Digestion is the breakdown of complex food molecules into smaller components that are used by the organism. This may occur outside of the organism as in bacteria and fungi, or the organism may have extracellular digestion. Most organisms take food into cells.
Digestion is a series of chemical reactions that use hydrolytic enzymes. The chemical digestion can be preceeded by mechanical fragmentation of food into smaller pieces. Breaking down the food mechanically increases the surface area of the food and makes the digestive enzymes more effective. Once the food is digested the organic molecules can cross the plasma and enter cells. The undigested materials are defecated.
A. Definitions
1.Holotrophs
organisms that ingest other organisms, dead or alive, whole or by the piece, or absorb organic molecules directly.
2. Herbivores
organisms that eat plants or algae
1. Carnivores
Organisms that ingest other animals
2. Omnivores
Organisms that ingest both plant/algae and animals
3. Suspension feeders
Organisms that sift small food particles from the water
4. Substrate feeders
Organisms that live in or on the food source
5. Fluid Feeders
Organisms that suck nutrient rich fluids of the living host
6. Bulk Feeders
Organisms that ingest relatively large pieces of food.
B.Comparative Digestion
The simplest compartment for digestion is a food vacuole. These can digest food without the enzymes mixing with the cytoplasm. Protists take food in through endocytosis and then perform the intracellular digestion.
Sponge: Food particles enter the choanocytes via phagocytosis. Digestion occurs in food vacuoles. The food vacuoles are transferred to amoebocytes which distribute the food to other cells.
Gastrovascular Cavities are digestive sas with a single opening. This cavity functions in both digestion and distribution of nutrients through the body. For example, hydra catches food using cnidocytes on the tentacles, ad then stuff food into a mouth. In the gastrovascular cavity specialized cells produce digestive enzymes that break the food down, flagellated cells spread the food particles in th cavity, and particles are taken into cells by phagocytosis. Planarians also have a gastrovascular cavity with one opening. An alimentary canal is a tube with two openings: mouth and anus. Parts of the tube are specialized, ie. Some parts digest the good and other parts absorb the food. In earthworms, food is taken in by the mouth and pharynx, passes to the esopogus, the crop, the gizzard, the intestine, on to the anus. This is similar to the foraging and digestive structures of fish, amphibians, reptiles and birds.
B.Foraging and Digestive Structures in Mammals
Teeth grow in sockets in the jaw. Usually mammals begin with temporary milk teeth that are replaced by the adult teeth. There are 4 types of teeth in mammals.
1. Incisor
Chisel shaped for cutting
2. Canines
Tearing food and defense
3. Premolars
Ginding food
4. Molars
Grinding food. In herbivores molars are large and flat.
Humans have teeth that are not specialized for any particular type of diet.
D. Human digestive system is a good representative of the Mammalian system
1.Oral cavity
the lips are essential in eating.
a. Close lips to swallow
b. Help food keep inside the mouth
2. Tongue
a. Moves food into position for chewing
b. Tells us when our food can be swallowed
c. Prevents us from swallowing
d. Contains chemoreceptors which can distinguish 4 different types of taste: salt, sour, and bitter. These receptors are in a specific concentration on specific sites of the tongue. The stimulation of the taste buds can also enhance saliva flow.
3.Three pairs of Salivary Glands
a. Parotids
Located in the front and below the ears at the angle of the jaw.
b. Submaxillaries
Found below the angle of the jaw
c. Sublinguals
Found below the tongue.
Saliva: 95% water, ions, lubricating mucus, and starch splitting enzyme amylase which starts digestion and digests the starch caught between the teeth. Dissolved in the saliva is a slippery glycoprotein called mucin. Mucin protects the mouth from abrasions and lubricates the food. Saliva also contains buffers, which help prevent dental cavities by neutralizing acid in the mouth, and antibacterial agents.
4. Pharynx
Near the rear of the oral cavity and forms a common passageway to the naval cavity. Below the tongue the pharynx divides into the larynx and laryngopharynx. When you swallow the top of the larynx moves so that the air passageway is blocked by a cartilaginous flap called epiglottis. This ensures that the bolus stays out of the respiratory system.
5. Esophogus
Moistens the food and mves it to the stomach and is made up of smooth muscle.
The esophagus serves two functions
a. Secretes mucus into the lumen and cavity of the digestive tract.
b. Moves food along- compressing food inta a bolus-muscles contract behind the food and push it along.
6. Stomach
Located on the left side of the abdominal cavity just below the diaphragm. This temporarily stores and helps digest the food.
a. food in the stomach
the stomach stretches easily and any reistance to the stretching will cause cramps. The stomach is closed off at either end by wo sphincters. The pyloric sphincter which is the bottom sphincter that opens to the small intestine. The cardiac sphincter is the top that opnes up into the esophagus. Food is churned without being pushed into the esophagus or intestine. Ulcers occur when the digestive system digests its own stomach or small intestine. The vomit center in the brain allows for the cardiac sphincter to relax, the pyloric sphincter to tighten, and the stomach to contract.
A complex layering of muscles allows for the twisting action and wringing and shortening of the stomach that grinds up the food.
After being ground up, the pyloric sphincter relaxes a bit and the food enters the small intestine.
About every 20 seconds the stomach contents are mixe by the churning of the smooth muscles. When the stomach is empty, the stomach churns and hunger pangs are felt. After food and gastric juices mix, acid chyme is produced. The pyloric sphincter opens and squirts some chyme into the small intestine. It takes 2-6 hours for the stomach to empty after a meal.
b. Chemistry of the stomach
The stomach is lined mucosa, mucus secreting tissue, the tubular glands, which secrete gastric juices. Chief cells secrete hydrochloric acid which activates pepsinogen to be converted to pepsin. Pepsin hydrolyzes proteins and works well in the low ph generated by the HCL. Other glands secrete water mucus, and rennin, which digests milk and gastric lipases. A low ph in the stomach, kills bacteria.
Gastric secretions are controlled by the nervous system and hormones. When we see and smell foods, the brain stimulates the stomach to secrete gastric juices. Certain substances will cause the stomach wall to realease a hormone called gastrin. Gastrin travels through the circulatory system to the stomach, causing the stomach to secrete more gastric juices.