Chapter 7

Carbohydrates

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, hemicellul

ose) 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 beta-glucans, 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 glucos

e-(!-4 )-α-glucoside linkage. • Amylose : glucose(1-4)-α-glucoside linkage linear polym

er of glucose, MW 15000 ~ 100000 D.Amylopectin: linear polymer with branches 10 - 100 milli

on D.• Maize contains 24% amylose and 76% amylopectin, Wa

xy maize contains 1%:99%• Rice and potatoes contain 20%:80%• The hydrolysis of starch in amylose and amylopectin to g

lucose 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.

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 )

Absorption and transport of monosaccharides

2.Mechanism• Sodium dependent active transport mech

anism 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) fastin

g, 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 co

ntent)• interaction of starch with protein, fat.• presence of antinutrient such as phytate, t

annin, saponins and enzyme inhibitors.

( 三 ) Metabolic conversions

• Homeostasis : maintaining the blood glucose concentration

glycogenesis insuline↑

blood glucose glycogen

glycogenolysis glucagon↑

Gluconeogenesis • glucose can be formed by body tissues fro

m 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 stage

s.

(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 (pr

oduce 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

• 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%.

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

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 toβ-hydroybutyrate 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, B-hydroxybutyrate) 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)

• hyperglycemia ( excessive blood glucose )

• three types :– insulin–dependent diabetes (type )Ⅰ– Non-insulin–dependent diabetes (type )Ⅱ– gestational diabetes

Diabetes mellitus

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

• Type :Ⅰ– destruction of the beta cell of Islets of Langerh

ans 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.

• carbohydrate, lipid, amino acid metabolism abnormal

↓

» negative N balance

» glucose loss in urine

» polyuria

» electrolyte loss

Insulin deficiency

Insulin – like growth factors (IGF)

• IGF I and IGF are small polypeptides that Ⅱcontrol the growth of several types of cells, have a structure very similar to that of insulin.

• IGF and bone growth Ⅰ 有關 IGF and fetal growth Ⅱ 有關• IGF and IGF and their binding protⅠ Ⅱ

eins 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

• Monosaccharide• Disaccharide• Oligosaccharide : 3-10 sugar unit• Polysaccharide : >10 sugar unit• Crude fiber• Dietary fiber• Neutral detergent fiber• Acid detergent fiber