Chapter
Carbohydrates
7
Function:
• Source of energy for life process.
• In Plant：
1.Simple sugars (glucose,ribose)
involved in energy transformation and
tissue synthesis.
2.Starch as energy reserve.
3.Insoluble fraction (cellulose,
hemicellulose) provided structural
support.
• Glycan: vary in shape and size, rangeing
from linear chains (polysaccharides)to
highly branched molecular
• Soluble components of the plant
cell:starch, hemicellulose and betaglucans, which are available to
fermentation.
Cellulose: glucose(1-4)-β-glucoside linkage.
• Microflora of the rumen of ruminants and
cecum and colon of nonruminants produce
cellulase to hydrolyze cellulose.
• Other nonrumianats utilize cellulose by
anaerobic fermentation in large intestine.
• Starch is soluble carbohydrate in plant
• Two important molecular of glucose: Linked via a
glucose-(!-4 )-α-glucoside linkage.
• Amylose : glucose(1-4)-α-glucoside linkage linear
polymer of glucose, MW 15000 ~ 100000 D.
Amylopectin: linear polymer with branches 10 - 100
million D.
• Maize contains 24% amylose and 76% amylopectin,
Waxy maize contains 1%:99%
• Rice and potatoes contain 20%:80%
• The hydrolysis of starch in amylose and amylopectin to
glucose is similar.
Carbohydrate Digestion
(一) preparation for absorption
• Only monosaccharides can be adsorbed
from GI tract, except in newborn animals.
• Poly-, tri-, disaccharides hydrolyzed (by
enzymes from host or microbe) to
monosaccharide
Plant cell wall digestion
• cellulose, hemicellulose, pectin, lignin為植
物細胞壁之主要成分, 即structure
polysaccharides, insoluble polysaccharide
之生物可利用率( bioavailability ), 即影響其
營養價值( nutritive value ).
• Anaerobic fermentation of carbohydrate
results in the production of large
quantities of volatile fatty acids (VFA, =
acetic acid, propionic acid and butyric
acid )and provides a large proportion of
the total energy supply.
Absorption and transport of
monosaccharides
1.Absorption:
• proximal section of the small intestine
(duodenum and jejunum) has greatest
capacity to absorb monosaccharides.
• glucose and galactose are absorbed very
efficiency. Mannose absorbed about 20%
of the efficiency of glucose, arabinose at
the lowest rate. (Table 7.3 )
2.Mechanism
• Sodium dependent active transport
mechanism transported against its
concentration gradient which is maintained
by membrane bound sodium-potassium
ATPase (Na- K-ATPase)
• Facilitative glucose transporters transport
glucose actively down a concentration
gradient.
• These two families of glucose transporters
are protein (500-600 amino-acids).
3. Factors that affect the absorption of
glucose
– Reduced by short-term (24 or 48h)
fasting, increased by restricted feed
intake.
– diabetic absorb more rapidly than normal
– adrenalectomy reduce in glucose
absorption but no effect on xylose
absorption.
– thyroidectomy and ovariectomy reduce
glucose absorption.
4.Digestive enzyme deficiency and
utilization of CHO
• Young mammals fed large amounts of
sucrose develop diarrhea and death
results from an insufficiency of sucrase.
• Ruminant species produce no sucrase.
• Some adults have lactase deficiency, like
pig and human.
• Xylose feeding of young pigs results in
depressed appetite and growth and
causes eye cataracts.
The absorption of soluble CHO exceeds 90%
Factors that affect the rate of starch digestion :
• particle size.
• nature of starch (amylose, amylopectin
content)
• interaction of starch with protein, fat.
• presence of antinutrient such as phytate,
tannin, saponins and enzyme inhibitors.
(三) Metabolic conversions
• Homeostasis : maintaining the blood
glucose concentration
glycogenesis
blood glucose
glycogenolysis
insuline↑
glycogen
glucagon↑
Gluconeogenesis
• glucose can be formed by body tissues
from noncarbohydrate metabolites,
including lipids, amino acids.
• Glucogenic amino acids such as
nonessential amino acids and several of
essential (arginine, methionine, cystine,
histidine, threonine, tryptophane, valine.)
• The amino acids used for gluconeogensis
or for energy enter TCA cycle (Fig. 7.2).
• Glucose as an energy source
• Glucose is metabolized to energy has two
stages.
(1) glycolysis : occure under anaerobic condition
and production of pyruvate. Referred to as the
Embden-meyerhof pathway ( in the cytosol).
