Biomembrane and Membrane Transport

S. Jonathan Singer and Garth Nicholson introduced the concept fluid mosaic model in 1972. According to this model, the lipids and integral proteins are disposed in a kind of mosaic arrangement. Both lipids and integral proteins are able to move within the layer. They are held together by weak interactions like ionic and hydrogen bonds.

They are membrane proteins present in plasma membrane. About 70% to 80% of membrane proteins are integral proteins. These are not easily extracted from membrane. Integral proteins are amphipathic. Their hydrophobic regions are buried in the lipid while the hydrophilic portions project from the membrane surface.

Collagen is the major insoluble fibrous protein in the extracellular matrix and in connective tissue. In fact, it is the single most abundant protein in the animal kingdom. Collagen, in the form of elongated fibrils, is mostly found in fibrous tissues such as tendon, ligament, and skin and is also abundant in cornea, cartilage, bone, blood vessels, the gut, and intervertebral disc. The fibroblast is the most common cell, which creates collagen.

Binding protein transport systems or ATP-binding cassette transporters are active in bacteria, archaea and eukaryotes. One of the major problems related with anti-cancer chemotherapy is the resistance against anti cancer drugs. The ATP-binding cassette transporters are a family of transporter proteins that are responsible for drug resistance and a low bioavailability of drugs by pumping a variety of drugs out of cells at the expense of ATP hydrolysis.

It is a form of active transport across a biomembrane in which the transporting molecule is altered in the course of the transport. This mechanism is utilised by bacteria to transport a compound into their cell by first allowing the compound to bind with protein on the cell surface, followed by altering its chemical structure during its passage across the membrane. The best known group translocation system is the phosphoenolpyruvate sugar phosphotransferase system. It transports a variety of sugars into prokaryotic cells while phosphorylating them using phosphoenolpyruvate as the phosphate donor.

The solution external to the cell is recognised as isotonic, hypotonic, or hypertonic. Solutions with lower solute concentration than the cytoplasm are called hypotonic. When a cell is placed in hypotonic solution, solvent enters into the cell by the process called endosmosis. When the animal cells like erythrocytes are placed in hypotonic solution, they swell rapidly and burst behind the ruptured membranes called ghosts. In plant cells, the rupturing is prevented by the rigid cell wall.

It simultaneously transports two substances across membrane in the same direction. It is also called co-transport. In this, one compound is being transported down the concentration gradient, the other against a gradient. It uses a common carrier mechanism to transport. Sodium and glucose transport is an example for symport. Here, the energy stored as proton gradient drives solute transport.

Endocytosis is the process of uptake of fluid or particles through the cell membrane and consequent formation of vacuoles. Pinocytosis is a type of endocytosis, which is similar to phagocytosis, but the substances ingested are in fluid form. It is also called cell drinking. The vacuole formed in pinocytosis is called pinosomes. It is noticed in amoeba. It requires energy.

The plasma membrane allows only lipid-soluble substances to pass through it. Water-soluble substances such as glucose, amino acids, ions and water can pass through the plasma membrane with the help of the various proteins. Amino acid permeases are integral membrane proteins involved in the transport of amino acids into the cell.

It is the main mechanism for the specific uptake of most macromolecules by eukaryotic cells. Depending on the cell type, mammalian cells take in hormones, growth factors, enzymes, serum proteins, antibodies, and even some viruses and bacterial toxins by this mechanism. The uptake of LDL occurs by receptor mediated endocytosis.

Cholesterol is an important constituent of cell membrane. It has a rigid ring system and a short branched hydrocarbon tail. It is largely hydrophobic. It is absent in prokaryotes. Cholesterol along with the phospholipids makes the membrane impermeable to water-soluble substances. It serves as a precursor for the biosynthesis of steroid hormones, bile acids, and vitamin D.

The sodium-potassium pump is the major electrogenic pump of the animal cells. The sodium pump is a type of active transport in which potassium ions are concentrated inside the cell must enter against a concentration gradient. The pump oscillates between two conformational states in a pumping cycle that transports three sodium ions out of the cell for every two potassium ions pumped into the cell. With each crank of the pump, there is a net transfer of one positive charge from the cytoplasm to the extracellular fluid, a process that stores energy in the form of voltage. The main electrogenic pump of plants, bacteria, and fungi is a proton pump, which actively transports hydrogen ions out of the cell.

The hydrocarbon tails of these two lipids result in steric limitations to their packing such that they will form disc-like micelles. The structure of these micelles results from the interaction of the hydrophobic tails of the lipids and the exposure of the polar head groups to the aqueous environment. Glycerophospholipids have a glycerol backbone. Phosphtidylinositol is an example for glycerophospholipids. Sphingolipids are derivatives of the lipid sphingosine. Cerebroside is an example for sphingolipids.