Studying Cells

Question 1

Starting with some leaves, describe how you would obtain a sample of undamaged
chloroplasts. Use your knowledge of cell fractionation and ultracentrifugation to answer this
question.

Answer 1

1. Chop up /homogenise the leaves
2. Cold;
3. Buffer solution;
4. Isotonic / same water potential;
5. Filter and centrifuge filtrate;
6. Centrifuge supernatant;
7. At higher speed;
8. Chloroplasts in (second) pellet;

Question 2

Scientists use optical microscopes and transmission electron microscopes (TEMs)
to investigate cell structure. Explain the advantages and the limitations of using a
TEM to investigate cell structure compared with an optical microscope

Answer 2

Advantages:
1) Small objects can be seen;

2) TEM has higher resolution becuase
3) Wavelength of electrons is shorter than light;

Limitations:
4) Cannot look at living cells;

5) Must be in a vacuum;
6) Must cut section / thin specimen;
7) Preparation may create artefact
8) Does not produce colour image;

Question 3

Contrast how an optical microscope and a transmission electron microscope work and contrast the limitations of their use when studying cells.

Answer 3

TEM use electrons and optical use light;

TEM allows a greater resolution;

(So with TEM) smaller organelles/named cell structure can be observed OR
greater detail in organelles/named cell structure can be observed;

TEM view only dead/dehydrated specimens and optical (can) view live
specimens;

TEM does not show colour and optical (can);

TEM requires thinner specimens;

TEM requires a more complex/time consuming preparation;

TEM focuses using magnets and optical uses (glass) lenses;

Question 4

Describe how you could make a temporary mount of a piece of plant tissue to observe the
position of starch grains in the cells when using an optical (light) microscope [4]

Answer 4

1. Add drop of water to (glass) slide;
2. Obtain thin section (of plant tissue) and place on slide / float on drop of water;
3. Stain with / add iodine in potassium iodide;
4. Lower cover slip using mounted needle;

Question 5

A scientist wanted to calculate the mean volume of
spores. Describe how she could use Figure 5 do this.
You may assume the spores are perfectly spherical.

Answer 5

Measure diameter of large number
of spores;

Divide measured values by 700 (to
find true diameter);

Calculate the volume of a sphere
using the formula 4/3πr^3;