Читать книгу Cell Biology - Stephen R. Bolsover - Страница 39

Our bodies are ~70% water. Water can readily diffuse into and out of cells through the process of osmosis. It is thus essential that we maintain correct water balance otherwise our cells would distort or even lyse. Osmosis can be defined as the movement of water across a membrane down its concentration gradient from a solution of low osmolarity to a solution of high osmolarity. Osmolarity is calculated by summing the molar concentration of all the solutes in a particular solution. The more concentrated a solution is, the lower its water concentration and the higher its osmolarity.

For most mammalian fluids, the osmolarity is approximately 300 mOsm/l. Hypertonic solutions have an osmolarity that is higher, whereas hypotonic solutions have an osmolarity that is lower. If cells were placed in a hypertonic solution, water would move out of the cell from the cytosol where the osmolarity is lower than the bathing medium. Cells shrink under these conditions. Conversely, water would move into the cell if placed in hypotonic solutions because the osmolarity of the cytosol is now higher relative to the bathing medium. This can cause lysis. Changes in osmolarity can occur in pathological situations.

Water is a hydrophilic molecule, obviously! Yet it can cross the hydrophobic membrane relatively quickly. Why? The answer is that membranes have water channels known as aquaporins. They facilitate the diffusion of water across the membrane. Aquaporins are expressed in all cells but are particularly abundant in red blood cells and kidney tubules. Consequently, the plasma membranes of these cells are highly permeable to water.

Figure 2.3. The nucleus and the relationship of its membranes to those of the endoplasmic reticulum.

Within the nucleus, it is usually possible to recognize discrete areas. Much of it is occupied by chromatin, a complex of DNA and certain DNA‐binding proteins such as histones (page 39). In most cells, it is possible to discern two types of chromatin. A central region of lightly staining euchromatin is that portion of the cell's DNA database that is being actively read out by being transcribed into RNA, another nucleic acid (Chapter 5). In contrast the peripheral, darkly staining heterochromatin is the inactive portion of the genome where no RNA synthesis is occurring. The DNA in heterochromatin is densely packed, leading to its dark appearance.