cell bio

• Plasma membrane


  Encloses the cell, defines its boundaries, maintains the essential differences between the cytosol and the extracellular environment and mediates cell signaling

  
• Fluid mosaic model


  Describes the structure of the plasma membrane as a mosaic of components—including phospholipids, cholesterol, proteins, and carbohydrates—that gives the membrane a fluid character

  
• The thickness of the plasma membranes range from 5 to 10 nm
• Plasma membrane


  Controls passage of various molecules—including sugars, amino acids, ions, and water—into and out of the cell. How easily these molecules can cross the membrane depends on their size and polarity
• Plasma membrane


  Contains proteins that act as sensors of external signals: transfer information
• Specialized cells


  • Have modifications to increase the surface area. E.g. enterocytes (microvilli)
• Lipid bilayer


  Basic structure for all cell membranes. Bilayer structure is attributable exclusively to the special properties of the lipid molecules, which assemble spontaneously into bilayers (due to amphipathic characteristics of phospholipids)

  
• Phospholipids


  The most abundant membrane lipids. They are amphiphilic, having a hydrophilic ("water-loving") or polar head and a hydrophobic ("water-fearing") or nonpolar tail

  
• Lipid bilayer


  • Phospholipids spontaneously pack together to minimize exposure of their hydrophobic tails to water and maximize exposure of their hydrophilic heads to water. This provides a self-sealing property

  
• Lipid bilayer


  • It is a two-dimensional fluid. Individual molecules (both lipids and proteins) are free to rotate and move in lateral directions. Thermal conditions will impact movements. Flip-flop rarely occurs since it is energetically the most unfavorable type of movement

  
• Membrane fluidity


  Depends on its composition. Saturated fatty acids tend to form compacted bilayers (rigid), while the presence of unsaturated fatty acids increase fluidity
• Lipid bilayer


  • Asymmetry is functionally important. Lipid asymmetry is a common feature of all eukaryotic membranes. Two sides of the plasma membrane have different biophysical properties which influences numerous cellular functions

  
• Phosphatidylserine (PS)


  An amino phospholipid and the most abundant negatively charged phospholipid in eukaryotic membranes. PS is mostly located in the inner leaflet of the plasma membrane in normal cells. During apoptosis, PS is externalized to the outer leaflet and acts as an "eat me" signal
• Phosphoinositides (PtdIns)


  Signaling lipids derived from phosphatidylinositol in where one or more phosphates are added to the inositol ring. They are exclusively located at the cytosolic leaflet of the plasma membrane and intracellular compartments. Crucial roles in cell signaling, membrane identity and dynamics (intracellular trafficking)
• Phospholipases


  Enzymes activated by extracellular signals to cleave specific phospholipids to release short-lived intracellular messages (aka second messengers)
• Lipid rafts


  • Small and transient regions of the plasma membrane (microdomains) enriched in cholesterol and sphingolipids. They have roles in receptor signaling on the plasma membrane and are essential to signal transduction and spatial organization of the plasma membrane

  
• Sterols


  Contain a single polar hydroxyl group (head), steroid ring and a short nonpolar hydrocarbon chain (tail). They orient themselves in the bilayer with their hydroxyl group close to the polar head groups of adjacent phospholipid molecules. Cholesterol is the major sterol found in animal cells

  
• Glycolipids


  Have a mono- or oligosaccharide bound to a lipid moiety. They are exclusively found on the surface of all eukaryotic plasma membranes or inner leaflet of some intracellular compartments (away of the cytosol always). They play critical roles in cell recognition and help maintain the stability of the plasma membrane

  
• Gangliosidoses are a heterogeneous group of lysosomal storage diseases which are characterized by the intracellular accumulation of gangliosides in several tissues, mainly in neurons
• Membrane proteins


  Approximately 50% of the dry mass. Proteins are Amphiphilic. Perform most of the membrane functions. Amount and type of proteins highly vary among different types of cells: great contribution to cell specialization and functions

