Cell membranes osmosis

He Z, He C, Zhang Z, Zou Z, Wang H (2007) Changes of antioxidative enzymes and cell membrane osmosis in tomato colonized by arbuscular mycorrhizae under NaCl stress. Colloids Surf B Biointerfaces 59 128-133... 

Osmotic pressure plays an important role in biological chemistry because the cells of the human body are encased in semipermeable membranes and bathed in body fluids. Under normal physiological conditions, the body fluid outside the cells has the same total solute molarity as the fluid inside the cells, and there is no net osmosis across cell membranes. Solutions with the same solute molarity are called isotonic solutions. 

If a sample of red blood cells is added to pure water, osmosis carries water into the cells. This process would continue until the internal pressure of the cell was 6.9 atm higher than the pressure on the outside of the cell. However, 6.9 atm is much more than the cell membrane can tolerate. Consequently, red blood cells burst when immersed in pure water. 

The concentrations of sodium, potassium (and chloride) ions in the body are high and make the largest contribution to the electrical charge of cells hence they are known as electrolytes. They have two important roles maintenance of the total solute concentration in the cell which prevents excessive movement of water into or out of cells through osmosis and the controlled movement of these ions across cell membranes acts as a signalling mechanism (e.g. the action potential in neurones and muscle. Chapter 14). Severe disruption of sodium or potassium levels in the body interferes with this signalling mechanism and with osmotic balance in cells. 

Central to the osmosis phenomenon is the semipermeable membrane (SPM), whose physical properties and species-selectivity directly govern the kinetics and thermodynamics of osmotic flow. Naturally occurring biomembranes of high selectivity, permeable to water but not to other solutes, are ubiquitous, for example, in macroscopic stomach linings and blood vessels, as well as in the microscopic cell membranes that encapsulate all known cell types. Some common synthetic membranes, such as Gore-Tex and cellophane, also exhibit selective permeability and osmotic activity. 

The cell membranes in cucumber cells are semipermeable, meaning water molecules pass back and forth through the membranes but solute molecules do not. A cucumber left in a concentrated salt solution shrivels up because water molecules leave the cells and enter the salt solution. Is this an example of osmosis or reverse osmosis ... 

The simple pore was originally considered in the context of osmosis as an explanation of how water might move across a biological structure (e.g. an epithelium) in the absence of solute movement. This notion introduced by Brucke in the mid 19th century, (see Hille, 1984) was subsequently extended by Boyle and Conway (1941) to consider the selective ionic permeability of the resting cell membrane. Here the explanation for the high membrane permeability to potassium and to chloride, as compared to sodium, was simple. The hydrated ionic radius of sodium was greater than that of either the hydrated potassium or chloride ion, hence the pores postulated to be present in the membrane would act as a molecular sieve and permit the movement of potassium and of chloride but not of sodium. 

Cells do have other transport options. Very small molecules can filter through the various channel proteins in the membrane. Drugs tend to be too large to pass by this method. The filtration of water is specifically called osmosis. Endocytosis is another method by which molecules may enter a cell. Endocytosis comes in many forms, all of which involve the indentation of a section of cell membrane that pinches off to form a new internal vesicle in the cell (Scheme 3.2). Once the cell breaks down the vesicle, the contents are released... 

Fig. 18. Diagrams illustrating the differences and difficulties during freezing of cells in suspension (a) and on surfaces (b, c and d). In both cases, large ice crystal formation must be avoided, this means that freezing must be rapid and often involves the use of cryo-protectants. In suspension, the use of hypertonic solutions to shrink cells by osmosis helps to avoid membrane rupture. But with cells fixed to surfaces, shrinkage can lead to rupture of the filopodia or to parts of cytoskeleton or cell membrane (c). Additionally, animal cells under stress (including this kind of osmotic stress) tend to build up into a spherical shape. This means they would lose many of their surface contacts before freezing and disappear into solution after re-thawing. Cryo-con-servation of adhered cells in defined positions requires very precise control of the conditions...

The process of dialysis (Figure 8.4) in cells differs slightly from osmosis. In dialysis through a cell membrane, water and small molecules and ions pass through... 

Any living skin tissue which is very, very thin is called a membrane. Like all living things, membranes are made of cells, the basic structural unit of plants and animals. Membranes seem to be strong and solid, but if you looked at one under a microscope, you would see that the walls of the cells are porous. Because of these pores, osmosis can take place. Osmosis is the process by which molecules pass through the pores in the cell walls of a membrane. Osmosis, in other words, is diffusion through a membrane. 

The process of osmosis is very important in our lives because all the air we breathe, the food we eat, and the wastes our bodies must remove pass through the cell membranes in this manner. 

In Chapter 13, you learned that diffusion is the mixing of gases or liquids resulting from their random motions. Osmosis is the diffusion of solvent particles across a semipermeable membrane from an area of higher solvent concentration to an area of lower solvent concentration. Semipermeable membranes are barriers with tiny pores that allow some but not all kinds of particles to cross. The membranes surrounding all living cells are semipermeable membranes. Osmosis plays an important role in many biological systems such as kidney dialysis and the uptake of nutrients by plants. 

As a final example of new discoveries being made about water s remarkable properties, consider the work of Kolesnikov et al. (2004), who have experimented with water confined to carbon nanotubes and foimd some highly unusual behavior that might be important for plant osmosis and the transport of protons across cell membranes. If these expectations are confirmed, it would be worthwhile to elucidate... 

If osmosis and simple diffusion were the only mechanisms for transporting water and ions across cell membranes, these concentration differences would not occur. One positive ion would be just as good as any other. However, the situation is more complex than this. Large protein molecules embedded in cell membranes actively pump sodium ions to the outside of the cell and potassium ions into the cell. This is termed active transport because cellular energy must be expended to transport those ions. Proper cell function in the regulation of muscles and the nervous system depends on the sodium ion/potassium ion ratio inside and outside of the cell. 

Because a cell membrane is selectively permeable, it is not always possible for solutes to pass through it in response to a concentration gradient. In such cases the solvent diffuses through the membrane. Such membranes, permeable to solvent but not to solute, are specifically called semlpermeable membranes. Osmosis is the diffusion of a solvent (water in biological systems) through a semlpermeable membrane in response to a water concentration gradient. 

Consider the following problem. A spherical cell consists of a thin membrane surrounding a salt solution. Outside of the cell membrane there is a solution that is isotonic with that within the membrane. The cell is removed instantaneously from its surroundings and placed into an environment of piue water. The action of osmosis immediately drives water through the... 

The plasma membranes of cells are semipermeable in other words they are permeable to water and a select group of small molecules, but are impermeable to large molecules and many ions. If the total concentrations of solutes on either side of a semipermeable membrane are different, water diffuses from the region of low solute concentration to the region of high solute concentration imtil the solute concentrations are the same on both sides. This process is called osmosis, and the difference in water pressiue across the cell membrane is called the osmotic pressure. 

By using a fluid of known density to suspend the mixture of creatures in one will find that those of greater density will sink and those with lighter density will rise to the surface. Centrifuging the mixture can increase the rate of settling . The real problem is what to use for the fluid. If one chooses to use salt solutions to increase the density of water then osmosis takes place as the salt tries to flow across the cell membrane of the creatures. The increase in salt content may kill the creature and they most assuredly increase their density making the separation difficult. By using colloidal silica suspensions... 

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