Cells: The Living Units

Introduction to Active Membrane Transport

Cells maintain concentration gradients of various chemicals across their plasma membranes, which is essential for cellular function. These gradients represent potential energy that cells use to perform work, especially in transport processes.

• Active transport and vesicular transport are the two major types of active membrane transport.

• Both require ATP to move solutes across the plasma membrane when:

  • Solute is too large for channels

  • Solute is not lipid soluble

  • Solute cannot move down its concentration gradient

Active Transport

Types of Active Transport

Active transport is classified based on the source of energy used to move substances against their concentration gradients.

• Primary active transport: Energy comes directly from ATP hydrolysis.

• Secondary active transport: Energy is obtained indirectly from ionic gradients created by primary active transport.

Primary Active Transport

In primary active transport, the hydrolysis of ATP causes a change in the shape of a transport protein, allowing solutes (such as ions) bound to the protein to be pumped across the membrane.

• Examples of pumps: Calcium pump, Hydrogen (proton) pump, Sodium-potassium (Na+-K+) pump

Sodium-Potassium Pump (Na+-K+ ATPase)

The sodium-potassium pump is the most studied example of primary active transport. It is an enzyme that pumps 3 Na+ ions out of the cell and 2 K+ ions back into the cell, maintaining essential gradients.

• Located in all plasma membranes, especially active in excitable cells (neurons and muscles).

• Functions as an antiporter (moves two different ions in opposite directions).

• Maintains electrochemical gradients (combining concentration and electrical gradients).

• Essential for muscle and nerve tissue function.

Inhibitors:

• Oleander tree toxin

• Digitoxin (foxglove) – arrow poison and cardiac drug

Key Facts Table: Sodium and Potassium Ion Concentrations

Secondary Active Transport

Secondary active transport uses energy stored in ionic gradients created by primary active transport. Carrier proteins (solute pumps) bind specifically and reversibly with substances being moved.

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

• Symporters: Transport two different substances in the same direction.

• Commonly transports sugars, amino acids, and ions into the cell.

Example: Low Na+ concentration inside the cell (maintained by the Na+-K+ pump) strengthens sodium's drive to enter the cell, which can drag other molecules with it via symporters.

Vesicular Transport

Overview of Vesicular Transport

Vesicular transport moves large particles, macromolecules, and fluids across membranes in membranous sacs called vesicles. This process requires cellular energy, usually ATP.

• Includes endocytosis (into the cell) and exocytosis (out of the cell).

• Transcytosis: Moving substances from one area of the cell to another.

Types of Endocytosis

Endocytosis is the process of taking substances into the cell via vesicles. There are three main types:

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

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

• Receptor-mediated endocytosis: Highly selective process where substances bind to specific receptors in clathrin-coated pits before being internalized. Used for uptake of enzymes, LDL, iron, insulin, and some viruses/toxins.

Comparison Table: Types of Endocytosis

Exocytosis

Exocytosis is the process by which material is ejected from the cell, usually activated by cell-surface signals or changes in membrane voltage. The vesicle's v-SNARE protein binds to target SNARE proteins on the membrane, triggering release.

• Substances exocytosed: hormones, neurotransmitters, mucus, cellular wastes

• Botulinum toxin blocks exocytosis of acetylcholine at neuromuscular junctions.

• Black widow spider venom causes massive, uncontrolled exocytosis of acetylcholine.

Membrane Potential

Resting Membrane Potential (RMP)

The resting membrane potential is the electrical potential energy produced by the separation of oppositely charged particles across the plasma membrane in all cells. The difference in electrical charge between two points is called voltage.

• Cells with a charge are said to be polarized.

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

• Typical membrane voltages range from -50 to -100 mV (negative sign indicates inside of cell is more negative).

Role of Potassium (K+) in RMP

K+ diffuses out of the cell through leakage channels down its concentration gradient, while negatively charged proteins remain inside. This makes the cytoplasmic side of the membrane more negative.

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

• Drive for K+ to leave is balanced by its drive to stay, establishing the RMP.

• Most cells have an RMP around -80 mV.

Electrochemical gradient of K+ sets the RMP.

Role of Sodium (Na+) in RMP

In many cells, Na+ also affects RMP. Na+ is attracted to the inside of the cell due to the negative charge. If Na+ enters, it can bring the RMP up to -70 mV. However, the membrane is more permeable to K+, so K+ is the primary influence on RMP.

Maintenance of Electrochemical Gradients

Electrochemical gradients are maintained by the Na+-K+ pump, which continuously ejects Na+ from the cell and brings K+ back inside. The steady state is maintained because the rate of active pumping of Na+ out equals its diffusion into the cell.

• Neurons and muscle cells can "upset" this steady state by intentionally opening gated Na+ and K+ channels.

Key Equations

• Na+-K+ Pump Reaction:

• Nernst Equation (for equilibrium potential):

Additional info: The Nernst equation is used to calculate the equilibrium potential for a particular ion across a membrane, considering its concentration gradient.