein, fat, and glycogen are all reservoirs of energy. In what order are they used during a prolonged fast?