The Digestive System

By: Natalie Jacquez

An animals requirements for survival

Food for fuel (chemical energy)

Organic raw materials used in biosynthesis

Carbon skeletons

Essential nutrients

ATP’s importance in homeostasis

It powers resting metabolism and regulates temperature in endotherms

It comes from the oxidation of organic fuel molecules- carbohydrates, proteins, and fats= in cellular respiration.

Fats are especially rich in ATP; 2x the amount of energy compared to carbohydrates and proteins

Glucose Regulation

If the animal isn’t growing or reproducing, the body tends to store surplus energy in energy depots.

In humans, the liver and muscle cells store energy in the form of glycogen (composed of many glucose units which fuel cell’s metabolism and regulate hormone action).

If not enough calories are consumed, fuel is taken out of storage depots and is oxidized.

Caloric Imbalance

Undernourishment- If the diet of a person is chronically deficient in calories in which the stores of glycogen and fat are used up and the body results in the breakdown of its own proteins for fuel.

Consequences: brain can become protein-deficient & muscles decrease in size. (This can result in death)

Overnourishment- Results from excessive food intake and cause obesity.

Obesity

Complex feedback mechanisms regulate fat storage and use.

An increase in adipose tissue increase leptin levels in the blood which cues the brain to depress appetite and to increase energy- consuming muscular activity and body-heat production result in weight gain.

Loss of body fat subsequently decreases leptin levels.

Essentials for Biosynthesis

To build the complex molecules it needs to grown and maintain itself, an animal must obtain organic precursors (carbon skeletons) from its food.

Essential Nutrients- materials that must be obtained in preassembled form because the animal’s cells cannot make them from any raw material. There are 4 tours of these nutrients: Essential Amino Acids, Fatty Acids, Vitamins, and Minerals.

Malnourished- An animal whose diet is missing one or more essential nutrients.

Essential Amino Acids

E.A.A- Must be obtained from food in prefabricated form. Eight amino acids are essential in the adult human diet.

A diet that provides insufficient amounts of one or more of these amino acids, causing a form of malnutrition known as protein deficiency. In children it can hurt their physical and mental development (most common form of malnutrition)

Reliable sources: meat, eggs, cheese

Proteins in animal products are “complete”, which means they provide all of the essential amino acids.

Essential Fatty Acids

E.F.A- Certain unsaturated fatty acids in which animals are incapable of creating themselves. Ex) Linoliec acid

Vitamins

Vitamins- Organic molecules required in the diet in amounts that are quiet small compared with the relatively large quantities of essential amino acids and fatty acids animals need.

13 Vitamins are essential to humans.

They are grouped as: water soluble and fat soluble

Water Soluble: Includes the B complex and Vitamin C which function as coenzymes in key metabolic processes. (Excess are excreted in urine)

Fat Soluble: Vitamins A, D, E, K (Excess are deposited as fat)

Minerals

Minerals- Simple inorganic nutrients required in small amounts.

Ex) Large quantities of calcium and phosphorus are needed for the construction and maintenance of bones.

Ingesting an excess of minerals can upset homeostatic balance and cause toxic side effects. Ex) Too much sodium leads to high blood pressure/ excess iron results in liver damage

Types of Feeders

Herbivores- Eat mainly autotrophs (plants, algae)

Carnivores- Eat other animals

Omnivores- Consume animals & plant matter

Most animals are opportunistic, eating foods that are outside their main dietary category when these foods are available.

Feeding Adaptations

Suspension Feeders- They swift small food particles from the water. Ex) clams/oysters/baleen whales

Substrate Feeders- They live in or on their food source, eating their way through the food. Ex) maggots/caterpillar

Deposit Feeders- They eat their way though the dirt and salvage partially decayed organic material consumed along with soil. Ex) earthworms

Fluid Feeders- They make their living by sucking nutrient rich fluids from a living host. (considered parasites) Ex) mosquitos/leeches/aphids

Bulk Feeders- They eat relatively large pieces of food. Contain tentacles, pincers, claws, poisonous fangs, and jaws and teeth to tear off the meat easily. Ex) python

Food Processing: The Four Stages

1) Ingestion- The act of eating in which complex arrays of molecules, including large polymers are consumed. Organic material in food consists mainly of proteins, fats, starches, and other polysaccharides that can’t be used due to their inability to pass through membranes/ their unidentical marcomolecules.

2) Digestion- The process of breaking down food into their component monomers which are small enough to absorb and can be used to make molecules and fuel for ATP producion.

Polysaccharides break down to simple sugars, fats to glycerol, proteins to amino acids, and nucleic acids to nucleotides.

Enzymatic hydrolysis- In which digestion breaks macromolecule’s bonds with the enzymatic addition of water.

Food Processing: The Four Stages Contd.

Chemical Digestion is usually followed by mechanical fragmentation of the food by chewing.

3) Absorption- After the food is digested, the animal’s cells absorb small molecules such as amino acids and simple sugars from the digestive compartment.

