chapter3a
DESCRIPTION
Spring 2010 chapter 3a PowerPoint for BIOL2401 (Human Anatomy & Physiology) at San Antonio College with AlbaTRANSCRIPT
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Chapter 3: Cells: The Living Units
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The Cellular Basis of Life: Cell Theory
• A cell is the basic structural & functional unit of living organisms
• 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
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Cell Diversity
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The Composite Cell
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The Plasma Membrane: Structure
• Very thin, selectively permeable phospholipid bilayer
• Membrane proteins: surface receptors, pumps, channels, cytoskeletal anchors
• Membrane carbohydrates- Cell recognition, “ID tags”
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The Plasma Membrane: Functions
• Defines the cell’s boundary
• Selectively permeable membrane that transports substances in and out of the cell
• Generates and maintains the resting membrane potential
• Mediates cell-cell and cell-environment interactions
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Transport:Protein channels and pumps transport specific substances into the cell
Enzymatic Activity:Some enzymes are at the membrane, supplying products where they are needed
Signal Transduction:Some membrane proteins are receptors that receive external signals and initiate cascades of chemical reactions
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Intercellular Joining:Cell adhesion molecules (CAMS) provide binding 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
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Plasma Membrane Special Structures
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Special Structures: Tight Junctions
• Interlocking junctional proteins fused together to form a relatively impermeable barrier around cells
• Prevents molecules from passing between adjacent cells
• Example: Digestive tract lining to prevent “seeping”
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Special Structures: Desmosomes
• Plaque & linker proteins interdigitate like a zipper
• Allow for cell sheets to form
• Also prevents cell sheets subjected to mechanical stress from tearing
• Examples: Skin, heart muscle
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Special Structures: Gap Junctions
• Communicating junction between cells
• Connexons are transmembrane proteins that form channels that span across adjacent cells
• Essential for excitable tissue that require rapid communication in order to synchronize activities
• Examples: Cardiac & smooth muscle
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Membrane Transport• Cells are surrounded by extracellular fluid/ interstitial fluid containing nutrients, amino acids, sugars, fatty acids, vitamins, etc.
• Cells need to extract essential substances from interstitial fluid in order to survive
•Traffic across the plasma membrane is constant but is also selective on what substances are introduced into the cell
•Transport is either passive or active
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Membrane Transport
• Passive Processes– Diffusion
• Simple Diffusion• Facilitated Diffusion (channel & carrier-mediated)• Osmosis
– Filtration
• Active Processes– Active Transport– Vesicular Transport
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Diffusion
• The tendency of molecules or ions to scatter evenly throughout the environment
• Molecules and ions move down their concentration gradients until equilibrium is reached
[High] [Low]
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DiffusionMolecules diffuse through the membrane if:1)Lipid-soluble2)Small3)Assisted by a carrier molecule
Types of Diffusion:1)Simple Diffusion: Unassisted diffusion2)Facilitated Diffusion: Assisted diffusion
a)Channel-mediatedb)Carrier-mediated
3) Osmosis: Diffusion of solvents (water)
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Simple Diffusion
• Small, non-polar, lipid-soluble substances diffuse directly through the lipid bilayer from regions of high concentration to low concentration
• Examples: Oxygen, Carbon dioxide, fat-soluble vitamins
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Channel-Mediated Facilitated Diffusion
• Protein channels form “tunnels” that allow small, lipid-insoluble molecules (water, ions) into the cell
• Channels are selective, only allowing molecules of certain sizes and charges to pass through
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Carrier-Mediated Facilitated Diffusion
• Large, polar, lipid-insoluble molecules such as sugars and amino acids bind to protein carriers and are “shuttled” across the membrane
• Carrier membrane proteins are molecule-specific
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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
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Diffusion –vs- Osmosis
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Tonicity Hypertonic: Osmotic pressure outside the cell is higher, water leaves cell faster than it enters
Isotonic: Osmotic pressure in and out of cell are equal, water enters and leaves cell at same rate
Hypotonic: Osmotic pressure inside cell the higher, water enters the cell faster than it leaves
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Filtration
• Movement of molecules through membranes from regions of high hydrostatic pressure to regions of lower hydrostatic pressure
• Separation of solids from fluids
• Generally applies only to capillary walls