Transport Across Membranes Flashcards
Simple Diffusion
- No membrane proteins involved
- Driven by concentration gradients
High concentration outside of the cell + low concentration inside
No energy input needed
Facilitated Diffusion
- Membrane proteins involved e.g. carrier or channel
- Driven by concentration gradients
High concentration outside of the cell + low concentration inside
No energy input needed
Concentration gradient in passive diffusion
Net movement is down the concentration gradient
What can diffuse via simple diffusion?
- Hydrophobic molecules e.g. O2, CO2, N2, benzene
- Small uncharged polar molecules e.g. H2O
- Large uncharged polar molecules e.g. glucose
Ions ,e.g. HCO3, Ca2+, does not
Channels in Facilitated Diffusion
- Membrane proteins that form hydrophilic pores (made of hydrophilic amino acids) through the plasma membrane
- Most (not all) are non-directional ion channels and/or gated
- Driven by electrochemical gradients
- Fast - up to 10^7 ions per second
Voltage-gated channels
Differences in voltage potential causes opening
Mechanically gated channels
Cytoskeleton cause physical opening of a channel
Ligand gated channels
Extracellular or intracellular binding that causes the channel to open
Carrier Proteins in Facilitated Diffusion
- Required for transport of almost all small organic molecules
- Highly selective since solute has to bind
- Relatively slow - less than 1000 molecules a second - due to time needed for binding
- Can be passive/with gradient or active/against gradient
Glut1 as an example of a carrier protein
(1) Glucose binds to protein that is open to the outside
(2) Transport protein shifts to alternative conformation
(3) Glucose is released to inside + protein returns to original conformation
How is gradient maintained?
- Conversion of glucose => glusose-6-phosphate
- Phosphorylation causes change in conformation so g6p won’t bind to Glut1
- Gradient is maintained + transport is unidirectional
