Cell Physiology: Plasma Membrane Transport

Cell Physiology: Plasma Membrane Transport

Introduction

The plasma membrane is a selectively permeable barrier that regulates the movement of substances into and out of the cell. Understanding the mechanisms of membrane transport is essential for comprehending how cells maintain homeostasis and interact with their environment.

Membrane Transport Overview

General Principles

• Plasma Membrane: Provides the medium through which substances enter and exit the cell.

• Selective Permeability: The membrane allows only certain molecules to pass while restricting others.

• Transport Mechanisms: Substances move across the membrane via active or passive transport.

• Concentration Gradient: Movement can occur down (from high to low concentration) or against (from low to high concentration) a concentration gradient.

Passive Transport

Definition and Types

Passive transport involves the movement of substances across the membrane without the use of cellular energy (ATP). It relies on the natural kinetic energy of molecules.

• Diffusion

• Facilitated Diffusion

• Osmosis

Simple Diffusion

• Definition: The net movement of molecules from an area of higher concentration to an area of lower concentration, down their concentration gradient.

• Mechanism: Lipid-soluble (hydrophobic) substances diffuse directly through the lipid bilayer.

• Examples: Oxygen (O2), carbon dioxide (CO2), fat-soluble vitamins.

• Energy Requirement: No energy required.

• Result: Equilibrium is reached when concentrations are equal on both sides.

Facilitated Diffusion

• Definition: The movement of larger or non-lipid soluble molecules across the membrane with the help of transmembrane proteins.

• Types:

  • Carrier-mediated facilitated diffusion: Integral proteins transport specific polar molecules (e.g., glucose, amino acids) that are too large for channels. Each carrier is specific to a particular molecule.

  • Channel-mediated facilitated diffusion: Substances move through water-filled channels formed by transmembrane proteins. Channels can be always open (leak channels) or gated (regulated by chemical or electrical signals).

• Specificity: Each carrier or channel is specific for certain substances (e.g., glucose carriers transport only glucose).

• Limitation: The rate is limited by the number of available carriers or channels.

• Direction: Always down the concentration gradient.

Osmosis

• Definition: The diffusion of water (solvent) through a selectively permeable membrane.

• Mechanism: Water moves through the lipid bilayer or via specific water channels called aquaporins.

• Driving Force: Occurs whenever water concentration differs on the two sides of the membrane.

• Equilibrium: Achieved when the concentration of water is equal on both sides.

Tonicity

Tonicity describes the ability of a solution to change the water volume of a cell by osmosis.

• Isotonic Solution: Solute concentration is equal inside and outside the cell. Water moves freely in both directions; the cell remains stable.

• Hypertonic Solution: Solute concentration is higher outside the cell. Water moves out, causing the cell to shrink (crenation).

• Hypotonic Solution: Solute concentration is lower outside the cell. Water moves in, causing the cell to swell and possibly burst (lysis/hemolysis).

Active Transport

Definition and General Features

Active transport requires cellular energy (usually ATP) to move substances against their concentration gradient (from low to high concentration).

• Used for: Substances that are too large for channels, not lipid soluble, or unable to move down their concentration gradient.

• Carrier Proteins: Integral proteins that specifically and reversibly bind to substances to transport them across the membrane.

Types of Active Transport

• Primary Active Transport: Energy comes directly from ATP hydrolysis. Example: Sodium-potassium pump (Na+/K+ ATPase), which pumps Na+ out of and K+ into the cell.

• Secondary Active Transport: Energy is derived indirectly from ionic gradients created by primary active transport. Includes cotransport of substances such as sugars, amino acids, and ions.

Cotransport Mechanisms

• Symporters: Transport two different substances in the same direction across the membrane. Example: Glucose cotransported with Na+ into the cell.

• Antiporters: Transport one substance into the cell while moving another out. Example: H+ cotransported out of the cell as Na+ moves in.

Sodium-Potassium Pump

• Function: Maintains the electrochemical gradients essential for muscle and nerve function.

• Mechanism: Pumps 3 Na+ ions out and 2 K+ ions into the cell per ATP molecule hydrolyzed.

Equation:

$3\,\mathrm{Na}^+_{\text{in}} + 2\,\mathrm{K}^+_{\text{out}} + \mathrm{ATP} \rightarrow 3\,\mathrm{Na}^+_{\text{out}} + 2\,\mathrm{K}^+_{\text{in}} + \mathrm{ADP} + \mathrm{P_i}$

Vesicular Transport

Definition and Types

Vesicular transport is a form of active transport that moves large particles, macromolecules, and fluids across the plasma membrane in membranous sacs called vesicles. This process requires ATP.

• Endocytosis: Movement of substances into the cell. Includes:

  • Phagocytosis: "Cell eating"; ingestion of large particles (e.g., white blood cells engulfing bacteria).

  • Pinocytosis: "Cell drinking"; ingestion of extracellular fluid and dissolved solutes (e.g., absorption in the small intestine).

  • Receptor-mediated endocytosis: Selective uptake of specific molecules via receptor binding.

• Exocytosis: Ejection of substances from the cell, enclosed in secretory vesicles (e.g., release of hormones, neurotransmitters, mucus).

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Examples
Simple Diffusion	No	Down gradient	O2, CO2, fat-soluble vitamins
Facilitated Diffusion	No	Down gradient	Glucose, amino acids, ions
Osmosis	No	Down water gradient	Water via aquaporins
Primary Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Secondary Active Transport	Yes (indirectly)	Against gradient (one substance)	Na+-glucose symport
Vesicular Transport	Yes (ATP)	In or out	Endocytosis, exocytosis

Key Terms

• Selective Permeability: Property of the membrane that allows some substances to pass while blocking others.

• Concentration Gradient: Difference in concentration of a substance across a space or membrane.

• Equilibrium: State in which concentrations are equal on both sides of the membrane.

• Aquaporins: Channel proteins that facilitate water movement across the membrane.

• Tonicity: The effect of a solution on cell volume.

Additional info: Some details, such as the specific mechanism of the sodium-potassium pump and the distinction between symporters and antiporters, were expanded for academic completeness.