Chapter 7

The cell membrane is a phospholipid bilayer with the hydrophobic portion of each layer oriented toward the inside of the membrane (note not the inside of the cell) and the hydrophilic portion toward the outside of the membrane. These phospholipids are in motion along the plane of the membrane, but rarely move from one layer to another because their hydrophilic regions would then have to cross the hydrophobic barrier in the middle of the membrane. The consistency of the membrane is approximately the same as salad oil. Floating in these phospholipids are proteins. These proteins must have hydrophobic regions if they extend into the hydrophobic center of the membrane. These proteins also may move along the plane of the membrane, but because they are larger they often move relatively slowly relative to the phospholipids. Proteins may be anchored to the cytoskeleton and also may be moved in a directed fashion by virtue of motor proteins along the cytoskeleton. Unsaturated fatty acids, which have kinks at sites of carbon-carbon double bonds, tend to space out the phospholipids allowing greater fluidity. Cholesterol another amphipathic molecule fits in between adjacent phospholipids within a layer further spacing out the phospholipids. While cholesterol doesn't increase the fluidity, it does reduce the temperature at which the membrane might potentially solidify. Membranes must be fluid to work appropriately.

Proteins that extend through the membrane are transmembrane proteins and are called integral proteins. They often have hydrophobic amino acids organized into alpha helices that are situated in the hydrophobic membrane regions. Peripheral proteins do not extend into the hydrophobic regions but usually bind to the integral proteins or sometimes to glycolipids. Remember the cell membrane is part of the endomembrane system and the outer layer of the cell membrane is the inner layer of endoplasmic reticulum, transport vesicles, and dictyosomes. The sugar portions of glycolipids and glycoproteins often serve are often significant sites for cell recognition.

One of the truly significant roles the cell membrane plays is selectively allowing material to move into and out of the cells. When we are trying to understand movement we have to consider both the driving force for this movement and the passageway of movement. The phospholipid bilayer allows small hydrophobic molecules, such as hydrocarbons, carbon dioxide and oxygen to cross easily. It also allows small, uncharged polar molecules (e.g., water and ethanol) to move between adjacent phospholipids. It is relatively impermeable to large, uncharged polar molecules (e.g., sugars) and ions.

The passageway for substances for which the membrane is impermeable involves transport proteins that are categorized as either carrier or channel proteins.

The most common driving force for molecules is diffusion. Diffusion results from the random motion of molecules at temperatures above absolute zero (-273 C) that tends to redistribute them from areas of high free energy to areas of low free energy. Often, but not always, concentration is the component of free energy that distinguishes the relative amount of free energy molecules possess in different regions. Regions where a molecule is in high concentration tend to be more orderly than regions where the molecule is in lower concentration have higher free energy. Movement of molecules due to diffusion across a membrane is referred to as passive transport. The passive transport of water across a selectively permeable membrane is called osmosis. When comparing two solutions the one with the lowest solute concentration is referred to as being hypotonic, while the one with the higher solute concentration is hypertonic. If the solute concentrations in two solutions are equal the solutions are isotonic.

Consider the U-tube experiment to appreciate that free energy differences that drives diffusion not concentration differences.

Osmotic movement is determined by the amount of particles per unit volume of water not the size or identity of the particles.

Examine figure 8.11 to consider the effect of placing animal and plant cells in hypertonic or hypotonic solutions. Note that animals must have adaptations for osmoregulation (Paramecium use contractile vacuoles).

If movement of molecules or ions would follow a diffusion gradient except for the fact that the phospholipid bilayer acts as a barrier, then transmembrane proteins sometimes can facilitate the movement across the membrane. Such movement is called facilitated diffusion. The transmembrane proteins are categorized as carrier or channel proteins. Carrier proteins loosely bind with these molecules, change shape and thereby move the substances across the membrane. Channel proteins provide canals through which usually small ions are able to pass from one side of the membrane to the other. These canals can be open or closed and their states are controlled by external factors and are referred to as gated channels. The gates may open due to external molecules (ligands) binding to the channels (ligand gated channels), voltage changes across the membrane exceeding a threshold (voltage gated channels) and mechanical force being applied to the channel (stress gated channels).

Active transport is the pumping of solutes against a free energy gradient. Often the energy to drive active transport comes from the dephosphorylation of ATP. Carrier proteins are necessary pathways. A good example is the Sodium-Potassium Pump. Typically animal cells expend more than 30% of their ATP in driving the Sogium-Potassium Pump. It exchanges Sodium (3 ions) from the cytosol to the extracelluar fluids for Potassium (2 ions) from the extracellular fluids to the cytosol using ATP to drive these exchanges against concentration gradients. By doing this it generates a voltage across the membrane because it is continuously moving 3 positive charges out and only 2 positive charges into the cell. Therefore the inner surface of the membrane is negative with respect to the extracellular side. This voltage is called the membrane potential and ranges from about -50 millivolts to -200 millivolts. Effectively this creates a battery. Since diffusion is driven by free energy gradients, this electrochemical gradient may influence the redistribution of ions due to diffusion. There is about an equal concentration of Potassium ions inside the cell as Sodium ions outside the cell due to the Sodium-Potassium Pump. Despite this the free energy gradient for Sodium ions is much higher due to the electrochemical gradient. This gradient of Sodium ions is used to drive the movement of other substances against their free energy gradients. Hydrogen ion gradients are established by plant and bacterial cells across their membranes and this hydrogen ion

gradient is used by plants in a similar fashion to the Sodium gradient in animals. The general term for transport proteins that establish membrane potentials is electrogenic pumps, but those that pump hydrogen ions to establish the gradient are called proton pumps.

If transport of a substance across a cell is independent of the transport of any other substance, the carrier is called a uniport. If transport of a substance is dependent on the simultaneous transport of another substance (often in animal cells Sodium) it is called cotransport. If the cotransport is in the same direction the carrier is called a symport, if the directions of transport are opposite the carrier is called an antiport.

Movement can also occur by the fusion of a cellular vesicle with the cell membrane thereby releasing the contents of the vesicle into the extracellular fluids (exocytosis). Endocytosis is the reverse process whereby a vesicle buds off the membrane thereby moving extracellular material from outside the cell to the inside. If the material is solid (e.g., a bacterium) the process is called phagocytosis, if the substance is in solution or suspension in the extracellular fluid it is called pinocytosis. Pinocytosis brings solutes into the cell in the same concentration that they occur in the extracellular fluid. Alternatively, receptor mediated pinocytosis selectively brings certain solutes into the cell in higher concentrations than their extracellular concentrations.

Exocytosis adds membrane to the cell membrane from the endomembrane system. Endocytosis removes membrane from the cell membrane.