Carbohydrates Flashcards
1
Q
Why are carbohydrates important
A
- Main source of energy
- Structural components (animals, plants, bacteria)
- Component of nucleic acids
- Contribute to protein structure and function
2
Q
What are peptidoglycans
A
- carbohydrates are involved in cross-linking peptide chains which forms a mesh to protect the cell (prokaryotes - bacteria)
3
Q
What are proteoglycans
A
- form the extracellular matrix in which all our cells are embedded, acts as a molecular sponge for water (eukaryotes - humans)
4
Q
Why is glucose important
A
- major source of metabolic energy
- Primary product of photosynthesis
- Building block - it is the primary monosaccharide so can form disaccharides and polysaccharides
- Makes cellulose, starch and glycogen
- Humans can’t digest cellulose as contains B 1-4 glycosidic linkages
5
Q
Describe Cellulose
A
- Source- plant
- Subunit- B-glucose
- Bonds- 1-4
- Branches- no
- Each alternate glucose is inverted
- Separate strands which form hydrogen bonds between each other
6
Q
Describe amylose
A
- Source- plant
- Sub-unit- a-glucose
- Bonds 1-4
- Branches- no
- Glucose forms straight chains- all facing same direction
- Coiled structure
- forms starch with amylopectin
7
Q
Describe amylopectin
A
- Source- plant
- Subunit- a-glucose
- bonds- 1-4 and 1-6
- Branches- yes
- Forms straight chain and branched chains
- Forms starch with amylose
8
Q
Describe glycogen
A
- Source- animal
- Subunit- a-glucose
- Bonds- 1-4 and 1-6
- Branches- yes (more than amylopectin)
- Very branched structure produced
- Main energy store in animals- accumulates in liver and muscles to be broken down when energy needed
9
Q
Describe sucrose
A
- Sucrose is a disaccharide of glucose and fructose
- Sweet taste is due to the fructose
- Sucrose is the main sugar we eat
- Glucose is absorbed by active transporters, therefore there is a maximum rate of glucose absorption when all active transporters are saturated
- Fructose is taken up by a different mechanism
- More energy is absorbed per unit time when eating sucrose as opposed to starchy foods because sucrose is hydrolysed into fructose and glucose which are then absorbed separately
10
Q
What are the two types of saccharides
A
- Aldose - in the aldehyde state
2. Ketose - in the ketone state
11
Q
Describe D-glyceraldehyde
A
- D-glyceraldehyde (an aldose)
- Has one chiral centre
- So has two optically isomeric forms (D-glyceraldehyde and L-glyceraldehyde)
- Can tell the difference between each enantiomer using optical rotation (plane polarised light)
- Glyceraldehyde is the molecule used in the fisher convention
12
Q
What is the Le Bel-van’t Hoff rule
A
- If a molecule has n chiral centres it has 2^n optical isomers
13
Q
Describe the structure of aldo-tetroses
A
- 2 chiral centres so 4 optical stereoisomers
- Monosaccharide with 4 carbon atoms. They have either an aldehyde functional group in position 1 or a ketone functional group in position 2.
- D (+) Erythrose D (-) threose L(+) erythrose L(-) threose
- ‘Erythro’ means substituents on the same side in the Fisher projection
- ‘Threo’ means substituents are on opposite sides in the Fisher projection
- Enantiomers are named due to the handedness of the most distal chiral centre from the carbonyl group
14
Q
What is the difference between hemiacetals and hemiketals
A
- Hemiacetals- alcohols react with aldehyde
2. Hemiketals- Alcohols react with ketone
15
Q
How is a ring formed
A
- A linear molecule forms a cyclic hemiacetal/hemiketal by the reaction of the aldehyde/ketone group and the alcohol group on carbon 5
- React intramolecularly
- There are two ways of forming the ring which means that 2 alternate anomers are formed (different conformations of the same molecule)
- These ring structures are not as strong as esters meaning they can form and unform (ring formation is reversible)
16
Q
What type of ring is glucose
A
- Hemiacetal = compound formed when an aldehyde reacts with an alcohol
- Hemiacetal is less chemically stable than an ester
17
Q
What type of ring is fructose
A
- Hemiketal = compound formed when a ketone reacts with an alcohol
- Can only form a furanose ring (5C) because the carbonyl group is on carbon 2
- Also has beta and alpha anomers
18
Q
What two types of ring can glucose form
A
- A six carbon ring (pyranose)
2. A five carbon ring (furanose)
19
Q
Why is each ring either beta or alpha
A
- The cyclisation of a monosaccharide results in a pair of diastereomers called anomers
- The hemiacetal or hemiketal carbon is referred to as the anomeric carbon
- This changes the 3D structure the rings adopt (several 3D forms e.g. boat, chair)
- beta anomers are more stable
- In free solution of glucose there are more pyranoses than furanoses
20
Q
How do you name a ring beta or alpha
A
- Haworth projection
- OH group on alpha anomers points down (carbon 1)
- OH group on beta anomers points up (carbon 1)
21
Q
What are epimers
A
- An epimer = a chirally opposite centre
- Galactose is the C4 epimer of glucose
- Mannose is the C2 epimer of glucose
- Differ only by the configuration about one C atom
22
Q
What determines stability in
A
- The repulsion between OH groups determines the stability of the anomers- hinderence etc
- The stability of anomers can be measured
- Start with a pure form
- Anomers will interconvert over a relatively short time period
- Measure optical rotation over time to see which anomer is favoured
23
Q
Which 3D structure is the most stable
A
- Chair form is the most stable- and the chair form which usually has less crowding among axial substituents is most stable
- It can also flip into boat form but more sterically crowded
- There are many other different conformers too
24
Q
Describe the stability of phosphate groups in ATP
A
- Kinetically stable: doesn’t spontaneously break down - implies a high activation energy.
- Thermodynamically unstable: because the phosphate bonds are high in energy
25
Q
Describe the hydrolysation of ATP
A
- ATP + H2O –> ADP + HPO4
- The negative charge on the phosphate anion can be distributed around all 4 oxygens which gives it more stability than when it is part of ATP
- ΔG is driven by entropic stabilisation of phosphate anion.
- Lots of energy is released
- ATP must be made constantly to maintain metabolic environment in the cell
