6.1 Digestion and absorption

Digestive juice! Who's in Charge?

3 major types 1.Cephalic 2.Gastric 3.Intestinal

Enzyme:

A globular protein that increases the rate of a biochemical reaction by lowering the activation energy threshold (i.e. a biologicalcatalyst)

Proton Pump Inhibitors

Acid in the stomach is produced by the combination of hydrogen ions (H+) and chloride ions (Cl-) produced by the epithelial cells (known as parietal cells) of the stomach lining. The release of hydrogen ions through protein carriers is coupled to the intake of potassium ions (K+) from the lumen of the stomach. This process requires energy in the form of ATP (Figure 2). Proton Pump Inhibitors (PPI) block the proton pump (hydrogen-potassium-ATPase pump), responsible for the secretion of hydrogen ions (H+) in the stomach. The decrease of H+ into the lumen of the stomach reduces the amount of hydrochloric acid (HCl) produced, which makes the stomach less acidic (Figure 4), making it a suitable treatment of gastric ulcers.

Identification of Exocrine Glands

Acinus : Groups of secretory cells that surround a duct branch

Endocytosis (Pinocytosis)

Antibodies from breast milkThe plasma membrane folds inward to form vesicles to absorb larger molecules without digesting them

Regulation of Gastric secretions

D.2 U3 The volume and content of gastric secretions are controlled by nervous and hormonal mechanisms●Both nerves and hormones involved in controlling secretion of digestive juices. ○Example: Gastric Juice○Starts with the sight or smell of food causes brain to send nerve impulses via the vagus nerve from the medulla ...

Lipases break down fats and lipids

Example: pancreatic lipaseSubstrate: triglycerides Product: fatty acids & glycerolSource: pancreas, delivered into small intestineOptimum pH: 7.2 - 7.5

Amylases break down carbohydrates

Example: pepsinSubstrate: polypeptides Product: amino acidsSource: stomachOptimum pH: 2

Amylases break down carbohydrates

Example: salivary amylase Substrate: starch Product: maltose Source: mouth (salivary glands) Optimum pH: 7-7.8

Exocrine vs. Endocrine glands

Exocrine Glands: secrete to the surface of the body or the lumen of the gut ●Secrete into DUCTSEndocrine Glands: secrete directly into the bloodstream Exocrine glands pass their secretions into a system of ducts that lead to the exterior of the body. Salivary glands, bile-producing glands, prostate gland, gastric gland and sweat glands Endocrine glands, in contrast, place their secretions into the blood. Adrenal glands, pituitary glands, thyroid gland, etc.

Nerves + Hormones control secretion of Digestive Juice

Fight or flight! ●If not necessary, digestion does not need to be occurring. Better to save energy in case "flight or fight" response is required.●Conserve energy for muscle use creates the need to divert energy from the digestive process.This is controlled by your autonomic nervous system

Facilitated Diffusion

Fructose, vitaminsWater-soluble (hydrophilic) molecules use channel proteins to pass phospholipid bilayerand enter the epithelial cells (down the concentration gradient)

Adaptations to Absorption

Getting digested food molecules into the blood from the lumen of the ileum. Many villi protrude into the lumen, greatly increasing the surface area for absorption. Single-cell layer of epithelial cells- Short path for diffusion. Microvilli on the surface of each cell increase surface area even further. Lacteals (lymph vessels)Allow for rapid absorption and transport of lipids. Capillaries close to epitheliumShort path for diffusion, rich supply of blood. Rich blood supplyMaintains concentration gradients between lumen and blood. Peptidases are embedded within the plasma membrane of the microvilli. Peptidases complete the digestion of peptides to amino acids Endopeptidase X exopeptidase: endopeptidase hydrolize peptide bonds within polypeptides and exopeptidase do so at the end of polypeptide chains. Once the digestion is completed in the SI, the products of digestion are absorbed into the blood stream through the villi found in the innermost cellular lining of the SI, the intestinal mucosa. Villi are the inner wall of the SI made up of thousand of finger-like extensions known as microvilli.

