Recall that phospholipids have a hydrophobic end and a hydrophilic end and that when placed in water they will orient themselves accordingly (5.11 pg 79). This is the basis for the plasma membrane of a cell. The cell membrane consists of a phospholipid bilayer with embedded proteins. We refer to the modern conceptual model of the cell membrane as the "fluid mosaic" model since the phospholipids are able to move about across the surface of the membrane (fluid) and the proteins are many and varied (mosaic) (5.12).
Attached to the some proteins and to some of the phospholipids are oligosaccharides (short polysaccharides). When a protein has an oligosaccharide attached it is called a glycoprotein. Glycolipids are phospholipids with the sugar chains added. These oligosaccharides are found on the outside of the membrane and are used in cell to cell recognition. They differ among species, among individuals and within individuals.
Membrane proteins can have a number of functions, such as transport proteins, enzymes (more on these shortly), receptor sites, cell adhesion, attachment to the cytoskeleton. (5.13)
The most important thing about membranes is that they regulate what moves in and out of a cell. The membrane is selectively permeable because substances do not cross it indiscriminately.
Some molecules, such as hydrocarbons and oxygen can cross the membrane. Many large molecules (such as glucose and other sugars) cannot. Water can pass through between the lipids. Ions such as H+ or Na+ cannot.
Transport proteins make passage possible for molecules and ions that would not be able to pass through a plain phospholipid bilayer. Some transport proteins have a hydrophilic tunnel through them which allows polar molecule or ions to pass. Others actually bind to the molecules and move them across the membrane. In either case transport proteins are very specific.
Diffusion and Osmosis
Diffusion is the tendency of molecules of any substance to spread out into the available space. Even though each molecule is moving at random, the spread is often directional since the molecules move from areas of high concentration to lower concentration. This is called moving along (or down) the concentration gradient. This requires no input of energy and when it happens across a cell membrane is called passive transport. Many substances move across cell membranes until there is an equal concentration on either side. (5.14)
Osmosis is a special case of diffusion. (5.15) First, imagine a semipermeable membrane, one that will allow water to pass through but keeps in dissolved molecules (called solutes). Second, imagine that there is a greater concentration of solutes in the water on one side of this membrane than on the other. The solutes can't move from one side to the other because of the membrane. But water can.
--A HYPOTONIC solution has a relatively lower concentration of solutes.
--Solutions of equal concentrations are said to be ISOTONIC. These terms are relative terms.
Osmoregulation is the control of water balance (5.16). A cell in an isosmotic environment doesn't have much to worry about, water goes in and water goes out at the same rate. But suppose that cell is in a hyperosmotic solution. Water will exit the cell, leaving behind a shriveled up cell. This is not good for the cell. In plant cells the plasma membrane actually shrinks back away from the wall (called plasmolysis) and the cell dies. If the cell is placed in a hypoosmotic solution water wants to get inside. This is also not good, at least for animal cells. Plant cells have cell walls which hold back the pressure of incoming water. They use this pressure to keep the cells turgid, which helps provide mechanical support of the plant.
Facilitated diffusion is a process by which solutes diffuse across membranes that they wouldn't normally get through on their own. They pass through with the aid of transport proteins.(5.17) The transport proteins are "substrate specific", which means they=re set up to transport just certain molecules or ions and block the rest. As with "regular" diffusion, solutes move along the concentration gradient.
Diffusion, osmosis and facilitated diffusion are passive means to get things across the membrane. There are energy consuming means also. These would fall under the heading of active transport.
Active transport uses the cell's energy to move substances against their concentration gradients. The content of a cell usually differs from the surroundings. Active transport is the means by which this is maintained. Transport proteins do the job. An example is the sodium-potassium pump used in the transfer of nerve impulses. Using ATP as an energy source, special transport proteins move Na+ out of a cell and K+ into the cell. (5.18 shows a hypothetical example and the role of the phosphate from the ATP)
Exocytosis and Endocytosis
The really big stuff (e.g. proteins and polysaccharides) does not get in and out of a cell by passing through the membrane. Exocytosis is the process by which large molecules leave the cell. Vesicles from inside fuse with the plasma membrane and empty their contents. In endocytosis the plasma membrane forms a vesicle around the particle. Examples of exocytosis: Secretory cells of the pancreas export insulin, nerve cells release chemical signals across synapse, plants make cell walls.
Endocytosis can be divide into three types. Phagocytosis, pinocytosis and receptor mediated endocytosis. Phagocytosis is the engulfing process we already talked about. In pinocytosis the cell "gulps" in a drop of the surrounding fluid. (5.19 a& b) Receptor mediated endocytosis (part 3 of 5.19c) is similar except the exterior part of the cell that gets drawn in has specific receptors which only bond to specific substances. This allows the cell to bring in only the substance it wants, often in much higher concentration than the surrounding fluid. (contrast this to pinocytosis) .