AS Biology Lesson 2 - Measuring Cells

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AS Biology Lesson 2 - Measuring Cells

  1. 1. X 1000 X 1000 (mm) (µm) (nm)2mm 2 2000 (2 x 103) 2000000 (2 x 106)130µm 0.13 130 130000 (1.3 x 105)0.032m 32 32000 32000000 (3.2 x 104) (3.2 x 107)7.25µm 0.00725 7.25 7250 (7.25 x 103) ÷ 1000 ÷ 1000
  2. 2. Learning Objectives- [PA] use an eyepiece graticule and stagemicrometer scale to measure cells and befamiliar with units (millimetre, micrometre,nanometre) used in cell studies;-[PA] calculate linear magnification ofdrawings and photographs;-(h) [PA] calculate actual sizes of specimensfrom drawings and photographs;
  3. 3. Measuring cells
  4. 4. To accurately measure the size of cellular structures we need a suitable scale:
  5. 5. The graticule a more suitable ‘ruler’ for measuring cells• The slide graticule:• The eyepiece graticule:
  6. 6. The stage graticule shows true lengths stage graticule
  7. 7. The eyepiece graticule has regular divisions. These need to be calibrated for each magnification eyepiece graticulee.g. x100 stage graticule
  8. 8. The eyepiece graticule has regular divisions. These need to be calibrated for each magnification eyepiece graticulee.g. x400 stage graticule
  9. 9. The eyepiece graticuleremains constant no matterwhat magnification the cellsare viewed at.
  10. 10. The eyepiece graticuleremains constant no matterwhat magnification the cellsare viewed at.
  11. 11. The eyepiece graticuleremains constant no matterwhat magnification the cellsare viewed at.
  12. 12. Eyepiece & stage graticulesLow magnification High magnification
  13. 13. Figure 4.3 Stage micrometer viewed at x100 magnification. The total length of the micrometer is 1mm total length = 1mm on this scale, 94 which is 1000μm divisions = 1000μmTherefore, 1 division on theeyepiece graticule represents1000 ÷ 94 = 10.6 μmat this magnification.
  14. 14. Figure 4.1 Cells of onion epidermis as viewed at x100 magnification with a graticule in the eyepiece of the microscopeWe know that at thisIn the two columns coveredmagnification, 1 divisionby the graticule there is anon the eyepiece graticuleaverage of five cells in therepresents 10.6 μmlength of the graticule 1060μm Therefore the averageTherefore the total lengthoflength of one cell is the eyepiece graticulerepresents = 212μm = 1060μm 1060 ÷ 5 10.6 x 100at this magnification
  15. 15. Figure 4.4 Part of the stage micrometer viewed at x400 magnificationremember thatshown so the length each on this scale, 90division here is 10μm by the bracket is 240μm divisions = 240μmTherefore, 1 division on theeyepiece graticule represents240 ÷ 90 = 2.67 μmat this magnification.
  16. 16. Figure 4.2 Cells of onion epidermis as viewed at x400 magnification with the same graticule in the eyepiece We know that at this magnification, each division of the eyepiece graticule represents 2.67μmThe length of the cell coveredby the graticule is 98 divisions,therefore the length of this cellis 2.67 x 98 = 262μm
  17. 17. We now have two measurements for the length of an onion cell; 212μm and 262 μm. Which of these is the more accurate estimate of the length of onion epidermal cells?• The answer from Q. 2 [212 μm]• because this is a mean of several cells.• Only one cell was measured in Q.3, and this one may not be representative.
  18. 18. Estimating cell width. Figure 4.5. Cells of the onion epidermis as viewed at x100 magnification with a graticule in the eyepiece of the microscopeRemember the total lengthof the eyepiece graticulerepresents 1060μmat this magnificationThere are approximatelythirteen cells in thelength of the graticuleTherefore the averagewidth of one cell is1060 ÷ 13 = 81.5μm
  19. 19. Figure 4.6. Cells of the onion epidermis as viewed at x400 magnification with the same graticule in the eyepiece of the microscope Remember, we know that at this magnification, each division of the eyepiece graticule represents 2.67μm 62 divisionsHere, two cells span 62 divisionson the eyepiece graticule. Thisrepresents 2.67 x 62 = 165.5 μmTherefore the averagewidth of one cell is165.5 ÷ 2 = 82.8μm

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