Answers about the Transport Mechanism

   On this page, I will try to explain some precise methods of transport that could be used for each of the arrows.  I am only giving you the possible methods that are described within chapter 12 of your book, or that I spoke about in class.  I may throw in a few extra, just so that you understand what must exist, or the variety of what must exist.  I also will be describing the methods in general terms as well as very specific terms wherever possible.

Sodium Ions

Into the cell:

    You figured out that passive transport is responsible for sodium ions entering the cell.  Passive transport includes:  1) simple diffusion; 2) facilitated diffusion; and 3) osmosis.  Which of these three is important for sodium ions (charged molecules)?  I hope you considered only facilitated diffusion!  Now that you know that facilitated diffusion will provide the path for sodium ions to cross the membrane, which specific membrane transporters are used?   Your book includes many examples, and I gave some in class.  Here is a list of some choices:
bulletSodium leak channel (a pore).  This channel exists, but is extremely rare.  Think about it... if there were lots of membrane pores available for sodium ions, they would flood into the cell and destroy the concentration gradient.
bulletSodium voltage-gated channel.  Under the right voltage conditions, this channel can open... once open, sodium will flow along its electrochemical gradient.
bulletAcetylcholine- (ligand-) gated channel (receptor).   When acetylcholine (a neurotransmitter) travels across the synapse, it activates the next cell through this channel; acetylcholine binds to the channel and that opens it up.  This channel is specific for sodium (and potassium ions, but your book only describes that in the figure legend), so once it opens, sodium can rush through along its electrochemical gradient.  There are also plenty of other ligand-gated channels for sodium.  (see Figure 12-18)
bulletStretch-gated sodium channel.  I mentioned this in class... where a physical tugging on the membrane, like what occurs when someone touches your skin and causes a mechanical deformation within your skin, can lead to the opening of stretch-gated channels for sodium.
bulletSodium/glucose symport carrier.  This is a bit more complicated, because not only does sodium go down its electrochemical gradient, but glucose goes against its concentration gradient.  So, the sodium half is like a passive transport channel, and the glucose half is like an active transport carrier.   Remember that the energy from sodium running down its gradient fuels the glucose transport.  But this symporter is only active when glucose is around.   Therefore, the presence of glucose is like the presence of a ligand to open a ligand-gated channel for sodium.  Once sodium goes through, that then powers the carrier to also transport its glucose.  (see Figures 12-13 & 12-14)
bulletSodium/hydrogen antiport carrier.  Like the sodium/glucose symport carrier, the sodium in the sodium/hydrogen antiport carrier runs down its electrochemical gradient, while the hydrogen ion runs against its electrochemical gradient.  Hydrogen ions inside the cell act as ligands for this carrier, and activate the sodium portion to allow sodium ions to flow down their electrochemical gradient; the flow of sodium ions powers the flow of the hydrogen ions against their electrochemical gradient.

Out of the cell:

        To move an ion against its concentration gradient requires active transport, and the only types of active transport are:  1) pumps/carriers; and 2) transport using vesicles.  Since ions are tiny, a pump or carrier is the way to go.  There is only one sodium pump described in your book, and I don't know of any others.  The pump is the sodium/potassium pump.  It only operates with the expenditure of ATP.

Potassium Ions

Into the cell:

    Active transport of potassium ions occurs through pumps/carriers, since potassium ions are tiny.  The active transport of potassium ions specifically uses the sodium/potassium pump.  For every three sodium ions kicked out of the cell, two potassium ions are shoved back in.

Out of the cell:

    Passive transport of potassium ions is something that we discussed more in class than is found in your book.  You should understand, however, that potassium ions can only cross the membrane through facilitated diffusion (not simple diffusion or osmosis, the only other ways of passive transport).   Here are some of the more specific channels for potassium ions:
bulletPotassium leak channels (pores).  We discussed how there are leak channels, or pores, for potassium... since the electrochemical gradient on potassium is not that strong.  When these pores are in cell membranes, they allow potassium ions to flow out of cells.  Not too much potassium leaks out-- only enough to be fixed with the sodium/potassium pump.
bulletPotassium voltage-gated channel.  Under the right voltage conditions, these voltage-gated channels can open to allow potassium to flow out of the cell.
bulletAcetylcholine- (ligand-) gated channel (receptor).   Potassium can flow out of a cell after acetylcholine binds to its channel.   Sodium flows in at the same time.  There are also other ligand-gated channels for potassium.  (see Figure 12-18)

Oxygen molecules entering the cell

    Oxygen is a nonpolar molecule.  Nonpolar molecules are soluble in lipid, so they can cross the membrane by simple diffusion; they do not require any sort of protein transporter.  Therefore, the most specific way to describe oxygen transport is simple diffusion directly through the lipid portion of the membrane, along the concentration gradient for oxygen.

Carbon dioxide molecules leaving the cell

    Carbon dioxide is a nonpolar molecule... It's internal bonds (O = C = O) are polar, but the entire molecule is not (this was mentioned in Figure 11-20).  So, just like with oxygen, carbon dioxide crosses the membrane by simple transport across the lipid portion of the membrane, in the direction of its concentration gradient.

Amino acids entering the cell

    Amino acids fit into the "large, uncharged polar molecules" category.  But they are at the small end of that category.  Because of their smaller size within this category, they can fit through protein transporters.  Therefore, amino acids enter by facilitated diffusion (see Figure 12-3).

