StemUp: AQA A level Biology 3.1.2 Carbohydrates

What three elements do carbohydrates contain? (1)

C, H and O

What are the three examples of monosaccharide monomers that

make up carbohydrates? (3)

- Glucose

- Fructose

- Galactose

What type of sugar is glucose? (1)

Hexose

Describe how disaccharides are formed (3)

- Two monosaccharides are joined together

- By a glycosidic bond

- In a condensation reaction

Glucose + Glucose ’ (1)

Maltose + Water

Glucose + fructose ’ (1)

Sucrose + Water

Glucose + Galactose ’ (1)

Lactose + Water

Describe how you would carry out a basic Benedict's test (2)

1. Heat sample with Benedict's reagent in water bath

2. If reducing sugar is present, the sample will form a brick red ppt

Describe how you would carry out a test for non reducing sugars?

(5)

1. Take a small sample of the solution you are testing and heat with

Benedict's reagent to confirm the negative result

2. Hydrolyse another sample by heating with dilute acid e.g. HCL

3. When cooled, neutralise the sample by adding an alkali e.g.

NaOH

4. Add the same volume of Benedict's reagent to another sample

of your solution and heat in a water bath

5. A positive brick red precipitate indicates a non-reducing sugar

(such as sucrose) was originally present in the sample

Describe how you would carry out a semi-quantitive Benedict's

test (2)

1. Perform Benedict's test as normal with two different test tubes

but ensure control variables are standardised

2. Resulting colour / amount of precipitate indicates amount of

reducing sugar

How do you ensure that control variables are standardised in

quantitive Benedict's tests (3)

- Use the same volume of each sample

- Use the same volume of Benedict's reagent

- Heat both samples at the same temperature for the same period

of time

What is a common issue that arises when analysing the results of

a semi-quantitive Benedict's test? (1)

Colour is subjective so different people will see different colours

Describe how you would carry out a quantitive Benedict's test (6)

1. Perform Benedict's test on reducing sugar solutions of known

concentrations

2. Standardise the controlled variables (see other flashcard)

3. Use a colorimeter to measure the absorbance value of each of

the known concentrations

4. Plot a graph of known concentration (x-axis) against ab-

sorbance value (y-axis) - a calibration curve

5. Repeat Benedict's test with your unknown samples (again stan-

dardising your controlled variables)

6. Use absorbance value of the unknown sample to read off the

calibration curve to find the concentration of reducing sugars

What are the three main examples of polysaccharides? (3)

- Starch

- Glycogen

- Cellulose

Which carbohydrates are reducing sugars? (3)

- All monosaccharides

- Lactose

- Maltose

Which carbohydrates are non-reducing sugars? (2)

- All polysaccharides

- Sucrose

Describe the structure of starch (2)

- Long branched chains of alpha-glucose molecules

- That are linked together by glycosidic bonds during condensation

reactions

Where is starch stored in a plant? (2)

- In the starch grains

- That are located in the cytoplasm of plant cells

How is starch adapted to its function as a storage carbohydrate

(4)

- Large, so it cannot cross the cell surface membrane and leave

the cell

- Insoluble and therefore osmotically inactive, i.e., it cannot draw

in water by osmosis, which could lead to cell damage

- Has a helical shape and therefore it forms a compact store

- Branched so that glucose can easily be released from its ends

for respiration

Describe how you would carry out the test for starch (2)

1. Add 2-3 drops of iodine/potassium iodide solution

2. If starch is present, the color will change from yellow/orange to

blue/black

Describe the structure of glycogen (2)

- Long branched chains of alpha glucose molecules

- That are linked together by glycosidic bonds during condensation

reactions

Where is glycogen stored in animals? (1)

Stored in large amounts in liver and muscle tissues

Describe how glycogen is adapted to its function as a storage

carbohydrate (2)

1. Insoluble and therefore, osmotically inactive

2. Similar structure to starch but with more branches

3. So that it can be hydrolysed more rapidly for respiration

Describe the structure of cellulose (2)

- Long straight chains of beta glucose molecules

- That are linked together by glycosidic bonds during condensation

reactions

Where is cellulose found? (1)

Cell wall of plants

How is cellulose adapted to its function as a structural compo-

nent? (5)

1. Long chains of beta glucose joined together by glycosidic bonds

in condensation reactions

2. Adjacent chains joined together by hydrogen bonds

3. This produces a 3D structure - microfibril

4. This provides strength and rigidity to the cell wall

5. Which prevents the cell from bursting