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Carbohydrate metabolism
• 60% of food carbohydrate
• Starch ,glycogen,sucrose,lactose and
cellulsoe are chief.
• Hydrolysed to hexose sugar (glusose,
galactose and fructose) in gastrointestinal
tract before they are absorbed.
• Hydrolysis of glycosidic bonds by
glycosidases monohexose components.
In the mouth
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Salivary α- amylase (Ptyline).
Salivary gland
Opt.pH 6.1
Activated by chloride ions (Clˉ).
Acts on starch and glycogen breaking α(14)bond  maltose
• Isomaltose (α1-6 linkage).
Cont.
• Food remains for a short time in the
mouth patial digestion of starch
,dextrins(amylodextrins,erthrodextrins,and
achroodextrins).
• So digestion in the mouthmaltose ,
isomaltose and starch and dextrins.
• In stomach  high acidity inactivates the
salivary α-amylase.
Digestion by intestinal
enzymes
• The final digestive process occur at the
mucosal lining and include the action of
several disaccharides.
• These enzymes are secreted through and
remain associated with the brush border of
the intestinal mucosal cells
enzymes
• Lactase (β-galactosidase)which hydrolyse
lactose into two molecules of glucose and
galactose.
• Lactose lactase
Glucose+ Galactose
• Maltase(α-glucosidase)
• Maltose maltase
Glucose + glucose
• Sucrase(α-fructofuranosidase)
• Sucrose sucrase
Glucose + frutose
Cont.
• Αlpha- dextrins (oligo-1,6 glucosidase)
• Isomaltose α-Dextrins Glucose+ Glucose
Digestion of cellulose
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Cellulose contains β(1-4)
In human no β-(1-4) glucosidase.
Cellulose passes as such in stool.
Cellulose helps water rentention during the
passage of food along the intestine
,producing larger ,softer feces.
Absorption of carbohydrates
• End products of carbohydrates are
monosaccharides .
• Active transport .
• Passive trasnport.
Active transport
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Sugar should have:
1. hexose ring
OH group at postiton 2 at the right side
Present in glucose and galactose.
Fructose does not contain OH to the right
side at position 2 is absorbed more slowly
than glucose and galactose by passive
transport.
Mechanism of active
transport
• Carrier proteins
• Two separate sites Na and glucose.
• Transport them from the intestinal lumen
across cell membrane to the cytoplasm.
• Glucose and Na are released in the
cytoplasm.
• Na is transported from high to low
concentration and at same time causes the
carrier to transport glucose against its
Inhibitors of transport
• Ouabain(cardiac glycosidase): inhibit
adenosine triphosphate necessary of ATP
which produces energy of sodium pump.
• Phlorhizin:inhibits the binding of sodium in
the carrier proteins.
• Passive transport (simple diffusion): Sugars
passes with concentration gradient .No
energy required.Fructose and pentoses are
absorbed by this mechanism.
Defects of carbohydrates
• Lactase deficiency: Lactose intolerance
• 1. congenital : which occurs very soon after
birth (rare).
• Acquired: which occurs later on life.
• Effect : presence of lactose causes.
• 1.increased osmotic pressure: water will be
drawn from the tissues into the large
intestine(osmotic diarrhoea).
Cont.
• 2. increased fermentation of lactose by
bacteria .: Intestinal bacteria ferment lactose
with subsequent production of CO2
gas..This causes distention and abdominal
cramps.
• Treatment: 1.By removing lactose form
diet.
• Tablets containing lactase enzyme.
Fate of absorbed sugar
• Uptake by tissues(liver): after absorption of
sugars are taken up by the liver ,where
galactose and fructose are converted by
glucose.
• Utilization by tissues: glucose under go
• Oxidation : through
• 1. Major pathway: glucolysis and kreb
cycle. Energy production
Cont.
• Hexose monophosphate shunt: production
of ribose , deoxyribose , and NADPH + H+
• Uronic acid pathway: production of
glucoronic acid which is used detoxication
and enter in the formation of
mucopolysaccharide.
Storage
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Glycogen : glycogenesis
Fat : lipogenesis
Conversion:
1. Ribose,deoxyribose: RNA,DNA
2. Lactose; Milk
3. Glucosamine,Galactosamine;M
ucopolysaccharides
• Glucuronic acid;
Mucopolysaccharides,conjugation.
Glucose oxidation
• Conversion of glucose to CO2 ,H2O and
energy .
• Occurs in cytoplasm(Glucose to pyruvate)
• Mitochondria (Pyruvate to acetyl CoA).
• Acetyl CoA enters a series of reaction
known as Kreb cycle to be oxidised
completely.
Embden-meyerhof pathway
• Oxidation of glucose or glycogen to give
pyruvate (in the presence of oxygen) or
lactate (in the absence of oxygen)
• Site .
