Cell Transport

Cell Membrane

• Homeostasis is maintained at the cellular level.

• Regulation occurs from the organ system level down to the cellular level.

Function of the Cell Membrane

• Much of cellular homeostasis is maintained by the cell membrane.

• The plasma membrane controls movement in and out of the cell.

Visualization of the Cell Membrane

• The cell membrane can be likened to a bubble that surrounds the cell.

Selective Permeability of the Cell Membrane

• The cell membrane is selectively permeable, meaning it is picky about what enters and exits.

• Composed of:

  • Glycoprotein

  • Carbohydrates

  • Glycolipid

  • Globular proteins

  • Cholesterol

• Transport classified as either:

  • Passive transport

  • Active transport

Passive Transport

• Requires no extra energy as molecules move from high to low concentration.

• Examples include:

  • Simple diffusion

  • Facilitated diffusion

  • Osmosis

Mechanisms of Passive Transport

• Diffusion: Movement down the concentration gradient (from high concentration to low).

  • Ex. Movement of O2 and CO2.

• Facilitated Diffusion: Involves transport proteins that assist molecules that cannot pass through the membrane.

• Osmosis: Diffusion of water across the cell membrane moving towards equilibrium.

Active Transport

• Requires extra energy (ATP) to move molecules against the concentration gradient (low to high concentration).

• Examples include:

  • Molecular pumps

  • Exocytosis

  • Endocytosis

Visualization of Transport Mechanisms

• Passive Transport: Like rolling down a hill (downhill movement with the concentration gradient).

• Active Transport: Like pushing uphill (uphill movement against the concentration gradient).

Simple Diffusion

• Molecules spread out across a membrane until they are equally concentrated.

• Equilibrium is achieved when concentration is equal on both sides.

Facilitated Diffusion

• Transport proteins facilitate diffusion for molecules that cannot normally pass through the cell membrane, moving from high to low concentration.

• Examples include glucose and polar molecules.

Osmosis

• The simple diffusion of water across the cell membrane.

• Water moves from high water concentration (low solute) to low water concentration (high solute) until equilibrium is reached.

Active Transport: Sodium-Potassium Pump

• Energy is used to pump sodium ions out and potassium ions into the cell against their concentration gradients.

• Process:

  • 3 sodium ions exit the cell, 2 potassium ions enter.

  • ATP is required for this process.

Active Transport Using Vesicles

• Endocytosis: Uses vesicles to move large particles into the cell.

  • Examples: Phagocytosis (cell eating) and Pinocytosis (cell drinking).

• Exocytosis: Uses vesicles to export materials out of the cell, such as neurotransmitters from nerve cells.