Edexcel A-Level Biology AS 2 Membranes Proteins Dna and Gene Expression Questions

Identify the molecular structure or process being tested, annotate the evidence precisely and link each bond, gradient or sequence change to its biological outcome.

Exam points

Interpret membrane and gas-exchange diagrams through structure, gradients and transport.
Explain protein and enzyme behaviour through bonds, shape and molecular interactions.
Apply DNA and gene-expression mechanisms to sequences, data and experimental evidence.

Covered syllabus points

2.1—Gas exchange surfaces and Fick's Law 2.2—Cell membrane structure and models 2.3—Core Practical 3 - membrane permeability 2.4—Osmosis and water potential gradients 2.5—Membrane transport mechanisms 2.6—Amino acids, proteins and protein structure 2.7—Enzyme action and specificity 2.8—Core Practical 4 - enzyme-catalysed reaction rate 2.9—Nucleotides, DNA, RNA and base pairing 2.10—DNA replication and Meselson-Stahl evidence 2.11—Genetic code 2.12—Gene as a DNA base sequence 2.13—Protein synthesis 2.14—DNA replication errors and mutations 2.15—Inheritance terminology and monohybrid inheritance 2.16—Cystic fibrosis gene mutation effects 2.17—Genetic screening uses and implications 2.18—Ethical and social issues in genetic screening

[Maximum number: 3]

Hydra are small animals that live in water.
They have a cylindrical body.
The drawing shows a hydra.

Figure for Question 1 — Edexcel A-Level Biology AS

A hydra has a hollow body that is made up of two layers of cells.

Explain how a hydra is adapted for gas exchange.

[Maximum number: 1]

All organisms contain one or more cells.

In which of the following is a cell membrane present?

animal cells only

animal and plant cells only

plant and prokaryotic cells only

animal, plant and prokaryotic cells

[Maximum number: 9]

Beetroot cells contain a red pigment called betalain. This pigment does not leak out of the cells, unless their membranes are damaged.

A student investigated the effect of detergent on beetroot cell membranes.
Discs of tissue were cut from a beetroot. Any betalain on the outside of the discs was removed.
One disc was placed into each of five test tubes, containing 0.2% detergent solution.
The five tubes were left for 30 minutes at $20^{\circ} \mathrm{C}$ .
The discs were then removed carefully.
Betalain had leaked from the discs and formed a red solution in each test tube.
Light was shone through this solution. The percentage of the light transmitted through the solution was recorded.

The same procedure was repeated using detergent concentrations of 0.0%,0.5%, 1.0% and 2.0%.

3(a)(i)
Concentration of detergent. (1)

3(a)(ii)
Any two of:
- temperature;
(1)
- pH;
(1)
- volume of detergent solution. (1)

3(a)(iii)
Accept answers from 3(a)(ii), for example:
- temperature: thermostatically controlled water bath / incubator / air-conditioned room / environmental chamber;
(1)
- volume of detergent solution: appropriate measuring device;
(1)
- pH: use of buffer. (1)

3(a)(iv)
An explanation including:
- to avoid physical/more damage to the discs/membranes/cells;
(1)
- which would cause more pigment release;
(1)
- causing the results to be invalid. (1)

3(a)(v)
An explanation including:
- to calculate standard deviation / measure reliability / variability;
(1)
- to identify anomalies. (1)

3(b)(i)
To allow any pigment leakage that is not due to detergent to be measured / as a control / for comparison / because the reading of 0% detergent is not 100% but only 97%. (1)

3(b)(ii)
A graph showing:
- axes correctly orientated and linear;
(1)
- axes correctly labelled with units: x-axis concentration of detergent solution, y-axis mean percentage of light transmitted through the solution;
(1)
- correct plotting;
(1)
- points joined with straight lines. (1)

3(b)(iii)
Any three of:
- discs that were in detergent will continue to lose pigment because membranes are permanently damaged;
(1)
- discs that were in 0.0% detergent solution will have no loss of pigment because membranes were not damaged;
(1)
- discs will lose pigment more slowly/stop because there is less/no pigment left in the beetroot;
(1)
- discs that were in higher concentrations of detergent will have the slowest rate of loss because they have the least pigment left. (1)
Accept reverse argument.

