carbs and lipids

Question 1

What is a monomer?

Answer 1

A monomer is a smaller / repeating) unit / molecule from which larger
molecules / polymers are made;

Question 2

A starch molecule has a spiral
shape. Explain why this shape is
important to its function in cells

Answer 2

Compact/occupies small space/tightly packed;

Question 3

Give one feature of starch and
explain how this feature enables it
to act as a storage
substance.

Answer 3

Helical / spiral So compact / tightly packed / can fit (lots) into a small
space;
Insoluble So no osmotic effect / does not leave cell / does not affect
water potential;
Large molecule / long chain (So) does not leave cell / contains large
number of glucose units;
Branched chains (So) rapid hydrolysis to remove glucose for
respiration;

Question 4

The structure of cellulose is
related to its role in plant cell
walls. Explain how. (3)

Answer 4

Long, straight, unbranched chains of Beta glucose;
(Joined by) many WEAK hydrogen bonds;
Form microfibrils / macrofibrils;
Provide rigidity/strength/support;

Question 5

Hydrogen bonds are important in
cellulose molecules. Explain why.

Answer 5

Holds chains/cellulose molecules together/forms cross links between
chains/cellulose molecules/forms microfibrils;
Providing strength/rigidity (to cellulose/cell wall);
Weak Hydrogen bonds provide strength in large numbers;

Question 6

Describe how lactose is formed
and where in the cell it would be
attached to a polypeptide to form
a glycoprotein

Answer 6

Glucose and galactose
Joined by condensation
Joined by glycosidic bond
Added to polypeptide in Golgi

Question 7

Describe how the student would
show that reducing sugars were
present in a solution.

Answer 7

Add Benedict’s;
2. Heat to 95°C;
3. Red/orange/yellow/green precipitate (shows reducing sugar present);

Question 8

Describe how you would test a
sample of food for the presence of
starch.

Answer 8

1. Add potassium iodide (KI) solution to the food sample;
2. Blue/black/purple indicates starch is present;

Question 9

Describe how you would test a
liquid sample for the presence of
lipid box and how you would
recognise a positive result.

Answer 9

(Mix / shake sample) with ethanol, then water and shake;
2. Cloudy White / milky (emulsion);

Question 10

Omega-3 fatty acids are
unsaturated. What is an
unsaturated fatty acid?

Answer 10

Double bond(s);
(Bonds) between carbon atoms within the hydrocarbon chain;

Question 11

Describe how an ester bond is
formed in a phospholipid
molecule

Answer 11

1. Condensation (reaction) OR Loss of water;
2. Between of glycerol and fatty acid;

Question 12

Some seeds contain lipids.
Describe how you could use the
emulsion test to show that a seed
contains lipids. (3)

Answer 12

1. Crush/grind;
2. With ethanol/ alcohol, to dissolve the lipid;
3. Then add water then shake;
4. Forms a white emulsion / goes white;

Question 13

What are the differences between
a triglyceride and a phospholipid?

Answer 13

1. Fatty acid removed;
2. Replaced with a phosphate group;

Question 14

Compare and contrast the
structure and properties of
triglycerides and phospholipids

Answer 14

Both contain ester bonds
Both contain glycerol
Fatty acids on both may be saturated or unsaturated
Both are insoluble in water
Both contain C, H and O but phospholipids are also contain P
Triglyceride has three fatty acids and phospholipids have two fatty
acids plus phosphate group
Triglycerides are hydrophobic/non-polar and phospholipids have
hydrophilic and hydrophobic region
Phospholipids form monolayer/micelle/bilayer but triglycerides don’t.

Question 15

Describe the biochemical tests
you would use to confirm the
presence of lipid, non-reducing
sugar and amylase in a sample.

Answer 15

Lipid
1. Add ethanol/alcohol then add water and shake/mix OR Add
ethanol/alcohol and shake/mix then pour into/add water;
2. White/milky emulsion OR emulsion test turns white/milky;
Non-reducing sugar
3. Do Benedict’s test and stays blue/negative;
4. Boil with acid then neutralise with alkali;
5. Heat with Benedict’s and becomes red/orange (precipitate);
Amylase
6. Add biuret (reagent) and becomes purple/violet/mauve/lilac;
7. Add starch, (leave for a time), test for reducing sugar/absence of
starch;

Question 16

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.

Answer 16

1. Filter and dry (the precipitate);
2. Find mass/weight;

Question 17

Describe how you would
produce a calibration
curve for a reducing
sugar of unknown
concentration and use it
to obtain results

Answer 17

1.
Make up several known concentrations of maltose
/ glucose / lactose / galactose / fructose;
2.
Carry out the Benedict’s test on each sample;
3.
Use a colorimeter to measure the colour intensity
of each solution and plot a calibration curve;
4.
Known Conc. on X axis and absorbance
transmission on Y.
5. Find the concentration of the unknown sample
using the calibration curve;