5.4: Energy and Nutrients

All organisms require energy and nutrients. Nutrients are the raw materials an organism must acquire from the environment to live. Biological molecules, those that are found in or produced by living organisms, are primarily made of carbon atoms bonded with other elements and/or other carbon atoms. Energy is used to power life’s processes and is derived from light, organic molecules or inorganic molecules. All living organisms need, at a minimum, a source of carbon for the synthesis of biological molecules, and a source of energy.

It is sometimes useful to classify organisms according to how they acquire energy and carbon. Organisms that use inorganic sources of both carbon and energy are called autotrophs. Because autotrophs produce their own food, they are sometimes called producers. Plants and algae are examples of producers because they use sunlight energy to produce carbohydrates in a process called photosynthesis. Heterotrophs are consumers that use organic molecules as a source of both carbon and energy. Animals are examples of consumers because they consume organic material by eating plants or other organisms. They then use a process called cellular respiration to extract energy from the food they eat.

Energy

Ultimately, the source of energy for many processes occurring on the earth's surface comes from solar radiation. But there are many exceptions to this rule as biology has been very clever at tapping a variety of forms of energy to construct and maintain living beings.

Scientists use the term bioenergetics to describe the concept of energy flow (Figure \(\PageIndex{1}\)) through living systems. Cellular processes such as the building and breaking down of complex molecules occur through stepwise chemical reactions. Some of these chemical reactions are spontaneous and release energy, whereas others require energy to proceed. Just as living things must continually consume food to replenish their energy supplies, cells must continually obtain more energy to replenish that used by the many energy-requiring chemical reactions that constantly take place. Together, all of the chemical reactions that take place inside cells, including those that consume or generate energy, are referred to as the cell’s metabolism.

Figure \(\PageIndex{1}\): Ultimately, most life forms get their energy from the sun. Plants use photosynthesis to capture sunlight, and herbivores eat the plants to obtain energy. Carnivores eat the herbivores, and eventual decomposition of plant and animal material contributes to the nutrient pool. (Copyright; author via open stax)

Producers capture energy from the sun

Plants, algae, and many types of bacteria are able to capture energy from the sun through a process known as photosynthesis. During photosynthesis, plants use energy (originally from sunlight) to convert carbon dioxide gas (CO2) into sugar molecules (like glucose: C6H12O6). They consume carbon dioxide and produce oxygen as a waste product. This reaction is summarized as:

6CO₂+6H₂O → C₆H₁₂O₆+6O₂

Sugar molecules have a great deal of energy stored within their bonds and many living things consume sugars as a major energy source. Most plants store the sugar molecules they produce in the form of starches or cellulose. Many animals consume plants to extract the energy stored in these molecules. Because they act as an energy source for animals higher in the food web plants (and other photosynthetic organisms are known as producers.

Consumers utilize the energy captured by producers

The reaction that harvests the energy of a sugar molecule in cells requiring oxygen to survive can be summarized by the reverse reaction to photosynthesis. In this reaction, oxygen is consumed and carbon dioxide is released as a waste product. This process is also known as cellular respiration. All animals, including humans, use cellular respiration to extract energy from the food they eat. The reaction is summarized as:

C₆H₁₂O₆+6O₂ → 6CO₂+6H₂O

Both of these reactions involve many steps. The processes of making and breaking down sugar molecules illustrate two examples of metabolic pathways. A metabolic pathway is a series of chemical reactions that takes a starting molecule and modifies it, step-by-step, through a series of metabolic intermediates, eventually yielding a final product. This is a classic example of one of the many cellular processes that use and produce energy.

Nutrients

In order to grow, an organism has to acquire materials to make itself bigger. These materials are sometimes considered 'food' or may be described as nutrients. In this section we will consider what nutrients are required by organisms and how they are acquired. We have already considered three nutrients, carbon, hydrogen and oxygen that play a role in the energetics of organisms, but they also are important materially. Organisms are built of more than just these three and need others in order to grow. The acquisition of required materials (i.e., organismal nutrition) is something that distinguishes most of the organisms considered in this book from animals. Nutrition is highly significant, not just to the success (i.e., growth and reproduction) of organisms but also to ecology and to interactions with other organisms. The nutrition of organisms is strongly affected by their lifestyle, in particular, whether they are an autotroph or heterotroph, and also by their evolutionary history.

The elemental composition of organisms reflects their molecular composition. Typically around 98% of the mass of any organism is composed of four elements, C, H, O, and N. But other elements are present and many of these are 'essential' , i.e., the organism must have them in order to survive, grow and reproduce.

Carbon

For all autotrophic organisms carbon is acquired as carbon dioxide, either from the atmosphere or dissolved in water. For heterotrophic organisms, carbon is acquired in a variety of biomolecules: carbohydrates, proteins, lipids. The exact mix of compounds depends on the dietary preferences of the organism.

Hydrogen

For autotrophs, hydrogen is acquired in water molecules and occasionally in other compounds. For heterotrophs, hydrogen is acquired in water, carbohydrates, proteins and lipids.

Oxygen

Some abiotic factors, such as oxygen, are important in aquatic ecosystems as well as terrestrial environments. Terrestrial animals obtain oxygen from the air they breathe. Oxygen availability can be an issue for organisms living at very high elevations, however, where there are fewer molecules of oxygen in the air.

Oxygen plays two roles in organisms: a structural role, being a part of most biomolecules (carbohydrates, proteins, nucleic acids) and a dynamic role, being an essential reactant in cellular respiration that is subsequently lost as water. For autotrophs, oxygen for the structural role is acquired as carbon dioxide that is incorporated into carbohydrates and subsequently into other important biological molecules. For heterotrophs, structural oxygen is acquired in the food that they consume. For both autotrophs and heterotrophs, oxygen for respiration is acquired as molecular oxygen (O₂) which, for terrestrial organisms, can be acquired directly from the atmosphere where it accounts for nearly 20% of the air's molecules. In aquatic systems, oxygen is obtained from the water where it usually is present as a dissolved solute.

