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

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

Answer 2

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

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

Hydrogen bonds are important in cellulose molecules. Explain why.

Answer 4

• 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 5

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

Answer 5

1. Add Benedict’s;
2. Heat to 95°C;
3. Red/orange/yellow/green precipitate

Question 6

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

Answer 6

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

Question 7

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

Answer 7

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

Question 8

Compare and contrast the structure and properties of triglycerides and phospholipids

Answer 8

• 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 9

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 9

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