Edexcel AS Bio Topic 2 Test

1 Osmosis is a special case of diffusion.a Describe two ways diffusion and active transport differ as mechanisms of membrane

transit.1.

2.

b Define osmosis.

c Explain how the free movements of many water molecules are hindered in the presence ofdissolved substances such as ions or glucose.

d Why is the plasma membrane described as a partially permeable membrane?

e When red cells were immersed in distilled water, the plasma membrane of these cellsburst, but plant cells in the same environment were unharmed. Explain this phenomenon.

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2 For the following examples of movements across plasma membranes, identify the precisemechanism that is described in each case.a In the lining of the small intestine, tiny droplets of partially digested lipids are taken into

cells of the epithelium layer.

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Edexcel Biology for AS Dynamic Learning CD-ROM © Hodder Education 2008

2 Cells, genes and health End-of-topic test


b ATP molecules (cell energy currency) are made in the interior of tiny cell organelles, butpass out into the rest of the cytoplasm at membrane protein sites that open as pores in thepresence of ATP.

____________________________________________________________________________

c Water molecules cross the lipid bilayer at permanently open channels.

____________________________________________________________________________

d Special macrophage cells take in and dispose of small debris particles that become attachedto their external surface.

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3 Read this introduction to amino acids, protein structure and enzyme action, and then answerthe following questions.

To understand how enzymes work, we need to understand the structure of proteins. A proteinis a long, ribbon-like molecule made by joining together many subunits – amino acids. Eachamino acid has a central carbon atom, attached to which is an amino group (–NH2), acarboxyl group (–COOH), and a side chain generally represented by R. The amino and acidicgroups are involved in joining the amino acids to one another to form the backbone of theprotein chain.

The protein molecule does not lie flat like a ribbon – far from it. In watery environments, suchas inside living cells, a protein chain will fold into a unique three-dimensional shape so thatits hydrophobic regions are (generally) towards the outside. Weak bonds form between Rgroups and these help stabilise the delicate shape. The shape of the enzyme molecule has to bejust right if the enzyme is to function. If the enzyme molecule is deformed, even slightly, it willnot work. Each different enzyme has its own highly specific shape with a pocket – the activesite. It is here the substrate will fit.

Only a few of the amino acids in the chain of an enzyme protein are involved in catalysis.These catalytic amino acids are not adjacent to one another in the chain. Instead they arequite spaced out along the length of the protein molecule. When the molecule folds up intoits three-dimensional shape, however, the specific folding brings all the catalytic amino acidresidues together in the active site.

Once the substrate has fitted into the active site, the reaction is catalysed. Generally, theactive site is lined with hydrophobic R groups and it contains groups that bind the substrateand other groups that catalyse the reaction. When the substrate binds, it disturbs the delicatebalance of the flexible protein chain of the enzyme, causing it to alter its shape. This is whywe refer to an induced fit of substrate to enzyme.

extract from ‘Enzymes: fast and flexible’ in Biological Science Review, 19.1 (September 2006),pages 2–5

a Show how two amino acids join together (line 5) by writing an equation for thecondensation reaction forming a dipeptide, using a generalised structural formula for thetwo amino acids. Identify and label the bond formed.

2 CELLS, GENES AND HEALTH – END-OF-TOPIC TEST


5

10

15

20

b In this passage, how does the author describe the primary structure of a protein?

c What do you understand by a ‘hydrophobic’ region of a protein (line 9)?

d Name or describe two types of weak bond that may form between different parts of theprotein chain (line 10), stabilising the folded structure of the protein.

e Enzymes are highly specific in their action; generally an enzyme reacts (combines) withonly one type of substrate molecule. By reference to the description of enzyme structuregiven (lines 21–23), explain how the specificity of enzyme action can be explained?

f Once an ‘induced fit’ state (enzyme–substrate complex) has been achieved (line 23), aninstantaneous reaction follows before the enzyme becomes ready to catalyse anotherreaction. Summarise the events of an enzyme-catalysed reaction in a simple equation,where E � enzyme, S � substrate, and Pr � product.

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4 The following account of DNA replication has certain key words omitted. Read the text andthen fill in the blanks, using the most appropriate term from this list:

cytosine semi-conservative complementary proof-readingbase-pairing antiparallel helicase thyminechromosome DNA polymerase exact hydrogen bonds

In replication, the DNA double helix must unwind, and the hydrogen bonds holding the

strands together must be broken. An enzyme, __________, is involved in these steps and

holds the strands apart whilst replication occurs.

