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A cell wall is a structural layer that surrounds some cell types, found immediately outside the cell membrane.It can be tough, flexible, and sometimes rigid. Primarily, it provides the cell with structural support, shape, protection, and functions as a selective barrier. [1]
While all bacterial cell walls (with a few exceptions such as extracellular parasites such as Mycoplasma) contain peptidoglycan, not all cell walls have the same overall structures. Since the cell wall is required for bacterial survival, but is absent in some eukaryotes, several antibiotics (notably the penicillins and cephalosporins) stop ...
All are spherical or nearly so, but they vary considerably in size. Members of some genera are identifiable by the way cells are attached to one another: in pockets, in chains, or grape-like clusters. These arrangements reflect patterns of cell division and that cells stick together.
The new wavefront for the o-ray will be tangent to the spherical wavelets, while the new wavefront for the e-ray will be tangent to the ellipsoidal wavelets. Each plane wavefront propagates straight ahead but with different velocities: V 0 for the o-ray and V e for the e-ray. The direction of the k-vector is always perpendicular to the ...
The Huygens–Fresnel principle (named after Dutch physicist Christiaan Huygens and French physicist Augustin-Jean Fresnel) states that every point on a wavefront is itself the source of spherical wavelets, and the secondary wavelets emanating from different points mutually interfere. [1] The sum of these spherical wavelets forms a new wavefront.
The plane wavefront is a good model for a surface-section of a very large spherical wavefront; for instance, sunlight strikes the earth with a spherical wavefront that has a radius of about 150 million kilometers (1 AU). For many purposes, such a wavefront can be considered planar over distances of the diameter of Earth.
Other tissue interactions that DAH offers an explanation for includes tissue hierarchy, where tissues with weaker surface adhesion surround tissues with stronger surface adhesion, the rounding of irregular cell masses to become spherical, and the cell sorting and construction of anatomical structures that occurs independent of the path taken. [3]
The cell envelopes of the bacterial class of mollicutes do not have a cell wall. [5] The main pathogenic bacteria in this class are mycoplasma and ureaplasma. [5] L-form bacteria are strains bacteria that lack cell walls derived from bacteria that normally possess cell walls. [6]