CELLS

• CELL STRUCTURE AND MICROSCOPY

  Uronema marinum is a single-celled eukaryotic organism. The diagram below is

  a photograph of U. marinum taken through an optical microscope.

  (a) Explain why it is not possible to determine the identity of the structures

  labelled X using an optical microscope. (2)

  1. Resolution (too) low;

  2. Because wavelength of light is (too) long;
• U. marinum cells ingest bacteria and digest them in the cytoplasm.

  Describe the role of one named organelle in digesting these bacteria. (3)

  1. Lysosomes;

  2. Fuse with vesicle;

  Accept phagosome for vesicle

  3. (Releases) hydrolytic enzymes;

  Accept lysozymes for "hydrolytic enzymes"

  Accept 'Ribosomes/ Rough endoplasmic reticulum

  form hydrolytic enzymes = 2 marks

  Accept 'Golgi body forms lysosomes' = 2 marks

  Accept 'Golgi body / ribosomes / rough

  endoplasmic reticulum' for 1 mark if no other mark

  awarded.
• Give two structures found in all prokaryotic cells and in all eukaryotic cells (2)

  1. Cell(-surface) membrane;

  2. Ribosomes;

  Ignore 70S

  3. Cytoplasm;

  4. DNA;
• All prokaryotic cells contain a circular DNA molecule and some prokaryotic cells

  contain plasmids.

  (b) Scientists have found that the rate of plasmid replication is faster in cells

  growing in a culture with a high concentration of amino acids than in a

  culture with a lower concentration of amino acids.

  Suggest one explanation for the faster rate of plasmid replication in cells

  growing in a culture with a high amino acid concentration. (2)

  1. (Amino acids used in) protein synthesis;

  Accept for 'protein synthesis', translation

  2. (So) more enzymes (for DNA/plasmid replication)

  OR

  (So) more DNA polymerase;

  3. (Amino acids used in) respiration;

  4. (So) more energy/ATP (for DNA/plasmid replication);
• What can you conclude from the figure above about a structural difference

  between the plasmids and the circular DNA? Explain your answer. (2)

  1. Circular DNA is bigger/heavier/denser;

  2. (Because band) moved further/is lower (in tube)/closer to bottom (of tube); Accept converse for plasmids
• Describe how a sample of chloroplasts could be isolated from leaves. (4)

  1. Break open cells/tissue and filter

  OR

  Grind/blend cells/tissue/leaves and filter;

  Accept homogenise and filter

  2. In cold, same water potential/concentration, pH controlled solution;

  Accept for 'same water potential/ concentration',

  isotonic

  Accept for 'pH controlled', buffered

  3. Centrifuge/spin and remove nuclei/cell debris;

  4. (Centrifuge/spin) at high(er) speed, chloroplasts settle out;
• Give one feature of the chloroplast that allows protein to be synthesised

  inside the chloroplast and describe one difference between this feature in

  the chloroplast and similar features in the rest of the cell. (2)

  1. DNA;

  2. Is not associated with protein/histones but nuclear DNA is

  OR

  Is circular but nuclear DNA is linear

  OR

  Is shorter than nuclear DNA;

  AQA Biology A-Level - Cell Structure and Microscopy MS PhysicsAndMathsTutor.com

  3. Ribosomes;

  4. Are smaller than cytoplasmic ribosomes;

  Accept: 70S ribosomes in chloroplast, but 80S

  ribosomes in cytoplasm
• Contrast the structure of the two cells visible in the electron micrographs shown in the figure above. (5)

  1. Magnification (figures) show A is bigger than B;

  2. A has a nucleus whereas B has free DNA;

  3. A has mitochondria whereas B does not;

  4. A has Golgi body/endoplasmic reticulum whereas B does not;

  5. A has no cell wall whereas B has a murein/glycoprotein cell wall;

  Accept peptidoglycan

  6. A has no capsule whereas B has a capsule;

  7. A has DNA is bound to histones/proteins whereas B has

  DNA not associated with histones/proteins

  OR

  A has linear DNA whereas B has circular DNA;

  8. A has larger ribosomes;

  Accept in all marking points, animal/eukaryote for A

  and prokaryote/ bacterium for B
• Eukaryotic cells produce and release proteins.

  Outline the role of organelles in the production, transport and release of

  proteins from eukaryotic cells.

