The Carbohydrates.
• BIOL1: Carbohydrates (6)
• Learning Outcomes: 3.1.2.
• 3.1.2 The digestive system provides an
interface with the environment. Digestion
involves enzymic hydrolysis producing
smaller molecules that can be absorbed and
assimilated.
Carbohydrate digestion:
Within this unit, carbohydrates should be
studied in the context of the following
• Starch, the role of salivary and pancreatic
amylases and of maltase located in the
intestinal epithelium
• Disaccharides, sucrase and lactase.
Biological molecules such as carbohydrates and
proteins are often polymers and are based on
a small number of chemical elements.
Monosaccharides are the basic molecular units
(monomers) of which carbohydrates are
composed.
• The structure of a-glucose as a-glucose and
the linking of a-glucose by glycosidic bonds
formed by condensation to form maltose and
starch.
• Sucrose is a disaccharide formed by
condensation of glucose and fructose.
• Lactose is a disaccharide formed by
condensation of glucose and galactose.
• Lactose intolerance.
• Biochemical tests using Benedict’s reagent for
reducing sugars and non-reducing sugars.
Iodine/potassium iodide solution for starch.
Small molecules into large ones
• Carbohydrate:
• Carbo = carbon molecule.
• Hydrate = combined with water.
• Carbon (and hydrogen) containing molecules
are organic molecules.
• Carbohydrates are like proteins and are made up of
chains of individual monomer components. Join
together to form a polymer.
• Single monomer = monosaccharide.
• 2 monosaccharides joined = disaccharide.
• 3 or more = polysaccharide.
• Elements in carbohydrates = Carbon, Hydrogen and
Oxygen.
Their general formula is (CH2O)n.
The Monosaccharides.
• Sweet tasting, soluble substances, that have
the general formula (CH2O)n
• n can be any number from 3 to 7.
• Three most common groups of
monosaccharides are shown.
C3H6O3
n = 3 = = a triose
e.g. Glyceraldehyde
n=4 = tetrose
C5H10O5
n = 5 = pentose
e.g. Ribose or Deoxyribose, (make up RNA & DNA).
C6H12O6
n = 6 = hexose
e.g. Glucose, Fructose (fruit sugar) or Galactose.
n =7 = heptose
The best known monosaccharide is
glucose C6H12O6.
• They are often drawn in
a linear form.
• What is assumed to be
at the end of the empty
bonds?
But the atoms actually form a ring.
• The arrangement of the atoms of carbon, hydrogen
and oxygen can vary to give alternative hexose
sugars such as Fructose or Galactose.
HEXOSE = 6 CARBONS not to do with shape!
• Also there are different ring forms of glucose known
as α- glucose and β-glucose.
α-glucose
Carbon molecules
are numbered
from the
molecules left
side.
In alpha-glucose
the hydroxyl (OH)
on carbon number
one is DOWN.
β-glucose
Beta-glucose.
The hydroxyl
group on
carbon number
1 is UP.
The hydroxyls
alternate “up
and down”
around the
molecule.
POLYSACCHARIDES are insoluble and
therefore OSMOTICALLY stable.
• α glucose combine = starch or glycogen
[polysaccharides used for energy storage].
• Β glucose combine = cellulose [ Structural
Polysaccharide].
Disaccharides
• Monosaccharide + Monosaccharide = Disaccharide.
• Soluble and sweet tasting.
• Formation of a glycosidic bond is between adjacent carbon
atoms.
• Two hydroxyls align and a molecule of water is lost =
condensation reaction.
• An oxygen atom is left to join the two molecules of
glucose.
• The bond = Glycosidic bond.
If water is added to a disaccharide, it breaks the
glycosidic bond and forms 2 monosaccharides.
= hydrolysis reaction.
Now reverse this process to show HYDROLYSIS
of a glycosidic bond.
Types of disaccharide:
GLUCOSE + GLUCOSE = Maltose - the sugar from grains.
Produced by breakdown of amylase, (= a component of
starch), in many germinating seeds.
