Nutrition: What Plants and Animals Need to Survive

Learning Objectives

  1. Distinguish essential, beneficial, macro- and micro-nutrient requirements for plants and animals
  2. Predict the symptoms of nutrient deficiencies in plants and animals
  3. Describe the diversity of adaptations for acquisition of nutrients in plants and animals

Living Cells Need Materials to Grow: Nutrients

The information below was adapted from OpenStax Biology 22.3, OpenStax Biology 23.2, and OpenStax Biology 24.1

Macronutrients

Cells are essentially a well-organized assemblage of macromolecules and water. Recall that macromolecules are produced by the polymerization of smaller units called monomers. For cells to build all of the molecules required to sustain life, they need certain substances, collectively called nutrients. When prokaryotes grow, they obtain their nutrients from the environment. Nutrients that are required in large amounts are called macronutrients, whereas those required in smaller or trace amounts are called micronutrients. Just a handful of elements are considered macronutrients—carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. (A mnemonic for remembering these elements is the acronym CHONPS.)

Why are these macronutrients needed in large amounts? They are the components of organic compounds in cells, including water. Carbon is the major element in all macromolecules: carbohydrates, proteins, nucleic acids, lipids, and many other compounds. Carbon accounts for about 50 percent of the composition of the cell. Nitrogen represents 12 percent of the total dry weight of a typical cell and is a component of proteins, nucleic acids, and other cell constituents. Most of the nitrogen available in nature is either atmospheric nitrogen (N2) or another inorganic form. Diatomic (N2) nitrogen, however, can be converted into an organic form only by certain organisms, called nitrogen-fixing organisms. Both hydrogen and oxygen are part of many organic compounds and of water. Phosphorus is required by all organisms for the synthesis of nucleotides and phospholipids. Sulfur is part of the structure of some amino acids such as cysteine and methionine, and is also present in several vitamins and coenzymes. Other important macronutrients are potassium (K), magnesium (Mg), calcium (Ca), and sodium (Na). Although these elements are required in smaller amounts, they are very important for the structure and function of the prokaryotic cell.

Micronutrients

In addition to these macronutrients, prokaryotes require various metallic elements in small amounts. These are referred to as micronutrients or trace elements. For example, iron is necessary for the function of the cytochromes involved in electron-transport reactions. Some prokaryotes require other elements—such as boron (B), chromium (Cr), and manganese (Mn)—primarily as enzyme cofactors.

Nutritional Needs and Adaptations in Plants

The information below was adapted from OpenStax Biology 31.1, OpenStax Biology 31.2, and OpenStax Biology 31.3

Essential Nutrients

Plants require only light, water and about 20 elements to support all their biochemical needs: these 20 elements are called essential nutrients. For an element to be regarded as essential, three criteria are required: 1) a plant cannot complete its life cycle without the element; 2) no other element can perform the function of the element; and 3) the element is directly involved in plant nutrition.

Essential Elements for Plant Growth
Macronutrients Micronutrients
Carbon (C) Iron (Fe)
Hydrogen (H) Manganese (Mn)
Oxygen (O) Boron (B)
Nitrogen (N) Molybdenum (Mo)
Phosphorus (P) Copper (Cu)
Potassium (K) Zinc (Zn)
Calcium (Ca) Chlorine (Cl)
Magnesium (Mg) Nickel (Ni)
Sulfur (S) Cobalt (Co)
Sodium (S)
Silicon (Si)

Macronutrients and Micronutrients

The essential elements can be divided into two groups: macronutrients and micronutrients. Nutrients that plants require in larger amounts are called macronutrients. About half of the essential elements are considered macronutrients: carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium and sulfur. The first of these macronutrients, carbon (C), is required to form carbohydrates, proteins, nucleic acids, and many other compounds; it is therefore present in all macromolecules. On average, the dry weight (excluding water) of a cell is 50 percent carbon. As shown below, carbon is a key part of plant biomolecules.

Three cellulose fibers and the chemical structure of cellulose is shown. Cellulose consists of unbranched chains of glucose subunits that form long, straight fibers.

 

Cellulose, the main structural component of the plant cell wall, makes up over thirty percent of plant matter. It is the most abundant organic compound on earth. Plants are able to make their own cellulose, but need carbon from the air to do so.

