three dietary categories
- herbivores: eat autotrophs such as plants and algae
- carnivores: eat other animals
- omnivores: eat both plants and animals
four mechanisms that animals use to obtain and ingest food
- suspension feeders: extract particles suspended in the surrounding water
- substrate feeders: these organisms live within or atop their food source
- they eat their way through the food source
- ex) earthworms - fluid feeders: suck nutrient rich fluids from their host
- host can be plant or animal
- ex) mosquitoes - bulk feeders: ingest large pieces of food
- utilizes utensils pieces of food
- kill prey
- tear off pieces
four stages of food processing
- ingestion
- digestion
- absorbtion
- elimination
ingestion
-this is the actual act of eating
digestion
- this is the breakdown of food into molecules small enough to be absorbed by the body
- this stage occurs in two stages:
1. mechanical breakdown: - occurs in the mouth of humans and many other animals
- teeth break food into smaller pieces, increasing surface area
2. enzymatic breakdown: - enzymes breakdown the chemical structure of the molecules
- occurs via the addition of water
absorption
- the products of enzymatic digestion are absorbed into the cells lining the digestive tract
- these products then enter the blood stream and are delivered to the cells of the body
elimination
-undigested material passes through the digestive tract
compartmentalized digestion
- digestion must occur in specialized compartments
- in order to avoid digestion of self tissues
- food vacuole is the smallest digestive compartment
- fuses with lysosome which contains digestive enzymes
- most animals contain a internal compartment
- permits extracellular digestion
- gastrovascular cavity: digestive cavity with only one opening the mouth)
steps in compartmentalized digestion
- gland cells lining the cavity secrete digestive enzymes
- enzymes break down soft prey tissues
- other cells present engulf small food particles
- food particles are broken down in food vacuoles
- undigested material is expelled through the mouth
alimentary canal
-has two openings a mouth and an anus
-most mammals
pharynx:
esophagus:
-may lead into a crop, a gizzard or a stomach
-stomach and gizzard may temporarily store food
-muscular
-churn and grind food
small intestine:
-bulk of enzymatic digestion and all nutrient absorption occurs here
anus:
-undigested materials are expelled through the anus
1/3 example of alimentary canals
- earthworm
- food enters pharynx from mouth
- passes through esophagus and is stored in the crop
- gizzard contains sand and gravel and digests the food
- digestion and subsequent absorption occurs in the intestine
- dorsal fold increases the surface area for absorption
2/3 example of alimentary canals
- grasshopper
- uses a crop to store food also
- digestion occurs in the mid-gut region
- gastric pouches here increases the surface area for absorption
- hind gut is to absorb water and compact waste
3/3 example of alimentary canals
- birds
- three separate chambers
- a crop
- a stomach
- a gizzard: filled with gravel to pulverize food
- chemical digestion and absorption occur in the small intestine
human digestion
- peristalsis propels food through the GI tract
- alternating waves of smooth muscle contraction and relaxation
- sphincters regulate the passage of food into and out of the stomach
- cardiac sphincter
- pyloric sphinter
stomach
- has accordion-like folds to increase absorptive surface area
- can stretch to accommodate up to 2L of food and liquids
- stomach secretes gastric juice:
- composed of mucus, strong acid and enzymes
- hydrochloric acid begins breakdown of chemicals bonds in food
- kills most bacteria that have entered with food pH=2
- mucus protects stomach lining from harsh acidity
- HCI also converts certain enzymes into their active form
- hormonal control of digestion
- gastrin is released from stomach cells into the blood in response to food
- circulation in the blood returning to the stomach to stimulate secretion of more gastric juice
small intestine
- remainder of digestion occurs here
- absorption of nutrients occurs here
- digestive enzymes used during digestion are synthesized by:
- the pancreas: pancreatic juice which is a mixture of digestive enzymes and bicarbonate
- the liver: produced bile which contains salts that emulsify fats
- bile is stored by the gall bladder until it is required in the small intestine
- small intestine also produce digestive enzymes which aid in digestion
- absorptive surface is increased by villi which are finger-like projections lined with microvilli
- extended into lumen of small intestine greatly increasingly surface area
- absorptive surface of the small intestine is approx 300m2
- roughly the size of a tennis court
liver
- liver is located between the intestine and the heart
- hepatic portal vein receives blood from capillaries of small and large intestine
- transport nutrients absorbed by the intestines directly to the liver
- main liver function is to remove excess glucose from the blood
- convert excess glucose to glycogen
- glycogen is stored in the liver
- liver also produces lipoprotiens that transport fats and cholesterol to the cells of the body
- liver is involved in detoxification and modification of substrate absorbed by the GI tract ex) alcohol and drugs
- then secreted by the kidney into urine
- produces bile
- processes nitrogen wastes from protein breakdown, forming main urine component, urea
large intestine
- 1.5m long
- outpocketings include a blind pouch called the caecum and the appendix
- main functions is to absorb any remaining water
- waste products are referred to as feces
- mainly undigestible plant fibers and normal flora from the colon
- especially E.coli
- feces are stored in the anus until elimination
evolutionary adaptations
-non-human animals exhibit different variations on this digestive system plan
-natural selection has favored adaptations that fit an animals digestive system to be structured to digesting the kind of food that the animal eats
-large, expandable stomachs: carnivores
-length of digestive tract: herbivores and omnivores
have longer digestive tracts
-vegetation is more difficult to digest
-longer canal provides more time for digestion
-herbivores also usually have special chambers to house lots of bacteria
-help the animal by digesting cellulose into simple sugars
nutrients supplied from diet
- essential nutrients must be supplied by animal diet
- animals cannot make these from any raw materials
- four classes:
1. essential fatty acids: linoleic acid used to synthesize the plasma membrane
2. essential amino acids: 8/20 cannot be synthesized- most common
3. vitamins
4. minerals - undernourishment: diet chronically deficient in calories
- malnourished: long term absence of one or more essential nutrients
