recap

5.3 recap

The hallmark of eukaryotic cells is compartmentalization. Membrane-enclosed organelles process information, transform energy, form internal compartments for transporting proteins, and carry out intracellular digestion. An internal cytoskeleton plays several structural roles.

learning outcomes

You should be able to:

  • Distinguish between prokaryotes and eukaryotes.

  • Compare the sizes, structures, typical cell numbers, and roles of subcellular structures.

  • Trace the path of a molecule as it moves between organelles in a eukaryotic cell.

Question 1

What are the major differences between prokaryotic cells and eukaryotic cells?

Prokaryotic cells are smaller than eukaryotic cells and lack organelles.

Question 2

Through how many membranes would a molecule have to pass in moving (a) from the interior (stroma) of a chloroplast to the interior (matrix) of a mitochondrion? (b) From the interior of a lysosome to the outside of a cell? (c) From one ribosome to another?

(a) Four membranes: two in the chloroplast and two in the mitochondrion. (b) Two membranes: the lysosomal membrane and the cell membrane (via vesicle; the molecules do not themselves cross any membranes). (c) No membranes: ribosomes do not have membranes. However, if the ribosomes were associated with the endoplasmic reticulum (ER), the answer would be two membranes: one into the ER and another out of the ER.

Question 3

The drug vincristine is used to treat many cancers. It appears to work by causing microtubules to depolymerize. Vincristine use has many side effects, including loss of dividing cells and nerve problems. Explain why this might be so.

Microtubules line the long axons of nerve cells, where they act as tracks for vesicles that carry substances down the neuron. Without microtubules, the contents of these vesicles cannot be delivered to their destination, which can result in nerve problems. Depolymerization of microtubules can interfere with cell divisions because microtubules are a key part of the mitotic spindle that is used to move chromosomes during cell division.

Question 4

The movements of newly synthesized proteins can be followed through cells using a “pulse–chase” experiment. During synthesis, proteins are tagged with a radioactive isotope (the “pulse”), then the cells are allowed to process the proteins for varying periods of time. The locations of the radioactive proteins are then determined by isolating cell organelles and quantifying their levels of radioactivity. What would be the expected pathway for (a) a lysosomal enzyme and (b) a protein that is released from the cell?

a) For a lysosomal enzyme, the pathway would be ribosome → interior of ER → Golgi apparatus → Golgi vesicles → lysosome.
b) For an extracellular protein (animal cell), the pathway would be ribosome → interior of ER → Golgi apparatus → Golgi vesicles → cell membrane → extracellular region.

104

All cells interact with their environments. Many eukaryotic cells are parts of multicellular organisms and must closely coordinate their activities with other cells. The cell membrane plays a crucial role in these interactions, but other structures outside that membrane are involved as well.