GLUCOSE + FRUCTOSE = Sucrose - table sugar and sugar found
in plants.
GLUCOSE + GALACTOSE = Lactose - the sugar from milk.
The hydrolysis of starch (Amylose) results in the disaccharide
MALTOSE forming.
Maltose can then be hydrolyzed to produce monosaccharides
of glucose.
Benedict’s test for a reducing sugar.
Theory
Need to know this for practical exam and written exams.
All monosaccharides and some disaccharides, (like maltose)
are reducing sugars.
Receiving an electron is reduction.
A reducing sugar is a sugar that can donate an electron to (or
reduce), another chemical, in this case Benedict’s reagent.
Benedict’s contains Copper II sulphate and is alkaline.
Cu2+ ions from the copper sulphate are reduced by the –CHO
Aldose or C=O Ketone groups in reducing sugars to form
Cu+ ions.
When heated with a reducing sugar pale blue Benedict’s
solution forms an insoluble red precipitate of Copper I
oxide.
Method
Mix 2cm3 sample + 2cm3 Benedict’s reagent.
Heat in gently boiling water for 5 min.
• Reducing Sugars –
all monosaccharides
and most
disaccharides but NOT
sucrose.
Semi-Quantitative methods.
Differences in colour mean it can be used to estimate
the approx amount of reducing sugar in a sample.
Concentration of reducing
sugar
Colour of solution and
precipitate
None
Blue
Very low
Green
Low
Yellowish green
Medium
Dark brown
High
Red
Question
1. Use table
Sample
Colour of solution
A
Yellowish brown
B
Green
C
Red
D
Dark brown
E
Yellowish green
a. Place the letters in sequence of the increasing amount of
reducing sugar in each sample.
b. Suggest a way other than comparing colour changes, in which
different concentrations of reducing sugar could be estimated.
c. Explain why it is not possible to distinguish between very
concentrated samples, even though their concentrations are
different.
2. What happens when maltose is hydrolysed?
Answers
1a. B, E, A, D, C.
b. Dry the precipitate in each sample and weigh it.
The heavier the precipitate the more reducing
sugar was present.
c. Once all the copper (II)| sulphate has been
reduced to copper (I) oxide, further amounts of
reducing sugar cannot make a difference.
2. It produces 2 glucose molecules.
More quantitative methods.
1.Colorimetry
Calibrate final colour against known glucose concentrations.
Use filter to get the filtrate (solution that runs through the
filter).
The more precipitate that has been formed and filtered away,
the less absorption of light occurs. High glucose
concentration gives a high transmission of light.
Use the colorimeter to record % transmission or absorbance.
Create a calibration graph.
Measure % transmission for the unknown specimen.
Read values of unknown against the calibration curve.
2. Measuring precipitate.
Filter and dry the precipitate (the residue caught in the
filter paper).
Measure the mass.
Calibrate known concentrations of glucose.
Measure the dry mass of the precipitate for the unknown
specimen.
Strong glucose concentration gives a large quantity of
precipitate.
High quantity of precipitate gives a high value of mass.
Read against the calibration curve.
Questions
3. Large molecules often contain carbon. Why is
this so?
4. What is the general name for a molecule that
is made up of many similar repeating units?
5. Why does Benedict’s reagent turn red when
heated with a reducing sugar?
Answers
3. Carbon atoms readily link to one and another
to form a chain.
4. Polymer.
5. Sugar donates electrons that reduce copper
(II) sulphate to orange copper (I) oxide.
Test for Non-Reducing Sugars
• Some disaccharides such as sucrose are nonreducing sugars. This means they do not
change the colour of Benedict’s when heated.
To test a non-reducing sugar it must be broken
down into its monosaccharide components by
hydrolysis. These monosaccharides can then
be tested with Benedicts as a reducing sugar.
The non-reducing sugar is first hydrolysed by
boiling with hydrochloric acid so that it will be
broken down into its monosaccharides.
