Biology 1 - Lesson 5: Membrane Structure and Transport
Overview
Every cell is bounded by a dynamic, selectively permeable membrane that controls the passage of substances in and out of the cell. Membranes not only separate the internal environment from the external one but also enable communication and energy transformation. In this lesson, we explore the fluid mosaic model of membrane structure, mechanisms of passive and active transport, and the electrochemical gradients vital for processes like nerve conduction and ATP production.
1. The Fluid Mosaic Model
1.1 Composition and Layout
Phospholipid Bilayer
Each phospholipid has a hydrophilic (water-loving) head containing a phosphate group and hydrophobic (water-fearing) tails made of fatty acids.
Arranged in two layers such that the hydrophobic tails face inward and the hydrophilic heads face outward, in contact with aqueous environments (inside and outside the cell).
Proteins
Integral (Transmembrane) Proteins: Span the entire lipid bilayer; can form channels or act as carriers.
Peripheral Proteins: Attached loosely to the membrane surface (either inner or outer side); often serve as enzymes, signaling molecules, or structural links.
Carbohydrates
Present as glycoproteins (carbohydrates attached to proteins) or glycolipids (carbohydrates attached to lipids).
Function in cell recognition and communication (e.g., determining blood types, facilitating immune responses).
Cholesterol (in Animal Cells)
Inserts between phospholipids; helps regulate membrane fluidity and stability, especially across temperature changes.
1.2 Fluidity and Mosaic Nature
Fluidity:
Lipids and some proteins can move laterally within the bilayer.
Unsaturated fatty acid tails create kinks, preventing tight packing and increasing fluidity.
Mosaic Aspect:
The membrane is often described as a “mosaic” of lipids, proteins, and carbohydrates embedded or attached to the surface.
1.3 Historical Note
Proposed by S. J. Singer and G. L. Nicolson in 1972, the fluid mosaic model revolutionized our understanding of the cell membrane’s dynamic and flexible nature. Prior models envisioned a more static, protein-coated structure.
2. Passive Transport
Passive transport does not require cellular energy (ATP). Movement occurs down a concentration gradient (from high to low concentration).
2.1 Simple Diffusion
Definition: Movement of small, nonpolar molecules (e.g., O₂, CO₂) directly across the phospholipid bilayer.
Example: Oxygen diffusing into cells for cellular respiration; carbon dioxide diffusing out as a metabolic waste product.
2.2 Osmosis
Definition: Diffusion of water across a selectively permeable membrane.
Tonicity:
Hypotonic Solution: Lower solute concentration outside the cell; water moves into the cell, causing it to swell.
Hypertonic Solution: Higher solute concentration outside; water exits the cell, causing it to shrink.
Isotonic Solution: Equal solute concentration inside and out; net water movement is balanced.
2.3 Facilitated Diffusion
Carrier Proteins: Bind specific molecules (e.g., glucose) and change shape to shuttle them across the membrane.
Channel Proteins: Form pores for ions or water (e.g., aquaporins for rapid water transport).
In each case, molecules move down their concentration gradient; the membrane proteins simply make the process more efficient or specific.
3. Active Transport
Active transport requires energy (often ATP) to move substances against their concentration gradient (from low to high concentration).
3.1 Primary Active Transport
Sodium-Potassium Pump (Na⁺/K⁺-ATPase)
Pumps 3 Na⁺ ions out of the cell and 2 K⁺ ions in per ATP molecule hydrolyzed.
Establishes electrochemical gradients essential for nerve impulse conduction and muscle contraction.
3.2 Secondary Active Transport (Cotransport)
Uses the gradient established by primary active transport as an energy source.
Example: The Na⁺ gradient created by the Na⁺/K⁺ pump drives the inward transport of glucose through a Na⁺-glucose symporter.
The sodium-potassium pump (Na+/K+-ATPase) is a key membrane protein that moves three sodium ions (Na+) out of the cell and two potassium ions (K+) in, powered by the hydrolysis of ATP. This process establishes crucial electrochemical gradients for nerve impulses, muscle contraction, and cell homeostasis.
- Ion Binding: Three Na+ ions attach inside the cell.
- ATP Hydrolysis: ATP provides energy for the pump to change shape and release Na+ outside.
