digestion and absorption

what does acquiring nutrients involve?

1. ingestion

2. physical digestion

3. chemical digestion

4. absorption

5. assimilation

digestion

the process in which larger insoluble molecules are hydrolysed by enzymes into smaller soluble molecules, which can be absorbed and assimilated

ingestion

food enters the mouth

physical digestion

breaking apart large fragments of food (teeth, stomach wall)

ingestion and physical digestion

provides a large SA for chemical digestion

chemical digestion

• enzymes are specific - more than 1 is required to hydrolyse a large molecule

• carbohydrases - hydrolyse carbohydrates into monosaccharides

• lipases - hydrolyse lipids into glycerol and fatty acids

• proteases - hydrolyse proteins into amino acids

carbohydrate digestion

• saliva enters the mouth via salivary glands, and contains salivary amylase

• amylase hydrolyses starch into maltose at a neutral pH

• food is swallowed and enters the stomach

• acidic conditions denature the amylase, preventing further hydrolysis

• food reaches the small intestine, and is mixed with pancreatic juice

• pancreatic amylase continues the hydrolysis of starch to maltose at a neutral pH

• food enters the ileum

• maltase is a membrane-bound disaccharidase (part of the cell surface membranes of epithelial cells lining the ileum)

• maltase hydrolyses maltose to alpha-glucose

where is sucrose found?

in natural food including fruits

where is lactose found?

in milk

what do amylase, maltase, sucrase, and lactase all do?

hydrolyse the glycosidic bonds

monosaccharide absorption

1. Na+ actively transported, maintaining a concentration gradient between lumen and epithelial cell

2. co-transport of Na+ and glucose via a carrier protein

3. glucose enters the blood from the epithelial cell via facilitated diffusion (absorption)

are fatty acids hydrophobic or hydrophilic?

hydrophobic

protein digestion

digested by peptidases (proteases)

• endopeptidases

• exopeptidases

• dipepidases

• the action of endopeptidases to split the one chain up into multiple chains increases the number of ends for exopeptidases to work on → this increases the rate of digestion

endopeptidases

hydrolyse peptide bonds between specific amino acids in the middle of a polypeptide

exopeptidases

hydrolyse peptide bonds between specific amino acids at the terminal ends of a polypeptide

dipeptidases

hydrolyse the peptide bond in a dipeptide

lipid digestion

• digested by lipases → hydrolyse ester bonds to form fatty acids + monoglycerides

• emulsification: bile salts emulsify lipids into tiny droplets called micelles → increases SA for lipases

• micelles brush against the villi of the ileum, breaking it down

• non-polar monoglycerides and fatty acids diffuse freely through the membrane and into the cell

• monoglycerides and FAs are transported to the ER, where they’re reformed into triglycerides

• transported to the golgi, packaged with cholesterol and other lipoproteins to form chylomicrons

• exocytosis of chylomicrons into lacteal vessels that form part of the intestinal lymph system

• chylomicrons pass via lymphatic vessels into the blood stream

• triglycerides in the chylomicrons are hydrolysed by lipase enzymes in the endothelial cells of the blood capillaries where they diffuse into cells

where are lipases produced?

in the pancreas

monoglyceride

glycerol + 1 fatty acid

where are bile salts produced?

liver

explain the function of ATP hydrolase

1. hydrolyses ATP to ADP + Pi

2. releases energy

3. energy allows active transport of ions

lipid absorption

• micelles carry fatty acids to the cell membrane

• fatty acids are absorbed and enter the cell via diffusion

• triglycerides reformed in the golgi apparatuses where they’re modified and proteins are also added

• golgi apparatus forms vesicles

• vesicles move to the cell membrane to enter lymph capillaries (exocytosis)

• chlyomicrons pass via lymphatic vessels into the blood system

• triglycerides in the chylomicrons are hydrolysed by lipase enzymes in the endothelial cells of capillaries where they diffuse into cells

micelles

• contain bile salts and fatty acids

• make fatty acids more soluble in water

• bring fatty acids to lining of the ileum

• maintain higher concentration of fatty acids to lining of the ileum

• formed of monoglycerides

how are fatty acids absorbed?

by diffusion

describe the process of facilitated diffusion

movement of polar molecules down a concentration gradient via channel protein

describe the process of active transport

movement of polar molecules against a concentration gradient via carrier protein using ATP

what are microvilli?

highly folded cell-surface membrane