Dietary Fibre

Indigestible plant carbohydrates●cellulose●lignin●pectinInsoluble fiber - woody + structural parts of plants, skin on fruit, coating of corn - speeds flow through the intestinesSoluble fiber- slows the passage and lowers cholesterol

Secretin

Inhibit gastrin secretion if pH in stomach too lowDuodenum

Somatostatin

Inhibit gastrin secretion if pH in stomach too lowpancreas and duodenum

Simple diffusion

Lipids Lipids are non-polar and therefore can pass freely through hydrophobic core of the plasma membrane into the epithelial cells (down the concentration gradient )

Helicobacter pylori

Look up:http://www.webmd.com/digestive-disorders/h-pylori-helicobacter-pyloriType of bacteria:Symptoms: Outlook:

The small intestine contains four distinct tissue layers from the lumen

Mucosa - inner lining, includes villi Submucosa - connective tissue (between the mucosa and muscle) Serosa - protective outer layer

Alimentary Canal is a smooth muscle

Muscles keep food moving through the alimentary canal. More specific, it is the smooth muscle that pushes food through the digestive system and it is controlled by autonomic nervous system (ANS), which are nerves that we do not voluntary control. There is two layer of smooth muscle: Circular and longitudinal These muscles help the movement of Peristalsis. Pushes food through the system. Happens in the stomach, Intestine and Esophagus.

Muscular layer

Muscular layer - inner circular and outer longitudinal muscle perform peristalsis

Human Digestive Enzymes

Remember: enzymes are specific to their substrates and each enzyme has its own optimum pH. Three main types of enzymes in human digestion:

Saliva

Salivary glandsWater, electrolytes, salivary amylase, mucus, lysosomes

Gastrin

Stimulates secretion of acid and pepsinogen by two types of exocrine glandsPart of stomach nearest to duodenum (more detailed NR: released by G cells, endocrine cells that are located in the gastric epithelium

oesophagus

a wave of muscle contractions (peristalsis) pushes the bolus into the stomach. Maltose (digested starch) + mucus = bolus Bolus leaves mouth and enter Esophagus. Trachea vs Esophagus Epiglottis block food from entering the lung. Inside Esophagus Peristalsis = smooth muscles contractions

Duodenum (small intestine)

bile from the liver and gall bladder neutralises acid and emulsifies fats. Pancreatic amylase and lipase digest carbohydrates and fats. Trypsin digests polypeptides to amino acids

Mouth

chewing (mechanical digestion) saliva moistens food to make a bolus for swallowing. Salivary amylase begins chemical digestion of starch. Food begins to be broken and enzymes begin the chemical digestion. Salivary glands: produce saliva that flows through ducts that lead from glands to free surface of the mouth. Saliva contains: 1. Salivary amylase - polysaccharide (starch) digestion (pH=7). Convert starch to maltose. 2. Bicarbonate - buffer keeping mouth pH close to neutral 3. Mucins - modified proteins that produces the bolus

Absorption and assimilation

digestion breaks down large food molecules into smaller molecules. Absorption is the uptake of these molecules into the blood. Once in the blood, they are carried to the tissues where they are assimilated taken in to be used Along with vitamins and minerals all products of digestion (monosaccharides, amino acids, fatty acids) are absorbed by the villi

Enzymes are essential in digestion

enzymes are biological catalysts- globular proteins that increate the rate of reaction by lowering activation energy. Digestive enzymes are released into the gut from glads are used in catabolic reactions- they break down larger molecules. By lowering the activation energy of the reaction, the reaction does not require high temperatures. This is ideal in living things- high temperatures would cause damage to cells and proteins. By using an enzyme reactions can occur more quickly at body temp.

Egestion

feces (containing undigested food, dead cells, and other waste, is forced out of the anus.

Enzymes digest most macromolecules in food into monomers in the small intestine.

large food molecules need to be digested before the nutrients can be absorbed. As food move through your body, many digestive enzymes are added along the way. Each enzymes is specific for a specific food type. Example: lipase→ lipid molecules amylase→ starch (amylose) Enzymes help catalyze hydrolysis reactions, speeds up the reaction without raising the normal body temperature 35oC, by decreasing the activation energy for the reactions to proceed Convert large macromolecules to smaller molecules that can be absorbed by the cell

Substrate (large molecule)

large molecules are usually insoluble, too large for diffusion across membranes in the blood.

