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Chapter3a

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Chapter3a Chapter3a Document Transcript

  • The Cellular Basis of Life: Cell Theory • A cell is the basic structural & functional unit Chapter 3: Cells: of living organisms The Living Units • The activity of an organism depends both the individual & collective activity of its cells • Biochemical activities of cells are dictated by the relative number of their specific subcellular structures • Continuity of life has a cellular basis 1 2 Cell Diversity The Composite Cell 3 4 The Plasma Membrane: Structure • Very thin, selectively permeable phospholipid bilayer • Membrane proteins: surface receptors, pumps, channels, cytoskeletal anchors • Membrane carbohydrates- Cell recognition, “ID tags” 5 6 1
  • Transport: The Plasma Membrane: Functions Protein channels and pumps transport specific substances into the cell • Defines the cell’s boundary • Selectively permeable membrane that Enzymatic Activity: transports substances in and out of the Some enzymes are at the membrane, supplying cell products where they are needed • Generates and maintains the resting membrane potential Signal Transduction: • Mediates cell-cell and cell-environment Some membrane proteins are receptors that interactions receive external signals and initiate cascades of chemical reactions 7 8 Intercellular Joining: Cell adhesion molecules (CAMS) provide binding Plasma Membrane Special Structures sites for cell-cell interactions Cell-Cell Recognition: Some glycoproteins serve as cellular “ID” tags that allow cells to recognize each other Attachment to the cytoskeleton and ECM: Some membrane proteins act as anchors for the cell, binding the cell to its environment and supporting the cytoskeleton 9 10 Special Structures: Tight Junctions Special Structures: Desmosomes • Interlocking junctional • Plaque & linker proteins proteins fused together to interdigitate like a zipper form a relatively • Allow for cell sheets to impermeable barrier around form cells • Also prevents cell sheets • Prevents molecules from subjected to mechanical passing between adjacent stress from tearing cells • Examples: Skin, heart • Example: Digestive tract muscle lining to prevent “seeping” 11 12 2
  • Special Structures: Gap Junctions Membrane Transport • Cells are surrounded by extracellular fluid/ • Communicating junction between cells interstitial fluid containing nutrients, amino acids, sugars, fatty acids, vitamins, etc. • Connexons are transmembrane proteins that form channels that span • Cells need to extract essential substances from across adjacent cells interstitial fluid in order to survive • Essential for excitable tissue that require rapid •Traffic across the plasma membrane is constant but communication in order to is also selective on what substances are introduced synchronize activities into the cell • Examples: Cardiac & smooth muscle 13 •Transport is either passive or active 14 Membrane Transport Diffusion • Passive Processes – Diffusion • Simple Diffusion • Facilitated Diffusion (channel & carrier-mediated) • Osmosis – Filtration • The tendency of molecules or ions to scatter evenly • Active Processes throughout the environment – Active Transport • Molecules and ions move down their concentration – Vesicular Transport gradients until equilibrium is reached 15 [High]  [Low] 16 Diffusion Simple Diffusion Molecules diffuse through the membrane if: 1)Lipid-soluble • Small, non-polar, lipid-soluble 2)Small substances diffuse directly 3)Assisted by a carrier molecule through the lipid bilayer from regions of high concentration Types of Diffusion: to low concentration 1)Simple Diffusion: Unassisted diffusion 2)Facilitated Diffusion: Assisted diffusion a)Channel-mediated • Examples: Oxygen, Carbon b)Carrier-mediated dioxide, fat-soluble vitamins 3) Osmosis: Diffusion of solvents (water) 17 18 3
  • Channel-Mediated Facilitated Diffusion Carrier-Mediated Facilitated Diffusion • Protein channels form • Large, polar, lipid- “tunnels” that allow small, lipid- insoluble molecules such insoluble molecules (water, as sugars and amino ions) into the cell acids bind to protein carriers and are “shuttled” across the membrane • Channels are selective, only allowing molecules of certain sizes and charges to pass • Carrier membrane through proteins are molecule- 19 specific 20 Osmosis Diffusion –vs- Osmosis • Specific to the movement of water (a solvent) through a membrane from regions of high water concentrations to regions of low water concentrations • Osmotic diffusion of water may be directly through the lipid bilayer or through aquaporins 21 22 Tonicity Filtration Hypertonic: Osmotic pressure outside the cell is higher, water • Movement of molecules leaves cell faster than it enters through membranes from regions of high hydrostatic pressure Isotonic: Osmotic pressure in and to regions of lower out of cell are equal, water enters hydrostatic pressure and leaves cell at same rate • Separation of solids from fluids Hypotonic: Osmotic pressure • Generally applies only inside cell the higher, water enters to capillary walls the cell faster than it leaves 23 24 4