  Do not include details of transcription and translation in your answer. (4)

  1. DNA in nucleus is code (for protein);

  2. Ribosomes/rough endoplasmic reticulum produce (protein);

  Accept rER for 'rough endoplasmic reticulum'

  3. Mitochondria produce ATP (for protein synthesis);

  4. Golgi apparatus package/modify;

  OR

  Carbohydrate added/glycoprotein produced by Golgi apparatus;

  Accept body for 'apparatus'

  5. Vesicles transport

  OR

  Rough endoplasmic reticulum transports;

  6. (Vesicles) fuse with cell(-surface) membrane;

  Accept exocytosis at cell membrane
• Suggest why a nucleus is not visible in above image (1)

  A section/slice (so nucleus in another part of cell)

  OR

  (Nucleus) not stained;
• Name the organelles labelled S and T in the image above. (1)

  S = Vacuole

  T = Chloroplast;

  Reject thylakoid/granum

  Reject incorrect spelling
• Give one advantage of viewing a biological specimen using a transmission electron microscope compared with using a scanning electron microscope. (1)

  Higher resolution

  OR

  View internal structures;
• Identify structures labelled D and E (2)

  D - Granum/grana/thylakoid(s);

  E - starch/lipid;
• The detail shown in the diagram above would not be seen using an optical microscope. Explain why. (2)

  1. Light has long(er) wavelength;

  Ignore: optical microscope has long(er) wavelength.

  2. (So) low(er) resolution;

  Accept poor resolution

  Ignore: weaker resolution

  Ignore references to magnification

  Accept correct references to values for resolution.

  E.g optical 0.2µm - 0.3 µm
• Name an organelle found in both a chloroplast and a prokaryotic cell. (1)

  (70S) Ribosome;
• A biologist separated cell components to investigate organelle activity. She

  prepared a suspension of the organelles in a solution that prevented

  damage to the organelles.

  Describe three properties of this solution and explain how each property

  prevented damage to the organelles. (3)

  1. (Ice) cold to prevent/reduce enzyme activity;

  For 1, 2 and 3 reject context of cell

  2. Buffered to prevent denaturing of enzyme/protein;

  Accept description of buffer.

  Accept: prevent change of tertiary structure.

  3. Same water potential/ Ψ to prevent lysis/bursting (of organelle);

  Accept: isotonic for same water potential.

  Reject: references to turgor or plasmolysis or crenation.
• Name the structures labelled W to Z in the diagram (2)

  W - (cell surface) membrane

  X - cell wall

  Y - capsule

  Z - flagellum

  Four correct = 2 marks.

  Three or two correct = 1 mark.

  Y - Ignore references to slime/mucus

  Y - Reject capsid

  Z - accept flagella
• Name the main biological molecule in W and X: (2)

  W - Phospholipids;

  X - Murein / glycoprotein;

  X - Accept peptidoglycans.

  Accept phonetic spellings
• Name the process by which prokaryotic cells divide (1)

  Binary fission;

  Reject binary fusion
• What type of microscope was used to obtain the image shown in the diagram above? Give one piece of evidence to support your answer.

  Type of microscope

  Evidence

  1. Scanning electron (microscope);

  2. 3D (image);

  Accept SE(M)

  2. Ignore any other correct features
• Contrast how an optical microscope and a transmission electron

  microscope work and contrast the limitations of their use when studying

  cells. (6)

  1. TEM use electrons and optical use light;

  2. TEM allows a greater resolution;

  3. (So with TEM) smaller organelles / named cell structure can be

  observed

  OR

  greater detail in organelles / named cell structure can be

  observed;

  4. TEM view only dead / dehydrated specimens and optical (can)

  view live specimens;

  5. TEM does not show colour and optical (can);

  6. TEM requires thinner specimens;

  7. TEM requires a more complex/time consuming preparation;

  8. TEM focuses using magnets and optical uses (glass) lenses;
• Name two structures present in plant cells that are not present in animal cells. (1)

  Golgi (body / apparatus);

  C Mitochondria / mitochondrion;
• Explain why the biologist used a blender and then filtered the mixture (steps 2 and 3) (2)

  1. Break open cells / homogenise / produce homogenate;

  2. Remove unbroken cells / larger debris;
• Name the organelle that made up most of the first pellet after centrifuging at a low speed (step 4). (1)
• The second centrifuge tube was spun at a higher speed to obtain the sample of organelles labelled C in the diagram (step 5). Suggest why (1)

  Nucleus / nuclei;
• 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.
• The cell-surface membrane can be seen with a transmission electron microscope but not with an optical microscope. Explain why. (1)

  Electron microscope has higher resolution (than optical microscope).
• No organelles are visible in the cytoplasm of this red blood cell. Suggest why (1)

  Cytoplasm of red blood cell filled with haemoglobin
• Before the cell was examined using the electron microscope, it was

  stained. This stain caused parts of the structure of the cell-surface

  membrane to appear as two dark lines.

