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

Lecture 5 Cell Membrane Transport


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


Restricted Materials

Selective permeability restricts materials based on: Size Electrical charge Molecular shape Lipid solubility


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


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


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


Diffusion vs. Osmosis


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


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


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


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)


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


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


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


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


Expends metabolic energy to pump protons across membranes

Proton Pump (in Mitochondrial Membranes)

Proton PumpPLAYPLAY


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)


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


Receptor-Mediated Endocytosis

Receptors (glycoproteins called clathrin) bind target molecules (ligands) Coated vesicle (endosome) carries ligands and receptors into the cell


Pinocytosis

Pinocytosis (cell drinking) Endosomes “drink” extracellular fluid


Phagocytosis

Phagocytosis (cell eating) pseudopodia (psuedo = false, podia = feet) engulf large objects in phagosomes


Exocytosis

Is the reverse of endocytosis


Summary

The 7 methods of transport

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Biology 11 Human Biology, TTh 8;00-9:20 Dr. Telleen Lecture 5 Cell Membrane Transport.

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