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Herbivore digestive System


Why do Herbivores have many different types of digestive Systems? for example a Rabbit has a mono-gastric Digestive system and a hippopotamus has a pseudo ruminant digestive system but they are both Herbivores.


This is kind of like asking why animals have so many different ways of moving quickly (dogs run, birds fly, kangaroos jump… ) There is a problem, and evolution has solved the problem in different ways.

In the case of herbivores (and in particular leaf and grass-eating herbivores) the problem is that their food is low in nutrients and high in indigestible cellulose. The particular solution depends on other evolutionary pressures. For many leaf-and-grass eaters the solution is to become big, with complex many-chambered digestive systems that allow for long slow processing of the food. Horses, hippos and ruminants all tend to be large. This means that the animal can eat a lot and hold the food in themselves for a long time, allowing enzymes and gut bacteria time to work.

Rabbits, as burrowing creatures, can't evolve a large size, so depend on a range of gut bacteria and coprophagia to pass food through their intestines twice.

Grasshoppers can produce their own cellulase to break down cellulose without microbial assistance.

There's more than one way to do it and as a rule, in evolution, for every possible solution, some animal will be using it.


Herbivore digestive System - Biology

Roughly 50% of the organic carbon on earth is tied up in cellulose. This represents an enormous source of energy, yet vertebrate cells do not produce the cellulases necessary to break down this abundant material.

Fortunately, many microbes produce cellulases which allow them to utilize dietary cellulose and other plant wall materials. Cellulolytic microbes inhabit the digestive tract of all animals, allowing the animal to siphon off and assimilate the end products of fermentation, particularly short chain or volatile fatty acids.

The relative value of fermentation to an animal's nutrition pretty much depends on the size of its fermentation vat. Fermentation occurs in the colon of dogs and humans, but their large bowel is rather small and the benefit they gain from fermentation is trivial. In contrast, herbivores make a living on cellulose by possessing massive fermentation vats as part of their digestive tract.


Structure and function of the digestive system in molluscs

The phylum Mollusca is one of the largest and more diversified among metazoan phyla, comprising many thousand species living in ocean, freshwater and terrestrial ecosystems. Mollusc-feeding biology is highly diverse, including omnivorous grazers, herbivores, carnivorous scavengers and predators, and even some parasitic species. Consequently, their digestive system presents many adaptive variations. The digestive tract starting in the mouth consists of the buccal cavity, oesophagus, stomach and intestine ending in the anus. Several types of glands are associated, namely, oral and salivary glands, oesophageal glands, digestive gland and, in some cases, anal glands. The digestive gland is the largest and more important for digestion and nutrient absorption. The digestive system of each of the eight extant molluscan classes is reviewed, highlighting the most recent data available on histological, ultrastructural and functional aspects of tissues and cells involved in nutrient absorption, intracellular and extracellular digestion, with emphasis on glandular tissues.

Keywords: Digestive gland Digestive tract Mollusca Salivary glands Ultrastructure.


Invertebrate Digestive Systems

Animals have evolved different types of digestive systems to aid in the digestion of the different foods they consume. The simplest example is that of a gastrovascular cavity and is found in organisms with only one opening for digestion. Platyhelminthes (flatworms), Ctenophora (comb jellies), and Cnidaria (coral, jelly fish, and sea anemones) use this type of digestion. Gastrovascular cavities, as shown in [link]a, are typically a blind tube or cavity with only one opening, the “mouth”, which also serves as an “anus”. Ingested material enters the mouth and passes through a hollow, tubular cavity. Cells within the cavity secrete digestive enzymes that break down the food. The food particles are engulfed by the cells lining the gastrovascular cavity.

The alimentary canal , shown in [link]b, is a more advanced system: it consists of one tube with a mouth at one end and an anus at the other. Earthworms are an example of an animal with an alimentary canal. Once the food is ingested through the mouth, it passes through the esophagus and is stored in an organ called the crop then it passes into the gizzard where it is churned and digested. From the gizzard, the food passes through the intestine, the nutrients are absorbed, and the waste is eliminated as feces, called castings, through the anus.



34.1 Digestive Systems

Animals obtain their nutrition from the consumption of other organisms. Depending on their diet, animals can be classified into the following categories: plant eaters (herbivores), meat eaters (carnivores), and those that eat both plants and animals (omnivores). The nutrients and macromolecules present in food are not immediately accessible to the cells. There are a number of processes that modify food within the animal body in order to make the nutrients and organic molecules accessible for cellular function. As animals evolved in complexity of form and function, their digestive systems have also evolved to accommodate their various dietary needs.

Herbivores, Omnivores, and Carnivores

Herbivores are animals whose primary food source is plant-based. Examples of herbivores, as shown in Figure 34.2 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. Herbivores can be further classified into frugivores (fruit-eaters), granivores (seed eaters), nectivores (nectar feeders), and folivores (leaf eaters).

