C4.The intracellular environment of cells differs in composition from the extracellular environment of cells.

Student Outcome: C4.1

Know that the proportions of chemicals in the intracellular environment of cells are different from those in the extracellular environment of cells.

• The cell is full of stuff. Proteins, ions, fats, etc.
• Ordinarily, these would cause huge osmotic pressures, sucking water into the cell.
• The cell membrane has no structural strength, and the cell would burst.
• Cells carefully regulate their intracellular ionic concentrations, to ensure that no osmotic pressures arise.
• As a consequence, the major ions Na+, K+, Cl- and Ca2+ have different concentrations in the extracellular and intracellular environments.

The following table gives you an idea of the differences in some key differences in ions between intracellualr and extracellular environments of three different organisms.

Source: http://www.ima.umn.edu/talks/workshops/6-16-27-2003short-course/keener/HH_dynamics.ppt

Student Outcome: C4.2

Understand why the internal composition of the cell is regulated.

Animals that persist in such arid regions do so largely because they are able to maintain a constant internal environment in the face of harsh and changing external conditions. In most vertebrates, when body temperature, water and salt concentrations, or a host of other factors stray from their optimum levels, regulatory mechanisms kick in to reestablish internal equilibrium. The cells of many mammals, for instance, live in a water-based solution that must be kept at relatively unvarying conditions of temperature, pH, and osmotic pressure with respect to the surrounding environment. The cell's internal chemical stability is maintained despite changing rates of nutrient input and waste output, as well as changing concentrations of dissolved minerals in the fluid surrounding the cell. This complex internal balancing act gets repeated above the cellular level as well, because tissues and organs, too, can function efficiently only within a narrow range of conditions. The general process of maintaining equilibrium in the body is called homeostasis.

Among vertebrates, mammals and birds are the masters of homeostasis. They are endotherms, or warm-blooded animals, generating their own body heat, and they can finely tune the thermal, water, and chemical balance of their bodies from minute to minute. That fine-tuning, and the self-containment it makes possible, has enabled mammals and birds to occupy many of the habitats on Earth, including deserts.

Source: http://www.nhmag.com/master.html?http://www.nhmag.com/1202/1202_feature2.html

Student Outcome: C4.3

Explain how selective exchange occurs at the cell membrane.

Cell membranes are composed of phospholipids and proteins. Phospholipids are polar on the end that contains the phosphate group and nonpolar through the rest of the molecule. The nonpolar ends are found in the center of the membrane, leaving the polar ends exposed to water on both surfaces. The nonpolar core of the membrane restricts the passage of water and water-soluble molecules and ions. Certain polar molecules do pass through the membrane. This specialized and selective transport is believed to be the presence of protein molecules, which serve as transport molecules across the membrane.

Source: science.csustan.edu

Here is a good presentation on membranes.

This video shows the structure of the cell membrane and the movement of some substances through it.