Practise AP Biology 2.1 questions by using organelle structure, protein-trafficking pathways and experimental controls to explain how eukaryotic cells function.
Most proteins that are secreted from a cell must be transported to the endoplasmic reticulum
(ER) either during translation or after translation.
Describe the function of ribosomes.
For proteins transported during translation, this process begins in the cytosol and pauses
when a specific sequence of amino acids is translated. The translation complex is then
transported to the surface of the ER where translation continues. Proteins that are transported
after translation are translated entirely in the cytosol and then transported to the ER. In both
instances, the translated proteins enter the ER through a protein channel in the membrane of
the ER.
Researchers studying the two types of protein transport identified that the ER membrane
protein SR is necessary for transport during translation, while the ER membrane protein Sec62
is necessary for transport after translation. To investigate which transport mechanism is used
for different proteins, researchers first created small interfering RNAs (siRNAs) that reduce
expression of either SR or Sec62. They then treated groups of cells with either the SR siRNA or
the Sec62 siRNA and determined the relative amount of SR and Sec62 protein in each group of
cells compared with cells treated with a control siRNA (Figure 1).
Figure 1. Average relative amounts of Sec62 and SR proteins in cells treated with control siRNA, Sec62 siRNA, or SR siRNA. Error bars represent \(\pm \mathrm{SE}_{x}\)
B.
Identify the dependent variable in the experiments shown in Figure 1.
Justify why the researchers included the control of measuring the relative amounts of
both Sec62 and SR proteins in cells that were treated with Sec62 siRNA only (data shown
in Figure 1).
Based on Figure 1, describe the effect on the production of SR protein when cells are
treated with Sec62 siRNA.
The researchers then measured the amount of each of three different proteins that was
transported to the ER in cells treated with Sec62 siRNA or SR siRNA. The researchers calculated
the percent transported relative to the cells treated with control siRNA (Figure 2).
Figure 2. Average relative amounts of three proteins that were transported to the ER when treated with control siRNA, Sec62 siRNA, or SR siRNA. Error bars represent \(\pm \mathrm{SE}_{x}\)
C.
Identify the independent variable in the researchers' second experiment (data shown in
Figure 2).
Based on Figure 2, identify the protein(s) that when treated with Sec62 siRNA showed an
increase in percent transport to the ER compared with the control.
D.
Researchers claim that protein 1 is the only tested protein that is transported to the ER
following its complete translation in the cytosol. Using data from Figure 2, support the
researchers' claim.