1. Bright-Field & Polarized Light
Each group will be given slices of potatoes. The recommended
thickness is a few centimeters, so that the flesh can be easily
scraped without being contaminated by the skin. Using a small
spatula, gently scrape the flesh of each potato slice and quickly
smear a thin coat on a slide. Place a drop of water over the smear
and cover with a cover slip. Observe with both bright-field and
polarized light. You will be able to clearly observe a “Maltese
cross” (McMahon, 2004) in the potato starch granules under
40× and 10× magnification (do not use the 100× magnification).
A sample image of starch granules under 10× and 40× magnification with both bright-field illumination and crossed polarized illumination is shown in Figure 1: the Maltese cross pattern, so
named because of its resemblance to the stylized eight-pointed
symbol of the Knights of Malta, can be seen in Figure 1C and 1D.
2. Lugol’s Iodine
Using a small spatula, gently scrape the flesh of each potato slice and
quickly smear a thin coat on a slide. Stain starch granules with Lugol’s
iodine solution. Place a cover slip over Lugol’s iodine solution. Staining should occur in 15–20 seconds. Do not overstain: a single drop of
the Lugol’s iodine solution is adequate for staining purposes. Observe
the staining with bright-field light. Do not use polarized light for this
experiment because the Lugol’s iodine stains the starch granules too
darkly to see the Maltese crosses.
You will need to prepare two solutions (control and experimental) for
this part of the experiment. The control solution contains 140 mM
NaCl and 10 mM CaCl2. The second solution contains the same ionic
content along with amylase. For the latter, we used a single capsule
(about 270 mg) of digestive supplement amylase, which was prepared
by cutting the capsule open with a razor and stirring the contents into
Use two vials: (1) control and (2) experimental. Add 5 mL of
control solution (140 mM NaCl and 10 mM CaCl2) to the control
vial and add 5 mL of the amylase solution to the experimental vial.
Then add 1 g of minced potato to the control and experimental vials.
Stir for 15 seconds. The potato is minced by scraping 1 g of flesh
from the interior of the potato and then grinding the flesh with mortar and pestle until it forms a smooth paste without any chunks in it.
Dip the first glucose test strip into the control solution and dip
the second glucose test strip into the experimental solution for
three seconds each. Place the glucose test strips (enzyme side up)
on the absorbent liner and wait about 60 seconds for color to
appear. The starting time is called “time zero.”
Incubate both the control and experimental vials at 38°C for
45 minutes, using either the water bath or the air incubator. Shake vials
every 10 minutes. At the end of the incubation period (45 minutes),
use two new glucose test strips. Dip the first test strip into the control
vial for three seconds and the second into the experimental vial for
three seconds each. Place the glucose test strips (enzyme side up) on
an absorbent liner and wait about 60 seconds for color to appear.
Compare the colors of the test strips at time zero to those after
45 minutes of incubation. Quantitate the glucose concentrations by
comparing the colors on the test strips to those on the glucose chart.
4. Starch Granules: Bright-Field & Polarized Light
Using separate pipettes, draw a small sample from the bottom of
the control vial and experimental vial, both at time zero and after
Table 1. The materials needed for this lab, what they are used to accomplish, where they can be
obtained, and cost.
Material Use Where It Can Be Obtained Estimated Cost
Potato Specimen Produce section of grocery store $1/potato
Two polarizers Starch indicator – viewing
Maltese cross in samples
before but not after digestion
Optics supplier (e.g., Edmund Optics, https://
$12/pair of 2 inch ×
2 inch sheets
Detecting the presence (or
absence) of glucose after (or
Biological classroom supplier (e.g., Carolina
Biological Supply, https://www.carolina.com/
$30 for 100
Starch indicator – dying
Carolina Biological Supply (https://www.
$6.30 for 100 mL
Amylase Digest starch into glucose Digestive supplement capsules in pharmacy
$14 for 90 capsules
NaCl and CaCl2 Buffering the amylase,
Brewing and vinting supplier or Amazon
$7.85 for 1 pound bag