Bio - Digestive System

Digestive System

What is the purpose of the digestive system?

to break down large macromolecules (carbs, lipids, nucleic acids, proteins) from our food into smaller molecules that can be absorbed by the body

Define digestion

the breaking down of food into smaller molecules usable by the cell

define ingestion

the process of taking food into the body

define motility

refers to the moving and mixing of food forward through the digestive tract

define secretion

the production and release of chemicals (hormones, etc) into the body

define absorption

the act of taking a substance into the cells of the body (where it will be used)

define egestion

the act of eliminating a substance from the body

define alimentation

the combined process of ingestion, digestion, and egestion (the digestive tract is also caused the alimentary canal)

define enzyme

biological catalysts (speed up chemical reactions, by lowering the activation energy of a reaction, without being used up)

what are the three main groups of enzymes?

amylase, protease, and lipase

what two major processes control ingestion?

• a nervous system (when)

• cultural conditioning (what)

what are the three factors involved in establishing the set point?

• the concentration of glucose in the blood

  • low glucose = hunger

  • high glucose = satisfies

• the distension of the stomach (feeling of being full)

• the amount of body fat (long term control)

what is the control centre for the digestive system in the brain called

the hypothalamus

what are the two kinds of digestion possible in humans and explain each

1) mechanical digestion

• physically breaking food down into smaller particles by chewing or churning

• the purpose is to maximize surface area upon which enzymes can work so that digestion becomes more efficient

2) chemical digestion

• using secreted enzymes to break large molecules into smaller ones

• the mouth is the first place in the digestive process

what does absorption refer to?

the movement of the structurally simple food-stuffs from the digestive tract into the blood

what are two reasons why food may have not been absorbed

• may have been unable (like when a dog swallows a ring or something)

• your body just didn’t digest

  • ex: starch never came in contact with amylase to break it down

what are two other names for the digestive tract and define it

• gastrointestinal tract (GI tract)

• alimentary canal

  • a simple system which moves food by peristaltic contractions from mouth to anus

approximately how long is the digestive tract

7-9m

define paristalsis

• the wave like contractions of the circular and longitudinal muscle layers of the digestive tract that mixes food with enzymes and moves it along the gut

what are the walls of the gut made of

smooth muscles

what are the four functions of the digestive system?

1) to physically tear food into smaller particles

2) to chemically breakdown large molecules into smaller molecules (by enzymes)

3) the absorption of nutrients into the circulatory system (to be brought to body cells)

4) eliminate undigested materials

what is the pathway of food?

mouth to pharynx to esophagus to stomach to duodenum to jejunum to ileum to caecum to ascending colon to transverse colon to descending colon to sigmoid colon to rectum to anus

what organs are considered accessory structures and what do they do?

includes: liver, gallbladder, pancreas, appendix

• add important enzymes and molecules to help digest the food (but food does not enter these organs)

what happens in the mouth?

food is taken in and ingested and the digestion process begins

what is the function of the teeth?

to mechanically break down food (mastication) into smaller pieces each tooth type assisting in the process

Bonus: name all the types of teeth and how many are in the adult mouth (and how many we have total)

incisor - 8

canine - 4

premolar - 8

molar - 12

adults have 32 permanent teeth

what is the purpose of saliva?

• starts chemical digestion

• has buffers that prevent tooth decay from acidic foods

• contains digestive enzymes secreted from three salivary glands

• contains a mucus that acts a lubricant and aids in swallowing

what is salivary amylase?

enzyme that catalyzes the breakdown of polysaccharides into smaller polysaccharides and maltose

what is the function of the tongue?

• to place food into the most appropriate teeth

• contains taste buds which send information to the brain about the taste of the food

• mixes food with saliva and rolls the food into a ball called a bolus

where is each taste (bitter, sweet, sour, salty) detected on the tongue?

bitter - back of the tongue

sour - sides of the tongue

sweet and salty - tip of the tongue

how does swallowing occur?

