The Digestive System
The digestive system exists within all organisms to allow nutritive particles to be taken in and broken down in a process termed catabolism to yield components that can be used as nutrients. Those components that are not utilizable are discarded from the body; excretion is considered part of the digestive system. Digestive systems range from the very simple systems of single-celled bacteria, to the complex, multi-organ systems used by vertebrates. Single-celled organisms such as amoeba engulf food particles with their outer membrane. The food particles are brought inside the cell; hence this type of digestion is referred to as intracellular. The food particles are contained within spherical structures composed of lipids (liposomes) that house potent chemicals. These structures are also called digestive vacuoles. Within a vacuole the particles are broken down to usable components. Waste products can be packaged in liposomes, which fuse with the cell membrane, liberating the waste to the exterior. More complex organisms may have a mouth which leads to a large, open body cavity. Sponges, for example, carry water into this cavity, where any tiny food particles are extracted and distributed throughout the cells of the sponge for digestion. The water and any waste is then sent back out through the mouth (World of Biology 2010).
Flatworms have a blind gut. This is a tube within which digestion takes place. It is equipped with a mouth opening, but is otherwise totally sealed off. Food enters the mouth and is partially digested by chemicals released into the gut. Because this type of digestion occurs within the cavity of the gut, and not within a cell, it is referred to as extracellular digestion. Once the food has been broken down through this extracellular digestion, the smaller bits can be absorbed by the cells lining the gut. Waste products are passed back out through the mouth (2010). As organisms become more complex, their digestive systems become correspondingly complex. Higher up the evolutionary ladder, the blind sac eventually leads to a separate opening for the passage of waste products, the primitive anus. This is evident in various worms called nematodes. In higher animals, outpouchings from the digestive tube become specialized organs such as the liver, pancreas, and gallbladder.
The basic schemata for vertebrate alimentary canals or digestive tracts involves structures which are responsible for receiving food, conducting and storing food, breaking food down and absorbing its useful nutrients, and absorbing water and eliminating wastes (defecation). The mouth is the opening responsible for receiving food. Within the mouth, food is often broken down into smaller bits by the mechanical actions of chewing with teeth and by the chemical action of substances found within saliva. Saliva helps to lubricate food, allowing it to pass into the esophagus. The esophagus conducts the food to its next destination. In some animals (certain birds and reptiles), an outpouching from the esophagus called the crop is used for storage of food. It may also contain small rocks, which help to grind tough bits of food, such as seeds (2010). The stomach is a muscular pouch which works to mix the food, usually with a highly acidic combination of chemicals which will further breakdown the food. The stomach sends food into the small intestine, where a variety of alkaline chemicals produced by the liver and pancreas continue the process of digestion. Enzymes and bacteria aid in the digestive process. As food becomes broken down into its component nutrients, these nutrients are absorbed into the bloodstream.
The components of food which are not absorbed continue passing down the large intestine, which works to absorb water from this mass. Ultimately, the mass of waste (feces), appropriately dehydrated by the absorption of water from it, arrives at the rectum, where feces are stored. When a sufficient quantity has accumulated, the feces are expelled through the anus.
Flatworms have a blind gut. This is a tube within which digestion takes place. It is equipped with a mouth opening, but is otherwise totally sealed off. Food enters the mouth and is partially digested by chemicals released into the gut. Because this type of digestion occurs within the cavity of the gut, and not within a cell, it is referred to as extracellular digestion. Once the food has been broken down through this extracellular digestion, the smaller bits can be absorbed by the cells lining the gut. Waste products are passed back out through the mouth (2010). As organisms become more complex, their digestive systems become correspondingly complex. Higher up the evolutionary ladder, the blind sac eventually leads to a separate opening for the passage of waste products, the primitive anus. This is evident in various worms called nematodes. In higher animals, outpouchings from the digestive tube become specialized organs such as the liver, pancreas, and gallbladder.
