Passive Transport and Membrane Permeability: Understanding Diffusion and Osmosis | Study notes Biology

Diffusion and Passive Transport

Slide 2 Biological membranes play a vital role in controlling how substances

move into and out of the cell. The phospholipid membrane and its embedded proteins act

as both a barrier to and a connection with the world outside the cell. In this module we

are concerned with the processes that move atoms, ions, and molecules across the

membrane without requiring any energy input. We will investigate passive transport

processes, including simple diffusion, osmosis, and facilitated diffusion.

Slide 3 In order to understand what makes substances move anywhere in the first

place, it is important to understand the concept of a gradient. In simplest terms,

whenever there is more of something in one place than in another, a gradient exists.

Think of your house on a cold day. On the outside of the wall there’s a lot of cold air.

On the inside of the wall there is warm air. A temperature gradient exists between the

cold air outside and the warm air inside. If you open the door, cold air will come into the

house, and warm air will go out of the house. Leave the door open long enough and the

air outside and inside will reach the same temperature. What has happened?

Temperature equilibrium has been reached through diffusion of the warm and cold air

molecules, and the temperature gradient has been erased. In the absence of any barriers

substances will tend to diffuse so that a state of equilibrium, or even distribution of the

substances, is reached. It is important to note that in the process of diffusion, no energy

input is needed. The random motion of the molecules will tend toward equilibrium.

Slide 4 Diffusion can occur across a membrane if the membrane is permeable to

the substance that is diffusing. Biological membranes are semi-permeable, or selectively

permeable, meaning that some substances can pass through the membrane, but others

can’t. In general molecules that are lipid-soluble will pass directly through the

phospholipids that make up a large part of the membrane. Polar and charged molecules

cannot pass directly through the phospholipid portion of the membrane. To move these

types of molecules across the membrane we will see that the proteins embedded in the

membrane have several important functions.

Even though it is a polar substance, water can readily diffuse across biological

membranes, but it must do so by way of protein channels. In some cases water moves

through special protein channels called aquaporins. Water may also be carried through

certain integral membrane proteins along with other molecules. Diffusion of water is

called osmosis. Remember that diffusion will always occur down the concentration

gradient from the area of higher concentration to the area of lower concentration. In the

case of osmosis, it is important to remember that it is the concentration of water that is

being considered, not the concentration of solute.

Consider the situation illustrated on this slide. A solute is dissolved in water. A

selectively permeable membrane separates two solutions with different solute

concentrations and is impermeable to the solute itself. The solution in the right half of

the tube initially contains a higher solute concentration than the solution in the left half of

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Diffusion and Passive Transport

Slide 2 Biological membranes play a vital role in controlling how substances move into and out of the cell. The phospholipid membrane and its embedded proteins act as both a barrier to and a connection with the world outside the cell. In this module we are concerned with the processes that move atoms, ions, and molecules across the membrane without requiring any energy input. We will investigate passive transport processes, including simple diffusion, osmosis, and facilitated diffusion.

Slide 3 In order to understand what makes substances move anywhere in the first place, it is important to understand the concept of a gradient. In simplest terms, whenever there is more of something in one place than in another, a gradient exists. Think of your house on a cold day. On the outside of the wall there’s a lot of cold air. On the inside of the wall there is warm air. A temperature gradient exists between the cold air outside and the warm air inside. If you open the door, cold air will come into the house, and warm air will go out of the house. Leave the door open long enough and the air outside and inside will reach the same temperature. What has happened? Temperature equilibrium has been reached through diffusion of the warm and cold air molecules, and the temperature gradient has been erased. In the absence of any barriers substances will tend to diffuse so that a state of equilibrium, or even distribution of the substances, is reached. It is important to note that in the process of diffusion, no energy input is needed. The random motion of the molecules will tend toward equilibrium.

Slide 4 Diffusion can occur across a membrane if the membrane is permeable to the substance that is diffusing. Biological membranes are semi-permeable, or selectively permeable, meaning that some substances can pass through the membrane, but others can’t. In general molecules that are lipid-soluble will pass directly through the phospholipids that make up a large part of the membrane. Polar and charged molecules cannot pass directly through the phospholipid portion of the membrane. To move these types of molecules across the membrane we will see that the proteins embedded in the membrane have several important functions.

Even though it is a polar substance, water can readily diffuse across biological membranes, but it must do so by way of protein channels. In some cases water moves through special protein channels called aquaporins. Water may also be carried through certain integral membrane proteins along with other molecules. Diffusion of water is called osmosis. Remember that diffusion will always occur down the concentration gradient from the area of higher concentration to the area of lower concentration. In the case of osmosis, it is important to remember that it is the concentration of water that is being considered, not the concentration of solute.

Consider the situation illustrated on this slide. A solute is dissolved in water. A selectively permeable membrane separates two solutions with different solute concentrations and is impermeable to the solute itself. The solution in the right half of the tube initially contains a higher solute concentration than the solution in the left half of

the tube. In this case, water will move across the membrane from the area of its higher concentration (but lower solute concentration) to the area of its lower concentration (but higher solute concentration). The resulting solutions have equal concentrations of both solute and water.

Slide 5 The selective permeability of biological membranes and the solute concentrations inside and outside of a cell have important implications for cell shape and function. If the solute concentrations inside and outside of a cell are too different, the cell may not be able to function.

An isotonic solution is a solution with the same solute concentration as is found inside the cell. A cell in an isotonic solution experiences no net movement of water into or out of the cell. Remember that water molecules are still diffusing across the membrane, but they are traveling equally in both directions, so the water and solute concentrations remain the same inside and outside the cell.

A hypotonic solution has a low solute concentration compared with the concentration inside the cell. A cell in a hypotonic solution experiences net movement of water into the cell, and the cell will swell. In cells with cell walls, such as cells of plants, prokaryotes, or fungi, the cell wall prevents the cell from bursting, and this swelling helps maintain turgor pressure in plants and helps the cell grow. In animal cells, on the other hand, there is no cell wall, and cells in a hypotonic solution may swell to the point of bursting.

A hypertonic solution has a high solute concentration compared with the concentration inside the cell. A cell in a hypertonic solution experiences net movement of water out of the cell, and the cell may shrivel and lose many of its cellular functions. As you can see, it is important for a cell to be in an environment with the proper tonicity in order to maintain proper shape and function.

Slide 6 Ions and molecules that cannot pass directly through the phospholipid membrane may move across the membrane through special protein complexes in a process called facilitated diffusion. Keep in mind that in facilitated diffusion, just as in simple diffusion and osmosis, the movement of molecules is always down the concentration gradient, and no energy input is needed. The protein complexes simply provide an appropriate opening in the membrane for specific molecules to pass through.

Ion channels provide a good example of protein channels. Each type of ion channel is specific to the type of ion that can move through it, and these channels are “gated,” meaning that some type of signal is needed to open them. When the appropriate signal is received, the shapes of the proteins in the channel change to allow the ion to pass through. The direction of net movement of the ions will depend on the relative concentrations of the ion inside and outside of the cell.

Slide 7 Carrier proteins also act in facilitated diffusion. These proteins actually bind the molecule that is being transported across the membrane, change shape in response to the presence of the molecule, and then release it on the other side of the

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Diffusion and Passive Transport