Recall Questions

• How are phospholipids arranged in a plasma membrane? [3]

  1.      Bilayer;​

  Accept double layer​

  Accept drawing which shows bilayer​

  2.      Hydrophobic / fatty acid / lipid (tails) to inside;​

  3.      Polar / phosphate group / hydrophilic (head) to outside;​

  2. &  3.  need labels​

  2. &  3.  accept water loving or hating 
• Describe the structure of a plasma membrane

  1. Phospholipids forming a bilayer/two layers;​
  2. The phosphate heads are on the outside of the bilayer ​the fatty-acid tails are on the ‘inside’ of the bilayer;​
  3. The membrane contains intrinsic proteins (which pass right through the membrane) and extrinsic proteins (which are confined to one layer)​
  4. The membrane has channel proteins and carrier proteins​
  5. There are cholesterol molecules to regulate the fluidity of the membrane​
  6. There are glycoproteins which are involved in cell recognition and signalling
• Explain how three features of a plasma membrane adapt it for its functions (part 1)

  1       phospholipid bilayer (as a barrier);​

  2       forms a barrier to water soluble / charged substances / allows non-polar substances to pass​

  OR    maintains a different environment on each side / compartmentalisation;​

  ​

  3       the bilayer is fluid;​

  4       so it can bend to form different shapes during phagocytosis / form vesicles / self repair;​

  ​

  5       intrinsic channel proteins through the bilayer;​

  6       let water soluble/charged substances through by facilitated diffusion;
• Explain how three features of a plasma membrane adapt it for its functions (part 2)

  ​

  7       carrier proteins (through the bilayer);​

  8       allow facilitated diffusion or active transport;​

  ​

  9       surface proteins / extrinsic proteins, glycoproteins / glycolipids;​

  10     cell recognition / act as antigens / receptors;​

  ​

  11     cholesterol;​

  12     regulates fluidity / increases stability;
• The structure of a cholera bacterium is different from the structure of an epithelial cell from the small intestine. Describe how the structure of a cholera bacterium is different.

  1       Cholera bacterium is prokaryote (and epithelial cell is eukaryotic);​

  2       Cholera does not have a nucleus/has DNA free in cytoplasm/has loop of DNA;​

  3        Cholera has no membrane-bound organelles / no mitochondria / no golgi / no endoplasmic reticulum / etc;​

  4       Cholera has small ribosomes only;​

  5       Cholera has a capsule / flagellum / plasmid / cell wall ;
• Contrast the structure of a bacterial cell and the structure of a human cell. [5] (part 1)

  1. Bacterial cell is much smaller than a human cell; ​
  2. Bacterial cells have a slime capsule but human cells do not;​
  3. Bacterial cell has a cell wall but human cell does not; ​
  4. Bacterial cell lacks a nucleus but human cell has a nucleus; ​
  5. Bacterial cell lacks membrane-bound organelles but human cell has membrane- bound organelles; ​
• Contrast the structure of a bacterial cell and the structure of a human cell. [5] (part 2)

  1. Bacterial ribosomes smaller than human ribosomes / bacteria have 70S ribosomes whereas humans have 80S ribosomes; ​
  2. Bacterial DNA is circular but human DNA is linear; ​
  3. Bacterial cells have plasmids but human cells do not;​
  4. Bacterial DNA is ‘naked’ whereas human DNA is bound to histones/proteins; 
• Starting with some leaves, describe how you would obtain a sample of undamaged chloroplasts. Use your knowledge of cell fractionation and ultracentrifugation to answer this question.