The basic schemata for vertebrate alimentary canals or digestive tracts involves structures which are responsible for receiving food, conducting and storing food, breaking food down and absorbing its useful nutrients, and absorbing water and eliminating wastes (defecation). The mouth is the opening responsible for receiving food. Within the mouth, food is often broken down into smaller bits by the mechanical actions of chewing with teeth and by the chemical action of substances found within saliva. Saliva helps to lubricate food, allowing it to pass into the esophagus. The esophagus conducts the food to its next destination. In some animals (certain birds and reptiles), an outpouching from the esophagus called the crop is used for storage of food. It may also contain small rocks, which help to grind tough bits of food, such as seeds (2010). The stomach is a muscular pouch which works to mix the food, usually with a highly acidic combination of chemicals which will further breakdown the food. The stomach sends food into the small intestine, where a variety of alkaline chemicals produced by the liver and pancreas continue the process of digestion. Enzymes and bacteria aid in the digestive process. As food becomes broken down into its component nutrients, these nutrients are absorbed into the bloodstream.
The components of food which are not absorbed continue passing down the large intestine, which works to absorb water from this mass. Ultimately, the mass of waste (feces), appropriately dehydrated by the absorption of water from it, arrives at the rectum, where feces are stored. When a sufficient quantity has accumulated, the feces are expelled through the anus.
The Digestive Process
5 basic activities involved with digestion:
- 1. Indigestion - the ability to take food in the mouth
- 2. Mixture and Movement of Food - involves muscular contractions that move food along the digestive tract (peristalsis)
- 3. Digestion - the break down of food mechanically and chemically
- 4. Absorption - the passage of food into the circulatory system
- 5. Defecation - the elimination of unusable materials
This image here is a picture depicting the digestive process with key details (The Gale Encyclopedia of Science 2010).
One-Way Digestive Tract
Cnidarians and flatworms have a gastrovascular cavity. Cnidarians, such as hydras, use their tentacles to move food through their mouth into their gastrovascular cavity. Then the cells lining this cavity excrete digestiveenzymes that start extracellular digestion and break the prey into smaller pieces. Any undigested remnants of the prey are expelled through the mouth opening. Like cnidarians, flatworms have a gastrovascular cavity with a single opening, but the cavity itself is highly folded. These folds greatly increase the surface area and extend throughout the body, bringing nutrients within the reach of all cells.
The gastrovascular cavity of cnidarians and flatworms allows them to digest larger prey than they could with intracellular digestion. However, the effectiveness of a gastrovascular cavity in supplying the animal with nutrients is limited. Because there is only one opening to the external environment through which prey is taken in and remnants are expelled, the animals have to complete digestion of the first prey and expel its remnants before taking in another prey. With the evolution of a second opening in the digestive system, the digestive system became a digestive tract, or alimentary canal, making it a one-way system between mouth and anus. Food could now be taken in and processed continuously, providing the animal with more nutrients. Most animals--including vertebrates, arthropods,mollusks, round worms, and earthworms--have this form of digestive tract (Animal Sciences 2009).
A one-way digestive tract is efficient because it allows the food to pass through a series of specialized regions. Such regions may be specialized for protein, fat, or carbohydrate digestion, making each step more efficient. Other regions may be used for food storage or for preparing the food for chemical digestion by physically grinding it into smaller pieces, which exposes more surface area to the action of digestive enzymes. These specialized regions eventually evolved into organs as parts of a complex digestive organ system. However, because nutrient dispersal, by the digestive system itself to all cells of the body, was no longer feasible with such a specialized digestive system (and animals became larger and bulkier), a separate cardiovascular system evolved to serve that function.
Simple animals, such as earthworms, suck soil into the mouth with the pharynx, pass it through the esophagus into the crop, where it is moistened and stored. From there it is moved into thegizzard, which contains small grains of sand that help grind down the food. The actual digestion andabsorption of nutrients takes place in the intestine, and anything that remains is excreted through the anus. Insects also move food from the mouth through the esophagus into a crop (all parts of the foregut) for food storage and moistening. From there it is moved to a midgut where digestion and nutrient absorption through specialized extensions, or ceca, takes place. The hindgut functions mainly to reclaim water and ions from the gut content that would otherwise be lost in the feces (2009).
