Carbohydrates Flashcards
simple carbohydrates
monosaccharides
- most common is glucose
- naturally occurring
- cannot be hydrolyzed into a smaller unit
- “reducing sugar” when the anomeric carbon is free
disaccharides
- most common is sucrose
- 2 monosaccharides joined by an acetyl bond (glycosidic)
complex carbohydrates
oligosaccharideds
- short polymers
polysaccharides
- homo and hetero exist
- glycogen (animal), starch and cellulose (plant)
ratio of organic molecules in carbohydrates
CHO = 1:2:1
most common monosaccharides
Triose (3C)
- metabolites of glucose
- broken down into trioses in glycolysis
Pentose (5C)
- components of DNA and RNA
- sugars in nucleic acids
Hexose (6C)
- nutritionally the most important
- give our body energy
what is an anomeric carbon
the carbon that is apart of the carbonyl (C=O) group
what is an aldose
a sugar that contains an aldehyde (e.g. D-glucose)
what is a ketose
a sugar that contains a ketone (e.g D-fructose)
what is sterioisomerism?
CHO with the same molecular formula and sequence but differ in 3D space due to chiral carbonds
- come in D and L forms
- enzymes used for digestion can recognize stereoisomers (steriospecific)
what is a chiral carbon?
a carbon attached to 4 DIFFERENT atoms or groups
different types of isomers
enantiomers: mirror image
diasteromers: not mirror images
Fischer projection
- linear form of a sugar
- counting of carbons begins at the anomeric carbon for an aldose
Stereoisomers and Fischer projections
- D or L for is determined by the -OH group on the highest chiral carbon (furthest from C=O)
- OH on the right = D
- OH on the left = L
- the number of stereoisomers is 2^n (n = # of chiral carbons)
why are D monosaccharides nutritionally important?
digestive enzymes are stereospecific for D sugars
converting a linear sugar to a cyclical sugar
- the anomeric carbon allows the linear structure to turn cyclical
- carbon 1 becomes a new chiral center
- hemiacetal: made from an aldose
- hemiketal: made from a ketose
- reaction occurs spontaneously (no enzymes involved and reversible)
alpha and beta configurations of cyclical sugars
- alpha = OH on anomeric carbon is down
- beta = OH on anomeric carbon is up
- beta is more common
- alpha is more nutritionally important for us
Haworth model of sugars
- the non-acetyl/ketal Ch2OH always points up (C#6)
- OH group right in fischer = down in haworth, left in fischer = up in haworth
- alpha = OH on C1 is down, beta = OH on C1 is up
disaccharides
- the most common oligosaccharide
- 2 monosaccharides attached by a glycosidic bond
- glycosidic bond can be alpha or beta
- configuration of -OH group on the anomeric carbon determines whether the disaccharide is alpha or beta
common disaccharides
sucrose (alpha(1-2))
- found in sugar cane and fruits
- glucose + fructose
- non-reducing (both anomeric carbons are used)
lactose (beta(1-4))
- found in milk
- galactose + glucose
- reducing - free anomeric carbon
maltose (alpha(1-4))
- found in beer and liquor
- glucose + glucose
- reducing - free anomeric carbon
polysaccharides
long strings or branches of monosaccharides (min 6) attached by glycosidic bonds
homo: all the same monosaccharides
hetero: different monosacharides
- both found in nature but homo ar more common in food
advantage of branching in polysaccharides
- provides a larger number of ends from which to cleave glucose when energy is needed
what is dietary fibre
- non-digestible CHO
- structureal part of plants
2 types: soluble and insoluble
insoluble fibre
- includes cellulose, lignin and hemicellulose
- remains intact through the intestinal tract
- doesn’t dissolve in water
- reduced transit time (moves quick through gut)
- increased fecal bulk
soluble fibre
- includes pectins, gums and mucilages
- forms a gel in intestinal tract
- dissolves in water
- delays gastric emptying (increased transit time)
- slows the rate of nutrient absorption
dietary fibres and solubility
characteristics of solubility are…
water holding ability (ability of a fibre to hold water, becoming a viscous solution)
absorptive ability (ability of a fibre to bind enzymes and nutrients)
why is dietary fibre beneficial for our gut health
dietary fibre feeds our gut microbiota, which reduced inflammation; less stress on host
cellulose
- both a dietary fibre (naturally occuring) and functional fibre (natural but added
- homopolysaccharide of B-1,4 glucose units in a linear chain
- poorly fermented by human gut bacteria ( because humans lack cellulose-fermenting microbes in their gut microbiome
- rich in bran, legumes, nuts, peas, etc.
hemicellulose
- heteropolysaccharide that varies between plants
- a mixture of alpha and beta glycosidic linkages
- can contain both pentoses and hexoses (xylose is most common)
- exists as both branched and linear structures
- the solubility and fermentability of hemicellulose depends on the sugar composition
- found in bran, whole grains, nuts and some vegetables and fruits