Carnivores are animals that eat other animals. The word carnivore is derived from Latin and literally means “meat eater.” Wild cats such as lions, shown in Figure 34.3a and tigers are examples of vertebrate carnivores, as are snakes and sharks, while invertebrate carnivores include sea stars, spiders, and ladybugs, shown in Figure 34.3b. Obligate carnivores are those that rely entirely on animal flesh to obtain their nutrients examples of obligate carnivores are members of the cat family, such as lions and cheetahs. Facultative carnivores are those that also eat non-animal food in addition to animal food. Note that there is no clear line that differentiates facultative carnivores from omnivores dogs would be considered facultative carnivores.

Omnivores are animals that eat both plant- and animal-derived food. In Latin, omnivore means to eat everything. Humans, bears (shown in Figure 34.4a), and chickens are example of vertebrate omnivores invertebrate omnivores include cockroaches and crayfish (shown in Figure 34.4b).

Invertebrate Digestive Systems

Animals have evolved different types of digestive systems to aid in the digestion of the different foods they consume. The simplest example is that of a gastrovascular cavity and is found in organisms with only one opening for digestion. Platyhelminthes (flatworms), Ctenophora (comb jellies), and Cnidaria (coral, jelly fish, and sea anemones) use this type of digestion. Gastrovascular cavities, as shown in Figure 34.5a, are typically a blind tube or cavity with only one opening, the “mouth”, which also serves as an “anus”. Ingested material enters the mouth and passes through a hollow, tubular cavity. Cells within the cavity secrete digestive enzymes that break down the food. The food particles are engulfed by the cells lining the gastrovascular cavity.

The alimentary canal , shown in Figure 34.5b, is a more advanced system: it consists of one tube with a mouth at one end and an anus at the other. Earthworms are an example of an animal with an alimentary canal. Once the food is ingested through the mouth, it passes through the esophagus and is stored in an organ called the crop then it passes into the gizzard where it is churned and digested. From the gizzard, the food passes through the intestine, the nutrients are absorbed, and the waste is eliminated as feces, called castings, through the anus.

Vertebrate Digestive Systems

Vertebrates have evolved more complex digestive systems to adapt to their dietary needs. Some animals have a single stomach, while others have multi-chambered stomachs. Birds have developed a digestive system adapted to eating unmasticated food.

Monogastric: Single-chambered Stomach

As the word monogastric suggests, this type of digestive system consists of one (“mono”) stomach chamber (“gastric”). Humans and many animals have a monogastric digestive system as illustrated in Figure 34.6ab. The process of digestion begins with the mouth and the intake of food. The teeth play an important role in masticating (chewing) or physically breaking down food into smaller particles. The enzymes present in saliva also begin to chemically break down food. The esophagus is a long tube that connects the mouth to the stomach. Using peristalsis, or wave-like smooth muscle contractions, the muscles of the esophagus push the food towards the stomach. In order to speed up the actions of enzymes in the stomach, the stomach is an extremely acidic environment, with a pH between 1.5 and 2.5. The gastric juices, which include enzymes in the stomach, act on the food particles and continue the process of digestion. Further breakdown of food takes place in the small intestine where enzymes produced by the liver, the small intestine, and the pancreas continue the process of digestion. The nutrients are absorbed into the blood stream across the epithelial cells lining the walls of the small intestines. The waste material travels on to the large intestine where water is absorbed and the drier waste material is compacted into feces it is stored until it is excreted through the rectum.

Avian

Birds face special challenges when it comes to obtaining nutrition from food. They do not have teeth and so their digestive system, shown in Figure 34.7, must be able to process un-masticated food. Birds have evolved a variety of beak types that reflect the vast variety in their diet, ranging from seeds and insects to fruits and nuts. Because most birds fly, their metabolic rates are high in order to efficiently process food and keep their body weight low. The stomach of birds has two chambers: the proventriculus , where gastric juices are produced to digest the food before it enters the stomach, and the gizzard , where the food is stored, soaked, and mechanically ground. The undigested material forms food pellets that are sometimes regurgitated. Most of the chemical digestion and absorption happens in the intestine and the waste is excreted through the cloaca.

Evolution Connection

Avian Adaptations

Birds have a highly efficient, simplified digestive system. Recent fossil evidence has shown that the evolutionary divergence of birds from other land animals was characterized by streamlining and simplifying the digestive system. Unlike many other animals, birds do not have teeth to chew their food. In place of lips, they have sharp pointy beaks. The horny beak, lack of jaws, and the smaller tongue of the birds can be traced back to their dinosaur ancestors. The emergence of these changes seems to coincide with the inclusion of seeds in the bird diet. Seed-eating birds have beaks that are shaped for grabbing seeds and the two-compartment stomach allows for delegation of tasks. Since birds need to remain light in order to fly, their metabolic rates are very high, which means they digest their food very quickly and need to eat often. Contrast this with the ruminants, where the digestion of plant matter takes a very long time.

Ruminants

Ruminants are mainly herbivores like cows, sheep, and goats, whose entire diet consists of eating large amounts of roughage or fiber. They have evolved digestive systems that help them digest vast amounts of cellulose. An interesting feature of the ruminants’ mouth is that they do not have upper incisor teeth. They use their lower teeth, tongue and lips to tear and chew their food. From the mouth, the food travels to the esophagus and on to the stomach.

