The document discusses carbohydrate metabolism, detailing the structure and classification of polysaccharides, the digestion and absorption processes occurring in the human body, and the physiological roles of various carbohydrates such as starch, glycogen, and cellulose. It emphasizes the importance of dietary fibers, their effects on digestion and health, and describes the mechanisms of monosaccharide absorption. Additionally, it covers lactose intolerance and its symptoms, treatment, and the metabolic fate of carbohydrates in the liver.

1. STORAGE AND CELL WALL
POLYSACCHARIDES
BCH 204 CARBOHYDRATE METABOLISM I
4th
Sept, 2024

2. POLYSACCHARIDES (GLYCANS)
• Carbohydrates composed of ten or more monosaccharide units or
their derivatives (such as amino sugars and uronic acids) are generally
classified as polysaccharides.
• monosaccharide units are joined together by glycosidic linkages.
Another term for polysaccharides is a “glycans”.
• Polysaccharides are sub-classified in two groups:
• 1. Homopolysaccharides (Homoglycans): polysaccharides made up of
several units of one and the same type of monosaccharide unit only.
• 2. Heteropolysaccharides (Heteroglycans): contain two or more
different types of monosaccharide units or their derivatives.

3. Homopolysaccharides or Homoglycans: Starch
• It is the storage form of glucose in plants, e.g. in potato, in grains and
seeds and in many fruits.
• Starch is composed of two constituent viz. amylose and amylopectin.
• Amylose Amylose is a linear polymer of D-glucose units joined by α-
1→4 glycosidic linkages.

4. HOMOGLYCAN CONTD…
• Amylopectin Amylopectin is structurally identical to those of amylose (α-
1→4 glycosidic linkages) but with side chains joining them by α-1→6
linkages.
• Thus, amylopectin is a branched polymer having both α-(1→4) and α-(1→6)
linkages. The branch points in amylopectin are created by α-1→ 6 bonds
and occur at an interval of 20 to 30 units of glucose.

5. Dextrin
• Partial hydrolysis of starch by acids or α-amylase (enzyme)produces
substances known as dextrins.
• These also occur in honey.
• They are oligosaccharides containing α-(1→4) and α-(1→6) linkages.
• All dextrins have few free aldehyde groups and can show mild
reducing property.

6. Glycogen
• Glycogen is the major storage form of carbohydrate (glucose) in
animals, found mostly in liver and muscle.
• The structure of glycogen is similar to that of amylopectin, except that
it is more highly branched, having α-(1 → 6) linkages at intervals of
about 8 to 10 glucose units.
• Function
• The function of muscle glycogen is to act as a readily available source
of glucose for energy within muscle itself.
• Liver glycogen is concerned with storage and maintenance of the
blood glucose.

7. Cellulose
• Cellulose is the chief constituent of cell wall of plants.
• It is an unbranched polymer of glucose and consists of long straight
chains which are linked by β-(1→4) glycosidic linkages and not α-
(1→4) as in amylose.
• Since humans lack an enzyme cellulase that can hydrolyse the β-(1→
4) glycosidic linkages, cellulose cannot be digested and absorbed and
has no food value unlike starch.
• However, the ruminants can utilize cellulose because they have in
their digestive tract microorganisms whose enzymes hydrolyse
cellulose.

8. Heteropolysaccharides or Heteroglycans
• Glycosaminoglycans (GAGs) or Mucopolysaccharides
• Structure of GAG
• A GAG is an unbranched heteropolysaccharide, made up of
repeating disaccharides.
• –– One component of which is always an amino sugar (hence
the name glycosaminoglycans), either D-glucosamine or D-
galactosamine.
• –– The other component of the repeating disaccharide(except
in the case of keratan sulfate) is a uronic acid, either L-
glucuronic acid or its epimer L-iduronic acid.
• Thus, GAG is a polymer of [uronic acid-amino sugar]n

9. DIGESTION, ABSORPTION AND
TRANSPORT OF CARBOHYDRATES
BCH 204
4TH
SEPT, 24

10.  Digestible carbohydrates:
 starch, glycogen, maltose, sucrose, and lactose
(oligosaccharides and polysaccharides).
 Ready-to-absorb carbohydrates:
 They are simple sugars, which do not need digestion and are
absorbed directly as such e.g., monosaccharides: glucose,
mannose, galactose, fructose and pentoses.
 Non-digestible carbohydrates:
 Which are called dietary fibers.
 These include cellulose, gums and pectins.
 They are important for adding bulk of stool, as they facilitate
gastric emptying (stools excretion). They have anticancer effects
and intestinal bacteria feed on it to release certain vitamins.

