The structure of biological membrane is known as fluid mosaic, the walls are made of phospholipid bilayer . The polar heads (phosphate groups) are hydrophilic and streams are at the ends, while the polar tails (fatty acids) hydrophobic are inside.
The fluidity is given by the lipid composition (length of chain fatty acids, saturation, polarity given by the phosphate groups) and the temperature.
A number between 0 and 20% is made up of cholesterol that prevents stiffening of the membrane at low temperatures and become too fluid to high temperatures.
The average density of protein is 1 to 25 phospholipid molecules and are divided into two types:
- the whole protein penetrate through the wall of phospholipid, those that cross completely are called transmembrane proteins.
- the peripheral proteins that are not immersed in the bilayer.
Some proteins are not free to move and are anchored by the cytoskeleton or lipid floating semi-fluid.
Phospholipids are synthesized in the endoplasmic reticulum proteins in smooth and rough endoplasmic reticulum.
Carbohydrates are located at the outer ends of the transmembrane protein and have the function of recognition sites and signaling. and glycolipids are glycoproteins.
membranes allow only be crossed by some substances, selective permeability. There are two ways for the transport of substances passive, requiring no additional energy, and active , requires ATP.
The case for passive diffusion that can be easily or facilitated.
Diffusion is the process by which a solution reaches an equilibrium state where there is a net movement of particles between a high concentration to a low concentration,
The diffusion rate is a function of diameter of the molecules, the temperature of the solution, the electric charge and concentration gradient is the change in concentration between different points. One solution may be isotonic , hypotonic or hypertonic of the comparison of solute relative to another.
more a molecule is soluble, therefore hydrophobic, so it penetrates more easily through the membrane.
If the substance is not soluble it passes through the membrane by the concentration gradient through specific proteins (facilitated diffusion). There
channel proteins, such as ion channels that allow the selective passage of one type of ion, and carrier proteins that help the transition of complex molecules.
Ion channels are central to every activity of the nervous system and can be adjusted (ie variable-access) from messengers intra / extracellular changes in electrical or mechanical stimuli.
The transport of substances against the concentration gradient occurs through carrier proteins that require ATP and each is specialized for one type of substance. They are divided into Uniport (one molecule each way), simporto (more molecules each way) and Antiporto (more molecules different directions). A carrier is very important Antiporto sodium-potassium pump, which puts two K + ions in and out 3 Na + ions with the result that the concentration of these is very different between the inside and outside the cell.
L ' endocytosis is the way that the cell has to introduce large molecules el' Exocytosis is the reverse process. Endocytosis can
be phagocytosis (ingestion of solids), pinocytosis (fluid intake) and receptor-mediated endocytosis or uptake.
Some proteins (eg clathrin) coat the exterior portions of specialized plasma membrane called coated pits the pit invaginates and forms a vesicle .
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