3(b)(iv)
Any three of:
Similarities:
- both cause pigment leakage. (1)
Differences:
- temperature leads to greater pigment leakage than detergent;
(1)
- temperature effect on pigment release is sudden above 40 °C / non-significant below 40 °C, but detergent effect is gradual across the range. (1)
Accept answers in terms of light transmission if the link with pigment release is clear.

3(b)(v)
- Would be 25%.
- From the graph at 60 °C and 70 °C / after 60 °C transmission is the same / has levelled off. (1)
- Because all pigment has leaked out / there is an equilibrium when the graph levels off. (1)

(a)

The table shows the results of this investigation.

Table for Question (a) — Edexcel A-Level Biology AS

[ 9 ]

(i)

Detergent permanently damages beetroot cell membranes.

Explain what will happen to the rate of loss of betalain if these discs are then transferred to test tubes of distilled water.

[ 3 ]

(ii)

In a second investigation, the effect of temperature on beetroot cell membrane permeability was studied.

The graph shows the results of the second investigation.

Compare and contrast the effects of detergent concentration and temperature on the leakage of betalain from beetroot cells.

Figure for Question (ii) — Edexcel A-Level Biology AS

[ 3 ]

(iii)

Estimate the percentage of light transmitted at 80 °C.

Use the information in the graph and your own knowledge to support your answer.

[ 3 ]

[Maximum number: 5]

In an investigation, the water potential of a potato was compared with that of a yam.

(a)

State what is meant by the term water potential.

[ 2 ]

(b)

The graph shows the results obtained in this investigation.

Figure for Question (b) — Edexcel A-Level Biology AS

[ 3 ]

(i)

Different concentrations of sucrose solutions have different water potentials.

The graph shows the water potentials for sucrose solutions of different concentrations.

Determine the water potential for the potato cylinders.
Use your completed graph and this graph to determine the water potential.

Figure for Question (i) — Edexcel A-Level Biology AS

Value read from completed graph
Water potential of the potato
kPa

[ 2 ]

(ii)

The water potential for the yam was -1890 kPa .

Suggest why there is a difference between the water potential of the yam and the water potential of the potato.

[ 1 ]

[Maximum number: 1]

The structure and properties of a cell membrane determine which molecules can pass through the membrane.

The diagram shows some structures in part of a cell membrane.

Figure for Question 5 — Edexcel A-Level Biology AS

Which structure allows ions to diffuse through the membrane?

[Maximum number: 3]

The butterfly is an insect that feeds on nectar produced by flowers.
The photograph shows a butterfly feeding on a flower.

Figure for Question 6 — Edexcel A-Level Biology AS

(a)

The nectar in flowers contains nutrients including sugars, amino acids and lipids.

[ 1 ]

(i)

Which row of the table describes how amino acids are joined together to form a protein?

□ A
□ B
□ C
□ D

Table for Question (i) — Edexcel A-Level Biology AS

[ 1 ]

(b)

The circulatory system of an insect is described as an open system. This means that the blood is not contained inside blood vessels but flows through cavities called sinuses.

The diagram shows part of the circulatory system of an insect.

Conducting vessel: blood flows out of the

[ 2 ]

(i)

The blood flowing through the sinuses of the butterfly is separated from the organs by collagen.

Describe the structure of collagen.

[ 2 ]

[Maximum number: 1]

The photograph shows a species of fungus growing on a dead tree in a woodland habitat.

Figure for Question 7 — Edexcel A-Level Biology AS

© www.pqpictures.co.uk / Alamy Stock Photo

Wood contains a high proportion of lignin and cellulose. Fungi can break down molecules in wood by releasing extracellular enzymes.

