transport

📌 General Physiology, Cell Growth, and Division

Main Topics:

  1. Membrane Transport

  2. Electrochemical Gradients & Membrane Potential

  3. Action Potentials

  4. Neurotransmission & Synaptic Function

  5. Myelin & Conduction Velocity

  6. Cell Growth & Division

1⃣ Membrane Transport

Cells must transport substances in and out to maintain homeostasis. There are two main types:

A) Passive Transport (No ATP Required)

✅ Moves molecules from high to low concentration (down the gradient).

  • Simple Diffusion → Small, nonpolar molecules (O₂, CO₂) pass directly through the membrane.

  • Facilitated Diffusion → Larger or charged molecules (e.g., glucose, Na⁺) use transport proteins (e.g., GLUT4 for glucose).

  • Osmosis → Water moves across membranes through aquaporins from low solute concentration to high solute concentration.

Tonicity & Osmosis:

  • Hypotonic solution → Water enters the cell → Swelling, possible lysis.

  • Hypertonic solution → Water leaves the cell → Cell shrinks (crenation).

  • Isotonic solution → No net water movement.

B) Active Transport (ATP Required)

✅ Moves molecules from low to high concentration (against the gradient).

  1. Primary Active Transport → Direct ATP use

    • Example: Na⁺/K⁺ ATPase Pump

      • Pumps 3 Na⁺ out, 2 K⁺ in to maintain resting membrane potential.

  2. Secondary Active Transport → Uses an existing gradient (indirect ATP use)

    • Example: Na⁺/Glucose Symporter

      • Uses Na⁺ gradient (from Na⁺/K⁺ ATPase) to move glucose into the cell.

2⃣ Electrochemical Gradients & Membrane Potential

What is the Membrane Potential?

The difference in charge across the membrane, created by ions.

  • Resting membrane potential (~ -60mV) is maintained by:
    ✅ Na⁺/K⁺ ATPase pump (3 Na⁺ out, 2 K⁺ in)
    ✅ K⁺ leak channels (K⁺ leaves, making inside more negative)

Key Ion Gradients in a Resting Cell:

Ion

Inside (mM)

Outside (mM)

Equilibrium Potential (Ex)

Na⁺

15

145

+61 mV

K⁺

120

4.5

-88 mV

Cl⁻

20

105

-44 mV

Ca²⁺

10⁻⁷

1

+123 mV

3⃣ Action Potentials (Nerve Signals)

Neurons send electrical signals (action potentials) by rapidly changing their membrane potential.

Phases of an Action Potential:

1⃣ Resting state (~ -60 mV)

  • Na⁺ and K⁺ channels are closed.

  • Maintained by Na⁺/K⁺ ATPase pump.

2⃣ Depolarization (+30 to +40 mV)

  • Voltage-gated Na⁺ channels open → Na⁺ enters the cell.

  • The membrane potential becomes more positive.

3⃣ Repolarization (-60 mV again)

  • Voltage-gated K⁺ channels open → K⁺ leaves the cell.

  • Membrane potential returns to negative.

4⃣ Hyperpolarization (~ -70 mV or lower)

  • K⁺ channels stay open too long, making the cell more negative than resting potential.

5⃣ Return to Resting State

  • Na⁺/K⁺ ATPase pump restores ion balance.

4⃣ Neurotransmission & Synaptic Function

Neurons communicate across synapses using neurotransmitters.

Steps of Neurotransmission:

  1. Action potential reaches presynaptic terminal.

  2. Voltage-gated Ca²⁺ channels open, allowing Ca²⁺ influx.

  3. Ca²⁺ triggers vesicle fusion via SNARE proteins, releasing neurotransmitters.

  4. Neurotransmitters bind to postsynaptic receptors → signal transmission.

  5. Neurotransmitters are removed (reuptake, degradation, or diffusion).

Neurotoxin Effects:

🚨 Tetrodotoxin (TTX) → Blocks Na⁺ channels, preventing action potentials.
🚨 Lidocaine → Blocks Na⁺ channels temporarily (local anesthetic).

5⃣ Myelin & Conduction Velocity

Myelin is a fatty sheath that surrounds axons, increasing conduction speed.

Why is Myelin Important?

✅ Increases conduction velocity via saltatory conduction.
✅ Reduces energy consumption (fewer Na⁺/K⁺ pumps needed).
✅ Prevents ion leakage, maintaining strong signals.

Disease: Multiple Sclerosis (MS)

Loss of myelin leads to:
🚨 Slower action potentials
🚨 Weakened signals

6⃣ Cell Growth & Division

Cells grow and divide to maintain tissues and repair damage.

Cell Cycle Phases:

1⃣ G1 Phase → Growth, preparing for DNA replication.
2⃣ S Phase → DNA replication occurs.
3⃣ G2 Phase → Growth, preparing for mitosis.
4⃣ M Phase (Mitosis) → Cell division occurs.

Mitosis Stages:

  1. Prophase – Chromosomes condense, spindle forms.

  2. Metaphase – Chromosomes align at the center.

  3. Anaphase – Sister chromatids separate.

  4. Telophase – Nuclear membrane reforms.

  5. Cytokinesis – Cytoplasm splits, forming two identical cells.

🔹 Summary of Key Points

✅ Na⁺/K⁺ ATPase maintains resting potential
✅ K⁺ leak channels set the resting membrane potential
✅ Depolarization = Na⁺ influx
✅ Repolarization & Hyperpolarization = K⁺ efflux
✅ Myelin speeds up conduction
✅ Ca²⁺ triggers neurotransmitter release
✅ Mitosis allows cell division