Biological membranes - functions, structure

 Biological membranes

....Protein-lipid membranes that surround the cell and some organelles....

Source

.....................................Biological membranes' functions:...............................................

↳ substance transport
↳ protection of cells and organelles against mechanical damage, microbes, changes in chemical composition
↳ receiving stimuli from the environment and transmitting inside the cell and organelles

...................................Which organelles have membranes?............................................

Organelles surrounded by two membranes:
cell nucleus
mitochondrion
chloroplasts
Organelles surrounded by one membrane:

smooth endoplasmic reticulum
rough endoplasmic reticulum
Golgi apparatus
lysosomes
vacuoles
Organelles without membrane:

ribosomes
centrosomes

..................................................Structure........................................................................

Source

Division of proteins due to the way they are associated with the bilayer

• Integral proteins transmembrane - it pierce the lipid bilayer on both sides, eg. Alpha-Helix protein (in the illustration), are associated to the membrane by strong hydrophobic interactions
• Other integral proteins - it penetrate only into one lipid monolayer, eg. peripherial protein (in the illustration), are associated to the membrane by strong hydrophobic interactions
• Surface proteins - there are on the surface of the membrane, are associated with the membrane by electrostatic interactions and hydrogen bonds

Division of proteins due to perform functions

• Transport protein - allow the transport of substances between the cell and the environment, and between different compartments of the cell
• Anchor protein - strengthens the cell membrane
• Receptor protein - receives signals from outside
• Enzyme protein - accelerates some processes in the cell

In animal cells' membrane occurs cholesterol, which builds up into hydrophobic fatty acid residues and stiffens the membrane. In plant cells' membrane can be found different phytosterols, but most often sitosterol. In contrast, in fungal cells' membrane occurs ergosterol.

..................................................Properties...............................................

SEMI-FLUID

Phospholipid molecules move within their individual monolayers, although sometimes exchange takes place between both monolayers. This phenomenon is called flip-flop.

Lipids with short and unsaturated hydrocarbon chains increase membrane fluidity, while long and saturated chain lipids do the opposite.

ASYMMETRY

Each of the monolayers has its own composition of lipids and proteins that build it. For example, the animal cell membrane is rich in glycolipids and glycoproteins, whose sugar chains form a sugar coat (glycocalyx), which has a protective function.

SEMIPERMEABLE

Only certain substances can penetrate it (polar molecules more difficult pass through the membrane than nonpolar ones).There is a relationship between permeability and fluidity of membranes. The more fluid the membrane, the greater its permeability.