Every living creature, be it a vegetable or animal, needs sustenance. Not just food - substances animals can shove down their gullet to still hunger pangs but actual nutrition that the organism needs to grow and thrive.
Note that only animals have gullets to shove food into; plants and some single-celled life forms make their own food. They are categorised as autotrophs. That word derives from Greek - 'autos' meaning 'self' and 'trophe' meaning 'nutrition'.
Understanding that 'hetero' means 'other', we can easily see that heterotrophs are organisms that eat other organisms. Animals and some plants eat other organisms, thus, they are classified as heterotrophs.
How these two broad classifications get their nutrition is a far more complex conversation; one that Superprof biology tutors take up with you now, just in time for your GCSE Biology review.
How Autotrophs Get Nutrition
Plants and some bacteria create their own sustenance from simple, natural ingredients: carbon dioxide that they pull in from the air and water that they pull up through their roots. Then, light energy gets absorbed by the organisms' chlorophyll; that energy converts to chemical energy to split water molecules into oxygen and hydrogen.
That is, essentially, the 'light reaction' phase of the process called photosynthesis. The 'dark reaction' phase nets the resulting products, one of which, sugar, is stored and becomes food while the other, water, is expelled as waste.
For photosynthesis to happen, four elements must be present: water, carbon dioxide, chlorophyll and light - typically sunlight but, as vertical gardens and other types of indoor growing ventures become more popular, fluorescent light works just as well.
Although every part of the plant that contains chlorophyll can perform photosynthesis, the process usually takes place in the leaves of plants because they are ideally suited for the photosynthetic process, as well as gas exchange and respiration. Nevertheless, stems and fruits, especially when they're tender and at their greenest, may also synthesize food.
Water-dwelling plants - algae and the like also convert their own food. As their entire structure is green, the whole plant is active in photosynthesis. Also, because they are underwater, they draw the necessary carbon dioxide from the supply found in their aquatic home.
Animals that photosynthesise also have chlorophyll; the product they create and store is called glycogen.
The sea slug is a particularly cunning example of such an organism. On its own, it has no chloroplasts but, as it consumes algae - a plant rich in chlorophylls, it diverts chloroplasts into its cells, thus giving them the necessary ingredient for photosynthesis. This process is known as horizontal genetic transfer - impossible in more evolved species but fairly common in bacteria.
What makes this animal so remarkable is that not only does it consume the food necessary to become a producer, but it lifts algae's genetic signature and passes it onto its offspring, so that they develop as natural autotrophs.
As autotrophs make their own food, they are labelled producers. Now, let's take a look at consumers - those organisms that consume others for their nutrition.
Heterotroph Food Intake
Any organism that consumes another organism is a heterotroph. That's true for the largest mammals - both on land and in the sea, as well as creatures you need a microscope to see. Included in that last category are non-green plants, such as moulds and fungi.
As you've been a heterotroph your whole life, it would seem the process is straightforward: you eat, you digest, you expel waste... right?
Well, not quite. The most vital step in heterotroph nutrition is the process of breaking down the food. That happens differently, depending on what type of consumer is involved.
Humans are classified as holozoic consumers. We do not degrade the substances we consume, nor do we have to wait until the food is decomposed before we can consume it. Indeed, should we eat decomposed food, we would end up very ill.
For humans and other holozoic consumers, enzymes are the catalysts that spur the breakdown of food and make possible the absorption of nutrients. If not for them, the food molecules would be too large to pass into our blood cells and the chemical reactions required to keep us alive would not happen fast enough.
Amoebas have a very interesting way to take in food. First, they identify a food source and then, they trap it within their 'false feet' - pseudopodia, projections of cytoplasm they cast out. The food is then ingested through the cell surface, forming a food vacuole within the amoeba's body. The enzyme lysosome breaks the food down; the nutrients are then transported throughout the cytoplasm.
Amoebas are also considered holozoic consumers.
Saprophytes eat dead organisms, as do humans and other animals. The difference is that saprophytes release enzymes onto their food before eating it. Once the saprophyte disgorges its enzymes, it must wait for the food to break down into component parts, which they then feed on. Moulds and some fungi are examples of saprophytic organisms.
Spiders and other animals that release enzymes onto their food prior to consuming it are called saprotrophs.
Parasites are all about the free ride. They cadge their entire existence off their host without returning them any benefit; indeed, a parasite actually harms their host. External parasites - fleas, leeches and the like, attach themselves to their host and draw nutrients from the host's blood.
Internal parasites like hookworms and tapeworms capture nutritional molecules destined to sustain the host, so that, even as the parasite grows, the host may become malnourished and lose dramatic amounts of weight before the parasite is found and removed.
Mixotroph Nutrition Systems
Along the spectrum of organisms requiring nutrition - with autotrophs at one end and heterotrophs at the other, we find a range of mixotrophs. These organisms may have chloroplasts that enable them to synthesise their food while others have none. Instead, they exist within a host and gain their nutrition from a symbiotic relationship.
How much of a mix these organisms employ to secure nutrition depends on whether they absolutely need as many ways to secure nutrition as they have available to them or whether they can change their means of securing food depending on the conditions they find themselves in.
- obligate mixotrophs use both hetero- and autotrophic means of securing nutrition to support growth and maintain homeostasis
- obligate autotroph/facultative heterotrophs: organisms grow and maintain themselves purely through autotrophy but may rely on heterotrophy should low-light conditions make photosynthesis impossible
- obligate heterotroph/facultative autotrophs: when prey is unavailable, these organisms sustain themselves through autotrophy
- facultative mixotrophs: the organism is primarily hetero- or autotrophic but draws on its mixotrophic capabilities only when necessary
Both plants and animals may be mixotrophs. On the plant side, flytraps and pitcher plants are particularly well-known examples of insect-eating plants that also photosynthesise nutrition; there are many others: flypaper plants with sticky resin coating their leaves, snap traps that can quickly close their leaves once they sense prey has landed, and bladder-trap plants that use an internal vacuum generated by the bladder.
Mixotrophy is less common on the animal side but there is at least one vertebrate animal that qualifies as a mixotroph: the spotted salamander, which feeds on algae. This amphibian has developed a symbiotic relationship with microalgae; when prey is scarce, the algae will do its part to generate nutrition for both organisms.
Reef coral, too, has built a symbiotic relationship with such microphytes - another word for microalgae, as have certain types of jellyfish and anemones. All of these animals are considered mixotrophs, as well.
The Trouble with Bacteria
Bacteria pose a classification problem in more ways than one. Some types of bacteria are consumers, such as those who gorge on decomposed organic material - the stuff you would toss in a compost pile, for instance, or the bacteria in your gut.
And then, there are the types of bacteria that are producers - whose nutrition mechanism is autotrophic. Cyanobacteria are a prime example of such.
Leave it to nature to find the middle ground between two distinct nutrition classifications!
Besides possibly being classified as either a producer or a consumer, different types of bacteria may fall anywhere along the autotrophic/heterotrophic spectrum, with a substantial number of types qualifying as mixotrophs. For instance, sulphur bacteria that live in geyser vents may draw on the air's carbon dioxide to fuel their autotrophic process or find sources of organic carbon to consume, thus initiating their heterotrophic process.
Nobody has a true count of exactly how many bacteria there are but all sources agree that they're the oldest species living on earth and they are the largest population of microorganisms on land and sea.
For all that they do, from nitrogen fixation - to improve air quality to aiding in the decomposition of organic matter, it stands to reason, that bacteria would defy classification as all of the above: consumer and producer, heterotroph and autotroph.
Do bacteria also play a role in animal and plant transport systems? You'll have to read this article to find out.
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