Cell Structure

Question 1

Describe the components and organisation of the eukaryotic cell membrane ?

Answer 1

-The eukaryotic membrane is a bilayer formed from two layers of phospholipids. These molecules have hydrophilic heads which face out of the bilayer and hydrophobic tails which face into the bilayer.
- The bilayer also has many proteins embedded in it. These proteins may be integral, meaning that they cross the entire bilayer, or they may be peripheral, meaning that they are located on only one side of the bilayer.
- These proteins can have many functions, including acting as ion channels or receptors that respond to external signals.
-The bilayer also has molecules of cholesterol embedded in it. Cholesterol acts as a buffer for membrane fluidity. This means at high temperatures, cholesterol prevents fluidity rising too high, and at low temperatures, cholesterol prevents the membrane becoming solid.

Question 2

What are the differences between eukaryotes and prokaryotes?

Answer 2

DNA - Prokaryotic DNA is circular and found free in the cytoplasm; eukaryotic DNA is linear and found in the nucleus.

Ribosomes - Prokaryotic ribosomes are smaller than eukaryotic ribosomes.

Membrane-bound organelles - Prokaryotes do not have any membrane-bound organelles such as the nucleus, mitochondria, chloroplasts or Golgi apparatus.

Cell wall - The cell wall of prokaryotes is different from that of plants or fungi. Bacterial cell walls are formed of peptidoglycan, plant cell walls of cellulose and fungal cell walls of chitin.

Cell division - Prokaryotes divide by binary fission; eukaryotes divide by mitosis.

Question 3

Describe the steps you might take to isolate mitochondria from a cell culture in the laboratory?

Answer 3

• homogenisation.
  The cells in the sample are broken open using a blender
• cell conditions
  Ice cold: reduces the activity of the enzymes.
  Isotonic solution: there is no water potential for osmosis to occur, which could cause the organelles to shrink or burst.
  Buffered solution: this keeps the pH at a constant level to prevent damage to protein structure.
• filtration
  The homogenised sample is filtered through a gauze. Filtration separates the larger components from the smaller organelles.
• ultracentrifugation
  The filtered samples are spun at a low speed in a centrifuge. It is important that each tube is balanced with a tube directly opposite. Heavier components are forced to the bottom of the tube. Lighter components rise to the top.
• repeat
  The supernatant from the first round of centrifugation is centrifuged again. The pellet this time will be the nuclei from the cells (this is the first pellet). The process is repeated and the mitochondria will be isolated in the fifth pellet.

Question 4

Having isolated the mitochondria, why would TEM be favourable to SEM to image the organelle and identify the double membrane structure?

Answer 4

Higher resolution - Transmission electron microscopy can achieve a higher resolution than scanning electrons microscopy which would be needed to identify the two separate membranes.

Can see internal structures - Electrons pass through the sample in TEM rather than being scattered off the sample in SEM. This means that TEM can visualise structures within the cell.

Greater magnification - Mitochondra are between 0.5 - 10 micrometers in size and so it requires very high magnification to visualise their internal consituents.