Exam mistakes PAPER 1 - Examiners' report

• Describe the structure of glycogen.

  • The majority of students scored mark point 2 for the idea that glycogen is a branched molecule or that it contains glycosidic bonds.
  • Although many students knew that glycogen is a ‘chain’ of alpha glucose, fewer could use the correct terminology to state that it is a polysaccharide or a polymer.
  • Students must take care to specify ALPHA glucose or α-glucose rather than a-glucose.
  • It was not uncommon for students to describe glycogen as if it were identical to starch and to include references to amylose and amylopectin in their answers.
• • It is important to note that, when referring to protein carriers/channels, only carriers can be used for active transport; either can be involved in facilitated diffusion.
  • Also, for a membrane to be adapted for rapid transport, it must have MORE! of these proteins, rather than them just being present.
• • It was not uncommon to see reference to “thin membranes”, indicating a clear confusion between the phospholipid bilayer structure of a cell-surface membrane and the epithelial layer of an exchange surface.
  • Reference to ‘villi’ also demonstrated similar confusion and prevented a student from obtaining mark point 1 - it is MICROVILLI - to INCREASE surface area for absorption
• High absorption of salt from the diet can result in a higher than normal concentration of salt in the blood plasma entering capillaries. This can lead to a build-up of tissue fluid. Explain how [2 marks]

  • Many students scored one mark for salt increasing the blood pressure (although it had to be clearly HIGHER! than normal; statements referring to ‘high’ pressure at the arteriole end were insufficient to gain credit).
  • Only a few could go on to suggest that, as a result, more fluid would move out at the arteriole end of the capillary.
• Explaining shape of graphs - enzyme curves at 50C and 60C

  • The mark least often awarded was for the explanation that a temperature of 60 ºC would denature the enzyme more quickly because of the increased (kinetic) ENERGY! at this temperature.
• Describe the action of these membrane-bound dipeptidases and explain their importance.

  • Many did not continue their answer to state the importance of this hydrolysis with reference to the passage of amino acids across the cell-surface membrane into the cells for absorption.
  • SO Amino acids can cross (cell) membrane / Maintain concentration gradient for amino acids / dipeptides cannot cross cell surface membrane
• • Some good understanding was demonstrated of how this universal enzyme ATP Synthasecould be different in different species.
  • Some students demonstrated understanding but their responses were insufficiently precise to fulfil the marking criteria;
  • they failed to reference the ‘TERTIARY’ structure or the ‘SHAPE’ of the active site.
  • Some students successfully suggested that the human enzyme would be found in the mitochondria and so be inaccessible to the antibiotic.
• • Those who did were those who fully understood that the significant aspect of the data was that
  • neither antibiotic killed all the bacteria and then went on to explain the consequences of this.
  • As with question 04.2, some students wrote extensive descriptions of the data; this is not creditworthy in this ‘explain’ question
• • Some good understanding was demonstrated here, but students often failed to gain credit due to poor use of language.
  • “Amount” and “level” are not accepted units
  • pH and light must be qualified, e.g. ‘soil pH’ and ‘light intensity’
  • ‘nutrients’ is not an acceptable alternative to mineral ion concentration at this level
• Describe how phagocytosis of a virus leads to presentation of its antigens. [3 marks]

  • Many students did not refer to the antigen being presented on the surface membrane of the phagocyte, so could not be awarded mark point 3.
• • Students needed to identify that the part of the immune response which would ‘attack’ the collagen would be the binding of specific antibodies, and then
  • use their knowledge of how an antigen-antibody complex leads to the destruction of the antigen (section 3.2.4 of the specification), i.e., human collagen in this case.
  • Credit could be gained for reference to agglutination or phagocytosis as methods of ‘attacking’ the human collagen
  • since these are the methods of antigen destruction named in the specification.

  
• It should be noted that this is the context of section 3.3.2 of the specification – adaptations of gas exchange surfaces…for efficient gas exchange and the limitation of water loss.

  • In order to explain successfully here, students needed to
  • identify the feature of the tracheal system,
  • go on to explain how this chosen feature allows for efficient gas exchange.
• IN TRACHEAL SYSTEM OF AN INSECT

  • Similarly, many students appreciated that a
  • large surface area is required but could not sufficiently explain how this is achieved in an insect with
  • highly branched/numerous tracheoles.
• There were some very good descriptions of lactate formation, causing water to be drawn out of the ends of the tracheoles, but few students could relate this to faster gas exchange as a result of diffusion through air, or a larger surface area
• Q8.1 Trachela system, 2017

  References to spiracles were not given credit as these are essential for limiting water loss but are not essential for efficient gas exchange.
• 3.3.1: ‘Students should be able to appreciate the relationship between surface area and volume ratio, and metabolic rate’.

  • Most students saw this question as an opportunity to write about their knowledge of gills rather than focusing onlife of the damselfly larvae.
  • Although this led some students to describe how more oxygen could be absorbed, the key to the question was about the increased activity of the damselfly and, therefore, its increased use of oxygen.
• • The command word here was contrast and so statements showing clear differences between the use of the two microscopes were required to gain credit.
  • Most students demonstrated sound knowledge of the optical and electron microscopes, but few managed to gain all six marks for relevant CONTRASTING statements.
  • Many suggested that no organelles could be seen with an optical microscope, rather than only LARGER ORGANELLES being visible.
  • Some referred to SEMs and 3D images; neither was relevant here. (Contrast TEM and optical)
• • Students often failed to gain mark point 3 because they stated that W contained half the genetic material of Z, rather than specifying half the DNA!.
  • Several students discussed these cells as if they were human cells rather than plant cells,
  • Some students named stages of meiosis in their answer. These were ignored as this is not expected knowledge
  • we were only looking for the outcomes of 1st and 2nd meiotic divisions.