2-11 The cell is organized into functional units

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• All cells are organized into functional units. Membranes, cytoplasm, ribosomes and nuclear regions are found in every cell.

Now that you have had an introduction to the chemicals that make up the typical cell, we will now look at how this chemistry combines to form major functional units. These can be thought of as the organizations that carry out the major business of the cell: growth, replication, feeding and movement. We will first start with membranes because so many things interact with them. Next internal structures in the cytoplasm will be described and finally structures outside of the membrane.

First, however, we should describe an important evolutionary hypothesis that will make sense of much of the following details. As you will see, there are a number of curious similarities and differences in the details of cellular structure among bacteria, archaea and eukaryotes. In general much of the machinery in a eukaryotic nucleus and in the cytoplasm looks rather a lot like what is present in the archaea. However, the organelles of eukaryotes, such as the mitochondria and chloroplasts, have properties that are much more similar to those of bacteria. How is this possible? One clue comes from observing organisms in nature. It is very common to find cooperative relationships between different species and this is also true in the microbial world. In some instances these relationships involved close physical contact between their participants, sometimes with one participant engulfing the other. In 1968 Dr. Lynn Margulis extended this observation and proposed that some of the organelles found in eukaryotes, specifically mitochondria and chloroplasts, were originally endosymbionts of their host. Originally these two microbes probably were able to live independently, but over time, the endosymbiont lost functionality that its host was already providing and then became dependent. Over the years ample evidence has accumulated to support this exceptional insight.

1. Both mitochondria and chloroplasts contain DNA that resembles the chromosomes of bacteria.
2. Both organelles are surrounded by two membranes reminiscent of gram-negative cell wall structure (see below) observed in a class of microbes.
3. Mitochondria and chloroplasts divide by a method that resembles binary fission.
4. Much of the internal structure and biochemistry of the photosynthetic organelle inside chloroplasts is very similar to that observed in cyanobacteria (a photosynthetic microbe).
5. The ribosomes of mitochondria and chloroplasts resemble those found in microbes and analysis of the sequence of the 16S rRNA of these ribosomes showed that the organelles are in fact closely related to proteobacteria (mitochondria) and cyanobacteria (chloroplasts) This last bit of proof is very strongest evidence substantiating Dr. Margulis's hypothesis.

In summary, it seems clear that the eukaryotic cell was born by the merger of an archaeal cell with a gram-negative proteobacteria. Photosynthetic eukaryotes arose from a second endosymbiosis, where the eukaryotic cell engulfed a cyanobacterium. Clearly, eukaryotes, including us, were the result of the cooperation of several bacterial species in the long distant past.

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