Inactive pepsinogen vs. Active Pepsin

look at slides -Surface mucus and mucus neck cells-secrete mucus that is 1-3mmthick and forms a protective barrier that prevents active digestionof proteins in the stomach by pepsin

stomach

muscular contrations continue mechanical digestion acid kills bacteria, pepsin begins digestion of proteins. Stomach: breaks down only protein Secretion of Gastric Juice that consists of: Hydrocloridic acid - pH=2, gives acidic environment. Pepsin - hydrolysis of proteins (pH 1 to 3) Solvent is water Mucus- lines the inside of stomach wall for protection. Hydrochloric acid (HCl) lowers pH of the gastric contents to about 2. HCl causes the hydrogen and ionic bonds between folded proteins to be destructed. Making it easy for protease enzymes break peptide bonds. HCl kills pathogens (fungus, bacteria) in food. Low pH also stops salivary amylase promotes activity of pepsin. Pepsin is a hydrolytic enzyme that acts on proteins to produce peptides. Its precursor is pepsinogen, once exposed to HCl, it becomes active in form of pepsin. 1st step: pepsinogen + HCL → pepsin 2nd step: protein + pepsin (H2O)→ peptides The contents of the stomach leave as an acidic chyme (digested proteins, maltose, etc). This mixture is washed with bicarbonate (comes from pancreas) as it enters the small intestines. Once food leaves stomach, opening sphincter valve, it signals the stomach wall to stop releasing gastric juice. Secretin (hormone) is also released into blood to tell brain to lower activity of gastric pits release. Stomach wall contains glandulars cells that are exocrine glands. There are three types of Glandulars cells located at the pits extending down into the inner lining of the stomach. Parietal cell= HCl 2) Chief cell= pepsinogen 3) Mucus cell= mucus

Epithelial cells -

single outer layer of cells on each villus

Exocrine Glands secrete to the surface of the body or the lumen of the gut

●Passage of mouth to anus = ALIMENTARY CANAL●Digestive juices added to food at several points●Exocrine glands secrete the juices

Acid Conditions of the stomach

-Acid secreted by the PARIETAL CELL in the STOMACH-strongly acidic fluid of the stomach kills many microbes found in found-HCL partially denatures (and unfolds) proteins in fluid-stimulates secretion of hormones that promote flow of bile and pancreatic juice-Pepsinogen is activated to form Pepsin (active at pH 2)which breaks long polypeptides into shorter peptides

Adaptations of Villi

-Folds of inner membrane of ileum make up villi-each epithelial cell adheres tightly to its neighbours assuring materials pass into the blood vessels lining the villi -microvilli make up the brush border on the lumen side to increase SA for absorption-large # of mitochondria, generate large amounts of ATP for active transport-many pinocytic vessels due to absorption by endocytosis -Apical surface = surface facing lumen-basal surface = surface facing blood vessels-BOTH contain different types of PROTEIN involved in material transport

Know how to label digestive system

.. Main pathway: mouth, esophagus, stomach, small intestine, large intestine, rectum, anus. Accessory glands: salivary glands; pancreas; liver; gall bladder * no food passes through the glands, these glands only help the digestive system by secreting enzymes or chemicals to speed the process.

The digested glucose is absorbed and then transported to various body tissues

1.Glucose is co-transported* with sodium ions into the epithelial cells (of the villus).2.Glucose moves by facilitated diffusion into the lumen of the villus.3.Glucose then diffuses a short distance into the adjacent capillaries where it dissolves into the blood plasma.4.Blood in the capillaries moves to to venules then to the hepatic portal vein which transports the glucose to the liver.5.The liver absorbs excess glucose which it converts to glycogen for storage.

Starch consists of amylose (by 1,4 bonds) and amylopectin (by 1,4 bonds and occasional by 1,6 bonds)

Amylase breaks 1,4 bonds in chains of four or more monomers producing maltose Maltase digests maltose into glucose monomers Dextrinase breaks the 1,6 bonds that amylase cannot deal with forming glucose monomers

Cholera

Cholera is a disease caused by the ingestion of food or water contaminated with the bacterium Vibrio cholerae. When cholera toxin is released from the bacteria in the infected intestine, it binds to the intestinal epithelial cells (enterocytes), triggering endocytosis of the toxin.The toxin then becomes an active enzyme which activates ions and water to leave the infected enterocytes, leading to watery diarrhea. There are approximately 4 million patients with cholera in the world every year. Symptoms include vomiting, muscle cramps and profuse diarrhea. Given that the volume of water lost by the body is very large, this causes severe dehydration that can lead to death (approximately 100,000 per year worldwide). The toxin attaches to the ion receptors in the small intestine which causes it to secrete chloride ions into the lumen. This then causes water to move in by osmosis (as the salt level is now higher in the lumen of the intestine) which can cause diarrhoea and dehydration.

