All algal types are eucaryotic, and therefore contain the internal organelles we encountered in Chapter 3, that is, nuclei, mitochondria, endoplasmic reticulum, ribosomes, Golgi body, and in most instances, chloroplasts. With the exception of one group (the Euglenophyta), all have a cellulose cell wall, which is frequently modified with other polysaccharides, including pectin and alginic acids. In some cases, the cell wall may be fortified with deposits of calcium carbonate or silica. This is permeable to small molecules and ions, but impermeable to macromolecules. To the exterior of the cell may be one or two flagella, with the typical eucaryotic 9 + 2 microstructure (see Figure 3.18), which may allow unicellular types to move through the water; cilia are not found in any algae.
The characteristics used to place algal protists into different taxa include the type of chlorophyll present, the form in which carbohydrate is stored, and the structure of the cell wall (Table 9.1). A group not considered here are the cyanophytes, previously known as the blue-green algae; although they carry out oxygenic photosynthesis, they are procaryotes, and as such are more closely related to certain bacteria. They are therefore discussed in Chapter 7.
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