APK 2105C Lecture Notes - Lecture 16: Active Transport, Electrochemical Gradient, Transmembrane Protein
23 May 2018
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Chapter 4, Lecture 2
Cell Membrane Transport
• Passive transport
o Simple diffusion
o Facilitated diffusion = transmembrane protein (protein carrier) is necessary for
molecule to move down its concentration gradient
▪ Protein carrier is sort of like a hinge—open to ECF or ICF
• Opens and closes at consistent rate
• Goes back to original conformation after it transports molecule into
cell
▪ Following conformational change, a molecule could be taken back across
the membrane
▪ Net flux depends on concentration gradient
▪ Glucose needs this type of diffusion
▪ Rate of facilitated diffusion
• Consider
o Transport rate of individual
carriers—some proteins are faster
than others and affect speed of net
flux
o Magnitude of concentration
gradient
o Number of carriers
• Leveling off due to saturation of carriers
• Carriers are working as hard as they can
• Increases in concentration gradient won’t help
o Diffusion through channels
▪ Acts as a door—either open or closed
▪ Multiple proteins form channel with pore through the middle
▪ Passive movement of ions through these channels—but still need
proteins to go through
▪ Examples
• Aquaporins = allow water to pass through
• Ion channels
• Active transport
o Pumps = transmembrane proteins that use energy to drive molecules in a
preferred direction
▪ Utilize energy to push something against concentration gradient
▪ Different from passive carriers
• Passive carriers
o Have equal affinity for molecule on either side of
membrane
o There is no difference in affinity
• Pumps
o Have greater affinity for a molecule on one side of the
membrane
o Want to move the molecule to one specific side of the
membrane
▪ Steady state = occurs when concentration gradient offsets affinity
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