.

• TEM microscopes work as:

  -electrons pass through specimen

  -detected as more dense structures

  -appear darker as they absorb more electrons

  -Focus image onto fluorescent screen or

  photographic plate using magnetic lenses.
• SEM microscope work as:

  -electrons focused by magnets

  -reflected electrons hit collecting device and produce a image
• Ultracentrifugation in cell fractionation consist of:

  1. homogenize (blend) tissue to break open cells &

  release organelles.

  2. Filter homogenate to remove debris.

  3. Perform differential centrifugation:

  a) Spin homogenate in centrifuge.

  b) The most dense organelles in the mixture form a pellet.

  c) Filter off the supernatant and spin again at a higher speed
• Cancer treatments control the rate of cell division as they:

  -prevent DNA replication by inibiting DNA helicase / polymerase in s phase of interphase

  -prevent the synthesis of enzymes needed for DNA replication

  -disrupts cell cycle= forcing cell to kill itself

  
  

  ● disrupt spindle formation = inhibit metaphase

  / anaphase

  NB: can also damage healthy cells
• Binary fission in bacteria:

  -Circular DNA replicates once and plasmids replicate in any amount in the cytoplasm

  -Cell elongates and cytoplasm splits by cytokinesis

  - (Circular) DNA moves to opposite poles of cell

  *produces two daughter cells = single copy circular DNA molecule and a variable number of plasmids
• Cells are adapted to maximise the

  rate of transport across membranes by having:

  ● many carrier/ channel proteins

  ● folded membrane increases surface

  area
• Fluid mosaic model as :

  
  

  -Fluid: phospholipid bilayer in which

  individual phospholipids can move so membrane has flexible shape.

  
  

  Mosaic: extrinsic & intrinsic proteins of different sizes and shapes are embedded.
• B cells respond by :

  -Complementary T H cells bind to foreign antigens on antigen-presenting T cells

  
  

  -Release cytokines that stimulate clonal selection and rapid mitosis of complementary B cells

  
  

  -B cells differentiate into plasma cells and memory cells

  
  

  -plasma cells secrete antibodies with complementary variable region to antigen
• T-cells respond by:

  -Complementary T h cells bind to foreign antigen on APC

  
  

  -T helper cells produce a clone of daughter cells that release cytokines

  
  

  -clonal selection of complementary T helper cells (rapid mitosis) become

  *memory T cells

  *Cytotoxic T cells

  *T helper cells= secrete cytokines that stimulate B cells to divide and form b plasma and memory cells
• Antigen variability is caused by:

  1. Random genetic mutation changes DNA base sequence.

  
  

  2. Results in different sequence of codons on mRNA

  
  

  3. Different primary structures of antigen = H-bonds,ionic bonds & disulfide bridges form in different places in tertiary structure.

  
  

  4. Different shape of antigen.
• Antigen variablity affects diseases as:

  ● Memory cells no longer complementary to

  antigen = individual not immune = can catch

  the disease more than once.

  
  

  ● Many varieties of a pathogen = difficult to

  develop vaccine containing all antigen types
• Vaccination leads to protection against a disease as:

  -Antigen on surface of microorganism/pathogen binds to surface receptor on a specific B cell

  -T helper cells secrete cytokines and stimulate B cells to divide by mitosis / produces clones

  
  

  -B plasma cells release antibodies;

  
  

  -(Some) B cells become memory cells;

  
  

  -Memory cells produce antibodies faster
• Direct ELISA test:

  detects presence of a specific antigen

  
  

  1. Monoclonal antibodies bind to bottom of test plate.

  2. Antigen molecules in sample bind to antibody. Rinse excess.

  3. Mobile antibody with ‘reporter enzyme’ attached binds to antigens that are ‘fixed’ on the monoclonal antibodies. Rinse excess.

  4. Add substrate for reporter enzyme. Positive result: colour

  change.
• HIV replication:

  -Attachment proteins bind to complementary CD4 receptors on T helper cell

  -Capsid and RNA enter the cell cytoplasm

  -Reverse transcriptase converts RNA to DNA

  -Viral enzymes make viral proteins and organelles from viral DNA

  -Virus assembled and released from the cell
• Indirect ELISA test:

  detects presence of an antibody against a specific antigen

  
  

  1. Antigens bind to bottom of test plate.

  2. Antibodies in sample bind to antigen. Wash away excess.

  3. Secondary antibody with ‘reporter enzyme’ attached binds to primary antibodies from the sample.

  4. Add substrate for reporter enzyme. Positive result: colour change.
• Co-transport of glucose and Na+:

  1. Na+ ions leave epithelial cell and enter blood;.

  2. (Transport out is by) active transport by carrier protein using ATP.