To help digest the large amount of plant material, the stomach of the ruminants is a multi-chambered organ, as illustrated in Figure 34.8. The four compartments of the stomach are called the rumen, reticulum, omasum, and abomasum. These chambers contain many microbes that break down cellulose and ferment ingested food. The abomasum is the “true” stomach and is the equivalent of the monogastric stomach chamber where gastric juices are secreted. The four-compartment gastric chamber provides larger space and the microbial support necessary to digest plant material in ruminants. The fermentation process produces large amounts of gas in the stomach chamber, which must be eliminated. As in other animals, the small intestine plays an important role in nutrient absorption, and the large intestine helps in the elimination of waste.

Pseudo-ruminants

Some animals, such as camels and alpacas, are pseudo-ruminants. They eat a lot of plant material and roughage. Digesting plant material is not easy because plant cell walls contain the polymeric sugar molecule cellulose. The digestive enzymes of these animals cannot break down cellulose, but microorganisms present in the digestive system can. Therefore, the digestive system must be able to handle large amounts of roughage and break down the cellulose. Pseudo-ruminants have a three-chamber stomach in the digestive system. However, their cecum—a pouched organ at the beginning of the large intestine containing many microorganisms that are necessary for the digestion of plant materials—is large and is the site where the roughage is fermented and digested. These animals do not have a rumen but have an omasum, abomasum, and reticulum.

Parts of the Digestive System

The vertebrate digestive system is designed to facilitate the transformation of food matter into the nutrient components that sustain organisms.

Oral Cavity

The oral cavity, or mouth, is the point of entry of food into the digestive system, illustrated in Figure 34.9. The food consumed is broken into smaller particles by mastication, the chewing action of the teeth. All mammals have teeth and can chew their food.

The extensive chemical process of digestion begins in the mouth. As food is being chewed, saliva, produced by the salivary glands, mixes with the food. Saliva is a watery substance produced in the mouths of many animals. There are three major glands that secrete saliva—the parotid, the submandibular, and the sublingual. Saliva contains mucus that moistens food and buffers the pH of the food. Saliva also contains immunoglobulins and lysozymes, which have antibacterial action to reduce tooth decay by inhibiting growth of some bacteria. Saliva also contains an enzyme called salivary amylase that begins the process of converting starches in the food into a disaccharide called maltose. Another enzyme called lipase is produced by the cells in the tongue. Lipases are a class of enzymes that can break down triglycerides. The lingual lipase begins the breakdown of fat components in the food. The chewing and wetting action provided by the teeth and saliva prepare the food into a mass called the bolus for swallowing. The tongue helps in swallowing—moving the bolus from the mouth into the pharynx. The pharynx opens to two passageways: the trachea, which leads to the lungs, and the esophagus, which leads to the stomach. The trachea has an opening called the glottis, which is covered by a cartilaginous flap called the epiglottis. When swallowing, the epiglottis closes the glottis and food passes into the esophagus and not the trachea. This arrangement allows food to be kept out of the trachea.

Esophagus

The esophagus is a tubular organ that connects the mouth to the stomach. The chewed and softened food passes through the esophagus after being swallowed. The smooth muscles of the esophagus undergo a series of wave like movements called peristalsis that push the food toward the stomach, as illustrated in Figure 34.10. The peristalsis wave is unidirectional—it moves food from the mouth to the stomach, and reverse movement is not possible. The peristaltic movement of the esophagus is an involuntary reflex it takes place in response to the act of swallowing.

A ring-like muscle called a sphincter forms valves in the digestive system. The gastro-esophageal sphincter is located at the stomach end of the esophagus. In response to swallowing and the pressure exerted by the bolus of food, this sphincter opens, and the bolus enters the stomach. When there is no swallowing action, this sphincter is shut and prevents the contents of the stomach from traveling up the esophagus. Many animals have a true sphincter however, in humans, there is no true sphincter, but the esophagus remains closed when there is no swallowing action. Acid reflux or “heartburn” occurs when the acidic digestive juices escape into the esophagus.

Stomach

A large part of digestion occurs in the stomach, shown in Figure 34.11. The stomach is a saclike organ that secretes gastric digestive juices. The pH in the stomach is between 1.5 and 2.5. This highly acidic environment is required for the chemical breakdown of food and the extraction of nutrients. When empty, the stomach is a rather small organ however, it can expand to up to 20 times its resting size when filled with food. This characteristic is particularly useful for animals that need to eat when food is available.

Visual Connection

Which of the following statements about the digestive system is false?

  1. Chyme is a mixture of food and digestive juices that is produced in the stomach.
  2. Food enters the large intestine before the small intestine.
  3. In the small intestine, chyme mixes with bile, which emulsifies fats.
  4. The stomach is separated from the small intestine by the pyloric sphincter.

