Читать книгу Molecular Mechanisms of Photosynthesis - Robert E. Blankenship - Страница 22

Green is all around us. The distinctive color of chlorophyll announces the presence of photosynthetic organisms, including trees, shrubs, grasses, mosses, cacti, ferns, and many other types of vegetation. But this is just the tip of the iceberg of photosynthetic life. In addition to these most visible organisms, there is a remarkable variety of microscopic life, including many types of algae and bacteria that carry out photosynthesis. This chapter will introduce the different types of photosynthetic and phototrophic organisms and will give some information about their cellular organization and structure.

All living things on Earth are related to each other. In some cases, the relationships are obviously close, such as between a dog and a coyote, or an orange tree and a lemon tree, while in other cases the relationships are apparent only upon close examination, such as between a bacterium and a human or an amoeba and a fish. To establish these less obvious relationships, it is necessary to look at a deeper level of analysis, down to the cellular and even the molecular levels (Alberts et al., 2014; Nelson and Cox, 2017). At these levels of organization, the unity of life is readily apparent. All organisms are organized in the same fundamental way, with DNA serving as the master copy of the information needed to construct the organism, RNA as the intermediate working copy, and proteins as the workhorses of the cell, carrying out almost all the chemical reactions that make up metabolism. This basic pattern of information flow and metabolic responsibilities is known as the central dogma of molecular biology. Although some exceptions are known, such as viruses that use RNA for information storage or RNA molecules that act as enzymes, the basic pattern applies to all life. The chemical structures of the building blocks of DNA, RNA, and proteins are exactly the same in bacteria and humans. The process of copying DNA into RNA is called transcription, and the translation of the nucleic acid code into proteins is called translation. This latter process takes place on large protein‐RNA complexes called ribosomes.

Cells are surrounded by membranes, which function as permeability barriers and also carry out many important functions. Membranes are composed of lipids, which are amphipathic molecules with a polar head group and nonpolar tail. The lipids are arranged in a bilayer structure, with the polar head groups toward the outside and inside of the cell, and the nonpolar tails pointed into the center of the bilayer. There are many types of lipids, which form several classes. Two of the most important are phospholipids and glycolipids. Phospholipids include a phosphate group that is esterified to a glycerol backbone. The most common type of lipids in chloroplasts are glycolipids, in which sugars are found in place of the phosphate groups. The nonpolar tails are long‐chain fatty acids that are esterified to the glycerol groups. They almost always contain one or more double bonds, which increase the fluidity of the membranes of which they are the principal components. The cell membrane is often called the cytoplasmic membrane, while the space enclosed is called the cytoplasm. Additional membranes are found in photosynthetic organisms, in particular the thylakoid membrane, which is the site of photosynthesis in chloroplasts and cyanobacteria.

Membranes also contain proteins, either integral membrane proteins, which span the lipid bilayer, or peripheral membrane proteins, which are associated with one or other side of the membrane but do not cross the bilayer. Many of the proteins essential for photosynthesis are membrane proteins. All cells also contain a variety of carbohydrates, or sugars, as well as many other small molecules essential for proper cellular function. When viewed in this way, the similarities among the various classes of life far outweigh the differences.

Despite the fundamental similarities just pointed out, life nevertheless comes in a remarkable variety of shapes and sizes. These differences form the basis of our mechanisms of classification of living things.