Calcium Ions

Out of the cell:

    Active tranport of calcium ions out of the cell can only be accomplished with pumps (they are too small for vesicular transport).   We talked about calcium pumps in class, especially those found in the membrane of the smooth endoplasmic reticulum.  But these pumps can also be in a cell membrane to pump out any excess calcium.  (described on pages 383-384 in your book, as well as Table 12-2).

Into the cell:

    Calcium will flood into the cell whenever it can... However, as a charged molecule, it requires a protein transported for facilitated diffusion.  We haven't discussed many calcium transporters... I only mentioned a calcium voltage-gated channel in class.  However, there are also ligand-gated channels for calcium as well.

Glucose

    As a monomer, glucose fits into the category of "large, uncharged polar molecules."  But, like amino acids, glucose is small enough to go through the membrane in protein transporters.

Out of the cell:

    Typically, glucose would not leave most cells... if it did, it would have to be by active transport, since it is against the concentration gradient for glucose.  This would require a glucose pump, like the sodium/glucose pump I described above for sodium... your book calls it the Na+-driven glucose pump.  However, this isn't really usually used for glucose transport out of a cell.  Why?

    You see, most cells use up glucose as soon as they get it.  The only real exception is in the cells lining your digestive tract.   They pick up as much glucose as possible, use only a regular-cell amount of it, and send the rest off into your blood for the rest of the cells of the body.  Therefore, the digestive tract epithelial cells are somewhat of an exception for glucose; they actually contain a high concentration of glucose within them, so when they send their glucose off into the blood, the glucose is going along its concentration gradient.   And when it picks up glucose from the digestive tract, it has to transport glucose in order to concentrate it inside the cell (in other words, transport against the concentration gradient).

    Keep this in mind when you review Figure 12-14... keep in mind that these epithelial cells are the exceptions to the rule, and that most cells don't have much free glucose floating about within them.  So most cells would need active transport to spit glucose out of them, and they don't really even do that.  But if they did, it would be by active transport, and it would use a pump rather than a vesicle, since glucose molecules are relatively small.

Into the cell:

    All cells need to pick up glucose.  This occurs passively, by means of facilitated diffusion.  You learned about the glucose carrier in your book.  This is different from the sodium/glucose pump.  The glucose carrier, shown in Figure 12-6 in your book, just shuttles the glucose across the membrane in the direction of its concentration gradient.

A whole cell into the cell

    An entire cell is HUGE!  The only way to bring something huge across a membrane is to shuttle it within a vesicle.  So active transport, specifically phagocytosis, would be necessary.

A large protein

    Any protein is a macromolecule, and is too large for transport through protein channels or carriers.  Therefore, whichever way the protein moves, it will have to be via vesicular transport.

Into the cell:

    Endocytosis!

Out of the cell:

    Exocytosis!

Hydrogen ions

    Your book talks about hydrogen ion transport mainly in reference to plant, fungal, and bacterial cells.  We haven't really discussed these cell types-- just animal cells.  Figure 12-17 is rather complicated, so I do not expect you to be able to explain all of it.  Keep in mind that as long as you try to understand the electrochemical gradient on hydrogen ions in all of the parts of that figure, you could explain it all... but don't worry about it.  Also, for the most part our cells move hydrogen ions more among cellular compartments (organelles) rather than across the cell membrane, so the "into" and "out of" the cell analogy isn't really so good here.

Into the cell:

    Hydrogen ions are charged, so they can only diffuse passively by facilitated diffusion.  That means that there must be protein transporters for hydrogen ions.  For the most part, the only hydrogen ion channels that we see in animal cells are not described in this chapter.  You see, they are mainly found in the mitochondrial membrane, and are important for generating ATP.

Out of the cell:

    Hydrogen ions are tiny, so they can cross the membrane through active transport in pumps, rather than in vesicles.  There is more than one type of hydrogen ion pump... one that you can imagine is shown in the lysosomal membrane in Figure 12-17A.  You know that the lumen of lysosomes is much more acidic than the cytoplasm, so here you can imagine that a hydrogen pump could be used to help concentrate the hydrogen ions within the lysosome.  This is not pumping hydrogen out of the cell.

Water entering the cell

    Water typically moves across the membrane by osmosis.  Water is a small, uncharged polar molecule, and so it can slip through the lipid portion of the plasma membrane.  There are some other possible mechanisms for moving water around, but we'll only consider osmosis.

    Water will enter the cell if the cytoplasm is hypertonic to the extracellular fluid; in other words, if there are more solutes inside the cell, water will enter.  Solutes include ions, so the more we regulate the solute concentrations within the cell by transporting these solutes, the less water will enter.

Steroid hormone entering the cell

    Steroid, a nonpolar molecule, will enter by simple diffusion directly across the lipid portion of the membrane.  It will only enter if there is a concentration gradient driving its movement.

Chloride ions entering the cell

    Without any particular electrochemical gradient, chloride doesn't tend to move by any means across the membrane.  If the electrochemical gradient shifted so that it would drive chloride ions into the cell, they could cross via leak channels, voltage-gated channels, or ligand-gated channels.  I don't know of any active transport mechanism for chloride ions, but I'll bet that one exists!

Do you have any other questions or comments?  If so, write me!

For more information on gated channels, see Figure 12-22 in your book.