Review of previous lecture:
carbohydrate > monosaccharide
nucleic acid> nucleotide
lipid> fatty acid +glycerol
protein> amino acid
Lec 3: Organelles in Cells
Endomembrane (EM) system is made up of endoplasmic reticulum (ER), golgi, lysosome (or vacuole) and
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
Slide 10: Isotopes are unstable, and disintegrate, which usually releases electromagnetic radiation
(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
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