Proteins

Draw the general structure of an amino acid.

Which statement describing protein structure is correct?

Antibodies are proteins produced and secreted by plasma cells.

Once activated, a single B cell divides and produces 4000 plasma cells.

Each one of these plasma cells produces 2000 molecules of antibody Q per second.

A human male has a blood volume of 5.4 dm³

Calculate the total number of molecules of antibody Q per cm³ of blood 24 hours after the production of these plasma cells.

Scientists investigated the effect of age on the immune response in healthy female mice.

The scientists used the following method.

1. Infected a large number of young and old mice with the same strain of influenza virus.

2. Removed the lungs from 6 young and 6 old mice on the day of infection (day 0).

3. Repeated step 2 on 8 different days after infection.

4. Determined the mean number of B cells present per gram of lung tissue for young and old mice.

Figure 3 shows the scientists’ results.

The scientists decided that further investigations would be required to conclusively

determine the effect of age on the immune response in healthy female mice.

Using all the information provided, suggest three reasons for the scientists’ decision.

Do not refer to statistical tests, repeats or sample size in your answer.

Describe how a quaternary protein is formed from its monomers.

Do not include the process of translation in your answer.

Describe the structure of DNA and the structure of a chromosome.

Mutation can result in an increase in genetic variation within a species.

Describe and explain the other processes that result in increases in genetic variation within a species.

Give two features of all prokaryotic cells that are not features of eukaryotic cells.

Many multicellular organisms produce antimicrobial polypeptides (APs) that protect them against prokaryotes. Figure 3 shows how one type of AP acts on the cell-surface membrane of prokaryotes.

This AP has a secondary structure in a helical shape.

Tick () the box to show which type of bond maintains the helical structure of the polypeptide.

The amino acids on one side of each AP helix have hydrophobic properties. The amino acids on the opposite side of each helix have hydrophilic properties. Figure 4 shows this.

Suggest how these properties of the APs allow them to become positioned across the membrane (as shown in Figure 3) and make a channel through which ions can pass.

Figure 5 shows further information about a channel formed in the cell-surface membrane by the APs.

Use Figure 5 to calculate the cross-sectional area of the channel through which ions can pass. Assume the cross-sectional area is circular. Use π = 3.14 in your calculation. Give your answer in nm² and to 1 decimal place.

The APs damage prokaryotic cells but do not damage the eukaryotic cells in the organisms that produce them. Prokaryotic cell membranes do not contain cholesterol.

Assess why the APs do not damage the eukaryotic cells of the organisms that produce them.

Scientists observed these APs on prokaryotes using a transmission electron microscope. They stained the APs using a monoclonal antibody with gold attached to it.

Suggest how these techniques allowed observation of APs on prokaryotes.

Two enzymes, P and Q, are proteins with quaternary structure which catalyse the same reaction, but they have different amino acid sequences.

Define the quaternary structure of a protein.

Explain how two enzymes with different amino acid sequences can catalyse the same reaction.

Scientists investigated the effect of pH 8.4 and pH 7.5 on the activity of enzymes P and Q.

Figure 8 shows their results.

Describe what the scientists should place in the control tubes in this investigation.

Give three conclusions you can make from Figure 8.

Amino acids are used to make proteins. Table 1 shows the R groups of six different amino acids.

Use Table 1 to identify the three different amino acids used to make the polypeptide shown in Figure 1.

Middle amino acid

Table 2 shows three statements and names of four biological molecules.

Put a tick () in each box where the statement is true for the biological molecule.

Figure 2 represents the structure of adult human haemoglobin.

The number of amino acids in the beta chains in Figure 2 is 3.546% greater than in the alpha chains. Each alpha chain contains 141 amino acids.

Calculate how many amino acids there are in total in the haemoglobin molecule shown in Figure 2. Give your answer to the nearest whole number.

When a substance called BPG binds to haemoglobin, it reduces the affinity of haemoglobin for oxygen.

Figure 3 shows an oxyhaemoglobin dissociation curve for haemoglobin in normal conditions.

Sketch a curve on Figure 3 to show the oxyhaemoglobin dissociation curve for haemoglobin when BPG binds to it.