Ulcers

Crater like lesion in a membrane, peptic ulcers are a result in the GI tract.Typically caused by...1.H. Pylori (bacteria)2.NSAID anti-inflammatory drugs such as aspirin3.hypersecretion of HCl How to cure?1.Antibiotics2.Proton pump inhibitor drugs ○omeprazole Ulcers due to H. pylori are treated with a combination of medications: Amoxicillin: antibiotic that directly inhibits the synthesis of bacterial cell walls. Clarithromycin: antibiotic that prevents bacteria from growing by inhibiting the translation of peptides in the ribosome, thus inhibiting their protein synthesis. Proton Pump Inhibitor (PPI): inhibits acidification of stomach. A stomach ulcer is an open sore in the stomach wall, where digestive juices - mostly acid and the enzyme pepsin - have begun to eat away the stomach lining. 80% caused by infection from a bacterium called Helicobacter pylori (the remaining 20 per cent is caused by over-use of anti inflammatory drugs such as aspirin and ibuprofen). Bacteria produces an enzyme called urease, which neutralizes stomach acid by producing ammonia and allows the bacterium to colonize the stomach's mucous lining, opening up the stomach wall to attack from digestive fluids.

Active Transport

Glucose, amino acids and mineral ions Protein pumps use ATP to move molecules against the concentration gradient into the epithelial cells

Dialysis (visking) tubing can be used to model absorption

The tubing is semi-permeable and contains pores typically ranging 1 - 10 nm in diameterInitially contains a mixture of starch and glucoseTest the solutions inside and outside the dialysis tubing for starch and glucose before and after at least 15 minutes have elapsed (see the Practical Biology link for details).

Lipid digestion:

Triglyceride → glycerol + fatty acid Inside the alimentary canal, the fluid is aqueous, yet lipids are hydrophobic and this cause them to pack together as a coalesced globules. Its with the help of lipase and bile, that lipids are broken down to small particles. Bile has hydrophobic and hydrophilic sides, this makes them capable of move in aqueous solution and also interact with hydrophobic particles like lipid. Bile tries to block the coalescing into large globules, by emulsification, where the globules or droplets become small for easy digestion with lipase.

ileum (small intestine)

lower half of small intestine absorbs nutrients into the blood via villi. Types of transport: Fructose - facilitated diffusion down its concentration gradient from lumen into epithelial cells and then capillaries Amino acids, small peptides, vitamins, glucose are pumped against concentration gradient by epithelial membranes, pass through epithelium, enter capillaries and carried away by blood stream - active transport. Glycerol and fatty acids - absorbed by epithelium, recombined into fats, mixed with cholesterol, coated with proteins forming chylomicrons - transported by exocytosis out of the epithelial cells into lacteals, then lymphatic system and eventually into large veins. In contrast with lacteals, the capillaries and veins that drain the nutrients away from villi all converge into hepatic portal vessel (liver vessel) Liver has first access to amino acids and sugar absorbed after meals are digested - vessels leaving the liver have different internal balance

Peristalsis

moves food through the alimentary canal. In the small intestine peristalsis also mixes food with enzymes and forces the products of digesiton into contact with the wall of the intestine. Therefore in the intestines the food is moved very slowly to allow time for digestion. 1. Contraction of longitudinal muscle expand the lumen in front of the food giving it space to move into. 2. Contraction of circular muscles behind the food propels it forwards.

