6.1 Digestion Flashcards
steps of nutrition. [5]
- ingestion: eating/taking in food into the alimentary canal
- digestion: mechanical and chemical breakdown of complex food molecules into their simpler forms.
- absorption: digested food substances are absorbed by cells in the alimentary canal and distributed around the body through blood circulation.
- assimilation: the products of digestion are taken up by different organs for different uses (eg. cell respiration, synthesis of new molecules)
- egestion: secretion of undigested waste
alimentary canal - structures and functions [4]
- esophagus:
hollow tube connecting oral cavity to stomach
food is mixed with saliva and moved in a bolus by peristalsis - stomach:
churning and mixing of food with secreted water and acid which kill pathogens and foreign bacteria
digestion of proteins begins - SI:
final stages of digestion of lipids, proteins, carbohydrates, nucleic acids
absorption of nutrients - LI:
absorption of water and minerals
accessory organs - functions [4]
salivary glands
- secrete saliva which moistens food and contains enzymes (amylase) to initiate digestion of starch
pancreas
- produces a broad spectrum of enzymes *amylase, protease, lipase) which are secreted (as pancreatic juice) into the small intestine via the duodenum
secretes hormones such as glucagon and insulin which regulate bg conc
liver
- uses nutrients absorbed in the SI to synthesize key chemicals
- synthesizes alkaline bile which neutralizes acidic chyme, bile contains bile salts which are used to emulsify (break up) fats
gall bladder
stores bile produced by liver
releases bile into the SI via bile duct
mechanical digestion [2]
chewing (oral cavity):
- food broken up by grinding of teeth (chewing)
- tongue pushes food to the back of throat where food travels down the esophagus as a bolus
churning (stomach):
- stomach lining contain muscles which squeeze and mix food with digestive juices
- food is digested in the stomach for several hours until it becomes a creamy paste (chyme)
- chyme eventually enters the duodenum where absorption occurs
movement of food [2]
peristalsis:
- movement of food down the esophagus, also occurs in stomach and gut
- continuous segments of longitudinal muscle rhythmically contract and relax
- food moves unidirectionally in a caudal (mouth to anus) direction
segmentation:
- contraction and relaxation of non-adjacent circular muscles in the intestines
- moves chyme in both directions, allowing greater mixing of chyme with digestive juices
* while segmentation helps to physically digest food particles, its bi-directional movement can slow down the overall movement
chemical digestion - stomach
chemical digestion (food broken down by chemical agents - enzymes, acids, bile)
stomach acids:
stomach releases gastric acids => low pH environment
low pH => denaturation of proteins and other macromolecules, aiding their digestion
* stomach epithelium contains mucous membrane which prevents acids from damaging gastric lining
* pancreas releases alkaline compounds (eg. bicarbonate ions) to neutralize acid as they enter the intestine
bile:
produced by liver, stored in gall bladder
contains bile salts which interact with fat globules to break them up into smaller droplets of fat (emulsification)
increases total SA available for enzyme activity (lipase)
enzymes:
gastric juices contains proteases (eg. pepsin) that function at low pH and initiates the break down of proteins
chemical digestion - small intestine [2]
SI produces enzymes to complete digestion (some are secreted in intestinal juices but most are embedded in the walls of the SI)
maltase + maltose -> glucose
lactase + lactose -> glucose + galactose
sucrase + sucrose -> glucose + fructose
dipeptidase + dipeptides -> AA
exopeptidase + short polypeptides -> dipeptides + AA
nuclease + nucleic acids -> nucleotides
pancreas secretes pancreatic juice into the lumen of the SI
amylase (pancreatic) + starch -> maltose
lipase + triglycerides -> fatty acids and glycerol
endopeptidase (eg. trypsin) + peptides -> AA
- no enzymes in humans to digest cellulose, cellulose is passed out in feces
Structure of SI (outside to center). [4]
serosa: outer protective covering composed of a layer of cells reinforced by fibrous connective tissue
muscle layer: outer layer of longitudinal muscle (peristalsis) + inner layer of circular muscle (segmentation)
submucosa: composed of connective tissue separating muscle layer from innermost mucosa
mucosa: highly folded inner layer which absorbs material through its surface epithelium from intestinal lumen
*SMSM
features of villi [6]
microvilli: ruffling of epithelial membrane further increases SA
