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The plasma membrane is what separates the internal and external environments of the cell. It regulates entrance and exit of molecules, creating a steady internal environment, which in done through diffusion, osmosis, facilitated transportation and active transportation. Along with all those things it gives the cell structure and protects it through its selective permeability.
The most common explanation for the structure of the plasma membrane is called The Fluid Mosaic Model, because the membrane is more like a fluid then a solid with many molecules so it looks like a mosaic. It's a phospholipid bilayer where proteins are either embedded partially or completely, creating a mosaic pattern. The membranes consistency is like a liquid but it stays in its shape. Cholesterol is found in the membrane of animal cells and similar steroids in plants and is used to stiffen and strengthen the membrane helping to regulate its fluidity.
Hydrophilic polar heads of the phospholipid molecules face the inside and outside of the cell where the water is found and the hydrophobic nonpolar tails face each other where there is no water. The proteins that make up the membrane can either be peripheral or integral. Peripheral proteins on the inside surface are held in place by cytoskeleton filaments, while integral proteins are embedded in the membrane and are able to move back and forth. Both phospholipids and proteins can have a carbohydrate chain attached. If they do they would be called glycolipids and glycoprotein's.
Proteins are what are used to transport the molecules through the membrane. There are many different types of proteins that are used for this process. They are all selective to a degree.
These proteins allow specific molecules to go through the plasma membrane freely. If there is a malfunction it can cause different diseases, Cystic Fibrosis is caused by a faulty chloride channel.
These proteins selectively combine with molecules to help it cross the membrane. Carrier proteins are needed for both active transportation and facilitated transportation. A faulty glucose carrier can cause diabetes mellitus.
Receptors are in a shape that allows a specific molecule to bond with it. Bonding with a molecule can cause it to change shape. The reason pygmies are short is because their growth hormone receptors are faulty and the two cannot bond.
Enzymatic proteins cause a specific reaction. Cholera bacteria can release a toxin that interferes with the functioning of the enzyme that helps control the sodium content of the cell. If this happens the person could die from severe diarrhea.
Peripheral proteins are attached to cytoskeleton filaments so they play a role in helping stabilize the cell.
Animal cells have carbohydrate chains of cell recognition proteins. These chains give the cell a "sugar coat" called the glycocalyx. The glycocalyx protects the cell and acts as an adhesive between the cells, receives the signal molecules, and cell-to-cell recognition. These chains are very diverse and no two cells have the same chain. They vary by length, sequence of sugars and how they are branched. The chains are like a fingerprint and each cell has its own unique fingerprint. This uniqueness is what makes it hard to transplant tissues. The immune system is able to recognize the fact that the new cells have a different carbohydrate chain. In Humans, carbohydrate chains are the basis for A, B, and O blood types.
DIFFUSION AND OSMOSIS
Diffusion is the movement of molecules from a higher concentration to a lower concentration through a membrane. The molecules must follow their concentration gradient. Diffusion ends when there is equal concentration.
This process can works with any molecule just as long as the membrane will allow it to flow through
Osmosis is the diffusion of water through a selectively permeable membrane and it only works with water.
Osmosis Within the Cell
Isotonic Solutions- This is where the solute concentration and water concentration are the same inside and outside the cell, and so there is no gain or loss of water.
Hypotonic Solutions- These solutions are able to make the cell swell or even burst because of the amount taken in to the cell. The solution will have a lower amount of solute then in the cell. When a cell is placed in the solution it will take in the water. Any solution with a salt concentration lower than 0.9 percent is considered hypotonic.
Hypertonic solutions- These solutions cause the cell to shrink. If the cell is placed in a solution with a salt concentration more more 0.9percent with allow its water to leave its self.
Facilitated transport is the transportation of molecules like glucose and amino acids that aren't lipid soluble. These carrier proteins are specific and because of that glucose is able to transport faster then the other sugars. Once the glucose has bonded to the open end of the protein that open end will close, as the glucose moves along, the closed part of the protein opens in front of the glucose allowing the protein to continue moving forward. When the glucose has excited the protein the process will start over again. This process can happen up to as many as 100 times per second. Facilitated transport doesn't require any energy usage.
Active transportation requires both carrier proteins and energy. Proteins used in this action are called pumps because they use energy to help them move molecules against their gradient just like a water pump does with water. Molecules move through the protein the same as in facilitation transport but as often or as fast.
EXOCYTOSIS AND ENDOCYTON
Both are types of active transportation because they require the use of energy. These processes transport macromolecules like polypeptides, polysaccharides and polynucleotides that are too big for carrier proteins in and out of the cell. The macromolecules are transported by vesicle formations so they don't mix with the ones in the cytoplasm.
Vesicles that have been produced by the golgi apparatus and contain proteins. The Vesicles will then encompass the macromolecules. Then fuse with the plasma membrane. Exocytosis occurs during cell growth so the membrane isn't too thick when the vesicle wall combines with it. The proteins then adhere to the cell surface, combine in the extracellular matrix or diffuse into the tissue fluid where it can nourish and signal other cells. Molecules transported by exocytosis include digestive enzymes, like ones from the pancreatic cells, and growth hormones. The Vesicle only releases the molecules after it has received a signal at the plasma membrane. For example, there could be a rise in blood sugar so the vesicles would release insulin. Ecocytosis only occurs when the cell needs it too.
Endocytosis is like Exocytosis except its taking macromolecules into the cell. A portion of the plasma membrane inverts to envelope the molecules the closes the open end off and creates a n intracellular vesicle. There are 3 ways for endocytosis to occur.
When a cell takes in a large particle like food or another cell, the cell would have used phagocytosis. This is the process unicellular organisms use but it is used by the human red blood cells also. They are able to ingest old red blood cells and bacteria. When the vesicle attaches to a lysosome, digestion occurs. Phagocytosis is necessary to the development of immunity towards bacteria. You are able to observe phagocytosis with a light microscope.
When a vesicle forms around a liquid or small particle its called pinocytosis. Red blood cells within the kidney tubules and intestinal wall use pinocytosis to envelope substances. Pinocytosis vesicles can only be seen with an electron microscope, they are no bigger then 0.1-0.2 um. But they still use a lot of the plasma membrane because it occurs continuously. The loss of plasma membrane is balanced by the return of vesicles through exocytosis.
This process id a form of pinosytosis that uses receptor proteins shaped in a specific way so a vitamin, peptide hormone or lipoprotein can bind to it. These receptors are found at a single location on the membrane. This area is called the coated pit because of the layer of proteins on the cytoplasmic side. When the vesicle is formed it is uncoated and is able to fuse with a lysosome. When the vesicle fuses with them membrane the receptors will return to their original location.
This process is selective and more efficient then regular pinocytosis. It is involved in the transfer between cells. These exchanges occur between maternal and fetal blood at the placenta.