Cell Biology: Membrane Transport

• Alberts - Essential Cell Biology, 6th Edition, Chapter 12 (pages 405-410, 412-425) and Chapter 11 (pages 397-398) provide readings for the topics

Permeability of the Lipid Bilayer

• A protein-free, artificial lipid bilayer is impermeable to most water-soluble molecules
• Cell membranes have membrane transport proteins to transfer specific molecules
• This is known as facilitated transport

Movement across the Lipid Bilayer

• Permeable artificial lipid bilayers allow movement via simple diffusion
• Transport occurs from high to low solute concentration, following the concentration gradient
• Hydrophobic or non-polar molecules experience faster diffusion across lipid bilayers
• Impermeable membranes need membrane proteins for transport

Proteins in Membrane Transport

• Transmembrane transport proteins create a protein-lined path across the cell membrane
• They transport polar and charged molecules like ions, sugars, amino acids, and nucleotides
• Each transport protein is selective and transports a specific class of molecules
• Different cell membranes have different complements of transport proteins

Classes of Membrane Transport Proteins

• There are two main classes of membrane transport proteins: channels and transporters
• Channel selectivity relies on the size and electric charge of the solute
• Transport involves transient interactions
• There are no conformational changes during transport in channels
• Transport occurs through an open channel
• Transporter selectivity relies on the solute fitting into the binding site
• Transport involves specific binding of the solute
• A series of conformational changes for transport occur

Passive and Active Transport

• Solutes cross cell membranes by either passive or active transport
• Passive transport moves down the concentration gradient (high to low)
• Active transport requires energy to move against the concentration gradient

Electrochemical Gradient

• The electrochemical gradient is the concentration gradient plus the membrane potential and results from the electrical gradient
• It includes passive and active transport for solutes with a net charge
• The direction and width of a green arrow represents the net driving force

Electrochemical Gradient Forms

• In the additive form are different forms of membrane transport influencing the membrane
• In the work against each other form concentration gradient no membrane potential

Overview of Membrane Transport Proteins

• Channel proteins allow passive transport
• Transporter proteins includes passive and active transport
• Passive transport is via uniport
• Active transport is via gradient-driven pumps (symport and antiport) or ATP-driven pumps (P-type, V-type proton, and ABC transporters)

Channel Proteins

• Channel proteins have a hydrophilic pore across the membrane
• Most channel proteins are selective
• Ion channels transport a specific ion based on ion size and electric charge
• They mediate passive transport of solutes
• Solutes passing through a channel experience transient interactions with the channel wall which leads to selectivity
• Channels enable faster transport than transporters

Types of Ion Channels

• Ion channels are found in animals, plants, and microorganisms
• Non-gated ion channels are always open, for example K+ leak channels
• K+ moves out of the cell through these channels, playing a major role in generating the resting membrane potential
• Gated ion channels require a signal for channel opening, and transport specific ion(s)

Types of Gated Ion Channels and Signals

• Mechanically-gated channels: signal is mechanical stress
• Ligand-gated (extracellular ligand) channels: signal is a ligand with neurotransmitters
• Ligand-gated (intracellular ligand) channels: signal is a ligand with ions, nucleotides
• Voltage-gated channels: signal is a change in voltage across the membrane

Transporter Proteins Function

• A transporter binds a specific solute and goes through a conformational change to transport the solute across the membrane
• Rate of simple diffusion and passive transport for the channel,
• Rate of simple diffusion and passive transport via a transporter
• Y-axis scale is not comparable; channels have a faster rate

Transport Proteins

• Proteins involved in this type of transport include: channel proteins, transporter proteins and pumps
• Passive transport includes: K+ Leak Channel, GLUT Uniporter
• Active transport includes: Na+-glucose symporter, Na+-H+ exchanger, Na+-K+ pump, H+ pump

Uniport Transporters

• Uniport transporters transport one solute
• Passive transport moves down the electrochemical gradient and the direction of transport is reversible
• Example is the glucose transporter (GLUT Uniporter)
• It transports glucose down the concentration gradient and can work in either direction

Active Transport with Pumps

• Active transport moves solutes against the electrochemical gradient - needing energy
• Gradient-driven pumps use the movement of one solute down its gradient to power the movement of another solute against its gradient
• ATP-driven pumps (ATPases) use ATP hydrolysis and move a solute against its gradient
• Light-driven pumps (bacteria) uses light energy

Gradient-Driven Pumps

• Symport: transports two solutes moved in the same direction
• Antiport: transports two solutes moved in opposite directions
• Free energy is derived from movement of the first solute, transporting the second solute against its electrochemical gradient

Symport Example

• The Na+-glucose symporter is an example of symport
• The Na+ electrochemical gradient provides energy
• It moves glucose against its concentration gradient
• Random oscillations between conformations occur where reversible
• Conformational changes occur only after both sites are occupied
• Cooperative binding of Na+ and glucose or both sites are empty and both Na+ and glucose dissociate

