Organismal Biology





Key Concepts




Chapter  14 - Animals -









In order to access nutrients, animals have to ingest or feed on materials that contain the nutrients and then digest those materials down to absorbable components, which will be used to build their own molecules.

Direct absorption of nutrients is a rare feeding method, depending upon digestion already having been done.  These animals may live with close associations with decomposers; some are parasites that live in digestive systems.

Filter feeding, sometimes called suspension feeding, involves drawing water through some sort of straining system that separates out particles for ingestion.  This is done by perhaps the simplest animals, the sponges, and one of the most complex, the baleen whales, as well as many other groups and species.

Deposit feeding can be done a few ways.  Earthworms and many other animals consume sediments and soil to process material from it;  snails are soemtimes considered deposit feeders as they scrape materials from surfaces.

Fluid feeding is done on the fluids from other living things.  Mosquitoes exist mostly on plant fluids, with females accessing blood for a nutrient needed for egg production.

Massed food feeding takes in larger bits of living material.  This can involve taking in whole individuals, as snakes and a lot of insect-eaters do, biting and swallowing pieces from larger individuals, such as with crocodiles, or taking pieces and grinding them to a pulp, found in many high-fiber plant eaters, or in mammals.









Breaking larger food molecules down to their absorbable components is done primarily with digestive enzymes, which attach to the molecules at the component-to-component links and promote that breakage, a chemical process called hydrolysis.  Some other chemicals help:  emulsifiers help break up (but don't break down) blobs of fatty, oily lipids to smaller blobs (enzymes can only access the outside of such blobs), and acids affect the internal attachments of large three-dimensional molecules like proteins, unwinding the strands so that they can be snipped apart by enzymes.

In a few cases, digestive may be intracellular, with particles taken by cells into food vacuoles broken down inside the cells.  Nutrients may then be shared with non-feeding cells.  This is done by sponges, and cnidarial gastrodermal cells sometimes do it.

Mostly, digestion is done extracellularly in the digestive cavities that are a basic feature of animals in general.  The cavities are lined with special secretion cells (mucus secretions prevent digestion of the lining), often modified to greatly increase absorption surface.  In cnidaria, platyhelminths, and a few smaller phyla, the cavity is a sac system where food is taken in, broken down, and "leftovers" leave through the entrance.  In the other phyla, a tube system is used.  Food enters through a mouth, is processed in stages along specialized parts of the tube that increase digestive efficiency, and the remains leave through an anus.  As food is processed, more food can be added to the system.

Food can be moved along very small systems by ciliary movement, but usually a traveling muscular squeeze, peristalsis, pushes food along the tube.  Some areas may be specialized for churning and mixing, and some chambers may house the fermentation processes necessary for plant fiber processing.  Systems commonly also hold symbiotic organisms with specialized digestive capacities.






The Basic Digestive Tube / Tract / Canal



Associated with the mouth are receiving regions.  Common structures here include a buccal cavity for temporary buildup and processing of food, mouthparts to help food intake and processing, a pharynx that often helps suck food in, salivary glands to do chemical pre-processing and/or produce a swallowable bolus of chewed-up food.

Beyond the receiving region is the conduction and storage region.  This often includes an esophagus to move food to the next region, but often a crop, a food storage chamber, is there.

A grinding and predigestion region may be integrated into the other  regions.  Grinding may be done by specialized teeth, very useful in preparing high-fiber plant food, or by gizzards, chambers with special ridges or embedded hard objects (birds take in small stones, dinosaurs took in fairly large stones that might have been gizzard stones).  Predigestion may be done in fermentation chambers (often diverticula), where symbionts help to process plant fibers, or in stomachs, which often expose large food molecules to chemicals like acids to "unwind" large molecules so that actual digestive enzymes can easily attach to them.  Such chambers can prevent entry to such pathogens as viruses - any organisms using the digestive system as a way into the host must be able to resist exposure to acids.

A terminal digestion and absorption region is where actual nutrients are processed to absorbable and usable components.  Surface may be increased  with folds, villi, and microvilli.  This is where most digestive enzymes and emulsifiers are active.  Associated with this region may be processors and detoxification structures such as livers, which can change absorbed materials into either more valuable or less poisonous molecules.

Terminal digestion requires the food to be in small bits in a watery "soup" for the chemicals to mix well.  Some materials are not easily processed this way, or may be difficult to mobilize for absorption into body fluids, and of course a large valuable part of food is its water.  The last part of the system is the water absortion and waste concentration region.  This part of the system is also heavily populated by micro-organisms, a system called the microbiome.  This includes symbionts, processing materials for the host or doing other useful functions, as well as many other commensal microbes (often anaerobic since such chambers may be very low-oxygen environments) that just live there.  Considerable study of the microbiome, which includes hundreds or thousands of species, is ongoing. 

Once the water is absorbed, what's left - a combination of undigested food, symbionts, and some metabolic wastes that have been added along the way - is feces.









Hunger is a response to nutritional needs.  It can be tied to circadian rhythms, daily timing periods, to just overall volume of food in the system, or very nutrient-specific, causing animals to seek very particular types of foods.

Nutrients include the components of the major biological molecules:  carbohydrates, such as sugars, starches, and fibers (absorbed as sugars), used mostly as an energy fuel;  proteins (absorbed as amino acids), used in a broad range of ways, including as enzymes;  lipids, fats, oils, waxes, and others, usable due mostly to their water-repellent qualities, which makes carriers a necessity to get the components to dissolve in body fluids for transport.

Minerals are used a few ways in animal systems.  Ions / salts / electrolytes are used in a number of processes, including the function of nerves and muscles.  Minerals are often a critical component of particular protein molecules, such as hemoglobin only being an oxygen carrier with iron atoms embedded in the molecules.

Vitamins are somewhat hard to define, since the "true" definition is very human-centered.  Enzyme systems often require small coenzyme molecules to function properly.  Most of these coenzyme molecules are made in the cells, but since they are used in many biological systems and are small enough to be absorbed without breakdown, they don't have to be made if enough is present in food.  Humans, being omnivores, have a long history of not having to make a long list of coenzymes, and if we once could make them, we have lost that ability (the unused genes have mutated beyond functionality).  Vitamins is particularly that list of coenzymes that humans need to obtain from food;  different animals with different food histories would have different lists.

Water, although absolutely necessary for the proper workings of all cell chemistry, is not generally considered a nutrient - we don't digest it, we don't really transform it, it's not used the way that other nutrients are.






Click on term to go to it in the text.
Terms are in the order they appear.



Direct Absorption

Filter Feeding
Deposit Feeding
Fluid Feeding
Massed Food Feeding
Intracellular Digestion
Food Vacuoles
Sac Digestive System
Tube Digestive System
Ciliary Movement
Receiving Region
Buccal Cavity
Salivary Glands
Conduction & Storage Region
Grinding & Predigestion Region
Fermentation Chambers
Acids in Digestion
Terminal Digestion & Absorption Region
Water Absorption & Waste Concentration Region

Water as Nutrient











Organismal Biology

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