Unit 4 Cell Membrane

Cell Membrane Structure

• Outer facing hydrophillic heads

• Inner-facing hydrophobic tails

• 4-5 nm thick

Transport across membrane

• Diffusion

• Endocytosis/exocytosis

• Membrane transport proteins

Diffusion

• Movement from high solute concentration to low solute concentration.

• Typically involves smaller polar/nnonpolar molecules.

Chemical Gradients

Uneven solute distribution.

Electrical Gradient

Uneven charge distribution

Cell Membrane Potential

• Formed from electrical gradient components.

• Differeneces in net charge across membrane

• measured in mV

Negative Membrane Potential

• Na+/K+ pump and other transporters force ions out of the cell.

• Push 3 Na+ out and 2 K+ in

• More - than + solute in cell.

0 mV Membrane Potential

Charged solutes distribute evenly.

-70 mV Membrane Potential

Charged solutes build up at membrane.

Dynamic Equilibrium

• Gradients remain relatively constant.

• Impacted by factors that impact transportation rate.

Resting Membrane Potential

• When dynamic equilibrium is reached

• Typically -70mV for most cells.

Active Transporters

• Pumps

• Use energy to force solute up electrochemical gradient.

• Conformational change

• Many are ATPases

• ex. Na+/K+ pump

Ca^2+Pump

Maintains low intracellular
concentration of Ca2+

Mg²⁺ Pump

Maintains high intracellular
concentration of Mg2+

V-ATPase

– Regulates intracellular pH
– Acidifies organelle lumens
• Ex. lysosome

Na+/K+ Pump

1) 3 Na+ bind to pump

2) ATP binds, Pump
hydrolyzes and attaches
phosphate to self
(autophosphorylation)

3) Pump releases Na+
outside cell

4) 2K+ bind pump

5) Pump
dephosphorylated

6) K+ released into cytosol

Passive Transporters

• Carriers

• Move solute down
  electrochemical gradient

• Conformational change

• No energy required

• Motion generated by movement of solute

• Fascilitated Diffusion

Gradient Driven Transporters

• Active Transporter

• Mix of active and
  passive

• Movement based on solute flow
  (passive)

• Use gradient energy to drive solute
  up gradient (active)

Symporters

Both solutes move in same electrochemical gradient across membrane.

Antiporters

Solutes move in opposite directions across membrane.

Passive Pores

• Channels

• Always passive

• Open/closed conformations

• Solute flow is much faster.

Specificity

• Nonselective cation
channels (NSCCs)
– Other specific to only one ion

Regulation

– Non-gated (always open)
– Gated (controlled by signal)
• Ligand-gated
• Voltage-gated
• Mechanically-gated

Selectivity Filters

• Control of solute flow by amino acids lining ion channel

• Hydrophillic ions, coated in hydration shell

• Hydration shell shed during transport and rebuilt on other side

Osmotic Pressure

• Uneven water concentrations across membranes

• Balancing takes time

• Can damage cell

Aquaporins

• Embedded in membrane

• Allow water to flow freely across membrane