Topic 2 question

• Key


  words in /slash/ are either in bold or underlined in the markscheme

  Sentences with a; at the end means that's the mark you get for it.
• A student prepared a stained squash of cells from the tip of an onion root and observed it using an optical microscope.

  During the preparation of the slide, he:

  • cut the first 5 mm from the tip of an onion root and placed it on a glass slide

  • covered this tip with a drop of stain solution and a cover slip

  • warmed the glass slide

  • pressed down firmly on the cover slip.

  He identified and counted nuclei in different stages of the cell cycle.

  Explain why the student:

  1. used only the first 5 mm from the tip of an onion root.

  2. pressed down firmly on the cover slip. [2]

  1. Where dividing cells are found/mitosis occurs;

  2. Single/thin layer of cells /so/ light passes through (making cells/nuclei visible);
• What is an antigen? [2]


  1. Foreign protein;

  2. (that) stimulates an immune response/ production of an antibody;
• What is an antibody? [2]

  1. A protein/ immunoglobulin specific to an antigen;

  2. Produced by B cells;
• High absorption of salt from the diet can result in a higher than normal concentration of salt in the blood plasma entering capillaries. This can lead to a build-up of tissue fluid.

  
  

  Explain how. [2]

  1. (Higher salt) results in low/er/ water potential of tissue fluid;

  2. (So) less /water/ returns to capillary by osmosis (at venule end);D
• DNA and RNA can be found in bacteria.

  Give two ways in which the nucleotides in DNA are different from the nucleotides in RNA. [2]

  1. DNA contains thymine /and/ RNA contains uracil;

  2. DNA contains deoxyribose /and/ RNA contains ribose;
• Cells lining the ileum of mammals absorb the monosaccharide glucose by co-transport with sodium ions. Explain how. [3]

  1. Sodium ions actively transported from ileum cell to blood;

  2. Maintains/ forms diffusion gradient for sodium to enter cells from gut (and with it, glucose);

  3. Glucose enters by facilitated diffusion;
• Describe and explain what the student should have done when counting cells to make sure that the mitotic index he obtained for this root tip was accurate. [2]

  1. Repeat count;

  2. to ensure figures are correct;
• Sodium ions from salt (sodium chloride) are absorbed by cells lining the gut. Some of these cells have membranes with a carrier protein called NHE3.

  NHE3 actively transports one sodium ion into the cell in exchange for one proton (hydrogen ion) out of the cell.

  Use your knowledge of transport across cell membranes to suggest how NHE3 does this. [3]

  1. Co-transport;

  2. Uses ATP;

  3. Sodium ions and proton bind to the protein;
• NMO is a disease that leads to damage to nerve cells in the spinal cord. A person with NMO produces anti-AQP4 antibody that attacks only these nerve cells. Explain why the anti-AQP4 antibody only damages these cells. [4]


  1. Antibody has a (specific) tertiary structure;

  2. Has binding site that only binds to one /antigen/;

  3. Antigen to this antibody (only) found on these nerve cells;

  4. So, antibody (only) forms antigen-antibody complex with these nerve cells (causing damage);
• A new treatment for NMO involves using a monoclonal antibody. The structure of the variable region of this monoclonal antibody is identical to the variable region of anti-AQP4 antibody, but the rest of its structure is different.

  
  

  Use this information and your knowledge of antigen-antibody complexes to suggest how this monoclonal antibody prevents anti-AQP4 damaging nerve cells. [2]


  1. The monoclonal antibody binds to the nerve cell antigen so less AQP4 can bind;

  2. When monoclonal antibody binds it doesn't cause damage to nerve cell;
• Describe a method the student could have used to monitor the temperature of the water in each tube. [1]


  1. Take reading/s/ (during the experiment) using a (digital) thermometer;
• The decrease in the percentage of light passing through the water between 25 °C and 60 °C is caused by the release of the red pigment from cells of the beetroot.

  Suggest how the increase in temperature of the water caused the release of the red pigment. [2]

  1. Damage to (cell surface) membrane;

  2. (membrane) proteins denature;

  3. Increased fluidity/ damage to phospholipid bilayer;
• Describe how you would use cell fractionation techniques to obtain a sample of chloroplasts from leaf tissue. Do not include in your answer information about any solutions. [3]

  1. Macerate/ homogenise/ blend/ break tissues/ cells (in solution);

  2. Centrifuge;

  3. At different/ increasing speeds until chloroplast fraction obtained;
• This is meant to be a table but I can't.

  Does a mitochondrion or chloroplast have a double outer membrane;

  Does a mitochondrion or chloroplast have starch grains;

  Does a mitochondrion or chloroplast have diffusion of oxygen into the organelle;

  1. Mitochondrion;

  2. Chloroplast;

  3. Diffusion of oxygen into the organelle;
• Give the function of a mitochondrion. [1]

  1. Site of aerobic respiration;
• Give three properties of water that are important in biology. [3]

  1. Large latent heat of vapourisation;

  2. Solvent;

  3. High heat capacity;

  4. Metabolite;
• Name the blood vessels that carry blood to the heart muscle. [1]

  1. Coronary arteries;
• Give two similarities in the movement of substances by diffusion and by osmosis. [2]

  1. Passive;

  2. (Movement) down a gradient/ from high concentration to low concentration;
• Name the process by which prokaryotic cells divide. [1]

  1. Binary fission;
• Human breast milk is produced and secreted by gland cells. These gland cells have adaptations that include many mitochondria and many Golgi vesicles. The milk contains a high concentration of protein.