Both strands of DNA act as templates in replication. New nucleotides with the appropriate

__________ bases line up opposite the bases of the exposed strands. Adenine pairs with

__________, __________ with guanine – a process called __________. The bases of the two

strands fit together only if the sugar molecules they are attached to point in opposite

directions – the strands are said to be __________.



__________ then form between these complementary bases, holding them in place. Finally,

the sugar and phosphate groups of adjacent nucleotides of the new strand condense together.

This reaction is catalysed by an enzyme called __________. Then each pair of strands winds

up into a double helix. One strand of each new double helix came from the original

__________ and one is a newly synthesised strand. This arrangement is known as

__________ replication because half the original molecule is kept the same.

DNA polymerase also has a role in __________ the new strands – any ‘mistakes’ that start to

happen (such as the wrong bases attempting to pair up) are corrected. Each new DNA double

helix is an __________ copy of the original.(12)

5 For a genetic condition such as cystic fibrosis, in utero detection of the condition in a fetusmakes it possible for society to offer the parents a legal termination of the pregnancy.Abortion in such circumstances raises issues for society and for the individuals involved.Identify two reasons in favour of genetic screening that you might make to potential parentsin this situation, and two reasons against it.

Reasons in favour of genetic screening:

1.

2.

Reasons against genetic screening:

1.

2.

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6 The term ‘gene’ may be correctly defined in a number of ways. Define the term gene from theviewpoints of:a a plant or animal breeder





b a molecular geneticist.

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7 The diagram below shows a section through part of the cell surface membrane.

inside of cell

a By reference to the properties of phospholipids, explain why these molecules are arrangedin a bilayer.

b Why is this structure referred to as a fluid mosaic model?

c In a cell, the plasma membrane is typically 5 nm thick, but in the diagram above it isshown as about 15 mm thick. Calculate the magnification of the lipid bilayer, showing yourworkings. Express your answer in standard notation.

d A channel protein may be a site of entry of water molecules into the cell. Describe theproperty of the amino acid residues that line the surface of such a pore.

e Outline one other role of membrane proteins.

f Describe a function of glycoproteins found on the outside of membranes.

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8 In an investigation of ion uptake by plant cells, samples of five thin discs of plant root tissuewere suspended in a solution of nitrate ions (10 mmol dm�3) for 10 hours. Identical tissuesamples were maintained at 0 �C, 10 �C, 20 �C and 30 �C, and a steady stream of air wasbubbled through the solution. At the end of the experiment the concentration of nitrate ionsin the cells of the tissue discs was measured, and is recorded in the table below.

Temperature/�C Concentration of nitrate ions in root tissue/ mmol dm�3

0 15

10 38

20 70

30 96

a Comment on the choice of especially thin tissue discs, and on the maintenance of a supplyof air to the discs throughout the experiment.

b Plot a graph of nitrate uptake by root tissue discs against temperature.

c Describe the relationship between nitrate uptake by root tissue discs and temperature.



d The experiment was repeated with nitrogen gas in place of air, and the nitrate ionconcentration was found to be about 15 mmol dm�3 in all tissue samples, after 10 hours.Describe the mechanism of nitrate ion uptake in plant tissue disc cells, and explain howthe evidence from the above experiments confirms your hypothesis.

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9 Beetroot tissue contains a coloured pigment in the solution that fills the large central vacuoleof the cells. If adverse external conditions are applied to beetroot tissue, the vacuolar solutionmay escape in quantities that can be detected.a In an experiment with beetroot tissue, cut cylinders of tissue were first washed in excess

water. Why is this necessary?

b If you were required to investigate the effect of immersion of a cylinder of beetroot tissue(say, 3 cm long by 0.5 cm diameter) in a 70% ethanol solution for 20 minutes at roomtemperature, exactly what control would be required and why?

c What structures within the plant cell normally cause the contents of the cell to beretained?

d Explain what effect submersion in ethanol solution will have on beetroot tissue samplesand why.

(8)



10 An efficient surface for gas exchange in living organisms requires a large, thin surface areaacross which a difference in concentration is maintained. Show how the mammalian lungtissue meets these criteria by means of a fully annotate diagram of a single alveolus and thesurrounding capillary. Make clear how the concentration gradient at the respiratory surface ismaintained.

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11 In an investigation of the effects of temperature on the rate of an enzyme-catalysed reaction, itwas found that at lower temperatures an increase in temperature increased the rate of reaction.However, at higher temperatures, increases in temperature had the reverse effect. Explain thisphenomenon by reference to the effects of heat on chemical reactions and on protein structure.