  Suggest an explanation for the appearance of the cell-surface membrane

  as two dark lines. (3)

  1. Membrane has phospholipid bilayer;

  2. Stain binds to phosphate / glycerol;

  3. On inside and outside of membrane.

  Accept phospholipid head / protein
• 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 / cellsurface membrane.
• Describe how you could make a temporary mount of a piece of plant tissue to observe the position of starch grains in the cells when using an optical (light) microscope. (4)

  1. Add drop of water to (glass) slide;

  2. Obtain thin section (of plant tissue) and place on slide / float on

  drop of water;

  3. Stain with / add iodine in potassium iodide.

  3. Allow any appropriate method that avoids

  trapping air bubbles

  4. Lower cover slip using mounted needle
• Give the name and function of the structures labelled W and Z.

  Name of W_________________________________________________

  Function of W_______________________________________________

  Name of Z _________________________________________________

  Function of Z __ (2)

  W - chloroplast, photosynthesis;

  Z - nucleus, contains DNA / chromosomes / holds genetic

  information of cell
• A transmission electron microscope was used to produce the image in the figure above. Explain why (2)

  1. High resolution;

  2. Can see internal structure of organelles.
• CELL CYCLE AND MITOSIS

  Describe binary fission in bacteria. (3)

  1. Replication of (circular) DNA;

  Accept nucleoid

  Reject chromosome

  Reject mitosis

  2. Replication of plasmids;

  3. Division of cytoplasm (to produce daughter cells);

  Ignore genetically identical
• Suggest and explain how two environmental variables could be changed to increase the growth rate of these cells. (4)

  Principle of marking pairs: Named environmental variable; Correct effect

  on growth rate;

  Examples

  1. Increased (concentration of) glucose;

  2. Increased respiration;

  3. Increased (concentration of) oxygen;

  4. Increased respiration;

  5. Increased temperature;

  6. Increased enzyme activity;

  7. Increased (concentration of) phosphate;

  8. Increased ATP/DNA/RNA;

  9. Increased (concentration of) nucleotides;

  10. Increased DNA synthesis;
• This question is about mitosis in cells.

  The image below shows the arrangement of the genetic material in a cell during

  prophase.

  (a) Describe and explain the arrangement of the genetic material shown in the

  above image.(2)

  1. Chromosomes (are) becoming visible/distinct;

  2. Because (still) condensing;
• Name the fixed position occupied by a gene on a DNA molecule. (1)

  Locus/loci;
• Name the three phases of mitosis shown by C, D and E on the figure

  above.

  Describe the role of the spindle fibres and the behaviour of the

  chromosomes during each of these phases. (5)

  1. C = prophase and

  D = metaphase and

  E = anaphase;

  2. (In) prophase, chromosomes condense;

  Accept chromatin for 'chromosomes' and for

  'condense', shorten and thicken

  3. (In) prophase OR metaphase, centromeres attach to spindle fibres;

  4. (In) metaphase, chromosomes/pairs of chromatids at

  equator/centre of spindle/cell;

  5. (In) anaphase, centromeres divide;

  6. (In) anaphase, chromatids (from each pair) pulled to (opposite)

  poles/ends (of cell);

  Accept for 'chromatids', chromosomes but reject

  homologous chromosomes

  7. (In) prophase/metaphase/anaphase, spindle fibres shorten;
• The offspring produced from farmed trout are sterile. Suggest and explain why. (2)

  1. Too many/extra set/three copies of chromosomes;

  Accept 1 paternal, 2 maternal

  2. (Homologous) chromosomes do not pair

  OR

  (Homologous) chromosomes do not separate (evenly);

  Accept divide for separate

  3. (So) no meiosis;
• Suggest why preventing the formation of spindle fibres (lines 2-3) stopped the cell cycle. (2)

  1. Chromosomes/centromeres cannot attach (to spindle)

  OR

  Chromosomes cannot line up (on spindle);

  2. (So, no) metaphase;

  OR

  3. Chromatids cannot separate (on spindle);

  Accept description of 'cannot separate' e.g cannot move

  to poles

  Ignore 'split