• after chewing food, it’s placed near the back of the tongue

• soft palate and uvula move to seal the nasal passage

• epiglottis seals the trachea to prevent food from entering the lungs

• the last voluntary step of the digestive system

what is the esophagus?

a muscular tube about 25 cm long and is about 25 cm wide that conducts food from the pharynx to the stomach

where is the esophagus located?

directly behind the trachea

what does the esophagus secrete?

a non-enzymatic mucous that acts like a lubricant

what three layers is the esophagus made of and describe them?

mucosa: inner most layer covered with mucous

submucosa: contains blood, lymph vessels, nerves

muscularis: 2 muscular layers

• circular muscle

• longitudinal muscle

what is at the end of the esophagus?

a tight ring of muscle called the gastroesophageal sphincter that prevents things from moving in either direction (bolus from moving back into the esophagus and gastric acid from splashing into the esophagus)

what causes movement of the bolus through the esophagus?

peristalsis (wave-like contractions)

where is the stomach located in the body?

on the left side of the abdominal cavity, below the diaphragm

what is the stomach?

a muscular sac that can expand in size to accommodate approximately 2 litres of food

How many layers are in the stomach and what are they?

1) mucosa

2) submucosa

3) muscularis

4) serosa

what is the mucosa in the stomach? what does it do?

• first layer of stomach lining

• has many folds called ruggae which hold gastric pits and gastric glands in between these folds which produce gastric juice

what is gastric juice?

a secretion that contains a variety of substances that aid in the digestion of food including enzymes that help in the breakdown of food?

what is the submucosa in the stomach and what does it do?

• second layer of stomach lining

• contains nerves and blood vessels that detect when food enters the stomach and initiates the release of the hormone gastrin into the blood stream

• the gastrin acts on the gastric glands, telling them to release gastric juice

what is the muscularis in the stomach and what does it do?

• third layer of stomach lining

• a layer of muscles that contract frequently, churning and mixing the bolus with gastric juices to produce a semi-solid mixture called chyme

• has three muscle layers:

  • circular

  • longitudinal

  • oblique

what is the serosa in the stomach and what does it do?

• fourth layer of stomach lining

• holds the stomach in place and secretes a lubricating fluid that eliminates friction between organs

what are the three contents of gastric juice and what do they do?

• hydrochloric acid

  • loosens tough fibrous material

  • kills bacteria that enters stomach with food

• pepsin - type of protease

  • breaks down proteins into amino acids and polypeptides

  • made as pepsinogen (inactive) and activated in the presence of HCl because it would destroy the cells that make it if it was active

• lipase

  • breaks down triglycerides into fatty acids and glycerol

True or False: Amylase from saliva can survive in the stomach

False, it is destroyed in the stomach as the pH is very acidic

what does the mucus secreted by epithelial cells of the inner stomach do?

protects the stomach lining from HCl

how often is the stomach lining replaced?

approximately every 2-3 days

what happens to the chyme as it moves to the small intestine?

• acidic chyme passes through the pyloric sphincter out of the stomach and into the duodenum

what is the small intestine and what are its two main functions?

main functions:

• site of most digestion

• absorption of nutrients

a long, coiled tube more than 6 m long in humans

• length gives it more time to be broken down and absorbed

attached by a thin membrane called the mesentery to the rear wall of the abdomen

consists of three parts:

• duodenum

• jujenum

• ileum

what is the duodenum?

• first 25 cm of the small intestine

• receives acidic chyme from the stomach

• chyme is mixed with secretions from the pancreas, liver, gall bladder, via the common bile ducts as well as gland cells from duodenal wall

what is the jujenum?

• about 3 meters long

• contains more intestinal folds and glands than the duodenum

• enzymes produced here remain attached to the intestinal wall and the peristaltic contractions bring the chyme in contact with the enzymes

• main function is the absorption of nutrients

• nutrient absorption is enhanced by the increased surface area

what are the three ways the surface area of the small intestine is increased

1) folds - increase surface area 3X

2) villus - increase surface area 10X

• villi are tiny finger-like projections along the internal surface of the intestine

• villi contain microvilli

  • function is to absorb small nutrient molecules back into circulatory via capillaries and lymphatic system

3) microvillus - increases surface area 20X

what is the ileum and what does it do?

• makes up about 4 m of small intestine

• fewer digestive enzymes than duodenum as well as smaller and fewer villi

• main function is absorption of nutrients

how long / wide is the long intestine?

