Nutritional Needs and Principles of Nutrient Transport

Learning Objectives

  1. Differentiate between photo-, chemo-, auto-, and hetero-trophies
  2. Distinguish between essential, beneficial, macro- and micro-nutrient requirements for plants and animals
  3. Recognize that both insufficient and excessive amounts of nutrients can have detrimental effects on organism’s growth and health
  4. Define and differentiate between diffusion, facilitated diffusion, ion channels, active transport, proton pumps, and co-transport, and explain their roles in the process of nutrient acquisition
  5. Explain the importance of surface area, distance, and concentration gradients in the process of nutrient acquisition

Living Cells Need Materials to Grow: Nutrients

The information below was adapted from OpenStax Biology 22.3

Recall from our discussion of prokaryotes metabolic diversity that all living things require a source of energy and a source of carbon, and we can classify organisms according to how they meet those requirements:

  • Classification by source of energy:
    • Phototrophs (phototrophic organisms) obtain their energy from sunlight
    • Chemotrophs (chemosynthetic organisms) obtain their energy from chemical compounds
  • Classification by source of carbon:
    • Autotrophs (autotrophic organisms) are able to fix (reduce) inorganic carbon such as carbon dioxide
    • Heterotrophs (heterotrophic organisms) must obtain carbon from an organic compound

These terms can be combined to classify organisms by both energy and carbon source:

  • Photoautotrophs obtain energy from sunlight and carbon from carbon dioxide
  • Chemoheterotrophs obtain energy and carbon from an organic chemical source
  • Chemoautotrophs obtain energy from inorganic compounds, and they build their complex molecules from carbon dioxide
  • Photoheterotrophs use light as an energy source, but require an organic carbon source (they cannot fix carbon dioxide into organic carbon)

Essential Macronutrients and Micronutrients

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

Cells are made of more than just carbon; for cells to build all of the molecules required to sustain life, they need certain substances collectively called nutrients. A nutrient is essential if an organism cannot synthesize it and must acquire it from another source. In contrast, a beneficial nutrient can stimulate growth and development but is not required and/or could be substituted by another nutrient.

Essential nutrients that are required in large amounts are called macronutrients, and those required in smaller or trace amounts are called micronutrients. Just a handful of elements are considered macronutrients across all domains of life: carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. (A mnemonic for remembering these elements is the acronym CHONPS.)

  • Carbon is the major element in all biological 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. Atmospheric (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.
  • Depending on the organism, other important macronutrients include potassium (K), magnesium (Mg), calcium (Ca), and sodium (Na).

In addition to these macronutrients, cells require micronutrients or trace elements, which are elements needed in small amounts. For example, iron (Fe) is necessary for the function of the cytochromes involved in electron-transport reactions.

Nutritional Needs in Plants

The information below was adapted from OpenStax Biology 31.1

Plants require light, water and about 20 essential nutrients (including both macro- and micronutrients) to support all their biochemical needs. Depending on the species, some plants have additional essential nutrients. Here we will describe only a handful of essential macronutrients that are common to all plants:

  • Carbon (C) from the air is used by plants to make glucose (sugar) which can be ultimately used to construct cellulose. Cellulose is the main structural component of the plant cell wall, and it is the most abundant organic compound on earth.
  • Nitrogen (N) is part of proteins and nucleic acids, and 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) 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 potassium is the third most common limiting factor for plant growth.

Based on the information above, you probably aren’t surprised to learn that nitrogen, phosphorus, and potassium are the three most common components in commercial fertilizers.

Both deficiencies and excessive amounts of nutrients, whether macronutrients or micronutrients, can adversely affect plant growth. Deficiencies occur when a plant does not have enough of a particular nutrient to support biological functions performed by that nutrient. Conversely, excessive amounts of some nutrients can be toxic to certain tissues or cell types.

Nutritional Needs in Animals

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

Like all forms life, animals require (at minimum) carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur as building blocks for biological molecules. Unlike photoautotrophic plants we have just described, which obtain their required energy from the sun and their required nutrients from carbon dioxide in the air and nutrients from the soil, animals are chemoheterotrophs and must obtain both their energy and their nutrients by eating other organisms. Animals can be classified based on the type(s) of organisms they eat (more on this concept later on in this reading):

  • Herbivores are animals whose primary food source is plant-based. Examples of herbivores 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, which require additional digestive processing steps compared to animal material. Herbivores can be further classified into frugivores (fruit-eaters), granivores (seed eaters), nectivores (nectar feeders), and folivores (leaf eaters).
  • Carnivores are animals that primarily eat other animals. Lions, tigers, snakes, sharks, and orcas (killer whales) are examples of vertebrate carnivores; invertebrate carnivores include sea stars, spiders, and ladybugs.
  • Omnivores are animals that eat both plants and animals. Humans, bears and chickens are examples of vertebrate omnivores; invertebrate omnivores include cockroaches and crayfish.