  1. Chop up /homogenise the leaves​
  2. 2. Cold;​

  3. Buffer solution;​

  4. Isotonic / same water potential;​

  5. Filter and centrifuge filtrate;​

  6. Centrifuge supernatant;

  ​7. At higher speed;​

  8. Chloroplasts in (second) pellet;
• Scientists use optical microscopes and transmission electron microscopes (TEMs) to investigate cell structure. Explain the advantages and the limitations of using a TEM to investigate cell structure compared with an optical microscope (part 1 - advantages)

  1       Small objects can be seen;​

  2       TEM has higher resolution becuase​

  3       Wavelength of electrons is shorter than light;
• Scientists use optical microscopes and transmission electron microscopes (TEMs) to investigate cell structure. Explain the advantages and the limitations of using a TEM to investigate cell structure compared with an optical microscope (Part 2 -limitations)

  4       Cannot look at living cells;​

  5       Must be in a vacuum;​

  6       Must cut section / thin specimen;​

  7       Preparation may create artefact​

  8       Does not produce colour image;
• Contrast how an optical microscope and a transmission electron microscope work ​

  and contrast the limitations of their use when studying cells. (part 1)

  • TEM use electrons and optical use light; ​
  • TEM allows a greater resolution; ​
  • (So with TEM) smaller organelles/named cell structure can be observed OR greater detail in organelles/named cell structure can be observed; ​
• Contrast how an optical microscope and a transmission electron microscope work ​

  and contrast the limitations of their use when studying cells. (part 2)

  
  

  • TEM view only dead/dehydrated specimens and optical (can) view live specimens; ​
  • TEM does not show colour and optical (can); ​
  • TEM requires thinner specimens; ​
  • TEM requires a more complex/time consuming preparation; ​
  • TEM focuses using magnets and optical uses (glass) lenses; ​
• 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;​

  4.      Lower cover slip using mounted needle; ​

   Allow any appropriate method that avoids trapping air bubbles
• A scientist wanted to calculate the mean volume of spores. Describe how she could use Figure 5 do this.​You may assume the spores are perfectly spherical. (magnification= x700)

  1. Measure diameter of large number of spores; ​
  2. Divide measured values by 700 (to find true diameter); ​
  3. Calculate the volume of a sphere using the formula 4/3pi r^3; 

  
•  Mitosis is important in the life of an organism. Explain why [3]

  1.      Growth / increase in cell number;​

  Ignore growth of cells​

  2.      Replace cells / repair tissue / organs /body;​

  Ignore repair cells​

  Reject bacteria​

  3.      Genetically identical cells;​

  ‘Produces 2 genetically identical cells’ does not reach MP1 as well as MP3​

  4.      Asexual reproduction / cloning;​

  Allow example or description
• Describe the behaviour of chromosomes during mitosis and explain how this results in the production of two genetically identical cells. [6](part 1)

  1       chromosomes shorten/thicken/supercoiling;​

  2       chromosomes (each) two identical chromatids/strands/copies ​(due to replication);​

  3       chromosomes/chromatids move to equator/middle of the spindle/cell;​

  4       attach to individual spindle fibres;​

  5       spindle fibres contract / centromeres divide / repel;​
• Describe the behaviour of chromosomes during mitosis and explain how this results in the production of two genetically identical cells. [6] (part 2)

  6       (sister) chromatids/chromosomes (separate)  move to opposite poles/ends of the spindle;​

  7       each pole/end receives all genetic information/ identical copies of each chromosome;​

  8       nuclear envelope forms around each group of chromosomes/ chromatids/at each pole;​

  9      Chromosomes unwind/uncoil
• Describe and explain the appearance of one of the chromosomes in the cell [3]

  • Chromosome is formed of two chromatids; ​
  • (Because) DNA replication (has occurred); ​
  • (Sister) chromatids held together by centromere; 
• Describe how bacterial cells replicate

  1. The circular DNA and plasmids in the cell replicate​
  2. the two copies of the DNA attach to different points on the cell’s membrane​
  3. The bacterial cell grows so that the attached DNA move further away from each other​
  4. the plasmids divide randomly between the two sides of the cell (moving by diffusion)​
  5. The cell membrane pinches inwards in the middle of the cell, so the cytoplasm divides​
  6. New cell wall forms so the cell is divided into two
• Describe how viruses replicate