  
• Membrane proteins

  • Can be associated with the lipid bilayer in various ways: Transmembrane proteins, Intrinsic proteins, Peripheral proteins

  
• Detergent-based lysis


  The membranes are solubilized, lysing cells and liberating their contents. These methods are used to release proteins that are strongly associated to the lipid bilayer (integral and some intrinsic)
• Osmotic shock


  Often combined with mechanical disruption, relies on changing from high to low osmotic medium to burst the cell and release peripheral proteins
• Transmembrane proteins


  In most transmembrane proteins, the polypeptide chain crosses the lipid bilayer in an α-helical conformation
• Transmembrane β-barrels


  • β-barrel membrane proteins are always arranged as a cylinder
  • β-barrel outer membrane proteins are transmembrane proteins (OMPs) founded only in the outer membranes of Gram-negative bacteria, mitochondria and chloroplast
  • The nonpolar side chains of a folded β-barrel are oriented outwardly towards the lipid bilayer of the membrane, while the polar side chains are exposed inwardly towards the interior of the barrel that often forms a channel
  • The two main activities of the OMPs are to permit transport/insertion of proteins and diffusion of solutes across the outer membrane
• Glycoconjugates (lipids and proteins) play important roles in interaction of cells with their microenvironment: protection (pH, enzymes), signaling (electrical properties-Ca2+), cell adhesion (lectins)

  
• Many pathogens, including viruses and bacteria exploit glycoconjugates to get inside the cells or as part of their pathogenic mechanisms Eg. Influneza virus, Vibrio Cholerae (cholera toxin)
• Intracellular galectins (lectins) act as cytosolic receptors that sense cytosolically exposed glycans as danger and mediate cellular responses (sensors of membrane damage)
• • The glycocalyx is the carbohydrate-rich zone on the cell surface that protects the membrane from the harsh and dynamic microenvironment that could damage the lipids and proteins
  • Keeps various other cells at a distance, preventing unwanted cell–cell interactions
  • Is made of intrinsic glycoconjugates (proteins and lipids) as well as it contains glycoproteins and proteoglycans that have been secreted into the extracellular space and then adsorbed onto the cell surface

  
• Many membrane proteins diffuse in the plane of the membrane, however most cells restrict the movement and confine membrane proteins to specific regions in a continuous lipid bilayer. This segregation has many functional implications
• Four ways of immobilizing specific membrane proteins through protein–protein interactions

  • Forming large self-assemble protein aggregates
  • Tethering them to macromolecular assemblies on either side of the membrane
  • Interacting with proteins on the surface of another cell
  • The cortical cytoskeleton gives membranes mechanical strength and restricts membrane protein diffusion
• Three ways in which membrane-bending proteins shape membranes

  • A hydrophobic region of the protein can insert as a wedge into one monolayer to pry lipid head groups apart
  • The curved surface of the protein can bind to lipid head groups and deform the membrane or stabilize its curvature
  • A protein can bind to and cluster lipids that have large head groups and thereby bend the membrane

  
• The plasma membrane encloses the cell, defines its boundaries, maintains the essential differences between the cytosol and the extracellular environment and mediates cell signaling

  
• The cell membrane regulates the concentration of substances inside the cell, including ions such as Ca++, Na+, K+, and Cl–, nutrients including sugars, fatty acids, and amino acids, and waste products, particularly carbon dioxide (CO2), which must leave the cell
• Selective permeability


  The phospholipids are tightly packed together, and the membrane has a hydrophobic interior. This structure causes the membrane to be selectively permeable, allowing only substances meeting certain criteria to pass through it unaided

  
• Transporters


  Binding to the solute triggers conformational changes that expose solute-biding sites in both sites of the membrane. More continuous interaction with the solute
• Channels


  Switches between an open and close conformation. When open, forms a pore that acts like a bridge for the molecules to be transported. Transiently interact with the solute
• Transport via channels occurs at a much faster rate compared to transporters
• Passive transport


  Movement of substances across the membrane without the expenditure of cellular energy
• Active transport


  Movement of substances across the membrane using energy