transport across cell membranes Flashcards
stages of carb breakdown (simple)
1) amylase (starch) : starch -> maltose
2) maltase: maltose -> α-glucose
products of carbohydrate break down
maltose -> glucose
main carbs (2)
starch
glycogen
how is glucose absorbed
1) facilitated diffusion of glucose into epithelial cell of villa
2) glucose diffuses through the cell down concentration gradient
3) facilitated diffusion into capillary (initiated by active transport by Na+/K+ ion pump)
where is maltase?
- type of enzyme?
membrane-bound enzyme - in cell surface membrane of epithelial cells
- membrane-bound disaccharidase
stages of carb break down
- saliva enters mouth - mixed salivary amylase with food
- salivary amylase: carbs -> maltose
(mineral salts in saliva keep pH around neutral - optimum temp for salivary amylase) - [swallowed] acid conditions in stomach denature salivary amylase - prevent further hydrolysis of starch
- [into s. intestine] - mixed w. pancreatic juice
- pancreatic amylase - hydrolysis of remaining starch -> maltose (alkaline slates keep pH neutral)
- muscles push food along ileum - membrane-bound maltase
- maltose -> α-glucose
endopeptidases
hydrolyse peptide bonds between amino acids in central region
exopeptidase
hydrolyse peptide bonds between terminal amino acids
dipeptidases
hydrolyse bond between 2 amino acids of dipeptides
adaptations of ileum (4)
- increased SA (villi, microvilli) for diffusion
- very thin-walled = short diffusion distance
- contain muscle - can maintain diffusion gradient because movement mixed contents
- well supplied with blood vessels to transport away absorbed molecules and maintain diffusion gradient
functions of surface proteins on plasma membrane (2)
- mechanical support
- act as cell receptors for molecules eg. hormones
functions of proteins spanning the plasma membrane
+ what are they
- channel proteins
- carrier proteins
movement of molecules across membrane
functions of plasma membrane (5)
- structural support
- act as receptors
- form cell-surface receptors for identifying cells
- help adhere cells together
- control entry and exit of substances to the cell
functions of cholesterol in plasma membrane (4)
- adds structural strength
- v. hydrophobic - prevents waterless + dissolved ions from leaving
- pulls together fatty acid tails of phospholipid molecules = makes membrane rigid and limits movement - reduces lateral movement of other molecules
- makes membrane less fluid at high temps
functions of glycolipids in plasma membrane (3)
- recognition site
- maintain stability of membrane
- help cells to attach to eachother and form tissues
what is a glycolipid
carbohydrate covalently bonded to lipid
situation of glycolipid on phospholipid bi-layer
carbohydrate portion extends from bilayer into environment outside cell
functions of glycoproteins in plasma membrane (3)
- recognition sites - hormones and neurotransmitters
- help cells to attach to eachother and form tissues
- allows cells to recognise each other - eg. lymphatic cells recognising organisms own cells
why dont molecules generally diffuse across plasma membrane? (4)
- not lipid soluble - cant pass through phospholipid bilayer
- too large to pass through channel proteins
- of same charge as channel proteins - even if they are small enough they are repelled
- electrically charge/polar - cant pass through hydrophobic tails of bi-layer
why fluid mosaic model?
fluid - individual phospholipid molecules move relative to eachother - flexible structure
mosaic - proteins embedded vary in size and shape
ways facilitated diffusion across a membrane occurs
- channel proteins
- carrier proteins
channel proteins
- water-filled, hydrophilic channels across the membrane
- selective - open in presence of specific ion - binds to protein, closing it to one side and opening to another
carrier proteins
- molecule binds to it, changing shape, opening to one side and closing to the other
how are amino acids and glucose absorbed?
diffusion and co transport
how are triglycerides absorbed?
- monoglycerides and fatty acids reman associated with bile salts in micelles
- micelles come into contact with epithelial cell lining of villi (due to movement of material in lumen) causing micelle to break down and release monoglycerides and fatty acids
- as are non-polar they diffuse easily across membrane
what happens to products of lipid absorbtion once inside epithelial cells?
- transported to endoplasmic reticulum where are reocmbined to make triglycerides
- continuing in Golgi body, are combined with cholesterol and lipoproteins to make CHYLOMICRONS
- chylomicrons move out of cell by exocytosis
- enter lymphatic capillaries
- pass into blood via lymphatic vessels
- triglycerides in chylomicrons are hydrolysed by an enzyme in endothelial cells of capillaries, then diffuse into cells
chylomicron?
special particles adapted for transport of lipids
how are proteins digested [4]
- hydrolysis of peptide bonds by peptidases
- endopeptidases break polypeptides into smaller polypeptide chains
- exopeptidase hydrolyse bonds between terminal amino acids producing amino acids
- dipeptidases hydrolyse bond between 2 amino acids in dipeptide, releasing amino acids
how is glucose absorbed (extended - incl. role of Na+)
- NA+ is actively transported out of epithelial cell into blood by Na+/K+ ion pump creating concentration gradient through cell
- Na+ diffuses down conc gradient into cell through protein carrier (co-transport protein) bringing glucose with it
- glucose diffuses through cell into blood by facilitated diffusion using another type of carrier protein
