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A. Urganelles of the endomembrane system
The various parts of the endomembrane system serve different functions in the cell. In this activity, you will identify the roles of each part of the endomembrane system. Drag each function to the appropriate bin.
a. protein synthesis
b. protein modification and sorting
c. macromolecule digestion
d. lipid synthesis
e. autophagy
f. cisternal maturation
g. calcium ion storage
h. poison detoxification
1. Smooth ER
2. Rough ER
3. Golgi apparatus
4. Lysosomes
B. Sites of protein synthesis
All proteins are synthesized by ribosomes in the cell. Some ribosomes float freely in the cytosol, while others are bound to the surface of the endoplasmic reticulum. Most proteins made by free ribosomes function in the cytosol. Proteins made by bound ribosomes either function within the endomembrane system or pass through it and are secreted from the cell. Which of the following proteins are synthesized by bound ribosomes?
A. Lysosomal enzyme
B. Insulin
C. ER protein
D. DNA polymerase
E. Ribosomal protein
C. Pulse-chase experiments and protein location
Scientists can track the movement of proteins through the endomembrane system using an approach known as a pulse-chase experiment. This experiment involves
the "pulse" phase: Cells are exposed to a high concentration of a radioactively labeled amino acid for a short period to tag proteins that are being synthesized.
the "chase" phase: Any unincorporated radioactively labeled amino acids are washed away and large amounts of the same, but unlabeled, amino acid are added.
Only those proteins synthesized during the brief pulse phase are radioactively tagged. These tagged proteins can be tracked through the chase period to determine their location in the cell.
The data below were obtained from a pulse-chase experiment in which cells were examined at different times during the chase period. The numbers represent the radioactivity (measured in counts per minute) recorded at each of the indicated sites. The higher the number, the greater the radioactivity.
Time ER Golgi Cytoplasm Lysosomes Extracellular Space
3 minutes 162 7 21 5 4
20 minutes 73 88 17 10 8
60 minutes 9 35 14 112 13
120 minutes 11 23 18 151 10
Based on these data, what is the most likely function of the cells in this experiment?
a. detoxification
b. muscle contraction
c. phagocytosis
d. secretion

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Answer:

A) 1. Smooth ER: d. lipid synthesis, g. calcium storage, h. poison detoxification

2. Rough ER: a. protein synthesis

3. Golgi apparatus: b. protein modification and sorting, f. cisternal maturation

4. Lysosomes: c. macromolecule digestion, e. autophagy

B) A. Lysosomal enzyme; B. Insulin; C. ER protein

C) c. phagocytosis

Explanation:

In eukaryotic cells, the smooth ER (smooth endoplasmic reticulum) is defined as a subset of the endomembrane system of the endoplasmic reticulum responsible for many cellular functions, including the synthesis of lipids and hormones (steroid hormones), the elimination of harmful metabolic compounds and the storage of calcium ions. The Rough ER is a series of connected flattened sacs that bound to the ribosomes and functions during the synthesis of proteins. The Golgi apparatus is another membrane organelle responsible for transporting, modifying, and packaging lipids and proteins. The lysosomes are also membrane organelles that are found in animal cells, these organelles contain digestive hydrolytic enzymes used to break down different types of molecules. Ribosomes are structures formed by both RNA and proteins which are found within all cells. The ribosomes play a major role during the synthesis of proteins, specifically during the translation from messenger RNAs (mRNAs) to proteins. These structures link amino acidic residues to form polypeptide chains. Finally, phagocytosis is a type of endocytosis where the cell engulfs a particle by using its plasma membrane, thereby producing a phagosome.

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