Lecture 5 Cell Membrane Transport

1
Lecture 5 Cell Membrane Transport
2
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

PLAY
Membrane Transport Fat- and Water-Soluble
Molecules
3
Restricted Materials

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

4
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

PLAY
Membrane Transport Diffusion
5
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

6
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

7
Diffusion vs. Osmosis
8
Effects of Osmosis on Cells
Tonicity how a solutions osmolarity affects
cell volume

• 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

9
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

10
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

11
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)

12
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

PLAY
Membrane Transport Facilitated Diffusion
13
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

PLAY
Active Transport
14
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

15
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

16
Proton Pump (in Mitochondrial Membranes)

• Expends metabolic energy to pump protons across
  membranes

PLAY
Proton Pump
17
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)

18
Vesicular Transport

• Also called bulk transport
• Transport of large particles and macromolecules
  across plasma membranes
• Directional 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 cells interior

19
Receptor-Mediated Endocytosis

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

20
Pinocytosis

• Pinocytosis (cell drinking)
• Endosomes drink extracellular fluid

21
Phagocytosis

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

22
Exocytosis

• Is the reverse of endocytosis

23
Summary

• The 7 methods of transport