carbs part 1 Flashcards
whats the carb general formula
Cm(H2O)n
what are carbs and what groups do they contain
hydrates of carbon though some contain other elements such as nitrogen
Synthesis by plants from CO2 & H2O + energy
they contain the carbonyl (C=O) and hydroxyl (OH) groups. which hydrogen bonds occur between
discuss alcohol solubility
Alcohols tend to be very soluble in water due to hydrogen bonding between water molecules and the polar OH groups.
name the 3 major carb groups
monosaccharide, oligosaccharide, and polysaccharide
monosaccharides NO. units and ex
they have 1 unit
e.g. ribose, glucose, fructose,
oligosaccharides unit NO. and ex
they have 2 or more units that don’t have to be same they can be different from each other
e.g. sucrose, maltose, lactose
polysaccharides NO. units and ex
they have many units (100 - 1000)
as poly means many
e.g. cellulose, starch, glycogen
what are the most common monosaccharides
pentoses (5 carbons) and the hexoses ( 6 carbons).
naming monosaccharides
Common names are used, ending in ‘ose’
what are monosaccharides classified as and how
Classified as aldoses or ketoses, depending on whether they contain an aldehyde or a ketone group.
what do stereoisomers have
multiple chiral carbons
in what shape are sugars found in nature
in a ring
in an aldose sugar what is attached to the first and last carbon
the first carbon is at top of chain and has an aldehyde group attached
the last carbon has the primary alcohol group attached (hydroxyl group)
how many chiral carbons does a hexose aldose sugar have
4
Therefore there are 2 to the power of 4 =16 stereoisomers , ie. 8 pairs of mirror images.
so 8 sugars
what does a chiral carbon have that a non chiral carbon doesn’t have
4 diff groups attached to the carbon
are ketoses rare or common in nature
rare.
fructose is the most interesting and common one
how many chiral carbons does a hexose ketose sugar have
3 chiral carbons …. Therefore there are 23=8 stereoisomers , ie. 4 pairs of mirror images.
what is D/L nomenclature is based on for
orientation of the hydroxyl group on the greatest number chiral Carbon, ex c6 on the aldose or ketose
as chiral carbons are numbered after the c=o group
D for right hand side
L foe left hand side
what are enantiomer optical isomers
non-superimposable mirror images of one another eg. D and L glucose
what are Diastereoisomer optical isomers
if not mirror images but their chiral carbons are connected to the exactly the same substrates but connected at differing configurations
what are epimer optical isomers
two diastereoisomers that differ only at one stereocenter eg. D-Arabinose and D-Ribose
what do cyclic forms result from
the intramolecular reversible reaction of their C=O group with an OH group (usually the OH on the highest numbered chiral carbon).
sugars in nature exist by large as a ring structure, as there most stable. But aren’t a ring structure in solution.
Its reversible reaction as ring opens and closes continuously but at very high speed
But theres always a time when we have a number of closed molecules at one time as they have to be in closed form in nature
Theyre intramolecular as it happens within the molecule
what forms when alcohol reacts with aldehydes and ketones
“hemi acetyls” or “hemi ketals”
what are pyranose form of a monosaccharide and furanose form
The pyranose form of a monosaccharide has a six-membered ring, while the furanose form has a five-membered ring.
what is the chair type structure
sugars exist in solution in the most thermodynamically favourable arrangement. For most 6-membered rings this is the chair-type structure.
what is Mutarotation (interconversion)
When sugars such as glucose are in the ring form, carbon 1 is chiral and is called the anomeric carbon.
The new stereoisomers, termed anomers, are designated a and b.
what is an alpha anomer
OH group at C-1 is on the opposite side of the ring from the CH2OH group…. shown here below the plane of the ring.
what does glucose crystallised from ethanoic acid give
pure beta form
[beta]D +19 degrees
what does glucose crystalised from methanol give
pure alpha form
[alpha]D +113 degrees
the alpha and beta anomers differ in the degree of rotation of what
plane-polarized light
what happens during recrystallisation and whats it used for
you take a compound eg glucose and you crystallise it in a lil bit of warm solvent, eg, ethanol, methanol, etc. and you leave it until the crystals form
Crystallisation is used to purify compounds
what can the change in the Mutarotation of glucose be monitored by
a polarimeter
what happens in mutarotation of glucose when the pure alpha and beta forms of glucose are dissolved in water
the rotation gradually changes until it reaches an equilibrium value of + 52 degrees
due to the slow conversion of pure anomers into a 37a:63b equilibrium mixture. . in This the example, the beta form is dominant for glucose
what happens in a polarimeter during glucose mutarotation
the light shines through the sample, this analyses what comes out and then you detect the results
whats the Most stable form of Glucose in solution
C1 conformation and the b anomer is preferred for D-glucose in solution and therefore this form predominates. this keeps all the hydroxyl groups apart from each other
what is oxidation reaction
the addition of oxygen
what are weak oxidising agents and what do they do
Tollens, Fehlings and Benedicts solutions oxidise the carbonyl group end of the monosaccharide.