- Conformational Change: The pump reconfigures to bind two K+ ions, bringing them back into the cell.
- 3 Na+ Out, 2 K+ In: This cycle repeats, maintaining the gradient needed for essential cellular functions.
4. Membrane Potentials and Electrochemical Gradients
4.1 Resting Membrane Potential
Most cells maintain a negative charge inside relative to outside, typically around −70 mV in neurons.
Key contributors:
Ion pumps (Na⁺/K⁺-ATPase)
Ion channels (e.g., K⁺ leak channels)
4.2 Importance
Nerve Impulses: Changes in membrane potential enable action potentials to travel along neurons.
Muscle Contraction: Depolarization of the muscle cell membrane triggers contraction.
Transport: Some substrates use ion gradients (e.g., H⁺ gradient in mitochondria for ATP synthesis via chemiosmosis).
5. Real-Life Applications
Medical Relevance
Intravenous (IV) Fluids: Must be isotonic to prevent red blood cells from lysing (hypotonic) or shriveling (hypertonic).
Cystic Fibrosis: Caused by defects in a chloride ion channel (CFTR), leading to thick mucus in the lungs and digestive tract.
Agricultural Practices
Correct soil osmolarity is crucial: Over-fertilizing can create hypertonic conditions, drawing water out of plant roots.
Pharmacology
Drug Delivery: Some drugs target ion channels or pumps, like calcium channel blockers for hypertension management.
6. Exercise: Investigating Osmosis with Household Materials
Objective: Observe osmosis in action using everyday items.
Materials
Two raw eggs
Vinegar (enough to submerge the eggs)
Corn syrup (or very sugary solution)
Water
Two clear containers
Procedure
Dissolve the Shell
Place each egg in vinegar for 24–48 hours. The acidic vinegar dissolves the shell, leaving the semipermeable membrane intact.
Set Up the Solutions
Place one egg in corn syrup (hypertonic) and the other in pure water (hypotonic).
Observe
Over several hours, the egg in corn syrup will shrink as water leaves the egg. The egg in water will swell as water enters.
Analyze
Compare final sizes and weigh each egg (if a kitchen scale is available).
Conclusion
Relate changes in egg size to hypertonic vs. hypotonic conditions.
7. Additional Learning Components
7.1 Historical Anecdote: Gorter and Grendel’s Lipid Bilayer Model
In 1925, Evert Gorter and F. Grendel proposed that cell membranes are a bilayer of lipids, based on their classic experiment measuring the surface area of red blood cell lipids. Although missing the integral proteins, it was a crucial step toward our modern model.
7.2 Researcher Spotlight: Peter Agre and Aquaporins
Nobel Laureate Peter Agre discovered aquaporins, the specialized channel proteins that facilitate rapid water movement. His work illuminated a fundamental mechanism for regulating water balance in cells.
7.3 Advanced Reading Suggestions
“Molecular Biology of the Cell” (Alberts et al.) – In-depth coverage of membrane dynamics and cellular transport.
Journal Articles in Nature Chemical Biology exploring novel ion channel structures and inhibitors.
7.4 Notable Breakthrough: Cryo-Electron Microscopy in Membrane Protein Study
Advances in cryo-EM allow scientists to visualize membrane proteins in near-atomic detail, revealing their conformational states and guiding drug design against diseases (e.g., antibiotic-resistant bacteria).
7.5 Interactive Concept
Online Simulations: Several educational websites offer interactive modules for osmosis and diffusion, allowing you to manipulate solute concentrations and watch how cells respond.
8. Recall Questions
Membrane Model: Summarize the fluid mosaic model. Why is cholesterol important in animal cell membranes?
Passive vs. Active Transport: Compare the energy requirements and mechanisms of passive and active transport. Provide one biological example of each.
Osmosis: What happens to a cell placed in a hypertonic solution? How about a hypotonic solution?
Ion Gradients: How do the sodium-potassium pump and secondary active transport work together to move substances across the membrane?
Home Experiment: If you performed the egg osmosis experiment, describe your observations and link them to the concept of osmosis.
Use these questions to test your understanding of membrane structure, function, and the various transport processes critical for cellular life.