The stomach is also the major site for protein digestion in animals other than ruminants. Protein digestion is mediated by an enzyme called pepsin in the stomach chamber. Pepsin is secreted by the chief cells in the stomach in an inactive form called pepsinogen . Pepsin breaks peptide bonds and cleaves proteins into smaller polypeptides it also helps activate more pepsinogen, starting a positive feedback mechanism that generates more pepsin. Another cell type—parietal cells—secrete hydrogen and chloride ions, which combine in the lumen to form hydrochloric acid, the primary acidic component of the stomach juices. Hydrochloric acid helps to convert the inactive pepsinogen to pepsin. The highly acidic environment also kills many microorganisms in the food and, combined with the action of the enzyme pepsin, results in the hydrolysis of protein in the food. Chemical digestion is facilitated by the churning action of the stomach. Contraction and relaxation of smooth muscles mixes the stomach contents about every 20 minutes. The partially digested food and gastric juice mixture is called chyme . Chyme passes from the stomach to the small intestine. Further protein digestion takes place in the small intestine. Gastric emptying occurs within two to six hours after a meal. Only a small amount of chyme is released into the small intestine at a time. The movement of chyme from the stomach into the small intestine is regulated by the pyloric sphincter.

When digesting protein and some fats, the stomach lining must be protected from getting digested by pepsin. There are two points to consider when describing how the stomach lining is protected. First, as previously mentioned, the enzyme pepsin is synthesized in the inactive form. This protects the chief cells, because pepsinogen does not have the same enzyme functionality of pepsin. Second, the stomach has a thick mucus lining that protects the underlying tissue from the action of the digestive juices. When this mucus lining is ruptured, ulcers can form in the stomach. Ulcers are open wounds in or on an organ caused by bacteria (Helicobacter pylori) when the mucus lining is ruptured and fails to reform.

Small Intestine

Chyme moves from the stomach to the small intestine. The small intestine is the organ where the digestion of protein, fats, and carbohydrates is completed. The small intestine is a long tube-like organ with a highly folded surface containing finger-like projections called the villi . The apical surface of each villus has many microscopic projections called microvilli. These structures, illustrated in Figure 34.12, are lined with epithelial cells on the luminal side and allow for the nutrients to be absorbed from the digested food and absorbed into the blood stream on the other side. The villi and microvilli, with their many folds, increase the surface area of the intestine and increase absorption efficiency of the nutrients. Absorbed nutrients in the blood are carried into the hepatic portal vein, which leads to the liver. There, the liver regulates the distribution of nutrients to the rest of the body and removes toxic substances, including drugs, alcohol, and some pathogens.

Visual Connection

Which of the following statements about the small intestine is false?

  1. Absorptive cells that line the small intestine have microvilli, small projections that increase surface area and aid in the absorption of food.
  2. The inside of the small intestine has many folds, called villi.
  3. Microvilli are lined with blood vessels as well as lymphatic vessels.
  4. The inside of the small intestine is called the lumen.

The human small intestine is over 6m long and is divided into three parts: the duodenum, the jejunum, and the ileum. The “C-shaped,” fixed part of the small intestine is called the duodenum and is shown in Figure 34.11. The duodenum is separated from the stomach by the pyloric sphincter which opens to allow chyme to move from the stomach to the duodenum. In the duodenum, chyme is mixed with pancreatic juices in an alkaline solution rich in bicarbonate that neutralizes the acidity of chyme and acts as a buffer. Pancreatic juices also contain several digestive enzymes. Digestive juices from the pancreas, liver, and gallbladder, as well as from gland cells of the intestinal wall itself, enter the duodenum. Bile is produced in the liver and stored and concentrated in the gallbladder. Bile contains bile salts which emulsify lipids while the pancreas produces enzymes that catabolize starches, disaccharides, proteins, and fats. These digestive juices break down the food particles in the chyme into glucose, triglycerides, and amino acids. Some chemical digestion of food takes place in the duodenum. Absorption of fatty acids also takes place in the duodenum.

The second part of the small intestine is called the jejunum , shown in Figure 34.11. Here, hydrolysis of nutrients is continued while most of the carbohydrates and amino acids are absorbed through the intestinal lining. The bulk of chemical digestion and nutrient absorption occurs in the jejunum.

The ileum , also illustrated in Figure 34.11 is the last part of the small intestine and here the bile salts and vitamins are absorbed into blood stream. The undigested food is sent to the colon from the ileum via peristaltic movements of the muscle. The ileum ends and the large intestine begins at the ileocecal valve. The vermiform, “worm-like,” appendix is located at the ileocecal valve. The appendix of humans secretes no enzymes and has an insignificant role in immunity.

Large Intestine

The large intestine , illustrated in Figure 34.13, reabsorbs the water from the undigested food material and processes the waste material. The human large intestine is much smaller in length compared to the small intestine but larger in diameter. It has three parts: the cecum, the colon, and the rectum. The cecum joins the ileum to the colon and is the receiving pouch for the waste matter. The colon is home to many bacteria or “intestinal flora” that aid in the digestive processes. The colon can be divided into four regions, the ascending colon, the transverse colon, the descending colon and the sigmoid colon. The main functions of the colon are to extract the water and mineral salts from undigested food, and to store waste material. Carnivorous mammals have a shorter large intestine compared to herbivorous mammals due to their diet.

Rectum and Anus

The rectum is the terminal end of the large intestine, as shown in Figure 34.13. The primary role of the rectum is to store the feces until defecation. The feces are propelled using peristaltic movements during elimination. The anus is an opening at the far-end of the digestive tract and is the exit point for the waste material. Two sphincters between the rectum and anus control elimination: the inner sphincter is involuntary and the outer sphincter is voluntary.

