Transport Across Cell Membranes Study Guide
Transport Across Cell Membranes
Chapter Overview
- Reference: Essential Cell Biology, Fifth Edition
- Copyright © 2019 W. W. Norton & Company
Principles of Transmembrane Transport
Channel Proteins
- Definition: Channel proteins form transmembrane pores that facilitate the passive movement of molecules and ions.
- Function: They allow small, water-soluble molecules and ions to move into or out of the cell or organelle.
- Characteristics:
- Discriminate based on size and charge.
- The solute must recognize a binding site within the channel.
Ion Channels and Membrane Potential
Properties of Ion Channels
- Definition: Ion channels are narrow, highly selective pores in the membrane.
- Selectivity Filter:
- The pore narrows to atomic dimensions, contributing to ion selectivity.
- Ions passing through must shed their water shell to interact with the polar groups lining the selectivity filter.
Gated Ion Channels
- Definition: Ion channels can be gated, meaning they change their conformation in response to specific stimuli.
- States of Gating: The channel can be either open or closed.
- Types of Stimuli: Different gated ion channels respond to various stimuli.
Membrane Potential
- Definition: Membrane potential is generated by differences in electrical charge across the membrane.
- Measurement: Animal cells typically range from -20 to -200 mV.
- Factors Influencing Membrane Potential:
- The state of the membrane’s ion channels.
- Ion concentrations on either side of the plasma membrane.
- At rest, the plasma membrane of animal cells is primarily permeable to K+ ions, which determine resting membrane potential.
Resting Membrane Potential
- Definition: The resting membrane potential is largely determined by the K+ electrochemical gradient.
- Stability: At rest, the K+ concentration gradient is balanced by the effect of the membrane potential, leading to no net movement of K+ ions across the membrane.
- Importance of Ions: Only a thin layer (less than 1 nm) of ions close to the membrane is needed to establish membrane potential.
- Example: Approximately 6,000 K+ ions crossing 1 μm² of membrane can shift the membrane potential by about 100 mV. There are about 70,000 K+ ions in 1 μm³ of cytosol, highlighting the minuscule fraction needed.
Experimental Techniques
Patch-Clamp Recording
- Definition: A patch-clamp technique is used to study ion channels and their currents.
Biological Applications
Biological Jet Propulsion System
- Example: Squid species, such as Loligo, utilize ion channels for movement.
Nerve Cell Excitability
- Study on an isolated axon from squid shows the role of K+ and Na+ ions in membrane potential.
- Modifying concentrations of these ions can affect resting membrane potential significantly.
Synapses
Definition
- A synapse is the connection between neurons to transmit signals.
Structure
- Components include:
- Presynaptic neuron (axon terminals)
- Postsynaptic neuron (dendrites)
- Synaptic cleft
- Synaptic vesicles
- Function: Convert electrical signals into chemical signals at nerve terminals.
- Process: The presynaptic nerve terminal converts electrical signals into secreted chemical signals upon activation.
Signal Transmission
Mechanism
- Postsynaptic transmitter-gated ion channels convert chemical signals back into electrical signals.
- Neurotransmitters can elicit excitatory or inhibitory responses, influencing neuronal communication.
Neurotransmitters
Effects on Synaptic Signaling
- Most psychoactive drugs impact synaptic signaling by binding to neurotransmitter receptors, illustrating the complexity of neurotransmitter action in the nervous system.
Ion Channels Summary Table
| Ion Channel | Location | Function |
|---|
| K+ leak channel | Plasma membrane of most animal cells | Maintenance of resting membrane potential |
| Voltage-gated Na+ channel | Plasma membrane of nerve cell axon | Generation of action potentials |
| Voltage-gated K+ channel | Plasma membrane of nerve cell axon | Return of membrane to resting potential after action potential |
| Voltage-gated Ca²+ channel | Plasma membrane of nerve terminal | Stimulation of neurotransmitter release |
| Acetylcholine receptor | Plasma membrane of muscle cell (neuromuscular junction) | Excitatory synaptic signaling |
| Glutamate receptor | Plasma membrane of many neurons (at synapses) | Excitatory synaptic signaling |
| GABA receptor | Plasma membrane of many neurons (at synapses) | Inhibitory synaptic signaling |
| Glycine receptor | Plasma membrane of many neurons (at synapses) | Inhibitory synaptic signaling |
| Mechanically-gated channel | Auditory hair cell in the inner ear | Detection of sound vibrations |
Special Case: Auditory Hair Cells
Function
- Respond to sound vibrations by causing the stereocilia to tilt, opening mechanically-gated ion channels and allowing the influx of cations.
Example: Venus Flytrap
- Mechanical stimulation of trigger hairs leads to the opening of ion channels, affecting electrical signaling and turgor pressure to close the leaf.
Light-Gated Ion Channels
Channelrhodopsin Example
- Found in photosynthetic green algae, used for sensing and navigation towards light.
- Activation: Upon exposure to blue light, channelrhodopsin allows Na+ to flow into the cell, depolarizing the membrane and affecting movement.
Conclusion
- Understanding ion channels is crucial for grasping neuronal function and signaling, impacting broader biological functions and potential therapeutic targets in neuroscience.