The next most abundant element in plant cells is nitrogen (N); it is part of proteins and nucleic acids. Nitrogen is also used in the synthesis of some vitamins. While there  is an overwhelming amount of nitrogen in the air (79% of the atmosphere is nitrogen gas), the nitrogen in the air is not biologically available due to the triple bond between the nitrogen atoms. Only a few species of bacteria are capable of “fixing” nitrogen to make it bioavailable; thus nitrogen is often a limiting factor for plant growth.

Phosphorus (P), another macromolecule, is necessary to synthesize nucleic acids and phospholipids. As part of ATP, phosphorus enables food energy to be converted into chemical energy through oxidative phosphorylation. Likewise, light energy is converted into chemical energy during photophosphorylation in photosynthesis, and into chemical energy to be extracted during respiration. Phosphorous is typically available in a form that is not readily taken up by plant roots; the form that is bioavailable is present in small quantities and rapidly “fixed” into the bioavailable form once again. Phosphorus is therefore often a limiting factor for plant growth.

Potassium (K) is important because of its role in regulating stomatal opening and closing. As the openings for gas exchange, stomata help maintain a healthy water balance; a potassium ion pump supports this process. Potassium may be present at low concentrations in some types of soil, and it is the third most common limiting factor for plant growth.

Other essential macronutrients: Hydrogen and oxygen are macronutrients that are part of many organic compounds, and also form water. Oxygen is necessary for cellular respiration; plants use oxygen to store energy in the form of ATP.  Sulfur is part of certain amino acids, such as cysteine and methionine, and is present in several coenzymes. Sulfur also plays a role in photosynthesis as part of the electron transport chain, where hydrogen gradients play a key role in the conversion of light energy into ATP.

Magnesium (Mg) and calcium (Ca) are also important macronutrients. The role of calcium is twofold: to regulate nutrient transport, and to support many enzyme functions. Magnesium is important to the photosynthetic process. These minerals, along with the micronutrients, which are described below, also contribute to the plant’s ionic balance.

In addition to macronutrients, organisms require various elements in small amounts. These micronutrients, or trace elements, are present in very small quantities. They include boron (B), chlorine (Cl), manganese (Mn), iron (Fe), zinc (Zn), copper (Cu), molybdenum (Mo), nickel (Ni), silicon (Si), and sodium (Na).

Deficiencies in any of these nutrients, particularly the macronutrients, can adversely affect plant growth. Depending on the specific nutrient, a lack can cause stunted growth, slow growth, or chlorosis (yellowing of the leaves). Extreme deficiencies may result in leaves showing signs of cell death.

Photo (a) shows a tomato plant with two green tomato fruits. The fruits have turned dark brown on the bottom. Photo (b) shows a plant with green leaves; some of the leaves have turned yellow. Photo (c) shows a five-lobed leaf that is yellow with greenish veins. Photo (d) shows green palm leaves with yellow tips. Nutrient deficiency is evident in the symptoms these plants show. This (a) grape tomato suffers from blossom end rot caused by calcium deficiency. The yellowing in this (b) Frangula alnus results from magnesium deficiency. Inadequate magnesium also leads to (c) intervenal chlorosis, seen here in a sweetgum leaf. This (d) palm is affected by potassium deficiency. (credit c: modification of work by Jim Conrad; credit d: modification of work by Malcolm Manners)

Plants obtain inorganic elements from the soil, which serves as a natural medium for land plants. Soil is the outer loose layer that covers the surface of Earth. Soil quality is a major determinant, along with climate, of plant distribution and growth. Soil quality depends not only on the chemical composition of the soil, but also the topography (regional surface features) and the presence of living organisms. In agriculture, the history of the soil, such as the cultivating practices and previous crops, modify the characteristics and fertility of that soil.

Plants obtain food in two different ways. Autotrophic plants can make their own food from inorganic raw materials, such as carbon dioxide and water, through photosynthesis in the presence of sunlight. Green plants are included in this group. Some plants, however, are heterotrophic: they are totally parasitic and lacking in chlorophyll. These plants, referred to as holo-parasitic plants, are unable to synthesize organic carbon and draw all of their nutrients from the host plant.

Plants may also benefit from microbial partners in nutrient acquisition. Particular species of bacteria and fungi have co-evolved along with certain plants to create a mutualistic symbiotic relationship with roots. This improves the nutrition of both the plant and the microbe. The formation of nodules in legume plants and mycorrhization can be considered among the nutritional adaptations of plants. However, these are not the only type of adaptations that we may find; many plants have other adaptations that allow them to thrive under specific conditions.

