carbohydrates Flashcards
give the 2 isomers of glucose and their structural difference
a-glucose, OHs are both on bottom
B-glucose, one OH on bottom, one OH on top
describe a biochemical test to show that a solution contains a non reducing sugar (3)
- heat with acid (HCL) and neutralise (NaOH)
- heat with benedict’s reagent
- forms red precipitate
suggest a method, other than using a calorimeter, to measure to quantity of reducing sugar in a solution after the benedicts test has been done (2)
- filter and dry the precipitate
- measure mass
why does using a calorimeter improve repeatability (1)
colour change can be subjective and difficult to see
give the monomers of:
-sucrose
-maltose
-lactose
- a-glucose & fructose
- a-glucose & a-glucose
- a-glucose & galactose
describe 2 differences between the structure of glycogen and cellulose (2)
- Glycogen has a-glucose whereas cellulose has B-glucose
- cellulose has only 1,4 glycosidic bonds, whereas glycogen has both 1,4 and 1,6 glycosidic bonds
describe and explain 5 features of starch that make it a good storage molecule (5)
- Insoluble- won’t affect water potential of cell
- coiled so molecule is compact- many molecules fit in small space
- Large so cannot cross the cell membrane
- polymer of a-glucose so provides glucose for respiration
- is branched so more ends for fast hydrolysis
explain how cellulose molecules are adapted for their function in plant cells (3)
- exist in long and straight chains
- chains link together by many hydrogen bonds to form fibrils
- provides strength to cell wall
hydrogen bonds are important in cellulose molecules, explain why (2)
- MANY hydrogen bonds are strong and forms cross linkages between chains
- makes the cellulose strong
in humans maltase breaks down maltose into glucose
explain why maltase:
- only breaks down maltose
-allows this reaction to occur at normal body temperature (5)
- Tertiary structure of maltase means
2.Active site is complementary to maltose
- Induced fit- maltase not complementary to maltose but changes shape once maltose binds to become complementary
4.Enzyme lowers activation energy so reaction can occur at normal body temperature
5.By forming enzyme-substrate complex
describe the structure of a cellulose molecule and explain how cellulose is adapted for it’s functions in cells (6)
- made from B glucose
- condensation reactions form glycosidic bonds between them
- 1,4 glycosidic bonds
- every other B-glucose is inverted
- Hydrogen bonds link long straight chains
- cellulose makes cell wall strong
- can resist turgor pressure
- bonds difficult to break
- not branched so resists action by enzymes
compare and contrast the structure of starch and cellulose (6)
compare:
1. both are polymers of glucose monomers
2. both have glycosidic bonds
3. both contain C,H,O
contrast:
4. starch has a-glucose, cellulose has B-glucose
5. starch has both 1,4 and 1,6 glycosidic bonds, cellulose just 1,4
6. starch is branched, cellulose is long and straight
7. starch has amylose & amylopectin, cellulose is 1 type of molecule
give the order of colours in the test for reducing sugars to show an increasing concentration of reducing sugar
- green
- yellow-green
- yellow-brown
- dark brown
- red
suggest a way other than comparing colours, in which different concentrations of reducing sugar could be estimated
dry each precipitate and weigh them
heavier precipitate= more reducing sugar
explain why it is difficult to distinguish between very concentrated samples of reducing sugars, even if they have different concentrations
once all the copper (II) sulfate has been reduced to copper(I) oxide, further amounts of reducing sugar cannot make a difference
name the bond which forms between 2 monosaccharides
glycosidic bond
give the monosaccharides of these disaccharides
- maltose
- sucrose
- lactose
a- glucose + a-glucose
a-glucose + fructose
a-glucose + galactose
what is the difference between a-glucose and B-glucose
a-glucose: both OHs are on the BOTTOM
B-glucose: one OH on top, one OH on bottom
is sucrose a reducing sugar or a non reducing sugar?
non-reducing
as it does not change colour when heated with benedicts reagent
how to test for non-reducing sugars
- grind up sample with water
- add 2cm3 of benedicts and heat in a water bath for 5 minutes
- if remains blue, then there is no reducing sugar present
- take another 2cm3 sample and heat with hydrochloric acid
- HCL hydrolyses the disaccharide into its monosaccharides
- neutralise with sodium hydroxide as benedicts will not work in acidic conditions- test with litmus paper to ensure its alkaline
- again add benedicts and heat in water bath for 5 minutes
- turns from blue to red due to the reducing sugars formed from hydrolysis of non-reducing sugar
describe the test for starch
- add 2cm3 of sample to test tube
- add 2 drops of iodine solution
- turns from orange to blue/black
give 4 features of glycogen that make it an efficient energy source for animals
- insoluble so does not affect water potential, prevents bursting
- insoluble so does not diffuse across the cell membrane
- compact so lots can be stored in a small space
- has many branches so more ends for enzymes to simultaneously act on and faster hydrolysis so glucose is released faster
what is the advantage of cellulose having no branches?
- chains run parallel
- hydrogen bonds between them forms cross linkages
- each hydrogen bond is weak but lots together is strong
- strengthens the cellulose- strong cell wall
- forms microfibrils
give 2 functions of cellulose in the cell wall
- provides rigidity
- prevents cell bursting as water enters by osmosis (exerts an inwards pressure which stops any more entry of water)
A student investigated the difference in the reducing sugar content of two fruit juices.
He performed a biochemical test on each fruit juice using Benedict’s solution. He
then used a colorimeter with each test result.
Describe how the results from the colorimeter can identify the fruit juice containing the
higher sugar content.
[1 mark]
higher absorbance has more sugar
The student controlled variables in the test using Benedict’s solution.
Give two variables the student controlled.
volume of benedicts
temperature of water bath
Iodine solution stains fresh apple tissue black. When iodine solution is added to
apples stored for a week, the stain is less black.
The water potential of apple juice decreases when apples are stored.
Suggest why the water potential of apple juice decreases when apples are stored.
[2 marks]
starch is hydrolysed
maltose is soluble so it decreases water potential
describe the biochemical tests to confirm presence of
- lipid
- non-reducing sugar
- amylase
lipid
- add ethanol and shake
- add water and shake
- milky emulsion
non-reducing sugar
- heat with acid and neutralise with alkaline
- heat with benedicts
- forms red precipitate
amylase
- add biuret reagent, positive result is a lilac colour
describe how lactose is formed and where in the cell it would be attached to a polypeptide to form a glycoprotein (4)
- glucose and galactose
- joined by condensation reaction
- joined by glycosidic bonds
- added to polypeptide in golgi apparatus
