Cell Membrane Transport, Membrane Potential, and Cell-Environment Interactions

Cell Membrane Transport

Active Membrane Transport

Active membrane transport refers to the movement of substances across the plasma membrane that requires energy, typically in the form of ATP. This process is essential for transporting solutes that cannot move passively due to size, charge, or concentration gradients.

• Types of Active Transport:

  • Active transport (using carrier proteins)

  • Vesicular transport (using membrane-bound vesicles)

• Requirements for Active Transport:

  • Solute is too large for channels

  • Solute is not lipid soluble

  • Solute is unable to move down its concentration gradient

Carrier-Mediated Active Transport

Carrier-mediated active transport uses specific proteins (solute pumps) to move substances against their concentration gradients.

• Carrier Proteins:

  • Bind specifically and reversibly with substances being moved

  • Some carriers transport one substance (uniporters)

  • Antiporters: transport two substances in opposite directions

  • Symporters: transport two substances in the same direction

• Energy Requirement: Movement against the gradient requires ATP

Types of Active Transport

• Primary Active Transport:

  • Energy comes directly from ATP hydrolysis

  • Example: Na+-K+ pump

• Secondary Active Transport:

  • Energy is obtained indirectly from ion gradients created by primary active transport

  • Example: glucose transport coupled to Na+ gradient

Primary Active Transport: Na+-K+ Pump

The Na+-K+ ATPase pump is a key example of primary active transport, maintaining cellular ion gradients essential for nerve and muscle function.

• Pumps 3 Na+ out and 2 K+ into the cell per ATP hydrolyzed

• Located in plasma membranes, especially in excitable cells (nerves, muscles)

• Maintains electrochemical gradients

Secondary Active Transport

Secondary active transport uses energy stored in ion gradients established by primary active transport to move other substances.

• Na+ concentration gradient maintained by Na+-K+ pump

• Na+ can drag other molecules into cell as it flows inward (symport)

• Some sugars, amino acids, and ions are transported via secondary active transport

Vesicular Transport

Vesicular transport involves movement of large particles, macromolecules, and fluids across membranes in membranous sacs called vesicles. It requires cellular energy (usually ATP).

• Types of Vesicular Transport:

  • Endocytosis: transport into cell

  • Exocytosis: transport out of cell

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

  • Vesicular trafficking: movement of substances from one area or organelle to another

Endocytosis

Endocytosis is the process of taking substances into the cell by forming vesicles from the plasma membrane.

• Formation of protein-coated vesicles

• Can be highly selective (receptor-mediated)

• Some pathogens hijack receptors for entry

• Vesicle may fuse with lysosome or undergo transcytosis

Phagocytosis

Phagocytosis is a type of endocytosis referred to as "cell eating." It involves engulfing large particles or pathogens.

• Membrane pseudopods surround particle, forming a phagosome

• Phagocytes include macrophages and certain white blood cells

• Uses amoeboid motion to move cytoplasm

Pinocytosis

Pinocytosis is "cell drinking," involving the intake of extracellular fluid and dissolved solutes.

• Plasma membrane infolds, bringing fluid inside

• Fuses with endosome

• Main method for nutrient absorption in small intestine

Receptor-Mediated Endocytosis

This process involves specific receptors and is used for selective uptake of molecules.

• Receptors embedded in clathrin-coated pits

• Examples: uptake of LDL, insulin, iron, viruses, toxins

• Caveolae: smaller pits for capturing specific molecules

Exocytosis

Exocytosis is the process of expelling substances from the cell by vesicle fusion with the plasma membrane.

• Used for secretion of hormones, neurotransmitters, mucus, and cellular wastes

• Triggered by cell-surface signals or changes in membrane voltage

• Protein called SNARE helps vesicle dock and fuse with membrane

Membrane Potential

Resting Membrane Potential (RMP)

The resting membrane potential is the electrical potential energy across the plasma membrane due to separation of oppositely charged particles. All cells have a RMP, but it is especially important in excitable cells.

• Cells are polarized: inside is more negative relative to outside

• Typical RMP values: -50 to -100 mV

• Voltage sign: negative inside cell

Role of K+ in RMP

Potassium ions (K+) play a key role in establishing the RMP.

• K+ diffuses out through leakage channels down its concentration gradient

• Negatively charged proteins cannot leave cell

• Electrical gradient pulls K+ back in

• RMP is established when K+ efflux is balanced by influx

• Most cells have RMP around -90 mV

• Electrochemical gradient of K+ sets RMP

Na+ Influence on RMP

Sodium ions (Na+) also affect RMP, but the membrane is less permeable to Na+ than K+.

• Na+ entry can bring RMP up to -70 mV

• Na+-K+ pump maintains RMP by pumping Na+ out and K+ in

Equation for RMP (Nernst Equation):

$E = \frac{RT}{zF} \ln \frac{[K^+]_{out}}{[K^+]_{in}}$

Additional info: The Nernst equation calculates the equilibrium potential for a particular ion based on its concentration gradient across the membrane.

Maintaining Electrochemical Gradients

• Na+-K+ pump continuously operates to maintain gradients

• Steady state is maintained by diffusion and active pumping

• Neurons and muscle cells can "reset" RMP by opening channels

Cell-Environment Interactions

Glycocalyx and Cell Adhesion Molecules (CAMs)

Cells interact with their environment via surface molecules, including the glycocalyx and CAMs.

• Glycocalyx: carbohydrate-rich area on cell surface involved in cell recognition

• Cell Adhesion Molecules (CAMs):

  • Glycoproteins projecting from membrane

  • Anchor cells to extracellular matrix or each other

  • Assist in movement and attraction of cells

  • Stimulate synthesis/degradation of junctions

  • Transmit intracellular signals for migration, proliferation, specialization

Plasma Membrane Receptors

Plasma membrane receptors serve as binding sites for chemical signals, facilitating cell communication.

• Contact Signaling: cells recognize each other by unique surface markers

• Chemical Signaling: interaction between receptors and ligands (chemical messengers)

  • Ligand binding triggers changes in cell activity

  • Examples: neurotransmitters, hormones, paracrines

• G Protein-Linked Receptors:

  • Ligand binding activates G proteins, which affect ion channels, enzymes, or release second messengers (e.g., cyclic AMP, calcium)

Summary Table: Types of Membrane Transport

Type	Energy Required?	Mechanism	Examples
Passive Transport	No	Diffusion, facilitated diffusion, osmosis	O2, CO2, water
Active Transport	Yes (ATP)	Carrier proteins (pumps)	Na+-K+ pump
Secondary Active Transport	Indirect (uses ion gradients)	Carrier proteins (symport/antiport)	Glucose-Na+ symport
Vesicular Transport	Yes (ATP)	Vesicles (endocytosis, exocytosis)	Phagocytosis, receptor-mediated endocytosis

Key Terms

• Active transport: movement of substances against their concentration gradient using energy

• Vesicular transport: movement of large particles or fluids via vesicles

• Resting membrane potential (RMP): voltage across the plasma membrane in resting cells

• Glycocalyx: carbohydrate-rich area on cell surface for recognition

• Cell adhesion molecules (CAMs): glycoproteins for cell attachment and signaling

• Ligand: chemical messenger that binds to a receptor

• G protein: intracellular protein activated by receptor-ligand binding