These can then reduce Benedict’s reagent in the
normal way.
So a non-reducing sugar is identified by a
negative reaction to Benedict’s before
hydrolysis and a positive result after
hydrolysis.
Method
Carry out the reducing sugar test.
If negative then in a boiling tube, add 2cm3 (10 drops) of
dilute hydrochloric acid (0.1M), to a fresh sample to be
tested, mix the solution and boil for 2-3 mins
Then add sodium hydroxide (0.1M), to the boiling tube until
the solution is neutral, (over neutralise with 12 drops), or
preferably alkaline. Use the pH paper to test for this. (This is
important because Benedict’s is not effective in acid
conditions).
Carry out the reducing sugar test again.
Result
• A negative result (solution remains blue),
after the first reducing sugar test, followed by
a positive result, (solution turns red/brown),
after the second reducing sugar test, is an
indication of a non-reducing sugar.
The polysaccharides.
These are polymers, of many monosaccharides all joined
by condensation reactions.
General formula: (C6H10O5)n
They are often folded and can also be branched.
They are not sweet, cannot be crystallised, unlike mono
and disaccharides. They are also insoluble, and
therefore osmotically stable. This makes them good
storage carbohydrates. E.g. glycogen in animal cells
and starch in plant cells.
Test for Starch
Method
Place two drops of the solution to be tested in a
test-tube.
Add a drop of Iodine (found in Potassium Iodide
Solution). (No heating is required).
Result
If starch is present the yellow-orange iodine in
potassium iodide solution becomes a blue-black
colour.
Questions
6. Which one, or more, monomer units make up each of the
following carbohydrates?
a. Lactose
b. Sucrose
c. Starch
7. Glucose (C6H12O6) combines with fructose (C6H12O6), to form
the disaccharide sucrose. From your knowledge of how
disaccharides are formed, work out the formula of sucrose.
8. To hydrolyse a disaccharide it can be boiled with hydrochloric
acid but if hydrolysis is carried out by an enzyme a much lower
temperature (40oC) is used. Why is this?
9. List 3 differences between a polysaccharide such as starch and
a sugar like glucose.
10. What is the main function of starch?
Answers
6a.Glucose + Galactose
b. Glucose + Fructose
c. Glucose only.
7. C12H22O11 (C6H12O6 + C6H12O6 - H20)
8. Enzymes are denatured at higher temperatures.
This prevents them functioning.
9. Polysaccharide – un-sweet, insoluble and not
crystallisable.
Glucose – sweet, soluble and crystallisable.
10. Respiratory substrate, (provide energy)
Carbohydrate Digestion
(a) Starch Digestion
Starch
(i) Starch → maltose via enzyme
amylase.
• Amylase is in mouth and pancreas. Via
hydrolysis reaction.
• Starch polysaccharide.
• Maltose = disaccharide.
• Mineral salts maintain pH 7.00 as optimum
temp for amylase here.
(ii) Food swallowed into stomach.
• Stomach acidic. Denatures amylase.
(iii) Chyme swallowed into small
intestine.
• Secretion called pancreatic juice mixed with
food here. Contains pancreatic amylase.
Continues hydrolysis of starch → maltose.
• Alkaline salts produced by pancreas and lining
of small intestine. Neutralises acidity of
stomach.
• Optimum pH here 7.4.
(iv) Maltose → α glucose
• Enzyme = maltase in epithelial lining of small
intestine.
• Maltose = disaccharide.
• Glucose = monosaccharide.
• Peristalsis pushed digested food along lumen
of gut.
So a number of different amylase enzymes
which all act on bonds at different points in
the starch molecule.
(b) Disaccharide Digestion.
a. Sucrose
In natural foods sucrose is found inside cells, so
cells must be broken down mechanically, by
teeth and churning of stomach.
Epithelial cells in lining of small intestine
produce enzyme sucrose.
Sucrose (disaccharide) → glucose + fructose (2
monosaccharides).
b. Lactose
sugar found in milk, yoghurt, cheese.
