3.3.3 Digestion and absorption Flashcards
What occurs during digestion?
Large biological molecules are hydrolysed to smaller molecules that can be absorbed across cell membranes
What do these produce when hydrolysed:
- lipids
- polysaccharides
- proteins
- starch
- lipids
- mono/disaccharides
- amino acids
- alpha glucose
What are the main 2 steps of digestion?
what occurs in step 1 ?
- PHYSICAL BREAKDOWN- no hydrolysis, inc SA.
food is broken down into small pieces by teeth.
food is churned by stomach muscle wall .
breakdown increases the SA of the food for chemical reactions
what occurs in step 2?
- CHEMICAL DIGESTION- catalysed by enzymes
- can be intracellular/extracellular
- all enzymes function via hydrolysis
Examples of enzymes.
- Amylase
- Membrane bound disaccharidases
- Lipases
- Endopeptidases
- Exopeptidases
- Membrane bound dipeptidases
Where are the smaller molecules absorbed?
Through the lining of the small intestine (ileum) into the blood
What substances are egested and why?
cellulose as it cannot be digested by human gut
so it passes out through anus
with bacteria, cells from gut lining
all egested as faeces
5 steps of digestion.
- INGESTION taking food into body
- DIGESTION physical, inc SA. chemical-hydrolysis
- ABSORPTION movement of molecules from lumen of the small intestine into the blood
- ASSIMILATION use of products made during digestion
- EGESTION removal of undigested material
what are the 2 parts of the small intestine? 2 functions of small intestine?
1st duodenum
2nd ileum
chemical digestion and absorption of products
What enzyme does the salivary gland secrete?
amylase
what does the liver do?
bile production
what does the gall bladder do?
stores bile
what is gastric juice?
pepsin (digests protein and HCL
what does the duodenum do?
connects stomach and small intestine together
what does the rectum do?
stores faeces
structure and function of stomach:
- 3 layers of muscle to churn food and mix it with gastric juices
- layer of mucus to prevent damage from gastric juices
- PH 1/2 opt pH for pepsin
- kills most bacteria swallowed with food
- digests protein
- absorbs water, alcohol and aspriri (simple molecules)
absorption of glucose
can be absorbed as soon as it reaches small intestine as it is small enough to be carried to the cell surface membrane of the cells that line intestine and enter blood
why cannot starch be absorbed, what occurs instead?
starch is large and insoluble
so it is converted into maltose then glucose
3 main enzymes for digestion of carbohydrates
salivary amylase
pancreatic amylase
maltase (mrmbrande bound)
step 1 of digestion of carbohydrates
SALIVARY AMYLASE starch- maltose
- saliva contains mineral salts to maintain PH
- PH 7 as its opt for salivary amylase
- short exposure
step 2 of digestion of carbohydrates
PANCREATIC AMYLASE starch- maltose
- food passed from stomach into small intestine
here it mixes with pancreatic juice (contains amylase) to continue hydrolysis of remaining starch to maltose
- alkaline salts maintain ph 7 so amylase can function
step 3 of digestion of carbohydrates
MALTASE maltose- glucose
- occurs in small intestine
- maltase is membrane bound and located in plasma membrane of epithelial cells of small intestine
- maltase hydrolyses maltose into 2 glucose
disaccharidases
MEMBRANE BOUND
- lactase, sucrase, maltase
- located in plasma membrane of epithelial cells lining epithelial cells
why are disaccharidases membrane bound?
so enzyme is not lost
shorter distance
efficient absorption
faster
Products of carbohydrate digestion.
glucose in lumen of small intestine
need to be moved into bloodstream
adaptations of small intestine, ileum.
- rich blood supply to maintain conc gradient
- short diffusion pathway, thin walled
- large SA and microvilli increase SA for absorption
how does glucose reach the blood from the lumen?