Different species of fungus can break down different molecules in wood. The effect of two species of fungus on the percentage loss in mass of wooden blocks was investigated. Species A was grown on one set of wood blocks and species B was grown on another set of wood blocks. The percentage loss in mass of the blocks was recorded every four weeks. The graph shows the results of the investigation.

Suggest one reason for the difference in breakdown of the wood by species A and species B.

[Maximum number: 7]

The enzyme invertase breaks down sucrose into its two component monosaccharides.

(a)

Suggest why the temperature was kept at 25°C.

[ 2 ]

An answer that makes reference to the following points:

- temperature affects \{action/ reaction rate/ number of (S and E) collisions\} of enzyme (1)

- $\left(25^{\circ} \mathrm{C}\right)$ \{may be / is\} optimum (for invertase) / gives rate which is not too fast or slow / (1)

ACCEPT so that only pH affects rate

ACCEPT above $25^{\circ} \mathrm{C}$ denatures enzyme

(2)

(b)

Explain why the initial rate of this reaction was determined.

[ 2 ]

(c)

Invertase is used in the food industry. Scientists wanted to increase the efficiency of the enzyme to save time and money.

One way is to immobilise the enzyme by binding it to a surface. This stops the enzyme molecules from moving.

The effect of temperature on immobilised invertase and non-immobilised invertase was investigated. The results are shown in the graph.

Figure for Question (c) — Edexcel A-Level Biology AS

Compare and contrast the effect of temperature on these two forms of invertase.

[ 3 ]

[Maximum number: 5]

Two types of polynucleotide, DNA and RNA, are found in the body.

(a)

Read through the following description of DNA. Complete the description by writing the most appropriate word or words on the dotted lines.

The mononucleotides that join together to form DNA consist of a base, a ______ and a ______ sugar. The mononucleotides are joined together by ______ bonds to form a strand of DNA.

[ 3 ]

An answer that makes reference to the following points :

- phosphate (ion / group) (1)

- deoxyribose (1)

- phosphodiester (1)

ACCEPT $\mathrm{PO}_{4}{ }^{3-}$

NB accept 'deoxyribose' if the second answer is {blank / says phosphate}

ACCEPT pentose / 5 C sugar DO NOT ACCEPT ribose

ACCEPT covalent

(b)

Two types of RNA are messenger RNA (mRNA) and transfer RNA (tRNA). The table shows some features of RNA. For each feature, put one cross ⊞ in the appropriate box, in each row, to show the features that are present in these types of RNA.

Table for Question (b) — Edexcel A-Level Biology AS

[ 2 ]

[Maximum number: 8]

When an organism grows or increases in number, cell division takes place.

(a)

When a cell is preparing to divide, DNA synthesis takes place. Explain how two identical molecules of DNA are produced.

[ 2 ]

(b)

The Voronoi diagram shows the proportions of molecules in a bacterial cell. The area of each labelled part of the square is proportional to the percentage of the dry mass of that molecule. The shape of each part has no meaning.

[ 6 ]

(i)

The proportions of these molecules change as a bacterial cell prepares to divide, as it divides into two new bacterial cells and as each new bacterial cell grows. Discuss how the appearance of this Voronoi diagram might change:
- as a bacterial cell prepares to divide
- in one of the new bacterial cells produced
- as one new bacterial cell increases in size.

Give reasons for these changes to support your answer.

