Lesson Explainer: Types of Nutrition Biology

In this explainer, we will learn how to identify and compare between autotrophic and heterotrophic nutrition.

All living things require some form of nutrition. Nutrition is how living organisms take in and use food materials. This provides them with the materials needed to carry out other life processes: reproduction, growth, repair and maintenance of their internal environment.

The fascinating thing about nutrition is the variety of how different organisms can obtain it. Some animals, like us, simply eat other organisms, but plants can obtain their own food by converting sunlight into chemical energy. There are even some amazing creatures that live deep under the sea in hydrothermal vents that obtain their energy from chemicals like hydrogen sulfide, which would be toxic to a human! However, this is how these creatures obtain their nutrition. All living organisms use a process called cellular respiration to release energy from their food.

There are two broad classifications of the methods of nutrition: autotrophic and heterotrophic. The suffix “-trophic,” means “food,” while “auto” means “self.” This explains why autotrophic organisms acquire their nutrients by making their own food, like how plants convert sunlight into sugars within their cells. “Hetero” means “different,” and heterotrophic organisms, such as most animals, acquire nutrients by consuming, or ingesting, a different organism. We are going to look into these two types of nutrition in more detail.

Let’s start with autotrophs, the name given to any organism that carries out autotrophic nutrition.

Green algae, some bacteria, and plants such as the sunflower in Figure 1 are an example of autotrophs, as they make their own food. They do this by converting light energy from the sun into chemical energy (glucose) in a process called photosynthesis. For this reason, they can be classified as photoautotrophs, the “photo” prefix meaning “light.” They also absorb minerals and water from the soil to help them build up large molecules in their cells and to carry out their life processes.

There are other types of autotrophs too. One such grouping is a chemotroph, “chemo” meaning “chemical” referring to their method of nutrition, which is converting one type of chemical into another, such as the bacterium in Figure 2. This process is called chemosynthesis. Chemoautotrophs are usually found in locations with little light, which are full of chemicals that would be toxic to other organisms. They make good use of these chemicals however, by converting them into sugars like glucose to be used in cellular respiration to release energy.

For example, the bacteria shown in Figure 2 live in deep-sea hydrothermal vents and convert hydrogen sulfide in the smoke from these vents into usable energy sources. These organisms are speculated to be one of the first groups of living things to live on Earth, and some scientists believe they may even exist on the dark and toxic moons of Jupiter.

Let’s discuss heterotrophs, the name given to any organism that obtains its nutrition heterotrophically, in more detail. Heterotrophs get their food by eating or absorbing another living, or once living, organism. This might be by eating plants, other animals, or breaking down parts of those plants and animals using enzymes and absorbing the remains.

The heterotroph must digest the organism it uses for its nutrition. Some heterotrophs like fungi digest, or break down, the organism before absorbing the nutrients. Others, like humans, ingest (eat) the organism first, and then digestion occurs thanks to the enzymes and organs of our digestive system breaking down the large molecules into smaller ones. Once digestion has occurred regardless of its method, the smaller molecules can then be used by cells to build up a variety of different useful molecules to carry out their functions.

There are a few different types of heterotrophs, and the first we will look at are parasites.

Parasites live on or inside another organism that is called its host. The parasite, for example, Plasmodium falciparum, which is responsible for causing malaria, will get its nutrition at the expense of the host. The Plasmodium parasite does this by infecting human red blood cells and absorbing their nutrients. Many parasites are associated with disease, as often in the process of gaining their nutrition, the host will either lose some of their own source of nutrition or be harmed in the process. Malaria is the cause of more than 400‎ ‎000 deaths per year, most of which are young children living in sub-Saharan Africa.

Some plants can also be parasites; for example, the tree in the picture below has another, parasitic plant’s roots surrounding its trunk. This parasitic plant is likely “stealing” the nutrients that the tree has made through photosynthesis or obtained from the soil. This is a common trick used by plants, lower down in the canopy, that cannot access enough light for photosynthesis or nutrients from nutrient-poor soil themselves.

