BIO 1007 Lecture Outlines | BIO 1007 Resources | Mrs. Caley Opsal's Home | IVCC Home | Contact Us
Structure of the Cell Membrane:
Fluid bathes the outer and inner surfaces of the cell membrane. You might think the cell membrane is rather solid, but it is actually more fluid-like. If you can imagine taking a microscopic pin and pushing it through a cell, then pulling it out, the cell doesn't lose all of its cytoplasm (the fluid on the inside). Instead the cell surface seems to flow over and seal the puncture. How can a fluid membrane remain distinct from the surrounding fluid? It has to do with the properties of lipid molecules.
Phospholipids are the most abundant type of lipid found in the CM:
polar (hydrophilic head = water loving)
nonpolar (hydrophobic tail = water dreading)
These phospholipids form a bilayer which forces 2 layers together with the tails on the inside opposing one another:
this minimizes the # of hydrophobic groups exposed to H20
(the fatty acid tails don't have to spend a lot of time fighting each other)
The reason the puncture automatically seals itself is because the puncture is energetically unfavorable because it leaves too many hydrophobic groups exposed.
Fluid Mosaic Model (proposed by Singer and Nicolson, 1972):
The three most common lipids in animal cell membranes are phospholipids (phosphate + lipid), glycolipids (carbohydrate + lipid), and sterols (cholesterol). These lipids are continuously moving, spinning, flexing their tails, etc...so the adjacent lipids don't get packed into a solid layer. There are also proteins in the membrane (mainly glycoproteins), so the membrane is a mosaic of lipids and proteins. The carbohydrates of the glycoproteins always extend outside the cell, never into the cytoplasm (their position can also shift in the plane of the membrane). The membrane is also asymmetric, which means the inside of the CM looks different than the outside.
Fluid Mosaic Model of Membrane Structure: a cell membrane is an asymmetrical mosaic of lipids and proteins. The membrane shows fluid behavior because of movements and packing variations among its lipids and proteins.
Two basic types of proteins embedded in the membrane include:
1. Integral membrane proteins
2. Peripheral membrane proteins
The functions/roles of the membrane proteins include the following:
1. Transport
2. Recognition
3. Receptor molecules
MOVEMENT OF SUBSTANCES ACROSS THE CELL MEMBRANE
Some substances do not require energy to cross the CM. These methods are termed passive transport processes. Examples of substances that do not require energy to move through the cell membrane are oxygen, carbon dioxide, and small electrically neutral molecules.
Other substances require energy to cross the cell membrane. This energy is usually in the form of ATP (adenosine triphosphate). Examples of substances that require energy to cross the CM are amino acids and ions (such as sodium).
TRANSPORT MECHANISMS:
1. Passive transport - does not require energy
-diffusion, osmosis, filtration
2. Active transport - requires energy (usually ATP)
-Na+-K+ pump, Ca++ pump, Endocytosis (pinocytosis, phagocytosis, and receptor-mediated endocytosis)
PROCESS OF DIFFUSION
Simple diffusion is the random movement of like molecules or ions down a concentration gradient.
Facilitated diffusion is the diffusion of molecules using a special carrier protein
Concentration is the number of molecules (or ions) of a substance in a given volume of fluid.
Concentration gradient refers to the situation where the distribution of ions or molecules in solution is unequal (a gradient is formed)
Molecules will tend to move from an area of greater ion/molecule concentration to an area of lesser ion/molecule concentration. Random collisions of molecules eventually send molecules outward from the region of greater concentration- this is termed the net movement. Gradients can also occur with differences in pressure, temperature or electric charge. In simple diffusion, each molecule will move down its gradient independent of any other substance that may be present.
PROCESS OF OSMOSIS
Osmosis is the movement of water across a differentially permeable membrane in response to solute concentration gradients, a pressure gradient or both (essentially it is the diffusion of water)
Using a red blood cell as an example: On a separate piece of paper, make a sketch of 3 beakers of solution and draw one red blood cell inside each of the beakers. We will discuss what happens to the red blood cell when it is put into 3 different types of solutions of varying concentrations of sodium chloride.
Osmotic movements across cell membranes are affected by tonicity: the relative concentrations of solutes in 2 fluids. (i.e. the cytoplasm and the water outside).
1. Isotonic - equal solute concentrations on both sides
2. Hypertonic - describes the fluid having more solutes
3. Hypotonic - describes the fluid having less solutes
*hint: water moves from a hypotonic fluid to a hypertonic one
*draw these situations out to make sure you get it correct!
FILTRATION
Filtration is the movement of substances (solids and water/fluid) through a membrane due to hydrostatic pressure.
Right now, your kidneys are filtering your blood at a rate of 125 mL/min or 180 L/day!! Anything small enough to fit through the membrane will get through (i.e. it becomes filtrate).
SPECIALIZATIONS OF THE PLASMA MEMBRANE
Some cells in the human body are separated by fluid-filled spaces called extracellular (intercelluluar) space. However, many cells are tightly packed together. In this instance, these cells may contain specializations of their plasma membranes. In other words, the surface and/or sides of the cells look different due to the specific function of those cells. Some important types of specializations of the plasma membrane are listed below:
1. Microvilli
2. Membrane junctions
a. Tight junctions
b. Desmosomes
c. Gap junctions
Last Updated 05/26/06
BIO 1007 Lecture Outlines | BIO 1007 Resources | Mrs. Caley Opsal's Home | IVCC Home | Contact Us