Cell Biology: Chapter 9- Channels and Carriers

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Integral proteins

• Embedded with in the membrane

• Portions of the protein associate with fatty acids

• Most are transmembranes (competley cross the membrane)

Peripheral Proteins

• Outside the membrane

• Non covalently attach to other proteins or lipids

• Covalently attach to lipids

More details on integral membrane proteins

• Most are glycoproteins (carb + protein)

• Removed with detergents (hard to remove)

Membrane Spanning Regions

• Composed of hydrophobic alpha helixes

Hydropathy

• Used to determine which amino acids are part of membrane spanning regions

• Hydrophobic amino acids have a value of +1

• Hydrophilic amino acids have a value of -1

Peripheral Membrane Proteins

• Non covalently attached to integral proteins or lipids

• May be removed with high salt

Lipid anchored proteins

• Protein covalently linked to lipid

• Extracellular: protein covalently links to GPI (attached to membrane)

  • Acetylcholinesterase

• Intracellular: protein covalently links to lipids such as isoprenyl

  • Ras: site for signal transduction

  • Rab: vesicular transport

Most transmembrane proteins are able to ___

Diffuse laterally

Dual fluorescence hybridoma

• Two cells are fused into a hybridoma

• Each cells proteins are labeled with a different color

• After some time, the different fluorescently labeled proteins have mixed together to prove that proteins can also diffuse laterally

Diffusion

Natural movement of molecules from high concentration to lower concentration until equilibrium is reached

Simple Diffusion

• Transports (mostly hydrophobic)

  • small polar molecules (H2O, glycerol)

  • small nonpolar moelcules (CO2)

  • large nonpolar molecules (steroids)

• Energy is diffusion is supplied by the molecules’ own kinetic energy

• Affected by size, lipid solubility, and concentration gradient

  • Smaller molecules diffuse faster than larger molecules

  • Hydrophobic molecules diffuse faster than hydrophilic ones because they can pass thru fatty acids easily

  • With concentration gradient

Facilitated diffusion

• Uses channels and carriers

  • Channels are mainly for ions

  • Carriers are mainly for larger molecules

• Will only let speicifc molecules through

  • Small polar molecules

  • Large polar molecules (glucose)

  • Ions (Na+, K+)

• Rate of this diffusion is greater than simple diffusion

Facilitated v Simple Diffusion (Graph)

• Facilitated diffusion (hyperbolic) happens at a quicker rate than simpel diffusion (linear)

• Faciliated diffusion can reach a plateau because the transporters can become saturated (they are all busy transporting something)

Channel protein structure

• Moves molecules through open passageway

• Alpha helizes interact and form a channel for small molecules

• Ex: aquaporin

  • Kidneys use aquaporins to bring in large amounts of water faster than simple diffusion

Ion Specificity

• The amino acisd surrounding the channels play a role in its selectivity and forms a selectivity filter

• Ex: In sodium potassium pump: the amino acids around the channel remove water molecules to make sure the ions can be brought in

Voltage gated channel

Regulated by changes in membrane potential

• When membranes depolarize, that changes the charge of the membrane and can activate/deactivate channels

Ligand gated channel

Neurotransmitter regulates channel

Covalently modified channel

• Epinephrine (neurotransmitter) binds to receptor on a cardiac muscle

• Receptor activates protein kinase

• Kinase phosphorylates Ca ++ channel (covalent modification)

• Phosphorylated channel opens more easily and increases Ca ++

Beta barrel

• A different type of membrane spanning region

• Found in mitochondria, chloroplasts, gram bacteria membranes

• Less selective

Carriers

• Bind to molecule and bring it across

• Alternate conformations

• Ex: GLUT1 is carrier

Types of carriers

• Uniport: transports only one molecule

• Symport: transports molecules in the same direction

• Antiport: transports molecules in different directions

Primary Active Transport

• Uses ATP to transport molecules against its concentration gradient

  • ATPases

• Moving uphill energetically

4 Types of active transport proteins

P-type: Transports cations; becomes phosphorylated

V-type: transport H+ into vacuoles

F-type: couples H+ transport to ATP synthesis

ABC-type: transports diverse cargo: ions, amino acids, polysaccharides

What is an example of an ATPase

• Sodium-Potassium Pump

• P-type (also a hydrolase)

• Antiport

• Pumps 2 K+ into cell, 3 Na+ out of cell

Secondary Active Transport

• AKA co-transport

• Transports molecule by coupling the movement of one molecule moving with its concentration gradient and the movement of one molecule moving against its concentration gradient

Na+ dependent cotransport

• Na+ binds to carrier protein

• Na+ moves with its concentration gradient

• The energy that produces is used to move glucose against its concentration gradient