Digested in small intestine, where cells lining
epithelial lining produce enzyme lactase.
Lactose (disaccharide) → glucose + galactose (2
monosaccharides).
(c) Lactose Intolerance
 Babies produce lots of lactase. Amount diminishes as get older.
 Lactase is found in the microvilli of the epithelial cells lining the
small intestine.
 Some people produce little or no lactase.
 Modern day we eat lots of milk products. Some cannot digest it all.
 So undigested lactose gets to large intestine, micro-organisms
break it down and produce loads of gas (methane).
 Hydrogen absorbed into blood.
 Result = bloating, nausea, diarrhoea and cramps.
 Some people can’t have any lactose, some only a little.
 Avoid foods with lactose in them.
 Problem, need to have calcium. So eat/drink lactose free products,
or add lactase to milk before eating it.
 Condition rare in babies but more serious as all they drink is milk.
Have special non-milk food, rich in calcium and vitamin D.
2 types:
(i) Primary lactase deficiency.
Develops as you age.
Usually inherited = fault in gene that codes for lactase production.
Affects more than half the world pop. Mostly non-Caucasians
75% Afro-Caribbean and 95% Asian people.
(ii)Secondary lactase deficiency
Person can’t produce enough lactase due to damage to small intestine through
injury or disease. E.g. coeliac disease, inflammatory bowel disease or
Crohn’s diseases.
Many can still eat fermented dairy products like buttermilk and yoghurt as
bacteria causing fermentation convert lactose to lactic acid before
consumption.
Lactose intolerance tests
(i) The Lactose tolerance test.
Patient fasts before test.
Then drinks drink of lactase.
Blood sample every 2 hours and measure glucose level in blood.
If lactose being broken down, then glucose level should rise. If intolerant,
then will stay low.
(ii) The Hydrogen breath test
Patient drinks fluid containing lactose. Patients breath measured for
hydrogen.
Normal breath has little.
If lactose intolerant then will have higher levels as bacteria in large intestine
are digesting the lactose and producing large amounts of hydrogen.
Hydrogen absorbed into blood then to lungs and exhaled.
(iii) Stool Acidity test
Measure level of acidity in stools. If lactose intolerant then fermented in
large intestine by bacteria to produce lactic acid and other fatty acids.
Questions
11 What is the final product of starch digestion in
the gut?
12. Name 3 enzymes produced in the epithelium of
the small intestine.
13. Which microorganisms produce the gas, which
lactose intolerant people produce, when they try
and digest lactose in their large intestine?
14. Suggest a reason why the gas is unlikely to be
carbon dioxide.
Answers
11. α glucose
12. Maltase, sucrose, lactase.
13. Respiration.
14. Carbon dioxide forms as a result of aerobic
respiration. Conditions in the large intestine
are anaerobic.
(d) Absorption of carbohydrates in
the small intestine.
(i) Villi and microvilli
Villi
Glucose is absorbed in small intestine. Always a high concentration of
glucose in the lumen of the gut as constantly being digested.
Here villi = finger-like projections.
Approx 1mm long. Part of thin epithelial cells.
Have thin walls, so less distance of diffusion.
Villi can move, so maintain diffusion gradient. Contain muscles which
contract and relax, so mix contents of lumen, means always glucose
rich food surrounding villi. Maintains conc gradient.
Well supplied with blood vessels = network of blood capillaries. These
carry absorbed food molecules away and so maintain diffusion
gradient.
Microvilli
• Finger-like projections of cell surface
membrane about 0.6µm in length.
• Called brush border – as look like head of a
brush.
• Villi increase surface area and so increase rate
of absorption.
(ii) Active transport
Not all glucose can be absorbed through
diffusion. To absorb all the glucose need active
transport as well.
Co-transport
• Where is ATP
needed in this
process?
• Which organelle
would you
expect to find in
high numbers to
supply ATP?
Active transport by co-transport
• Glucose Is absorbed in small intestine with sodium as well.
