3.1.2 - Carbohydrates Flashcards
Topic 1
What are monosaccharides? Give 3 common examples
● Monomers from which larger carbohydrates are made
● Glucose, fructose, galactose
Describe the difference between the structure of α-glucose and β-glucose
● Isomers - same molecular formula but differently arranged atoms
● OH group is below carbon 1 in α-glucose but above carbon 1 in β-glucose
What are disaccharides and how are they formed?
● Two monosaccharides joined together with a glycosidic bond
● Formed by a condensation reaction, releasing a water molecule
List 3 common disaccharides
- Maltose
- Sucrose
- Lactose
What monosaccharides are the disaccharides maltose sucrose and lactose made from?
- Maltose
> Glucose + Glucose - Sucrose
> Glucose + Fructose - Lactose
> Glucose + Galactose
What are polysaccharides and how are they formed?
● Many monosaccharides joined together with glycosidic bonds
● Formed by many condensation reactions, releasing many water molecules
Describe the basic function and structure of starch
- Starch Energy store in
plant cells
● Polysaccharide of α-glucose
● Some has 1,4-glycosidic bonds so is unbranched (amylose)
● Some has 1,4- and 1,6-glycosidic bonds so is branched (amylopectin)
Describe the basic function and structure of glycogen
- Glycogen Energy store in
animal cells
● Polysaccharide made of α-glucose
● 1,4- and 1,6-glycosidic bonds → branched
Explain how the structures of starch (amylose) relates to its function
● Helical → compact for storage in cell
● Large, insoluble polysaccharide molecule → can’t leave cell / cross cell membrane
● Insoluble in water → water potential of cell not affected (no osmotic effect)
Explain how the structures glycogen (and starch amylopectin) relates to its function
● Branched → compact / fit more molecules in small area
● Branched → more ends for faster hydrolysis → release glucose for respiration to
make ATP for energy release
● Large, insoluble polysaccharide molecule → can’t leave cell / cross cell membrane
● Insoluble in water → water potential of cell not affected (no osmotic effect)
Describe the basic function and structure of cellulose
Function
● Provides strength and structural support to plant / algal cell walls
Structure
● Polysaccharide of β-glucose
● 1,4-glycosidic bonds so forms straight, unbranched chains
● Chains linked in parallel by hydrogen bonds, forming microfibrils
Explain how the structure of cellulose relates to its function
● Every other β-glucose molecule is inverted in a
long, straight, unbranched chain
● Many hydrogen bonds link parallel strands
(crosslinks) to form microfibrils (strong fibres)
● Hydrogen bonds are strong in high numbers
● So provides strength to plant cell walls
Examples of reducing sugars
monosaccharides, maltose, lactose
Describe the test for reducing sugars
- Add Benedict’s solution (blue) to sample
- Heat in a boiling water bath
- Positive result = green / yellow / orange / red precipitate
Examples of non-reducing sugars
sucrose
Describe the test for non-reducing sugars
- Do Benedict’s test (as above) and stays blue / negative
- Heat in a boiling water bath with acid (to hydrolyse into reducing sugars)
- Neutralise with alkali (eg. sodium bicarbonate)
- Heat in a boiling water bath with Benedict’s solution
- Positive result = green / yellow / orange / red precipitate
Suggest a method to measure the quantity of sugar in a solution
● Carry out Benedict’s test as above, then filter and dry precipitate
● Find mass / weight
Suggest another method to measure the quantity of sugar in a solution
- Make sugar solutions of known concentrations
(eg. dilution series) - Heat a set volume of each sample with a set
volume of Benedict’s solution for the same time - Use colorimeter to measure absorbance (of
light) of each known concentration - Plot calibration curve - concentration on x axis,
absorbance on y axis and draw line of best fit - Repeat Benedict’s test with unknown sample and
measure absorbance - Read off calibration curve to find concentration
associated with unknown sample’s absorbance
Describe the biochemical test for starch
- Add iodine dissolved in potassium iodide (orange / brown) and shake / stir
- Positive result = blue-black
