bio factsheet- absorption in small intestine
TRANSCRIPT
Bio Factsheet
1
Number 140www.curriculum-press.co.uk
Absorption in the Small IntestineThis topic appears regularly in exams of all the Boards. This is not becauseit is particularly difficult – in fact it’s pretty straightforward - but because:
1. it can be used to illustrate the fundamental principle of the adaptationof structure to function;
2. Students keep confusing folds of the intestine with villi and microvilli.Some even refer to cilia!!
This Factsheet explains the basic principles underlying absorption anddescribes the commonest Questions and student errors.
Large food molecules such as proteins cannot be absorbed; they are too bigand they are insoluble. Thus, digestion breaks them down into smaller andsoluble molecules which can be absorbed.
90% of all absorption occurs in the small intestine, the remaining 10%occurs in the stomach and large intestine.
The first thing to do is to get an overview of what is going on. The productsof digestion are absorbed across the epithelial cells of the small intestine.Substances such as glucose and amino acids then pass into the capillariesin the villi. From there they are passed to the mesenteric veins and then thehepatic portal vein which delivers them to the liver. The liver convertsexcess glucose into glycogen and breaks down or deaminates excess aminoacids. The amine groups pass to the kidney for excretion.
Chylomicrons (triglycerides combined with cholesterol and phospholipid)leave the epithelial cells and pass into lacteals rather than blood vessels.The lacteals eventually empty into blood vessels and once there thechylomicrons are broken back down into fatty acids and glycerol whichcan then enter cells for lipid synthesis.
These processes are summarised in Table 1 and Fig 2.Fig 3. overleaf summaries all of the ways in which the small intestine isadapted for absorption. - this is the most common exam question in thistopic - learn it!
Food substance
Glucose
Amino acids
Short fatty acids
Long fatty acids and glycerol
Absorption into epithelial cell
Binds to sodium and is pulled inacross a carrier protein
Binds to sodium and is pulled inacross a carrier protein
Diffusion
Diffusion
Absorption into blood capillary
Facilitated diffusion
Facilitated diffusion
N/a
N/a
Absorption into lacteal
Diffusion
3 fatty acids plus a glycerol moleculecombine to form a triglyceride.Triglycerides combine with cholesteroland phospholipid to form chylomicronswhich pass into lacteals by exocytosis
Table 1
Fig 2. Absorption accross epithelial cells
passivediffusion
water,small water-solublesubstances (e.g. glycerol)lipid-soluble substances(e.g. fatty acids)
glucoseamino acids
activetransport
epithelilal cell
Tis
sue
flui
d
Tis
sue
flui
d
capillary
transport in blood plasma
glucose, amino acids, otherwater soluble substances
lacteal
susp
ensi
on o
f ch
ylom
icro
ns→
lym
ph f
luid
"m
ilky
whi
te"
transport in lymphcholesterol, fattyacids,resythesisedtriglycerides,chylomicrons
fatty acids + glycerol arerecombined in the epithelial cellsto from triglycerides which arecoated in protein and combinedwith phospholipid and cholesterol
chylomicrons
nucleus
Bio Factsheet
2
140 Absorption in the small intestinewww.curriculum-press.co.uk
small intestine
mucosa: folded inner
wall w
ith villi
columnar epithelial cells secrete
mucus and alkaline fluid to protect
the epithelia from m
echanical injuryand stom
ach acid
columnar
epithelial cell
microvilli
wall of intestine is folded to increase surface
area for absorption
SUR
FAC
E OF IN
TE
STIN
AL W
AL
L
villi
villi
villivilli
0.5 - 1.0 mm
Crypt of L
ieberkühn:secretion of ions and w
ater
20 - 45 villi/mm
2 which have
den
se sy
stem
of
blo
od
capilleries and lacteals to absorbproducts of digestion
epithelium - one cell thick so rapid
absorption of water, glucose,
dipeptides, amino acids and ions such
as Na
+ etc.
carrier proteins e.g forabsorption of glucose,am
ino acids
mitochondria
provide AT
P for activetransport of food m
olecules
rough endoplasmic reticulum
ribosomes are site of enzym
e synthesis
the epithelial cells haveenzym
es such as maltase
and dipeptidases embedded
in their surface - digestioncan occur right next to them
!
smooth endoplasm
ic reticulumcan detoxify organic m
olecules that areabsorbed
blood capillary - absorb glucose , amino acids
etc. from w
here they are transported tom
esenteric veins →liver via hepatic portal vein
venu
le
arteriole
muscle - m
oves the contents ofintestine along bringing digestedm
aterial to the villi
microvilli
to increase surfacearea
lacteal - absorbs chylomicrons (triglycerides
combined w
ith cholesterol and phospholipid)and individual fatty acids. T
he bile saltsem
ulsify the fats - breaks them into droplets
so their surface area is increased - speeding upthe action of lipases
circular muscle
longitu
din
alm
uscle
blood vesselnerve plexus
Acknow
ledgements:
This Factsheet w
as researched and written by K
evin Byrne.
Curriculum
Press, B
ank House, 105 K
ing Street, Wellington, Shropshire, T
F1 1N
U.
Bio F
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permission of the publisher. ISSN
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single epithelial cell
lymph duct
Structure to function: absorption in the ileum
muscles in w
all of intestinecontract and relax enabling villito m
ove from side to side so
increase amount of m
oleculesthey com
e into contact with
and can absorb