Cells - Microscopy + Studying Cells

Question 1

Compare and contrast the transmission electron microscope (TEM) and the scanning electron microscope (SEM). (4 marks)

Answer 1

1. TEM provides higher resolution than SEM. (1)
2. TEM produces 2D images, while SEM produces 3D images. (1)
3. TEM requires very thin specimens, but SEM does not. (1)
4. SEM only shows the surface of specimens, while TEM allows internal structures to be seen. (1)

Question 2

Describe how a transmission electron microscope (TEM) works and explain why it has a higher resolution than a light microscope. (4 marks)

Answer 2

1. A beam of electrons is passed through a thin specimen. (1)
2. Denser areas absorb more electrons and appear darker, while less dense areas appear lighter. (1)
3. Electrons have a shorter wavelength than light. (1)
4. This gives a higher resolution, allowing smaller organelles to be seen. (1)

Question 3

Why do specimens need to be thin when using a transmission electron microscope? (2 marks)

Answer 3

• So that electrons can pass through the specimen. (1)
• To allow internal structures (1)

Question 4

Describe how to prepare a temporary mount of a specimen for an optical (light) microscope. (4 marks)

Answer 4

• Add a drop of water to a glass slide. (1)
• Place a thin section of the specimen on the slide. (1)
• Stain the specimen with an appropriate dye (e.g., iodine for starch, methylene blue for DNA). (1)
• Place a cover slip over the specimen, avoiding air bubbles. (1)

Question 5

Define the terms magnification and resolution. (2 marks)

Answer 5

Magnification: How much larger an image appears compared to the actual object. (1)

Resolution: The ability to distinguish between two separate points as distinct objects. (1)

Question 6

Calculate the actual size of a cell that appears 50 mm in an image with a magnification of ×5000. Show your working. (2 marks)

Answer 6

Actual size = Image size / Magnification
= 50 mm / 5000 = 0.01 mm (or 10 µm) (2)

Question 7

Explain the purpose of differential centrifugation in cell fractionation. (3 marks)

Answer 7

• Separates organelles based on their density. (1)
• Heavier organelles form a pellet at lower speeds, while lighter organelles remain in the supernatant. (1)
• This allows isolation of specific organelles for further study. (1)

Question 8

What is the order of organelle separation in differential centrifugation? (3 marks)

Answer 8

Nuclei

Chloroplasts (if carrying out cell fractionation of plant tissue)

Mitochondria

Lysosomes

Endoplasmic reticulum

Ribosomes

Question 9

Why is a cold, isotonic, buffered solution used in cell fractionation? (3 marks)

Answer 9

Cold: Reduces enzyme activity to prevent organelle breakdown. (1)
Isotonic: Prevents osmotic damage to organelles. (1)
Buffered: Maintains pH to prevent enzyme denaturation. (1)

Question 10

A student wants to estimate the size of a cell using an eyepiece graticule and stage micrometer. Describe the process. (3 marks)

Answer 10

Calibrate the eyepiece graticule using a stage micrometer. (1)
Measure the cell in eyepiece units. (1)
Convert the measurement using the calibration factor to determine the actual size. (1)