Test 3: Carbohydrates Flashcards
What are carbohydrates? What are they composed of? What are they also called?
- major energy source
- composed of: C, H, and O
- called “saccharides” meaning sugar
Explain the formation and distruction of carbohydrates.
formation: photosynthesis
- eg. glucose in plants from CO2, H2O and ATP (sun).
destruction: respiration (oxidation)
- produces CO2, H2O and energy
Give a precise definition of carbohydrates.
- polyhydroxyaldegydes,
- polyhydroxyketones
- substances that give such compounds on hydrolysis.
What are the different types of carbohydrates? What do they form when water is added?
monosaccharides:
- simplest carbohydrates
- + H2O –> no hydrolysis
- cannot be hydrolysed
disaccharides:
- two monosaccharides
- + H2O –> two monosaccharide units
olgosaccharides:
- 2+ monosaccharides
polysaccharides:
- 100-1000 monosaccharides
- + many H2O –> many monosaccharide units
How are monosaccharides classified?
- number of carbon atoms (3-6 usually)
- the carbonyl group: aldehyde or ketone
What is the colour of monosaccharides? Are they soluble? What taste do they have?
- colourless
- crystalline solids
- soluble in water
- insoluble in nonpolar solvents
- sweet taste
State the two major types of monosaccharides.
- ketoses
- aldoses
What are aldoses?
monosaccharides with:
- aldehyde group (H-C=O)
- many hydroxyl groups
What are ketoses?
monosaccharides with:
- ketone group (C=O)
- many hydroxyl groups
What structure is used to represent carbohydrates? What are the two types? How to tell them apart?
fisher projection:
- L-isomer: OH- is on the left of the chiral carbon
- D-isomer: -OH is on the right of the chiral carbon
What are the principal rules of the fisher projection?
- most oxidised group on top
- chiral carbon shown at the intersection of vertical and horizontal lines
- -OH of chiral carbon determines L-/D- isomer
What is D-glucose?
- aldohexose
- C6H12O6
- blood sugar
- monosaccharide of starch, cellulose, and glycogen
What is the normal level of glucose in the blood?
4-6mmol/L
What is D-fructose?
- ketohexose
- C6H12O6
- sweetest carbohydrate
- converts to glucose in the body
What is D-galactose?
- aldohexose
- C6H12O6
- not found free in nature
- obtained from lactose
- essential for the formation of certain glycoproteins and cerbrosides
What are the most important pentoses?
D-ribose and D-deoxyribose
What is the function of ribose?
- coenzyme A
- ATP
- cyclic AMP
- monomeric units of RNR
What is teh name of the other structure of carbohydrates (not fisher)?
W. Haworth projection
Explain cyclic structures. When do they form? What are they?
- two forms of monosaccharides with 5 or 6 atoms
form when the hydroxyl group on C-5 reacts with the aldehyde/ketone group
How are stable cyclic hemiacetals formed?
- C=O group of ketone or aldehyde group
- -OH group of C’5
cyclic structure of D-isomer has CH2OH above the ring.
How to draw a W. Haworth projection?
- turn chain clockwise (90 * )
- bend chain to make hexagon
- ## draw new -OH on C1
What is the difference between alpha and beta glucose?
- alpha and beta glucose have identical configurations except at C-1
alpha glucose: -OH on bottom
beta glucose: -OH on top
What is a reducing sugar?
- monosaccharide
- carbonyl group which oxidises to form carboxylic acid
- undergo fehlings reaction, benedicts reagent
- eg. glucose, galactose and fructose
What reactions do reducing sugars undergo?
- with Fehling’s reagent (Cu2+ complexed with tartrate ion) –> deep blue colour achieved
- with Benedict’s reagent (Cu2+ complexed with citrate ion
State monosaccharide examples of reducing sugars.
- glucose
- galactose
- fructose
When do ketoses qualify as reducing sugars?
if they can isomerate to aldoses
What do monosaccharides form when they react with alcohols?
acetals
- called: glycosides
- with glycosidic bonds
What is a disaccharide?
- two monosaccarides
- glycosidic bond (between anomeric carbon and hydroxyl group)
What are the three important disaccharides?
- maltose
- lactose
- sucrose
What is maltose?
- malt sugar
- 2 D-glucose molecules
- hydrolysis of starch
-
alpha-1,4-glycosidic bonds:
alpha-OH on C’1 & -OH C’4 of
In what forms is maltose found in?
both alpha and beta
Why is maltose a reducing sugar?
it closes and opens
What bond does maltose have? Between what molecules?
2 alpha glucoses
- alpha-1,4-glycosidic bond
What is lactose? What bonds does it contain? What monosaccharides is it formed from? Is it a reducing sugar? Why?
beta-D-galactose and alpha/beta-D-glucose
- beta-1,4-glycosidic bond
- reducing sugar: opens and closes
What is sucrose? What bonds does it contain? What monosaccharides is it formed from? Is it a reducing sugar? Why?
- sugar cane and sugar beets
- alpha-D-glucose + beta-D-fructose
- alpha,beta-1,2-glycosidic bond
- NOT a reducing sugar
- easily hydrolysed
What does the hydrolysis of sucrose form?
alpha-glucose and beta-fructose
What are polysaccharides?
- polymers of** D-glucose**
- amylose, amylopectin, starch, glycogen made of alpha-D-glucose
- cellulose made of beta-D-glucose
What is starch? How can it be seperated?
- energy storing carbohydrate
- plants (glucose storage)
- seperated into 2 fractions: amylose and amylopectin
What is amylose? What bonds are present? What does it form?
- polymer of alpha-D-glucose units
- alpha-1,4-glycosidic bonds
- continuous unbranched chain (forms coils ‘amyl”O”se’)
What is amylopectin? What bonds are present? What does it form?
- polymer of glucose units
- branched polysaccharide chain
- alpha-1,4-glycosidic bonds (in horizontal lines)
- alpha-1,6-glycosidic bonds (between branches)
Explain the function of the starch iodine test.
starch: deep blue colour complex with iodine
- iodine + amylose helix = large complex
(other polysaccharides give other colours (ie. glycogen- red))
What is glycogen? What is it composed of? What is its function?
- energy storing carbohydrate
- made of 1,4- and 1,6- linked glucose units
- helps maintain glucose balance in the body
- removes and stored excess glucose
- branched!
What is cellulose? What is it composed of? What is its function?
- polysaccharide of glucose
- unbranched chains
- beta-1,4-glycosidic bonds
- cannot be digested by humans
Why can cellulose not be digested by humans?
Humans cannot break down beta-1,4-glycosidic bonds.
What are the two types of starch?
- amylose
- amylopectin
What are chitins?
- nitrogen containing polysaccharides
- forms crustaceans shells + insect exoskeletons
What are pectins?
polysaccharides
- used for jellies
- monomer: galacturonic acid