Nutrients from Other Sources

Some plants cannot produce their own food and must obtain their nutrition from outside sources. This may occur with plants that are parasitic or saprophytic. Some plants are mutualistic symbionts, epiphytes, or insectivorous.

Parasitic Plants

A parasitic plant depends on its host for survival. Some parasitic plants have no leaves. An example of this is the dodder, which has a weak, cylindrical stem that coils around the host and forms suckers. From these suckers, cells invade the host stem and grow to connect with the vascular bundles of the host. The parasitic plant obtains water and nutrients through these connections. The plant is a total parasite (a holoparasite) because it is completely dependent on its host. Other parasitic plants (hemiparasites) are fully photosynthetic and only use the host for water and minerals. There are about 4,100 species of parasitic plants.

Saprophytes

A saprophyte is a plant that does not have chlorophyll and gets its food from dead matter, similar to bacteria and fungi (note that fungi are often called saprophytes, which is incorrect, because fungi are not plants). Plants like these use enzymes to convert organic food materials into simpler forms from which they can absorb nutrients. Most saprophytes do not directly digest dead matter: instead, they parasitize fungi that digest dead matter, or are mycorrhizal, ultimately obtaining photosynthate from a fungus that derived photosynthate from its host. Saprophytic plants are uncommon; only a few species are described.

Photo shows a plant with light pink stems reminiscent of asparagus. Bud-like appendages grow from the tips of the stems. Saprophytes, like this Dutchmen’s pipe (Monotropa hypopitys), obtain their food from dead matter and do not have chlorophyll. (credit: modification of work by Iwona Erskine-Kellie)

Symbionts

A symbiont is a plant in a symbiotic relationship, with special adaptations such as mycorrhizae or nodule formation. Fungi also form symbiotic associations with cyanobacteria and green algae (called lichens). Lichens can sometimes be seen as colorful growths on the surface of rocks and trees. The algal partner (phycobiont) makes food autotrophically, some of which it shares with the fungus; the fungal partner (mycobiont) absorbs water and minerals from the environment, which are made available to the green alga. If one partner was separated from the other, they would both die.

 

Epiphytes

An epiphyte is a plant that grows on other plants, but is not dependent upon the other plant for nutrition. Epiphytes have two types of roots: clinging aerial roots, which absorb nutrients from humus that accumulates in the crevices of trees; and aerial roots, which absorb moisture from the atmosphere.

 

Insectivorous Plants

An insectivorous plant has specialized leaves to attract and digest insects. The Venus flytrap is popularly known for its insectivorous mode of nutrition, and has leaves that work as traps. The minerals it obtains from prey compensate for those lacking in the boggy (low pH) soil of its native North Carolina coastal plains. There are three sensitive hairs in the center of each half of each leaf. The edges of each leaf are covered with long spines. Nectar secreted by the plant attracts flies to the leaf. When a fly touches the sensory hairs, the leaf immediately closes. Next, fluids and enzymes break down the prey and minerals are absorbed by the leaf. Since this plant is popular in the horticultural trade, it is threatened in its original habitat.

Photo shows a Venus flytrap. Pairs of modified leaves of this plant have the appearance of a mouth. White, hair-like appendages at the opening of the mouth have the appearance of teeth. The mouth can close on unwary insects, trapping them in the teeth.

A Venus flytrap has specialized leaves to trap insects. (credit: “Selena N. B. H.”/Flickr)

Nutritional Needs and Adaptations in Animals

The information below was adapted from OpenStax Biology 34.0, OpenStax Biology 34.1 OpenStax Biology 34.2

Most animals obtain their nutrients by the consumption of other organisms. At the cellular level, the biological molecules necessary for animal function are amino acids, lipid molecules, nucleotides, and simple sugars. However, the food consumed consists of protein, fat, and complex carbohydrates. Animals must convert these macromolecules into the simple molecules required for maintaining cellular functions, such as assembling new molecules, cells, and tissues. The conversion of the food consumed to the nutrients required is a multi-step process involving digestion and absorption. During digestion, food particles are broken down to smaller components, and later, they are absorbed by the body.

Animals obtain their nutrition from the consumption of other organisms. Depending on their diet, animals can be classified into the following categories: plant eaters (herbivores), meat eaters (carnivores), and those that eat both plants and animals (omnivores). The nutrients and macromolecules present in food are not immediately accessible to the cells. There are a number of processes that modify food within the animal body in order to make the nutrients and organic molecules accessible for cellular function. As animals evolved in complexity of form and function, their digestive systems have also evolved to accommodate their various dietary needs.