4) Elimination- Undigested material passes out of the digestive compartment.

Intracellular Digestion

Food Vacuoles- organelles in which hydrolytic enzymes break down food without digesting the cell’s own cytoplasm are the simplest digestive compartments

Intracellular Digestion- Newly formed vacuoles fuse with lysosomes which mix the food with enzymes allowing digestion to occur safely within a compartment enclosed by a protective membrane.

Ex) Sponges/ heterotrophic protists solely rely on this process to digest their food entirely

Extracellular Digestion

Extracellular Digestion- In which hydrolysis occurs and food is broken down outside of cells. Having extracellular cavities for digestion therefore enables an animal to devour larger prey than can be ingested by phagocytosis and intracellularly.

Gastrovascular Cavities- function in both digestion and the distribution of nutrients throughout the body. The gastrodermis, tissue layer that lines the cavity, secrete digestive enzymes that break the soft tissues of the prey into tiny pieces. Ex) hydra, sponges, flatworms, cnidarians

Complete Digestive Tracts (Alimentary Canals)- Possessing digestive tubes extending between two openings, the mouth and the anus. Ex) nematodes, annelids, mollusks, echinoderms, chordates

Food ingested through the mouth and pharynx passed through the esophagus that leads to a crop, gizzard, or stomach and next enters the intestine where digestive enzymes hydrolyze the food molecules and absorbs nutrients across the lining of the tube into the blood.

Mammalian Digestive System

Consists of: alimentary canal and glands that secrete digestive juices into the canal

Peristalsis- rhythmic waves of contraction by smooth muscles in the wall of the canal that push food along the tract

Sphincters- Ring-like valves that close off the tube to regulate the passage of material between chambers of the canal.

Accessory Glands- Salivary glands, pancreas, liver, and the gallbladder (stores digestive juice)

Oral Cavity

Both physical and chemical digestion begins in the mouth

The presence of food in the oral cavity triggers a nervous reflex that causes the salivary glands to deliver saliva through ducts to the oral cavity.

Saliva contains glycoprotein (carbodhyrdrate-protein complex) which protects the soft lining of the mouth from damage, lubricating the food & preventing tooth decay

Chemical digestion of carbohydrates begins in the oral cavity

Salivary Amylase- an enzyme that hydrolyzes starch and glycogen which produces the disaccharide maltose and smaller polysaccharides

Bolus- Is the ball the tongue shapes the food into to push into the pharynx

The Pharynx

Pharynx- a junction that opens to both the esophagus and the windpipe (trachea).

When we swallow, the top of the windpipe moves up so that its opening, the glottis, is blocked by a cartilaginous flap, the epiglottis.

It ensures that a bolus will be guided into the entrance of the esophagus

The Esophagus

Esophagus- conducts food from the pharynx down to the stomach by peristalsis. The muscles at the very top are voluntary however, the contraction by smooth muscles are involuntary.

The Stomach

Stomach- Is located in the upper abdominal cavity, just below the diaphragm. It can stretch to store food and perform digestive functions by secreting a digestive fluid called gastric juice which mixes with the food by the churning action of the smooth muscles in the stomach wall.

Gastric juice contains a pH of 2 to disrupt the extracellular matrix that binds cells together in meat and plant material all while killing bacteria swallowed with food.

It also contains pepsin, an enzyme that begins the hydrolysis of proteins. It breaks peptide bonds adjacent to specific amino acids, making proteins into smaller polypeptides. (works in acidic env.)

How it prevents self-digestion

Pepsinogen- Pepsin’s inactive form by specialized cells called chief cells located in gastric pits. It’s activation in the lumen of the digestive tract is an example of positive feedback.

Parietal cells- secrete hydrochloric acid which converts pepsinogen to active pepsin by removing a small portion of the molecule and exposing its active site

Epithelial cells- secrete a coating of mucus that help protect the stomach lining. Mitosis generates enough cells to completely replace the stomach lining every 3 days.

Acid chyme- As a a result of mixing and enzyme action what was once a swallowed meal becomes nutrient-rich broth.

Pyloric Sphincter- At the opening from the stomach to the small intestine that helps regulate the passage of chyme into the intestine.

Small Intestine

Small intestine- longest section of the alimentary canal in which most of the enzymatic hydrolysis of food macromolecules and absorption of nutrients into the blood occurs.

Duodenum- the first 25cm of it where acid chyme mixes with digestive juices from the pancreas, liver, gallbladder, and glad cells of the intestinal wall itself

Pancreas- makes hydrolytic enzymes & an alkaline solution full of bicarbonate that acts as a buffer to offset the chyme’s acidity

Liver- makes bile, a mixture of substances that is stored in the gallbladder until needed. It contains bile salts which aid in the digestion and absorption of fat.

Carbohydrate Digestion

The digestion of starch and glycogen begins by salivary amylase in the oral cavity to the small intestine.