Accessory Organs

The organs discussed above are the organs of the digestive tract through which food passes. Accessory organs are organs that add secretions (enzymes) that catabolize food into nutrients. Accessory organs include salivary glands, the liver, the pancreas, and the gallbladder. The liver, pancreas, and gallbladder are regulated by hormones in response to the food consumed.

The liver is the largest internal organ in humans and it plays a very important role in digestion of fats and detoxifying blood. The liver produces bile, a digestive juice that is required for the breakdown of fatty components of the food in the duodenum. The liver also processes the vitamins and fats and synthesizes many plasma proteins.

The pancreas is another important gland that secretes digestive juices. The chyme produced from the stomach is highly acidic in nature the pancreatic juices contain high levels of bicarbonate, an alkali that neutralizes the acidic chyme. Additionally, the pancreatic juices contain a large variety of enzymes that are required for the digestion of protein and carbohydrates.

The gallbladder is a small organ that aids the liver by storing bile and concentrating bile salts. When chyme containing fatty acids enters the duodenum, the bile is secreted from the gallbladder into the duodenum.


What is Herbivores Digestive System?

Herbivorous animals have a special type of digestive system since they only depend on plant matter. The energy requirements, the nutrients and other essential compounds necessary for the survival of herbivores are fulfilled by plants. Plant materials contain cellulose. Hence, a special type of digestive mechanism is needed since cellulose is only digested by the enzyme cellulase. The teeth of herbivorous animals are flat since they need to grind plant material in the buccal cavity to complete mechanical digestion. The typical digestive system of a herbivore is composed of a single stomach and a long intestine along with a large cecum.

Herbivores teeth are highly specific to eat plant matter. The molars of herbivores are usually flat and wide which assist them to break and grind plants that they ingest. The herbivore incisors are not present on both upper and lower jaws, but they are sharp to tear the plant material. Many herbivores like goat, cow, and horse possess jaws which could be moved sideways. In their large pouch-like cecum, millions of bacteria reside that contains cellulase enzyme. This helps in the digestion of cellulose. This is the exact reason why herbivorous animals possess a longer intestine than carnivorous animals. Herbivores such as cow, goat, and sheep possess multiple stomachs. These are called ruminant species which they possess four stomachs. This allows these animals to swallow partially chewed plant matter mixed with saliva which is known as a bolus. The partially chewed plant matter first enters the first two stomachs that are, the rumen and the reticulum respectively. Here the plant matter is stored until taken for later use.

Figure 01: Parts of the Herbivores Digestive System

When the animal is at rest, it can cough up the partially chewed food back into the buccal cavity and chew it completely forming another bolus of food. This bolus enters the third and fourth stomach omasum and abomasum. In the omasum, the liquid part of the bolus which contains water and minerals are absorbed into the bloodstream. The abomasum is similar to the human stomach where chemical digestion of food takes place, and the digested nutrients are absorbed in the small intestine.


Evolution

Before we get into evolutionary arguments. Let’s just get some common misconceptions clear first. Humans did not evolve from the chimpanzee or any other now-living primate. Humans didn’t evolve from great apes to be something else we are great apes – or Hominidae. Humans share a common ancestor with chimpanzees, actually humans share common ancestors with every single organism ever existed if you go back long enough. Since chimpanzees and humans are closely related, humans share a much more recent ancestor with chimpanzees than for example a horse.

In the image above, the species A, B and C is different species. A and B is more closely related than A and C, and B and C. Where the arrow points is the most recent common ancestor between A and B. That doesn’t mean that A evolved from B, but that A and B evolved from an ancestral species that diverged into A and B. This species didn’t look neither like A nor B. At the very root of the tree lived a species that is the common ancestor between A, B and C. One important point though is that A, B and C are equally distant from the species at the root of the tree.

Lastly, and this might be the most important one: Evolution is not teleological. Evolution does not have a purpose, aim or goal. There is no such thing as more evolved or de-evolution. Humans are not more evolved than chimpanzees we just diverged in different direction. The quantitative unit of evolution is time, and as far as I know humans has not evolved longer than chimpanzees. Evolution is the change of living organisms over time that depend on many factors, but a lot less chance than some people think, and no planning ahead what so ever. No are created to be food.

Human evolution = loss of sanity

What I mean with that subtitle is that, when people reflect over human evolution to construct an argument, often they lose the ability to view the human species objectively and transform humans into something completely separated from the rest of the vast number of species on earth. Humans are unique, but so are every single species on earth. Some people even claim that human evolution has stopped, and that of course is utterly ridiculous.

I’d like to present something I’d like to call the ‘Alien David Attenborough thought experiment‘: Imagine that you are an alien biologist travelling from a distant planet to study life on earth (I like to use David Attenborough’s voice to narrate this, that’s all). You study all the different species, describing behavior, diet and appearance. When you start describing humans, what diet would you assign humans to have? What behaviors would you ascribe humans? If I would do it, I would certainly not say: “Homo sapiens diet has for thousands of years included meat in some populations, and less in some, but really, they are made for fruit” neither would I say: “Humans live in artificial buildings and wear fabric clothing, this is however a very unnatural state for the Homo sapiens species”. I think that this could be a nice strategy to get away from an anthropocentric prison of mind.

