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6.4.1. Chemical composition of the membrane

Lipids

They form the structure of the lipid bilayer. They include:

  • PhospholipidsStructural function. They are the fundamental components of the plasma membrane. They provide great fluidity to the membrane. They are arranged in a bilayer. Due to their amphipathic nature, on both sides they present a hydrophilic zone, the polar heads (glycerin or glycerol in phosphoglycerides), and a hydrophobic zone (fatty acids), which form the apolar tail, which is oriented towards the interior of the layer.

Detalle de la membrana celular

By Jpablo cad (Own work) [CC BY 3.0], via Wikimedia Commons

Sección de la bicapa lipídica de la membrana plasmática

By Bensaccount [Public domain], via Wikimedia Commons

The lipids are not arranged in the same way in each of the two layers, so it has an asymmetrical arrangement .

As we have already said, the membrane is not a static structure, but the  phospholipids have a certain movement, which gives the membrane fluidity.

The movements that lipids can perform are:

  • Rotational: the phospholipid  rotates around its major axis. It is very frequent and largely responsible for the other two movements.
  • Lateral diffusion: it is the most frequent movement, the phospholipids can move laterally within the bilayer.
  • Flip-flopflipase enzymes allow one lipid molecule to move to the other monolayer. It is the least frequent movement, because it is very unfavorable energetically.

Difusión por flip flop

By A09231315 (Own work) [CC BY-SA 3.0], via Wikimedia Commons

The fluidity of the membrane by having these movements allows it to repair itself (if it breaks), to fuse with any other membrane, or by endocytosis, to form a vesicle from the membrane.

What does the fluidity of the plasma membrane depend on?

The main factors that determine the fluidity of the plasma membrane are:

  • Temperature: the higher the temperature, the greater the fluidity in the membrane.
  • Nature of lipids: the presence of unsaturated and short-chain fatty acids makes the membrane more fluid.
  • CholesterolThe presence of cholesterol reduces the mobility of phospholipids, thereby reducing the fluidity and permeability of the membrane.

Protein

They provide the membrane with its specific functions and are characteristic of each species. Like lipids, they also have lateral diffusion movements, which gives the membrane greater fluidity. Most proteins are globular in structure. There are two kinds:

  • Peripheral or extrinsic. They are found on one side or the other of the lipid bilayer, loosely linked by non-covalent bonds. They are soluble.
  • Integral or intrinsic. Embedded in the lipid bilayer, they cross the membrane one or more times, poking out on one or both sides (transmembrane proteins); or by means of covalent bonds with a lipid or a carbohydrate of the membrane. Isolating it requires the rupture of the bilayer.

Membrane proteins are involved in the exchange of substances .

Carbohydrates

Located on the outer side of the membrane, they intervene in cell recognition functions. They are attached to lipids or proteins forming glycolipids and glycoproteins.

Carbohydrates constitute the glucocalyx, a secretion membrane in animal cells made up of two layers: one, next to the plasma membrane with an amorphous texture, and the other, external, with a fibrous appearance and variable thickness.

Each face of the plasma membrane has a different chemical composition, which is why it is said that the membranes are asymmetric, being able to distinguish the inner and outer face of the membrane by its composition.

Fundamental ideas about the composition of the plasma membrane

Chemical composition of the plasma membrane :

  • Lipids:
    • Phospholipids.
      • Arranged in bilayer for its amphipathic character.
      • Fundamental component of the membrane.
    • Glycolipids.
      • Recognition of signals between cells.
      • They only appear on the outer side of the plasma membrane.
    • Sterols
      • It gives stability to the lipid bilayer.
  • Proteins:
    • Two types:
      • Peripheral or extrinsic.
      • Integral or intrinsic.
    • They are involved in the exchange of substances.
  • Carbohydrates.
    • On the outer side of the plasma membrane.