IMAT Biology Prep 🔬
Topic: Passive Transport
Movement Across Membranes Without Energy.
1. The Principles of Passive Transport
Passive transport is the movement of substances across a cell membrane without the use of cellular energy (ATP). It relies entirely on the natural tendency of substances to move down their concentration gradient—from an area of higher concentration to an area of lower concentration. Think of it like a ball rolling downhill; it happens spontaneously.
Simple Diffusion
The direct movement of small, nonpolar molecules (like O₂, CO₂, and lipids) straight through the phospholipid bilayer. The membrane doesn't need to help. The rate depends on the steepness of the gradient and the molecule's properties.
Osmosis: The Diffusion of Water
Osmosis is a special case of diffusion specifically for water molecules across a selectively permeable membrane. Water moves from an area of higher water concentration (lower solute concentration) to an area of lower water concentration (higher solute concentration). This concept of tonicity is crucial:
• Isotonic: Solute concentrations are equal; no net water movement.
• Hypertonic: Higher solute concentration outside; cell loses water and shrivels.
• Hypotonic: Lower solute concentration outside; cell gains water and may burst (lyse).
Facilitated Diffusion
This is still passive transport (no ATP), but it requires the help of a membrane protein. It's for molecules that can't easily cross the lipid bilayer, such as ions (e.g., Na⁺, K⁺) and large polar molecules (e.g., glucose).
• Channel Proteins: Form a hydrophilic tunnel for specific ions to pass through.
• Carrier Proteins: Change shape to shuttle specific molecules across the membrane.
2. Visualizing Transport Mechanisms
Simple Diffusion
Small, nonpolar molecules move directly across the membrane.
Osmosis
Water moves to the area of higher solute concentration.
Facilitated Diffusion
Molecules move through a protein channel.
3. 🧠 Medical Case Study: Oral Rehydration Therapy
A patient in a rural clinic is suffering from severe diarrhea due to cholera. They are dangerously dehydrated. Instead of using an IV drip, the healthcare worker administers an Oral Rehydration Solution (ORS), a simple mixture of water, salt (NaCl), and sugar (glucose).
Question: How does this simple drink use the principles of transport to save lives?
Answer & Explanation:
ORS is a brilliant application of membrane transport principles, specifically the link between active and passive transport.
- The Problem: In cholera, the intestine loses massive amounts of water and electrolytes, but the mechanism for absorbing glucose and sodium together remains intact.
- The Mechanism: In the small intestine, there is a co-transporter protein (SGLT1) that actively transports sodium ions (Na⁺) and glucose into the epithelial cells together. This is a form of secondary active transport.
- Creating the Gradient: The active uptake of sodium and glucose from the intestine into the cells makes the inside of the cells hypertonic (higher solute concentration) compared to the fluid in the intestine.
- Osmosis to the Rescue: Because the cells are now hypertonic, water follows the solutes from the intestine into the cells via osmosis (a passive process). This rehydrates the cells and, subsequently, the entire body.
- Takeaway: ORS works because glucose absorption is coupled to sodium absorption. By providing both, we kickstart an active transport process that creates an osmotic gradient, allowing the passive transport of water to rehydrate the patient effectively.
4. 📊 Concept Check: True or False?
5. 📝 IMAT-Style Practice Questions
6. 🧾 Key Takeaways Summary
- Passive Transport: No ATP required. Movement is always down a concentration gradient (high to low).
- Simple Diffusion: For small, nonpolar molecules (O₂, CO₂) that pass directly through the lipid bilayer.
- Osmosis: The diffusion of water across a selectively permeable membrane, driven by differences in solute concentration (tonicity).
- Facilitated Diffusion: For ions and large polar molecules (glucose). Still passive, but requires a specific channel or carrier protein.
- Tonicity is Key: A cell in a hypertonic solution loses water; in a hypotonic solution, it gains water; in an isotonic solution, there is no net change.