Chapter 55: Digestion of Carbohydrates

How are carbs digested in the large intestine?

-They are not actually digested in the large intestine but serve other purposes. -these carbs mostly FIBER serve as fuel for "bacterial metabolism" short chain fatty acids, lactate, gas -fiber also helps break down(chelate) cholesterol which has benefeicial effects lower cholesterol -colon cancer prevention

2 types of glucose transporters

SGLT 1 (against concentration gradient) GLUT (going along the concentration gradient)

starch

Starch represents the major form of stored carbohydrate in plant cells, where it is found in the cytosol as a mixture of amylose and amylopectin (figure 55.1). Foods such as flour, rice, pasta and potatoes are good sources of dietary starch, and these constitute a major source of caloric intake in humans.

GLUT II (glucose transporter)

glucose transporter in: 1.liver 2. kidneys 3. beta cells of pancreas 4. small intestine 1. LIVER insensitive to insulin It has a low affinity for glucose, this suggest it corresponds with blood sugar levels. Never saturated/completely on in blood. However, this allows hepatocytes to take up glucose for reserves 3.BETA CELLS of PANCREAS allows pancreas to make correct amount of insulin in response to glucose 4. small intestine helps enterocytes transport glucose intestinal epithelium and the blood stream also transport galactose and fructose

Where are dissacharidase enzymes most active?

jejunum and gradually decrease towards in of small intestine

How are monosaccharides absorbed by intestinal epithelial cells?

pages 303-304 last paragraph SGLT 1 / Sodium Glucose transport system draw out and label the mechanisms of transport

How is dietary starch digested?

salivary & pancreatic "amylase" enzymes

lactose intolerance

the absence or reduced activity or lactase enzyme on intestinal cell surface inherited by autosomal recessive or occurs through damaging the intestinal area

Digestion in the small intestine

- Requires first pancreatic amylase enzyme (polysacharides. -> disachharides) - second, it requires "disaccharidases" enzyme (disach. -> monosach) -third it requires carrier mediated transport systems , ex. glucose

SGLT glucose transport

- goes against concentration gradient - symport transport of glucose,(galactose but not fructose) & sodium across intestinal epithelium & reasorption by the kidneys

complex carbohydrate

- such as starch and fiber. -Starch is more readily digested than the non-starch polysaccharides such as dietary fiber (cellulose) that passes through the gastrointestinal tract and has no nutritive value since it is not digested.

amylase enzyme & glycemic index

-Amylase enzyme are salivary and pancreatic enzymes that break down dietary starches. -Amylase breaks the 1,4 links making smaller polysaccharides maltose & limit dextrin -Amylase breaks down amylose & amylopectin at different rates. -Amylose=brown rice, whole grain=digested slowly=low glycemic index -amylopectin=white bread,white rice, pasta, mash potatoes=digest fast= high glycemic index because will increase circulating glucose and insulin levels in blood conclusion: western culture has an abundance of amylopectin starches in diet which may lead to increase risk of type II diabetes

GLUT 1 (glucose transport)

-Responsible for basal glucose uptake in cells -highly affinity for glucose active transporter, completely saturated at physiological levels found in: 1.erythrocytes RBC's 2.placenta 3.fetal tissue 4. fat 5. brain found in small amounts in liver and muscle tissue

primary lactase deficiency

-carbohydrate malabosrption syndrome -lactase is digestion into CO2, H, short chain Fatty acids by intestinal gut bacteria causing bloating, gas, diarehia -occurs in latin people & children

summary of livers function in carbohydrate metabolsim

CHAPTER FIFTY-FIVE; DIGESTION OF CARBOHYDRATES IN REVIEW 1. Dietary carbohydrate may take various forms. These differ in their digestibility, from the more readily digested starch and glycogen, to the non-starch polysaccharides such as dietary fiber. 2. Digestion of dietary carbohydrate occurs in the mouth, through the action of salivary amylase, and continues in the small intestine, through the action of pancreatic amylase, and membrane bound enzymes. Some carbohydrate digestion occurs in the large intestine, through secreted bacterial enzyme activity. 3. Following a carbohydrate-rich meal, the circulating levels of glucose become elevated, insulin secretion is stimulated and cellular glucose uptake is enhanced. 4. Monosaccharides enter cells via mediated transport mechanisms. 5. The liver is the center of metabolic activity, and it is ideally positioned to perform this role. The liver can function as a 'buffer', soaking up glucose after a meal, when it is in abundance, and liberating glucose when other tissues require it. 6. Circulating glucose may enter the adipocyte and can be stored as triacylglycerol

