Active and Vesicular Transport & Membrane Potential Part 2

Active Membrane Transport

• Requires ATP to move solutes across the plasma membrane if:

  • Solute is too large for channels

  • Solute is not lipid soluble

  • Solute is unable to move down the concentration gradient

Active Transport

• Requires carrier proteins (solute pumps)

  • Bind specifically and reversibly with the substance being moved

  • Some carriers transport more than one substance

    • Antiporters: transport one substance into the cell while transporting a different substance out

    • Symporters: transport two different substances in the same direction

• Moves solutes against their concentration gradient (from low to high)

  • Requires energy (ATP)

Types of Active Transport

• Primary active transport

  • Required energy comes directly from ATP hydrolysis

• Secondary active transport

  • Required energy is obtained indirectly from ionic gradients created by primary active transport

Primary Active Transport

• Energy from hydrolysis of ATP causes a change in the shape of the transport protein.

• The shape change causes solutes (ions) bound to the protein to be pumped across the membrane.

• Examples of pumps: calcium, hydrogen (proton), $Na^+-K^+$ pumps

Sodium-Potassium Pump

• Most studied pump

• An enzyme, called $Na^+-K^+$ ATPase, that pumps $Na^+$ out of the cell and $K^+$ back into the cell

• Located in all plasma membranes, but especially active in excitable cells (nerves and muscles)

• Leakage channels cause leaking of $Na^+$ into the cell and $K^+$ out of the cell

  • Both travel down their concentration gradients

• The pump works as an antiporter, pumping ions against their concentration gradients

• Maintains electrochemical gradients

  • Essential for functions of muscle and nerve tissues

Secondary Active Transport

• Depends on the ion gradient created by primary active transport.

• Energy stored in gradients drives the transport of other solutes.

• Low $Na^+$ concentration inside the cell (maintained by the pump) strengthens sodium's drive to enter the cell.

• $Na^+$ can drag other molecules with it through symporters.

• Some sugars, amino acids, and ions are transported this way.

Vesicular Transport

• Involves transport of large particles, macromolecules, and fluids across the membrane in vesicles

• Requires cellular energy (ATP)

• Processes:

  • Endocytosis: transport into the cell

    • Types:

      • Phagocytosis

      • Pinocytosis

      • Receptor-mediated endocytosis

  • Exocytosis: transport out of the cell

  • Transcytosis: transport into, across, and out of the cell

  • Vesicular trafficking: transport from one area/organelle to another

Endocytosis

• Involves formation of protein-coated vesicles

• Selective process involving receptors

• Substance being pulled in must bind to a unique receptor.

• Pathogens can hijack receptors for transport into the cell.

• Once inside, vesicles may:

  • Fuse with a lysosome

  • Undergo transcytosis

Phagocytosis

• "Cell eating"

• Membrane projections (pseudopods) form and flow around solid particles.

• Forms a vesicle (phagosome) that is pulled into the cell.

• Used by macrophages and certain white blood cells.

• Phagocytic cells move by amoeboid motion.

Pinocytosis

• "Cell drinking" (fluid-phase endocytosis)

• Plasma membrane infolds, bringing extracellular fluid and solutes inside.

• Fuses with endosome.

• Used to "sample" the environment.

• Main way nutrient absorption occurs in the small intestine.

• Membrane components are recycled.

Receptor-Mediated Endocytosis

• Endocytosis and transcytosis of specific molecules.

• Cells have receptors in clathrin-coated pits.

• Examples of molecules taken in: enzymes, LDL, iron, insulin, viruses, diphtheria, and cholera toxins.

• Caveolae have smaller pits and different protein coats but still capture specific molecules and use transcytosis.

Exocytosis

• Material is ejected from the cell.

• Activated by cell-surface signals or changes in membrane voltage.

• Substance is enclosed in a secretory vesicle.

• v-SNARE on the vesicle hooks up to t-SNARE proteins on the membrane.

• Docking triggers exocytosis.

• Examples of substances exocytosed: hormones, neurotransmitters, mucus, cellular wastes

Membrane Potential

• Resting membrane potential (RMP): electrical potential energy produced by separation of oppositely charged particles across the plasma membrane in all cells

  • Voltage: difference in electrical charge between two points

  • Cells with a charge are polarized

  • Voltage occurs only at the membrane surface; the rest of the cell and extracellular fluid are neutral

Key Role of Potassium in RMP

• $K^+$ diffuses out of the cell through leakage channels down its concentration gradient.

• Negatively charged proteins cannot leave.

• The cytoplasmic side of the membrane becomes more negative.

• $K^+$ is pulled back by the negative interior due to its electrical gradient.

• When the drive for $K^+$ to leave is balanced by its drive to stay, RMP is established (around $-90 mV$).

• Electrochemical gradient of $K^+$ sets the RMP.

Other Factors Affecting RMP

• $Na^+$: also attracted to the inside of the cell due to the negative charge; if it enters, it can bring RMP up to $-70 mV$

• Membrane is more permeable to $K^+$ than $Na^+$, so $K^+$ has a primary influence on RMP.

• $Cl^-$: does not influence RMP because its concentration and electrical gradients are balanced.

Maintenance of Electrochemical Gradients

• RMP is maintained by the $Na^+-K^+$ pump, which ejects 3Na+ out and brings 2K+ in.

• Steady state is maintained because the rate of active pumping of $Na^+$ equals the rate of diffusion into the cell.

• Neurons and muscle cells intentionally open gated channels to "upset" the steady state RMP.

Cell-Environment Interactions

• Cells interact with their environment by responding to other cells or extracellular chemicals.

• Interactions involve:

  • Glycocalyx

  • Cell adhesion molecules (CAMs)

  • Plasma membrane receptors

Cell Adhesion Molecules (CAMs)

• Functions:

  • Anchor cells to the extracellular matrix or each other

  • Assist in the movement of cells past one another

  • Attract WBCs to injured or infected areas

  • Stimulate synthesis or degradation of adhesive membrane junctions

  • Transmit intracellular signals to direct cell migration, proliferation, and specialization

Plasma Membrane Receptors

• Binding sites for chemical signals

  • Contact signaling: cells recognize each other by surface receptors

    • Used in normal development and immunity

  • Chemical signaling: interaction between receptors and ligands

    • Triggers enzyme activation or opens chemically gated ion channels

    • Examples of ligands: neurotransmitters, hormones, and paracrines

• The same ligand can cause different responses in different cells.

• Ligand binding activates the receptor protein.

• Activated receptors become enzymes or open/close ion gates.

• G protein-linked receptors:

  • Indirectly cause cellular changes by activating G proteins

  • G proteins affect ion channels, activate enzymes, or release internal second messengers (e.g., cyclic AMP or calcium)