The gastrovascular cavity of cnidarians and flatworms allows them to digest larger prey than they could with intracellular digestion. However, the effectiveness of a gastrovascular cavity in supplying the animal with nutrients is limited. Because there is only one opening to the external environment through which prey is taken in and remnants are expelled, the animals have to complete digestion of the first prey and expel its remnants before taking in another prey. With the evolution of a second opening in the digestive system, the digestive system became a digestive tract, or alimentary canal, making it a one-way system between mouth and anus. Food could now be taken in and processed continuously, providing the animal with more nutrients. Most animals--including vertebrates, arthropods,mollusks, round worms, and earthworms--have this form of digestive tract (Animal Sciences 2009).
A one-way digestive tract is efficient because it allows the food to pass through a series of specialized regions. Such regions may be specialized for protein, fat, or carbohydrate digestion, making each step more efficient. Other regions may be used for food storage or for preparing the food for chemical digestion by physically grinding it into smaller pieces, which exposes more surface area to the action of digestive enzymes. These specialized regions eventually evolved into organs as parts of a complex digestive organ system. However, because nutrient dispersal, by the digestive system itself to all cells of the body, was no longer feasible with such a specialized digestive system (and animals became larger and bulkier), a separate cardiovascular system evolved to serve that function.
Simple animals, such as earthworms, suck soil into the mouth with the pharynx, pass it through the esophagus into the crop, where it is moistened and stored. From there it is moved into thegizzard, which contains small grains of sand that help grind down the food. The actual digestion andabsorption of nutrients takes place in the intestine, and anything that remains is excreted through the anus. Insects also move food from the mouth through the esophagus into a crop (all parts of the foregut) for food storage and moistening. From there it is moved to a midgut where digestion and nutrient absorption through specialized extensions, or ceca, takes place. The hindgut functions mainly to reclaim water and ions from the gut content that would otherwise be lost in the feces (2009).
Diagram of the gastrointestinal tract (Anissimov 2013)
Process of Digestion
Ingestion:
Begins in the mouth (breaks up food, digests starch, kills germs, and moistens food):
Epiglottis
Groups of specialized secretory cells
Functions
Digestive enzymes
Function
Most chemical digestion takes place here
Function
Absorption occurs through villi and microvilli
Function
Begins in the mouth (breaks up food, digests starch, kills germs, and moistens food):
- Mechanical digestion - teeth and tongue break up food
- Chemical digestion - saliva (amylase) digests starch, mucin, buffers, and anti-bacterial chemicals
Epiglottis
- flap of cartilage
- closes the trachea
- involuntary muscle contractions to move food along
- Back of the throat
- Larynx - passage for air
- 15 cm long
Groups of specialized secretory cells
- Sublingual gland, parotid gland, and submandibular gland
Functions
- food storage ( stretch to fit about 2L of food)
- disinfect food (HCl kills bacteria)
- chemical digestion (pepsin breaks down proteins)
- Pouch structure located near the liver which concentrates and stores bile
- Bile emulsifies lipids (physically breaks apart fats)
- Bile is bitter, greenish-yellow alkaline fluid
- Stored in the gall bladder between meals and upon eating is discharged into the duodenum
Digestive enzymes
- digest proteins (trypsin, chymotrypsin)
- digest starch (amylase)
- neutralizes acid from stomach
- Insulin - diabetics
Function
- produces bile (stored in gall bladder until needed)
Most chemical digestion takes place here
Function
- chemical digestion (digestion and absorption)
- absorption through the lining (6 meters long)
- 3 sections:
- Duodenum: most digestion
- Jejunum: absorption of nutrients and water
- Ileum: absorption of nutrients and water
Absorption occurs through villi and microvilli
- finger-like projections
- increase surface area for absorption
Function
- Re-absorb water (about 9 L of water)
- Solid materials pass through (indigestible solids, fibers)
- Vitamins K and B are reabsorbed with the water
- Rectum - solid wastes exit the body
Villi located in the small intestines