Biology Ultracentrifugation

Exam style questions

Describe how a sample of chloroplasts could be isolated from leaves

Grind/blend cells/tissue/leaves and filter;

In cold, same water potential/concentration, pH-controlled solution;

Centrifuge/spin and remove nuclei/cell debris;

(Centrifuge/spin) at high(er) speed, chloroplasts settle out

A biologist separated cell components to investigate organelle activity. She prepared a suspension of the organelles in a solution that prevented damage to the organelles. Describe three properties of this solution and explain how each property prevented damage to the organelles.

(Ice) cold to prevent/reduce enzyme activity;

Buffered to prevent denaturing of enzyme/protein;

Same water potential/ Ψ to prevent lysis by osmosis/bursting (of organelle);

A biologist prepared a sample of organelles labelled C from liver. He used the following method.

1. Added to the liver tissues an ice-cold, buffered solution with the same water potential as the liver tissue.

2. Mixed the liver and solution in a blender.

3. Filtered the mixture from the blender.

4. Spun the filtered liquid in a centrifuge at a low speed. A pellet appeared in the bottom of the centrifuge tube.

5. Poured off the liquid above the pellet into a second centrifuge tube and spun this at a higher speed to obtain the sample of organelles labelled C

Explain why the biologist used a blender and then filtered the mixture (steps 2 and 3).

1. Break open cells / homogenise / produce homogenate;

2. Remove unbroken cells / larger debris;

Name the organelle that made up most of the first pellet after centrifuging at a low speed (step 4).

) Nucleus / nuclei;

The second centrifuge tube was spun at a higher speed to obtain the sample of organelles labelled C in the diagram (step 5). Suggest why

Mitochondria / organelle C less dense than nucleus / organelle in first pellet;

What can you conclude from the figure above about a structural difference between the plasmids and the circular DNA? Explain your answer.

1. Circular DNA is bigger/heavier/denser;

2. (Because band) moved further/is lower (in tube)/closer to bottom (of tube);