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Chapter 6: Physiology
6.1: Digestion and absorption
Structure of the wall of the small intestine allows it to move, digest and absorb food
Organ Function
Mouth ● Chewing (mechanical digestion): Saliva moistens food to make a bolus for swallow
● Salivary amylase begins chemical digestion of starch
Oesophagus ● Wave of muscle contractions (peristalsis) pushes the bolus into the stomach
Stomach ● Muscular contractions continue mechanical digestion
● Acid kills bacteria
● Pepsin (protease: breaks down proteins) begins digestion of proteins
○ Little chemical digestion
Duodenum ● Bile from the liver and gall bladder neutralises acid and emulsifies fats
(small intestine) ● Pancreatic amylase and lipase digest carbohydrates and fats
● Trypsin digests polypeptides to amino acid
Ileum ● Lower half of small intestine absorbs nutrients into the blood via the villi that
(small intestine) absorb monomers formed by digestion and mineral ions and vitamins
Large intestine ● Water is reclaimed and returned to the blood, leaving semi-solid feces.
○ Stored in the rectum
Egestion ● Feces ( undigested food, dead cells and other waste) is forced out of the anus
,Alimentary canal (directly transfers food): Accessory organ (supports digestive processes):
● Oesophagus: ● Salivary glands: Mointens food bolus
○ Food tract from mouth to stomach → Starch digestion
● Stomach: Storage with low pH ● Pancreas: Secrete enzymes (amylase, lipase,
→ protein digestion endopeptidase) into lumen of small intestine
● Small intestine: ● Liver: metabolises absorbed nutrients
○ Site of nutrient absorption → Produces bile
● Large intestine ● Gall bladder: stores and secretes bile
○ Absorbs water and dissolved mineral → Emulsifies fats
Human digestive enzymes break down:
Protease → Proteins (polypeptides) Ex: Pepsin (amino acids to polypeptide)
Lipase → Fats and lipids Ex: Pancreatic lipase (triglycerides to
fatty acids & glycerol)
Amylase → Carbohydrates Ex: Amylase (starch to maltose)
Small intestine
● Digestion of food molecules
● Chyme enters duodenum
● Bile from gallbladder and liver is emptied in duodenum,
neutralising acid and emulsifying fats
● Pancreatic enzymes are released (amylase, trypsin)
● Enzymes are further released in jejunum
● Ileum (last stage): Digested food molecules absorbed
● Villi increase surface area for absorption and have rich
blood supply
● Peristalsis: Wave of muscle contractions keeps mixture
of digested & undigested food moving through intestine
Pancreas: synthesizes 3 main digestive enzymes (amylase, lipase, protease). Pancreatic
juice contains all enzymes and is released in duodenum (start of small intestine) via the
pancreatic duct. Small intestine is where final stages of digestion occurs
Tips:
● Lumen = empty inner space in the body
● Chewing breaks down food so that there's more surface area exposure for enzymes to
break the food down
● Acid doesn't break down food: just provides optimum conditions for enzyme to do it
Segmentation:
● Bidirectional mixing of food in small intestine
● Caused by contraction of non-sequential circular muscle
, Peristalsis moves through the alimentary canal, wave of muscle contractions
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.
● Small intestine: peristalsis mixes food with enzymes and forces the products
of digestion into contact with the wall of intestine
● Hence, in intestines the food is moved slowly to allow time for digestion
Mechanical Digestion: Physical break down Chemical digestion: Digestion via chemical agents:
● Chewing (teeth) ● Stomach acids (low pH environment)
● Churning (squeezing stomach content) ● Bile (emulsification of fat into droplets)
● Segmentation (intestinal contractions) ● Enzymes (catalyse hydrolysis reaction)
Starch hydrolysis:
Starch is composed of glucose monomers and is linear (amylase) or branched (amylopectin)
● Amylase (salivary or pancreatic): Digests starch
○ Digests amylose in maltose disaccharides or amylopectin in dextrin chains
● Pancreas: Regulates uptake of glucose
○ Insulin increases glucose uptake by cells or glucagon decreases glucose
● Liver: Responsible for glucose storage
○ Glucose is stored as glycogen
Large food molecules to be digested before the nutrients can be absorbed (catabolic)
→ Hydrolysis through enzymes, which are globular proteins increasing reaction rate
Hydrolysis breaks down large organic molecules into monomers and requires enzymes
Large molecules are: Products of digestion are
● Usually insoluble ● Usually soluble
● Too large for diffusion across ● Small enough for absorption in blood
membranes into the blood and later assimilation in tissues
Starch, glycogen, lipids & nucleic acids digested in monomers but cellulose is undigested
4 tissues in small intestine from the lumen:
Mucosa: Inner lining with villi submucosa, connective
tissue between mucosa and muscle
Epithelial cells: Single outer cell of cells on each villus
, Absorption and assimilation
Absorbed components undergo assimilation in the cell to become fluid / solid part of organism
Many villi protrude in the lumen: Increase surface area for absorption of epithelial
● Microvilli on surface increase surface area more and single-cell layer = fast diffusion
● Lacteals (lymph vessels): Rapid absorption and transport of lipids (middle part)
● Capillaries close to epithelium: Short diffusion path, rich blood supply
● Rich blood supply: maintains concentration gradients between lumen and blood
Digestion: breaks down large food molecules into smaller molecules
Absorption: the uptake of these molecules into the blood
When molecules in the blood, they are carried to tissues to be
assimilated, taken in for use
Emulsification: breaks big fat blobs into lots of small fat
blobs to increase the surface area of the fats so that the enzymes can attack
Role of membrane transport:
Method of transport Nutrients Outline
Simple diffusion Lipids (triglyceride & Lipids are non-polar → can pass freely through
fatty acids) hydrophobic core of plasma membrane into the
epithelial cells (down the concentration gradient )
Facilitated Fructose, vitamins Water-soluble (hydrophilic) molecules use channel
Diffusion proteins to pass phospholipid bilayer and enter the
epithelial cells (down the concentration gradient)
Active Glucose, amino acids Protein pumps use ATP to move molecules against the
Transport and mineral ions concentration gradient into the epithelial cells
Endocytosis Antibodies from The plasma membrane folds inward to form vesicles
(Pinocytosis) breast milk to absorb larger molecules without digesting them
In small intestine: digestion of starch and transport of digestion products to the liver
● Starch consists of amylose and amylopectin
● Amylase breaks 1,4 bonds in chains of 4+ monomers producing maltose
● Maltase digests maltose into glucose monomers
● Dextrinase breaks 1,6 bonds that amylase can't deal w/ → Forms glucose monomers
Digested glucose is absorbed and then transported to various
body tissues:
1. Glucose is co-transported (active transport) with
sodium ions into 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.
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