9.1 Nutrition and Energy Production

Plants can take inorganic nutrients from their environment (soil and atmosphere) and synthesize all of the organic molecules they need to survive, grow and reproduce. They are known as autotrophs or producers. Animals, on the other hand, must obtain the organic molecules they need from other organisms through feeding. They are known as heterotrophs or consumers. Given the diversity of animal life on our planet, it is not surprising that the animal diet would also vary substantially. The animal diet is the source of monomers needed for building DNA, proteins, and other macromolecules needed for growth, maintenance, and reproduction. The diet is also the source of materials for energy production in the cells. The diet must be adapted to the digestive functions of each animal and balanced to provide minerals and vitamins that are required for cellular function.

Food Requirements

What are the fundamental requirements of the animal diet? The animal diet should be well balanced, providing any and all necessary nutrients for bodily function. It should also feature the minerals and vitamins required for maintaining good health and reproductive capability. These requirements for a human are illustrated graphically in Figure 9.1.1.

The first step in ensuring that you are meeting the food requirements of your body is an awareness of the food groups and the nutrients they provide. To learn more about each food group and the recommended daily amounts, explore this interactive site by the U.S. Department of Agriculture.

Everyday Connection: Let’s Move! Campaign

Obesity is a growing epidemic, and the rate of obesity among children is rapidly rising in the United States. To combat childhood obesity and ensure that children get a healthy start in life, former First Lady Michelle Obama launched the Let’s Move! campaign. The goal of this campaign is to educate parents and caregivers on providing healthy nutrition and encouraging active lifestyles to future generations. This program aims to involve the entire community, including parents, teachers, and healthcare providers to ensure that children have access to healthy foods—more fruits, vegetables, and whole grains—and consume fewer calories from processed foods. Another goal is to ensure that children get physical activity. With the increase in television viewing and stationary pursuits such as video games, sedentary lifestyles have become the norm. Learn more at Let’s Move. It should be noted, however, that structural issues such as lack of access to health care and economically accessible nutrition also contributes to the rise in obesity, particularly in children of low socioeconomic background. Other policy strategies beyond increasing physical activity are needed to combat these structural barrier to healthy eating.

Extracting Energy from Organic Matter

The organic molecules required for building cellular material and tissues must come from food. Carbohydrates, or sugars, are the primary source of organic carbons in the animal body because they are broken down to form glucose. Complex carbohydrates, including polysaccharides, can be broken down into glucose through biochemical modification; however, animals do not produce the enzyme cellulase and most animals lack the ability to derive glucose from the polysaccharide cellulose. Since cellulose remains as a polymer in the digestive track, it is not absorbed. In fact, it is the bulk of what nutritionists refer to as fiber in our diet and is required for moving waste through the digestive track.

The intestinal microbes in the gut benefit from the food we consume and the organisms whose gut these microbes inhabit benefit from their metabolism. Some bacteria in animal guts can produce vitamins that are critical to the health of the organism. These vitamins are also absorbed by into the organism’s bloodstream and contribute to many biological functions in different cells including the processes that extract energy from absorbed monomers.

Building up Organic Matter

Once macromolecules are digested in the gut, the monomers that are produced can be absorbed into the body where they are then distributed to the many cells that make up multicellular organisms. Excess sugars in the blood are absorbed by liver and skeletal muscle cells where they are converted into glycogen and stored for later use. Glycogen stores are used to fuel prolonged exertions, such as long-distance running, and to provide energy during food shortage. Excess fatty acids can be absorbed by adipocytes where they are converted to triglyceride, a lipid that can store energy for extended periods of time (Fig. 9.1.2). Fats are also required in the diet to aid the absorption of fat-soluble vitamins and in the production of fat-soluble hormones.

Another important requirement is that of nitrogen. Protein catabolism provides a source of organic nitrogen in the form of amino acids. The carbon and nitrogen derived from these become the building block for nucleotides, nucleic acids, proteins, cells, and tissues. Excess amino acids can be built up into proteins in muscle cells. During periods of malnourishment when nitrogen inputs are low, skeletal muscle becomes the source of amino acids for the rest of the body (Fig. 9.1.2) as those proteins are broken down.

Essential Nutrients

While the animal body can synthesize many of the molecules required for function from organic precursors, there are some nutrients that need to be acquired from food. These nutrients are termed essential nutrients, meaning they must be eaten, and the body cannot produce them.

Vitamins are a class of essential organic molecules that are required in small quantities for many enzymes to function and, for this reason, are considered to be coenzymes. Absence or low levels of vitamins can have a dramatic effect on health. Both fat-soluble and water-soluble vitamins must be obtained from food.

Minerals such as calcium, magnesium, and iron are inorganic essential nutrients that must be obtained from food. Among their many functions, minerals help in structure and regulation and are considered cofactors. Certain amino acids also must be procured from food and cannot be synthesized by the body. These amino acids are the “essential” amino acids. The human body can synthesize only 11 of the 20 required amino acids; the rest must be obtained from food.

