1. Carbohydrates
Lecture3

2. Polysaccharide

3. Polysaccharides and sácchar means "sugar“
From the Greek:  poly means “many",
and  sácchar means "sugar“
Polysaccharides are complex carbohydrates, composed of 10 to up to several thousand monosaccharides arranged in chains.
The most common monosaccharides that appear as parts of polysaccharides are glucose, fructose, galactose and mannose.
Three main polysaccharides related to the human nutrition include:
Starch ─ an energy source and storage form of glucose in plants
Glycogen ─ an energy source and a storage form of glucose in the human liver and muscles
Cellulose ─ a structural polysaccharide in plants

4. Polysaccharides Starch ─ an energy source obtained from plants:
Starch is (polymer of glucose) a polysaccharide comprising glucose monomers joined by glycosidic bonds.
It consists of two types of molecules: amylose and amylopectin.
Amylose : is the simplest form of starch; it is the linear polymer of glucose monomers joined in α (1,4) linkages
Amylopectin: is the branched form. Branching takes place with α(1→6) bonds occurring every 24 to 30 glucose units, resulting in a soluble molecule that can be quickly degraded

6. Polysaccharides Starch ─ an energy source obtained from plants:
Starch is the most common carbohydrate in the human diet and is contained in many staple foods. The major sources of starch intake worldwide are the cereals (rice, wheat, and maize) and the root vegetables (potatoes and cassava).
The enzymes that break down or hydrolyze starch into the constituent sugars are known as amylases.
Alpha-amylases are found in plants and in animals.
Human saliva is rich in amylase, and the pancreas also secretes the enzyme:
** secreted Alpha-amylases hydrolyse dietary starch into disaccharides and trisaccharides which are converted by other enzymes to glucose to supply the body with energy

7. Polysaccharides
2. Glycogen ─ an energy source and a storage form of glucose in human
Glycogen is the analogue of starch!
It is a glucose polymer that functions as energy storage in human.
It has a structure similar to amylopectin (a component of starch), but is more extensively branched and compact than starch.

8. Polysaccharides
2. Glycogen ─ an energy source and a storage form of glucose in human
Glycogen is mainly stored in muscles and liver
>> it provides the body with a readily available source of energy if blood glucose levels decrease.
Glycogen breakdown into glucose:
(glycogen -> G1P -> G6P -> Glucose).
>> Glycogen Phosphorylase catalyzes breakdown of glycogen into Glucose-1-Phosphate (G1P)
>> G1P then into G6P under the action of Phosphoglucomutase enzyme
>> G76P into Glucose under the action of glucose phosphatase enzyme

10. Polysaccharides 3. Cellulose ─ a structural polysaccharide in plants:
Cellulose, a major constituent of plant cell walls,
It is a linear chain of several hundred to many thousands of glucose joined with ß (1,4) linkages. Every other glucose is flipped over, due to ß linkages.
Cellulose is considered insoluble because it does not bind with water nor change form in the digestive tract.

11. Polysaccharides 3. Cellulose ─ a structural polysaccharide in plants:
Humans are unable to digest cellulose because the appropriate enzymes to breakdown the beta linkages are lacking.
Despite the fact that humans (and many other animals) cannot digest cellulose, cellulose is a very important part of the healthy human diet.
>> This is because it forms a major part of the dietary insoluble fiber that we know is important for proper digestion.
>> insoluble fiber (like cellulose) travels through the digestive system unchanged,
>> it helps move waste through your digestive tract, which prevents constipation.

13. Polysaccharides
Three main polysaccharides related to the human nutrition

14. Carbohydrate Digestion
Begins in mouth
chewing releases saliva
enzyme amylase hydrolyzes starch to maltose
Stomach
no enzymes available to break down starch
acid does some breakdown
fibers in starch provide feeling of fullness

15. Small intestine majority of carbohydrate digestion takes place here.
pancreatic amylase reduces carbohydrates to glucose chains or disaccharides.
specific enzymes finish the job
maltase
maltose into 2 glucose
sucrase
sucrose into glucose and fructose
lactase
lactose into glucose and galactose

