Selective Permeability

 Changes in environmental conditions greatly impact what enters and exits a cell.  The plasma membrane adjusts to these changes in many ways.  Integral proteins embedded within the plasma membrane act as channels(pumps) allowing materials to pass through the membrane. Carbohydrates, attached to lipids or proteins found on the exterior surface, help the cell bind to substances needed in the extracellular fluid.

So, what can enter and exit a cell?  Plasma membranes have both hydrophilic and hydrophobic regions. This helps the movement of certain materials through the membrane and hinders the movement of others.  Size and solubility are major determining factors in what can pass. Generally, small, uncharged particles have no trouble in passing, while large molecules and ions tend to face issues.  In those cases, special mechanisms are used in order to penetrate the plasma membrane.   Molecules of oxygen and carbon dioxide pass through quite easily.

Polar substances, with the exception of water, present problems for the membrane. While some polar molecules connect easily with the outside of a cell, they cannot readily pass through the lipid core of the plasma membrane.  In these instances, special mechanisms are again necessary.

Diffusion

Diffusion is a passive process.   A single substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across the space.  For example, what happens when someone opens a bottle of perfume in a room?  The perfume is at its highest concentration in the bottle and at its lowest in the room. The perfume vapor will diffuse, or spread away, from the bottle.  Gradually, the vapor will spread.  Materials move within the cell’s cytosol by this means of transport (Figure 1). Diffusion requires no energy use and stops when equilibrium is reached.  The molecules do not stop movement but instead, maintain equilibrium.

Figure 1. Diffusion through a permeable membrane follows the concentration gradient of a substance, moving the substance from an area of high concentration to one of low concentration. (credit: modification of work by Mariana Ruiz Villarreal)

Each separate substance in a medium has its own concentration gradient, independent of the concentration gradients of other materials. Additionally, each substance will diffuse according to that gradient.

Several factors affect the rate of diffusion.

• Concentration gradient:  the greater the difference in concentration, the more rapid the diffusion;  as equilibrium gets close, diffusion slows
• Molecular mass:  larger molecules move more slowly,  it is more difficult to move between the molecules of the substance they are moving through
• Temperature:  higher temperatures increase the energy and molecular movement, increasing the rate of diffusion

Facilitated transport/diffusion

In facilitated transport, also known as facilitated diffusion, material moves across the plasma membrane from high to low concentration without the need for ATP.  However, the use of integral proteins provide the advantage of reaching equilibrium easier and faster. The substances are passed to specific integral proteins that facilitate their passage by forming channels(pores) to allow certain substances to pass through the membrane. The integral proteins involved in facilitated transport are referred to as transport proteins and function as either channels or carriers.  Substances that undergo facilitated transport might not pass through otherwise.

Osmosis

Figure 2. In osmosis, water always moves from an area of higher concentration (of water) to one of lower concentration (of water). In this system, the solute cannot pass through the selectively permeable membrane.

Osmosis is the diffusion(high to low) of water through a semipermeable membrane.  The membrane limits the diffusion of solutes in the water. Osmosis is a special case of diffusion. Water, like other substances, moves from an area of higher concentration to one of lower concentration. Imagine a beaker with a semipermeable membrane, separating the two sides(Figure 2).  The water(solvent) is the same on both sides of the membrane but there are different concentrations on each side of a dissolved substance(solute) that cannot cross the membrane. If the volume of the water is the same, but the concentrations of solute are different, then there are also different concentrations of water on either side of the membrane.

A principle of diffusion is that the molecules move around and will spread evenly throughout the medium if possible.  Only the material capable of getting through the membrane will diffuse.  In this example, the solute cannot diffuse through the membrane, but the water can. Water has a concentration gradient in this system. Therefore, water will diffuse down its concentration gradient, crossing the membrane to the side where it is less concentrated. This diffusion of water through the membrane, osmosis, will continue until the concentration gradient of water goes to zero. Osmosis is actively involved in all living things.

Tonicity

Tonicity describes the amount of solute in a solution.  Three terms—hypertonic, isotonic, and hypotonic—are used to relate the tonicity of a cell to the tonicity of a cell’s extracellular fluid(Figures 3 and 4)/

In a hypertonic solution, the extracellular fluid contains less water than the cell. Because the cell has a lower concentration of solutes, the water will leave the cell. In effect, the solute is drawing the water out of the cell. In animal cells, this may result in the cell undergoing crenation(shrivel). Plants will undergo the process of plasmolysis(wilt).

In an isotonic solution, the extracellular fluid has the same tonicity as the cell. If the concentration of solutes of the cell matches that of the extracellular fluid, there will be no net movement of water into or out of the cell. In animal cells, this the considered the best tonicity. Plants can withstand this tonicity but prefer a hypotonic solution. Blood cells in hypertonic, isotonic, and hypotonic solutions take on characteristic appearances.

In a hypotonic solution, the extracellular fluid has a lower concentration of solutes than the cell.   The extracellular fluid has a higher concentration of water than does the cell. In this situation, water will follow its concentration gradient and enter the cell.  In animal cells, the cell may swell(lysis).  Due to the large, central vacuole and the cell wall, plants prefer this form of tonicity, referred to as turgor.

Art Connection

Figure 3. Osmotic pressure changes the shape of red blood cells in hypertonic, isotonic, and hypotonic solutions. (credit: modification of work by Mariana Ruiz Villarreal)

A doctor injects a patient with what is thought to be an isotonic saline solution. The patient dies.  An autopsy reveals that many red blood cells have been destroyed. Do you think the solution the doctor injected was really isotonic?

Figure 4. The turgor pressure within a plant cell depends on the tonicity of the solution that it is bathed in. (credit: modification of work by Mariana Ruiz Villarreal)

Section Summary

The passive forms of transport, diffusion and osmosis, move material without the need for energy.  Substances diffuse from areas of high concentration to areas of low concentration, and this process continues until the substance is evenly distributed in a system. In solutions of more than one substance, each type of molecule diffuses according to its own concentration gradient. Many factors can affect the rate of diffusion, including concentration gradient, molecular size, and temperature.

In living systems, diffusion of substances into and out of cells is mediated by the plasma membrane. Some materials diffuse readily through the membrane while others are hindered.  Their passage is only made possible by protein channels and carriers. The chemistry of living things occurs in aqueous solutions, and balancing the concentrations of those solutions is an ongoing problem.