Nutrition and Energy Production

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 materials needed for building DNA and other complex molecules needed for growth, maintenance, and reproduction; collectively these processes are called biosynthesis. The diet is also the source of materials for ATP production in the cells. The diet must be balanced to provide the 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 and provide nutrients required for bodily function and the minerals and vitamins required for maintaining structure and regulation necessary for good health and reproductive capability. These requirements for a human are illustrated graphically below.

Organic Precursors

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. During digestion, digestible carbohydrates are ultimately broken down into glucose and used to provide energy through metabolic pathways. Complex carbohydrates, including polysaccharides, can be broken down into glucose through biochemical modification; however, humans do not produce the enzyme cellulase and lack the ability to derive glucose from the polysaccharide cellulose. In humans, these molecules provide the fiber required for moving waste through the large intestine and a healthy colon. The intestinal flora in the human gut are able to extract some nutrition from these plant fibers. The excess sugars in the body are converted into glycogen and stored in the liver and muscles for later use. Glycogen stores are used to fuel prolonged exertions, such as long-distance running, and to provide energy during food shortage. Excess glycogen can be converted to fats, which are stored in the lower layer of the skin of mammals for insulation and energy storage. Excess digestible carbohydrates are stored by mammals in order to survive famine and aid in mobility.

Another important requirement is that of nitrogen. Protein catabolism provides a source of organic nitrogen. Amino acids are the building blocks of proteins and protein breakdown provides amino acids that are used for cellular function. The carbon and nitrogen derived from these become the building block for nucleotides, nucleic acids, proteins, cells, and tissues. Excess nitrogen must be excreted as it is toxic. Fats add flavor to food and promote a sense of satiety or fullness. Fatty foods are also significant sources of energy because one gram of fat contains nine calories. Fats are required in the diet to aid the absorption of fat-soluble vitamins and the production of fat-soluble hormones.

Essential Nutrients

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

The omega-3 alpha-linolenic acid and the omega-6 linoleic acid are essential fatty acids needed to make some membrane phospholipids. Vitamins are another 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, as outlined in the tables below. Both fat-soluble and water-soluble vitamins must be obtained from food. Minerals, listed further below, 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. The essential amino acids are listed below.

Water-soluble Essential Vitamins

Fat-soluble Essential Vitamins

Minerals and Their Function in the Human Body

*Greater than 200mg/day required

Essential Amino Acids

*The human body can synthesize histidine and arginine, but not in the quantities required, especially for growing children.

Food Energy and ATP

Animals need food to obtain energy 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 food.

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, or ATP, is the primary energy currency in cells; ATP stores energy in phosphate ester bonds. ATP releases energy when the phosphodiester bonds are broken and ATP is converted to ADP and a phosphate group. 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 collectively called cellular respiration. For example, glycolysis is a series of reactions in which glucose is converted to pyruvic acid and some of its chemical potential energy is transferred to NADH and ATP.

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 uses the excess ATP and excess glucose to produce molecules called glycogen. 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.

Summary

Animal diet should be balanced and meet the needs of the body. Carbohydrates, proteins, and fats are the primary components of food. Some essential nutrients are required for cellular function but cannot be produced by the animal body. These include vitamins, minerals, some fatty acids, and some amino acids. Food intake in more than necessary amounts is stored as glycogen in the liver and muscle cells, and in fat cells. Excess adipose storage can lead to obesity and serious health problems. ATP is the energy currency of the cell and is obtained from the metabolic pathways. Excess carbohydrates and energy are stored as glycogen in the body.

Key terms

• essential nutrient — nutrient that cannot be synthesized by the body; it must be obtained from food.
• mineral — inorganic, elemental molecule that carries out important roles in the body.
• vitamin — organic substance necessary in small amounts to sustain life.

Practice

Explain why an animal’s diet should be balanced and meet the needs of the body

Define the primary components of food

Describe the essential nutrients required for cellular function that cannot be synthesized by the animal body

Explain how energy is produced through diet and digestion

Describe how excess carbohydrates and energy are stored in the body

This section is adapted from Biology 2e, Section 34.2: Nutrition and Energy Production by Mary Ann Clark, Jung Choi, Matthew Douglas, and OpenStax, © OpenStax, licensed under CC BY-NC-SA 4.0. Access the original for free at openstax.org. Changes: figures re-encoded as WebP; Figure_B34_02_01 re-kinded from the manifest’s file-extension “photo” guess to “diagram” (a flat plate infographic/logo, not a captured photograph); both figure references in the prose (“illustrated graphically in [link],” “listed in [link]”) rewritten as descriptive references to the figure or table below rather than a print number; the Link to Learning note and both Everyday Connection notes (Let’s Move! Campaign, Obesity) rendered as callouts, each Everyday Connection keeping its bold name followed by its source title in italics; the Let’s Move! note’s link kept pointing at the module’s own openstax.org/l/Letsmove redirect, with the visible link text set to the destination URL the module itself prints as the link’s wording; the four vitamin/mineral/amino-acid tables (34.1–34.4) transcribed in full as Markdown tables, each preceded by its source spanning title row rendered as a bold line (Markdown tables cannot span a header cell across columns); the asterisk footnote below Table 34.3 ("*Greater than 200mg/day required") and below Table 34.4 (the histidine/arginine note) kept as plain text below their tables rather than as a table row, for the same reason; vitamin B-numbers set as Unicode subscripts and NAD/NADP charges as a trailing Unicode superscript, matching the book’s chemical-notation convention; the end-of-section Review Questions and Critical Thinking Questions adapted into the closing interactive Practice block (multiple choice and self-check respectively); rubric checkpoints added to each self-check, decomposing its model answer (the source solution) into check-off clauses with no new claims; three key-term recall items (essential nutrient, mineral, vitamin) added from the glossary; two cloze recall items (balanced; energy currency) and one select-the-term multiple choice (“primary components”) added from the section summary to strengthen the “balanced diet” and “primary components” objectives, which the glossary and exercise sets do not test directly; two local cloze items built strictly from the section’s own sentences — “cellular function” from its opening sentence, to cover the “balanced diet” objective a third time since the module keys only one Critical Thinking question to it, and “glucose” from its digestion sentence, to cover the “primary components” objective a third time; a sortbins exercise added, built from the fat-soluble/water-soluble split across Tables 34.1 and 34.2 (ten of the thirteen listed vitamins, interleaved), per the module’s own category distinction between the two tables. One table cell is corrected with a visible Source note: blood clotting is dropped from the vitamin D row, where the table’s own vitamin K row places it (erratum 441).