Maltose- completes the digestion of maltose, splitting it into two molecules of the simple sugar glucose. (In the family: disaccharides which cover the intestinal epithelium, also known as the site of sugar absorption)

Protein Digestion

Digestion of proteins occurs in the small intestine where the process begun by pepsin in the stomach is completed

Trypsin & Chymotrypsin- specific for peptide bonds adjacent to certain amino acids, and like pepsin, break large polypeptides into shorter chains (secreted by pancreas)

Dipeptidases- Attached to the intestinal lining, split small peptides

Carboxypeptidase- Splits off one amino acid at a time beginning at the end that has a carboxyl group (secreted by pancreas)

Aminopeptidase- It is secreted by the intestinal epithelium and works in the opp. direction as Carboxypeptidase. (team work between the two speed up the process)

Tetropeptidase- Directly/ Indirectly triggers activation of these enzymes within the intestinal lumen

Nucleic Acid Digestion

Nucleases- enzymes that hydrolyze DNA and RNA into their component nucleotides.

Other hydrolytic enzymes than break nucleotides down further into nucleosides, nitrogenous bases, sugars, and phosphates.

Fat Digestion

Fat molecules are insoluble in water.

Emulsification- Bile salts from the gallbladder secreted into the duodenum coat fat droplets to prevent them from hardening

Lipase- an enzyme that hydrolyzes the fat molecules

Peristalsis moves the mixture of chyme and digestive juices along the small intestine

Jejunum & Ileum- Parts of the small intestine that function mainly in the absorption of nutrients and water

Absorption of Nutrients

Most of the absorption of nutrients occurs in the small intestine

Villi- large fingerlike projections that contain appendages called microvilli, which are exposed to the intestinal lumen

Penetrating the core of each villi are capillaries and lacteals- small vessel in the lymphatic system

Nutrients are absorbed across the intestinal epithelium and then across the unicellular epithelium of the capillaries or lacteals. (these two layers separate nutrients in the lumen of the intestine from the bloodstream)

Active transport- allows the intestine to absorb high amounts of nutrients

Amino acids/sugars pass through the epithelium, enter the capillaries, and are carried away from the intestine by the bloodstream. After the glycerol/fatty are absorbed by the epithelial cells, they are recombined into fats and are mixed with cholesterol/ coated with proteins that form small globules called chylomicrons. Most are transported out of the epithelial cells and into the lacteals which converge into large vessels of the lymphatic system.

Hepatic Portal vessel- In which the capillaries and veins that drain the nutrients away from the villi all converge and are lead directly to the liver. This ensures the livers access to amino acids and sugars. Consequently, the liver regulates the level of glucose molecules in the blood and from there travels to the heart.

Digestive Cost

Because digestion depends on peristalsis, enzyme secretion, and active transport, there is a substantial energy cost to processing food.

It may require one to expend 3-30% of the chemical energy available from the meal

Hormones help regulate digestion

Gastrin- Certain substances in food stimulate the stomach wall to secrete this hormone. As it gradually recirculates in the bloodstream back to the stomach wall, the hormone stimulates further secretion of gastric juice

If the pH of the stomach is too low, the acid will inhibit the release of gastrin, decreasing the secretion of gastric juice. (ex: of negative feedback system)

Enterogastrones- Hormones secreted by the wall of the duodenum. This signals the pancreas to release bicarbonate, which neutralizes the acid chyme.

Secretin- The acidic pH of the chyme that enters the duodenum stimulates cells in the wall to release this hormone

Cholecystokinin (CKK)- A second enterogastrone which is secreted in response to the presence of amino acids or fatty acids. This causes the gallbladder to contract and release bile into the small intestine. It also triggers the release of pancreatic enzymes.

Chyme- If rich in fats, causes the duodenum to release other enterogastrones that inhibit peristalsis in the stomach.

Large Intestine

Large Intestine (colon)- Is connected to the small intestine at a T-shaped Junction, where a sphincter controls the movement of material

Cecum- One “arm” of the T is this pounch.

Appendix- A fingerlike extension of the cecum

The colon’s major function is to recover water in the lumen that has entered the alimentary canal as the solvent of the various digestive juices

Together the small intestine and colon retain 90% of that water that entered

Feces- waste of the digestive tract that become more solid as they are moved along the colon by peristalsis which contain masses of bacteria and undigested materials.

Escherichia coli- common inhabitant of the colon and the presence of it in lakes is an indication of contamination

Intestinal bacteria live on unabsorbed organic material and produce gases as byproducts of their metabolism. While some produce vitamins including biotin, folic acid, vitamin K, and vitamin B.

Rectum- terminal portion of the colon where feces are stored until they can be eliminated

Symbiotic microorganisms

Possessing large populations of symbiotic bacteria and protists in special fermentaton chambers in their alimentary canals helps herbivorous animas hydrolyze cellulose. They can therefore digest it to simple sugars and other compounds the animal can absorb. Most are held in the cecum.