11. Digestion of Carbohydrates
• The principal sites of carbohydrate digestion are the mouth and small
intestine. The dietary carbohydrate consists of:
• Polysaccharides: Starch, glycogen and cellulose
• Disaccharides: Sucrose, maltose and lactose
• Monosaccharides: Mainly glucose and fructose
• Monosaccharides need no digestion prior to absorption, whereas
disaccharides and polysaccharides must be hydrolyzed to simple
sugars before their absorption

12. Digestion in Mouth
• Digestion of carbohydrates begins in the mouth.
• Salivary glands secrete salivary α-amylase (ptylin), which
initiates the hydrolysis of a starch.
• Its optimum pH is 6.7 - 6.8 and is activated by chloride ions.
• During mastication, salivary α-amylase is an -glucosidase
which acts on dietary starch in random manner breaking
some α-(1 4)
→ bonds.
• α-amylase hydrolyzes starch into maltose and dextrins.
• Salivary amylase cannot digest -1,4-glucosidic linkage in
cellulose.

13. Digestion in Stomach
 Salivary amylase continues to act on starch, glycogen or
dextrins for 2 - 3 minutes only in the stomach (acidic pH 1 -
2).
 Thus, carbohydrate digestion halts temporarily in the
stomach because the high acidity inactivates the salivary α-
amylase.

14. Digestion in Small Intestine
• Further digestion of carbohydrates occurs in the small intestine by
two juices.
• There are two phases of intestinal digestion.
• 1. Digestion due to pancreatic α-amylase
• 2. Digestion due to intestinal enzymes: Sucrase, maltase, lactase,
isomaltase

15. Digestion in Intestine contd…
1. Digestion due to pancreatic Juice
 Pancreatic juice contains Pancreatic amylase, an -glucosidase.
 It has an optimum pH 7.1 and is also activated by chloride
ion.
• The function of pancreatic α-amylase is to degrade glycogen and
starch dextrins further into a mixture of maltose, isomaltose (three α-
glucose residues linked -1,4 bonds ), α-limit dextrin (a mixture of
branched oligosaccharides) and non-branched
oligosaccharides and some glucose.
• The α-limit dextrins are smaller oligosaccharides containing 3 to 5
glucose units.

16. 2. Digestion due to intestinal enzymes
• Enzymes responsible for the final phase of carbohydrate digestion are
located in the brush-border membrane.
• The enzymes and the reactions they catalyze are as follows:
• The end products of carbohydrate digestion are glucose, fructose
and galactose which are readily absorbed through the intestinal
mucosal cells into the bloodstream

17. Lactose Intolerance
• Intolerance to lactose (the sugar of milk) not to milk. This is the most common
disorder due to deficiency of enzyme lactase.
• an inherited or age-dependent decline of enzyme expression or an
acquired medical problem due to intestinal diseases such as colitis and
gastroenteritis.
• In this condition, lactose accumulates in the gut which undergoes bacterial
fermentation in the large intestine with the production of H2 and CO2 gases and low
molecular weight acids like acetic acid, propionic acid and butyric acid which are
osmotically active.
• Symptoms include: Abdominal cramps and flatulence results from the accumulation
of gases and the osmotically active products that draw water from the intestinal cells
into the lumen resulting in diarrhea and dehydration.
• Treatment for this disorder is simply to remove lactose from the diet or
consumption of live-culture yogurt..

18. Dietary fibers
 The -1,4 glycosidic linkage of Cellulose is not hydrolyzed
by human digestive enzymes.
 Hemicellulose, gums, pectins and pentosans are also
indigestible.
 Cellulose and other dietary fibers passes as it is in stools,
increasing bulk of intestinal contents by adsorbing water
and stimulates peristaltic movements to reduces stool
transit time and prevents constipation.
 They bind and dilute bile acids.
 The more soluble fibers found in legumes and fruit, e.g.,
gums and pectins, lower blood cholesterol, possibly by
binding bile acids and dietary cholesterol.

