Last time we discussed the fluid mosaic model and some of the experimental data that has been obtained to support this model. We also started our discussion of the permeability of biological membranes. For example, some b iologically important molecules that can pass through the phospholipid bilayer, such as: H₂O, O₂, CO₂ and hydrophobic molecules . Ions or large polar molecules (eg sugars), however, cannot pass through the ph ospholipid bilayer.

Therefore, if a cell wants to transport molecules that cannot free diffuse through the phospolipid bilayer into the cell, it must have a spec ific channel in the membrane through which these solutes can pass. These channel s are invariably in the form of an integral membrane proteins.

The next question we need to address concerns the direction i n which the molecules will move. That is, what will determine if a molecule will move into or out of the cell.

Last time time we discussed that the random motion of molecul es ensure that molecules will diffuse from an area of high concntration to low c oncentration. So if we start with a molecule that is at a high concentration on one side of the cell, and this molecule can pass through the membrane, at equili brium, the concentration of this molecule on both sides of the membrane will be the same.

Let’s now consider a slightly different situation: one in whi ch a solute molecule is more concentrated on one side of the cell membrane, but in this new situation, the solute molecule cannot pass through the membrane.

The side of the membrane with the higher concentration of sol ute is said to be hypertonic, while the other side is said to be hypotonic.

Although the solute molecule in this case cannot pass through the membrane, we know that water can. So what will happen is that the water fro m the side of the membrane with the low concnetration of solute will move to the side with the higher concentration of solute.

Why is water diffusing? The diffusion of water is called osmo sis, and it occurs to lower the free energy of the system. A situation in which a solute is more concentrated on one side than the other is a low entropy state. Water will move to balance the concentration of the solute on both sides of the membrane because this state has a higher entropy.

Osmosis has profound effects on cell shape and integrity. Let ’s consider what will happen if we drop an animal cell into a hypotonic, isotoni c or hypertonic solvent. In the hypotonic solvent, the water concentration insid e the cell will be lower than the surrounding, so water will rush in and the cel l will lyse. In the hypertonic solution, water will be more concentrated in the cell and will therefore rush out leaving the cell shriveled. It is only in a iso tonic solution that the animal cell can maintain it shape.

These osmotic considerations plays a large role in determinin g how the cell will store different molecules. Recall the cell stores gluose not as monomers, but as a large polymer of glycogen. This is at least partly due to the fact that one molecule of glycogen with a hundred residues puts less osmoti c pressure on the cell than 100 molecules of glucose.

Plant cells are designed to exists in a hypotonic environment . They have a cell wall, such that even when they are exposed to pure water, the cell wall supports the membrane and prevents the cell from exploding. The press ure cell on the cell wall gives the plant tugor. Tugor is what makes plants crun chy: when you bite into a stick of fresh celery, the sound that you hear is all the cells popping as the pressure is released.

The movement of molecules through the membrane in response to concentration gradients is called diffusion. If the movement is through the lip id bilayer it is called free diffusion. If it occurs via a protein molecule in t he membrane it is called facilitated diffusion.

As you might imagine, the cell needs to accumulate certain mo lecules while removing or pumping out others. In instances in which a solute mus t be moved across the membrane against its concentration gradient, a special mem brane protein that can perform active transport is required.

The accumulation of molecules on one side of the cell generat es a situation with a high free energy. This situation therefore will not happen spontaneously, and require an input of energy. As in many other reactions, this energy is often supplied by the hydrolysis of ATP. Membrane proteins that use e nergy to move molecules against a concentration gradient are often called pumps.

Let’s consider a specific example of concentration gradients maintained by the cell. In animal cells Na ions are very low, relative to the su rrounding environment, while K ions are high, relative to the outside.

The gradient of these two ions is maintained by the sodium-po tassium pump. This pump uses the enrgy supplied by ATP hydrolysis to pump out 3 sodium atoms, while importing 2 potassium.

An important point to note about the Na-K pump, is that 3 pos itive charges are pumped out for every 2 that are transported in. The Na-K pump therefore generates not only a concentration gradient, but an electrical gradien t as well (it is therefore referd to as an electrogenic pump). The combination o f both a electrical and chemical gradient is refered to as an electrochemical gr adient.

The gradient of Na across the membrane represents a very high free energy state. Na "wants" to get back into the cell not only because of the steep concentration gradient, but because of the electrical gradient as well (n egative on the inside).

As it turns out, all cells maintain an electrical gradient ac ross their membran. In animal cells, the Na-K pump is important for maintaining the electrical gradient (refered to as the membrane potential). In plants and ba cteria, a proton pump maintains a high pH inside, and a net negative charge insi de.

The electrochemical gradient across the cell membrane serves a number of purposes. In nerve cells, releasing this charge difference activates the cell to release signal molecules which in turn activate other nerve cells.