Antiport Example

• The Na+-H+ exchanger is an example of antiport
• Cytosolic pH needs to be regulated for optimal enzyme function
• Excess H+ occurs in the cytosol and need to be removed as excess H+ from acid-forming reactions and leaks out of lysosome
• Transporters maintain cytosolic pH
• Na+ moves down its electrochemical gradient providing the energy to move H+ against its electrochemical gradient
• A drop in cytosolic pH leads to transporter activity which increases where H+ transported out

Sodium Electrochemical Function

• Sodium electrochemical gradients are maintained by symport and antiport
• sodium drives down its electrochemical gradient to provide energy
• It is enhanced by the continuous action of gradient-driven pumps to drive the actions
• Gradients are also created via the Na+-K+ pump (plasma membrane ATP-driven pump)

ATP - P Type Pumps

• ATP-driven pumps include P-type pumps which use ATP and are phosphorylated during the pumping cycle (transporting ions)
• Animal plasma membrane Na+-K+ pumps move Na+ and K+ against their electrochemical gradients
• 3 Na+ out and 2 K+ in
• Na+ gradient is used to transport nutrients into cells and maintain pH

Active Transport: V-type Proton Pumps

• Also includes ABC (ATP-Binding Cassette) transporter with 2 ATP to pump small molecules across the cell membrane
• V-Type pumps use ATP to pump H+ into organelles to acidify the lumen in lysosome, plant vacuole
• Related is F-Type ATP synthase, which its structurally related to V-type proton pump in that Structurally related to V-type proton pump; but opposite mode of action

Types of ATP - Proton Transport

• V-type pumps use ATP to transport H+ from cytosol to lumen, against the electrochemical gradient
• F-type ATP Synthase uses the H+ electrochemical gradient to produce ATP within the intermembrane space
• Reversible reactions depend on ATP concentration and the H+ electrochemical gradient

Critical Cellular Processes

• Transport proteins are involved in many critical cellular processes
• It can be observed in the transcellular transport of glucose by transporters
• As well as through the generation of membrane potentials

Transcellular Glucose Transport by Transporters

• Transcellular transport of glucose by transporters, including ATP and channel proteins

Intestinal Glucose Absorption

• Transporters work together to transfer glucose from the intestine to the bloodstream
• Glucose and sodium enter the apical portion of the cell. the cytosol
• On the Basolateral side, glucose is released to the blood stream

Epithelial Tight Junctions and Glucose

• The apical membrane uses Na+-glucose symporter
• The basolateral membrane uses GLUT2 uniporter, a passive transporter, and the Na+-K+ pump
• Tight junctions inintestinal epithelium restricts the movement of transport proteins

Establishing Membrane Potential

• Membrane potential relies on the channel proteins (passive transport)
• Transport also from active transport from atp proteins with establish membrane potential

Membrane Potential Differences

• Membrane potential results from an electrical charge difference on two sides of the membrane
• They can carry out used gradient-driven pumps to carry out active transport
• As well as assist with electrical signaling for cells
• They can be seen acting in nerves as well as carnivorous plants

Membrane Potential and Leak Channels

• K+ leak channels play a major role in membrane potential

Animal Cells

• Na+-K+ pump is responsible for ~10% of membrane potential by
• Maintaining:Na+ gradient (low cytosolic Na+) and K+ gradient (high cytosolic K+)
• Is electrogenic: meaning 3 Na+ out and 2 K+ ions are pumped into cell at -1 charge

Membrane Potential - Balance Result

• The result is generally that Cells generally balance electrical charges inside and outside of cell where
• K+ flows out → K+ leak channels
• Ions diffusing from [high] to [low]
• Na+-K+ pump for a Net: 1 (+) ion pumped out

Membrane Cells - Concentrations

• Creates a bit more (+) on outside (Na+, K+)
• a bit more (-) on inside (Cl- and fixed anions)
• Which help forms membrane potential
• equilibrium = resting membrane potential
• animal cells: vary from -20 mV to -200 mV
• a bit more (+) : outside (Na+, K+)
• a bit more (-) : inside (Cl- and fixed anions)

Plant Cell Potential

• Plants use plasma membrane with P-type pump
• It can create H+ and can electrochemical gradient
• H+ results in -120 to -160 mV
• As well as drive Active Transport of Driven Pumps via and Regulate the pH
• It can allow Electrical Signaling

Week 2 Objectives

• Understand the properties of cell membranes that control permeability
• Differentiate between active and passive transport
• Distinguish between the different classes of transport proteins
• Understand how a solute is transported by the different classes of transport proteins
• Predict the role of the electrochemical gradient on the transport of a solute by a transport protein
• Understand how a membrane potential is established