1.5 m in length and 7.6 cm in diameter

what is the path of the long intestine?

caecum to ascending colon to transverse colon to descending colon to sigmoid colon to rectum to anus

what is the pathway of feces in the long intestine?

• receives secretions from the ileum via the ileocaecal sphincter

• feces first enters the caecum and then moved through the ascending, transverse, and descending organ by rhythmic contractions into the rectum

what is the appendix and what happens if food gets trapped?

• extends from the caecum and is a vestigial organ (no known function)

• if food gets trapped, bacteria will build up and release toxins causing it to swell (appendicitis)

• if the appendix bursts and releases toxins into the abdominal cavity, this is called peritonitis

what are the three major functions of the large intestine?

1) reabsorption of water

• most of the 20L of fluid passed through the large intestine daily is reabsorbed

  • this fluid is mainly maid up of saliva, digestive juices, and fluid from food

2) vitamin production

• vitamin k and some b vitamins are made by harmless bacteria that live in the intestines

• these bacteria also produce a methane, carbon dioxide, and hydrogen sulfide gas mixture which is either absorbed or released as farts

3) egestion

• formation and expulsion of feces

• feces is stored in the rectum until it is full, sending a nerve signal to the brain causing you to have to shit

• elimination is controlled by the anal sphincter (asshole)

what are accessory digestive organs?

• organs outside the gastrointestinal tract

• they create or store and secrete enzymes, hormones, and chemicals that are required for digestion

where is the pancreas located?

between the stomach and the duodenum

when does the pancreas release pancreatic juices

when chyme enters the duodenum, the hormone CCK (cholecystokinin) is secreted by the duodenum walls and causes the pancreas to release pancreatic juices and the liver to release bile

what is the hormone that the duodenum walls secrete to initiate the secretion of pancreatic juices and bile?

CCK (cholecystokinin)

what three things does the pancreas do to aide in digestion?

1) release pancreatic juice

• travels from pancreas to duodenum via the pancreatic duct

• contains 28 different enzymes

  • amylases: starch to sugar

  • lipases: lipids to glycerol and fatty acids

  • trypsin/peptidase: proteins into amino acids

    • peptidase breaks down polypeptides (proteins)

2) neutralize acidic chyme

• pancreatic juices are alkaline due to high bicarbonate concentration

• changes chyme from pH 2 to pH 8

• if this doesn’t happen, ulcers can occur

3) release hormones

• Insulin:

  • controls blood sugar levels by allowing glucose to enter the cells, storing glucose as glycogen

• Glucagon:

  • when more glucose is needed in the blood, glucagon allows glycogen to be broken into glucose again

  • think: glucagone out the stored cells

what is secretin and what does it do?

• a hormone produced by the duodenal walls

• when pH in duodenum is too acidic, it is released into the blood stream causing:

  • the production of bicarbonate ions from pancreas

  • inhibition of gastric juices in the stomach

what is a gland?

an organ that secretes a substance like a hormone or an enzyme

what is an endocrine gland?

• secretes hormones directly into the blood stream

  • ex: pituitary glands, adrenal glands, ovaries, testes

• think of endothermic reactions: just as energy goes into the cells, hormones go right into the blood stream

what is an exocrine gland?

• secretes into a duct

  • ex: sweat and digestive glands

• think exothermic reactions: energy moves out of the reactants as hormones move out of the gland and blood stream (idk this one isn’t as straight forward)

what type of gland is the pancreas?

• both endocrine and exocrine

  • endocrine: secretes insulin and glucagon into the blood stream

  • exocrine: secretes pancreatic juices into the pancreatic duct

what do secretory cells in exocrine glands do?

produce the product and secrete them into the acinus

what is the acinus in an exocrine gland and what does it do?

• the hollow space surrounded by secretory cells that collect the product

• send product to the duct

what two locations can ducts lead to?

• a lumen

• outside the body

what two secretions does the pancreas make, what enzymes do those secretions contain, and what is the enzyme function? also, where is the site of release for these secretions?

