Cellular Transport Mechanisms and Concentration Gradients

Posted on Oct 6, 2025 in Biotechnology

Solution Concentration and Particle Movement

Many essential chemical reactions take place in the cytoplasm and aqueous solutions.

  • Solution: A solute dissolved in a solvent.
  • Concentration: The amount of solute in proportion to the amount of solvent in a solution.
  • All particles in a solution possess kinetic energy, resulting in continuous random movement.

The Principle of Diffusion

Molecules and ions in a fluid are in continuous, random motion due to their kinetic energy. Diffusion is the net movement of these particles:

  • Movement occurs from an area of high concentration to an area of low concentration (moving down the concentration gradient).
  • Movement continues until the particles are evenly spread out, reaching an equilibrium.

Passive Transport Mechanisms

Passive transport requires no energy (ATP) and moves substances down the concentration gradient.

Simple Diffusion

The passive net movement of small, non-polar particles across a membrane.

  • Particles: Small, non-polar molecules (e.g., oxygen, carbon dioxide).
  • Movement: Down the concentration gradient (high to low concentration).
  • Pathway: Directly across the membrane, moving between the phospholipids.
  • Result: Particles are evenly distributed (equilibrium).

Osmosis: Water Movement

Osmosis is the passive net movement (diffusion) of water molecules only through a semipermeable membrane.

  • Movement Direction: Down the water concentration gradient (from a region of higher water concentration to lower water concentration). This is equivalent to moving up the solute concentration gradient.
  • Goal: Water is evenly distributed (equilibrium).
  • Cellular Importance:
    • All cells carry out osmosis, as biochemical processes occur in aqueous solution in the cytoplasm.
    • Cells requiring large water absorption (e.g., kidney cells and root hair cells) utilize transport proteins called aquaporins.
    • Animal Cells: Excessive osmosis can cause damage.
    • Plant Cells: The rigidity of the cell wall prevents excessive water absorption.

Facilitated Diffusion

The passive net movement of small, polar molecules or ions across a membrane using transport proteins.

  • Particles: Small, polar molecules or ions (e.g., glucose molecules, sodium ions, potassium ions).
  • Movement: Down the concentration gradient (high to low concentration).
  • Pathway: Through specific transport proteins embedded in the membrane.
  • Result: Particles are evenly distributed (equilibrium).

Active Transport

Active transport requires energy in the form of ATP to move highly polar molecules or ions across a semipermeable membrane.

  • Movement Direction: Up the concentration gradient (from low particle concentration to high particle concentration).
  • Mechanism: Uses specialized protein pumps.

Example: The Sodium-Potassium Pump

This pump is crucial for the transmission of nerve impulses along the axon of a neuron:

  • It transports 3 sodium ions (Na+) from the inside to the outside of the axon.
  • It transports 2 potassium ions (K+) from the outside to the inside of the axon.

Bulk Transport: Endocytosis and Exocytosis

These processes involve the movement of very large or highly polar particles and require active transport (energy is required).

Endocytosis (Cell Import)

The process by which the cell imports or takes in particles from outside to the inside. The cell membrane surrounds the particle, forming a vesicle.

  • Pinocytosis: “Cell drinking” (importing liquid particles).
  • Phagocytosis: “Cell eating” (importing solid particles).

Exocytosis (Cell Export)

The process by which the cell exports or releases particles (e.g., excretion of waste or secretion of enzymes/hormones).

  • The particle is carried within a vesicle.
  • The vesicle fuses with the cell membrane.
  • The particle is released outside the cell.