carbohydrates

Question 1

what are carbohydrates? state their general formula

Answer 1

substances that contain carbon, hydrogen and oxygen
Cₓ(H₂O)ᵧ where x and y are variable numbers

Question 2

define monosaccharides and state its general formula?

Answer 2

carbohydrates which cannot be hydrolysed to simpler carbohydrates
(CH₂O)n

Question 3

what are some common examples of monosaccharides (the 3 needed to know) and what kind of sugars are they?

Answer 3

glucose, fructose, galactose (hexose sugars)

Question 4

how are disaccharides formed?

Answer 4

condensation reaction between 2 monosaccharides, in which one molecule of water is removed from the pair of monosaccharides

Question 5

give the equations of how 3 different disaccharides are formed

Answer 5

glucose + glucose -> maltose
glucose + fructose -> sucrose
glucose + galactose -> lactose

Question 6

what are polysaccharides?

Answer 6

polymers of monosaccharides (mainly glucose)

Question 7

what are common examples of polysaccharides? state if it is a storage/structural polysaccharide, and where it is found

Answer 7

starch - storage polysaccharide in plant cell
glycogen - storage polysaccharide in animal cell
cellulose - structural polysaccharide in plant cell

Question 8

describe the structure of starch, and its components

Answer 8

• polymer of glucose
• has two components: amylose and amylopectin

1) amylose
- straight chain structure with helical shape
- consists of several thousand glucose units joined by glycosidic bonds

2) amylopectin
- highly branched and hence compact structure
- glucose units within and between branches are held by glycosidic bonds
- twice as many glucose units as amylose

specifity to glucose is important

Question 9

what is the general function of carbohydrates?

Answer 9

source of energy

Question 10

explain the functions of sucrose and storage polysaccharides

Answer 10

1) sucrose is a good transport sugar in plant phloem
- it is very soluble and therefore can move in high concentrations
- chemically unreactive

2) polysaccharides like starch and glycogen are good storage molecules
- large and insoluble and therefore indiffusable through partially permeable membranes
- compact shape which allow more glucose units to be stored in cell
- easily hydrolysised into glucose

for (2), “glucose units” and “glucose” indicate specifity. unless stated otherwise, be specific rather than saying “carbohydrates” and “monosaccharides”. (IP3 2024 WA1)

Question 11

explain the functions of cellulose and monosaccharides

Answer 11

1) cellulose is a good structural molecule
- found in all plant cell - a good cell wall material
- has good tensile strength
- permeable to water and solutes

2) monosaccharides are required for synthesis of:
- nucleic acids
- disaccharides and polysaccharides

Question 12

which monosaccharides and disaccharides are reducing and non-reducing sugars?

Answer 12

reducing: glucose, fructose, galactose, lactose, maltose
non-reducing: sucrose

Question 13

describe the test for reducing sugars

Answer 13

1) to 2cm3 of test sample in a test tube, add an equal volume of Benedict’s solution.
2) mix well and place the test tube in a boiling water bath for 2 minutes.
3) (observe any colour change)

since the Benedict’s test is semi-quantitative, hence the greater the intensity of red in final mixture with greater amount of reducing sugar

expected results:
with reducing sugar: the colour of final solution turns from blue to green/yellow/brown/red
without reducing sugar: the colour of solution remains blue

Question 14

describe the test for non-reducing sugars

Answer 14

if the reducing sugar test turns out negative…
1) to 2cm3 of test sample in a test tube, add a few drops of dilute hydrochloric acid.
2) heat the mixture for 2 minutes in a hot water bath.
3) after 2 minutes, neutralise the acid by adding sodium hydrogen carbonate until effervescence stops
4) carry out the Benedict’s test again
5) if there were non-reducing sugars present in original sample, the test will now be positive. (and the solution will turn from blue to brown/red)

adding acid and heating the sample hydrolyses any sucrose present, splitting the molecules to give fructose and glucose

Question 15

describe the test for starch

Answer 15

1) add a few drops of iodine-potassium iodide solution to test sample in a test tube / on a white tile
2) (observe any colour change)

expected results:
starch present: brown/yellow solution turns dark blue
starch absent: solution remains brown/yellow

this happens because the iodine takes up position in center of amylose helix to form a dark blue starch-iodine complex

Question 16

why is benedict’s test semi-quantitative?

Answer 16

it can be used to determine the relative amount of reducing sugar present in solution, but the range of glucose standards is limited and hence the precise concentration of glucose may not be able to be determined.

Question 17

why may an excess amount of benedict’s solution be added?

Answer 17

ensures all the reducing sugars in sample has reacted with benedict’s solution to form red precipitate [1]
therefore the colour changes observed would accurately reflect the total amount of reducing sugars present [1]

Question 18

draw the condensation reaction between 2 glucose
molecule

Answer 18

https://drive.google.com/file/d/1qPGha3pL89t__imT_osSUtaCTytUmgaj/view?usp=sharing

Question 19

draw the hydrolysis reaction of lactose
molecule

Answer 19

answer