Post on 12-Jan-2016
Biology 11 Human Biology, TTh 8;00-9:20 Dr. Telleen
Lecture 5 Cell Membrane Transport
Biology 11 Human Biology, TTh 8;00-9:20 Dr. Telleen
Overcoming the Cell Barrier
The cell membrane is a barrier, but: Nutrients must get in Products and wastes must get out
Permeability determines what moves in and out of a cell
A membrane is: Impermeable if it lets nothing in or out Freely permeable if it lets anything pass Selectively permeable if it restricts movement
Cell membranes are selectively permeable: Allow some materials to move freely Restrict other materials
Membrane Transport: Fat- and Water-Soluble MoleculesPLAYPLAY
Biology 11 Human Biology, TTh 8;00-9:20 Dr. Telleen
Restricted Materials
Selective permeability restricts materials based on: Size Electrical charge Molecular shape Lipid solubility
Biology 11 Human Biology, TTh 8;00-9:20 Dr. Telleen
Diffusion in Solutions
All molecules are constantly in motion Molecules in solution move
randomly Random motion causes mixing
Concentration is the amount of solute in a solvent
Concentration gradient: More solute in one part of a solvent
than another
Solutes move down a concentration gradient: Molecules mix randomly Solute spreads through solvent Eliminates concentration gradient
Membrane Transport: DiffusionPLAYPLAY
Biology 11 Human Biology, TTh 8;00-9:20 Dr. Telleen
Factors Affecting Diffusion Rates
Distance the particle has to move Molecule size:
Smaller is faster
Temperature: More heat, faster motion
Gradient size: The difference between high and low concentration
Electrical forces: Opposites attract, like charges repel
Biology 11 Human Biology, TTh 8;00-9:20 Dr. Telleen
Osmosis
Osmosis is the movement of water across the cell membrane
• Osmotic Pressure is the force of a concentration gradient of water
• Equals the force (hydrostatic pressure) needed to block osmosis
Biology 11 Human Biology, TTh 8;00-9:20 Dr. Telleen
Diffusion vs. Osmosis
Biology 11 Human Biology, TTh 8;00-9:20 Dr. Telleen
Effects of Osmosis on Cells
Isotonic – solutions with the same solute concentration as that of the cytosol
Hypertonic – solutions having greater solute concentration than that of the cytosol; water leaves the cell causing crenation (shrinkage)
Hypotonic – solutions having lesser solute concentration than that of the cytosol; water enters the cell causing swelling and potential lysis
Tonicity – how a solution’s osmolarity affects cell volume
Biology 11 Human Biology, TTh 8;00-9:20 Dr. Telleen
Hydrostatic and Osmotic Pressure
Hydrostatic pressure = water pressure
Filtration is the passage of water and solutes through a membrane by hydrostatic pressure
Pressure gradient pushes solute-containing fluid from a higher-pressure area to a lower-pressure area
Osmotic pressure can create an important counter force against hydrostatic pressure
Biology 11 Human Biology, TTh 8;00-9:20 Dr. Telleen
KEY CONCEPT
Concentration gradients tend to even out In the absence of a membrane, diffusion eliminates
concentration gradients When different solute concentrations exist on either side of
a selectively permeable membrane, osmosis moves water through the membrane to equalize the concentration gradients
Biology 11 Human Biology, TTh 8;00-9:20 Dr. Telleen
Transport Through Cell Membranes
Transport through a cell membrane can be: Active (requiring energy and ATP) Passive (no energy required)
3 categories of transport Diffusion (passive) Carrier-mediated transport (passive or active) Vesicular transport (active)
Biology 11 Human Biology, TTh 8;00-9:20 Dr. Telleen
Diffusion and the Cell Membrane
Diffusion can be simple, channel, or carrier mediated Channel & carrier mediated diffusion is:
Specific: to size, charge, & interaction with the channel Subject to saturation: making the channels rate limiting
Membrane Transport: Facilitated DiffusionPLAYPLAY
Biology 11 Human Biology, TTh 8;00-9:20 Dr. Telleen
Active Transport
Active transport proteins: Move substrates against concentration gradient Require energy, such as ATP Ion pumps move ions (Na+, K+, Ca+, Mg2+) Na+-K+ Exchange Pump moves both of these ions at the
same time, each in the opposite direction (called antiport or countertransport)
Proton Pump uses photosynthesis or food energy to create a proton concentration gradient that then is used to manufacture ATP
Active TransportPLAYPLAY
Biology 11 Human Biology, TTh 8;00-9:20 Dr. Telleen
Sodium-Potassium Exchange Pump
Active transport, carrier mediated: 1 ATP moves 3 Na+ out 2 K+ in This creates an electrical potential across the membrane
Called the Transmembrane Potential
Biology 11 Human Biology, TTh 8;00-9:20 Dr. Telleen
Transmembrane Potential
Voltage across a membrane
Resting membrane potential – the point where K+ potential is balanced by the membrane potential Ranges from –20 to –200 mV
Results from Na+ and K+ concentration gradients across the membrane
Differential permeability of the plasma membrane to Na+ and K+
Steady state potential is maintained by active transport of ions
Biology 11 Human Biology, TTh 8;00-9:20 Dr. Telleen
Expends metabolic energy to pump protons across membranes
Proton Pump (in Mitochondrial Membranes)
Proton PumpPLAYPLAY
Biology 11 Human Biology, TTh 8;00-9:20 Dr. Telleen
Types of Active Transport
Primary active transport: hydrolysis of ATP phosphorylates the transport protein causing conformational change
Secondary active transport: use of an exchange pump (such as the Na+-K+ pump) indirectly to drive the transport of other solutes
Symport system – two substances move across a membrane in the same direction (also called cotransport)Antiport system – two substances move across a membrane in opposite directions (also called countertransport)
Biology 11 Human Biology, TTh 8;00-9:20 Dr. Telleen
Vesicular Transport
Also called bulk transport Transport of large particles and macromolecules across plasma
membranesDirectional Descriptive Terms Exocytosis – moves substance from the cell interior to the extracellular
space Endocytosis – enables large particles and macromolecules to enter the cell
Receptor-mediated Pinocytosis Phagocytosis
Functional Descriptive Terms Transcytosis – moving substances into, across, and then out of a cell Vesicular trafficking – moving substances from one area in the cell to
another Phagocytosis – pseudopods engulf solids and bring them into the cell’s
interior
Biology 11 Human Biology, TTh 8;00-9:20 Dr. Telleen
Receptor-Mediated Endocytosis
Receptors (glycoproteins called clathrin) bind target molecules (ligands) Coated vesicle (endosome) carries ligands and receptors into the cell
Biology 11 Human Biology, TTh 8;00-9:20 Dr. Telleen
Pinocytosis
Pinocytosis (cell drinking) Endosomes “drink” extracellular fluid
Biology 11 Human Biology, TTh 8;00-9:20 Dr. Telleen
Phagocytosis
Phagocytosis (cell eating) pseudopodia (psuedo = false, podia = feet) engulf large objects in phagosomes
Biology 11 Human Biology, TTh 8;00-9:20 Dr. Telleen
Exocytosis
Is the reverse of endocytosis
Biology 11 Human Biology, TTh 8;00-9:20 Dr. Telleen
Summary
The 7 methods of transport