[ 6 ]

Indicative content:

- DNA would increase

- because DNA synthesis / replication will take place / each new cell will need a copy of the DNA

- DNA would decrease

- as only one of each molecule in each cell

- description of change in glycogen / lipids / other molecules

- explanation for change in glycogen / lipids linked to energy requirements (once)

- glycogen decreases as broken down into glucose

- change in lipids linked to role in membranes

- change in glucose linked to energy requirements

- change in phospholipids linked to membrane requirements

- increase in proteins

- needed (as enzymes) in DNA synthesis/ structural proteins

- increase in mRNA

- as there will be more transcription for protein synthesis

- increase in tRNA / rRNA

- as there will be more for translation

Stage 1: Bacterial cell preparing to divide

- proportion of other components would decrease - if increase in DNA

Stage 2: One of the two daughter cells formed

- proportion of components would be the same as in the cell preparing for division - once DNA split between two cells

Stage 3: One of the daughter cells as it increases in size

- proportion of DNA will decrease as other components increase

Cells at end of process / cell prepares to divide again

- Voronoi diagram would look identical to the one in diagram / original as cells are genetically identical

Level 1 Simple description / explanation

1 mark = one description of a change

2 marks = one simple explanation of a change

Level 2 Explanations of changes, not necessarily linked clearly to stages / proportions

3 marks = explanations of changes in two {molecules / Voronoi diagram} 4 marks = explanations of changes in three {molecules / Voronoi diagram}

Level 3 Explanations linked to a stage and proportion on the diagram

5 marks = explanations of changes in four {molecules / Voronoi diagram} with some link made to the stages

6 marks = four explanations of {molecules / Voronoi diagram} with some link made to the stages and either includes 'other' molecules or the relative changes in proportion / area in diagram

[Maximum number: 1]

Stents are used in the treatment of atherosclerosis.
Stents widen the diseased coronary artery so that blood can flow through to the heart muscle.

The diagram shows a stent.

Figure for Question 11 — Edexcel A-Level Biology AS

Stents can damage the endothelial cells lining the artery and trigger the formation of a blood clot.

Patients who have a stent fitted are given anticoagulants.

The photograph shows a tropical bont tick.

This bont tick produces a peptide called variegin.
Variegin binds to the active site of thrombin and prevents blood clotting.

Variegin is a peptide consisting of 32 amino acids.

Which is the minimum number of bases in the gene that codes for this peptide?

[Maximum number: 1]

Cystic fibrosis is an inherited recessive disease caused by mutations in a gene on chromosome 7.

Give the meaning of the term gene.

[Maximum number: 2]

The nature of the genetic code is important in determining the sequence of amino acids in a polypeptide chain. The diagram shows the genetic codes for each amino acid and the stop codons found in human DNA.

Figure for Question 13 — Edexcel A-Level Biology AS

(a)

The diagram shows the sequence of bases in part of a DNA template (antisense) strand for serine and arginine. DNA template strand

mRNA

Figure for Question (a) — Edexcel A-Level Biology AS

[ 2 ]

(i)

Complete the diagram to show the sequence of bases in the mRNA molecule formed during protein synthesis.

[ 1 ]

(ii)

Give the sequence of bases that would be found on the anticodon of a tRNA molecule that would bind to the first codon in this mRNA molecule.

[ 1 ]

[Maximum number: 1]

Sickle cell disease is caused by a gene mutation affecting the $\beta$ -globin chain of haemoglobin.

The mutation occurs in the seventh triplet code of this gene.

This mutation results in the amino acid Glu being replaced with the amino acid Val.

The table shows the sequence of bases in the first part of the DNA in a person who does not have sickle cell disease. It also shows the corresponding sequence of amino acids in the $\beta$ -globin chain.

Table for Question 14 — Edexcel A-Level Biology AS

Name the type of mutation that causes sickle cell disease.

[Maximum number: 4]

Most Bengal tigers are orange with black stripes but there is a very small number of Bengal tigers that are white with black stripes.

The photograph shows a white Bengal tiger with black stripes.

Figure for Question 15 — Edexcel A-Level Biology AS

White tiger offspring are produced by two Bengal tigers that each carry at least one recessive allele for a gene affecting coat colour.

The pedigree diagram shows the phenotypes in one family of tigers, bred in captivity.

Key female orange tiger female white tiger male orange tiger male white tiger

(a)

The phenotype is affected by the genotype.

State what is meant by the term genotype.

