Carbohydrates Flashcards
What is the general formula for carbohydrates?
CnH2nOn
So they contain carbon, hydrogen and oxygen
Describe carbohydrates
Group of organic compounds associated with energy, but also with structural applications in cell membranes and cell walls
Describe carbon
- present in all biological molecules except for water
- an form 4 bonds in a tetrahedral arrangement
- used as scaffolding for the molecules
Carbohydrate functions
- provide energy-rich nutrients to organisms
* used to built their body structures
Examples of carbohydrates
- sugars
- starch
- glycogen
- cellulose
Describe monosaccharides
- simplest -> ‘simple sugars’ 3-7 carbons
- white crystalline solids
- soluble in water -> dissolve to form sweet tasting solutions
Name the hexoses
- α-glucose
- β-glucose
- fructose
- galactose
Define isomers and give an example of a pair
Same molecular formula, but a different structural formula
E.g.: α-glucose and β-glucose
Name the pentoses:
- ribose
* deoxyribose
Name a triose
• glyceraldehyde
Describe reducing sugars
- all monosaccharides are reducing sugars (+ some disaccharides)
- have an aldéhyde functional group
- can donate electrons to reduce other chemicals
Test for reducing sugars
- add 2cm3 of a food sample in liquid form (pestle and mortar)
- add 2cm3 of Benedict’s reagent
- heat in water-bath for 5 mins
Describe the results of the reducing sugars test
- blue -> none
- green/yellow -> traces of reducing sugars
- orange/red -> moderate amount of reducing sugars
- brick red -> large amount of reducing sugars
Describe the differences between α-glucose and β-glucose
- positioning of hydrogen and hydroxyl groups on carbon atom 1
- α-glucose: OH group is below carbon atom 1
- β-glucose: OH group is above carbon atom 1
Describe the similarities between α-glucose and β-glucose
- ring structures
* CH2OH group is above the ring
What are the consequences of the differing structures of α-glucose and β-glucose
- α-glucose: combines to form starch
* β-glucose: combines to form cellulose
Describe how to make calibration curves:
1) create a dilution series of glucose using distilled water. Use six boiling tubes to make concentrations ranging from 0-10mol/dm3
2) place 2cm3 of each of the unknown samples in separate boiling tubes
3) add 2cm3 of Benedict’s solution to all boiling tubes
4) place boiling tubes in water bath at 90°C for 4 minutes
5) use tongs to take the boiling tubes out of the water bath. Leave to cool
6) zero the colorimeter using a cuvette with distilled water (reference cuvette). The colorimeter should be set to the red filter
7) place known samples into cuvettes and measure the absorbance of each, using the colourimeter
8) make a calibration curve, plotting the absorbance against the known glucose concentrations
What is the function of glucose?
- major source of energy for most animals
- 1g yields approximately 16kJ energy when fully broken down in respiration (a lot!)
- main form in which carbohydrates are transported around the body in animals
What are the common disaccharides
- sucrose
- lactose
- maltose
Which of the common disaccharides are reducing sugars
- lactose
* maltose
Which of the common disaccharides is not a reducing sugar?
sucrose
Describe sucrose
- α-glucose + α-fructose
- stored and moved from one region to another in plants such as sugar cane
- table sugar
Describe lactose
- α-glucose + β-galactose
* milk sugar
Describe maltose
- α-glucose + α-glucose
- malt sugar
- produced by the breakdown of amylose in many germinating seeds
Describe condensation reactions
- a larger molecule is formed BT bonding two smaller molecules together
- a new covalent bond is formed (in this case, the bond is glycosidic)
- a water molecule is released
Describe glycosidic bonds
- 1,4 -> bonds between carbon 1 and carbon 4; form straight chains
- 1,6 -> bonds between carbon 1 and carbon 6; cause branching
Describe hydrolysis reactions
- reverse reaction to condensation: breaks down polymers into their constituent monomers
- a larger molecule is broken down into smaller molecules
- a new covalent bond is broken (in this case, the bond is glycosidic)
- a water molecule is used
Describe non-reducing sugars
- do not contain and aldehyde functional group
* lise reducing capability; cannot donate electrons
Describe the test for non-reducing sugars:
1) carry out the test for reducing sugars; if there is no colour change (blue), there are no reducing sugars present
2) add 2cm3 food sample to 2cm3 dilute HCl and heat in boiling water for 5 mins
3) slowly add sodium hydrogencarbonate to the test tube to neutralise the acid, which will allow the Benedict’s reagent to work
4) test solution with pH paper to ensure that it is alkaline
5) take 2cm3 of the solution and add it to 2cm3 Benedict’s reagent
6) perform the reducing sugars test again
7) positive result = orange/brown
Define polymer
Large molecule made up of repeating units
How are polysaccharides formed
When multiple monosaccharides join together by the glycosidic bonds in the condensation reaction to form a long chain of monosaccharides; polymerisation
Define oligosaccharide
A molecule with between 3 and 10 monosaccharide units
Describe conjugated molecules and give an example
- can be formed when monosaccharides monomers combine with other types of molecule
- e.g. glycolipids, glycoproteins
What is the general formula for polysaccharides
C6nH10nO5n
Describe the properties of polysaccharides
- relatively insoluble in water
- not sweet
- cannot be crystallised
- dépend on the number and type of monomer it contains and how they are joined together
What is starch?
- polysaccharide
* made up of amylose and amylopectin
How is starch formed
By joining 200-100,000 α-glucose molecules through condensation reactions creating glycosidic bonds
How does the structure of starch relate to its function
- insoluble -> doesn’t affect water potential; water isn’t drawn in via osmosis
- large -> doesn’t diffuse out of cells
- α-hélices cause it to be compact -> stored in small places
- hydrolysed into glucose for respiration
- can be branched -> many ends allow multiple enzymes to act on it simultaneously; glucose can be released quickly
Describe amylose
- polysaccharide
- α-glucose
- contains 1,4 glycosidic bonds
- does not contain 1,6 glycosidic bonds
- straight chain (compact)
- 20% of starch
- insoluble
- used for energy storage
Describe amylopectin
- polysaccharide
- α-glucose
- contains 1,4 glycosidic bonds
- contains 1,6 glycosidic bonds
- branched chain (still quite compact)
- 80% of starch
- not very soluble
- also used for energy storage, but better for energy release
Test for starch
HELP
Describe glycogen
Very similar to starch, but only found in animals, never in plants
How does the structure of glycogen differ to the structure of starch
- shorter chains
- more branched -> allows glucose to be broken down more rradily; useful because animals have higher metabolic rate- glucose is needed for respiration; less dense, more soluble
- stored as smaller grains in muscle and liver cells
- mass is small -> fat is the main storage in animals
How is cellulose different to starch and glycogen
- β-glucose not α-glucose
* large differences in both structure and function
Describe the structure of cellulose
- straight (unbranched) chains run parallel to each other
* every alternate β-glucose molecule is inverted (because of the glycosidic bonds); allows hydrogen bonds to form
Describe hydrogen bonds and their effect on the function of cellulose
Although hydrogen bonds are relatively weak, the huge amount in cellulose allows it to be strong; hence, it is used as a structural molecule (major constituent in plant cell walls)
Describe the properties of cellulose
- completely permeable (allows water and dissolved substances to enter and leave plant cells freely)
- becomes impermeable when the gaps between the fibres are filled with impermeable substances
- cannot be hydrolysed easily
Describe cellulase
- the enzyme which digests cellulose
- produced by microorganisms in the guts of cows and elephants, so they can digest grass
- humans and most other animals do not produce cellulase; they cannot obtain the nutrient content from plant cells
