BIOLOGICAL MEMBRANES

(Chapter 3, p 26-33)

We cannot live without them!

• Membrane are barriers which give cells their outer boundaries (plasma membrane) and their inner compartments (organelles).

• Controls the movement of molecules in and out of the cell.

• They also control the flow of information between cells.

• They are involved in capturing and release of energy, for example photosynthesis takes place on membranes.

Three Membrane Components• Biological membranes are made of 3 major components: lipids, proteins

and sugars.

• General structure shows a two-layered sheets of lipid molecules having proteins embedded in them.

• Structure is highly fluid and most of the lipid and protein molecules can move about in the plane of the membrane.

• Lipid and protein molecules are held together mainly by non covalent interactions. Sugars are attached by covalent interactions to some lipid and protein molecules.

Components-Lipids• Lipids are the most abundant type of

macromolecule present in membranes (60-80%).

• Lipids provide both basic structure and the framework of the membrane.

• Three types of lipids present: phospholipids, cholesterol and glycolipids.

• PHOSPHOLIPIDS: compounds that are amphiphatic i.e. having both hydrophilic and hydrophobic.

• Some examples: phosphotidylcholine, phosphotidylserine .

• The polar heads of phospholipids extend outward and the fatty acid tails extend inward.

• The fatty acid chains normally undergo motions such as flexion (bending), rotation, transverse and lateral movement.

Components-Lipids• CHOLESTEROL: It also is an

amphipathic, containing polar hydroxyl head as well as hydrophobic steroid ring attached hydrocarbon.

• It is present between phospholipids.

• Cholesterol decreasing movement of lipids thus increasing stability of the membrane.

• GLYCOLIPIDS: Lipids attached to carbohydrates- glycolipids are found in low concentration in the membrane.

• The sugar portion of glycolipids is oriented towards the outside of the cell

• They are mainly involved in cell-to-cell interactions.

• Proteins are largely responsible for many biological functions of the membrane.

• Type of proteins within the plasma membrane vary slightly depending on the cell type.

• All proteins are associated with membrane as: transmembrane protein, lipid-anchored protein or peripheral membrane protein.

Components- Proteins

transmembrane

• TRANSMEMBRANE PROTEINS: They are embedded within the lipid bilayer of the membrane with the structures that extend from the environment to the cytosol.

• Some contain one transmembrane region while others contain several.

• All transmembrane proteins are amphipathic in nature.

• These proteins are oriented with their hydrophilic portions in contact with the extracellular fluid or the cytosol, whereas the hydrophobic portions in contact with the fatty acid tails of the phospholipids.

• Proteins cross cellular membrane by adopting a structure containing one or more α helices.

Components- Proteins

• LIPID-ANCHORED PROTEINS: They are attached covalently to a portion of a lipid of the bilayer of the membrane.

• Both transmembrane and lipid-anchored proteins are integral membrane proteins since they can only be removed by disruption of the membrane.

• PERIPHERAL MEMBRANE PROTEINS: They are located on both the sides of the membrane and are only indirectly attached to the lipid of the membrane.

• They bind to the other proteins that are attached to the lipids.

Components- Proteins

Membrane Protein Functions• They enable cells to function as members of a tissue. For Example,

they enable cell-to-cell contact.• Other membrane proteins function as ion channels and transport

proteins to enable molecules to enter and exit the cell.

• Some as ligand receptors enabling the cell to respond to hormones and other signalling molecules.

• Peripheral proteins like cytoskeletal proteins that attach to the membrane and regulate its shape and stabilize its structure.

• A lipid bilayer, also known as the phospholipid bilayer, is a sheet of lipids two molecules thick, arranged so that the hydrophilic phosphate heads point “out” to the water on either side of the bilayer and the hydrophobic tails point “in” to the core of the bilayer.

• This arrangement results in two “leaflets” that are each a single molecular layer.

Bilayer Arrangement

• Lipids self-assemble into this structure because of the hydrophobic effect, which creates an energetically unfavorable interaction between the hydrophobic lipid tails and the surrounding water.

Fluid Mosaic Model• The fluid mosaic model was given by S.J. Singer and G.L. Nicolson

(1972).

• According to it biological membranes can be considered as a two-dimensional liquid in which lipid and protein molecules diffuse more or less easily.

• Amphipathic lipids form a lipid bilayer, with hydrophobic regions of lipidsfacing each other at the core of the bilayer and their hydrophilic regions facing outward.

• In the lipid bilayer some proteins are embedded, some protrude from one or the other face of membrane while some span its entire width.

• The orientation of proteins is asymmetric.• The individual lipid and protein subunits in a membrane form a fluid

mosaic; its pattern is free to change constantly because of interactions between lipids and proteins are weak bonds.

• The model explains the structure of membrane as,” icebergs of proteins in sea of lipids.”

Membrane Asymmetry• Membrane lipids are asymmetric in

their distribution on the two leaflets of the bilayer i.e. certain lipids are typically found in the outer face of the bilayer and others in the inner (cytoplasmic) face.

• Similarly each membrane protein has specific orientation in the bilayer.

• Glycoproteins are invariably situated with their sugar residues on the outer surface and on the inner face.

•

Fluid Mosaic Model

Summary• Membrane are barriers which give cells their boundaries.• Biological membranes are made of 3 major components: lipids, proteins and

sugars.• Three types of lipids present: phospholipids, cholesterol and glycolipids.• All membrane proteins are associated with membrane as: transmembrane

protein, lipid-anchored protein or peripheral membrane protein.• Amphipathic lipids form a lipid bilayer, with hydrophobic regions of lipids

facing each other at the core of the bilayer and their hydrophilic regions facing outward.

• The individual lipid and protein subunits in a membrane form a fluid mosaic; its pattern is free to change constantly because of interactions between lipids and proteins are weak bonds leaving them some degree of freedom to move in the plane of the membrane.