At lower temperatures:

At higher temperatures:

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12 The nucleic acids found in cells, RNA and DNA, are built from mononucleotides condensedtogether. Each mononucleotide consists of phosphate, a pentose sugar, and an organic base.a Using a table, record the differences between DNA and RNA.



b The percentage composition of the bases in the DNA of three unrelated organisms wasfound to be:

Adenine Thymine Guanine Cytosine

cow 28.7 27.2 22.2 20.7

sea urchin 32.8 32.1 17.7 17.3

yeast 31.3 31.9 18.7 18.1

b What do these results suggest about base-pairing in the DNA molecule?

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13 The genetic code for a selection of amino acids involved in protein synthesis is shown in thetable below. These triplet codons are those of messenger RNA (mRNA), where uracil Ureplaces thymine T.

alanine (Ala) GCU, GCC

glutamic acid (Glu) GAA, GAG

leucine (Leu) CUU, CUC

proline (Pro) CCA, CCG

valine (Val) GUU, GUC

initiation codon AUG

stop codons UAA, UGA, UAG

a Give the sequence of amino acids of the polypeptide chain that is coded for by this part ofthe mRNA.

A U G G A A C C G G U U C U U G C C

b In the long sequence of codons that typically comprises a single gene, where would thisshort sequence occur?

c Give the sequence of bases in the corresponding coding (antisense) strand of DNA fromwhich this mRNA was transcribed.

d The information in mRNA becomes translated into a sequence of amino acids in ribosomesin the cytoplasm. Specific transfer RNA (tRNA) molecules with a corresponding aminoacid attached are involved in translation. The anticodon of a tRNA is a sequence of threebases complementary to a codon of the mRNA. What are the tRNA anticodons for thefollowing amino acids coded for in this sequence?

i glutamic acid _______________________

ii valine _______________________(11)





14 Meselson and Stahl provided important evidence that DNA is replicated in the nucleus‘semi-conservatively’.a Explain in your own words what you understand by ‘semi-conservative replication’.

b Meselson and Stahl’s experimental technique involved isotopes of nitrogen. What is anisotope?

c How did they arrange for the cultures of the bacterium E. coli used at the start of theirexperiment to exclusively contain the heavy isotope of the element nitrogen?

d Outline how the composition of DNA in their experimental populations of E. coli changedover two generations, thereby establishing that replication was as predicted.

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15 The pedigree chart shows the inheritance of cystic fibrosis in two families related by marriage.Cystic fibrosis is due to a mutation of a single gene.

where is an unaffected male, and is an unaffected female,and affected individuals are represented by filled symbols

1 2 3 4A

1 2

1 2

43 65 87 109

3 4 5 6 7 8 9 10 11

B

C

a Who are the uncles of C 4? __________

b Who are the aunts of C 5? __________

c Who in this chart has parents unknown to us? __________

d How many male grandchildren do A 3 and A 4 have? __________

e Individuals C 6 and 7 have cystic fibrosis. What evidence do you have that this gene isrecessive?

f Explain why we can be confident that one of the grandparents A 1 and 2 is a carrier of thegene for cystic fibrosis, and so too is one of the grandparents A 3 and 4.

g Describe and explain the effects of cystic fibrosis on gas exchange in the lungs.

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16 Leopards and panthers were thought to be separate but closely related species, but we nowknow their different appearances are due to one pair of alleles. The panther is a black-coatedvariety of the leopard, which has a spotted coat. The allele for ‘spotted’ is dominant; aleopard may be homozygous (SS) or heterozygous (Ss) for coat appearance, whereas thepanther is homozygous recessive (ss).

Construct and label a genetic diagram to show the possible F2 offspring of a cross between ahomozygous leopard and a panther (P generation), followed by a sibling cross between two ofthe F1 progeny.

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17 In gene therapy, corrected copies of genes are introduced into a patient with a genetic disease.Outline the main steps by which a healthy CF gene may be made available in the lungs of acystic fibrosis patient. Present your answer as a flow diagram.

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18 Genetic screening of potential parents as carriers of alleles for a genetic disease, and thedetection of a genetic disease in the unborn child are outcomes of the Human GenomeProject (HGP).a What did the HGP involve?



b Apart from cystic fibrosis, name two other genetic diseases for which screening is possibleand sometimes advised.

c Outline the diagnostic technique by which the presence of an allele for a specific geneticdisease may be diagnosis in a DNA sample taken from a fetus or parent.

d Screening of potential parents also involves investigations of their family histories by agenetic counsellor. What is the main focus of such an enquiry?

e Using an annotated diagram, explain what is involved in the screening of a fetus by meansof amniocentesis.

f An alternative approach to amniocentesis is chorionic villus sampling. What advantagesdoes this latter technique offer to parents and doctor?

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Edexcel AS Bio Topic 2 Test

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