27. Regulation of Digestion Flashcards
Why is digestion necessary?
- Major food nutrients are large macromolecules which cannot pass through the lining of the intestine
- They must be broken down to small molecules that can pass through cell membranes
What does physical digestion involve?
The physical breakdown of food into small particles by grinding or chewing to increase surface area
What is enzymatic digestion?
The breakdown of complex molecules by hydrolytic enzymes usually secreted into the gut lumen
What does physical digestion require?
Jaws and strong jaw muscles
- Teeth of different types
What are the three types of teeth?
Incisors, canines, molars and premolars
What is the first component of the digestive tract?
The oral cavity
What is the purpose of the oral cavity?
Physical breakdown or mystification of ingested food
Why is the physical breakdown of food in the oral cavity necessary?
Necessary for efficient chemical digestion (increases surface area)
What enzyme is involved in the oral cavity?
Salivary amylase which is secreted from salivary glands acting on the substrate (starch/glycogen) to produce maltose (end product)
What is the second component of the digestive tract?
The oesophagus
What does the oesophagus do?
It is a muscular tube which transports food through the thorax and diaphragm to the stomach
What is the epiglottis?
A covering that blocks the trachea so that food goes straight down into the stomach
What are the two sphincters?
The lower esophageal sphincter and the pyloric sphincter
What is the purpose of the sphincters?
They are control points to ensure food enters the stomach at the correct point and exists when it needs to (no back flow or premature entry)
What is the lower oesophageal sphincter between?
Oesophagus and the stomach
What is the pyloric sphincter between?
Stomach and small intestine
What are the three types of cells in the gastric pit in the stomach?
Chief cells, parietal cells and epithelial cells
What do chief cells do?
They secrete pepsinogen
What do parietal cells do?
Produce hydrochloric acid
What do epithelial cells do?
Secrete mucus which protects the tissues from the acids and enzymes
How is pepsin secreted?
- Chief cells secrete pepsinogen, the inactive form or zymogen of the protease pepsin
- The low pH of the stomach converts it to the active form
- Pepsin activates other pepsinogen molecules in a process called autocatalysis
What is the third component of the digestive tract?
The stomach
What component of the stomach churns food?
Thick muscular walls churn food
What is the substrate and end product of the enzyme pepsin?
The substrate is polypeptide bonds and the end product is large peptides
What is the fourth compartment of the digestive tract?
The small intestine
What are the three different regions of the small intestine?
Duodenum Jejunum and ileum
What is the purpose of the small intestine?
- Moves chyme forward from the stomach
- Continues enzymatic digestion with secretions from cells in its walls and from accessory digestive glands
- Absorbs products of digestion through villi
What does the gallbladder do?
Stores bile which aids in digesting lipids
What does the liver do?
Synthesises bile salts from cholesterol and secretes them as bile
What does the pancreas do?
Produces digestive enzyme and bicarbonate solution
What is the purpose of bicarbonate?
Bicarbonate is required to neutralise the pH of the chyme released from the stomach
What is the pathway of the movement of bile from the liver?
Bile flows through the HEPATIC DUCT to the DUODENUM and through the CYSTIC DUCT to the GALL BLADDER where bile is stored
What does bile do?
- Bile salts emulsify fats in the chyme
How are micelles formed?
- One end of the bile salt molecule is lipophilic and the other end is hydrophobic
- The lipophilic ends merge with fat droplets and keep them from sticking together
- This enlarges the surface area exposed to lipase that digest fat
What is the action of lactase?
Breaks down lactose into galactose and glucose
What is the action of sucrase?
Breaks down sucrose into fructose and glucose
What is the fifth compartment of the digestive tract?
The large intestine (colon)
What is the function of the large intestine?
- Absorption of water and inorganic ions
- Formation and storage of faeces from undigestable material. These are periodically excreted from the rectum
What nervous system controls the constriction of the muscles in the large intestine?
Enteric nervous system
What are the names of the two layers of smooth muscle that are outside the submucosa?
The circular muscle layer and the longitudinal muscle layer
What is the circular muscle layer?
Innermost cells oriented around the gut which CONSTRICT the gut
What is the longitudinal muscle layer?
Outermost cells oriented along the gut which shorten the gut
How does the absorption of breakdown products of carbohydrates occur in the small intestine?
- Carbs are absorbed as monosaccharides into the capillaries of the villi
- Glucose is taken up by active transport
- Blood vessels drain into hepatic portal vein which carries blood to the liver
- The liver converts carbs into glycogen which is stored in the liver
How does the absorption/breakdown of proteins occur in the small intestine?
- Absorbed into the capillaries as amino acids
- Taken up directly by cells which are synthesising proteins
- Excess is deaminated by the liver which uses nitrogen to form urea for excretion
Which breakdown products of the small intestine may be absorbed directly into underlying blood vessels irrespective of need?
Water salts and alcohol
- Excess excreted by kidneys
How does the absorption of fats occur in the small intestine?
- Fats are absorbed as glycerol and neutralised fatty acids into lacteals and then via lymphatics back to major veins near the heart
- Excess stored in adipose tissue
What is the role of gastrin?
- Food entering the stomach stimulates release of gastrin hormone from the stomach mucosa into the blood
- Causes the release of HCl and pepsin from the stomach mucosa
What is the role of secretin?
- Acid in duodenum causes the release of secretin from intestinal mucosa
- This causes the pancreas to release bicarbonate and the gall bladder to release bile