  
  

  Explain the role of these cell adaptations in the production and secretion of breast milk. [2]

  1. (Many mitochondria) release energy /for/ movement of vesicles;

  2. (Many Golgi) vesicles transport protein;
• Give two ways in which the arrangement of prokaryotic DNA is different from the arrangement of the human DNA. [2]

  1. Circular (as opposed to linear)

  2. Not associated with proteins;
• What is a homologous pair of chromosomes? [1]

  1. carry the same genes;
• Describe and explain how cell fractionation and ultracentrifugation can be used to isolate mitochondria from a suspension of animal cells. [5]

  Any five from:

  1. Cell homogenisation to break open cells;

  2. Filter to remove (large) debris / whole cells;

  3. Use isotonic solution to prevent damage to mitochondria / organelles;

  4. Keep cold to prevent / reduce damage by enzymes / use buffer to prevent protein / enzyme denaturation;

  5. Centrifuge (at lower speed / 1000 g) to separate nuclei / cell fragments / heavy organelles;

  6. Re-spin (supernatant / after nuclei / pellet removed) at higher speed to get mitochondria in pellet / at bottom.
• Describe the principles and the limitations of using a transmission electron microscope to investigate cell structure. (5)

  Principles:

  1. Electrons pass through / enter (thin) specimen;

  2. Denser parts absorb more electrons;

  3. (So) denser parts appear darker;

  4. Electrons have short wavelength so give high resolution;

  
  

  Principles: Allow maximum of 3 marks

  
  

  Limitations:

  5. Cannot look at living material / Must be in a vacuum;

  6. Specimen must be (very) thin;

  7. Artefacts present;

  8. Complex staining method / complex / long preparation time;

  9. Image not in 3D / only 2D images produced;
• The events that take place during interphase and mitosis lead to the production of two genetically identical cells. Explain how. (4)

  1. DNA replicated;

  2. (Involving) specific / accurate / complementary base-pairing;

  3. (Ref to) two identical / sister chromatids;

  4. Each chromatid / moves / is separated to (opposite) poles / ends of cell;
• Describe and explain how centrifuging the culture allowed the scientists to obtain a cell-free liquid. [3]

  1. Large / dense / heavy cells;

  2. Form pellet / move to bottom of tube (when centrifuged); 3. Liquid / supernatant can be removed.
• The scientists measured cell damage by measuring the activity of lysosomes. Give one function of lysosomes. [1]

  Break down cells / cell parts / toxins;
• Name two structures in a eukaryotic cell that cannot be identified using an optical microscope. [1]

  Two of the following for one mark:

  Mitochondrion / ribosome / endoplasmic reticulum / lysosome / cell-surface membrane.
• Describe how you could use cell fractionation to isolate chloroplasts from leaf tissue. [3]

  1. How to break open cells and remove debris;

  2. Solution is cold / isotonic / buffered;

  3. Second pellet is chloroplast.
• The scientists carried out a further investigation. They treated the liquid from strain A with a protein-digesting enzyme before adding it to a culture of human cells. No cell damage was recorded. Suggest why there was no damage to the cells. [3]

  1. Enzyme (a protein) is broken down (so no enzyme activity);

  Accept hydrolyse / digested for 'broken down'.

  2. No toxin (as a result of protein-digesting enzyme activity); Must be in the correct context.

  3. (So) toxin is protein.
• MM is caused by a faulty receptor protein in cell-surface membranes.

  Cells in MM tumours can be destroyed by the immune system.

  
  

  Suggest why they can be destroyed by the immune system. [3]

  1. Faulty protein recognised as an antigen/ as a 'foreign' protein;

  2. T cells will bind to faulty protein/ to (this) 'foreign' protein;

  3. (Sensitied) T cells will stimulate clonal selection of B cells;

  4. (Resulting in) release of antibodies against faulty protein;
• The same variety of barley was used in all the the plots. Why was this important? [2]

  1. Idea of a confounding variable;

  2. (So) genetically similar;

  3. (So) have similar salt tolerance/ response to salt water/ response to watering treatment;

  4. (So) have similar yield/ mass of seeds;
• Explain what is meant by the independent variable. [1]

  1. Variable that is changed;
• Give two ways in which pathogens can cause disease. [2]

  1. (Releases) toxins;

  2. Kills cells / tissues.
• Putting bee honey on a cut kills bacteria. Honey contains a high concentration of sugar.

  Use your knowledge of water potential to suggest how putting honey on a cut kills bacteria. [3]

  1. Water potential in (bacterial) cells higher (than in honey) / water potential in honey lower (than in bacterial cells);

  
  

  Q candidates must express themselves clearly 1. Must be comparative e.g. high WP in cell and low WP in honey

  
  

  2. Water leaves bacteria / cells by osmosis;

  3. (Loss of water) stops (metabolic) reactions.
• Explain why antibody A attaches only to the protein found in species of Plasmodium. [2]

  1. Antibody has tertiary structure;

  2. Complementary to binding site on protein.
• HSV infects nerve cells in the face (line 1). Explain why it infects only nerve cells. [3]

  1. Outside of virus has antigens / proteins;

  2. With complementary shape to receptor / protein in membrane of cells;

  3. (Receptor / protein) found only on membrane of nerve cells.
• The percentage of the population vaccinated does not need to be 100% to be effective in preventing the spread of whooping cough. Suggest why. [2]

  1. More people are immune / fewer people carry the pathogen;

  If neither point 1 or 2 awarded Herd immunity = 1 mark

  2. So susceptible / unvaccinated people less likely to contact infected people;
• Explain how the monoclonal antibody would prevent the regulator protein from working (lines 7−8). [2]

  1. Lipid soluble / hydrophobic

  2. Enters through (phospholipid) bilayer

  OR

  3. (Protein part of) LDL attaches to receptor

  4. Goes through carrier / channel protein.

  
  

  4. Accept by facilitated diffusion or active transport

  4. Reject active transport through channel protein