In aquatic systems, the concentration of dissolved oxygen is related to water temperature and the speed at which the water moves. Cold water has more dissolved oxygen than warmer water. In addition, salinity, current, and tide can be important abiotic factors in aquatic ecosystems. Dissolved oxygen can also depend on biological activity. Photosynthetic organisms produce oxygen but this effect is limited to the region of the water column that receives light. Throughout the water column, oxygen is consumed by all aerobic organisms. How much the oxygen levels are lowered by this action depends on the amount of living things, their rate of oxygen consumption (this is a strong function of temperature), and the rate of oxygen delivery to the system.

Nitrogen

For heterotrophs, nitrogen is obtained from the food that they eat, primarily from proteins, but also from nucleic acids and nucleotides. Some fungi and bacteria can acquire nitrogen as nitrate (NO₃–) or ammonia (NH₃)/ammonium ion (NH₄+). For plants, nitrogen is always acquired as nitrate or ammonia dissolved in water. Although nitrate and ammonia are considered to be 'inorganic' molecules they are almost always produced from biological molecules as a consequence of biological processes:

Phosphorus

Phosphorus is a key component of phospholipids (found in all biological membranes such as those surrounding cells), and nucleic acids (such as DNA). For autotrophs, phosphorus is generally acquired as the phosphate anion (PO 4 – ) which is made available by the action of heterotrophs who break down organic material and release phosphate. Phosphate is a key nutrient in aquatic systems and often regulates the amount of autotroph biomass and primary production.

Sodium

Sodium is unusual because it is not an essential element for most plants but it is essential for animals. In spite of not being essential for plants, sodium is generally present in plants in amounts sufficient to supply the needs of most heterotrophs. Sodium is common in most water sources, even 'fresh water' sources and sodium is essential for some types of plants.

Resource Acquisition

Plants obtain the majority of the nutrients they need, including water, nitrogen, phosophorus, etc from the soil through their roots. The only exception is carbon, which is taken up in the form of CO₂ from the atmosphere. Resource acquisition operates somewhat differently in animals than in plants. As heterotrophs, animals must consume all nutrients they require, including glucose. Animals have a variety of acquisition strategies available to them, depending on which trophic level and ecosystem they inhabit.

Types of Feeders

Animal resource acquisition strategies can be categorized in several ways. First, we might consider categorizing animals by what types of foods they eat. For example, herbivores consume plant material, carnivores consume animal material, and omnivores consume both. Each of these categories can be broken down more specifically. Among carnivores, there are piscivores which eat only fish and insectivores which eat only insects, as well as many other categories. Similarly, among herbivores, there are frugivores which eat only fruit, and granivores which eat only seeds, as well as many other categories. Another important grouping is the detritivores which consume detritus, or decaying dead plant and animal material. Detritivores are very important in nutrient cycling, and include organisms such as fungi, bacteria, earthworms, Each of these strategies are associated with adaptations for increasing the effectiveness of that strategy. For example, herbivores often have symbiotic bacteria in their gut and particularly long intestinal tracts in order to break down the cellulose found in plant cell walls. Additionally, carnivores have a wide variety of adaptations for detecting, capturing, and consuming prey.

Animals can also be categorized by how they obtain the foods they consume. For example, suspension or filter feeders filter particles of food from the surrounding water. Deposit feeders consume material deposited in the soil or sediment and are often also detritivores. Symbionts live in close association with another organism (their host) and obtain their nutrients from that organism. Symbionts can be parasitic (such as tapeworms) or mutualistic (such as the cellulose-digesting bacteria in the gut of herbivores). Perhaps the most well-known strategy for obtaining food is predation in which animals use stealth or active hunting to kill and consume other animals. This category can also be broken down into more specific strategies. For example, ambush predators use stealth and/or camouflage to hide from their prey and attack only when prey accidentally come near enough for the predator to capture or ambush. Spiders that hunt with webs are an example of ambush predators. Pursuit predators, however, actively chase fleeing prey. These two strategies have their own trade-offs; pursuit predation requires intensive energy investment in chasing down prey without guarantee of capture, while ambush predators must wait for potentially long time periods until prey come into their reach.

As can be seen in these definitions, various levels of specificity exist in both our definitions and in the strategies animals employ. Some animals are specialists, focusing on a particular strategy and/or food source. For example, pandas eat only bamboo and termites eat only wood (though from a variety of species). Other species are generalists and employ a wide variety of food sources and strategies. Bears, for example, are famous for their predation and also their love of berries. Goats and sharks notoriously eat things that are not even digestible food. The diets of many reptiles, such as lizards and snakes are what scientists call ‘gape-limited’ meaning they consume whatever fits inside their gaping mouths. Generalism and specialism have their own trade-offs. A specialist may struggle to find food that they can consume; however, they can also tailor their strategy to most effectively take advantage of a single food source. The anteater, for example, would have a difficult time consuming most types of prey, but is extremely effective at collecting ants. A generalist, on the other hand, can likely eat many things that it comes across in the ecosystem; however, it may not use those food sources as efficiently or effectively as a specialist.

Attribution

This page is a modified derivative of:

• Energy by LibreTexts Biology, license CC-NC-SA 3.0
• Energy and Metabolism by Open Stax, license CC-BY 4.0
• Nutrition and Nutrients by George M. Briggs, license CC-NC-SA 3.0