digestive system + absorption

• what is physical breakdown

  food is broken down into smaller pieces

  = increases surface area

  = eg. chewing, stomach churning
• what is chemical digestion

  Hydrolysis of large insoluble molecules into smaller soluble molecules

  = hydrolysis of covalent bonds

  = enzymes!
• what does carbohydrases break down

  carbohydrates into sugars

  = hydrolyse glycosidic bonds
• what does protease break down

  proteins (polypeptides) into amino acids

  = hydrolyse peptide bonds
• what does lipase break down

  triglycerides into fatty acids + glycerol

  = hydrolyse ester bonds
• what are the 2 types of amylase

  salivary and pancreatic
• what does amylase break down

  starch into maltose
• examples of disaccharides


  sucrose, lactose, maltose
• what is disaccharides digested by

  membrane bound disaccharidases
• where are disaccharidases found

  on the cell membrane of epithelial cells lining the small intestines

  = ie. not released into the small intestine lumen
• What does maltase break down?

  maltose→alpha glucose + alpha glucose
• What does sucrase break down?

  sucrose→alpha glucose + fructose
• What does lactase break down?

  lactose→alpha glucose + galactose
• examples of polysaccharides


  starch, glycogen, cellulose
• what are proteins digested by

  series of proteases which hydrolyse peptide bonds at different positions in the polypeptides
• endopeptidases


  hydrolyse peptide bonds in the central region of a polypeptide

  = forms shorter peptides

  = eg. pepsin (produced in the stomach)
• exopeptidase


  hydrolyse peptide bonds at the ends (acting on the terminal AA's) of polypeptides

  = forms dipeptides + single amino acids
• dipeptidases


  - hydrolyse peptide bonds between 2 amino acids of a dipeptide

  - dipeptidases are bound to the membrane of epithelial cells lining the ileum (small intestines)
• what are triglycerides made of

  Glycerol + 3 fatty acids.
• what are triglycerides digested by

  lipase which hydrolyse ester bonds= triglycerides→fatty acids + monoglyceride
• what is a monoglyceride

  one glycerol + one fatty acid tail
• what happens to lipids before digestion

  they are split into tiny droplets (micelles) by bile salts

  = emulsification

  = increases the SA of lipids so lipases can work quicker
• where is bile salt formed + stored

  formed in the liver + stored in the gall bladder
• what are micelles


  - tiny vesicles

  - water soluble
• how are micelles formed

  bile slats associating with fatty acids + monoglycerides

  = because bile salts have hydrophilic end sticking out, fat droplets don't stick to each other
• adaptations of a villus

  1. microvilli - epithelial cells on surface of villus is highly folded in microvilli

  = increases SA for faster diffusion/absorption

  
  

  2. epithelial cells are very thin

  = short diffusion distance

  = faster diffusion/absorption

  
  

  3. blood supply + capillaries close to surface

  = moving blood maintains a steep concentration gradient

  = faster diffusion/absorption

  
  

  4. lots of mitochondria

  = ATP production

  = provides energy for active tranpsort

  
  

  5. many ribosomes to produce carrier/channel proteins
• ileum (small intestines)


  absorption of digested food (glucose, AA's, fatty acids + glycerol) move into the blood by simple diffusion, facilitated diffusion + active transport- surface is covered in millions of tiny villi= increased SA for diffusion/facilitated diffusion/active transport= very long→increased SA + time for absorption to occur
• what is the role of diffusion

  there is a greater concentration of glucose/AA in the ileum than the blood

  = these molecules can move down the concentration gradient into the blood by facilitated diffusion
• what is facilitated diffusion

  The passive movement of a particle across a cell membrane via a channel/ carrier protein

  = changes shape to allow diffusion

  = opens if an ion is present
• role of active transport

  all glucose + amino acids should be absorbed into the blood

  = against the concentration gradient using co transport
• stage of co transport

  1. sodium ions (Na+) are actively transported from epithelial cell into blood by the sodium potassium pump + a carrier protein

  = requires ATP to hydrolyse ADP

  
  

  2. this reduces the conc. of Na+ in the epithelial cell + creates a conc. gradient for Na+ from ileum into epithelial cells

  
  

  3. Na+ moves into epithelial cell from ileum by facilitated diffusion + carries a glucose (or an AA) with it

  
  

  4. glucose (or AA) moves into blood by facilitated diffusion lowers a concentration gradient
• why is glucose carried not diffused

  against the concentration gradient
• what does active transport only occur with

  a carrier protein

  but facilitated diffusion can be a carrier or channel protein
• what are micelles


  - tiny structures in diameter

  - formed from fatty acids, monoglycerides + bile salts

  = make fatty acids more soluble in water
• what do micelles do in the ileum - absorption of triglycerides

  carry the fatty acids/monoglycerides to the epithelial cells lining the ileum

  = micelles break down, releasing monoglycerides + fatty acids

  = these are non polar so diffuse across the membrane into the epithelial cells
• what happens inside the cell - absorption of triglycerides

  - monoglycerides + fatty acids are transported to the SER where they recombine to form triglycerides

  - then travel to the Golgi apparatus, where the triglycerides associate with cholesterol + lipoproteins to form chylomicrons

  = they move out of the epithelial cells by exocytosis + enter the lymphatic capillaries called lacteal

  - chylomicrons pass via lymphatic vessels into the bloodstream

  - triglycerides in the chylomicrons are hydrolysed (products diffuse into cells) by an enzyme that is in the endothelial cells of the blood capillaries, which surround all tissues
• what are chylomicrons


  special particles adapted for the transport of lipids
• what is exocytosis


  vesicle fuses with the membrane + ejects contents from cell
• explain the advantages of lipid droplets and micelle formation (3)

  - Droplets increase surface areas for lipase / enzyme action

  - (So) faster hydrolysis of triglycerides / lipids)

  - Micelles carry fatty acids and monoglycerides to / through membrane / to cell
• how is the Golgi apparatus involved in the absorption of lipids (3)

  - modifies/processes triglycerides

  - combines triglycerides with proteins to form chylomicron

  - these are packaged for exocytosis