Passive and Active Transport

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In this lesson, we will explore the difference between passive and active transport. Molecules are constantly moving. In our bodies, this movement becomes particularly important because molecules need to be able to move from our lungs and digestive system, for example, into our blood, around our bodies, and ultimately into and also out of our cells. We will focus this discussion on the latter part, movements into and out of our cells. Much of this movement is dependent on concentration gradients. A concentration gradient occurs when the concentration of a substance is different in two adjacent areas. Consider this example where there is a higher concentration of substance A outside of the cell. This is a concentration gradient. Molecules tend to move towards equilibrium, which occurs when there is no longer a concentration gradient. When equilibrium is established, the molecules don't just stop. They keep moving but they maintain about equal spacing. When there is a concentration gradient, molecules tend to move from the area of higher concentration to the area of lower concentration to try to attain equilibrium. We refer to this as moving down or along the concentration gradient always from the area of higher concentration to the area of lower concentration. This directional movement occurs spontaneously and does not require any energy. In other words, it is a passive process, so we call this passive transport. In passive transport, molecules spontaneously move down their concentration gradient. Simple diffusion is an example of passive transport. In this case, molecules merely move down their concentration gradient, spreading out to fill their space. This may be as simple as a drop of food coloring dispersing through a beaker of water, or small molecules passing directly through a cell membrane. Oxygen, for example, enters our bodies by diffusing across the walls of tiny air sacs called alveoli and then, once in the blood, it travels throughout the body and then again will diffuse now across cell membranes. Facilitated diffusion works the same way. Molecules move down their concentration gradient but now they require some help. The chemical makeup of a cell membrane determines what can pass directly through and what cannot. Non-polar molecules, such as oxygen, can diffuse directly through the membrane, but larger or polar molecules and most ions cannot. To diffuse through the cell membrane, molecules that move by facilitated diffusion first must bind to a protein in the cell membrane that helps transport them across the membrane, acting as a carrier or a channel. Glucose, for example, requires one of these proteins to help it diffuse through the cell membrane. Thus, glucose moves by facilitated diffusion. Our final example of passive transport is osmosis, which is an even more specific for of diffusion. Osmosis is specifically the diffusion of water through a selectively permeable membrane. If a cell contains either too little or too much water, it'll change size and not be able to function optimally. Osmosis is critical to maintaining the proper amount of water inside of our cells. To summarize, all three of these movement processes - simple diffusion, facilitated diffusion, and osmosis - are passive transport processes. Each occurs spontaneously, requiring no energy, and moves a substance down its concentration gradient. But sometimes substances need to move up or against their concentration gradient. This requires work and work requires energy. Think about this. If you have a closet into which you have crammed a lot of junk, what would happen if you opened the door and put more junk into the closet? The junk already inside would try to come out. Right? Moving down the concentration gradient. So you would have to do some work to cram more junk in there; maybe put an arm up to block stuff from falling out while you cram more in. And that would take energy. When a substance is moved against its concentration gradient, energy is required and this is referred to as active transport. For example, sodium ions are usually in higher concentration outside of the cell and potassium ions are usually more concentrated inside the cell. To maintain this, the two ions are pumped to their respective sides of the cell membrane by a protein molecule in the cell membrane called the sodium-potassium pump. This specialized protein allows the ions to move in opposite directions, sodium out and potassium in, and against their concentration gradients. This requires energy and the cellular energy that is used for this is ATP. Thus, these ions are moved against their concentration gradient by active transport.