19.  The soluble fibers also slow the stomach emptying and
attenuate the post-prandial rise in blood glucose that
spares insulin.
 This effect is beneficial to diabetics and to dieters
because it reduces the rebound fall in blood glucose that
stimulates appetite
 It induces establishment of normal colon bacteria with
several benefits including fermentation of fibers and
production of vitamins (e.g., vit. K)

20. Absorption of Carbohydrates
• Carbohydrates are absorbed as monosaccharides from the upper part of the
small intestine (jejunum) in the intestinal lumen.
• Very small amount is absorbed in the stomach or large intestine.
 Carbohydrates are absorbed via portal vein to the liver, i.e., blood
stream chiefly in the form of hexoses (glucose, fructose,
mannose and galactose) and as pentose sugars (ribose).
• Two mechanisms are responsible for the absorption of monosaccharides:
• 1. Active transport against a concentration gradient, i.e. from a low glucose
concentration to a higher concentration.
• 2. Facilitative transport, with concentration gradient, i.e. from a higher
concentration to a lower one.

21. Active Transport
• This absorption occurs against concentration gradient, i.e., sugars are absorbed from
low to high concentration .
• requires the presence of an OH group on C2 at the right side of a pyranose ring and a
methyl group or a substituted methyl group at C5. This applies to glucose and
galactose.
• The transport of glucose and galactose across the brush border membrane of mucosal cells occurs by
an active transport.
• Active transport is an energy requiring process that requires a specific transport protein and the
presence of sodium ions.
• It utilizes a sodium-dependent glucose transporter, SLGT1. A mobile carrier protein
molecule (glucose transporters) presents in the cell membrane of all cells including
intestinal cells.
• The Glucose transporter has 2 sites, one for sodium and the other for glucose,
symporting sodium down its concentration gradient and glucose against its
concentration gradient across cell membrane.
• i.e Na+ is transported down its concentration gradient (higher concentration to lower concentration)
and at the same time glucose is transported against its concentration gradient.

22. • Both sodium and glucose are released within mucosal
cells, allowing the carrier to recycle for more cargo.
• Sodium is pumped out again by ATP-dependent Na+-K+-
exchange pump.
• The ratio of Na+/glucose transported varies according to
type of transporter to be 1:1 or 3:1 ratio.
• The free energy required for this active transport is obtained from
the hydrolysis of ATP linked to a sodium pump that expels Na+ from
the cell in exchange of K+.

23. Facilitative Transport
• Fructose and mannose are transported across the brush border by a
Na+ independent facilitative diffusion process, requiring specific
glucose transporter, GLUT-5.
• Movement of sugar in facilitative diffusion is strictly from a higher
concentration to a lower one until it reaches an equilibrium.
• Absorption is derived by concentration gradient of sugar in
the intestinal lumen, i.e., sugars passes from high
concentration in lumen to lower concentration in mucosal
cells then to blood.
• The same transport can also be used by glucose and galactose if the
concentration gradient is favorable.

24. Transport of Carbohydrates
• The sodium independent transporter, GLUT-2 facilitates transport of
sugars out of the mucosal cells, thereby entering the portal circulation
and being transported to the liver.

27. Metabolic Fate of Carbohydrates
• After being absorbed from the intestinal tract the monosaccharides are carried by the portal
circulation directly to the liver.
• In the liver most of the entering free D-glucose is phosphorylated to glucose-6-phosphate and
sugar is trapped within the cell and it cannot diffuse back out of the cell because its plasma
membrane is impermeable to the glucose-6-phosphate.
• The remainder of the glucose passes into the systemic blood supply.
• Other dietary monosaccharides D-fructose and D-galactose are phosphorylated and may be
converted into glucose in the liver.
• Glucose-6-phosphate is an intermediate in several metabolic pathways that uses glucose in the
liver depending upon the supply and demand. This includes:
• Glycolysis
• Pentose phosphate pathway
• Glycogenesis and
• Glycogenolysis.