Movement Through the Plasma Membrane

Substances that must be able to enter a cell are water, oxygen, and nutrients. On TV medical dramas, you'll often see technicians administering a saline drip to a patient who has lost blood or is dehydrated. This saline solution maintains a specific concentration of dissolved substances in the blood and body fluids. When dehydrated, our concentration of dissolved substances increases, upsetting the balance in our cells. Water can freely move in and out of cells to maintain the same water pressure on both sides of the plasma membrane. Oxygen is important because cells are undergoing cellular respiration. A cell takes chemical bond energy and converts it to a form of energy that it can use--a molecule of ATP. ATP contains small amounts of energy appropriate to powering cellular processes. This process of energy conversion requires oxygen (we will discuss this in more detail in Lesson 4). For aerobic cellular respiration to occur inside this cell, oxygen must move through the plasma membrane.

Some nutrients enter freely; others are controlled. Cells must also export the products that they make. (We'll discuss how cells make proteins in Lesson 7.) The cells in your liver are amazing: they make many, many proteins that leave the liver cells to be transported to cells in other parts of your body. Waste products must also leave a cell. For instance, during cellular respiration, carbon dioxide is released as a waste product. It goes back into our blood stream and eventually is exhaled from our lungs. So cells must interact with their environment yet maintain fairly constant internal conditions. There are three ways that substances move across the plasma membrane: (1) diffusion, (2) facilitated diffusion, and (3) active transport.

Diffusion

The simplest method of moving substances across the membrane is diffusion, the random movement of particles from an area of higher concentration to an area of lower concentration. Diffusion follows a concentration gradient (Figure 3.4) and will occur across the plasma membrane as long as there is no restriction (e.g., size or charge of molecule). Non-polar lipids and small molecules such as oxygen and carbon dioxide are able to pass freely through the membrane. For example, because oxygen is used for cellular respiration, there is always a higher oxygen concentration outside the cell and a lower concentration inside. As oxygen follows this gradient from higher to lower concentration, oxygen molecules are always diffusing into the cell. Carbon dioxide also undergoes diffusion but in the opposite direction because there is always a higher concentration of carbon dioxide inside than outside the cell. Other small molecules, like ethanol, also can diffuse freely through the plasma membrane, which is why alcohol hits your system fairly quickly: it diffuses from your digestive system into your bloodstream and then is carried to all of your cells. It affects these fairly rapidly and evenly, diffusing  into them until the cellular concentration is approximately equal to that in your bloodstream. Diffusion does not require the input of energy on the part of the cell.

 Graphic showing the diffusion of a lump of sugar in four steps: Step 1, the sugar is dropped into a beaker of water; Step 2, the sugar molecules beging to spread throughout the water; Steps 3 and 4, the sugar molecules continue to spread out in the water.

Figure 3.4. Diffusion of Sugar in Water
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Osmosis

Osmosis is the term for a special type of diffusion, the diffusion of water, and is based on the concentration of dissolved substances (solutes), either in the fluid within the cell or in your blood stream, which cannot cross the membrane. In the figure below (Figure 3.5), a beaker is shown to demonstrate the movement of water within a cell by osmosis.

Water molecules, which can cross the membrane, will diffuse to the side with the lower water concentration (higher solute concentration). The first frame is isotonic, meaning the water molecules are evenly distributed between the two sides of the beaker. In the center frame, you add a nondiffusible solute to the right side. With the addition of these molecules, there is less water pressure, so water will flow from the left side of the beaker to the right, until the water pressure is equal on both sides, illustrated by the third frame. Water will move into your cells or out of your cells depending upon the concentration of solutes, like salt, in your body tissues and in your blood stream. Therefore, fluid replacement for an injured person must match the bloodstream's dissolved solute concentration, as is true of isotonic saline, which will not cause water to leave or enter cells too rapidly. Although the movement of water is given a special name, osmosis follows a concentration gradient (its own) and does not require the input of energy.

Graphic depicting osmosis.  In step 1 a permeable membrane in a beaker of water causes water to distribute equally on either side of the membrane.  In step 2, solute molecules that cannot cross the membrane at added to one side of the beaker.  The water molecules on the solute side bind to the solute which decreases the number of water molecules on that side.  In step 3, diffusion causes the free water molecules on the non-solute side to move to the solute side.