Food Energy and ATP

Animals need food to obtain energy and grow and maintain homeostasis. Homeostasis is the ability of a system to maintain a stable internal environment even in the face of external changes to the environment. For example, the normal body temperature of humans is 37°C (98.6°F). Humans maintain this temperature even when the external temperature is hot or cold. It takes energy to maintain this body temperature, and animals obtain this energy from the food they consume.

The primary source of energy for animals is carbohydrates, mainly glucose. Glucose is called the body’s fuel. The digestible carbohydrates in an animal’s diet are converted to glucose molecules through a series of catabolic chemical reactions.

Adenosine triphosphate (ATP), is the primary energy currency in cells. ATP releases energy when it transfers its terminal phosphate group to other molecules, converting it into adenosine diphosphate (ADP). ATP is produced by the oxidative reactions in the cytoplasm and mitochondrion of the cell, where carbohydrates, proteins, and fats undergo a series of metabolic reactions that are collectively called cellular respiration.

ATP is required for all cellular functions. It is used to build the organic molecules that are required for cells and tissues; it provides energy for muscle contraction and for the transmission of electrical signals in the nervous system. When the amount of ATP is available in excess of the body’s requirements, the liver utilizes the excess ATP and glucose to produce glycogen molecules. Glycogen is a polymeric form of glucose and is stored in the liver and skeletal muscle cells. When blood sugar drops, the liver releases glucose from stores of glycogen. Skeletal muscle converts glycogen to glucose during intense exercise. The process of converting glucose and excess ATP to glycogen and the storage of excess energy is an evolutionarily important step in helping animals deal with mobility, food shortages, and famine.

Everyday Connection

Obesity is a major health concern in the United States, and there is a growing focus on reducing obesity and the diseases it may lead to, such as type 2 diabetes, cancers of the colon and breast, and cardiovascular disease. How does the food consumed contribute to obesity?

Fatty foods are calorie-dense, meaning that they have more calories per unit mass than carbohydrates or proteins. One gram of carbohydrates has four calories, one gram of protein has four calories, and one gram of fat has nine calories. Animals tend to seek lipid-rich food for their higher energy content.

The signals of hunger (“time to eat”) and satiety (“time to stop eating”) are controlled in the hypothalamus region of the brain. Foods that are rich in fatty acids tend to promote satiety more than foods that are rich only in carbohydrates.

Excess carbohydrate and ATP are used by the liver to synthesize glycogen. The pyruvate produced during glycolysis is used to synthesize fatty acids. When there is more glucose in the body than required, the resulting excess pyruvate is converted into molecules that eventually result in the synthesis of fatty acids within the body. These fatty acids are stored in adipose cells—the fat cells in the mammalian body whose primary role is to store fat for later use.

It is important to note that some animals benefit from fat stores. Polar bears and seals need body fat for insulation and to keep them from losing body heat during Arctic winters. When food is scarce, stored body fat provides energy for maintaining homeostasis. Fats prevent famine in mammals, allowing them to access energy when food is not available on a daily basis; fats are stored when a large kill is made or lots of food is available.

References

Argilés, Josep M., Nefertiti Campos, José M. Lopez-Pedrosa, Ricardo Rueda, and Leocadio Rodriguez-Mañas. 2016. “Skeletal Muscle Regulates Metabolism via Interorgan Crosstalk: Roles in Health and Disease.” Journal of the American Medical Directors Association 17 (9): 789–96.

Figure descriptions

Figure 9.1.1. Image from ChooseMyPlate.gov showing the basic components of a balanced meal. This includes fruits, vegetables, grains, protein, and dairy. [Return to Figure 9.1.1]

Figure 9.1.2. The image shows two parts labeled A and B, illustrating the effects of nutrient states on the human body. In the top section (A), there’s a comparison between the ‘Fed state’ and ‘Fasted state.’ On the left, labeled ‘Fed state,’ various colored circles are targeted to specific organs, describing places of high use or storage. Red amino acids go primarily to the skeletal muscles where they are built up and stored as proteins. Yellow lipids are primarily stored in adipose tissue but also metabolized in the kidneys and the liver. Blue glucose is used by nearly every organ since it is metabolized for energy, especially the brain, which has a high metabolic need. On the right, labeled ‘Fasted state,’ a human silhouette is shown with arrows pointing from the organs to the body’s ‘Whole body metabolism.’ This represents where different monomers are extracted from different storage locations. Kidneys can make sure to not excrete glucose to keep more circulating in the bloodstream and glucose is released by the liver when it breaks down glycogen. Amino acids are extracted from the breakdown of proteins in skeletal muscles.

The bottom section (B) focuses on the ‘Malnourished state,’ with an illustration of the skeletal muscle and a human figure on the left. Arrows point from the muscle to labels on the right showing that amino acids (depicted as red circles) are extracted from all muscle cells to support the following activities: ‘Tissue structure & function,’ ‘Organ structure & function,’ ‘Immune responses,’ ‘Skin integrity,’ and ‘Neural function.’

Licenses and Attributions

“9.1 Nutrition and Energy Production” is adapted from “34.2 Nutrition and Energy Production” by Mary Ann Clark, Matthew Douglas, and Jung Choi for OpenStax Biology 2e under CC-BY 4.0. “9.1 Nutrition and Energy Production” is licensed under CC-BY-NC 4.0.