Herbivores, Omnivores, and Carnivores

Herbivores are animals whose primary food source is plant-based. Examples of herbivores, as shown below, include vertebrates like deer, koalas, and some bird species, as well as invertebrates such as crickets and caterpillars. These animals have evolved digestive systems capable of handling large amounts of plant material. Herbivores can be further classified into frugivores (fruit-eaters), granivores (seed eaters), nectivores (nectar feeders), and folivores (leaf eaters).

Herbivores, like this (a) mule deer and (b) monarch caterpillar, eat primarily plant material. (credit a: modification of work by Bill Ebbesen; credit b: modification of work by Doug Bowman)

Carnivores are animals that eat other animals. The word carnivore is derived from Latin and literally means “meat eater.” Wild cats such as lions and tigers are examples of vertebrate carnivores, as are snakes and sharks, while invertebrate carnivores include sea stars, spiders, and ladybugs. Obligate carnivores are those that rely entirely on animal flesh to obtain their nutrients; examples of obligate carnivores are members of the cat family, such as lions and cheetahs. Facultative carnivores are those that also eat non-animal food in addition to animal food. Note that there is no clear line that differentiates facultative carnivores from omnivores; dogs would be considered facultative carnivores.

Carnivores like the (a) lion eat primarily meat. The (b) ladybug is also a carnivore that consumes small insects called aphids. (credit a: modification of work by Kevin Pluck; credit b: modification of work by Jon Sullivan)

 

Omnivores are animals that eat both plant- and animal-derived food. In Latin, omnivore means to eat everything. Humans, bears and chickens are example of vertebrate omnivores; invertebrate omnivores include cockroaches and crayfish.

Omnivores like the (a) bear and (b) crayfish eat both plant- and animal-based food. (credit a: modification of work by Dave Menke; credit b: modification of work by Jon Sullivan)

Animal Nutritional Requirements (Human Focus)

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 co-enzymes. 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 are inorganic essential nutrients that must be obtained from food. Among their many functions, minerals help in structure and regulation and are considered co-factors. 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 in the form of protein. When eaten, proteins are broken down into their amino acid building blocks and are then used almost immediately to synthesize new proteins needed by the body. The essential amino acids are listed below (note, you are not required to memorize vitamins and minerals included in these tables).

