Biological Molecules

Question 1

Describe the difference between the structure of a phospholipid and a triglyceride:

Answer 1

One fatty acid is replaced with a phosphate in a phospholipid

Question 2

Describe how you would test for the presence of a lipid in a sample of food

Answer 2

Add ethanol then add water. Shake to see if a white precipitate forms (indicates the presence of a lipid)

Question 3

How is a saturated fatty acid different from an unsaturated one?

Answer 3

A saturated fatty acid has no double bonds between carbons.

Question 4

Describe how ATP is resynthesised in cells

Answer 4

ADP and a phosphate ion will bind by ATP synthase during respiration or photosynthesis

Question 5

Give two ways in which the hydrolysis of ATP is used in cells

Answer 5

1) To provide energy for other reactions (such as active transport)
2) To add phosphate to other substances and make them more reactive through changing their shape

Question 6

Name the following processes:

a) Transport through a channel protein
b) Transport of small, non-polar molecules

C) Transport of glucose with sodium ions

Answer 6

a) Facilitated diffusion
b) Simple diffusion (through th phospolipid bilayer)
c) Co-transport

Question 7

In the cell wall, bonds hold the cellulose molecules together side by side.

What type of bond holds the cellulose molecules together side by side?

Answer 7

Hydrogen

Question 8

Name the products of the hydrolysis of sucrose.

Answer 8

Glucose and Fructose

Question 9

Describe the induced-fit model of enzyme action

Answer 9

Active site is not complementary to the substrate before the reaction. The shape of active site changes as
enzyme-substrate complex forms which distorts bonds in the substrate.

Question 10

Explain the shape of a graph plotting concentration of substrate against time

Answer 10

High initial rate (steep graph) as plenty of E-S complexes are formed.
Rate slows (gradient steepness decreases) because most enzymes active sites become occupied.
The graph plateaus as all the substrate is used up

Question 11

A quantitative Benedict’s test produces a colour whose intensity depends on the concentration of reducing sugar in a solution.
A scientist used quantitative Benedict’s tests to produce a calibration curve of colorimeter reading against concentration of maltose.
Describe how the scientist would have produced the calibration curve and used it to obtain the results in the graph.
Do not include details of how to perform a Benedict’s test in your answer.

Answer 11

1) Make maltose solutions of different concentrations
(and carry out quantitative Benedict’s test on each).
2) Use a colorimeter to measure colour of each
solution and plot calibration curve
3) Find concentration of sample from calibration curve

Question 12

What are the features of starch and how do these enable it to act as a storage substance.

Answer 12

• Helical so it is compact
• Insoluble so it does not affect water potential
• Long chain so it cannot leave the cell

Question 13

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

Answer 13

Cellulose is made from beta glucose monomers that form straight unbranched chains joined by hydrogen bonds. This forms microfibrils which provide rigidity and strength.

Question 14

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

Answer 14

Unsaturated fatty acids contain C=C double bonds.

Question 15

Name the products of the hydrolysis of sucrose.

Answer 15

Fructose and Glucose

Question 16

Describe the induced-fit model of enzyme action

Answer 16

Before the reaction the active site does
not fit substrate. As a substrate binds to it’s complementary enzyme, the shape of the active site alters till it becomes complementary to the substrate. This distorts the substrates bonds, leading to the reaction.

Question 17

When HIV infects a human cell, the following events occur.
• A single-stranded length of HIV DNA is made.
• The human cell then makes a complementary strand to the HIV DNA.
The complementary strand is made in the same way as a new complementary strand is made during semi-conservative replication of human DNA.
Describe how the complementary strand of HIV DNA is made.

Answer 17

Free nucleotides bind through complementary base pairing to HIV DNA. DNA polymerase will then bind the nucleotides together by forming phosphodiester bonds.

Question 18

Contrast the structures of DNA and mRNA molecules to give three differences.

Answer 18

mRNA is single stranded whereas DNA is double stranded.
DNA has the base Thymine which is replaced by uracil in mRNA.
DNA is a long strand where as mRNA is very short.

Question 19

The addition of a respiratory inhibitor stops the absorption of amino acids.

Answer 19

No ATP will be produced so there will be no active transport so sodium cannot move out of the cell into the ileum and therefore there won’t be a concentration gradient for sodium to move back in with amino acids

Question 20

A student investigated the distribution of stomata on leaves from two species of plant. She removed small pieces from the lower surface of the leaves of each plant species. She mounted these pieces on separate microscope slides. She then counted the number of stomata in several parts of the epidermis on each piece of leaf tissue using an optical microscope.
The pieces of leaf tissue examined were very thin.
Explain why this was important

Answer 20

To ensure there is a single layer of cells to allow light to pass through.

Question 21

Give two reasons why it was important that the student counted the number of stomata in several parts of each piece of leaf tissue.

Answer 21

• There is an uneven distribution of stomata so counting on several parts will give you a more representive sample
• Repeats allow you to obtain a reliable mean

Question 22

One of the two plant species used by the student in this investigation was a xerophyte.
Other than the distribution of stomata, suggest and explain other xerophytic features the leaves of this plant might have.

Answer 22

• Rolled up leaves in order to trap moist air outside of the leaf so there is not a significant water potential gradient between outside and inside the leaf
• Hairy leaves in order to trap still air on the surface of leaves so there is a reduced water potential gradient between the inside and outside of the cell
• Stomata in pits and grooves to trap water vapour and reduce water potential gradient
• Thick wax layer to increase diffusion distance
• Needles so reduces surface area to volume ratio

Question 23

The student then compared the rate of transpiration (evaporation of water) from the two species of plant. She did this by measuring the rate of water uptake by each plant species.
Suggest reasons why the rate of water uptake by a plant might not be the same as the rate of transpiration.

Answer 23

• Some water is used during photosynthesis
• Some water is used to make cells turgid
• Water is used for hydrolysis
• Some water is produced during respiration

Question 24

Phospholipids are one of the storage molecules found in cherry seeds.
Name the type of reaction used to break down phospholipids to release phosphate.

Answer 24

Hydrolysis

Question 25

The scientists concluded that an increase in phosphate in the embryo was linked to growth of the embryo.
Suggest two reasons why an increase in phosphate can be linked to growth of the embryo.

Answer 25

• Phosphates are used for phosphorylation

- USed to make DNA.

Question 26

he chilling requirement of seeds of certain plant species is considered to be an adaptation for survival in countries with seasonal changes in environmental conditions.
Suggest how this adaptation may enable these plant species to survive and respond to seasonal changes.

Answer 26

The seeds will be able to remain dormant during the winter and then develop later in spring when light intensity increases so they can photosynthesise.

Question 27

Suggest how amyloid-precursor protein can be the substrate of two different enzymes,
α-secretase and β-secretase

Answer 27

The amyloid precursor protein may be complementary to different parts of the α-secretase and β-secretase which come from different parts of the amino acid sequences.

Question 28

One product of the reaction catalysed by β-secretase is a smaller protein.
Describe what happens in the hydrolysis reaction that produces the smaller protein from amyloid-precursor protein.

Answer 28

A peptidase breaks a peptide bond using water.

Question 29

Describe how the structures of starch and cellulose molecules are related to their functions.

Answer 29

Starch:

• Alpha helix structure means the molecule is more compact a
• Can be branched so enzymes can work simultaneously and therefore energy can be released faster for respiration.
• Large so it cannot leave a cell.

Cellulose:

• Straight unbranched chains of B glucose that forms fibrils.
• Many hydrogen bonds form cross linkages between chains that provides strength.