Suggest and explain when it would be an advantage to a human for BPG to bind to haemoglobin.

Describe how one amino acid is added to a polypeptide that is being formed at a ribosome during translation.

Table 2 shows:

• mRNA codons and the amino acid coded for by each codon

• the type of bond formed by the R group of some of the amino acids.

Crystallin is a structural protein found in the human eye. An inherited disease that leads to blindness is caused by changes in properties of crystallin. The replacement of the amino acid Arg with the amino acid Gly causes these changes.

Use information in Table 2 to suggest why this amino acid replacement changes the properties of crystallin.

The amino acid replacement of Arg with Gly is caused by a single base substitution mutation in the DNA. The non-mutant DNA triplet is TCC.

Complete Table 3.

Give:

• the mRNA codon complementary to the non-mutant DNA triplet

• the mutated mRNA codon that could cause the change from Arg to Gly in the crystallin protein

• the DNA triplet complementary to this mutated mRNA codon.

Figure 1 shows a cell from the lining of the ileum specialised for absorption of products of digestion.

SGLT1 is a carrier protein found in the cell-surface membrane of this cell, it transports glucose and sodium ions (Na⁺) into the cell.

The action of the carrier protein X in Figure 1 is linked to a membrane-bound ATP hydrolase enzyme.

Explain the function of this ATP hydrolase.

The movement of Na⁺ out of the cell allows the absorption of glucose into the cell lining the ileum.

Explain how.

Describe and explain two features you would expect to find in a cell specialised for absorption.

Figure 2 is a diagram of one SGLT1 carrier protein.

Figure 2

Draw phospholipids on Figure 2 to show how the carrier protein, SGLT1, would fit into the cell-surface membrane.

Do not draw more than eight phospholipids.

Figure 2 shows the SGLT1 polypeptide with NH₂ at one end and COOH at the other end.

Describe how amino acids join to form a polypeptide so there is always NH₂ at one end and COOH at the other end.

You may use a diagram in your answer.

Space for diagram:

Explain five properties that make water important for organisms.

Describe the biochemical tests you would use to confirm the presence of lipid, non-reducing sugar and amylase in a sample.

Describe the chemical reactions involved in the conversion of polymers to monomers and monomers to polymers.

Give two named examples of polymers and their associated monomers to illustrate your answer.

Draw the general structure of an amino acid.

Table 1 shows mRNA codons and the amino acids coded for by each codon. It also shows some properties of the R group of each amino acid.

The genetic code is described as degenerate.

What is meant by this? Use an example from Table 1 to illustrate your answer.

A scientist investigated changes in the amino acid sequence of a human enzyme resulting from mutations. All these amino acid changes result from single base substitution mutations. This enzyme is a polypeptide 465 amino acids long.

Table 2 shows the result of three of the base substitutions.

What is the minimum number of bases in the gene coding for this polypeptide?

A change from Glu to Lys at amino acid 300 had no effect on the rate of reaction catalysed by the enzyme. The same change at amino acid 279 significantly reduced the rate of reaction catalysed by the enzyme.

Use all the information and your knowledge of protein structure to suggest reasons for the differences between the effects of these two changes.

Broken bones are repaired by cells called osteoclasts and osteoblasts.

Osteoblasts secrete a hormone called osteocalcin in an inactive form. Osteocalcin is a protein. The active form of osteocalcin binds to a receptor on beta (β) cells in the pancreas, stimulating them to release insulin. Osteoblasts have receptors for insulin.

Figure 1 shows how the production of osteocalcin by osteoblasts is controlled by positive feedback.

The secretion of osteocalcin (in an inactive form) by osteoblasts is controlled by positive feedback.

Use information from Figure 1 to explain why this is positive feedback.

The acidic pH conditions created by osteoclasts cause the inactive form of the protein osteocalcin to change into the active form of osteocalcin.

Suggest how.

Binding of insulin leads to an increase in the rate of respiration in cells such as osteoblasts.

Explain how.

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Write an essay on one of the topics below.

EITHER

The importance of nitrogen-containing substances in biological systems.

OR

The importance of diffusion in organisms.