Читать книгу Molecular Mechanisms of Photosynthesis - Robert E. Blankenship - Страница 25

An older, but still very useful, concept to distinguish among living things is the division into prokaryote and eukaryote. Prokaryotes are the structurally simplest life forms, including bacteria and Archaea. No Archaea that carry out chlorophyll‐based photosynthesis have yet been found, so our discussion of them will be limited. Both these groups of organisms are nearly always single‐celled and have a relatively simple cellular organization without a nucleus or other subcellular organelles. A bilayer lipid cytoplasmic membrane surrounds the cell and serves as the main permeability barrier. In Gram‐negative bacteria, including most types of phototrophic bacteria, a second, more permeable, outer membrane is present, as well as a tough cell wall that provides mechanical stability (Madigan et al., 2017). The space between the outer surface of the cytoplasmic membrane and the inner surface of the cell wall is called the periplasm. This region contains a number of soluble proteins, including some cytochromes and chemosensory binding proteins. These proteins are actually topologically outside the cell, but are prevented from being lost by the cell wall. The cell wall has several layers and a complex chemical structure consisting of lipids, proteins, and polysaccharides. Nutrients pass into the periplasm from outside the cell through pores, which are made of proteins called porins. A porin is an integral membrane protein that forms a small hole in the outer membrane. Ions and small molecules, such as sugars and amino acids, can easily pass through the pore, but larger molecules cannot. Bacteria are almost always submicroscopic cells, with typical dimensions on the order of one to a few micrometers. They usually divide by binary fission, producing two daughter cells.

Eukaryotes, which are identical to the eukarya domain discussed earlier, are more sophisticated cells than prokaryotes. They are usually much larger than bacteria, up to hundreds of microns in size. In many cases, eukaryotic cells make up multicellular organisms, in which different cells are highly specialized or differentiated. Eukaryotic cells also contain a number of internal structures called organelles. These organelles are surrounded by membranes and include the nucleus, the mitochondrion, the endoplasmic reticulum, and, most importantly for photosynthesis, the chloroplast. At least two of these organelles, the mitochondrion and the chloroplast, have complex evolutionary histories, having been acquired by an early eukaryotic cell by a process called endosymbiosis (Margulis, 1993).