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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actsheets may be reproduced, stored in

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itted, in any other form or by any other m

eans, without the prior

permission of the publisher. ISSN

1351-5136

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