Lecture 6: Membranes and Cell Transport Biological Membranes I. Fluid Mosaic Model A. Biological membranes are lipid bilayers with associated proteins 1. Characteristics a. Phospholipids form bilayers in water b. Phospholipids have two fatty acid chains linked to a glycerol molecule c. Hydrophilic portion of the molecule is the phosphate bonded to the glycerol d. Fatty acids make up the hydrophobic portion of the molecule e. Molecule is amphipathic B. Biological membranes are two-dimensional fluids 1. Phospholipids act as liquid crystals, and the molecules move laterally a. Fluidity is affected by temperature, among other factors 2. Molecules rarely move from one side of the membrane to the other 3. Various bonds may occur between hydrocarbon chains a. Saturated fats lack double bonds in the fatty acid chains, causing the membrane to be less fluid
C. Biological membranes fuse and form closed vesicles 1. Membrane fusion is due to the liquid crystalline state 2. When vesicles and another membrane fuse, their bilayers and lumens become continuous 3. Endocytosis and exocytosis are products of membrane fusion. D. Membrane proteins include integral and peripheral proteins 1. Integral proteins are firmly bound to the membrane, or are bound to other lipids that are part of the membrane 2. Transmembrane proteins span the entire bilayer a. Transmembrane proteins are amphipathic b. Most commonly alpha helix 3. Peripheral proteins are bound to the hydrophilic ends of the integral proteins F. Proteins are oriented asymmetrically across the bilayer 1. Proteins are produced in the ER, then pass to the Golgi, then pass via a vesicle to the plasma membrane a. Various chemical modifications occur along the way
G. Membrane proteins function in transport, information transfer, and as enzymes 1. Membrane proteins are formed by free ribosomes 2. Proteins are involved in transport of small molecules 3. Enzymes may be embedded in the membrane 4. Receptor proteins are embedded in the exterior surface of the membrane a. Signal molecules convert an extracellular signal into an intracellular signal via signal transduction 5. Membrane proteins can serve as identification tags functioning in cell-to-cell recognition; others form junctions between adjacent cells
II. Movement of materials through cell membranes: Cell membranes are selectively permeable A. Most biological membranes are permeable to small or lipid-soluble molecules 1. Water molecules may pass the lipid bilayer 2. Gases, small polar molecules, and large hydrophobic substances may also pass 3. Other molecules move through special channels, primarily through membrane transport proteins. **See Plant Transport Lecture for general discussion of types of transport B. Exocytosis and endocytosis: large particles are transported by vesicles or vacuoles 1. In exocytosis, the cell expels wastes or a secretory product a. Exocytosis also results in growth of the cell membrane
2. In endocytosis, a cell takes up materials a. Phagocytosis involves taking in solid materials b. Pinocytosis involves taking in liquid droplets
3. Receptor-mediated endocytosis involves the bonding of receptor proteins to the material to be engulfed a. Molecules bind to receptors, called ligands, concentrated in coated pits i. Coated pits are formed by a protein, clathrin b. Coated pits move inwards, via endocytosis, and are then termed coated vesicles c. Coated vesicle ultimately becomes an endosome when the coating is lost III. Cell Signaling A. Cell signaling involves six steps 1. Synthesis and release of the signaling molecules a. Signaling molecules may be neurotransmitters, hormones, or other types of molecules b. Signaling molecules are ligands (often called the first messengers), which specifically bind to receptors on the target cells c. Some ligand-receptor complexes bind to and activate specific integral membrane proteins: the G proteins 2. Transport to target cells 3. Reception of the information by target cells a. Transmembrane protein with a functional portion on the extracellular surface b. Three domains i. External domain for docking by the signaling molecule
ii. Domain extending through the plasma membrane iii. Domain resembling a tail that extends into the cytoplasm 4. Signal transduction a. Cells convert and amplify an extracellular signal into an intracellular one 5. Response by the cell a. The tail undergoes a conformational change i. Activates a sequence of proteins that initiate changes within the cytoplasm b. G proteins may pass the message to the second messenger, often cyclic AMP c. Second messengers typically activate protein kinases 6. Termination of signaling IV. Cell Junctions are specialized contacts between cells A. Anchoring junctions connect epithelial cells 1. Desmosomes a. Points of attachment between some animal cells b. Oppose mechanical stresses c. Composed of intermediate filaments which span the gap between two cells
2. Tight Junction a. Seal off intercellular spaces between some animal cells b. Seal cells tightly with protein links
3. Gap junctions a. Permit transfer of small molecules and ions b. Contain pores that connect cells c. Proteins form the pores d. Allow rapid chemical and electrical communication between cells e. May be controlled
4. Plasmodesmata a. Allow movement of certain molecules and ions between plant cells b. Plasma membranes are continuous through the plasmodesmata c. Molecules, but not organelles, pass through plasmodesmata