2.2: Biological molecules (EXAM QUESTIONS)

Question 1

DNA is one of many substances which will dissolve in water. Explain why water is a good solvent. (2 MARKS)

WATER

Answer 1

molecules are polar (1)

(polarity) enables (water) molecules
to, attract / bind to, solute molecules
(1)

Question 2

Water has many properties that are essential for living organisms.
Explain how properties relating to the density of water contribute to the survival of organisms. (3 MARKS)

WATER

Answer 2

ice, is less dense than water
/ floats ✓

ice, provides habitat for
some species / AW ✓

(floating) ice insulates (water
below) ✓

any three

Question 3

Outline the properties of water which make it an ideal habitat for an amphibian. (2 MARKS)

WATER

Answer 3

(good) solvent ✓,
high specific heat (capacity) /
temperature stability

OR

described ✓

(high) density (so frog floats /
buoyant) ✓

ice is less dense than water ✓

two max

Question 4

Many multicellular organisms need to be able to convert monosaccharides into polysaccharides and back
again.
Mammals convert the monosaccharide glucose into a highly branched polysaccharide called glycogen, which
gets stored in liver cells.
Explain why mammals store glycogen instead of glucose. (3 MARKS)

CARBOHYDRATES

Answer 4

• insoluble, so has no
  effect on, water
  potential / Ψ (of cell)
• metabolically inactive
• compact / lots can be
  stored in a small space
• able to store, large
  amounts / lots, of
  energy
• (highly branched so)
  has lots of ends
  for, adding / removing,
  glucose (when needed)
  or
  can be broken down,
  fast / quickly / rapidly, to
  release glucose

Any three

Question 5

Phospholipid molecules are similar to triglycerides but they also contain the element phosphorus as
part of a phosphate group.
Explain how the structure of phospholipids allows them to form the bilayer of a plasma membrane. (3 marks)

LIPIDS

Answer 5

hydrophilic head and hydrophobic
tails (1)

hydrophobic part / tails, repelled /
AW, by water (1)

head / hydrophilic part, forms H
bonds with water (1)

idea that medium outside / inside
plasma membrane is aqueous (1)

idea that hydrophobic nature of
tails results in their facing towards
each other (1)

Question 6

Living organisms have many uses for triglycerides, one of which is the production of
phospholipids.

Name three other functions of triglycerides in living organisms. (3 MARKS)

LIPIDS

Answer 6

energy source for respiration /
respiratory substrate ✓
energy storage ✓
thermal insulation ✓
electrical insulation ✓
buoyancy ✓
idea of: (physical) protection ✓

Question 7

Triglycerides are a type of lipid molecule that can be broken down during hydrolysis
reactions.
Using the structure of triglyceride molecules as an example, explain what is meant by
hydrolysis. (2 MARKS)

LIPIDS

Answer 7

uses / AW, water ✓

(to) break 3 ester bonds ✓

lysis means splitting and fatty
acids are, split / AW, from glycerol
✓

Question 8

Cells are surrounded by a plasma membrane that contains phospholipids.
Explain how the structure of phospholipid molecules allows for the formation of plasma
membranes. (3 MARKS)

Answer 8

phosphate (on head), is
hydrophilic / bonds with water (molecules) ✓

(two) fatty acid tails are
hydrophobic ✓

heads orientate towards water /
tails orientate towards other fatty
acids / tails orientate away from
water , (so a bilayer forms)✓