The ancestors of Homo sapiens cooked their food, cooking has been around for approximately a million year (that is around 500 000 years longer than the human species has existed) (Berna et al., 2012 Organ, Nunn, Machanda, & Wrangham, 2011). Traces of humans eating meat is also ancient and seems to have been around for as long as our species existed (Pobiner, 2013). One of our closest relatives the chimpanzee also eats an omnivorus diet with mainly fruits, but occasionally eats animals (McGrew, 1983).

Reflecting to my previous discussion, saying that meat-eating is unnatural because we need to cook it (which we don’t) is a flawed argument. Likewise is the claim that we need to be able to hunt down grazing prey with our bare hands,kill and eat it raw a flawed argument. Due to our highly developed brain, we don’t need that, we find other ways. That trait is no stranger than a lions teeth.

The whole idea of finding an ancient diet that we are “made” for, is just absurd, we are not exactly the same as pre-historic humans. The changes in our environment have led to several adaptations regarding diet. For example, mammals give their young mother’s milk (that is the very definition of mammal). This stops at a certain age and the offspring is able to eat as their parents. Milk contains lactose and mammals have an enzyme called lactase to digest lactose. When the child stops receiving milk, the expression of this enzyme is turned off. However, in some human populations this enzyme remains active through adulthood, which is referred to as lactase persistence. This is thought to be an adaptation to the habit of drinking milk from domesticated animals (Tishkoff et al., 2007).

A different relatively recent human adaptation is a duplication of the gene AMY1 that encodes an enzyme called amylase that digests starch. Duplication of genes typically result in an increased production of the enzyme, thus this is hypothesized as being an adaptation to the use of agriculture which would increase the amount of starch in the diet (Perry et al., 2007). For these adaptations we are talking about, we are in a time frame of

Here I really like to emphasize that the naturalistic fallacy of equating a ‘is’ with a ‘should’, is something we really should avoid. The fact that humans have eaten meat and drinking milk is no argument that we should, unless we had to (we don’t).


Matches of GI System Biochemistry (Enzymes and Transporters) to Changes in Diet Composition

General principles

There is overwhelming evidence that the digestive and absorptive function of the GI tract of animals can vary with diet composition. This flexibility is exhibited at two levels: anatomical, including the overall size and architecture of the GI tract (Section “Models help in understanding the diversity of digestive systems and guide mechanistic, integrative research”) and biochemical, especially the activity of digestive enzymes and transporters. The biochemical flexibility is generally considered to maximize the acquisition of carbon for energy production and essential nutrients for maintenance and growth, while protecting against the acquisition of excessive, potentially toxic, amounts of certain dietary constituents (e.g., iron). Any nutritional imbalance that might arise from this strategy is widely considered to be corrected postabsorption, so that the retention and use of certain nutrients are optimized, while surplus metabolites can be eliminated (249, 416). In this section, the relationship between diet composition and digestive enzyme activity is addressed first, followed by consideration of transporters in the GI tract.

Flexible adjustment of digestive enzymes to diet change

Many studies on vertebrates have demonstrated that the production of digestive enzymes increases with availability of substrate in the gut lumen. For example, this effect has been confirmed in rodents for all of the major pancreatic enzymes (amylase, lipase, and proteases) and enzymes of the intestinal brush border (sucrase-isomaltase, maltase-glucoamylase, and aminopeptidase-N) (246). Other data relate to a variety of mammals, birds, reptiles and fish, as well as a number of invertebrates [reviewed in reference (249)]. This mode of regulation both maximizes the digestibility of substrates and minimizes the cost of synthesizing excess enzyme when the substrate is at low levels. The mechanistic basis of the impact of diet on digestive enzyme activity has not been investigated in most species but, where studied, there is persuasive evidence that differential enzyme activity is underpinned by changes in gene expression. For example, the elevated expression of intestinal sucrase-isomaltase gene in the intestine of rats and mice fed on high-carbohydrate diets is controlled by the transcription factors Cdx-2 and HNF-1 (36) and the recruitment of these transcription factors to the promoter region is correlated with the acetylation of histones H3 and H4 associated with this gene (215). In Drosophila, the activity of amylase in the midgut is significantly higher in larvae feeding on starch diets than sugar diets, and the 5’ cis-regulatory region that regulates gene expression of the amylase genes has been identified (226).

Adaptive variation in digestive enzyme activity with diet composition is crucial to the lifestyle of many animals. For example, female Aedes aegypti mosquitoes feed on both sugar-rich nectar and protein-rich vertebrate blood. The gut protease activity is undetectable in individuals feeding on a sugar meal but, within hours of taking a bloodmeal, the digestive protease activity in the midgut increases rapidly, reaching a maximum after about 2 days. A. aegypti has three trypsin genes expressed in the midgut. The synthesis of two trypsins, known as the late trypsins, is regulated by dietary protein content. Initial production (within 3 h of feeding) is from a preformed mRNA, in response to protein in the blood and subsequent production (8� h after feeding) comes from de novo trypsin gene expression, induced by amino acid products of trypsin-mediated digestion of blood proteins (146). The other midgut trypsin, called early trypsin, is synthesized constitutively.

