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By contrast, the envelope of gram-negative bacteria is more complex and consists (from inside to outside) of the cytoplasmic membrane, a thin layer of peptidoglycan, and an additional outer membrane, also called the lipopolysaccharide layer. Other bacterial cell surface structures range from disorganised slime layers to highly structured capsules.
The cell itself consists of two halves, each containing an essentially flat plate, or valve, and marginal connecting, or girdle band. One half, the hypotheca, is slightly smaller than the other half, the epitheca. Diatom morphology varies. Although the shape of the cell is typically circular, some cells may be triangular, square, or elliptical.
Navicula diatoms are highly motile and move through a gliding movement [3] [4] [5] This is done through excretion of extracellular polymeric substances (EPS). One form of EPS surrounds the outside of the cell and another is excreted through a slit in the frustule called a raphe, allowing the cell to glide along a track.
Examples range from the propulsion of single cells such as the swimming of spermatozoa to the transport of fluid along a stationary layer of cells such as in a respiratory tract. Though eukaryotic flagella and motile cilia are ultrastructurally identical, the beating pattern of the two organelles can be different.
Bacterial gliding is a process of motility whereby a bacterium can move under its own power. Generally, the process occurs whereby the bacterium moves along a surface in the general direction of its long axis. [5] Gliding may occur via distinctly different mechanisms, depending on the type of bacterium.
Cells are rectangular in girdle view (when in colonies), and lanceolate in valve view. Raphe system is slightly keeled and runs from pole to pole. Two large plate-like chloroplasts are present, one near each end of the cell. The nucleus is located centrally. Cells are yellow-brown in colour.
Going one step even further back, the chromerids, the peridinin dinoflagellates and the heterokont algae have been argued to possess a monophyletic plastid lineage in common, i.e. acquired their plastids from a red alga, [20] and so it seems likely that the common ancestor of alveolates and heterokonts was also photosynthetic.
Bacillaria paxillifer was originally described under the name Vibrio paxillifer by Otto Frederick Müller in 1786. It is the first diatom species known to be described. [5] It was separately described two years later (1788) by Johann Friedrich Gmelin as Bacillaria paradoxa.