Cellular Transport Mechanisms and Membrane Potential: Study Notes for Human Anatomy & Physiology

Cells: The Living Units

Active Membrane Transport

Active membrane transport is essential for moving substances across the plasma membrane against their concentration gradients. This process requires energy, typically in the form of ATP, and is necessary when solutes are too large, not lipid-soluble, or unable to move down their concentration gradient.

• Active Transport: Utilizes carrier proteins (solute pumps) to move substances against their concentration gradient. Includes antiporters (move substances in opposite directions) and symporters (move substances in the same direction).

• Vesicular Transport: Involves the movement of large particles, macromolecules, and fluids via membranous sacs called vesicles.

Types of Active Transport

• Primary Active Transport: Energy is derived directly from ATP hydrolysis. The classic example is the sodium-potassium (Na+-K+) pump, which maintains cellular electrochemical gradients by pumping Na+ out and K+ into the cell.

• Secondary Active Transport: Relies on the energy stored in ionic gradients created by primary active transport. For example, the Na+ gradient established by the Na+-K+ pump can drive the cotransport of glucose into the cell.

Key Equations

• Na+-K+ Pump: For each ATP hydrolyzed, 3 Na+ are pumped out and 2 K+ are pumped in.

• Electrochemical Gradient: $\Delta G = RT \ln \left( \frac{[C]_2}{[C]_1} \right) + zF\Delta \Psi$

Vesicular Transport

Vesicular transport is responsible for moving large substances into and out of cells via vesicles. This process is ATP-dependent and includes several mechanisms:

• Endocytosis: Transport into the cell. Types include:

  • Phagocytosis: "Cell eating"; the cell engulfs large particles using pseudopods, forming a phagosome. Used by macrophages and some white blood cells.

  • Pinocytosis: "Cell drinking"; the cell engulfs extracellular fluid and dissolved solutes in small vesicles. Main method for nutrient absorption in the small intestine.

  • Receptor-mediated Endocytosis: Specific molecules bind to receptors in clathrin-coated pits, allowing selective uptake.

• Exocytosis: Ejection of substances from the cell, often triggered by signals or changes in membrane voltage. Involves the fusion of secretory vesicles with the plasma membrane.

• Transcytosis: Transport into, across, and out of the cell.

• Vesicular Trafficking: Movement of substances from one area or organelle to another within the cell.

Comparison of Endocytosis Types

Type	Description	Specificity
Phagocytosis	Engulfs large particles using pseudopods	Specific (receptors involved)
Pinocytosis	Engulfs extracellular fluid and solutes	Nonspecific
Receptor-mediated	Engulfs specific molecules bound to receptors	Highly specific

Exocytosis

Exocytosis is the process by which cells expel materials. It is initiated by signals or voltage changes, and involves the docking and fusion of vesicles with the plasma membrane, mediated by SNARE proteins.

• Examples of substances released: hormones, neurotransmitters, mucus, and cellular wastes.

Membrane Potential

Resting Membrane Potential (RMP)

The resting membrane potential is the electrical potential energy resulting from the separation of oppositely charged particles across the plasma membrane. Most cells have an RMP between –50 and –100 mV, with the inside of the cell being more negative than the outside.

• K+ is the key player: K+ diffuses out of the cell, making the inside more negative. The balance between the chemical and electrical gradients of K+ establishes the RMP.

• Na+ also contributes: Na+ entry can make the RMP less negative (e.g., –70 mV).

• Cl–: Does not significantly influence RMP due to balanced gradients.

Key Equation

• Nernst Equation (for K+): $E_K = \frac{RT}{zF} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$

Role of the Na+-K+ Pump in RMP

The Na+-K+ pump maintains the electrochemical gradients by continuously ejecting 3 Na+ ions out and bringing 2 K+ ions in, which is essential for nerve and muscle function.

Cell-Environment Interactions

Cell Adhesion Molecules (CAMs)

CAMs are glycoproteins on the cell surface that mediate interactions with the extracellular matrix and other cells. They play roles in anchoring, cell movement, immune response, and signaling.

• Anchor cells to the extracellular matrix or each other

• Assist in cell movement and migration

• Attract immune cells to sites of injury or infection

• Transmit signals for cell proliferation and specialization

Plasma Membrane Receptors

Membrane receptors are proteins that bind chemical signals (ligands) and initiate cellular responses. They are involved in contact signaling (cell recognition) and chemical signaling (response to neurotransmitters, hormones, etc.).

• Ligand binding can activate enzymes or open ion channels

• G protein–linked receptors activate intracellular signaling cascades via second messengers like cyclic AMP or calcium

Summary Table: Types of Membrane Receptors

Type	Function	Example
Ion channel–linked	Open/close ion channels	Neurotransmitter receptors
Enzyme-linked	Activate intracellular enzymes	Insulin receptor
G protein–linked	Activate G proteins and second messengers	Adrenergic receptors

Additional info: G proteins act as intermediaries between extracellular first messengers (ligands) and intracellular second messengers, amplifying and diversifying cellular responses.