Nevertheless, some studies have found that the secretion of digestive enzymes does not vary in a simple fashion with substrate concentration. For some insects feeding on a nutritionally unbalanced diet, such that one dietary component is in excess, the enzymes mediating the degradation of that dietary component can be downregulated. For example, locusts Locusta migratoria feeding on diets with excess protein or carbohydrate display reduced activity of digestive α-chymotrypsin and α-amylase, respectively (93) ( Fig. 14 ). These data suggest that an insect has the capacity to regulate digestive enzymes homeostatically, such that enzymes yielding nutrients in excess are secreted at lower rates than enzymes that generate nutrients in deficit. The production of some digestive enzymes appears to be regulated by integrated sensing of both the nutrients available in the gut and the nutritional requirements of the animal. This complexity may not be revealed in the nutritionally sufficient diets that are commonly used for laboratory maintenance of animals, but could be important for animals in the field with access diets of variable and often suboptimal composition.

The activity of α-chymotrypsin and α-amylase in the gastrointestinal tract of the locust L. migratoria fed on diets of different composition: PC (21% protein:21% carbohydrate), pc (10.5% protein: 10.5% carbohydrate), Pc (35% protein: 7% carbohydrate), and pC (7% protein: 35% carbohydrate). The enzyme activities were downregulated in insects on diets containing an excess of the substrate. [Data from Fig. 1 C and D of Clissold et al. (93).]

Flexible adjustments of transporters to diet change

Current understanding of the matching of transporter function to diet composition derives largely from the classic work of Diamond and colleagues (120, 149) conducted on isolated intestine preparations of mice. The transport of nutrients that are metabolized for energy production increase with increasing dietary supply, while those mediating the uptake of essential but non energy-yielding nutrients tend to decrease with increasing dietary supply. Thus, transporter activity for sugars (e.g., glucose and fructose) and nonessential and essential amino acids and peptides increase with their content in the diet, but transport of most vitamins and minerals decrease with dietary content. Interestingly, the uptake of dietary essential amino acids, such as histidine, lysine, leucine, and methionine, tends to increase slightly at low dietary levels (the reverse of the response to nonessential amino acids), indicating the central role of dietary essential amino acids for protein synthesis and use of nonessential amino acids as a respiratory substrate. How this differential response to essential versus nonessential amino acids is achieved despite the overlapping substrate specificities of the various amino acid transporters ( Table 3 ) is not fully understood.

Kinetic analyses of nutrient uptake indicate that the diet-dependent variation in sugar and amino acids transporter activity is mediated predominantly by changes in the density of transporters on the apical membrane (149). Two processes can mediate increased transporter function: recruitment of preexisting transporter protein in the cytoplasm to the membrane (as occurs for GLUT2 in response to dietary glucose, see Section �sorption of carbohydrates”), and elevated gene expression. Most research has focused on expression response to dietary nutrients. For example, in response to high dietary supply of sugars, the expression of genes encoding the transporters SGLT1 (for glucose) and GLUT5 (for fructose) is increased. GLUT5 expression is elevated in isolated rat intestine preparations perfused with fructose (425) horses fed on diets with high levels of digestible carbohydrate display elevated expression of SGLT1 in both the duodenum and ileum (133) and piglets raised on isoenergetic diets with different concentrations of digestible carbohydrate exhibit elevated expression of SGLT1 when fed on diets with more than 50% digestible carbohydrate (330) ( Fig. 15 ).

The effect of dietary soluble carbohydrate on the transcript abundance of the glucose transporter gene SGLT1 in (A) the mid-intestine of 28-day-old piglets and (B) the duodenum of horses fed sequentially on different diets including hay (essentially starch-free) and grain (containing 0.3% starch). [Data from Fig. 1A of reference (330) and Fig. 3A of reference (133).]

The activity of the Pept-1 peptide transporter in the intestine is elevated by high dietary protein. In the rat intestine, the Pept-1 mRNA is elevated twofold in the intestine of rats fed on high-protein diet (50% protein), relative to low-protein diet (4%), and this effect of high dietary protein can be replicated by a dietary supplement of a single dipeptide Gly-Phe (142, 400).

The expression of various transporter genes is regulated in anticipation of food. Adult rats exhibit diurnal variation in expression of sugar transporters in the intestine, with induction of GLUT2 (glucose transporter), GLUT5 (fructose transporter), and Pept-1 expression 3 to 4 h before the onset of peak feeding by the animal (100, 371, 402). Diurnal variation of GLUT2 and Pept-1 is regulated by the vagus nerve, and GLUT5 by paracrine and endocrine signals in the intestine (371, 427). In addition, preexisting pools of transporter proteins, probably localized in the cytosol, are likely localized to the membrane this can achieve more rapid changes in transporter activity than changes in gene expression.

The central role of transporters in the modulation of absorption with diet raises important questions about the capacity of an animal to regulate uptake of nutrients with significant levels of passive absorption. For these nutrients, uptake is predicted to increase monotonically with concentration in the gut lumen. The uptake of the vitamins pantothenic acid, ascorbic acid, and choline conforms to this expectation. Absorption of these vitamins is predominantly passive and, unlike other essential nutrients, it is not upregulated in response to low dietary supply (418). Nutrients that are taken up by the paracellular route are also predicted not to be tightly regulated. This effect is important, for example, for the uptake of various solutes by passerine birds, for which paracellular absorption is significant (Section “Paracellular transport of organic molecules”).