This electrochemical gradient also plays an important role in a number of co-transport systems. Let’s take for example sucrose transport in p lant cells. The sucrose concentration inside the plant cell is always higher tha n it is outside. However, the cell often requires more sucrose to be imported, s o energy is required to import sucrose against the concentration gradient.

In plants, this energy is derived from the proton electrochem ical gradient with the use of a co-transport system. A proton pump initiates the process by establishing the gradient (using ATP hydrolysis). A specialized co-t ransporter then uses the energy stored in this gradient to import sucrose: for e very molecules of sucrose imported against its concentration gradient, a proton is imported.

Energy from proton import is available not only because of th e proton gradient is driving protons into the cell, but also because to the elec trical gradient. Again, the combination of these two forces is refered to as the electrochemical gradient.

The final aspect of membrane transport we will discuss is the transport of large molecules: molecules that are two large to transport by an i ntergral membrane protein.

The two general mechanisms used by the cell to move large mol ecules through the membrane are exocytosis and endocytosis. Exocytosis of course refers to the movement of molecules out of the cell by the fusion of a membrane vesicle with the plasma membrane, whereas endocytosis involves the formation an d engulfment of a vesicle derived from the plasma membrane.

The vesicle for exocytosis will be generated for example by t he Golgi apparatus, and we have already discussed how these are formed.

There are three types of endocytosis: phagocytosis, pinocytos is, and receptor-mediated endocytosis.

Phagocytosis is a relatively non-specific event in which cell s with a general ability to engulf food particles and debris wrap their plasma m embrane around a particle and engulf it. The engulfed vesicle is often delivered to the lysosome for digestion.

Pinocytosis, or "cell drinking" is also characterized by the formation of membrane vesicles, but in the case of pinocytosis, particles are no t observed in the particle.

Finally, their is receptor mediated endocytosis. This type of endocytosis is very particle specific, and is used to import certain macromolec ular complexes into the cell.

Receptor mediated endocytosis begins with a highly specific i nteraction with a particle at the surface of the cell with an integral membrane protein. Once this binding reaction has occured, the cell begins to build a "coa ted pit" around the receptor-ligand complex. This coated pit starts as a small c oncave surface, but continues to assemble into a spherical structure that eventu ally pinches off the plasma membrane to form a coated vesicle.

Note that coated pits are not formed during phagocytosis. In phagocytosis, the cytoskeleton is responsible for piching off the vesicle from t he plasma membrane.

What particles are imported by receptor-mediated endocytosis? As ot turns out, cholestrol is imported into animal cells through receptor-medi ated endocytosis.

Cholesterol travels through the blood as part of a low-densit y-lipoprotein. These LDLs are composed of cholesterol, other lipids, and specifi c proteins. The LDL is recognized specifically by a receptor of cells. This inte raction triggers the import of the LDL the receptor mediated endocytosis.

Interestingly, an inherited genetic disorder: hypercholestero lemia arises when the LDL receptor protein is defective. This defective receptor protein precludes the import of the LDL particle, and results in increased leve ls of plasma LDL in pateints. The increased LDL levels in the plasma cause blood flow problems.

We are going to turn back to the problem of bioenergetics. Pr imarly we are going to concern ourselves with how chemical energy is harvested t o generate ATP from ADP and P. As we shall see, our discussion of biological mem branes will be important for understanding how ATP is generated in the cell.

When we discussed the structure of the mitochondria, I indica ted that this organelle was one of the primary sites of ATP synthesis. I also me ntioned that the inner mitochondrial membrane was very important in this process . As it turns out, a very steep electrochemical gradient is generated across the inner mitochondrial membrane, and this electrochemical gradient is used as the energy source to make ATP.

Before we get into the specifics of mitochondrial function, w e must first understand something about redox reactions.

As I have drawn out a number of times, in cellular respiratio n, H atoms are transfered from foods (such as sugars) to O₂ to make C O₂ and water. This is a spontaneous reaction, and the release of ener gy is used by the cell to drive ATP synthesis.

This reaction is called a redox reaction. Since H carries an electron with it, O₂ is reduced to water in this reaction. Conversely , since sugar is losing electrons, it is said to be oxidized in the reaction.

This reaction is driven because O is highly electronegative. Its high affinity for electrons makes the transfer of electrons to it highly fav orable.

H is not transfered directly to O₂, but requires i ntermidates. For example NAD+ reduced to NADH.

Other H carriers include FAD and ubiquinone. The electron com ponent can be carrier by the iron of heme (Fe3+ -> Fe2+).

So for example H-C-OH --> C=O would reduce NAD+ to NADH + H+.

Next time we will discuss how the flow of electrons is couple d to ATP synthesis.