Pancreatic juice

pancreas Water, bicarbonate, enzymes including: amylase, lipase, carboxypeptidase, trypsinogen

Products

products of digestion are usually soluble, small enough for absorption into the blood and later assimilation in the tissues

Gastric juice Control of the Gastric Juice & secretion

stomachWater, mucus, enzymes including pepsin, rennin, HCl Any smell or sight of food initiate the secretion of gastric juice through autonomic nervous system, that tells your medulla oblongata. The medulla oblongata respond with parasympathetic nerve, through vagus nerve directly to stomach. Causing release of acid and enzymes. Once the food has entered the stomach, the stomach walls are stimulated and send sensory signals to the brain. This leads the brain to tell the stomach to secrete even more gastric juice. The distension (bloating) of the stomach cause production of gastrin, a hormone that causes more release of the gastric juice (HCl). Gastric juice is needed mainly for the digestion of protein by pepsin The stomach begins its production of gastric juice while the food is still in the mouth. Nerves from the cheeks and tongue are stimulated and send messages to the brain. The brain in turn sends messages to nerves in the stomach wall, stimulating the secretion of gastric juice before the arrival of the food. The second signal for gastric juice production occurs when the food arrives in the stomach and touches the lining. This mechanism provides for only a moderate addition to the amount of gastric juice that was secreted when the food was in the mouth.

Small intestine

the SI completes digestion of food molecules. Chyme enters the duodenum. Bile from gallbladder and liver is emptied into the DD neutralizing the acid and emulsifying fats. Pancreatic enzymes are released (amylase, trypsin) Enzymes are further released into jejunum. The ileum is the last stage of the small intestine. Here absorption of digested food molecules takes place. Villi (finger-like projections) increase the surface area for absorption and have rich blood supply.. A wave of muscle contraptions (peristalsis) keeps a mixture of digested and undigested food moving through the intestine. Small Intestine: site of food absorption. The first portion of the SI is duodenum, where all the necessary final breakdown of food into smallest particles are completed. The other two sites are jejunum and ileum. Small Intestine (also has its own enzymes): Maltase, sucrase and lactase: breakdown of disaccharides Nuclease: breakdown of nucleic acids Some enzymes of SI are added to the fluid in the lumen by the glandular cells, while some stay attached to the villi cells ex: maltase. Liver & gallbladder: Bile - body soap essential for the emulsification of fat. It is stored in the liver and gall bladder, acts in the small intestine. Pancreas: Trypsin= break small peptides Lipase= break fat Amylase= break sugar into monosaccharide Bicarbonate= neutralize the chyme from the stomach.

large intestine

water is reclaimed and returned to the blood, leaving semi-solid faeces. This is stored in the rectum. Major function of the large intestine is the absorption of water. It is also the home of the bacteria E. coli. We provide nutrients, water, and warm environment for bacteria, while they synthesize vitamin K and keep environment of the L.I. healthy for us. (mutualism). No, cellulose is not digested. Characteristics: Polysaccharide Tough material, insoluble Found in fibers, paper, fresh fruits, vegetables and leaves Most abundant organic compound on Earth No enzymes inside human body to hydrolyze cellulose Stimulates peristalsis, adds bulk (helps movement in SI Good for constipation, as it leaves the body by picking up some fat or other molecules. What leaves??? Some food are not digested, they end up leaving through your feces, they are: Cellulose- plants cell walls Lignin- plant cell walls Bile pigments- from bile Bacteria- live in your digestive tract Intestinal cells- break off as food move through the lumen

The pancreassynthesises the three main types of digestive enzyme:

•amylase to digest carbohydrates, e.g. starch•lipases to digest lipids, e.g. triglycerides•proteases to digest polypeptidesPancreatic juice containing the enzymes is released into the upper region of the small intestine (duodenum) via the pancreatic ductThe small intestine is where the final stages of digestionoccur.

ADDITIONAL NOTES: Adaptations of the Villus

●Each epithelial cell covering the villus adheres to its neighbours throughTIGHT JUNCTIONS → which ensure that most materials pass through blood vessels lining the villi●The collection of microvilli on the intestinal side of the epithelial cells is termed BRUSH BORDER → function = to increase surface area. Microvilli - Ruffling of epithelial membrane further increases surface area Rich blood supply - Dense capillary network rapidly transports absorbed products Single layer epithelium - Minimises diffusion distance between lumen and blood Lacteals - Absorbs lipids from the intestine into the lymphatic system Intestinal glands - Exocrine pits (crypts of Lieberkuhn) release digestive juices Membrane proteins - Facilitates transport of digested materials into epithelial cells