Biology Digestive Systems Essay Example

Even thought its not part of the digestive system teeth are very resourceful for aiding digestion, in all Herbivores they have a Horny pad which helps initially crush the food and a meeseter that is larger than that of a carnivore so it can chew for longer, they have cainines at the front and molars and pre molars that grind at the back, so that the foodis crushed and soft making the break down and digestion of food easier. Example: Cow chews its food for a long time to crush it andmake it easier to digest, then the food moves down the oesophagus to the Rumen (the first partof the stomach where the fermentation occurs, then the next stage it moves to the honeycomb like chamber called the Reticulum (which prepares food for regurtitation), then it regurgitates it back up and starts again, once its been through this stage enough the third part is the omasum, then finally moves onto the abomasum (rennet stomach).

Carnivore: Carnivores have very different digestive systems because of the different diet, carnivores eat predominantly meat, so therefore there is more protein and less cellulose, so the digestion will occur in the stomach and further more finish in the duodenum, (part of the small intestine just under the stomach) so the insides need to have different purposes for the digestion,absorbtion of nutrients occurs largelyin small intestine. They have an average sized stomache in relation to their size, alot of the digestion occurs in the stomach, they have a very short colon and a short and wide small intestine, and their caecum and appendix is poorly developed and for all intensive purposes are not aknowledged.

To aid digestion prior to entering the stomach (even though its not part of the digestive system) their teeth, they have long sharp canines for trapping and locking prey in their mouth, and for stabbing, then theres the premolars for cutting and molars for grinding and the carnissals to cut and break. Example: A Tasmanian devil will eat its prey which is swallowed and follows down the oesophagus to the stomach then follows the small intestine to the caecum, then lastly passes through the large intestine,to the anus. Nectar feeding animal: Nectar feeders eat the nectar from plants so therefore they dont have use for teeth, they have a long papillae (hair like tounge) to pick up the nectar.

They have a very small stomach because of the nectar only diet, (Nectar being mostly made up of sugar) they dont need to do alot of digestion so stomach isnt put to alot of use, and their intestines arent very long mainly used for degestion of pollen. Example: Honey Possum has a much simpiler digestive system,it only consists of the stomach, intestine then the anus. Vertebrate Herbivore is an animal with a backbone that eats plant matter, it would have a digestive system that consists of a long small intestine, a well developed large caecum, and a long colon, with two types of digestion, depending on the different herbivores hindgut and foregut digestion.

The long small intestine is so, because it contains a lot of cellulose and fiber makes up their diet these components are difficult to digest, so therefore herbivores need longer digestive systems to cope with the nutrients. The well developed large caecum is so that the cellulose from the plants can be stored and fermented. The long colon in herbivorous animals, tends to be a highly advanced organ involved in water and electrolyte absorption, vitamin absorbtion and production, and fermentation of fibrous plant materials. The colons of herbivores are usually wider than their small intestine. Vertebrate Carnivore is an animal with a backbone that eats mostly meat, its digestive system would be made up of large stomach, a small non exsistant caecum and appendix and the small intestine, and large intestine. A carnivore’s saliva does not contain digestive enzymes.

Carnivores have a simple (single-chambered) stomach as opposed to Herbivores which can have 4 chambers for example cows. Since most carnivores average a kill only about once a week, a large stomach volume is a plus because it allows them to fill up when eating, taking in as much meat as possible at once which can then be put off to be digested at a later time. A carnivore’s stomach also secretes powerful digestive enzymes with about 10 times the amount of hydrochloric acid than a human or herbivore. A carnivore’s small intestine is a tool designed for rapid elimination of food that rots quickly. A carnivore’s large intestine is relatively short.

This passage is also relatively smooth and runs fairly straight so that fatty wastes high in cholesterol can be easily eliminated out before they start to putrefy. The large stomach is useful and big because their diet is mostly protein so most of the digestion will occur in the stomach, and it needs to be large enough. The small non existent caecum and appendix are because those are only used in Herbivores because the meat (protein) in the carnivores diet doesnt need to be fermented like the herbivores they dont require use of the caecum. The large intestine (colon) of carnivores is simple and very short, as its only purposes are to absorb salt and water.


Examples of omnivores

Omnivores such as the (a) bear and (b) crayfish eat both plant- and animal-based food. While their food options are greater than those of herbivores or carnivores, they are still limited by what they can find to eat, or what they can catch.

Carnivores are animals that eat other animals. The word carnivore is derived from Latin and means "meat eater." Wild cats, such as lions and tigers, are examples of vertebrate carnivores, as are snakes and sharks, while invertebrate carnivores include sea stars, spiders, and ladybugs . Obligate carnivores are those that rely entirely on animal flesh to obtain their nutrients examples of obligate carnivores are members of the cat family. Facultative carnivores are those that also eat non-animal food in addition to animal food. Note that there is no clear line that differentiates facultative carnivores from omnivores dogs would be considered facultative carnivores.


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