Cell Biology: Transporter Systems and Membrane Potential

Cell Biology: Transporter Systems, Part 2 (Lecture 5)

Review: Glucose Transporters and Absorption

• Food Processing Steps:

  1. Ingestion

  2. Digestion

  3. Absorption

  4. Transport

  5. Storage or Use

• Transporting Glucose Across Intestinal Epithelium: This is a key process for absorbed glucose to enter the bloodstream.

  • Basolateral Na$\text{Na}^{+}$/K$\text{K}^{+}$ Pump:

    • Uses ATP (primary active transport).

    • Generates an electrochemical gradient:

      • Low cytosolic Na$\text{Na}^{+}$ concentration (cytosolic $[\text{Na}^{+}]$ is low).

      • High cytosolic K$\text{K}^{+}$ concentration (cytosolic $[\text{K}^{+}]$ is high).

  • Apical Na$\text{Na}^{+}$/Glucose Cotransporter (SGLT):

    • Secondary active transport.

    • Moves both Na$\text{Na}^{+}$ and glucose into the cell, leveraging the Na$\text{Na}^{+}$ gradient created by the Na$\text{Na}^{+}$/K$\text{K}^{+}$ pump.

  • Basolateral GLUT2 Uniporter:

    • Facilitated diffusion (passive transport).

    • Allows glucose to diffuse from the cytosol into the extracellular fluid (and subsequently into the blood) down its concentration gradient.

Overview of Transport Proteins

A) Transporter Proteins (also known as Carriers)

• a) Passive Transport by Transporter Proteins: Movement down the electrochemical gradient without direct ATP hydrolysis.

  • I. Uniporter: Moves a single type of solute across the membrane (e.g., GLUT uniporter for glucose).

• b) Active Transport by Transporter Proteins: Requires energy to move solutes against their electrochemical gradient.

  • 1) Coupled Transporters (Secondary Active Transport): Use the energy stored in an ion gradient to move another solute.

    • I. Symporter: Moves two different solutes in the same direction across the membrane (e.g., Na$\text{Na}^{+}$/glucose symporter).

    • II. Antiporter: Moves two different solutes in opposite directions across the membrane (e.g., Na$\text{Na}^{+}$/H$\text{H}^{+}$ antiporter).

  • 2) ATP-driven Pumps (Transport ATPases - Primary Active Transport): Directly use the energy from ATP hydrolysis to pump solutes across the membrane.

    • I. P-type ATPase: Undergoes phosphorylation during the transport cycle.

      • Examples: Na$\text{Na}^{+}$/K$\text{K}^{+}$ pump, H$\text{H}^{+}$ pump (found in the plasma membrane), Ca$\text{Ca}^{2+}$ pump.

    • II. V-type ATPase: Pumps H$\text{H}^{+}$ ions; found in some organelles like lysosomes and vacuoles.

    • III. F-type ATPase: ATP Synthase; primarily functions in reverse to synthesize ATP using a H$\text{H}^{+}$ gradient (found in mitochondria and chloroplasts).

    • IV. ABC Transporters (ATP-Binding Cassette): A large superfamily with diverse functions, often involved in efflux of molecules.

      • Examples: MDR (Multi-Drug Resistance protein), CFTR (Cystic Fibrosis Transmembrane conductance Regulator).

B) Channel Proteins (Passive Transport)

• Form hydrophilic pores allowing specific ions or small molecules to pass rapidly down their electrochemical gradient.

• Movement of ions down their concentration gradient through their ion channel is a passive diffusion process.

Membrane Potential

• Definition: The difference in electrical charge across the two sides of a membrane.

• Physical Basis:

  • Excess positive ions predominantly outside the cell (mostly Na$\text{Na}^{+}$ and Ca$\text{Ca}^{2+}$).

  • Excess negative charges predominantly inside the cell (Cl$\text{Cl}^{-}$ and various phosphates and negatively charged proteins).

• Utility: Essential for various cellular processes:

  • Driving co-transporters (e.g., Na$\text{Na}^{+}$/glucose symporter).

  • ATP generation (e.g., in mitochondria).