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 transport of digested material to epithelial cells
*MR SLIM
structure of villus epithelium [4]
tight junctions:
occluding associations between plasma membrane of adjacent cells => impermeable barrier
keeps digestive fluids separated from tissues and maintains CG by ensuring one-way movement
microvilli:
significantly increases SA => faster rate of absorption
membrane embedded with immobilized digestive enzymes and channel proteins to assist in material uptake
mitochondria:
epithelial cells of intestinal villi posses large numbers of mitochondria to provide ATP for AT mechanisms
ATP may be needed for primary AT (against gradient), secondary AT (so-transport) and pinocytosis
pinocytotic vesicles:
pinocytosis - non-specific uptake of fluids and dissolved solutes (quick way of translocating in bulk)
materials are ingested by breaking and reforming of membrane => contained within vesicle
starch digestion
amylase digests amylose into maltose subunits (disaccharides) and digests amylopectin into branch chains (dextrins)
maltose and dextrin are digested by maltase fixed in the epithelial lining of the SI => formation of glucose monomers
role of pancreas:
- produces amylase released from the exocrine glands (acinar cells) into the intestinal tract
- produces hormones insulin and glucagon released from the exocrine glands (islets of Langerhans) into the bloodstream
* insulin lowers BG levels - increases glycogen synthesis and storage in liver and adipose tissue
* glucagon increases BG levels - limits synthesis and storage of glycogen in liver and adipose tissues
Lipid digestion
- being hydrophobic (= insoluble in aq environments), lipids group together to form larger globules of fat
- lipase is generally soluble; only hydrophobic at AS
=> lipase can only bind to lipid globules at lipid-water interface (ie. outer extremity of the globule)
=> slow rate of digestion of lipids because the interior of lipid globules are inaccessible to lipase in this form
role of bile:
- bile salt molecules have both a hydrophobic surface and a hydrophilic surface
- hydrophobic end interacts with lipid, hydrophilic end faces out and prevents lipids from coalescing (combining)
=> divides fat globules into smaller droplets (emulsification), increasing SA for enzyme activity
absorption - membrane transport mechanisms [5]
- simple diffusion
- hydrophobic molecules (lipids) freely pass through the hydrophobic portion of the plasma membrane
- once absorbed, lipids pass first into the lacteals instead of being transported via the bloodstream - pinocytosis
- form of endocytosis => involves vesicle formation => energy-dependent
- (process) cell takes in fluids along with dissolved materials
- allows material to be ingested all together => takes less time than shuttling via membrane proteins - osmosis
- absorption of water (occurs in SI and LI) - facilitated diffusion
- channel proteins help hydrophilic food molecules pass through the hydrophobic portion of the plasma membrane
- channel proteins are often situated around specific membrane-bound enzymes => create a localized gradient
* method of transport of some monosaccharides (fructose), vitamins and minerals - secondary AT
- transport protein couples the translocation of one molecule to the passive movement of another (co-transport)
- glucose and AA are co-transported across the epithelial membrane by the active translocation of Na ions
* glucose and AA can be moved against their CG because Na is moving down its CG. Na Cg is generated by AT of Na out of the epithelium cell by a pump protein
- SPOFS
lipid absorption
- fatty acids absorbed into the epithelial cells of the intestinal lining, combined to form triglycerides
- triglycerides combine with proteins in the golgi apparatus to form chylomicrons
- chylomicrons are released by epithelial cells and transported to the liver via lacteals
liver:
- chylomicrons can be modified => different lipoproteins
- low density lipoproteins: transports lipids via bloodstream to cells
- high density lipoproteins: scavenge excess lipids in the bloodstream and tissues and return them to the liver
sections of the gut [4]
SI:
duodenum
- first segment of SI which is fed by digestive fluids from the pancreas and gall bladder
- bile emulsifies fat globules into smaller fat droplets, pancreatic juice contains digestive enzymes
- sodium bicarbonate is released to neutralize the stomach acids
jejunum
- second segment of SI where digestion is largely completed
-pancreatic enzymes and enzymes released from intestinal glands complete the breakdown of carbs, proteins and lipids
jejunum