  3. So, Na+ conc. in cell is lower than in lumen (of gut)

  4. Na+ ions enter by facilitated diffusion

  5.Glucose absorbed with Na+ ions against their concentration
• the length of a cell using a microscope can be estimated by:

  • measuring diameter of field with ruler
  • measure the length with graticule
  • calibrate against stage micrometre
• in prophase

  • chromosomes condense
  • centrioles move to opposite poles and spindle fibres form
  • nuclear envelope breaks down (chromosomes free in cytoplasm )
• in metaphase

  • chromosomes line up at equator
  • spindle fibres attach to centromeres and pull to end poles
• In anaphase

  • centromeres divide
  • chromatids separate and are pulled to opposite poles of the cell
  • spindle fibres contract
  • Using energy from ATP hydrolysis
• in telophase

  • chromosomes decondense becoming visible
  • nuclear envelope form around each set of chromosomes
  • cytokinesis as cytoplasm divide
• Phagocytosis destroys pathogens as:

  1. Phagocyte recognises foreign antigen

  
  

  2. Phagocyte moves round and engulfs pathogen by endocytosis to form phagosome

  
  

  3. Phagosome fuses with lysosome and is enclosed in the vacuole (phagolysosome)

  
  

  4. Lysozymes digest pathogen (hydrolysed)

  
  

  5. Presents antigen on surface = antigen presenting cell (APC)
• Compare and contrast passive and active immunity:

  ● both involve antibodies

  ● can both be natural or artificial

  
  

  active= exposure to antigen, take while for protection to develop, memory cells produced

  • long term protection as antibodies produced complementary to antigen

  
  

  pass= doesn't require exposure to antigen, protection immediate, memory cells not produced

  • short term protection as antibodies broken down
• In cell fractionation the solution is:

  -ice cold: slow the action of enzymes so they don't digest organelles

  
  

  buffered: maintain constant pH. (protein/enzymes 3* not denatured

  
  

  isotonic: so they all have the same water potential and prevent osmosis of organelles/ prevent cell bursting or lysis
• Name and describe five ways substances can move across the cell-surface membrane into a cell

  
  

  • Simple diffusion of small/non-polar molecules down a concentration gradient
  • Facilitated diffusion down a concentration gradient via protein carrier/channel
  • Osmosis of water down a water potential gradient
  • Active transport against a concentration gradient via protein carrier using ATP
  • Co-transport of 2 different substances using a carrier protein
• Describe the appearance and behaviour of chromosomes during mitosis

  • prophase
  • Chromosomes condense so become visible
  • Chromosomes appear as 2 sister chromatids joined at the centromere
  • ----------------------------------------------
  • metaphase
  • Chromosomes line up on the equator
  • Chromosomes attached to spindle fibres by their centromere
  • ------------------------------------------
  • anaphase
  • centromere divides
  • Sister chromatids are pulled to opposite poles
  • -----------------------------
  • telophase
  • Chromosomes unwind
• one way the cell is adapted for photosynthesis:

  • Chloroplasts absorb light
• structure of a mitchondrian

  A) matrix

  B) crista

  C) ribosome

  
• cell fractionation:

  • ice cold- reduce/ prevent enzyme activity
  • Prevent osmosis/ movement of water, so organelles don't burst
• virus strucutre

  A) capsid

  B) attachment protien

  C) reverse transcriptase

  D) RNA

  E) viral envelope

  
• oxygen and carbon dioxide can diffuse across membranes as

  
  

  -Lipid part of membrane is non-polar/hydrophobic

  -Oxygen and carbon dioxide small/ non-polar molecules

  -Oxygen/carbon dioxide can diffuse through molecules in this layer

  -Down a concentration gradient
•  why the student soaked the root tips in hydrochloric acid

  • To break down / hydrolyse / cellulose/ cell wall
  • Allowing the stain to /diffuse into the cells
  • To stop mitosis
• Pressing the coverslip downwards enabled the student to observe the stages of mitosis clearly. Explain why

  • To create a thin layer of cells
  • So that light could pass through
• Other students in the class followed the same method, but calculated different mitotic indices.

  Apart from student errors, suggest two explanations why.

  • Garlic roots are a different age
  • Root tips from different (garlic) species
  • Single field of view is not representative of a root
  • Cells / roots undergo mitosis / cell division at different times/rates;
• Homologous chromosomes

  2 chromosomes that carry the same gene
• Explain why a second stain would be needed to stain the red blood cells.

  Suggest which molecule the stain could bind to in the red blood cells.

  • Red blood cells do not have a nucleus
  • Haemoglobin
• Explain how two enzymes with different amino acid sequences can catalyse the same reaction.

  1. Both active sites have similar tertiary structures
  2. So form enzyme-substrate complexes with the same substrate
• Describe viral replication.

  1. Attachment proteins attach to receptors
  2. Viral nucleic acid enters cell
  3. Reverse transcriptase makes DNA from RNA
  4. Cell produces viral protein and enzymes
  5. Virus assembled and released from cell
• Action of cholesterol

  1. Cholesterol stabilises the membraneby restricting the movement of molecules/phospholipids/fatty acid (tails) making it less fluid
• Explain how the use of antibiotics has led to antibiotic-resistant strains of bacteria becoming a common cause of infection acquired when in hospital.

  Some bacteria have alleles for resistance

  2. So Non-resistant bacteria die

  3. More antibiotics used in hospital compared with elsewhere