• Sodium ions are actively pumped out of epithelial cells, via
sodium-potassium pump and into blood. Happens via
protein carrier molecule in cell surface membrane of
epithelial cells.
• Now is higher conc of sodium in lumen of small intestine
than inside epithelial cells.
• So sodium diffuses down conc gradient from lumen into
epithelial cells. Go through special co-transporter proteins,
taking glucose with them.
• So glucose enters epithelial cell by facilitated diffusion
using another type of carrier.
Theory behind administering ORS for
Cholera treatment.
• Sodium moves down conc gradient from
lumen into epithelial cell.
• Glucose molecules move up their conc
gradient. It is sodium ion conc and not ATP
directly that powers movement of glucose into
cells.
• So is indirect and not direct active transport.
Questions
15. State 2 ways in which a glucose concentration
gradient is maintained between the inside of the small
intestine and the capillaries in the villi.
16. Why is the term co-transport” used to describe the
transport of glucose into cells?
17. In each of the following events in the glucose cotransport system, state whether the movements are
active or passive:
a. Sodium ions move out of epithelial cell.
b. Sodium ions move into epithelial cell.
c. Glucose molecules move into the epithelial cell.
Answers
15. Heart beat ensures blood with high glucose
conc is moved away from capillaries inside villi.
Muscles in the villi keep the contents of digestion
moving, so glucose rich contents come into
contact with villi for absorption.
16. Because glucose molecules and sodium ions
move into the cell coupled together.
17a. Active
b. Passive
c. Passive
Cellulose
Module: BIOL2
• Syllabus objectives
• 3.2.4 The variety of life is extensive and this
is reflected in similarities and differences in
its biochemical basis and cellular
organisation.
Carbohydrates
• The structure of b-glucose as b-glucose and
the linking of b-glucose by glycosidic bonds
formed by condensation to form cellulose.
• The basic structure and functions of starch,
glycogen and cellulose and the relationship of
structure to function of these substances in
animals and plants.
(a) Starch
• Found in plants as small
gains, especially in
seeds and storage
organs, like potato
tubers.
• Is an energy source in
plants and is used by us
as an important
component of food, as
an energy source.
Structure = is a mixture of 2 polysaccharides, both are
chains of α-glucose monosaccharides, called amylose
and amylopectin.
Amylose = long unbranched chain of α-glucose. It has a coiled shape, wound
tight into a coil, making the molecule very compact and so a good energy
store. Not so good for releasing energy though as it only has 2 ends for
reaction to occur at.
Amylopectin – long branched chain of α-glucose. Its side braches allow the
enzymes that break down the molecule to get to the glycosidic bonds
easily. This means glucose can be released quickly, due to their being lots
of ends.
They are linked by glycosidic bonds formed by condensation reactions. For
the main branches, the linkage is α-1, 4 linkage. This means that the bond
formed = α-1, 4-glycosidic bond, as it forms between carbon atoms 1 and
4 of 2 α-glucose molecules.
It then has α-1, 6 linkage at side branch points.
Amylose
(“Heads up” always)
Characteristics which make it a good energy store:
•Insoluble and so does not draw water into or out of a cell by osmosis.
•Compact, so can store a lot in a small space.
•When hydrolysed it forms monosaccharides of α-glucose,
which is easily transported and used in respiration.
Question
18. Describe how the structure of
starch makes it suited to its function.
(6 marks)
18. Answer
Each point is 1 mark
• Amylose is a long unbranched chain.
• Which forms a coiled shape.
• This coiled shape is compact and so makes it good for
storage.
• Amylopectin is a long branched chain.
• Its side branches make it good for storage as the enzymes
that break it down can reach the glycosidic bonds easily.
• Starch is insoluble in water.
• This means it can be stored in large quantities
• Without the cell being affected by osmosis.
(b) Glycogen
The animal cell equivalent of starch. Found in meat but not normally
eaten. It is the form in which animals store their carbohydrates.
