Introduction to Cell Membranes
The cell membrane is an essential feature of all living cells. This thin (approximately 10nm), complex arrangement of phospholipids and proteins is ultimately the only barrier between the highly ordered interior of the cell and its disorganized external environment. Complete separation, however, is not the membrane's function. Rather, cell membranes must be selective barriers that allow transport of essential materials into and out of the cell while at the same time locking in required biochemicals and locking out hazardous ones. By performing the experiments in the exercises to follow, you will gain an appreciation for, and insight into, how ions, molecules, and water move into and out of cells and the role that cell membranes play in regulating and controlling these movements.
Specifically, in this module you will study four membrane-related activities:
Each activity illustrates a different aspect of membrane function. After completing this module, you will have a new appreciation for what membranes do, how they do it, and how their structure is directly related to their function.
Diffusion is a physical phenomenon that leads to a net movement of molecules, ions, or atoms from regions of high concentration to regions of low concentration. A difference in concentration between two areas is referred to as a concentration gradient. Diffusion is driven by random thermal motion and requires no external source of energy. Given a concentration gradient, it simply happens. At equilibrium, movement does not cease, but no net change in concentration occurs because the concentration gradient no longer exists, i.e., the concentration of diffusing particles is the same in both areas (Exercise I). Diffusion is routinely utilized by cells and organisms to move essential ions and molecules into the cell, out of the cell, and between cells without the need to expend energy. Diffusion across the cell membrane is one mechanism by which cells can extract essential solutes from their environment or remove toxic materials from the cytoplasm, as long as the concentration gradient of the diffusing substance remains favorable. Indeed, concentration gradients can be created by the normal metabolic activity of the cell. As essential chemicals are utilized in metabolic reactions, their concentration within the cell falls. The concentration gradient thus created allows the diffusion of more of these chemicals into the cell.
A special case of diffusion is Osmosis, which involves the relatively rapid diffusion of water into or out of a compartment that is separated from an adjacent compartment by a selectively permeable membrane. A selectively permeable membrane, by definition, is permeable to water, the solvent of living systems, while being relatively impermeable to ions and molecules dissolved in water. Thus, water is free to diffuse across a membrane following its concentration gradient, while the ions and molecules dissolved in the water (solutes) are not. This tendency for water to move down its own concentration gradient can be measured as a "pressure" referred to as Osmotic Pressure. Physically, this characteristic of a solution is a measure of how much pressure (measured in atmospheres) would have to be applied to prevent water from diffusing into a membrane-bound compartment with a higher osmotic pressure than the surrounding solution. Solutions with high solute concentrations have high osmotic pressures. Osmotic pressure gradients and the type of water movement they can cause are easily illustrated using an Osmometer (Exercise II).
Using osmometers to demonstrate osmosis has biological significance, because living cells behave as osmometers (Exercise III). They are complex, highly concentrated solutions (the cytoplasm) separated from their environment by a selectively permeable membrane (the cell membrane). Thus, a cell's osmotic environment is critical to maintaining normal cellular activity. For example, cell size is determined by the amount of water in the cell; if water diffuses into the cell, the cell expands; if water flows out of the cell, it shrinks. A favorable concentration gradient, however, is not the only factor affecting diffusion and the time it takes to reach osmotic equilibrium. Membrane thickness and chemical composition can also influence diffusion rates. In fact, the Permeability Coefficient (Kpermeability) is a parameter that reflects the ease with which diffusing substances (e.g., water), can cross a membrane. Kpermeability has units of distance/time (m/sec). As Kpermeability increases, the membrane becomes more permeable to water and the rate of diffusive movement across a membrane increases.
Diffusion as a mechanism for the movement of solutes across a cell membrane can only operate if the concentration gradient is favorable (i.e., if the concentration of the desired solute is higher outside the cell than inside). Frequently, this is not the case. Usually, movement of essential materials, either into or out of cells, is against a concentration gradient. This movement from an area of low concentration to an area of high concentration requires that energy be expended. Essentially, these dissolved solutes must be pumped across the membrane. The pumping is referred to as Active Transport, the energy for which is provided by adenosine triphosphate (ATP). When the terminal phosphate group of an ATP molecule is enzymatically removed, enough chemical energy is released to pump solutes (e.g., sugars, ions) across a membrane against their concentration gradient. These solutes generally have low Kdiffusion values, indicating that they cannot leak out very quickly. Kdiffusion is a constant that measures the ease with which a solute can diffuse through a membrane. Thus cells can regulate the cellular concentrations of essential biomolecules and ions by actively pumping them into the cytoplasm. The phenomenon of active transport is illustrated in Exercise IV.
Diffusion, Osmosis, and Active Transport are all activities associated with living cells and the membranes that surround them. Using these as well as other transport mechanisms, cells regulate and control the movement of water, ions, and molecules across their membranes. In the exercises to follow, you will have the opportunity to study and observe these processes in greater detail.
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