  1. Virus attaches to a host cell​
  2. Virus injects its genetic material into the host cell​
  3. The host cell transcribes and translates the viral genes​
  4. These proteins form new virus particles​
  5. The virus particles burst out of the host cell, so the host cell is destroyed
• Why does each virus only infect a specific type of cell? [3]

  • Outside of virus has antigens/proteins; ​
  • With complementary shape to receptor/protein in membrane of (target) cells; ​
  • (Receptor/protein) found only on membrane of that specific type of cells; 
• Some substances can cross the cell-surface membrane of a cell by simple diffusion through the phospholipid bilayer. Describe other ways by which substances cross this membrane (part 1)

  By osmosis (no mark)​

  1.     From a high water potential to a low water potential / down a water potential gradient;​

  2.     Through aquaporins / water channels;​

  By facilitated diffusion (for large, polar, water-soluble molecules)​

  3.      through channel or carrier protein;​

  4.      Down concentration gradient;
• Some substances can cross the cell-surface membrane of a cell by simple diffusion through the phospholipid bilayer. Describe other ways by which substances cross this membrane (part 2)

  By active transport (no mark)​

  5.      through Carrier proteins​

  6.     Against a concentration gradient;​

  7.     Using ATP / energy (from respiration);​

  By phagocytosis / endocytosis (no mark)​

  8.     Engulfing by cell surface membrane to form vesicle / vacuole;​

  By exocytosis​

  9.     Fusion of vesicles (from the Golgi) with cell surface membrane and releasing the contents;
• Describe the processes involved in the absorption of the products of starch digestion. 

  1. Sodium ions are actively transported out of epithelial cell by sodium-potassium pump;​
  2. Into blood;​
  3. So the sodium ion concentration in the cell is lower than in the lumen of the gut;​
  4. So, sodium ions enter the cell by facilitated diffusion (from the gut)​
  5. And glucose is absorbed with Na+ against its concentration gradient through a symport protein;​
  6. Glucose moves from the epithelial cell into the blood by facilitated diffusion;
• Describe the processes involved in the absorption of the products of protein digestion. 

  1. Amino acids moves in with sodium (into epithelial cell);​
  2. Via (carrier/channel) protein/symport;​
  3. Sodium removed (from epithelial cell) by active transport/sodium-potassium pump;​
  4. Into blood;​
  5. Maintaining low concentration of sodium (in epithelial cell) / maintaining sodium concentration gradient (between lumen and epithelial cell);​
  6. Amino acid moves into blood;​
  7. By (facilitated) diffusion;
• Describe how substances move across cell-surface membranes by facilitated diffusion. [3]

  • Carrier/channel protein; ​
  • (Protein) specific/complementary to substance; ​
  • Substance moves down concentration gradient; ​

  ​
• Contrast the processes of facilitated diffusion and active transport. [3]

  1. Facilitated diffusion involves channel or carrier proteins whereas active transport only involves carrier proteins; ​
  2. Facilitated diffusion does not use ATP / is passive whereas active transport uses ATP; ​
  3. Facilitated diffusion takes place down a concentration gradient whereas active transport can occur against a concentration gradient; 
• Compare and contrast the processes by which water and inorganic ions enter cells. [3]

  1. Comparison: both move down concentration gradient; ​
  2. Comparison: both move through (protein) channels in membrane; ​
  3. Contrast: ions can move against a concentration gradient by active transport; 
• Oxygen and chloride ions can diffuse across cell-surface membranes. The diffusion of chloride ions involves a membrane protein. The diffusion of oxygen does not involve a membrane protein. ​Explain why the diffusion of chloride ions involves a membrane protein and the diffusion of oxygen does not. 

  • Chloride ions water soluble/charged/polar; ​
  • Cannot cross (lipid) bilayer (of membrane); ​
  • Chloride ions transported by facilitated diffusion OR diffusion involving channel/carrier protein; ​
  • Oxygen not charged/non-polar; ​
  • (Oxygen) soluble in/can diffuse across (lipid) bilayer;