Cell Structure & Function unit test
A test on this unit alone, marked as a percentage and a letter grade — for the test your class is actually sitting, rather than for May. Answer everything, then submit once: seeing the answer to question 3 before attempting question 4 makes the final percentage meaningless.
Passive transport
Surface area to volume ratio
Mitochondrial structure and function
Lab: Diffusion and Osmosis
Cotransport
Water potential
Cholesterol in membranes
Smooth ER
Sodium-potassium pump
Ribosome
Plasmodesmata
Rough ER
Short answer 1. Define or explain: Hypotonic, hypertonic, isotonic
3 ptsShort answer 2. Define or explain: Nucleolus
3 ptsShort answer 3. Define or explain: Osmosis
3 ptsShort answer 4. Define or explain: Nuclear envelope and pores
3 ptsFree response
9 ptsMost proteins secreted from a cell must be transported to the endoplasmic reticulum (ER) either during translation or after translation. For proteins transported DURING translation, the process begins in the cytosol and pauses when a specific sequence of amino acids has been translated; the translation complex is then carried to the surface of the ER, where translation continues. Proteins transported AFTER translation are translated entirely in the cytosol and then carried to the ER. In both cases the protein enters the ER through a protein channel in the ER membrane. Researchers found that the ER membrane protein SR is necessary for transport during translation, while the ER membrane protein Sec62 is necessary for transport after translation. They made small interfering RNAs (siRNAs) that reduce expression of either SR or Sec62, treated groups of cells with one of them, and measured the relative amount of each protein compared with cells treated with a control siRNA. FIGURE 1. RELATIVE PROTEIN CONTENT, % OF CONTROL (± SE of the mean) siRNA added to cells Sec62 protein SR protein Control siRNA 101 ± 1 101 ± 1 Sec62 siRNA 23 ± 11 169 ± 17 SR siRNA 126 ± 49 13 ± 5 The researchers then measured how much of each of three different proteins was transported to the ER in cells treated with Sec62 siRNA or SR siRNA, as a percent of the amount transported in control-siRNA cells. FIGURE 2. AMOUNT OF PROTEIN TRANSPORTED TO THE ER, % OF CONTROL (± SE of the mean) siRNA added to cells Protein 1 Protein 2 Protein 3 Control siRNA 100 ± 1 100 ± 3 100 ± 4 Sec62 siRNA 45 ± 10 115 ± 6 111 ± 4 SR siRNA 81 ± 16 48 ± 14 7 ± 5
(a) Describe the function of ribosomes.
(b)(i) Identify the dependent variable in the experiments shown in Figure 1.
(b)(ii) 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.
(b)(iii) Based on Figure 1, describe the effect on the production of SR protein when cells are treated with Sec62 siRNA.
(c)(i) Identify the independent variable in the researchers’ second experiment (data shown in Figure 2).
(c)(ii) 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.
(c)(iii) Protein 1 is encoded by 234 nucleotides, while protein 2 is encoded by 495 nucleotides. Assuming all nucleotides for both proteins encode amino acids, calculate the difference in the number of amino acids between the two proteins.
(d)(i) 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.
(d)(ii) For any protein that enters the ER, researchers claim that amino acids close to the protein’s amino terminus determine how likely the protein is to pass through the protein channel within the ER membrane. Justify the researchers’ claim based on your understanding of factors that affect the transport of proteins across membranes.