what are tests for reducing sugars and explain the test
With Fehling’s solutions, glucose reduces the copper Cu2+ ion (blue) to the Cu+ (red) ion.
We get oxidation of the aldehyde to a carboxylic acid and the sugars are called reducing sugars.
This is used as a test for reducing sugar
Fehling’s is copper sulphate in sodium hydroxide
The oxidising agent is reduced to copper oxide, from blue to orange. So Fehling’s solution is reduced to copper oxide
So we go from aldose sugar to an acid sugar
So glucose is a reducing sugar cos of what happened to Fehling’s solution
what do sugars need in order to be reducing
a free aldehyde group.
this isn’t present when in ring form
how is glucose able to react with Fehling’s, Benedict’s and Tollen’s reagents.
because even though Glucose is mainly in the cyclic form in solution, the equilibrium to the open chain form is established quickly enough to allow it to react with the Fehling’s, Benedict’s and Tollen’s reagents.
how can A ketose undergo oxidation reactions even though it does not possess an aldehyde group
because in alkaline solution the ketose form is in equilibrium with the aldose form through an enediol intermediate.
So if put fructose in water with Fehling’s reagent then fructose will reducing sugar, even tho there’s no aldehyde, because of the intermediate
what does a strong oxidising agent do and give an ex of a strong oxidising agent
Such as dilute nitric acid, can oxidise both ends of a monosaccharide at the same time - so is why its called strong. (carbonyl group + terminal primary OH group) to produce a dicarboxylic acid.
Eg. glucaric acid
what groups are most easily oxidised by a strong oxidising agent
Aldehyde groups most easily oxidised, followed by the primary alcohol group
what can enzyme systems do
oxidise the primary alcohol end of an aldose such as glucose, without oxidising the aldehyde group to give a uronic acid.
enzymes are selective so that’s why there good
what are uronic acids
Often components of polysaccharides. Glucuronic acid is, for example, a component of hyaluronic acid, the main component of vitreous humour of the eye.
what happens during reduction
The carbonyl group (CHO) can be reduced to an hydroxyl group (OH), using hydrogen as the reducing agent to produce a polyhydroxy alcohol (sugar alcohol).
what are examples of reduction made sugars
glucitol - occurs naturally in some fruits and berries and is sold commercially as the bulk sweetener “Sorbitol”.
sweetener in “tooth friendly” gum.
moisturising agent in food and cosmetics
what are deoxy sugars
one of the OH groups is completely reduced to H
how are phosphate esters formed
The hydroxyl group of a monosaccharide can react with inorganic oxyacids such as phosphoric acid to form inorganic esters.
acid + alcohol -> ester + water
what are the important roles of phosphate esters
carbohydrate metabolism and is a component of DNA and RNA.
how are glycosides formed
Cyclic forms of monosaccharides react with alcohols or with amines to form glycosides.
how do glycosides bond
through the anomeric carbon
how are cyclic molecules that reacat with another alcohol such as disaccharides bound
by glycosidic linkage
how are foxgloves toxic glycosides
Foxgloves (Digitalis purpurea) provide the important heart stimulant, digitalin.
They become harmful when the glycon portion is stripped off during digestion.
amino sugars of what are common in nature
glucose, mannose and galactose
how does the amino sugars structure differ from other sugars
Hydroxyl group on carbon-2 is replaced with an amino group (NH2) (enzymic reaction). this occurs under enzymes in nature
what is good about amino sugars being present in the biological marker on red blood cells
it allows us to distinguish blood types
explain the glycosidic linkage/bond in oligosaccharides
The cyclic form of one monosaccharide can react with an alcohol group from another to form a disaccharide.
