Biological membrane

Biological Membrane

A biological membrane is a dividing layer that can either seperate Oranells from the Cytoplasm (intracellular) or a cell from extracellular matrix/ other cells (intercellular). It is not just a divider, it also actively participates in the exchange of information and transport of molecules.

Structure

Biomembranes consist of a phospholipid bilayer and different embedded Proteins. The Phospholipidbilayer is made up out of two amphophil phsopholipids. While the tails, which are hydrophobic ( or lipophil) face each other the hydrophilic heads build the outer parts of the membrane. Proteins can be linked to carbohydrate chains and are mainly responsible for the activity of the membrane. They are not bound to certain parts of the cells but float around freely and so keep changing the face of the membrane. Biomembranes can be characterized from their density which lies between 1,12 and 1,22 g.cm-³. It reprensents the relation of the weight between lipids and proteins. The higher the density, the higher is the part reprensenting proteins.

Membrane Proteins

Different kinds of membrane proteins are responsible for the different traits of each biomembrane. For exmaple glycoproteins which often function as receptors and are very important in the immune response. Even the membranes which are facing outward (e.g. extracellular matrix) and those facing inward (e.g. cytoplasm) can differ very much in their functions although they are part of the same membrane. Generally we can divide the proteins in three different types : Intergral proteins, periphery proteins and lipid bound proteins. Integral proteins are mainly responsible for transport of molecules. As the phospholipids they are also amphipathic, their aminoacid tail interacts with the lipidchains while the hydrophilic heads interacts with the watery surface. Periphery Proteins are outside of the membrane. They are linked to the peripheral proteins or lipids on the membrane but do not reach the hydrophobic core. Lipidbound proteins are also on the outside of the membrane but are kovalently linked with a lipid embedded in the membrane. Most of them are GPI-anchored. The main functions of membrane proteins are transport of nutrients, extracellular signal reception and also the selective transport of ions and other molecules.

Functions

- compartmentation ( divides cells from extracellular matrix and organells from cytoplasm, so that these can maintain their own specific environment)

- transport ( through the selective permeability they only allow certain substances to gothrough the membrane )

- receptors ( the biomembrane are important receptors for external signals and also redirects these signals inside so they arrive where they are needed)

In general one can say that the biomembrane plays an important role in protect the cell or organells and is also an important place for the exchange of molecules.

http://www.uta.edu/biology/wilk/classnotes/cellphys/biomembranes.pdf Junqueiras Basic Histology 12th edition Lecture by Mgr. Eva Filová, Ph.D. about Biophysics of Cell and Tissue http://en.wikipedia.org/wiki/Biological_membrane http://web.mit.edu/semrau/www/Homepage_Stefan_Semrau/Research_files/semrau2009b.pdf