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Carbohydrates
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Carbohydrate – (hydrated carbon)
Carbohydrates have empirical formula Cx(H2O)y.
Most abundant carbohydrate is glucose, C6H12O6.
Glucose is a 6 carbon aldehyde sugar and fructose is a 6
carbon ketone sugar.
• The alcohol side of glucose can react with the aldehyde
side to form a six-membered ring.
• Glucose and Fructose are the main substrate for
respiration, releasing energy for all cell processes.
• Most glucose molecules are in the ring form.
• Note the six-membered rings are not planar.
• Focus on carbon atoms 1 and 5: if the OH groups are on
opposite sides of the ring, then we have -glucose; if
they are on the same side of the ring, then we have glucose.
• The - and - forms of glucose form very different
compounds.
Monosaccharides
Simplest form of carbohydrates
Classified according to number of carbons
Triose – 3C’s
Pentose – 5C’s
Hexose – 6C’s
All have 2 or more alcohols and a carbonyl group
General Formula – CH2O
These simple sugars that cannot be broken down
by hydrolysis with aqueous acids.
2 Common Monosaccharides – both
isomers
C6H12O6
Each contain
many OH groups
which make these
molecules soluble
in water.
This are the straight
chain form of
these sugars.
Aldose sugar
Ketose sugar
Monosaccharides
When these sugars are put in an (aq) solution,
they undergo and internal reaction which results in
the more familiar ring structure
The ring form of sugars make it possible for another type
of isomer.
Disaccharides
• Disaccharides are sugars formed by the condensation of
two monosaccharides.
• Examples: sucrose (table sugar) and lactose (milk
sugar).
• Glycoside linkage – “ether” bond formed when
monosaccharides combine to form disaccharides or
polysaccharides (C-O-C).
• Disaccharides can be converted into monosaccharides by
treatment with acid in aqueous solution.
Disaccharides
Lactose – glucose + glactose – found in milk
Maltose – glucose + glucose – product from starch digestion
Sucrose – glucose + fructose – table sugar
All have same molecular formula – C12H12O11
glycosidic linkage
Polysaccharides
• Polysaccharides are formed by condensation of several
monosaccharide units.
• All polysaccharides are insoluble so they are ideal for
storage.
Starch
• Main storage carbohydrate in plants – this is why food
from plants are rich sources of starch – Ex. potatoes,
rice, flour…
• Starch is a polymer of -glucose.
• Made from two polysaccharides – amylose (straight
chain) and amylopectin (branched)
• Starch contains 1,4 and 1,6 linkages.
Starch
Amylose – 1,4 linkage of -glucose monomers
Amylopectin – 1,4 and 1,6 linkage
of -glucose monomers
Glyogen
• Also a polymer of -glucose.
• Sometimes called “amimal starch” as it is the main storage carb
in animals – found in the liver and muscles
• Contains many 1,6 branches
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Cellulose
Cellulose is a structural material in plant cell walls.
Cellulose is a polymer of -glucose. It is linear.
Every other monomer is upside down – enables to
hydroxyl groups to to form hydrogen bonds
The 1,4 linkage can be hydrolyzed by cellulase which is
absent in most animals, including mammals.
Major Functions of Carbohydrates
1. Energy sources – (glucose)
2. Energy reserves (glycogen)
3. Structure (cellulose)
Dietary Fiber – mainly plant material that is not hydrolyzed by
enzymes secreted in the human digestive tract. (may be
digested by microflora in the gut) Most is excreted intact by
the body. Ex. Cellulose, hemicellulose, lignin and pectin
Starch and glycogen are easily broken down and absorbed in
the body.
Importance –as fiber passes through the digestive system it
helps stimulate the lining to produce mucus which helps
smooth passage of undigested foods. May help prevent
diverticulitis, IBS, colorectal cancer, constipation, obesity,
Crohn’s disease, hemorrhoids, and diabetes mellitus.