 In this video I will define diffusion and explain the factors that affect the rate of diffusion, define the following terms, and recognize examples of each, passive transport, simple diffusion, facilitated diffusion, osmosis, hypertonic, hypotonic, and isotonic, and explain the effects of hypertonic, hypotonic, and isotonic solutions on cells in the direction solutes and water would diffuse. Diffusion is the movement of solute down its concentration gradient. If we had a flask filled with a chemical on one side connected to an empty flask on the other side as shown in the illustration here, and a barrier between the two flasks was opened to allow a channel to connect the two flasks, the chemical would spontaneously spread or diffuse from the area where there was a high concentration of that chemical to the other side where there was initially a no chemical or very low concentration of that chemical. So similarly in the photo on the bottom here we can see the idea if you had a test tube filled with water and you were to place a drop of a red dye into that water, initially there would be a high concentration right in the location where the drop of dye met the water and over time that dye would spread out until it was uniformly distributed through the water and this mechanism where the dye would spread out is diffusion. The concentration gradient has a major effect on the speed of diffusion. If there is a larger concentration gradient that will lead to an increased rate of diffusion. The permeability of a barrier between two compartments can also influence the rate of diffusion. If there's a lower permeability, that is if there's less spaces for the chemicals to move through then there will be a slower rate of diffusion. In contrast if we increase the permeability then the rate of diffusion will increase. Cells have the ability to regulate the permeability of the plasma membrane in order to regulate the rate at which molecules can pass through by facilitated diffusion. The molecular weight of the chemical that is diffusing will also influence the rate of diffusion. A larger molecule will diffuse more slowly compared to a smaller molecule. For example here we have an experiment where a crystal of potassium permanganate and a crystal of methylene blue were both placed into a petri dish that contains an agarose gel. Over time both of these chemicals will diffuse through the gel. The larger molecule, methylene blue, diffuses more slowly compared to potassium permanganate. Temperature will also influence the rate of diffusion. Here we have an experiment where two teabags have been placed into two different cups of water. The cup on the left is hot water and the cup on the right is ice cold water. You can see here that the color from the tea has diffused through the hot water much more rapidly than through the cold water. Passive transport refers to any movement of chemicals across the plasma membrane that does not require ATP in order to derive that transport. And so a passive transport mechanism will be a type of diffusion. There's two broad categories of passive transport. Simple diffusion is when a solute can diffuse across the membrane unassisted. A small nonpolar molecule is able to cross the plasma membrane by simple diffusion. However larger molecules and polar molecules are not able to pass through the plasma membrane by simple diffusion because the fatty acid tails in the phospholipid bilayer create a barrier that will prevent polar molecules from being able to enter or exit the cell. Examples of small nonpolar molecules that enter and exit cells by simple diffusion include oxygen gas and carbon dioxide gas. The plasma membrane contains lots of embedded proteins, integral membrane proteins, and many of these function as channel proteins that can enable diffusion through a mechanism we call facilitated diffusion. Facilitated diffusion is when an integral membrane protein carries or assists the solutes in order to enable diffusion across the plasma membrane. There are two major types of proteins that enable facilitated diffusion. Channel proteins enable solutes to flow through an opening like a pore that will just allow those chemicals to flow through the open pore. These channel proteins can be opened and closed in order to regulate the permeability of the membrane. Channel proteins are often the way that ions will cross the plasma membrane. For example, there are sodium ion channels that will enable facilitated diffusion of sodium across the plasma membrane. Sodium will be moving from a higher concentration in the extracellular fluid to a lower concentration within the cytoplasm. Carrier proteins are a little more selective than channels. The carrier protein will bind to the specific structure of the molecule that's being transported across the plasma membrane. Carrier proteins are typically required to enable facilitated diffusion of larger molecules like glucose, amino acids, or proteins. Hosmosis is the diffusion of water through a semi-permeable membrane down its concentration gradient. Tonicity is the ability of a solution to change the shape of a cell by altering its internal water volume. The tonicity of a solution depends on the concentration of non-penetrating solutes, that is, solutes that are not able to cross the plasma membrane will influence the tonicity of a solution. Isotonic solutions contain an equal concentration of solutes as is found in the cytosol of the cell. Therefore, if we place a cell into an isotonic solution, it will have no effect on the volume of the cell. There may be a small amount of osmosis carrying water into the cell, and that will be equally balanced by the small amount of osmosis carrying water out of the cell, so there's no net movement of water into or out of the cell. In contrast, a hypotonic solution contains more solutes than is found in the cytosol of the cell. If we place a cell into a hypotonic solution, the volume of the cell would decrease as water moves out of the cytosol into the extracellular fluid by osmosis. A hypotonic solution contains less solutes than the cell. If we place a cell into a hypotonic solution, the volume would increase as water would move into the cytosol by osmosis. The cell would swell, and if too much water enters the cell, it could cause the cell to rupture, or lice, also known as hypotonic lices, when a cell bursts as a result of being placed in a solution with too low of a solute concentration.