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I am trying to learn about basic cell biology, and have what is probably an extremely simple question.

So this is how I understand it so far: Proteins are made from amino acids. This process is called protein biosynthesis, which is carried out by the ribosomes. So proteins are made by the ribosomes in every single cell.

So are the proteins made by the ribosomes to stay in the cell? Or can a protein move to other cells, and if so, what's the reason behind it?

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Yes. All proteins actually begin to get synthesized on cytoplasmic ribosomes but if they are going to be used for extracellular purposes, they are tagged and whole ribosome is taken to ER where protein synthesis is completed. The proteins are exocytosed with help of Golgi body, the post office tagging and packaging organelle (the Golgi body packages these proteins into the vesicles that fuse with the plasma membrane).

Examples of this process are: the pancreas secreting insulin for blood sugar control, digestive enzymes for digestion, the immune system telling the body there's a bacterium we must fight it and calls more immune cells (cytokines). Most proteins of course stay in the cell, and are necessary to make the cell or carry out metabolism or DNA transcription.

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  • $\begingroup$ Extremely good answer which really helped. Thanks! $\endgroup$
    – Lars Holdgaard
    May 20, 2013 at 21:05
  • $\begingroup$ My pleasure Lars! $\endgroup$
    – AndroidPenguin
    May 20, 2013 at 21:10
  • $\begingroup$ Thanks Alan :) how did I forget Adrenaline is a steroid hormone... $\endgroup$
    – AndroidPenguin
    May 20, 2013 at 23:10
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    $\begingroup$ Other examples are neuropeptides produced by neurons in the brain (e.g. endogenous opioids, various neurohormones such as GnRH, GHRH, vasopressin, oxytocin etc etc) $\endgroup$
    – nico
    May 21, 2013 at 16:27
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    $\begingroup$ Collagen and elastin are other examples of extracellular proteins. $\endgroup$
    – Chinmay Kanchi
    May 24, 2013 at 14:34
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I would like to complement dd3's answer a bit. It is about bacteria

  1. Bacteria secret enzymes outside. The bacterial membrane has import size limits and it is necessary for them to chop the big molecule into smaller pieces before import. Examples are glycoside hydrolases and protease. Bacteria take part in the clean-up after the annual algal and jellyfish bloom. It is especially important for the jellyfish because they have few predators and if it were not for the bacteria, their gelatinous body will pile up at our sea bottoms.

  2. Bacteria release protein toxin, either to defend, or use to recognize the host if they are symbionts.

  3. Gram-positive bacteria use peptide as autoinducer to do quorum sensing.

  4. Last but not least, bacteria sometimes form aggregates. The glue among them is chemically very complex (in other words, we don't know exactly what it is). It is believed sugars are there and, proteins too.

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