[ 1 ]

(b)

Determine the expected phenotypic ratio of orange tigers to white tigers born to the parents shown in this pedigree diagram.

Use a genetic diagram to support your answer.

Answer

[ 3 ]

[Maximum number: 6]

Cystic fibrosis is an inherited disorder.

The diagrams show a cross section through a trachea from a person without cystic fibrosis and from a person with cystic fibrosis.

Figure for Question 16 — Edexcel A-Level Biology AS

Explain why cystic fibrosis causes the differences shown in the diagrams.

Indicative content:

Descriptions of diagrams (D points)

- reduced diameter of lumen (in person with CF)

- thicker or inflamed muscle layer (in person with CF)

- more mucus (in person with CF)

Cystic fibrosis (C points)

- CF results in the production of very \{sticky / thick\} mucus (credit this point only once)

- CF is due to a \{mutation in the CFTR gene / faulty CFTR allele\}

- (resulting in) \{the CFTR protein not functioning properly / abnormal or defective CFTR protein / abnormal channel proteins / CFTR channel blocked\}

- this affects transfer of chloride ions out of cells

- ( $\mathrm{Cl}^{-}$ accumulate in cells so) \{water moves out of mucus / water remains in cytoplasm\} / mucus is dehydrated \}

Change in lumen diameter (L points)

- CF results in the production of very \{sticky / thick\} mucus (credit this point only once)

- diameter of lumen in patient with CF is reduced

- because the mucus builds up

- (because the) cilia cannot move / beat (idea of cilia normally clearing mucus)

- (because the) cilia cannot move mucus away (it is too thick) (credit this only once)

- Inflammation of muscle (narrows lumen) (credit this only once)

Inflammation (I points)

- cilia cannot move mucus away (it is too thick) (credit this only once)

- mucus (is very sticky and) traps bacteria

- bacteria have ideal growth conditions (in mucus)

- bacteria can cause (chest) infections

- CF characterised by coughing \{to remove mucus / as a result of infection\}

- \{coughing / infection\} damages \{cells / tissues\} (lining airways)

- (leading to) inflammation of muscle (credit this only once)

(6)

Level

Marks

0

No awardable content

1

1-2

Level 1: description of diagrams

1 mark: one D or one C point

2 marks: second D or C point

2

3-4

Level 2: explanation of either change in diameter of lumen or inflammation linked to CF

3 marks: three L points or 3 I points

4 marks: as 3 marks plus additional C point

3

5-6

Level 2: explanation of both change in diameter of lumen and inflammation linked to CF

5 marks: three L points and 3 I points

6 marks: as 5 marks plus additional C point

[Maximum number: 3]

Cystic fibrosis is an inherited disorder.

The graph shows the number of babies with cystic fibrosis per 10000 births in one country, from 1990 to 2008.

Number of babies with cystic fibrosis per 10000 births

Figure for Question 17 — Edexcel A-Level Biology AS

Explain how the identification of carriers, using genetic screening, could have contributed to the downward trend shown in this graph.

[Maximum number: 3]

Cystic fibrosis is an inherited disorder.

The graph shows the number of babies with cystic fibrosis per 10000 births in one country, from 1990 to 2008.

Number of babies with cystic fibrosis per 10000 births

Figure for Question 18 — Edexcel A-Level Biology AS

Describe the ethical issues relating to genetic screening.

All question bank results loaded

Covered syllabus points

Question 1

Question 2

Question 3

Question (a)

Question (i)

Question (ii)

Question (iii)

Question 4

Question (a)

Question (b)

Question (i)

Question (ii)

Question 5

Question 6

Question (a)

Question (i)

Question (b)

Question (i)

Question 7

Question 8

Question (a)

Question (b)

Question (c)

Question 9

Question (a)

Question (b)

Question 10

Question (a)

Question (b)

Question (i)

Question 11

Question 12

Question 13

Question (a)

Question (i)

Question (ii)

Question 14

Question 15

Question (a)

Question (b)

Question 16

Question 17

Question 18