Bio AQA A-Level (AS P1) Exam Revision

• Describe and explain how the Counter-current system leads to efficient gas exchange across the gills of fish:

  • Blood flow is in the opposite direction of water flow
  • This maintains the oxygen concentration gradient between water and blood
  • Along the whole length of the Lamaella
• The volume of water passing over the gills increases if the temperature of the water increases. Suggest why:

  • Increased respiration due to increased enzyme activity
  • Less oxygen (dissolved in water)
• Give 2 possible causes of variation that results from meiosis during gamete formation:

  • Crossing over
  • Independent Assortment
• Explain the relationship between concentration of Herbecide and the mean index of diversity of insects:

  • Reduced biodiversity
  • Concentration of herbicide removes weeks which leads to fewer food sources
  • Fewer habitats available for insects.
• A starch molecule has a spiral shape. Explain why this shape is important to its function in cells:

  • Starch is helical -> more compact so more efficient storage
  • More glucose can be stored
• Some seeds contain lipids. Describe how you could use the emulsion test to show that a seed contains lipids:

  • Crush seeds with ethanol
  • then add distilled water
  • If lipids are present a cloudy white emulsion is formed
• Explain the difference in the structure of starch molecules and cellulose molecules:

  • Starch Is made from Alpha glucose - Cellulose from Beta Glucose
  • Hydroxyl group and hydrogen atoms on Carbon 1 of Cellulose are inverted.
• Describe competitive and Non-competitive inhibition of an enzyme:

  (5m)

  Competitive:

  • Inhibitors reduce binding of substrate and enzyme = reduced rate
  • Same shape as substrate so it binds to active site.
  • Can be overcome by more substrate.

  Non-competitive:

  • Binds to another site on the enzyme (other than active site)
  • Causes active site to change shape - no longer complimentary to substrate
  • Cannot be overcome by more substrate
• Explain one way which starch molecules are adapted for their function in plant cells:

  • Starch molecules are insoluble in water
  • So it doesn't affect water-potential
• How is Cellulose adapted for its function in plant cells:

  • Long and Straight chains
  • In between layered chains there are hydrogen bonds - forms a microfibrilis
  • Provides strength for function as cell wall.
• Describe the structure of proteins (5m)

  • Primary Structure: Condensation reactions forms peptide bonds between Amino Acids
  • Secondary: Hydrogen bonds form between amino acids - chain folds
  • Tertiary: Further folding of secondary structure due to disulphide bridges (between sulphur groups), Ionic bonding (between charged R groups) and Hydrogen bonds
  • Quaternary: protein made of more than 1 polypeptide chain.
• Describe and explain how centrifuging a culture allows scientists to obtain a cell-free liquid:

  • Centrifugation forms pellets = organelles of high density.
  • Supernatant formed above pellets.
  • Super-natant can be removed by filtering off.
• Describe how you could use cell fractionation to isolate chloroplasts from leaf tissue:

  • Place leaf tissue in buffer, ice-cold, isotonic solution.
  • Homogenize with blender - to break open cells and reveal organelles. Then filter to remove cell-debris
  • Centrifuge - spin sample. Filter to remove nucleus then repeat for second time at higher speeds to remove chloroplasts.
• Contrast how an optical microscope and a transmission electron microscope work and contrast the limitations of use when studying cells:

  Optical:

  • Uses light
  • Live specimen can be used.
  • Easy preparation
  • Uses lenses to focus.

  TEM:

  • Uses electron beams
  • Greater resolution & Magnification - Optical is poorer.
  • Solution done in a vacuum
  • TEM does not show colored images
  • Specimen can only be dead or artifact.
  • More time consuming preparation.
  • Uses electromagnets to focus on specimen.
• Explain why in cell fractionation biologists use ice-cold, buffered and isotonic (same water potential) solutions: (3m)

  • Ice-cold: To reduce / slow enzyme activity
  • Buffer: Maintains pH so enzymes do not denature
  • Isotonic / Same water potential: to prevent osmosis and shrinkage of organelles.
• Describe and explain two features you would expect to find on cells specialized for absorption:

  • Microvilli - increases Surface Area : Volume ratio for more effective diffusion
  • More carrier / channel proteins - for more facilitated diffusion nd active transport.
• The movement of substances across the cell membrane is affected by membrane structure. Explain how (5m):

  • Phospholipid bilayer allows movement of lipid-soluble , non-polar substances.
  • Phospholipid bilayer prevents movement of lipid-insoluble, polar substances through it.
  • Carrier proteins allow active transport.
  • Channel protein allows facilitated diffusion and co-transport.
  • Shape of protein channel determines the substance that will move through it.
  • Cholesterol affects rigidity / Fluidity of membrane.
• The movement of Na+ out of the cell allows the absorption of glucose into cell lining of Ileum:

  • Maintains concentration gradient.
  • Na+ moving in by facilitated diffusion brings glucose with it, so absorption from high to low concentration from protein to ileum.
• Give 2 similarities in the movement of substances by Osmosis and Diffusion:

  • Down the concentration gradient - from high to low concentration.
  • Passive movement - without ATP.
• Compare and contrast similarities and differences between prokaryotic and eukaryotic DNA:

  Eukaryotic:

  • Membrane-bound in nucleus and associated with histones
  • Very long & Linear
  • In the nucleus.

  Prokaryotic:

  • Not membrane-bound or associated with proteins. Made compact by super-coiling.
  • In the cytoplasm.
  • Circular and short.
• Explain how the considerable length of a DNA molecule is compacted into a chromosome:

  
  

  • Protein bound - DNA is wrapped around Histones
  • Double-helix
• Describe the role of tRNA in the process of translation (3m):

  • tRNA molecule contains an Amino Acid at one end and a sequence of 3 bases called an anti-codon on the other end.
  • At the ribosome, the anti-codon on tRNA binds to complimentary codon on mRNA.
  • Another tRNA anticodon attaches to mRNA and a peptide bond is formed between amino acids.
• Transcription is the process where genetic information encoded in DNA is copied into mRNA in the nucleus of a cell.
• Transcription involves the separation of two strands of DNA, with one serving as a template strand.
• Transcription results in the production of a single-stranded RNA molecule.
• Translation is the process where information on mRNA is used to synthesize a protein, involving ribosomes and tRNA in the cytoplasm.
• Translation involves decoding the sequence of codons on mRNA by tRNA molecules that carry specific Amino Acids.
• Translation involves the synthesis of a polypeptide chain and production of proteins.
• How is genetic diversity created during Meiosis?

  
  

  Crossing Over:

  • During Prophase 1 the sections of chromatids of homologous pairs wrap around each other. The gene breaks off and re-joins on the non-sister chromatid.
  • The swapping between maternal and paternal alleles creates a new combination of alleles in each chromosome.

  Independent Assortment:

  • During Metaphase 1, when the homologous pairs align, maternal and paternal chromosomes align randomly.
  • The genes of the daughter cells are random = Each gamete would have a different allele.
• Land-dwelling insects lose water from their gas exchange surfaces. Use Fick's law to explain why they lose less water in air when humid:

  • Higher concentration of water in air than within insect.
  • Reduces rate of diffusion.
• Explain why the structure of the gill makes O2 uptake efficient:

  • Many lamellae = large surface area to volume ratio
  • Thin Lamellae wall = shorter diffusion pathway.
• Describe and explain 3 features in xerophytic plants that helps reduce water loss by evaporation:

  • Hairs = traps humid / moist air near surface of leaf - reduces diffusion.
  • Thick cuticle = reduces evaporation via leaf surface / increases diffusion pathway.
  • Sunken stomata = traps moist air near leaf surface - reduces transpiration.
• Explain how the lining of the Trachea / Bronchi and bronchioles provide protection against pathogens:

  • Cilia traps pathogens in mucus produced by goblet cells.
  • Mucus wafts up to mouth where it is swallowed.
  • Pathogen and mucus destroyed by HCL in stomach
  • Mucus provided by goblet cells provide barrier for ciliated epithelium cells from pathogens.
  • Blood vessels bring macrophages to engulf and destroy pathogens.
• How can genome sequencing (gene / base code) be used to establish evolutionary relationships between species:

  • Base sequence of organisms can be compared to other organisms.
  • The more similar the base sequence, the more closely related the organisms are.
  • Genome sequencing helps to clarify evolutionary relationships = common ancestry.
• Define Isomer:

  Molecules with same empirical formula but different structural formula.
• What is meant by the term tertiary structure?

  Further folding / coiling of the secondary structure due to interactions between R groups (ionic bonding / disulphide bridges)
• Explain the effect of temperature on Lipase activity:

  • As temperature increases, the Average Kinetic energy of the substrate increases.
  • More successful collisions = more enzyme-substrate complexes formed.
  • But at high temperatures, the active site of Lipase denatures.
  • Due to breaking of hydrogen bonds, ionic bonds and disulphide bridges in tertiary structure.
• Explain how the structure of cellulose is related to its function:

  • Straight and long chains
  • Hydrogen bonds formed between layers of chains - forms a microfibrilis
  • Many hydrogen bonds provides strength for cell wall.
• Mitochondria is found in large numbers in cells which take up substances by active transport. Explain why?

  Because energy, in the form of ATP, needed to transport substances against the concentration gradient (from low to high concentration)
• Give two reasons why an electron microscope is better than a light microscope:

  • Greater magnification
  • Greater resolution