Transport Across Membranes

Introduction

• This lecture covers the movement of molecules across cellular membranes.
• Builds on previous lessons about cell surface and organelle membranes.

Plasma Membrane Recap

• Plasma membrane consists of a phospholipid bilayer.
• Lipid-soluble molecules and very small molecules can diffuse through.
• Polar, ion substances, or large molecules cannot diffuse through easily.

Types of Membrane Transport

1. Simple Diffusion

• Movement: High concentration → Low concentration.
• Continues until equilibrium is reached.
• No ATP required; relies on kinetic energy.
• Occurs in liquids and gases.
• Requires molecules to be small and lipid-soluble.

2. Facilitated Diffusion

• Also a passive process (no ATP required).
• Utilizes proteins within the membrane.
  • Protein Channels: Water-filled tubes allow passage of water-soluble ions or molecules.
  • Carrier Proteins: Shape-specific binding changes protein shape, facilitating molecule transport.
• Used for ions, polar molecules, and large molecules.

3. Osmosis

• Definition: Movement of water from higher to lower water potential through a partially permeable membrane.
• Water Potential: Pressure created by water molecules; measured in kilopascals (kPa).
• Pure water = 0 kPa (highest potential), solutes lower the potential making it negative.

Osmosis Context

• Water moves from less negative to more negative water potential.
• Terminology:
  • Isotonic: Equal water potential on both sides of a membrane.
  • Hypotonic: Solution with higher (less negative) water potential compared to the cell.
  • Hypertonic: Solution with lower (more negative) water potential.
• Effects on Cells:
  • Animal Cells: Burst in hypotonic; shrivel in hypertonic solutions.
  • Plant Cells: Become turgid in hypotonic; also shrivel in hypertonic solutions.

4. Active Transport

• Movement from lower to higher concentration (against gradient).
• Requires ATP and carrier proteins (acting as pumps).
• Highly selective; only specific molecules bind to carrier proteins.
• ATP is hydrolyzed to release energy required for protein shape change.

Conclusion

• Overview of membrane transport types: simple diffusion, facilitated diffusion, osmosis, and active transport.
• Emphasized the role of ATP in active transport and the selectivity of carrier proteins.
• Encouragement to watch linked videos for more detailed explanations on specific topics.