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Passive Transport - Opening the Floodgates
Passive Transport - Opening the Floodgates
Recall that we likened a concentration gradient to a dam holding back a torrent of water. The water, much like the solutes on the hypertonic side of a barrier, 'wants' to move past the barrier and into the hypotonic side.
If you remove the barrier separating two solutions (or if particles can bypass the barrier), particles will move down their concentration gradient, as it is called. This means that the net particle movement is from hypertonic solution to hypotonic solution (from the area with lots of the solute to the area with less solute.
When particles move down their concentration gradient, no energy needs to be added to the system - the system will do this on its own. This is referred to as passive transport (aka diffusion)- passive because no energy needs to be added to the system and transport because particles are moving.
There are two types of passive transport and all require no additional energy. These two types are known as simple diffusion and facilitated diffusion. These are represented by this image and explained below.
A. Simple Diffusion
A. Simple Diffusion
Simple diffusion occurs when a particle can pass through the plasma membrane on its own (hence simple). Recall that this generally only occurs with nonpolar molecules and uncharged, small polar molecules (but this is much slower).
To observe this, look at the GIF shown and keep track of the small molecules with the blue and white atoms. You can see that the GIF begins with a high concentration of this molecule on the left side (hypertonic) and a small concentration on the right side (hypotonic). These particles can move through the plasma membrane (unlike the green particles), and so will naturally over time spread out until each side is isotonic for that solute.
Please notice that even though net movement is from the left to the right side, some particles still move from the right side to the left. It is just that more particles move from left to right until equilibrium is reached.
B. Facilitated Diffusion
B. Facilitated Diffusion
Facilitated diffusion is still passive transport, so it still doesn't require any additional energy. However, this classification described when particles use the help (facilitation) of a protein to pass the membrane. This is where those channel and carrier proteins we learned about recently come into play.
A channel protein is a simple pathway for these particles to pass through. A carrier protein is one that contorts its shape in order to carry molecules across the membrane. Anything that cannot pass through the membrane by itself requires the assistance of a protein. These would be polar molecules, large molecules, and charged ions. Remember, these proteins are very specific about what molecules they can allow through.
Osmosis - Passive Transport of Water: It's All About Balance
Osmosis - Passive Transport of Water: It's All About Balance
Generally when we discuss diffusion, we do so in the context of solutes. But sometimes, the solutes cannot pass through a membrane and there are no channel or carrier proteins to move them, then the concentrations will remain unbalanced. However, if water molecules can make it through, (perhaps through their own proteins known as aquaporins - literally 'water holes'), the water will move in order to equalize the concentrations. In this case, the water moves from the hypotonic solution to the hypertonic solution.
Effectively, the water is trying to bring down the concentration of the hypertonic solution using the extra water from the hypotonic solution. When water moves across a membrane in order to equalize the concentrations of a solute, it is known as osmosis. Osmosis is passive transport as well, but only applies to water.
This becomes particularly important in the context of cells. If you place a cell into water that has no solutes (pure water), the inside of the cell is hypertonic and the water is hypotonic. But those solutes in the cell can't leave the cell. So, water will flow into the cell in order to dilute all of those solutes. If too much water enters the cell, it causes the cell to be lysed (or basically pop). This is why you get a saline (salt) solution via an IV at hospitals. If they gave you plain water, your blood cells would burst. Plants actually use this phenomenon to their advantage, however. Basically they use all of that water in combination with their cell wall to remain rigid.
If a cell is placed into a hypertonic solution, the opposite occurs. Water will leave the cell and enter the hypertonic solution surrounding the cell. It is only in an isotonic solution that a cell will have no net movement of water.
Active Transport - Energy Required
Active Transport - Energy Required
Active transport is a process that requires energy. Recall that in cells, the most common 'currency' of energy is ATP, adenosine triphosphate. That molecule will be providing pump proteins with energy so that they can pump particles against their concentration gradient (from hypotonic solution to hypertonic solution).
This process is useful when a cell has a lot of a solute, but still needs more. Because there is already a high concentration of the solute within the cell, they 'want' to leave the cell. So if the cell is to gather even more of that solute, it must expend energy to pump more in.
Overview of Active and Passive Transport
Overview of Active and Passive Transport
Remember, active transport requires additional energy as an input in order to move particles because they are not moving down their concentration gradients. That is why energy is required. This requires the use of a pump protein. The energy is supplied via ATP in cells.
Passive transport (aka diffusion) has two different kinds - simple and facilitated diffusion. Simple diffusion occurs with molecules that can bypass the cell membrane. Facilitated diffusion occurs with molecules that need help crossing the membrane.
Using One Battery to Charge Another
Using One Battery to Charge Another
Remember that ATP is just like a battery because it provides energy within a cell. So, this battery is what powers active transport. Active transport is an incredibly important process in AP Biology, and it is particularly crucial when we get into cellular respiration and photosynthesis. One thing that is a little unintuitive about it, however, is that active transport is often used to create a gradient that will be used later. Recall that we likened concentration gradients as batteries as well because they contain potential energy just like ATP.
Effectively, something that will become common is active transport (powered by ATP, one battery) will be used to form a concentration gradient (another kind of battery). Essentially, we are spending one battery to power another. Why would we do this? Well, because we can use this new battery to power other processes when we allow those particles to move down their concentration gradients (passive transport). We will get a lot of practice with this during cellular respiration and photosynthesis, but it is important to think about this now.
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