• pancreatic juice

  • pancreatic amylase: breakdown of amylose into maltose

  • trypsin: breakdown of proteins into smaller polypeptides

  • lipase: breakdown of lipids into glucose and fatty acids

• bicarbonate ions (not an enzyme)

  • neutralizes stomach acid

• site of release: duodenum

what is trypsin released as and why? what catalyzes its conversion into active trypsin and what secretes it?

• released as trypsinogen (inactive precursor) so it doesn’t break down the proteins and cells that made it

• the enzyme enterokinase secreted by the duodenum catalyzes its conversion to active trypsin

why are lipids difficult to digest and how are they digested?

• difficult to digest because they are insoluble in water

• lipid molecules will group together to form fat droplets

• bile produced by the liver (stored in the gallbladder) will emulsify these fat droplets by breaking them into smaller ones

  • this produces are larger surface are for lipids to work on as they act on the surface of lipid spheres

where is bile produced and stored, and how does it enter the duodenum?

produced: liver

stored: gallbladder

when food enters the duodenum, the gallbladder will contract, pushing bile into the bile duct where it will enter the duodenum

what do the small intestinal walls produce to digest indigested materials and where are they located in the intestine?

• a variety of enzymes

  • some secreted in intestinal juice that mixes with chyme

  • majority are immobilized in the intestinal walls

name the 6 enzymes that are located in the intestinal walls and their functions? (what are their substrates and products)

• maltase: maltose into glucose

• sucrase: sucrose into glucose and fructose

• lactase: lactose into glucose and galactose

• nuclease: nucleic acids (DNA and RNA) into nucleotides

• exopeptidase: proteins by removing one amino acid at a time

• dipeptidase: dipeptides into two amino acids

what are the epithelial cells in the small intenstine?

• cells that line cavities and features

• create the villi that line the small intestine lumen and increase surface area

• contain microvilli that increase surface area

what are the SI epithelial cell features that allow for absorption?

• increased surface area due to villi and microvilli

• tight junctions: connections between epithelial cells which fuse adjacent cells together at their membranes to form a seal so nutrients don’t get lost between cells

• rich in mitochondria to produce ATP for active transport for absorption

• pinocytotic vesicles to aide in endocytosis

how does absorption by villi work?

• the epithelium that covers the villi forms a barrier to harmful substances but also must be permeable to allow for useful nutrients to pass through into villi

• will absorb:

  • monosaccharides, amino acids, fatty acids, monoglycerides, glycerol, nitrogenous bases, mineral ions, vitamins

  • some harmful and some harmless but unwanted substances will be absorbed as well

what are the methods of absorption into epithelial cells

1) passive transport:

• transport across a membrane that does not require energy

• driven by concentration gradient

• simple diffusion:

  • the molecules move through the membrane moving from high to low concentration

  • ex: lipids

• facilitated diffusion:

  • the molecules are too large to pass through the membrane on their own

  • they enter the membrane via a pore or a carrier protein or protein channel

  • ex: amino acids and glucose

2) active transport

• requires ATP energy

• often moves molecules against the concentration gradient

• transport proteins/protein pumps

  • proteins embedded in the cell membrane that require ATP to activate them

• endocytosis

  • the plasma membrane forms a vesicle around the nutrient molecules to bring them into the cell

    • pinocytosis: for liquids and small molecules

    • phagocytosis: for larger molecules

outline triglyceride absorption

• triglycerides are digested into monoglycerides and fatty acids

  • monoglycerides are absorbed via simple diffusion

  • fatty acids absorbed by facilitated diffusion

• once inside epithelium cells, fatty acids combine with mono glycerides to reform triglycerides

• they they form lipoprotein droplets by combining with cholesterol and being surrounded by phospholipids

• lipoproteins exit epithelial cells by exocytosis and will either enter a lacteal and be carried away by lymph or a capillary and enter the blood stream

outline glucose absorption

• a sodium-glucose symport will move Na⁺ and glucose together into the cell from the intestinal lumen

• This occurs because of the sodium-potassium pump that moves Na⁺ out of the villus epithelial cells, resulting in a low Na⁺ concentration in the cell

• this is considered facilitated diffusion because it moves with the Na⁺ concentration gradient

• the glucose protein channels will move glucose into interstitial spaces inside the villi and into the capillaries

what four molecules cannot be broken down and are therefore egested in feces?