Figure 3.5. Movment of Water by Osmosis
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Facilitated Diffusion

Many substances will follow a concentration gradient, but are too large to get through the membrane on their own. These substances need some kind of carrier molecule to help them. They must move through a protein that is imbedded in the plasma membrane. In the process of cellular respiration, we take chemical bond energy and turn it into a form of energy that the cells can use. One of the molecules whose chemical bonds are broken down is glucose. For example, when you eat a potato, which contains lots of starch, as it goes through your digestion system, the polymers of starch are broken down into glucose monomers in your small intestine, then glucose is absorbed across the small intestine and goes into your blood stream. That glucose is used by all of the cells in your body to provide energy. The concentration of glucose is usually higher outside a cell than it is inside, because once it enters the cell it is broken down. So, you have a concentration gradient, but glucose is too big to move freely through the membrane. There are carrier molecules in the cell membrane that are specific to glucose. They do not allow other molecules through. There are other carrier proteins for molecules such as amino acids. This is part of the complexity of cell membranes, because you have to have different carrier molecules for different substances that are going to be brought into or excreted from the cell. See Figure 3.6 for an illustration of facilitated diffusion.

Proteins in the plasma membrane act as gates to allow movement of large molecules into and out of a cell.  In step 1, a molecule binds a particular protein that is embedded in the plasma membrane.  In step 2, the protein helps or facilitates the movement of the molecule through the plasma membrane.  In step 3, the molecule is released on the other side of the membrane.  The same protein will move the molecule in either direction, and this type of movement across the plasma membrane does not require energy.

Figure 3.6. The Movement of a Substance Through the Plasma Membrane Using a Carrier Molecule
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Active Transport

The last type of general mechanism that cells use to transport materials is more complicated because it requires the cells to expend energy. That is why it is called active transport. In this case, substances are not going to go with the concentration gradient, they're actually going against the concentration gradient. When substances are going against their gradient, energy must be used. We call these proteins where this energy is used pumps because they are pumping substances against the concentration gradient. There are many different pumps for different molecules; one that has been well-studied is the sodium-potassium pump.

Sodium and potassium are extremely important molecules, particularly in animals. In our nervous system, the balance between these two ions, sodium and potassium, allows the transmission of nerve impulses. For instance, if I think that I am going to move my hand, it is the gradient created by this pump that causes the message to be propagated down a nerve, allowing my hand to move. So this is an extremely important pump. It has been estimated that about 30 percent of the energy in our cells is used to maintain the concentration of sodium against its gradient. Normally, there is more sodium inside of a cell than outside. A transport protein in the membrane has specific receptors for sodium ions. Sodium ions inside the cell attach to these proteins, as do ATP molecules which are the energy currency of the cells. When the ATP molecule splits, it provides energy to change the shape of the protein channel. When the protein changes shape, it traps the sodium ions and they are pushed to the other side of the membrane (see Figure 3.7).

Proteins in the plasma membrane act as gates to allow movement of large molecules into and out of a cell.  In step 1, a molecule binds a particular protein that is embedded in the plasma membrane.  In step 2, the protein helps or facilitates the movement of the molecule through the plasma membrane.  In step 3, the molecule is released on the other side of the membrane.  The same protein will move the molecule in either direction, and this type of movement across the plasma membrane does not require energy.

Figure 3.7. The Sodium-Potassium Pump
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On the outside of the membrane, two potassium ions bind and are then transported to the inside of the cell. This creates a concentration gradient with the higher concentration of sodium outside the cell, and potassium inside the cell.

This concentration gradient is also used to help bring other substances into the cell against their concentration gradients, through coupled channels (Figure 3.8). In this process a facilitated diffusion channel allows the diffusion of sodium ions back into the cell but only if it is accompanied by another particular molecule. There are specific coupled channels for many needed molecules such as sugars and amino acids. For instance, there is a coupled channel protein that allows diffusion of sodium ions into the cell if it is coupled to glucose. So when the sodium ions diffused back into the cell, it "pulls" glucose along into the cell even though it is against the concentration gradient of the glucose. It is important for the function of the protein channel to have this high sodium concentration outside of the cell.

These pumps are important to maintaining cellular conditions, allowing a number of different processes, including transmission of nerve impulses and transport of nutrients into the cells. The sodium potassium pump is the most active, but there are many other ions that are pumped in and out of cells to establish concentration gradients for different reasons. These three methods, diffusion (including osmosis), facilitated diffusion, and active transport, allow cells to regulate what can, or cannot, cross the plasma membrane.

Graphic depicts how the sodium-potassium channel is linked to another protein in the plasma membrane called a coupled channel.  The sodium-potassium pump creates a concentration gradient where there are more sodium ions outside of the cell.  However, these sodium ions can get back into the cell via the coupled channel as long as the pass through that channel with another molecule, such as sugar.

Figure 3.8. A Coupled Channel
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