  • Nerve electrical signals (action potentials).

Sources of Membrane Potential

1. Na$\text{Na}^{+}$/K$\text{K}^{+}$ Pump (Electrogenic Pump)

• This pump is a P-type ATPase that uses ATP to establish ion gradients.

• Pumps out $3$ Na$\text{Na}^{+}$ ions and pumps in $2$ K$\text{K}^{+}$ ions for each ATP hydrolyzed.

• Electrogenic Nature: Because more positive charges are pumped out than in (net export of $1$ positive charge per cycle), it directly contributes to the membrane potential by making the outside more positive relative to the inside.

2. K$\text{K}^{+}$ and Na$\text{Na}^{+}$ Concentration Gradients

• K$\text{K}^{+}$ Gradient: The Na$\text{Na}^{+}$/K$\text{K}^{+}$ pump maintains a high cytosolic $[\text{K}^{+}]$.

• Na$\text{Na}^{+}$ Gradient: The Na$\text{Na}^{+}$/K$\text{K}^{+}$ pump maintains a low cytosolic $[\text{Na}^{+}]$.

3. Movement of K$\text{K}^{+}$ Ions Down the K$\text{K}^{+}$ Gradient through K$\text{K}^{+}$ Leak Channels

• K$\text{K}^{+}$ ions tend to diffuse out of the cell through always-open K$\text{K}^{+}$ leak channels, moving down their steep concentration gradient.

• As K$\text{K}^{+}$ ions (positive charges) leave the cell, they leave behind unpaired negative counterions (e.g., proteins, phosphates) inside the cell.

• This outward movement of K$\text{K}^{+}$ significantly contributes to the excess positive charge outside the cell and the excess negative charge inside, establishing the resting membrane potential.

• The positive and negative charges accumulate in a thin layer along the membrane surfaces due to electrostatic attraction.

4. Ca$\text{Ca}^{2+}$ Concentration Gradient and Ca$\text{Ca}^{2+}$ Pump

• The Ca$\text{Ca}^{2+}$ pump (a P-type ATPase) uses ATP to establish a low cytosolic $[\text{Ca}^{2+}]$.

• Extra Ca$\text{Ca}^{2+}$ ions in the extracellular space contribute to the membrane potential, although K$\text{K}^{+}$ and Na$\text{Na}^{+}$ are the major contributors.

Resting Membrane Potential

• Characteristics in Animal Cells:

  • High $[\text{K}^{+}]$ in the cytosol drives K$\text{K}^{+}$ outward flow through K$\text{K}^{+}$ leak channels.

  • K$\text{K}^{+}$ outflow is resisted by:

    • Repulsion from the excess positive charge accumulating outside the membrane.

    • Attraction to the excess negative charge accumulating inside the membrane.

  • An equilibrium is reached where the electrical force opposing K$\text{K}^{+}$ efflux balances the chemical force (concentration gradient) driving K$\text{K}^{+}$ efflux.

  • The value of the resting membrane potential in animal cells varies from $-20 \text{ mV}$ to $-200 \text{ mV}$, typically around $-70 \text{ mV}$.

  • Net result: More positive charges on the outside (Na$\text{Na}^{+}$ and K$\text{K}^{+}$), more negative charges on the inside (Cl$\text{Cl}^{-}$ and fixed anions).

Neuronal Structure and Synaptic Communication

• General Structure of a Neuron:

  • Dendrites: Receive signals from other neurons.

  • Cell body (Soma): Contains the nucleus and cellular machinery.

  • Axon: Transmits electrical signals over long distances.

  • Axon terminals: Form synapses with other cells.

• Synapse: A specialized junction where neurons communicate with other neurons or target cells (e.g., muscle cells).

  • Involves the axon terminal of one neuron and the dendrite (or cell body) of another neuron, or a muscle cell.

  • Neurotransmitter: Chemical messenger released at the synapse to communicate between cells.

Chemical Synapse Example: Neuromuscular Junction

• Process of Synaptic Transmission: Often depicted with a motor neuron communicating with a muscle cell, using Acetylcholine (ACh) as the neurotransmitter.