• cellulose

• lignin

• bile pigments

• bacterial and intestinal cells

why is cellulose unable to be broken down by the body?

cellulose has beta-1,4 bonds and is therefore indigestible by the body

explain the process of starch digestion

• amylose breaks down the alpha-1.4 glycosidic bonds into maltose or maltotriose but cannot break down the alpha-1,6 glycosidic bonds in amylopectin

• the alpha-1,6 glycosidic bonds are called dextrins

• the following three enzymes are on the membrane of the microvilli and will complete digestion:

  • maltase: maltose to glucose

  • dextrinase: dextrins to glucose

  • glucosidase: maltotriose to glucose

what are the five roles of dietary fiber

1) increases feelings of satiety, making you feel satisfied and prevent overeating

2) prevent constipation because it draws water into the intestine and the greater the water content, the faster the movement of feces

3) fiber content in feces determines its rate of movement in the large intestine

4) slows down absorption of sugar (prevents type II diabetes)

5)increases the bulk of materials passing through the intestines

what is the function of the liver?

• to produce bile which emulsifies fats in small intestine

• helps regulate level of nutrients in blood and maintain homeostatic ranges

what are the two blood vessels that the liver receives blood from and what types of blood do they carry?

• hepatic artery: carries oxygenated blood to liver tissues

• hepatic portal vein: carries deoxygenated, nutrient rich blood from the digestive tract to the liver

• both carry blood into the sinusoids of the liver

through which blood vessel does blood leave the liver?

• hepatic vein which will join the inferior vena cava and bring deoxygenated blood back to the heart

what is the function of the sinusoids?

• O₂ rich blood from the hepatic artery and nutrient rich blood from the hepatic portal vein flow into the sinusoids of the liver

• this is where the exchange of materials will occur between blood and hepatocytes

name the three components of the sinusoids and their functions

• hepatocytes:

  • remove some things from the blood and add some

  • produce bile which will move in the opposite direction of blood towards the bile ducts

• endothelial cells:

  • form the wall of sinusoids

  • spaces allow plasma direct contact with hepatocytes

• kupffer cells:

  • cells that line sinusoids

  • ingest foreign particles and are involved in the breakdown of old erythrocytes

how does detoxification in the body occur?

• liver is the main organ for this

• toxins such as pesticides, medicines, food additives, and alcohol are removed from the bloodstream and broken down by hepatocytes and converted into non/less toxic substances

• will take multiple blood circuits for all chemicals to be acted on by the liver because the hepatocytes cannot grab all the chemicals in one go

how is alcohol detoxified in the body and what are it’s long term effects?

• hepatocytes attempt to remove alcohol from blood stream each time it passes through the liver

• the breakdown of alcohol creates free radicals which can interfere with proteins, lipids, and DNA

• alcohol —> acetaldehyde —> acetic acid —> carbon dioxide

• long term effects:

  • interferes with normal liver metabolism of proteins, fats, and carbs

  • can lead to cirrhosis, fat accumulation, and inflammation

what four things are stored in the liver?

• carbs are stored as glycogen

• iron

  • after rbc are broken down, their iron is stored in the liver until it can be reused

• vitamins but mainly:

  • vitamin A

  • vitamin D

how does the liver aide in the synthesis of plasma proteins?

plasma proteins: blood proteins produced by ER and Golgi in the hepatocytes

• albumins: regulates osmotic pressure of fluids in the body and is a carrier protein

• fibrinogen: involved in blood clotting

• globulins: antibodies

how is the liver involved in the synthesis of cholesterol and what is it used for?

• cholesterol can be ingested or made by the liver

• used for:

  • the production of bile

  • production of vitamin D

  • production of cell membrane

  • production of lipid based hormones

• the liver:

  • regulates amount of circulating lipids (including cholesterol) by:

    • synthesizing them as required

    • breaking them down

    • secreting them in bile

how are erythrocytes recycled?

• after approx. 120 days, the cell membranes get weaker until they rupture which usually occurs in the spleen

• spleen:

  • when rbc ruptures, they release millions of hemoglobin into the bloodstream and eventually pass through the liver where they are absorbed by kupffer cells via phagocytosis