Hi, and welcome to this video on passive transport, an important set of processes that allow substances to pass in and out of cells.
Before we jump into the specific science behind the mechanisms, let’s first think about why we need cellular transport.
Cells provide organization and protect essential substances (like DNA) from the hustle and bustle of the whole organism. A cell’s first line of defense is a membrane that encapsulates the entire cell. However, in order for cells to function, they need a way to acquire materials and get rid of waste. So, to maintain the protective barrier but still allow the passage of materials, cells have specialized methods of transport.
Cellular transport is split into two categories: methods that require energy, called active transport, and methods that do not require energy, called passive transport. In this video, we’ll focus on diffusion, facilitated diffusion, and osmosis, three types of passive transport.
Diffusion is the spontaneous tendency of a substance to move from an area of high concentration to low concentration. You’ve probably seen this in action before. For example, when you add a drop of food coloring to water, immediately, the food coloring begins to spread (or diffuse) throughout the water, eventually reaching equilibrium where all areas have the same concentration of coloring:
Diffusion on a Molecular Level
Let’s think about this on a molecular level. Initially, there’s a high concentration of the solute molecules (the color) in the drop and a low concentration in the rest of the water. This is called a concentration gradient. Remember, the particles in gases and liquids are in constant random motion. Once the drop is in the water, the solute molecules will start to move into the water as part of that chaotic motion. Since there is more “room” for the solute in the low concentration of the water, more molecules will move in that direction than back toward the high concentration of the drop. This shift continues until there is a uniform distribution of color throughout and you can no longer see where the drop was.
It may be helpful to think about the probability of this process happening in reverse: if you have a solution of uniform color, it would be extremely unlikely for solute molecules to randomly gather up into one concentrated drop. We refer to the diffusion of molecules from areas of high concentration to low concentration as moving down the concentration gradient.
Diffusion happens in liquids and gases—the two phases in which particles have translational kinetic energy. And it can also happen across the cellular membrane. If there’s a high concentration of a substance outside of the cell, more substance will diffuse into than out of the cell, until the concentrations are equal. Conversely, if the concentration of a substance is higher in the cell, the substance will diffuse out of the cell at a higher rate than into the cell.
However, this type of simple diffusion is only possible for a small set of substances. After all, if every molecule could pass through the membrane, it wouldn’t provide much protection for the cell. Large polar molecules are not able to cross the membrane because of the hydrophobic properties of the membrane’s core. Small non-polar molecules are not affected by this and can cross the membrane. The fact that some substances can cross the cell membrane and others cannot means that the membrane is semipermeable.
Now we know that some molecules can enter and leave the cell simply by diffusing across the cell membrane. However, the cell still needs a way to get the large, polar substances across the membrane. For this, there are proteins embedded in the membrane that create pathways to shield substances from the hydrophobic core, allowing them to pass in and out of the cell. This is called facilitated diffusion. It is important to note that substances are still moving from an area of high concentration to low concentration, meaning no energy is required for the facilitated diffusion.
The proteins are substance-specific, allowing only one type of molecule or ion through. There are carrier and channel proteins. Carrier proteins change shape when bound to their target substance, moving them from one side of the cell to the other. Channel proteins create a molecular tunnel through the membrane.
Some channels are always open, while others are gated, opening and closing in response to a cellular signal. Consequently, unlike in normal diffusion, an equal concentration inside and outside of the cell is not always reached through facilitated diffusion, which is beneficial as this allows for more control over the concentration of substances inside the cell.
Finally, let’s look at osmosis.
Consider a cell with a large concentration of substance A in the extracellular fluid. The system would like to achieve an equilibrium, where the concentration of A is the same on the inside and outside of the cell. However, A is too polar to diffuse through the cell membrane and there are no proteins embedded in the membrane to allow facilitated diffusion for A. What happens?
Water, which can quickly pass through the membrane via channel proteins, will begin to leave the cell at a higher rate than it enters, with a net movement to the area with a high concentration of A, thus lowering the concentration. This is an example of osmosis.
The formal definition of osmosis is “the process by which a solvent tends to pass through a semipermeable barrier from a less concentrated solution to a more concentrated one.” In our example, the solvent is water, the cell membrane is the semipermeable barrier, and the solution is highly concentrated outside of the cell.
It’s important to note here that the semipermeable membrane allows for fast diffusion or facilitated diffusion of the solvent molecules, but slow or no movement of the solute molecules.
Osmosis is incredibly important to biological systems because it can do work. For example, in plant cells, if the intracellular fluid has a high concentration of a solute, water will flow into the cells, so much so that it pushes on the cell wall, creating pressure, which is what keeps plants from wilting.
Because it involves the concentration of a solute, osmosis may seem like a special case of diffusion, however, the mechanism behind osmosis is a bit more complex. What’s important to remember is that osmosis, unlike diffusion, focuses on the movement of solvent molecules in relation to the concentration of a solute and can actually do work for a system. But like diffusion, it is the natural tendency of the system and therefore, does not require additional energy.
Let’s wrap up with a quick review. We covered three methods of passive transport: diffusion, facilitated diffusion, and osmosis, all of which are important for cellular functions. While the exact mechanisms driving each of these transport methods are different, they are united in the fact that they do not require energy.
Thanks for watching, and happy studying!