2-16 The cytoplasm is the area inside the membrane

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• The prokaryotic cytoplasm, while visually unadorned, is structured and is the site of most of the important metabolic reactions in the cell.
• There is little free water in the cytoplasm. This gives it a gel-like consistency.

The cytoplasm or protoplasm is the portion of the cell that lies within the cytoplasmic membrane. The cytoplasmic matrix is defined as substances within this membrane, excluding the genetic material. In most prokaryotes, it appears to be relatively featureless by electron microscope, but that simply means that there are no large structures in the matrix. This is in contrast to eukaryotic cells, which have mitochondria, and typically other visible organelles that exist for different specific functions. Despite this visual simplicity, the prokaryotic cytoplasm is the site of almost all of the important metabolic functions in the cell.

The cytoplasm has a gel-like consistency, with rather different properties than the simple solutions that we typically make up in the laboratory. This is because there is surprisingly little free water in the cell. Rather than picturing the cytoplasm as a pool of water with the occasional large molecule floating around, it is better to think of it as a bag of proteins and other macromolecules, each coated with a layer of water, and with a modest number of free water molecules bouncing around in between. In fact, there is so little free water in the cell that one-third of all water molecules are making hydrophilic contacts with the macromolecules in the cell. Given this difference between our lab solutions and the actual nature of the cytoplasm, it is a bit surprising that the biochemical analyses we perform in the lab mimic the behavior observed in the cell.

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