It is a polysaccharide, with shorter chains of α-glucose than starch, so
is even more easily hydrolysed. Similar structure to amylopectin
except it has more side branches, which means that stored glucose
can be released quickly.
In animals it is stored as small granules in muscles and liver.
Like starch, glycogen chains are linked by glycosidic bonds formed by
condensation reactions. For the main branches, the linkage is α-1,
4 linkage. This means that the bond formed = α-1, 4-glycosidic
bond, as it forms between carbon atoms 1 and 4 of 2 α-glucose
molecules.
It then has α-1, 6 linkage at side branch points.
Glycogen
(c) Cellulose
Found in plant cellulose cell walls, (fruit and vegetables)
Made up of monomers of β-glucose.
• 2 β-glucose molecules combined in a condensation
reaction = cellobiose.
• Bond formed = β 1, 4-glycosidic bond, as it forms
between carbon atoms 1 and 4 of 2 β-glucose
molecules.
• Many molecules link together like this to form
cellulose.
“heads up – heads down”
• The position of the -H group and the –OH
group on a single carbon are reversed. In the
β-glucose the –OH group is above rather than
below the ring. So to form a glycosidic link,
each β-glucose molecule must be rotated 180o
compared to its neighbor.
• Result = the –CH2OH group on each β-glucose
molecule alternates between being above and
below the chain, (“heads up – heads down”)
Cellulose.
Cellulose
• These do not form coils of cellulose. Instead
chains of cellulose run parallel to each other, with
hydrogen bonds between the chains forming
cross linkages.
• Each hydrogen chain is weak but there are many,
so together the structure is strong. This is known
as a microfibril.
• The cellulose fibres are not all aligned in the
same direction but “criss-cross” each other for
strength and elasticity.
• The cellulose molecules are grouped together
to form microfibrils. These are cemented
together by substance called hemicelluloses.
Microfibrils are like iron rods in reinforced
concrete.
• These microfibrils join together to form fibrils,
which group together to form fibres.
Functions of cellulose
•
•
•
•
Major component of plant cell walls.
Provides rigidity to plant cell.
Prevents cell bursting when water enters by osmosis.
It exerts an inward pressure that stops any further
water entering. As a result plant cells are rigid and the
cells push against each other.
• Rigid cells are important in stems and leaves. Means
they are turgid for support and in the leaf to maximize
surface area for max photosynthesis.
• It allows water and substances dissolved in water to
pass freely into and out of cell.
Digestion of cellulose
• Cellulose can’t be easily hydrolysed by
amylases, as these enzymes active site does
not fit the glycosidic bonds between the βglucose monomers, like it does the α-glucose
monomers of starch and glycogen.
Herbivores like cows and
elephants can digest
cellulose as they have
micro-organisms in their
gut that produce cellulases
that digest cellulose.
Humans can’t do this but
cellulose is important in
our diet as a source of
fibre. This helps peristalsis
occur and reduces colon
cancer occurrence.
Lignin.
• In wood cellulose is
further
strengthened by
lignin= noncarbohydrate
polymer.
• Used in xylem cells,
where lignifications
makes cells
impermeable to
water. It increases
the strength of the
cell walls.
19. Questions
From the following list
choose 1 or more
that closely fit each
of the statements:
α-glucose
β-glucose
starch
cellulose
glycogen
a. Stains deep blue with iodine
solution.
b. Is known as an animal starch.
c. Found in plants.
d. Are polysaccharides.
e. Monosaccharides found in starch.
f. Has a structural function.
g. Can be hydrolysed.
h. Easily diffuses in and out of cells.
19.Answers
a. Starch
e. α-glucose.
b. glycogen.
f. cellulose.
c. α-glucose, β-glucose,
starch and cellulose.
g. starch, cellulose,
glycogen.
d. starch, cellulose,
glycogen.
h. α-glucose and β-glucose
Make certain that you know structure
and function of the following
polymers...
• Collagen
• Glycogen
• Cellulose
Students often muddle these three due to
similarity of the names and it is a costly
mistake to make!