COTRANSPORT PROCESS
as carbohydrates are being digested continuously so there is a greater coc of gluc in small intestine than blood
blood is constantly being circulated so gluc is removed quickly which helps maintain the gradient
Digestion of proteins, where does it start?
starts lumen of stomach, small intestine and cytoplasm of cells
poteases- endopeptidases, exopeptidases and dipeptidases used
digestion
digestion of proteins step 1
ENDOPEPTIDASES
- hydrolyse peptide bonds in midlle of polypetide and chunk it
- stomach, endopeptidases and hcl. low ph
digestion of proteins step 2
EXOPEPTIDASES
- small intestine, contains pancreatic juice which neutralises acid mixture
- exopeptidases in this hydrolyse end of polypeptide chain forming dipeptides
digestion of proteins step 3
DIPEPTIDASES
- cell surface membrane of epithelial cells
these hydrolyse the dipeptides and release amino acids into cytoplasm of cells
absorption of proteins
COTRANSPORT INSTEAD OF GLUCOSE IT IS AA
in the membrane of epithelial cells of small intestine there are amino acid carrier proteins which rely on sodium ions
absorption of proteins steps
- NA+ out cell via AT into blood
- AA ad NA+ bind to carrier protein
- As Na+ diffuse into epithelial cell from the lumen of gut AA carried in too
- AA diffuse to the other end of the cell and are transferred into capillaries by FD
Lipid digestion site.
LUMEN OF SMALL INTESTINE
Physical breakdown of lipids
- in stomach solid lipids are churned into fatty droplets
- fatty liquid enters 1st part of small intestine, bile from gall bladder which is connected to the liver is secreted
- in small intestine bile salts bind to fat droplets and break them down into smaller fat droplets. EMULSIFICATION this increases SA for lipase
Chemical digestion of lipids
- lipase (secereted by pancreas) hydrolyses lipid into fatty acids and glycerol, monoglycerides
- MISCELLES (hydrophobic/poorly soluble) formed then transport these to surface of epthelial cells where they can be absorbed
- FA,G,M can diffuse through phospholipid bilayer of epithelial cell (nonpolar)
- short FA chains can diffuse directly into blood from lumen of small intestine via epthelial cells
- long FA chains, glycerol and monoglycerides diffuse into epthelial cells where they recombine in the smooth ER to form triglycerides
- triglycerides are packaged with cholesterol and lipoprotein to form watersoluble fat droplets (chylomicrons)
- chylomicrons enter the lacteal and are transported away from the intestine and eventually enter bloodtream
How are miscelles formed and what is inside them?
phospholipids and bile salts associate
fatty acids, glycerol and monogylcerides
Why does lactose intolerance occur?
due to deficiency in the production of lactase
What is primary lactase deficiency?
- young babies only have milk so produce lots of lactase however milk production becomes a smaller part of diet so lactas production naturally decreases
- in some people lactase reduction is so great they end up deficient
What is secondary lactase deficiency?
- lactase production decreases due to damage to the small intestine through injury or disease e.g coelic , crohns and inflammatory bowl disease
Symptoms of lactose intolerance?
- lactose remains undigested, cannot be absorbed into bloodstream
- high conc of lactose so low water potential so water drawn in via osmosis to the bowel which leads to osmotic diarrhea
- bacteria living in gut feed on lactose producing gas and acids which irritate bowels causing cramps
How to control lactose intolerance?
diet, avoid lactose rich foods.
why can lactose intolerant people consume yogurt?
bacteria digests lactose
what does large intestine do?
absorb water
Where is amylase produced and where is it released to?
salivary glands- mouth
pancreas- small intestine
where are lipases produced and where do they work?
pancreas and small intestine
how does the villi increase efficiency of absorption?
- inc SA for diffusion
- very thin walls, short diffusion pathway
- micro villi further inc SA of villi
- well supllied with blood vessels so can carry away absorbed molecules and maintain diffusion gradient
what happens when monoglycerides and fatty acids have been absorbed?
they are transported to ER where they are recombined to form triglycerides, then associating with cholestrol and lipoproteins in the golgi to form chylomicrons
what are chylomicrons?
special particles adapted for the transport of lipids
what happens to chylomicrons?
chylomicrons move out epithelial cell via exocytosis and enter the lacteal and are transported away from the intestine and eventually enter bloodtream
where are bile salts made?
liver
name 2 ways the concentration gradient between the inside of the ileum and blood is maintained?
- constant circulation of blood
- ileum has muscles which uses peristalsis which keeps food moving along the gut
Explain presence of of bile in the colon (large intestine) ?
- secreted by liver
- bile passes unchanged from small intestine to colon
suggest how chylomicrons leave epithelial cells and why?
exocytosis as it is too large to leave by other methods
suggest an advantage of having muscle cells in the villi?
enables villi to move and increases contact with food