The mono units are linked by glycosidic bond, which is covalent so strong bond
how big are oligosaccharide chains
big , usually 25 onwards
whats the diff between hydrolysis and condensation reactions
Hydrolysis is splitting apart a molecule usually in water
condensation is the opposite and water is produced
what are the Properties of glycosidic bonds
Stable under normal conditions
Hydrolysed by acid + heat or specific enzymes
explain the occurrence of free oligosaccharides
Free oligosaccharides other than disaccharides, are rarely found in biological systems and are usually associated with proteins or lipids.
disaccharides examples
Sucrose: Main food sweetener. Digestible by humans.
Lactose: 5-8% in milk. Digestible by many humans.
Cellobiose: From cellulose hydrolysis. Does not occur naturally. Not digestible by humans.
Maltose: From starch hydrolysis, used in food fermentations. Digestible by humans. Common ingredient in baby food.
what does lactose structure consist of and hows it bonded
Lactose consists of β-D-galactose and D-glucose molecules bonded through a β1-4 glycosidic linkage.
The C1 on the glucose unit is not involved in bonding
what do reducing sugars do
open the ring
how can the disaccharides of lactose, cellobiose and maltose, possess a free aldehyde group
The glucose unit, shown on the right, in lactose, cellobiose and maltose can exist in the open chain form in solution
They are therefore reducing and can exist in both alpha and beta anomeric forms in solution.
explain the glycosidic bond structure of sucrose
The glycosidic bond in sucrose is an a, b (1-2) linkage and involves the anomeric carbons of both its constituent sugar residues.
its made up of 2 anomeric carbons, not alcohol
what does the locked closed ring of glucose and fructose cause for the disaccharide
disaccharide does not possess a free aldehyde group in solution and sucrose is non reducing.
enzymes are specific so hydrolyse the linkage in which 3 structures, and dont hydrolyse the linkage in which 1 structure
Human enzymes can hydrolyse the linkage in sucrose, lactose and maltose but not the b 1-4 linkage in cellobiose (a derivative of cellulose).
sucrose can be hydrolysed by acids or invertase to produce what
glucose and fructose, this is invert sugar
what can invert sugar be used for
Crystallisation control
Texture softening
Moistness
Increased viscosity
Reduced freezing point
what are the general properties of polysaccharides
Contain many, often tens of thousands of monosaccharide units joined by glycosidic linkages. so very long
Each addition carried out by a specific enzyme.
Linear or branched.
May be homogeneous (contain similar sugar units in chain), or heterogeneous. (heterogeneous contain diff sugar units in their chain)
Commonly food reserves and structural components of cells.
Often insoluble and difficult to purify.
how do we name polysaccharides
Originally named according to their source, properties or function.
Now named on the basis of their monosaccharide units:
- homopolysaccharides - replace “ose” of sugar with “an” eg. glucan.
- heteropolysaccharides - use structural unit of main chain, prefixed with names of other units. eg. galactomannan
what are examples of common glucans
Starch - stored polysaccharides in plants
Glycogen - stores polysaccharides in animals such as humans
cellulose - in structure in plants
why store glucose as a polysaccharide
Osmotic pressure is proportional to the number of solute molecules … So 1000 glc molecules has a 1000 x higher osmotic pressure than 1 molecule of glucose polymer with 1000 units.
carb sugars are stored as polysaccharides
what is the glucan - starch
Is the main food reserve in plants eg. Cereals, potatoes, rice etc. (present as granules)
whats the structure of amylose
Linear with all the glucose units linked via a(1-4) linkages [200-2000 glc units long].
its soluble in cold water
what chape does amylose form
helix
what colour does amylose form with iodine
blue complex
what is the structure of amylopectin
Branched, having a small no. of a(1-6) linkages at various points along an a(1-4) chain [up to 100,000 units].
Branching is similar to the veins of a leaf, starting with a single strand which branches out every 20-25 units.
what percentage do most starches (maize, potato, rice) contain of amylose
20 - 30%
some cereals contain only 0 - 5% (waxy mutants).
what is glycogen
Energy reserve polysaccharide, abundant in the liver and in muscle cells.
explain the structure of glycogen
Similar to amylopectin in structure but more highly branched, with shorter branches (average of 12 glucose units). Around 60,000 glucose units .
how is glycogen built up and broken down
by enzymic reactions
where is cellulose found
Structural component of plants cell walls.
what is the structure of cellulose
Composed of linear chains of 2000-3000 glucose units, joined by b(1 → 4) linkages.
whats the importance of linearity in cellulose in plants
Linearity - allows polymers to line up in fibres with a great deal of hydrogen bonding between adjacent chains (strong) and little interaction with water (insoluble).
