CELLBIO Membranes

• An essential feature of every cell is the presence

  of membranes, structures that define the boundaries of the cell and any internal compartments such as the nucleus, mitochondria, and chloroplasts of eukaryotic cells.
• Roles of Biological Membranes.

  1. They define the boundaries of the cell and its organelles and act as permeability barriers.
  2. They serve as sites for specific biochemical functions, such as electron transport during mitochondrial respiration or protein processing and folding in the ER.
  3. Membranes also possess transport proteins that regulate the movement of substances into and out of the cell and its organelles.
  4. Membranes contain the protein molecules that act as receptors to detect extracellular signals.
  5. They provide mechanisms for cell-to-cell contact, adhesion, and communication.
• The interior of the cell must be physically separated from the surrounding environment, not only to keep desirable substances in the cell but also to keep undesirable substances out. Membrane serves this function well because the hydrophobic interior of the membrane’s phospholipid bilayer blocks the passage of polar molecules and ions and is thus an effective permeability barrier for these substances.
• The permeability barrier for the cells is the plasma (or cell) membrane, a membrane that surrounds the cell and regulates the passage of materials both into and out of the cells.
• Intracellular membranes serve to compartmentalize functions within eukaryotic cells.
• One key function of membrane proteins is to carry out and regulate the transport of substances into and out of cells and their organelles.
• Most substances needed by the cell are hydrophilic (polar or ionic) and require transport proteins that recognize and transport a specific molecule (for example, glucose) or a group of chemical species (for example, cations).
• Transporters

  Cells - to import glucose, amino acids, or other nutrients

  Nerve Cells - transmit electrical signals when Na and K ions are transported across the plasma membrane by specific ion channel proteins

  Muscle cells - move calcium ions across membrane via specific transporter for the phosphate ions needed for ATP synthesis

  Mitochondrion - for intermediates involved in aerobic respiration
• Aquaporin is a transporter for water that can rapidly transport water molecules through membranes of kidney cells to facilitate urine production.
• Cells receive information from their environment, usually in the form of electrical or chemical signals that contact the outer surface of the cell.
• Signal transduction is the process of transmitting signals from the outer surface of cells to the cell interior.
• Many chemical signal molecules bind to specific membrane proteins known as receptors on the outer surface of the plasma membrane.
• Many plant cells have a transmembrane receptor protein that detects the gaseous hormone ethylene and transmits a signal to the cell that can affect a variety of processes including seed germination, fruit ripening, and defense against pathogens.
• Membrane receptors allow cells to recognize, transmit, and respond to a variety of specific signals in nearly all types of cells.
• Membrane proteins mediate adhesion and communication between adjacent cells.
• Some membrane proteins in animal tissues form adhesive junctions.
• During embryonic development, specific cell-to-cell contacts are critical and, in animals, are often mediated by membrane proteins known as cadherins.
• Other membrane proteins form tight junctions, which form seals along surface of epithelial tissues that block the passage of fluids through spaces between cells.
• Membrane proteins are connected to the cytoskeleton, lending rigidity to tissues.
• Gap junction in animal cells and plasmodesmata in plant cells allow direct connections for the exchange of at least some cytosolic cellular components between cells.
• Fluid Mosaic Model - proposed in the 1970s, envisions a membrane as two fluid layers of phospholipids, with proteins localized within and on the bilayers.
• The fluid mosaic model is described first as a fluid because the lipids and proteins can easily move laterally in the membrane and also as a mosaic because of the presence of proteins within the membrane.
• Charles Ernest Overton in 1890s worked with cells of plant root hairs and observed that nonpolar, lipid-soluble substances penetrate readily into cells, whereas polar, water-soluble substances do not.
• Irving Langmuir dissolved purified phospholipids in benzene and layered samples of them onto a water surface. He reasoned that the phospholipids orient themselves on water such that their hydrophilic heads face the water and their hydrophobic tails protrude away from the water.
• Evert Gorter and Francois Grendel extracted the lipids from a known number of erythrocytes and found that the area of the lipid film on the water was about twice the estimated total surface area of the erythrocyte. They concluded that the erythrocyte plasma membrane consists of two layers of lipids.
• Hugh Davson and James Danielli suggested that proteins are present in membranes. Their model, a protein-lipid-protein “sandwich” was the first detailed representation of membrane organization.
• When membranes were stained with osmium, a heavy metal that binds to membranes, they appeared as pairs of parallel dark lines separated by a lightly stained central zone.
• J. David Robertson suggested that all cellular membranes share a common underlying structure, the unit membrane.
• Robertson suggested that the space between the two dark lines of the trilaminar pattern, which resists staining, comprised the hydrophobic region of the lipid molecules.
• S. Jonathan Singer and Garth Nicolson proposed the fluid mosaic model envisions a membrane as a mosaic of proteins embedded in or attached to a fluid lipid bilayer.
• Transmembrane segments anchor the protein to the membrane and hold it in proper alignment within the lipid bilayer.
• Bacteriorhodpsin is the first membrane protein shown to possess transmembrane proteins. It is a plasma membrane protein found in archaea of the genus Halobacterium, where its presence allows cells to obtain energy directly from sunlight.
• The main classes of membrane lipids are phospholipids, glycolipids, and sterols.
• Phospholipids are the most abundant lipids which consist of a backbone moiety to which are attached two fatty acids, a negatively charged phosphate group, and a charged or polar head group that is attached to the phosphate.
• Amphipathic characteristic of phospholipid is due to the combination of a highly polar head and two non-polar tails.
• The backbone of most membrane phospholipids is either a glycerol, a three-carbon alcohol, or sphingosine, a derivative of the amino acid serine with a fatty acid attached.
• Phosphogylcerides - glycerol-based phosphoglycerolipids
• Phosphosphingolipids - sphingosine-based phosphoglycerolipids
• A common phosphosphingolipid is sphingomyeline, one of the main phospholipids of animal plasma membranes but is absent from the plasma membranes of plants and most bacteria.
• It is the amphipathic nature of phospholipids in the membrane bilayer that causes them to form a bilayer with a hydrophobic interior.