Water-soluble Essential Vitamins
Vitamin Function Deficiencies Can Lead To Sources
Vitamin B1 (Thiamine) Needed by the body to process lipids, proteins, and carbohydrates Coenzyme removes CO2 from organic compounds Muscle weakness, Beriberi: reduced heart function, CNS problems Milk, meat, dried beans, whole grains
Vitamin B2 (Riboflavin) Takes an active role in metabolism, aiding in the conversion of food to energy (FAD and FMN) Cracks or sores on the outer surface of the lips (cheliosis); inflammation and redness of the tongue; moist, scaly skin inflammation (seborrheic dermatitis) Meat, eggs, enriched grains, vegetables
Vitamin B3 (Niacin) Used by the body to release energy from carbohydrates and to process alcohol; required for the synthesis of sex hormones; component of coenzyme NAD+ and NADP+ Pellagra, which can result in dermatitis, diarrhea, dementia, and death Meat, eggs, grains, nuts, potatoes
Vitamin B5 (Pantothenic acid) Assists in producing energy from foods (lipids, in particular); component of coenzyme A Fatigue, poor coordination, retarded growth, numbness, tingling of hands and feet Meat, whole grains, milk, fruits, vegetables
Vitamin B6 (Pyridoxine) The principal vitamin for processing amino acids and lipids; also helps convert nutrients into energy Irritability, depression, confusion, mouth sores or ulcers, anemia, muscular twitching Meat, dairy products, whole grains, orange juice
Vitamin B7 (Biotin) Used in energy and amino acid metabolism, fat synthesis, and fat breakdown; helps the body use blood sugar Hair loss, dermatitis, depression, numbness and tingling in the extremities; neuromuscular disorders Meat, eggs, legumes and other vegetables
Vitamin B9 (Folic acid) Assists the normal development of cells, especially during fetal development; helps metabolize nucleic and amino acids Deficiency during pregnancy is associated with birth defects, such as neural tube defects and anemia Leafy green vegetables, whole wheat, fruits, nuts, legumes
Vitamin B12 (Cobalamin) Maintains healthy nervous system and assists with blood cell formation; coenzyme in nucleic acid metabolism Anemia, neurological disorders, numbness, loss of balance Meat, eggs, animal products
Vitamin C (Ascorbic acid) Helps maintain connective tissue: bone, cartilage, and dentin; boosts the immune system Scurvy, which results in bleeding, hair and tooth loss; joint pain and swelling; delayed wound healing Citrus fruits, broccoli, tomatoes, red sweet bell peppers
Fat-soluble Essential Vitamins
Vitamin Function Deficiencies Can Lead To Sources
Vitamin A (Retinol) Critical to the development of bones, teeth, and skin; helps maintain eyesight, enhances the immune system, fetal development, gene expression Night-blindness, skin disorders, impaired immunity Dark green leafy vegetables, yellow-orange vegetables fruits, milk, butter
Vitamin D Critical for calcium absorption for bone development and strength; maintains a stable nervous system; maintains a normal and strong heartbeat; helps in blood clotting Rickets, osteomalacia, immunity Cod liver oil, milk, egg yolk
Vitamin E (Tocopherol) Lessens oxidative damage of cells, and prevents lung damage from pollutants; vital to the immune system Deficiency is rare; anemia, nervous system degeneration Wheat germ oil, unrefined vegetable oils, nuts, seeds, grains
Vitamin K (Phylloquinone) Essential to blood clotting Bleeding and easy bruising Leafy green vegetables, tea
Minerals and Their Function in the Human Body
Mineral Function Deficiencies Can Lead To Sources
*Calcium Needed for muscle and neuron function; heart health; builds bone and supports synthesis and function of blood cells; nerve function Osteoporosis, rickets, muscle spasms, impaired growth Milk, yogurt, fish, green leafy vegetables, legumes
*Chlorine Needed for production of hydrochloric acid (HCl) in the stomach and nerve function; osmotic balance Muscle cramps, mood disturbances, reduced appetite Table salt
Copper (trace amounts) Required component of many redox enzymes, including cytochrome c oxidase; cofactor for hemoglobin synthesis Copper deficiency is rare Liver, oysters, cocoa, chocolate, sesame, nuts
Iodine Required for the synthesis of thyroid hormones Goiter Seafood, iodized salt, dairy products
Iron Required for many proteins and enzymes, notably hemoglobin, to prevent anemia Anemia, which causes poor concentration, fatigue, and poor immune function Red meat, leafy green vegetables, fish (tuna, salmon), eggs, dried fruits, beans, whole grains
*Magnesium Required co-factor for ATP formation; bone formation; normal membrane functions; muscle function Mood disturbances, muscle spasms Whole grains, leafy green vegetables
Manganese (trace amounts) A cofactor in enzyme functions; trace amounts are required Manganese deficiency is rare Common in most foods
Molybdenum (trace amounts) Acts as a cofactor for three essential enzymes in humans: sulfite oxidase, xanthine oxidase, and aldehyde oxidase Molybdenum deficiency is rare
*Phosphorus A component of bones and teeth; helps regulate acid-base balance; nucleotide synthesis Weakness, bone abnormalities, calcium loss Milk, hard cheese, whole grains, meats
*Potassium Vital for muscles, heart, and nerve function Cardiac rhythm disturbance, muscle weakness Legumes, potato skin, tomatoes, bananas
Selenium (trace amounts) A cofactor essential to activity of antioxidant enzymes like glutathione peroxidase; trace amounts are required Selenium deficiency is rare Common in most foods
*Sodium Systemic electrolyte required for many functions; acid-base balance; water balance; nerve function Muscle cramps, fatigue, reduced appetite Table salt
Zinc (trace amounts) Required for several enzymes such as carboxypeptidase, liver alcohol dehydrogenase, and carbonic anhydrase Anemia, poor wound healing, can lead to short stature Common in most foods
*Greater than 200mg/day required
Essential Amino Acids
Amino acids that must be consumed Amino acids anabolized by the body
isoleucine alanine
leucine selenocysteine
lysine aspartate
methionine cysteine
phenylalanine glutamate
tryptophan glycine
valine proline
histidine* serine
threonine tyrosine
arginine* asparagine
*The human body can synthesize histidine and arginine, but not in the quantities required, especially for growing children.

This video provides a summary of human nutrition needs: