YouTube past paper biology

• Figure 1.1 shows part of a cell in the groin region of a plant
• Protein A

  Auxin receptor
• Protein B

  Proton pump
• Protein C

  Potassium ion channel
• Effect on cell wall of hydrogen ions moving into cell wall

  1. Hydrogen ions make cell wall more acidic

  2. Loosens cellulose linkages

  3. Activates expansins

  4. Causes cell wall expansion and plant elongation
• Interpupillary distance (IPD) is the distance between the centers of the pupils
• Continuous variation

  Non-distinct variation distributed over a range
• Genes and environment contribute to IPD variation

  • Many genes have additive effects and interact
  • Environment can affect gene expression e.g. diet
• Pax3 gene


  Regulatory gene that controls expression of other genes involved in embryonic development
• Using microarray analysis to identify genes switched on by Pax3

  1. Extract mRNA from embryonic cells

  2. Reverse transcribe to cDNA and tag with fluorescent labels

  3. Hybridize cDNA to microarray

  4. Detect fluorescent signals to identify expressed genes
• Chimpanzees have higher Pax3 expression than humans


  Leads to differences in facial structure between species despite similar DNA
• to say that there might be a small genetic difference um in the pax 3 uh sequence in humans and chimpanzees that results in them having you know this differences so just one small change in alleles can result in big phenotypic differences because the pax 3 controls so many other genes it might then have a different effect in chimpanzees than it would have in humans and also the fact that the pax 3 regime is higher in concentration in chimpanzees might then be the result for their eyes being close together because remember it said uh when we were reading the question earlier that you the more pax three like the pax region controls distance between the eyes so you can look at this chimpanzee the image is also there too for you to observe and sort of make an inference on what you might put as your answer but sometimes suggest questions can be quite difficult for students but by all means think about it and just think of how you would apply what you have learned and what you've learned also from the question because most of the time the answers are hidden in the question okay this one looks a little bit small but i hope that we can see it but i'm just going to read it out anyway so it says here some neurons in the brain produce a neurotransmitter known as dopamine i'm sure you've heard of dopamine before and it says parkinson's disease or cause when the neurons that produce dopamine die a person with the disease may experience difficulty in coordinating movement often seen as tremors in different parts of the body parkinson's disease typically occurs in people older than 55 years younger people with these symptoms are said to have early onset parkinson's disease recessive mutations in the gene known as pink one located on chromosome 1 and autism are believed to be one cause of eopd so that's early onset parkinson's disease a person with this form of eopd has a homozygous recessive genotype
• Homozygous recessive

  Having two recessive alleles
• Heterozygous

  Having one dominant and one recessive allele
• Dominant allele


  Represented by a capital letter
• Recessive allele


  Represented by a lowercase letter
• Genetic cross

  1. Parental genotypes

  2. Gametes

  3. Offspring genotypes
• Phenotype


  The observable characteristics or traits of an organism
• The ratio of offspring phenotypes is 3:1 (normal:early onset Parkinson's disease)
• The link reaction and Krebs cycle take place in the mitochondrion
• Stages of the link reaction and Krebs cycle

  1. Correct order

  2. Pyruvate enters mitochondrial matrix

  3. Pyruvate is decarboxylated and dehydrogenated

  4. Acetyl group combines with coenzyme A

  5. Acetyl CoA enters Krebs cycle

  6. Citrate is formed

  7. Citrate is decarboxylated and dehydrogenated

  8. Hydrogen atoms accepted by NAD and FAD

  9. Oxaloacetate is regenerated

  10. ATP is made by substrate-level phosphorylation
• NAD

  Coenzyme that accepts hydrogen from dehydrogenation and carries it to the electron transport chain for ATP production
• Carbon dioxide is removed from compounds in the link reaction and Krebs cycle by decarboxylation
• Each pyruvate molecule that goes through the link reaction and Krebs cycle yields 3 CO2 molecules
• Cellular respiration

  1. CO2 removed

  2. Pyruvate becomes acetyl

  3. Acetyl combines with CoE

  4. Goes into Krebs cycle

  5. Loses 2 more CO2 molecules
• Total CO2 lost in link reaction and Krebs cycle is 3
• 1 glucose molecule from glycolysis splits into 2 pyruvate molecules
• Each pyruvate molecule yields 1 CO2 in link reaction, and 2 extra CO2 in Krebs cycle
• Total CO2 produced from 1 glucose is 6
• CO2 transport in blood

  Mainly as hydrogen carbonate, not as carbonic acid
• Reason for CO2 transport as hydrogen carbonate

  • Carbonic acid would dissociate due to catalysis by carbonic anhydrase
  • Hydrogen carbonate ions prevent decrease in blood pH, acting as a buffer
• Researchers investigated the extent to which founder effect and natural selection affected evolutionary change in lizards
• Lizards

  • Brown and anole species
  • Live on Caribbean islands
  • Feed on invertebrates and small animals
• Perch diameter

  Width of branch that lizard rests on
• Perch diameter

  Positively correlated with hindlimb length of lizard
• Longer hindlimbs allow lizards to run faster on vegetation with larger diameter, shorter hindlimbs provide stability on smaller diameter vegetation
• Reestablishing lizard populations

  1. Hurricane destroyed lizards on 7 islands

  2. Researchers selected 7 male and 7 female lizards from nearby island

  3. Mated pairs placed on each of the 7 islands to form new populations
• Vegetation


  • Larger on source island, smaller on experimental founder islands
• Speciation

  Allopatric speciation - geographically isolated populations evolve differently
• Experimental founder populations at high risk of extinction