  1. Action Potential Conduction: An action potential (nerve impulse) arrives at the axon terminal, causing a brief depolarization of the membrane.

  2. Voltage-Gated Ca$\text{Ca}^{2+}$ Channel Opening: The depolarization of the axon terminal membrane opens voltage-gated Ca$\text{Ca}^{2+}$ channels.

  3. Ca$\text{Ca}^{2+}$ Influx: Ca$\text{Ca}^{2+}$ ions, being more concentrated outside the cell, flow into the axon terminal down their electrochemical gradient.

  4. Vesicle Fusion and Neurotransmitter Release: Increased cytosolic Ca$\text{Ca}^{2+}$ triggers synaptic vesicles (containing neurotransmitters like Acetylcholine) to fuse with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft.

  5. Neurotransmitter Diffusion and Receptor Binding: Acetylcholine diffuses across the synaptic cleft and binds to specific receptors on the post-synaptic membrane (e.g., the muscle cell membrane).

  6. Change in Postsynaptic Membrane Potential: Binding of Acetylcholine to its receptor (which is an ACh-gated cation channel) causes the channel to open.

  7. Na$\text{Na}^{+}$ Influx and Depolarization: Na$\text{Na}^{+}$ ions, being more concentrated outside the cell, flow into the muscle cell through the open channel. This influx of positive ions causes the muscle cell membrane to depolarize.

  8. Muscle Contraction: This depolarization of the muscle cell membrane ultimately triggers muscle contraction.

  9. Neurotransmitter Inactivation and Re-absorption: After inactivation (e.g., by acetylcholinesterase), neurotransmitter molecules are re-absorbed into the presynaptic terminal, where they are stored in vesicles for future release.

Gated Ion Channels

• Definition: Ion channels that open or close in response to specific stimuli.

• Types:

  • i) Voltage-Gated Ion Channel: Opened by a change in the electrical voltage (membrane potential) across the membrane.

    • Example: Voltage-gated Ca$\text{Ca}^{2+}$ channels at the axon terminal.

  • iii) Ligand-Gated Ion Channel: Opened by the binding of a specific extracellular ligand (a chemical messenger).

    • Example: The Acetylcholine receptor is an ACh-gated cation channel.

Diffusion and Ion Flow through Gated Channels

• General Diffusion: Random thermal motion of molecules promotes their random distribution in space. This applies to molecules that can cross the membrane (e.g., O$\text{O}<em>{2}$, CO$\text{CO}</em>{2}$, lipid-based hormones like cortisol) without the need for channels.

• Ion Diffusion: Ions cannot cross the lipid bilayer directly due to their charge. They can only cross the membrane through their specific channels, and only when those channels are OPEN.

Voltage-Gated Calcium Channels and Neurotransmitter Release

• State of Voltage-Gated Ca$\text{Ca}^{2+}$ Channel:

  • CLOSED: At normal resting membrane potential (net positive outside).

  • OPEN: When the membrane potential significantly lowers (depolarizes), causing a change in voltage across the membrane. This allows Ca$\text{Ca}^{2+}$ to enter the cytosol, moving down its steep Ca$\text{Ca}^{2+}$ gradient.

• Role in Neurotransmitter Release: The increase in cytosolic Ca$\text{Ca}^{2+}$ concentration caused by the opening of these channels is the critical signal that causes neurotransmitter-packed vesicles to fuse with the presynaptic cell membrane, releasing their contents into the synaptic cleft.

Acetylcholine's Receptor in the Muscle Cell Membrane

• Mechanism:

  • Acetylcholine diffuses across the synaptic cleft and binds to its receptor on the postsynaptic (muscle cell) membrane.

  • The Acetylcholine receptor is an ACh-gated cation channel.

  • Upon binding, the channel opens, allowing Na$\text{Na}^{+}$ to flow into the muscle cell from the extracellular space (where Na$\text{Na}^{+}$ concentration is high) down its electrochemical gradient.

• Effect: The influx of positive Na$\text{Na}^{+}$ ions causes the muscle cell membrane to depolarize. This change in membrane potential is the initiating event that leads to muscle contraction.