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Biological Sciences
Aarti Ashok

Review of previous lecture: carbohydrate > monosaccharide nucleic acid> nucleotide lipid> fatty acid +glycerol protein> amino acid Cell Bio Lec 3: Organelles in Cells Endomembrane (EM) system is made up of endoplasmic reticulum (ER), golgi, lysosome (or vacuole) and endosome Organelles have unique functions defined by their unique proteins Peroxisome was debated on its functions until 10 years ago Slide 8: Image is a good representation of the EM system, but it doesn’t show how it functions  Albert Claude and Georde Palade helped solve this, by labeling radioactive isotopes (atoms that differ by the number of neutrons, ex protium, deuterium, tritium) and detecting them using autoradiography Slide 10: Isotopes are unstable, and disintegrate, which usually releases electromagnetic radiation 35 35 (radioactivity). For most experiments, proteins are labeled by S. For this experiment, S was used as well, because most amino acids are composed of S (disulfide bonds). Slide 12: The radioactive atoms are inserted in the cell, and as it’s taken up, it is processed into amino acids and eventually proteins  radioactive Radioactive isotopes are detected by a radioactive-sensitive emulsion, which deposits the silver grains in radioactive areas (arrow in picture) autoradiography Slide 13: Setup The pancreatic cell is used, which produces digestive enzymes Slide 14: ‘pulse-label’- cell incorporates ‘pulses’ of radioactivity. They are fixed after three minutes, emulsion is applied, and EM is observed. The results show that the new proteins are synthesized in ER, because the silver grains are found in that area. There are rarely free floating amino acids in the pancreatic cell, since it constantly active in making proteins. Slide 15: ‘chase’- everything in the cell is washed away and normal proteins are inserted. This way only the radioisotopes of the first three minutes are present in the medium, and are ‘chased’, or observed. After 20 minutes, most proteins are found in the Golgi and vessels, and are in secretory granules and outside the c ell after 120 minutes. Therefore, the EM is a site of secretion of synthesized proteins. Slide 18: The lumen is a space that separates the cytosol from the ER. Smooth ER have no ribosomes. Instead, they synthesize and store other material like calcium and hormones. Rough ER. The spaces with no dots are the lumen. Only RER have translocons and ribosomes moving to it. SER does not have these features. Slide 20: The proteins in ER are mostly made to be secreted. There are also transmembrane proteins that move to the plasma membrane. ‘free’ ribosome= free- floating in cytosol. The peripheral membrane protein is membrane associated, and is not made in the ER. Slide 21: first picture= plasma cell, specifically activated B cell, attacks antigens, by producing large number of antibodies (high concentration of RER). Second picture= T-lymphocyte, which has T-cell receptors that detect antigens and infected cells, and once they’re captured, the lymphocyte signals other antibodies to fight it. It doesn’t have much ER. Slide 22+23: An mRNA (template) is taken from DNA, and moves to the cytosol to produce amino acids. The start codon is used to initiate production, which associates free-floating ribosomes (note= ribosomes in the RER are not static; they are dynamic in that they can come on and off the ER). The amino acids that are strung then result in a protein. BUT: how do the proteins know that they need to move to the RER? Slide 24: Signals in the protein sequences indicate this Slide 25: 4.a) proteins move from the cytosol (1) In the cytosol, the mRNA moves into the free ribosome, and a nascent protein is produced. The red part is the signal to dock the protein into
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