17. >> then they are transferred to the blood stream.
Large intestine
Sugars and starches usually take 1-4h to be totally digested and absorbed by small intestine
>> then they are transferred to the blood stream.
Carbohydrates that are not fully digested or absorbed (fibers), they are eliminated from the body.
All undigested carbohydrates move from the small intestine to the large intestine and to the colon, where elimination finally occurs.
Fiber can attract water, which softens stool
Bacteria ferment some fibers

18. Carbohydrate Absorption
glucose can be absorbed in the mouth. majority absorbed in small intestine.
There are two major ways that molecules can be moved across a membrane, and the distinction has to do with whether or not cell energy is used.
I. Passive mechanisms (use no energy):
simple diffusion: the transportation of molecules across the cell membrane depending on their concentrations gradient (from low to high) such as glucose and galactose.
B. facilitated diffusion: is diffusion that is facilitated by a membrane transport channel.
These channels are almost always specific for either a certain molecule or a certain type of molecule.
Example: FRUCTOSE- (GLUT5)
Active transport (requires energy from the cell):
Sometimes the body needs to move molecules against their gradient.
The transportation of molecules across a specific transmembrane protein such as fructose.
Example: Glucose and Galactose-(Na+/glucose cotransporter)

19. Lactose Intolerance more lactose is consumed than can be digested
lactose molecules attract water
cause bloating, abdominal discomfort, diarrhea
intestinal bacteria feed on undigested lactose to produce acid and gas

20. Carbohydrate Metabolism
When we eat – intake glucose increased therefore, the liver condenses extra glucose to glycogen.
When blood glucose falls, the liver hydrolyzes glycogen to glucose
1/3 of body’s glycogen is stored in liver and released as glucose to bloodstream
Inadequate supply of carbohydrates
ketone bodies (fat fragments) are an alternate energy source during starvation
excess ketones can lead to ketosis: imbalance of acids in body
when carbohydrates are severely restricted, the liver converts fat into fatty acids and ketone bodies to be used as an alternative energy source in a process called ketosis

21. Glucose Homeostasis
Maintaining an even balance of glucose is controlled by insulin and glucagon
insulin
moves glucose from the blood
glucagon
brings glucose out of storage

22. 1 2 3 4 5 6 7 Intestine Fat cell Muscle Pancreas Glucose Insulin
When a person eats,
blood glucose rises. 1 2
Insulin stimulates the uptake of
glucose into cells and storage
as glycogen in the liver and
muscles. Insulin also stimulates
the conversion of excess
glucose into fat for storage. 3 4 5 6 7
Blood glucose begins to
rise.
a The stress hormone
epinephrine and other hormones
also bring glucose out of storage.
Glucose
Insulin
Glucagon
Glycogen
Glucagon stimulates liver
cells to break down glycogen
and release glucose into the
blood.a
Liver
Low blood glucose stimulates
the pancreas to release
glucagon into the bloodstream.
As the body's cells use
glucose, blood levels decline.
Pancreas
Fat cell
Muscle
High blood glucose stimulates
the pancreas to release insulin.

23. Imbalance
diabetes
after food intake, blood glucose rises and is not regulated because insulin is inadequate.
hypoglycemia
blood glucose drops dramatically
too much insulin, activity, inadequate food intake, illness.

24. REFERENCES
Murray, R., Bender, D., Botham, K., Kennelly, P., Rodwell, V., Weil., P. (2012) Harper’s illustrated biochemistry, 29th edition. Publisher: McGraw Hill Lange.
Nelson, D. and Cox, M. (2008) Lehninger principles of biochemistry, 5th edition. Publisher: W.H. Freeman and company, New York.
Harvey, R. and Ferrier, D. (2011) Lippincott’s illustrated reviews: biochemistry, 5th edition. Publisher: Lippincott Williams & Wilkins.
Moorthy, K. (2008) Fundementals of Biochemical Calculations, 2nd edition. Publisher: CRC Press.
Weil, P.A., (2012) The diversity of the endocrine system. Chapter 41 in HARPER’S Illustrated biochemistry, 29th edition. Edited by: Murray, R.K., Bender, D.A., Botham, K.M., Kennelly, P.J., Rodwell, V.W., Weil, P.A.
Sam, A. and Meeran, K. (2009) Lectures notes in endocrinology and diabetes. Publisher: Wiley-Blackwell.
Preston, R.R. and Wilson, T.E. (2013) Endocrine system in Physiology. Publisher: Lippincott Williams & Wilkins.