dissacharidase enzymes

alpha glucosadase sucrase lactase maltase

carbohydrate metabolism in the liver

-center of metabolic activity within the body - recieves pancreatin hormones insulin and glucagon 1st -liver acts as a buffer soaking up glucose when there is excess and releasing it when other tissues need it -liver cells express GLUT II transport protein -glucose is phosphorylated by "glucokinase" -glucokinase does not follow mechailis mentin kinetics -glucose-6-phosphate can be turned into glycogen or go to CAC to make ATP In summary then, the liver can stimulate glycogen synthesis using glucose as the sole 'stimulant'. This is logical since digested material from the intestine reaches the liver via the hepatic portal vein, and glucose can be promptly taken up by the GLUT II-mediated mechanism. Hepatic glucokinase and an active glycogen synthesizing machinery can be activated immediately, without a need for the pancreas to secrete insulin. When appropriate, the liver is also able to break down glycogen by reciprocal activation of glycogen phosphorylase and inhibition of glycogen synthase. This mechanism is promoted by glucagon and opposed by insulin and glucose. The liver is also capable of synthesizing new glucose (in gluconeogenesis), since it possesses the enzyme glucose 6- phosphatase that can liberate glucose from glucose 6-phosphate.

GlUT glucose transports

-family of transporters that move glucose down its concentration gradient -structure: single polypeptide chain with 12 alpha helical structures

GLUT III (glucose transporter)

-found in all cells, mostly kidney, brain and nerve cells -has high affinity for glucose does not care what blood glucose levels are, glut III eats it up all day -works in cahoots with GLUT I

carbohydrate metabolism by adipocytes

-function of white adipose tissue is storage and mobilization of triacylglyrides -glucose enters adipocytes through GLUT I & IV transporters -TRAPPED in adipocytes by "hekokinase catalyzed phosphorylation" forming glucose 6 phosphate then it will enter pentose phosphate pathway or glycolyisis through the pyruvate dyhydrogenase complex 'normal' western diet do not produce significant amounts of fat from carbohydrate, since the large amount of fat in this diet down-regulates the expression of the acetyl-SCoA carboxylase and fatty acid synthase.

Cellulose

-linked by beta-1, 4 glycosidic bonds -It is a major polysaccharide in plants and forms the basis of dietary fiber. -Humans lack the "beta-1, 4 glycosidase" and are unable to digest cellulose that therefore increases stool bulk.

simple sugars

-monosaccharides and disaccharides such as glucose, fructose, sucrose, and lactose -The simple sugars are rapidly absorbed and require little digestion, rapidly increasing blood sugar level.

GLUT IV

-moves glucose into: 1.skeletal muscle 2.heart muscle 3.adipose tissue -sensitive to insulin (GLUT 1 is insensitive to insulin) insulin tells GLUT IV to go to surface increasing glucose uptake significantly in insensitive cells GLUT IV (not stimulated by insulin, unlike above) these transports are located intracellularly in cytoplasmic vesicles. a insulin deficiency could lead to an increase amount of GLUT IV glucose transport in cytoplasm contributes to hyperglycemia seen in diabetes

GLUT V

-present on apical membrane of : 1. enterocytes 2. brain 3. muscle -primarily responsible for fructose uptake! -works in cohoots with SGLT 1 glucose/sodium symport

Where does the majority of Carbohydrate digestion occur?

The majority of carb digestion occurs in the small intestines by the "pancreatic" amylase enzyme pancreatic amylase breaks down polysacchirides

How is starch digested in the stomach?

Very little starch digestion goes on in the stomach. "salivary" Amylase enzyme cannot function in high acidic/low ph environments. In fact, starch contained in the stomach likes to sit around for a while and may delay gastric emptying.

when do hepatocytes and erythrocytes start metabolizing carbohydrates

after eating a heavy meal rich in carbs the blood glucose is so high that the pancreas releases beta cells which secrete insulin stimulating the uptake of glucose by organs and tissue other than the GI system