3.1.2 — Carbohydrates Flashcards
What are monosaccharides?
Monosaccharides are simple sugars. They are the monomers from which larger carbohydrates are made.
Examples of monosaccharides?
Glucose, galactose and fructose
What bond forms in a condensation reaction of two monosaccharides?
Glycosidic
How do we produce maltose?
two alpha glucose molecules
How do we produce sucrose?
Glucose and fructose
How do we produce lactose?
glucose and galactose
What is the difference between alpha & beta glucose?
They are isomers.
Alpha-glucose has the OH groups at the bottom.
Beta glucose has the OH groups not on the same plane.
How are glycogen and starch formed?
Condensation reaction of alpha glucose
How is cellulose formed?
Condensation reaction of beta glucose
A biochemical test for reducing sugar produces a negative result with raffinose solution.
Describe a biochemical test to show that raffinose solution contains a non-reducing sugar.
- Heat with acid and neutralise;
Accept boil/water bath for heat
Accept named alkali for neutralise
Accept named examples, eg HCl, NaHCO3 - Heat with Benedict’s (solution);
- Red precipitate/colour;
Accept other colours eg orange/ brown/green
A precipitate is produced in a positive result for reducing sugar in a Benedict’s test.
A precipitate is solid matter suspended in solution.
A student carried out the Benedict’s test. Suggest a method, other than using a colorimeter, that this student could use to measure the quantity of
reducing sugar in a solution.
- Filter and dry (the precipitate);
Accept: correct reference to evaporation after filtration - Find mass/weight;
Use of a colorimeter in this investigation would improve the repeatability of the student’s results.
Give one reason why.
- Quantitative
OR
(Colour change is) subjective;
Accept: accurate/precise - Standardises (the) method;
In Test 1, the student used a measuring cylinder to measure 15 cm3 of solution from a beaker. The measuring cylinder gives a volume with an uncertainty of ±1 cm3. She used a graduated syringe to measure 5.0 cm3 of Benedict’s solution.
The graduated syringe gives a volume with an uncertainty of ± 0.5 cm3. She mixed these volumes of liquid to do the biochemical test.
(e) Calculate the percentage error for the measurements used to obtain a 20
cm3 mixture of the solution from the beaker and Benedict’s solution. Show your working.
16.67 − 17 = 2 marks;
(cumulative percentage error of both measuring vessels)
If incorrect final answer, accept for 1 mark:
0.167 − 0.17 (not a percentage)
Lactulose is a disaccharide formed from one molecule of galactose and one molecule of fructose.
Other than both being disaccharides, give one similarity and one difference between the structures of lactulose and lactose.
Similarity
1. Both contain galactose / a glycosidic bond;
Difference
2. Lactulose contains fructose, whereas lactose contains glucose;
Glycogen and cellulose are both carbohydrates.
Describe two differences between the structure of a cellulose molecule and a glycogen molecule.
- Cellulose is made up of β-glucose (monomers) and glycogen is made up of α-glucose (monomers);
- Cellulose molecule has straight chain and glycogen is
branched; - Cellulose molecule has straight chain and glycogen is coiled;
- glycogen has 1,4- and 1,6- glycosidic bonds and cellulose has only 1,4- glycosidic bonds;
Starch is a carbohydrate often stored in plant cells.
Describe and explain two features of starch that make it a good storage molecule.
** Any two from:**
1. Insoluble (in water), so doesn’t affect water potential;
- Branched / coiled / (α-)helix, so makes molecule compact;
OR
Branched / coiled / (α-)helix so can fit many (molecules) in small area; - Polymer of (α-)glucose so provides glucose for respiration;
- Branched / more ends for fast breakdown / enzyme action;
- Large (molecule), so can’t cross the cell membrane
How do we test for the presence of starch?
Iodine/potassium iodide;
In mammals, in the early stages of pregnancy, a developing embryo exchanges substances with its mother via cells in the lining of the uterus. At this stage, there is a high concentration of glycogen in cells lining the uterus.
(a) Describe the structure of glycogen.
- Polysaccharide of α-glucose;
- (Joined by) glycosidic bonds
During early pregnancy, the glycogen in the cells lining the uterus is an important energy source for the embryo.
Suggest how glycogen acts as a source of energy.
Do not include transport across membranes in your answer.
- Hydrolysed (to glucose);
- Glucose used in respiration;
Name the monomers from which a maltose molecule is made.
Glucose (and glucose);
Name the type of chemical bond that joins the two monomers to form maltose.
(α1,4) Glycosidic;
Explain the difference in the structure of the starch molecule and the cellulose molecule.
- Starch formed from α-glucose but cellulose formed from β-glucose;
- Position of hydrogen and hydroxyl groups on carbon atom 1 inverted.
Starch molecules and cellulose molecules have different functions in plant cells. Each molecule is adapted for its function.
Explain one way in which starch molecules are adapted for their function in plant cells.
- Insoluble;
- Don’t affect water potential;
OR - Helical;
Accept form spirals - Compact;
OR - Large molecule;
- Cannot leave cell.
Explain how cellulose molecules are adapted for their function in plant cells.
- Long and straight chains;
- Become linked together by many hydrogen bonds to form fibrils;
- Provide strength (to cell wall).
