Cellular Transport Mechanisms

General Principles of Movement

• Substances always move from an area of high concentration to an area of low concentration to achieve equilibrium.

Diffusion

• Movement of solute molecules down their concentration gradient (from $\text{high} \to \text{low}$ concentration).

Osmosis

• Specific movement of $\text{water}$ across a semi-permeable membrane.
• Water moves from an area of high water concentration (low solute) to an area of low water concentration (high solute).
• Often occurs simultaneously with diffusion, but in opposite directions for solvent and solute.

Passive Transport

• Does not require cellular energy ($\text{ATP}$).
• Includes diffusion, osmosis, and facilitated diffusion.

Facilitated Diffusion

• Uses channel or carrier proteins.
• Does not require $\text{ATP}$ (energy).
• Moves substances down their concentration gradient.

Osmotic Pressure and Tonicity

• Osmotic Pressure: The effect of solute concentration on water movement.
• Tonicity: Describes the relative solute concentration outside a cell compared to its inside.
  • Isotonic: Equal solute concentration outside and inside the cell.
  • Hypotonic: Lower solute concentration outside the cell than inside.
  • Hypertonic: Higher solute concentration outside the cell than inside.

Effects of Tonicity on Animal Cells

• Isotonic: Ideal conditions; the cell maintains its normal shape.
• Hypotonic: Water enters the cell, causing it to lyse (burst).
• Hypertonic: Water leaves the cell, causing it to shrivel.

Effects of Tonicity on Plant Cells

• Isotonic: Functional but not optimal; cell wall not maximally inflated.
• Hypotonic: Water enters the cell, creating turgor pressure; ideal conditions for plants.
• Hypertonic: Water leaves the cell, causing plasmolysis (shriveling).

Active Transport

• Requires cellular energy ($\text{ATP}$).
• Moves substances against their concentration gradient (from $\text{low} \to \text{high}$ concentration).
• Uses carrier proteins.

Bulk Transport

• Used for moving large molecules or large quantities of substances.
• Requires $\text{ATP}$ (energy).
• Exocytosis: Releases substances out of the cell (e.g., waste, hormones) via vesicles fusing with the plasma membrane.
• Endocytosis: Takes substances into the cell via vesicle formation around the substance.
  • Phagocytosis: "Cell eating"; engulfs large molecules, cellular debris, or bacteria.
  • Pinocytosis: "Cell drinking"; engulfs fluids and dissolved small molecules.