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Passive Transport

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Notes

Passive Transport

Sections

PASSIVE TRANSPORT

Requires no energy input because all molecules still move down their concentration gradient
All channels and many transporters work via passive transport

CHANNELS

Somewhat specific, only allowing a molecule through if it is the right size and charge
Allow for faster transport than transporters or diffusion
Some channels require a signal before solutes can travel through
Aquaporins are an example of channels – increase the rate of water travel

TRANSPORTERS

Also called carrier proteins
Very specific for the molecules that they transport
Transporters can reverse direction if the concentration gradient flips
Glucose transporter is an example

ELECTROCHEMICAL GRADIENT

Concentration gradient is important for movement of molecules, but voltage difference between the two sides of the membrane can also play a role

Electrochemical gradient

Combined force due to the membrane voltage and concentration gradient

Electrochemical gradient when molecules are non-charged

Movement based on concentration gradient ONLY, voltage difference plays no role

Electrochemical gradient when molecules are charged and voltage and concentration gradient work together

Larger electrochemical gradient when voltage and concentration gradient work in the same direction

Electrochemical gradient when molecules are charged and voltage and concentration gradient oppose one another

Smaller electrochemical gradient when voltage and concentration work in opposite directions

Full-Length Text

Here we will learn about passive transport across cellular membranes.

First, start a table to summarize key features of passive transport.

Denote that passive transport requires no energy input.

Denote that molecules move down their concentration gradient from areas of high concentration to areas of low concentration, which is why passive transport is sometimes referred to as facilitated diffusion.

Denote that all channels and many transporters work via passive transport.

Now let's discuss the membrane proteins involved in passive transport.

Write that channels are the first class of membrane proteins we will focus on.
- An examples of which are aquaporins, which increase the rate of water permeability.

Draw a lipid bilayer.

Label outside the cell extracellular.

And inside the cell cytosol.

Now show the channel protein extend through the lipid bilayer.

Now, off to the side, let's draw the concentration gradient.

Draw multiple solute molecules above and a few below the bilayer.

Draw an arrow from the high concentration above to the low concentration below to indicate the direction of the concentration gradient.

Then, outside of the channel, draw a few, positively-charged solute molecules.

Show one pass from outside to the cytosol through the channel.

Then, draw bigger, negatively-charged solute molecules that are incapable of traversing the channel.
- This is because channel proteins are somewhat specific, only allowing molecules across that have appropriate size and charge.

To the side, write that channels allow for faster movement than transporters (which we'll discuss, next), and that some channels must first receive a signal to "open" and allow the movement of molecules.

Moving on, indicate that the next class of membrane proteins we will discuss are the transporters (aka carrier proteins).
- An example is the glucose transporter.

Again, set up a plasma membrane and label the extracellular and cytosolic sides.

Draw the transporter protein traversing the plasma membrane, closed at the bottom.

Again, set up a concentration gradient with more solute on the extracellular side than the cytosolic side.

Next, within the transporter, label one of the solute binding sites.

Draw solute molecules above the transporter.

Show one pass into the transporter to the binding site.

Next, show a conformational change:

Redraw the transporter protein traversing the plasma membrane but with the tops close together.

Show that the solutes are now able to leave the binding site and travel into the cytosol.

Write that transporters are very specific for the molecules that they transport.

Also write transporters can reverse direction if the concentration gradient flips.

Although the concentration gradient is important for the movement of molecules across the cell membrane, the voltage difference between the two sides of the membrane also plays a role, collectively, the concentration gradient and voltage difference form the electrochemical gradient.

Denote that the electrochemical gradient is the combined force due to membrane voltage and concentration gradient.

To understand how the electrochemical gradient affects movement across a membrane, let us look into three situations:
- Movement of non-charged particles.
- Movement of charged particles when the voltage and concentration gradient work together.
- Movement of charged particles when the voltage and concentration gradient oppose one another.

First, movement of non-charged particles.

Draw the lipid bilayer.

Label the extracellular and cytosolic sides.

Indicate that the extracellular side of the cell membrane has a slight positive charge while the cytosolic side has a slight negative charge.
- We explore how this voltage difference is created elsewhere.

Draw non-charged solute molecules such that the concentration gradient will allow the molecules to move from outside the cell to inside.

Draw a somewhat thick arrow to denote this movement.

Write that since the molecules are non-charged, the voltage difference plays no role in the movement which is based solely on the concentration gradient.

Next, let us illustrate the movement of charged particles when voltage and concentration gradient work together.

Again draw the lipid bilayer and indicate that the extracellular side has a slight positive charge and the cytosolic side a slight negative charge.

Draw the solute molecules with the same concentration gradient as before, but this time, also indicate that they are positively charged.

Draw a large, thick arrow in the direction of movement because in this case, both the concentration gradient and the voltage (positively charged solutes attracted to the negative charge of the inside of the cell) work in the same direction to produce a large electrochemical gradient.

Finally, we will look at what happens when the concentration gradient and voltage oppose.

Again, draw the lipid bilayer as before.

This time, draw the positively charged solutes such that there is a greater concentration inside the cell than outside.
- In this situation, the solutes would try to move outside based on the concentration gradient but try to stay inside due to the voltage.

Draw a thin arrow out of the cell because the driving force on the solute is lessened because the concentration gradient and voltage are now opposing one another.

As a clinical correlation, when muscles contract, calcium channels are opened allowing calcium ions to diffuse into the cytoplasm down their electrochemical gradient to activate proteins.

UNIT CITATIONS:

Campbell, N. A. & Reece, J. B. Biology, 7th ed. (Pearson Benjamin Cummings, 2005).

Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K. & Walter, P. Molecular Biology of the Cell, 5th ed. (Garland Science, 2008).

Alberts, B., Bray, D., Hopkin, K., Johnson, A., Lewis, J., Raff, M., Roberts, K. & Walter, P. Essential Cell Biology, 3rd ed. (Garland Science, 2010).

Lodish, H., Berk, A., Kaiser, C. A., Krieger, M., Scott, M. P., Bretscher, A., Ploegh, H. & Matsudaira, P. Molecular Cell Biology, 6th ed. (W. H. Freeman and Company, 2008).

Seifter, J., Sloane, D. & Ratner, A. Concepts in Medical Physiology. (Lippincott Williams & Wilkins, 2005).

Marieb, E. N. & Hoehn, K. Human Anatomy & Physiology, 10th ed. (Pearson, 2016).

Roberts, M., Reiss, M. J. & Monger, G. Advanced Biology. (Nelson Thornes, 2000).

Lewin, B. Cells. (Jones & Bartlett Learning, 2007).

Segev, N. (edited by). Trafficking Inside Cells: Pathways, Mechanisms and Regulation. (Springer Science & Business Media, 2010).