6.1 Digestion

Which organs are in the alimentary canal

esophagus, stomach, small and large intestine

Which organs are the accessory organs

salivary glands, pancreas, liver, gall bladder

Distinguish between alimentary canal organs and accessory organs

Alimentary canal: food actually passes through these

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Accessory organs: aid in digestion but do not transfer food

Describe the structure and function of the esophagus

structure: hollow tube connecting the oral cavity to the stomach, separated from the trachea by the epiglottis

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function: food mixed with saliva and moved in a bolus via peristalsis

Describe the structure and function of the stomach

structure: lined by gastric pits that release digestive juices which create acidic environment (pH less than 2)

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function: temporary storage tank where food is mixed by churning, protein digestion begins

Describe the structure and function of the small intestine

structure: long, highly folded tube consisting of three sections (duodenum, jejunum, ileum)

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function: usable food substances (nutrients) absorbed here

Describe the structure and function of the large intestine

structure: final section of the alimentary canal, consists of ascending, transverse, descending, sigmodial colon, rectum

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function: water and dissolved minerals (ions) absorbed here

Describe the structure and function of the salivary glands

structure: includes the parotid gland, submandibular gland, sublingual gland

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function: releases saliva to moisten food, contains enzymes to initiate starch breakdown

Describe the function of the pancreas

produces a broad spectrum of enzymes that are released into the small intestine via the duodenum, secrets hormones (insulin and glucagon) to regulate blood sugar concentration

Describe the function of the liver

takes raw materials absorbed by small intestine and uses it for key chemicals; responsible for detoxification, storage, metabolism, bile production, hemoglobin breakdown

Describe the function of the gall bladder

stores bile produced by the liver because bile salts are used to emulsify fats, bile is released into th small intestine via the common bile duct

Know how to draw and label a diagram of the digestive system

List the processes of mechanical digestion

chewing (mouth), churning (stomach), segmentation (small intestine)

Describe the movement of food during chewing

* grinding action of the teeth breaks down food
* tongue pushes food to back of the throat
* food travels down esophagus as a bolus
* epiglottis prevents the bolus from entering the trachea
* uvula prevents the bolus from entering the nasal cavity

Describe the movement of food during churning

* stomach lining has muscles that squeeze and mix the food with strong digestive juices
* food is digested within the stomach for hours
* food turns into a creamy paste called chyme
* chyme enters the small intestine through the duodenum for absorption

Describe peristalsis

a movement of the esophagus, stomach, and gut

* continuous segments of longitudinal move rhythmically contracting and relaxing
* food is moved unidirectionally from mouth to anus

Describe segmentation

occurs in the intestines

* contraction and relaxation of non-adjacent segments of circular smooth muscle
* moves chyme in both directions to allow for greater mixing of food with digestive juices
* bidirectional propulsion can slow overall movement

Describe how stomach acids aid in chemical digestion

* stomach has gastric glands to release digestive acids and create a low pH
* acidic environment denatures proteins and other macromolecules to help with digestion
* the stomach epithelium has a mucous membrane to prevent the acids from damaging the gastric lining
* pancreas releases alkaline compounds to neutralize the acids as they enter the intestine

Describe how bile aids in chemical digestion

* bile is stored and concentrated in the gall bladder
* bile has bile salts which interact with fat globules and divide them into small droplets (called emulsification)
* this increases total surface area available for enzyme activity by lipase

Describe how enzymes aid in chemical digestion

* speed up the rate of chemical reaction by lowering the activation energy
* allow digestive processes to occur at body temperatures and at sufficient speeds for survival
* Enzymes are specific for a substrate and so can allow digestion of certain molecules to occur independently in distinct locations

Describe carbohydrate digestion

* salivary glands release amylase in the mouth
* the pancreas also secretes amylase to continue carbohydrate digestion in the small intestine
* enzymes for disaccharide hydrolysis are immobilized on the epithelial lining of the small intestine
* humans can not digest cellulose

Describe the digestion of proteins

* begins in the stomach with the release of proteases
* function optimally in acidic pH
* smaller polypeptide chains are broken down by endopeptidases (released by pancreas) in the small intestine
* work optimally in neutral environments