Regardless of whether an animal is an herbivore, carnivore, or omnivore, animals obtain their energy and nutrients from three primary organic precursors:

  • Carbohydrates (including both simple and complex sugars) are the primary source of organic carbon for herbivores and omnivores; carnivores must rely on protein and lipids as sources of carbon
  • Proteins are dietary sources of nitrogen and sulfur (and carbon for carnivores)
  • Fats are also significant sources of chemical energy, producing more energy per gram than carbohydrates. Dietary fats are also necessary to aid the absorption of fat-soluble vitamins and the production of fat-soluble hormones

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 because the animal cannot synthesize them. Essential nutrients vary among animal species (for example, almost all mammals can synthesize vitamin C; exceptions include humans, non-human primates, guinea pigs, and bats which must have a dietary source of vitamin C). The four classes of essential nutrients are:

  • Essential amino acids are necessary as protein building blocks. For humans, all essential amino acids are available from meat sources; because there are no plants that contain all essential amino acids, vegetarians and vegans must eat sufficient quantities of both grain and legume protein sources
  • Essential fatty acids are necessary as fat building blocks (also used as signaling molecules and many other functions). For humans, there are only two essential fatty acids, as we are able to synthesize nearly all of the fatty acids needed
  • Vitamins are organic compounds needed in small amounts. Vitamins may be water-soluble (such as vitamin C), meaning they are easily excreted in urine, or fat-soluble (such as vitamin D), meaning that they can accumulate in adipose (fat) tissue
  • Minerals are inorganic compounds needed in small amounts. Minerals include a variety of elements such as calcium, phosphorus, sulfur, potassium, sodium, iron, and many others

As with plants, both deficiencies and excessive amounts of nutrients can adversely affect animal growth and health. Deficiencies occur when an animal does not have enough of a particular nutrient to support biological functions performed by that nutrient. Conversely, excessive amounts of some nutrients can be toxic to certain tissues or cell types.

The video below provides an overview of the nutritional needs of humans:

Processes and Structures that Move Molecules into and out of Cells

Acquisition of nutrients, whether from the soil or from the digestive tract, relies on movement of molecules across a cell membrane. There are several structures and process that control this movement of molecules across cell membranes:

  • Diffusion occurs when molecules move from an area of high concentration to an area of low concentration. Diffusion does not require energy because the molecules move “down” their concentration gradient (from areas of high to low concentration).
  • Facilitated diffusion is diffusion that occurs through a protein channel embedded in a cell membrane. Each protein channel is highly specific for its given molecule. Facilitated diffusion is necessary for molecules that are too large or too polar to cross the nonpolar cell membrane.
  • Active transport is movement of a molecule using energy and is typically used to move a molecule against its concentration gradient. Active transport always requires an expenditure of energy, often (but not always) from ATP.
  • Proton pumps are protein complexes that use energy from ATP to “pump” protons from one side of a membrane to other. This process creates an electrochemical gradient, with a high concentration of positively-charged protons on one side of a plasma membrane. This electrochemical gradient can then be used as a source of energy to move other molecules against their concentration gradients via co-transporters.
  • Co-transport is the movement of two molecules at the same time: one molecule is transported along (“down”) its concentration gradient, which releases energy that is used to transport the other molecule against its concentration gradient. Co-transport occurs through protein channels called co-transporters. As with all protein channels, co-transporters are highly specific for the given molecules that they can transport.

The video below discusses the principles of active and passive transport (start at 1 min 7 sec and watch through 5 min 14 sec):

Physical Factors Influence Nutrient Acquisition

Several physical parameters influence the ability of an organism to bring nutrients into its body, and evolution has selected for specific physiological adaptations for nutrient absorption in response to these parameters:

  • Concentration gradient: Molecules will naturally diffuse from an area of high concentration to low concentration, meaning energy is not required. The higher the concentration gradient on either side of a membrane, the faster the rate of diffusion across the membrane. Living things have a number of adaptations to manipulate concentration gradients for the purposes of nutrient acquisition:
    • In plant roots, proton pumps move protons outside of the root tissue and into the soil environment to create a positive ionic gradient that helps to drive positively charged nutrients into plant roots.
    • In animal digestive tracts, as nutrients accumulate inside the intestines as a result of digestion, these nutrients move from an area of high concentration in the intestine to an area of lower concentration in the bloodstream or interstitial fluid adjacent to the intestines
  • Distance: Diffusion rate is influenced by distance; the longer the distance, the slower the rate of diffusion. Evolution has selected for adaptations that minimize the distance that molecules must travel across the nutrient absorption surface:
    • Cnidarians, such as jellyfish, bodies composed of only two thin tissue layers, meaning nutrients absorbed from the digestive cavity can diffuse throughout the body.
    • In organisms with circulatory systems that transport nutrients throughout the body, the capillaries that absorb nutrients from the digestive tract are immediately next to the nutrient absorption surface which minimizes the distance that nutrients must move from the digestive tract into the bloodstream.
  • Surface area: The larger the surface area where nutrient absorption can occur, the more nutrients the organism can absorb. Organisms have a variety of adaptations to maximize the surface area in their bodies where nutrient absorption occurs:
    • In plant roots, the presence of microscopic root hairs increase the surface area of the roots that can absorb water and nutrients.
    • In mammalian digestive tracts, the lining of the small intestine, where nutrient absorption occurs, is highly folded, with each fold containing multiple micro-projections called microvilli that increase the total surface area of the small intestines.