• Muscle during exercise : due to lack of
oxygen
• RBC’s :due to absence of mitochondria.
• One mol of glucose gives 2 mol of
glyceraldehyde-3- phosphate and 2 ATP
Glycolysis…
• What happens when you burn sugar?
• How do we extract the energy from the food
(fuel) that we eat without burning ourselves
up?
• The answer: Glycolysis and the TCA/Krebs
Cycle!
• Glycolysis and the TCA cycle is what we
use to convert sugar (glucose) into usable
chemical energy for our cells.
Cellular Respiration
• All organisms on this planet need energy to
survive. As we already know, almost all
most use trapped energy from the sun for
their metabolic needs. Animals that can trap
the energy themselves are called autotrophs;
they make their own food. The rest of us are
called heterotrophs; we need to eat other
organisms for our energy.
What is Glycolysis?
• Glycolysis is a series of chemical reactions
that make a little bit of ATP from the partial
breakdown of sugar into energy.
• Organisms usually choose one of two paths
after glycolysis: Fermentation or Aerobic
Respiration.
Glycolysis:
• In glycolysis,
organisms take in
glucose, or some other
6 carbon sugar, and
turn it into two three
carbon molecules. It
makes a little ATP in
the process, which the
organism can use for
it’s metabolic needs.
The Net Equation:
• Glycolysis’ net equation is this:
Glucose + 2 ATP + 2 NAD+ ------------->
2 Pyruvic acid + 4 ATP + 2 NADH + 2 H+
• Pyruvic acid is 3 carbon molecule. NAD+ is
a compound that accepts electrons so that
they may be used elsewhere (similar to
NADP+ in photosynthesis).
Fermentation
• Once the organism has performed
glycolysis it can metabolize the pyruvic
acid in several pathways, the first being
fermentation.
• Fermentation occurs in the absence of
oxygen. Fermentation does not make
energy, but it does regenerate NAD+ so that
glycolysis can continue.
Fermentation
• Our own bodies carry out fermentation. When you
are working out really hard and your legs start to
burn, that burning is the breakdown of glucose
into pyruvic acid and then the fermentation of
pyruvic acid into lactic acid. When your body is
working hard, the oxygen that your lungs intake is
inadequate to carry on normal aerobic respiration.
Your body, in an attempt to keep things going
(just in case it’s a bear chasing you) starts lactic
acid fermentation.
Why Fermentation?
• Well, if your body couldn’t ferment pyruvic
acid into lactic acid and NAD+, then you
would quickly run out of NAD+ and
glycolysis would stop. If you couldn’t
perform glycolysis, then not only would
those muscles of yours stop, but your heart
and brain would run out of fuel as well and
you would pass out or pass out and die.
As a note…
• When you eventually stop running, or when
you are in good or great physical condition,
you body can take that lactic acid that you
made while working out and convert it back
into pyruvic acid and send it off to the TCA
cycle as normal. Well conditioned athletes
can maintain aerobic conditions longer
than untrained athletes, thus they can “go”
longer and harder than those who are out of
shape.
Some other kinds of
Fermentation…
• Lactic acid fermentation is not the only
game in town. There are thousands of kinds
of fermentations. Some make ethyl alcohol,
like in beer, wine, vodka, gin, etc. Ethyl
alcohol (EtOH) is drinking alcohol. Some
other organisms make rubbing alcohol,
cheese, nasty petroleum distillates, etc.
Glycolysis is Not Very
Efficient
• Glycolysis is not very
efficient. It does not
do a good job of
extracting energy from
sugar. In fact, it only
obtains about 3.5% of
the available energy
from sugar.
Energy gain of glycolysis
• In the absence of O2
• ATP produced
• 2 ATP from 1,3
biphosphoglycerate
• 2 ATP from
phophoenolpyruvate
• ATP lost
• 1 ATP from glu-6 p
• 1 ATP from fru-1,6
biphoshatte
• In the presence of O2
• ATP produced
• 2ATP
1,3biphosphoglcerate
• 2 ATP
phosphoenolpyruvate
• 4 ATP or 6 ATP form
oxidation of 2NADH
+H+
• ATP lost
Importance of glycolysis
• Only the source of contracting muscle
during muscular exercise due to lack of O2
and to RBC due to absence of mitochondria.
• Gives 8 ATP
• 2,3 biphosphoglycerate which may be
produced in glycolysis decrease the affinity
of haemoglobin to O2. O2 is delivered in
tissues especially in case if hypoxia.
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Cont.
• Glycolysis provides the mitochondria with
pyruvic acid ,which gives acetyl CoA (kreb
cycle).
• Dihydroacetone phophate which may be
converted to α-glycerophophate.Later is an
important compound in lipogenesis.
• The amino acid serine may be formed from
3 phosphoglycerate.