Describe the breakdown of lipids

* occurs in the lipids with the release of bile from the gall bladder that enables the emulsification of fat globules
* smaller fat droplets are digested by lipases from the pancreas

Describe the breakdown of nucleic acids

* pancreas releases nucleases to digest nucleic acids into smaller nucleosides

List the four main tissue layers of the small intestine

serosa, muscle layer, submucosa, mucosa

State the structure of the serosa layer of the small intestine

protective outer covering composed of a layer of cells reinforced by fibrous connective tissue

State the structure of the muscle layer of the small intestine

outer layer of longitudinal muscle (peristalsis) and inner layer of circular muscle (segmentation)

State the structure of the submuscosa layer of the small intestine

composed of connective tissue that separate the muscle layer from the innermost mucosa

State the structure of the mucosa layer of the small intestine

a highly folded inner layer which absorbs material through the surface epithelium from the intestinal lumen

Label the cross section of the small intestine tissue

List the features of intestinal villi

Microvilli

* ruffling of epithelial membrane increases surface area further

Rich Blood Supply

* dense capillary network rapidly transports absorbed products

Single Layer Epithelium

* minimizes diffusion distance between lumen and blood

Lacteals

* absorbs lipids from the intestine into the lymphatic system

Intestinal glands

* exocrine pits release digestive juices

Membrane proteins

* facilitates the transport of digested materials into epithelial cells

List the structural features of villus epithelium

tight junctions, microvilli, mitochondria, pinocytotic vesicles

Describe the function of the tight junctions of the epithelial lining of villi

* Occluding associations between the plasma membrane of two adjacent cells, creating an impermeable barrier
* They keep digestive fluids separated from tissues and maintain a concentration gradient by ensuring one-way movement 

Describe the function of the microvilli of the epithelial lining of villi

* Microvilli borders significantly increase surface area of the plasma membrane (>100×), allowing for more absorption to occur
* The membrane will be embedded with immobilised digestive enzymes and channel proteins to assist in material uptake 

Describe the function of the mitochondria of the epithelial lining of villi

* Epithelial cells of intestinal villi will possess large numbers of mitochondria to provide ATP for active transport mechanisms
* ATP may be required for primary active transport (against gradient), secondary active transport (co-transport) or pinocytosis

Describe the function of the pinocytotic vesicles of the epithelium lining of villi

* Pinocytosis (‘cell-drinking’) is the non-specific uptake of fluids and dissolved solutes (a quick way to translocate in bulk)
* These materials will be ingested via the breaking and reforming of the membrane and hence contained within a vesicle

Label the following structures

purple: tight junction

blue: pinocytotic vesicles

green: microvilli

red: mitochondria

Describe how secondary active transport enables digestion in the small intestine

* transport protein couples the active translocation of one molecule with the passive movement of another (co transport)
* glucose and amino acids are co transported across the epithelial membrane by the active transport of sodium ions

Describe how facilitated diffusion enables digestion in the small intestine

* channel proteins often located near membrane-bound enzymes to create a localized concentration gradient
* certain monosaccharides like fructose, vitamins, some minerals transported by facilitated diffusion

Describe how osmosis aids in digestion

absorption of water and dissolved ions occurs in small and large intestine

Describe how simple diffusion aids in digestion i

hydrophobic materials (lipids) pass through the hydrophobic portion of the membrane

* after absorption, the lipids pass first into the lacteals instead of being transported via blood

Describe how pinocytosis aids in digestion

allows materials to be ingested en masse, hence takes less time than membrane protein transport

Explain why starch digestion only occurs in the mouth and the intestines

amylase is optimal in neutral pH

Describe how amylase breaks down starch

* digests amylose into maltose subunits (a disaccharide) and amylopectin into branched chains called dextrins
* maltose and dextrins are digested by enzymes like maltase which are fixed to the epithelial lining of the small intestine
* results in the formation of glucose monomers which can be used to produce ATP or stored as glycogen

Describe the two functions of the pancreas in breaking down starch

* produces amylase which is released from the exocrine glands into the intestinal tract
* produces insulin and glucagon which are released from endocrine glands into the blood
* insulin lowers blood glucose levels by increasing glycogen synthesis
* glucagon increases blood glucose levels by limiting synthesis and storage of glycogen