(2)oxidation of pyruvate: pyruvate produced by
glycolysis is transported into the mitosol and is
oxidized to CO2 and H2O with production of
energy
• Energetics of glucose catabolism
– 1 mole of glucose to 2 mole of pyruvate
(produce 8 mole ATP(10-2))
– 2 mole of pyruvate to 2 mole of acetyl-CoA
(produce 6 mole)
– 2 mole acetyl-CoA to CO2 and H2O
(produce 24 mole)
Energetics of glucose catabolism
異化1mole glucose
產生38mole ATP
glycolytic pathway
8 mole of ATP
pyruvate to acetyl CoA
6 mole of ATP
acetate to CO2 and H2O
24 mole of ATP
Totle
38 mole of ATP
• One mole of ATP has a value of 8Kcal/mole
• 1 mole glucose 氧化產生之能量為
8Kcal x 38 = 304Kcal (45% of efficiency).
• C6H12O6 → 6CO2 + 6H2O + 673 Kcal
• The conversion of the free energy of the oxidation of
glucose has an efficiency of 40-65%.
Acetic acid as an energy source
• Acetic acid is the major product of
carbohydrate digestion in the rumen and
only present in the peripheral blood. It is
used as a source of energy by various
tissues
Propionic acid as an energy
• In ruminant propionate are produced from
carbohydrate breakdown in the rumen. →
• Pass across the rumen wall to the liver→
• Change into glucose. : The first stage is
conversion to succinyl-COA.(fig.9.7)
Butyric acid as an energy source
• Butyric acid is converted toβhydroybutyrate in its passage across the
rumen walls and used as a source of
energy by number of tissues
Abnormal carbohydrate metabolism
(一) Ketosis:
• 因碳水化合物及脂肪代謝異常, 使過量之酮
體(acetone, acetoacetate, Bhydroxybutyrate)累積於體組織。
• 血中濃度增加稱Ketonemia(acetomia), 出現
在尿中則稱Ketoneuria.
• 特別容易出現於反芻動物開始泌乳(牛)或懷
孕期(羊)，對葡萄糖需要量大時，Vit B12缺
乏時。
hypoglycemia, depleted liver glycogen
↓
elevated mobilization of adipose tissue lipids
↓
increased production of ketones and
Lipemia
↓
Increase breakdown of tissue protein for energy
↓
• body weight loss (breakdown of tissue protein)
milk production decrease
abortion in pregnant animals
water consumption increase (loss body fluid )
loss of electrolytes (Na, K)
• Treatment :
injection of intravenous glucose, adrenal
corticoid hormones, or adenocorticotropic
hormone(ACTH)
Diabetes mellitus
• hyperglycemia ( excessive blood glucose )
• three types :
– insulin–dependent diabetes (typeⅠ)
– Non-insulin–dependent diabetes (typeⅡ)
– gestational diabetes
insulin
•
•
•
•
promote glucose utilization
increase protein synthesis
decrease protein catabolism
increase the entry of amino acids into the
cell
• stimulates formation of triglycerides (TG)
• inhibit TG breakdown
Diabetes mellitus
• TypeⅠ:
– destruction of the beta cell of Islets of
Langerhans in the pancrease (缺insulin)
– appear any age (genetic)
– require insulin administration
Diabetes mellitus
• Type Ⅱ:
– defect in utilization of insulin by liver and other
tissue (insulin作用被破壞), defective cellular
uptake of glucose and impaired release of
glucose from liver, 造成高血壓。
Diabetes mellitus
• Type 3:
– increased tissue resistance to the action of
insulin and increase the blood glucose.
Insulin deficiency
• carbohydrate, lipid, amino acid metabolism
abnormal
↓
» negative N balance
» glucose loss in urine
» polyuria
» electrolyte loss
Insulin – like growth factors (IGF)
• IGF I and IGFⅡ are small polypeptides
that control the growth of several types of
cells, have a structure very similar to that
of insulin.
• IGFⅠand bone growth 有關
IGFⅡand fetal growth 有關
• IGFⅠand IGFⅡand their binding
proteins are major links between
nutrient intake and anabolic response
of cells.
Plant fiber as energy source
• Plant fiber :cellulose, hemicellulose, liglin,
gum, and β-glucan are present as cellular
membrane and storage form of energy in
the plant.
• Tannin present in the seed coat of dark
seed (sorghum), it is unpalatable to
animal and birds.
• Van Soest partition plant tissues into
two fractions.
– Cell wall constituents
• Lignin, cellulose, hemicellulose
• Gum: pectin, glucan
• －Cell contents
• Simple sugars, starches
Summary
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Monosaccharide
Disaccharide
Oligosaccharide : 3-10 sugar unit
Polysaccharide : >10 sugar unit
Crude fiber
Dietary fiber
Neutral detergent fiber
Acid detergent fiber