Biological cells Cytoplasm

H)2D3 exerts its influence within target tissues through high-affinity sterol-specific intracellular receptor proteins. The D3 receptor, similar to steroid receptor systems, translocates the hormone from the cell cytoplasm to the nucleus, where biological response is initiated via transcription and translation (Fig. 66.3). 

A constant exchange of liquid substances, ions, gases can be seen between these three volumes. In the body, the main parameters of Uquid, such as pH, osmotic pressure and volume, are strictly controlled. The pH levels of the extracellular fluid and blood are the same, and are normally about 7.36. Several biological conformations of macromolecules and the catalytic activity of enzymes are dependent upon pH. The osmotic pressure of the extracellular fluid and blood plasma are similarly dependent. Hyaluronic acid of the extracellular matrix plays a very important role in maintaining these parameters within the physiological norms. As an intermediate between the external media and the cells, extracellular matrix performs the coordinative functions between the cells and different cellular structures. The functioning of the cell cytoplasmic membrane is directly dependent on the extracellular matrix. 

Figure 12. Phase of the Ez light component in the near field of a biological cell containing only cytoplasm and membrane. It is clearly shown that refractive index matching (graphs in the right) significantly enhances the phase contrast of the cell membrane. Cell radius - 0.72 pm, membrane thickness - 0.06 pm, cytoplasm ref. index - 1.36, membrane ref. index - 1.47. Asymmetry is due to the z-polarization of the incident light.

The cytoskeleton is a network of different biological filaments that crisscross the cell cytoplasm. These filaments form part of different cellular structures as they associate with each other and other related proteins. Different elements of the cytoskeleton have a variety of specialized roles in cell motility, transport, and division—without a cytoskeleton, cells would not be able to function. There are three main types of filament associated with the cytoskeleton filamentous actin (F-actin), microtubules, and intermediate filaments. Each of these filament types performs different functions within the cellular machinery and as a result have very different mechanical properties. 

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