Diffusion

concentrate their root cells with solute through active transport mechanisms, allowing for osmosis to drive the water in from the soil. Evaporation of water from the leaves draws more water up the stem; this evaporation of water is called transpiration.

As mentioned, the osmotic pressure is determined by the concentration of solutes in the solution. A substance that dissolves in water to make sodium and chloride will have twice the osmotic pressure as the same number of moles of dissolved sugar—because sodium chloride dissolves into two solutes. The osmotic gradient is the difference in the concentration of two solutions on either side of the membrane.

DIFFUSION

Diffusion can be passive or “facilitated”. Passive transport involves the movement of a substance across the cell membrane without the addition of energy. The rate of transport, depends on how permeable the membrane is to the substance. The different types of passive transport include filtration, facilitated diffusion, diffusion, and osmosis.

Simple diffusion involves the net transport of a substance from an area of high concentration to an area of low concentration. It does not necessarily involve a membrane as substances can diffuse across open areas of a solvent. The driving force is the concentration gradient. Solutes travel down their concentration gradient until it no longer exists. Simple diffusion and osmosis are similar; the main difference is that in diffusion, the substance moving is the solute, while in osmosis, the substance moving is the solvent.

Facilitated diffusion or facilitated transport is movement across a membrane through transmembrane integral proteins that does not require ATP energy. Molecules will still pass from their highest concentration to their lowest concentration. Figure 28 shows facilitated diffusion across a membrane:

Figure 28.

The main differences between simple diffusion and facilitated diffusion are that facilitated diffusion depends on binding to the membrane-embedded channel protein. This is saturable, which is not true of simple diffusion. Finally, facilitated diffusion is more dependent on temperature, which is not the case with simple diffusion.

Polar molecules and large ions cannot diffuse freely across the plasma membrane because the membrane is hydrophobic. Only small and nonpolar substances can diffuse