3.3.3 Digestion and Absorption Flashcards
What happens during digestion?
Large biological molecules are hydrolysed to smaller molecules that can be absorbed across cell membranes
Molecules that are digested in mammals
- Carbohydrates - by amylases + membrane-bound disaccharidases
- Lipids - by lipase, including the action of bile salts
- Proteins- by endopeptidases, exopeptidases, and membrane-bound peptidases
Major parts of the digestive system
- Oesophagus
- Stomach
- Ileum
- Large intestine
- Rectum
- Salivary glands
- Pancreas
Oesophagus
Carries food from mouth->stomach
Stomach
- Muscular sac with an inner layer that produces enzymes
- Stores + digests food
- Has glands that produce enzymes that digest proteins
Ileum
- Long muscular tube
- Food is further digested here by enzymes that are produced by its walls and glands
- Inner walls are folded into villi (which have microvilli) = large S.A.
Why are villi and microvilli important?
Increases S.A. that adapts ileum for its purpose of absorbing the products of digestion into the bloodstream
Large intestine
Absorbs water
Rectum
- Final section of the intestines
- Stores faeces before being periodically removed via the anus
Egestion
Removal of faeces from rectum
Salivary glands
- In mouth
- Pass their secretion via a duct (contains amylase)
Pancreas
- Large gland situated below the stomach
- Produces pancreatic juice
What does pancreatic juice contain
Proteases
Lipase
Amylase
By what process do enzymes break down a large molecule into its products?
- Hydrolysis
Types of digestion
- Physical digestion
- Chemical digestion
Physical digestion
Large food is broken down into smaller pieces mechanically (by teeth and churning in the stomach)
Chemical digestion
Hydrolyses large, insoluble molecules into smaller, soluble ones
How do enzymes work
Multiple enzymes are needed to break down a large insoluble molecule
- 1 enzyme will break a large molecule into smaller sections
- Another enzyme will break these sections into its monomers
Types of digestive enzymes
- Carbohydrases
- Lipases
- Proteases
Carbohydrases
Hydrolyses carbohydrates -> monosaccharides
Lipases
Hydrolyse lipids -> fatty acids + monoglyceride
Proteases
Hydrolyses proteins -> amino acids
Carbohydrate digestion
- Amylase in mouth + pancreas: starch -> maltose (disaccharide). Does this by hydrolysing glycosidic bonds in starch
- Maltase in ileum: maltase -> alpha glucose
Process of carbohydrate digestion in humans
- Salivary amylase in mouth hydrolyses starch -> maltose
- Food enters stomach. HCl in stomach denatures amylase
- Food enters small intestine: mixes with pancreatic juice (contains pancreatic amylase)
- Muscles in intestinal wall push food along ileum (peristalsis).
- Maltase is membrane bound in the intestine. Maltose -> alpha glucose
Other common disaccharides that are digested:
- Sucrase -> hydrolyses single glycosidic bond in sucrose -> glucose + fructose
- Lactase -> hydrolyses single glycosidic bond in lactose -> glucose + galactose
Lipid digestion
- Lipases - hydrolyse ester bond in triglycerides -> fatty acids + glycerol
Monoglyceride
Glycerol + 1 fatty acid
Emulsification
- Bile salts split up lipids into micelles (tiny droplets)
- Surface area -> for lipase action
Protein digestion
Peptidases (proteases)
- Endopeptidases
- Exopeptidases
- Dipeptidases
Endopeptidases
Hydrolyse peptide bond between amino acids in the central region of a protein
Exopeptidases
Hydrolyse peptide bond on the terminal amino acids of the peptide molecule formed by the endopeptidases
Dipeptidases
Hydrolyse peptide bond between 2 amino acids of a dipeptide
Characteristic of dipeptidases + disaccharidase
Membrane-bound: part of the cell-surface membrane of the epithelial cells lining the ileum
Villi in the ileum
- Increase S.A. for diffusion
- Thin walls: short diffusion distance
- Contain muscle: so can move, which maintains diffusion gradient
- Good blood supply: maintains diffusion gradient
Absorption of amino acids + monosaccharides
Co-transport
Absorption of triglycerides
- Micelles (contain bile salts and fatty acids) come into contact with epithelial cells (lining the villi)
- Micelles break down -> releasing monoglyceride + fatty acids
- These are non-polar so easily diffuse across the cell-surface membrane into the epithelial cells by simple diffusion
- Inside the epithelial cells, the monoglycerides + fatty acids are transported to E.R. where they recombine to form triglycerides
- Triglycerides associate with cholestrol + lipoproteins to form chylomicrons
- Chylomicrons move out of the epithelial cells by exocytosis and move to cell membrane as vesicles
- They enter lacteals
- They pass into blood system
- Triglycerides are hydrolysed by an enzyme in the endothelial cells of blood capillaries
Lacteals
Lymphatic capillaries found at the centre of a villus