Another form of heterotroph is called a saprophyte. “Sapro” means “rotting” and “phyte” relates to feeding, as saprophytes, which are typically bacteria or fungi, usually get their nutrition from dead and rotting material. Saprophytes do this by releasing enzymes from their cells to the extracellular environment when they are in contact with decaying organic matter, like dead leaves or dead tree stumps. Thus, saprophytes play the role of decomposers because they break down dead organisms.

Once the organic matter has been digested by the enzymes released by the saprophytes, the smaller particles are then absorbed by the saprophyte, as you can see in Figure 4. This is called extracellular digestion, as it occurs outside the saprophyte itself. Saprophytes are well adapted to spread far and embed themselves deep into a material to break it down effectively, so you should never eat a piece of bread when you can see even a little fungus on it, as that fungus has definitely spread further than your eyes can see!

The last form of heterotrophic nutrition we will look into is called holozoic nutrition, and it will likely be the one you are most familiar with. Holozoic nutrition involves anything characterized by ingesting and digesting solid, liquid, or gaseous food particles internally.

There are three main types of holozoic nutrition: carnivores, herbivores, and omnivores. Humans, dogs, and many other animals exhibit omnivorous feeding habits, which means that they gain their nutrition from consuming and internally digesting both plant and animal matter. Carnivores, such as lions, snakes, sharks, and even spiders, gain their nutrition from consuming animal matter only. Herbivores, such as goats, cows, rabbits, elephants, and mice, gain their nutrition from consuming plant matter only. Some humans classify themselves as herbivores too!

A simple example of a holozoic organism you may not have “seen” before is an amoeba. These are single-celled eukaryotic organisms that ingest food particles, digest them using enzymes within their cells, absorb the necessary nutrients to build into whatever molecules they need, and then egest whatever they do not need. This process, as seen in Figure 5, is the basic procedure that all holozoic organisms follow regardless of their size and the manner in which they absorb, digest, and egest their food.

Example 4: Identifying a Class of Heterotrophic Nutrition

Leeches, shown in the picture, attach to the body of another organism and obtain their nutrition by consuming some of that organism’s blood. What classification of heterotrophic organisms do they belong to?

1. Saprophytes
2. Parasites
3. Holozoic

Answer

Heterotrophic organisms acquire their nutrients by the ingestion of other organisms. There are three main types of heterotrophs: saprophytes, parasites, and holozoic organisms.

Saprophytes are organisms, usually fungi, which acquire their nutrients by absorption, mainly of decaying material. They do this by secreting extracellular enzymes that digest the material outside the saprophytes’ cells. The digested food is then absorbed into the cells of the saprophyte to be used.

Parasites are organisms that live on or in a host organism and get their food from or at the expense of its host, either by withdrawing a source of food from them or causing them physical damage.

Holozoic nutrition is a type of heterotrophic nutrition characterized by the ingestion and internal digestion of gaseous, liquid, or solid food particles, usually done by animals. Some single-celled organisms like amoeba also carry out holozoic nutrition.

The information in the question tells us that leeches attach to the body of another organism and consume their blood to gain nutrition. We can tell that they are not saprophytes, as most saprophytes are fungi and this leech is an animal. Furthermore, saprophytes mainly break down dead or decaying material, but leeches take blood from living organisms. They live on a host and feed to the detriment of the host itself by consuming its blood, so these are not holozoic, as the host is alive.

Therefore, our correct answer is B: parasites.

From this explainer and as shown in Figure 6 below, you can see that the energy stored in organic matter circulates between living organisms as a result of their different types of nutrition.

Autotrophs are at the bottom of the pyramid (green layer in Figure 6) because they are the primary producers of organic matter, using a physical source of energy (light in this example). Then, heterotrophs (yellow, orange, and red layers in Figure 6) are all the organisms whose nutrition depends either on autotrophs or on other heterotrophs. Saprophytes (brown circle) play the role of decomposers that can use the energy left by all of them once they are dead.

Let’s recap some of the key points we have covered in this explainer.