nucleus

• Plasma membrane


  Encloses the contents of the entire cell
• Plasma membrane


  • Allows specialized activities to proceed without external interference
  • Enables cellular activities to be regulated independently of one another
• Scaffold for biochemical activities

  Plasma membrane provides the cell with an extensive framework or scaffolding within which components can be ordered for effective interaction
• Selectively permeable barrier


  • Plasma membrane promotes the selective movement of elements into and out of the cell
  • Provides the means of communication between the compartments they separate
• Transporting solutes


  • Plasma membrane contains the machinery that allows the transport of substances from one side of the membrane to another, often down a concentration gradient
  • Plasma membrane also allows endo- and exocytosis
• Responding to external signals

  Plasma membrane is involved in signal transduction by containing receptors that combine with specific, complementary ligands
• Intercellular interaction


  • Plasma membrane mediates the interaction between a cell and its neighbors
  • Cell-to-cell recognition and signaling
  • Cell adherence
  • Exchange of cellular materials and information
• Energy transduction


  • Membranes are involved in the transformation of one form of energy to another
  • Conversion of solar energy to chemical energy during photosynthesis with the aid of membrane-bound pigments
  • ATP production via the membrane-bound ATP synthases in chloroplasts and mitochondria
• Plasma membrane contains a lipid bilayer
• Fluid-mosaic model


  The accepted model of plasma membrane structure, proposed by Singer and Nicolson in 1972
• The fluid-mosaic model depicts the plasma membrane as a fluid structure with proteins penetrating the lipid bilayer
• The plasma membrane is called a 'fluid-mosaic' because it is a fluid structure with a mosaic of proteins embedded in it
• Phosphoglycerides


  • Phospholipids built on a glycerol backbone
  • Diacylglycerides (DAG) with an additional group linked to the phosphate, forming the polar head group (amphipathic)
  • Examples: Phosphatidylcholine (PC), Phosphatidylethanolamine (PE), Phosphatidylserine (PS), Phosphatidylinositol (PI)
• Sphingolipids


  • Less abundant class of membrane lipids
  • Consist of a ceramide (sphingosine linked to a fatty acid)
  • Have additional groups esterified to the terminal alcohol of the sphingosine moiety
  • Examples: Sphingomyelin, Glycolipids
• Cholesterol


  • In certain animals, it constitutes up to 50% of the plasma membrane lipid molecules
  • Absent from the plasma membrane of most plant and all bacterial cells
  • Orients with the small hydrophilic hydroxyl group toward the membrane surface, and the remainder of the molecule embedded in the lipid bilayer
  • Functions: Prevents water-soluble molecules from moving across the membrane, Reduces membrane fluidity
• Membrane carbohydrates


  • All face the extracellular matrix (ECM)
  • Functions: Mediation of cell-ECM interaction, Sorting of membrane proteins to different cellular compartments
  • Glycosylation: The addition of carbohydrate to another molecule
  • Glycoproteins: More than 90% of membrane carbohydrates
  • Glycolipids: The remaining membrane carbohydrate
• Blood types


  Determined by the carbohydrates of the glycolipids of the red blood cell plasma membrane
• Integral proteins


  • Transmembrane proteins that pass entirely through the lipid bilayer
  • Functions: Membrane receptors, Channels or transporters, Electron transfer
• Peripheral proteins


  • Located entirely outside the bilayer, either on the cytoplasmic or extracellular side
  • Functions: Anchor for integral membrane proteins, As enzymes, As factors that transmit transmembrane signals
• Lipid-anchored proteins


  • Located outside the bilayer, either on the cytoplasmic or extracellular side, covalently linked to a membrane-bound lipid molecule
  • Functions: Signal transduction, For proper protein function
• Passive transport of substances through membranes

  • Simple diffusion through the lipid bilayer
  • Simple diffusion through an aqueous, protein-lined channel
  • Facilitated diffusion
• Osmosis


  The movement of water readily through a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration
• Ion channels


  • Membrane openings that are permeable to specific ions
  • Highly selective, allowing only 1 particular type of ion to pass through
  • Gated, existing in either an open or closed conformation
  • Can move millions of ions per second
• Types of gated channels

  • Voltage-gated: Conformational state depends on the difference in ionic charge on the two sides of the membrane
  • Ligand-gated: Conformational state depends on the binding of a specific molecule (the ligand)
  • Mechano-gated: Conformational state depends on mechanical forces (e.g., stretch tension) applied to the membrane
• Facilitated diffusion


  • Downhill movement of a substance across membranes through a highly specific membrane-spanning protein called a facilitative transporter
  • Solute binds to the transporter on 1 side of the membrane, triggering a conformational change that enables the solute to be transported to the other side
  • Can move only hundreds to thousands of solute molecules per second across the membrane
• Active transport of substances through membranes

  • Movement of substances across membranes through selective integral proteins (called pumps), and against a concentration gradient
  • Energy-requiring
• Coupling active transport to ATP hydrolysis


  The Na/K-ATPase pump uses the energy released by ATP hydrolysis to transport sodium ions out of the cell and potassium ions into the cell against their concentration gradients
• Bulk transport


  Uses vesicles to transport substances across membranes
• Active Transport


  Movement of substances across membranes through selective integral proteins (called pumps), and against a concentration gradient
• Active Transport


  • E-requiring
• Active Transport of Substances Through Membranes

  1. Coupling Active Transport to ATP Hydrolysis

  2. Sodium ions bind to protein on inside of membrane

  3. ATP is hydrolyzed, phosphate transferred to protein

  4. Protein conformation changes, allowing sodium ions to be expelled

  5. Potassium ions bind to protein

  6. Phosphate group is lost, protein snaps back to original conformation, allowing potassium ions to diffuse into cell
• Bulk Transport of Substances Through Membranes (More details in Chapter4)
• Extracellular Matrix (ECM)


  An organized network of extracellular materials that is present beyond the immediate vicinity of the plasma membrane
• Extracellular Matrix (ECM)


  • Takes diverse forms in different tissues and organisms, but composed of similar biomolecules
• Basement membrane or basal lamina


  A continuous sheet 50-200 nm thick, located around nerve fibers, muscles, and fat cells, beneath the basal surface of epithelial tissues, and beneath the inner endothelial lining of blood vessels
• Basement membrane or basal lamina


  • Provides mechanical support for the attached cells
  • Generates cell signals
  • Serves as substratum for cell migration
  • Separates adjacent tissues within an organ
  • Acts as barrier to the passage of biomolecules
• ECM Proteins


  • Typically extended and fibrous
  • Capable of self-assembling into an interconnected 3D network
  • Functions as trail markers, scaffolds, girders, wire, and glue
• Collagen


  Fibrous glycoproteins present only in ECM throughout the animal kingdom, single most abundant protein in the human body, each type is restricted to particular locations within the body, but 2 or more types are often present in the same ECM, has high tensile strength, produced by fibroblasts in connective tissues
• Collagen


  • All collagen molecules are trimers consisting of 3 polypeptide chains, called α chains
  • Along at least part of their length, the 3 polypeptide chains of a collagen molecule are wound around each other to form a unique, rodlike triple helix
• Proteoglycans


  A protein-polysaccharide complex, has a core protein with covalently attached glycosaminoglycans (GAGs), each GAG has a repeating disaccharide, and sulfate and carboxyl groups → acidic/- charged, assemble into gigantic complexes thru linkage between its core and a hyaluronic acid molecule, a nonsulfated GAG