Molecular Traffic Worksheet
Cell Biology Lab Series | Membrane Dynamics & Transport
Molecular Traffic Worksheet
Investigating Concentration Gradients, Passive Transport Modes, and Osmotic Tonicity
Name:
Date:
Period:
1
The Phospholipid Barrier & Concentration Gradients
Fundamental Concept
Extracellular: High Solute [ ● ● ● ● ] Phospholipid Bilayer Intracellular: Low Solute [ ● ] Passive flux flows down the concentration gradient
A. Define concentration gradient and why molecules spontaneously move down it:
B. When concentrations equalize across the membrane, explain dynamic equilibrium:
2
Passive Transport: Simple Diffusion vs. Facilitated Diffusion
Zero ATP Required
A. Simple Diffusion Direct Bilayer Pass
Small, uncharged, nonpolar molecules slip directly between hydrophobic fatty acid tails.
Examples: Oxygen (\(O_2\)), Carbon Dioxide (\(CO_2\)), Lipids.
Why are ions (\(Na^+\), \(Cl^-\)) unable to pass directly through the bilayer?
B. Facilitated Diffusion Protein Assisted
Polar or charged solutes move down their gradient through membrane channel or carrier proteins.
Examples: Glucose (GLUT carrier), Water (Aquaporins), \(K^+\) channels.
How does a carrier protein differ functionally from an open channel protein?
3
Osmosis & Tonicity Investigation
Water Equilibrium
Hypotonic Solution Lower solute outside than inside
Swollen
Net Water Flux:
Animal Cell Effect:
Plant Cell State:
Isotonic Solution Equal solute outside & inside
Normal
Net Water Flux:
Animal Cell Effect:
Plant Cell State:
Hypertonic Solution Higher solute outside than inside
Shriveled
Net Water Flux:
Animal Cell Effect:
Plant Cell State:
Plant vs. Animal Tonicity Check: Why do plant cells prefer hypotonic environments (turgor pressure), whereas human red blood cells undergo osmotic lysis?
Molecular Traffic Worksheet • Part II
Active Transport Mechanics & Synthesis
Page 2 of 2
4
Active Transport: Pumping Against the Gradient
Requires ATP Hydrolysis
ATP → ADP + Pi [High Solute] [Low Solute] Pumping solutes from LOW to HIGH concentration
A. Why does moving solutes against their gradient require ATP energy?
B. In the human \(Na^+/K^+\) pump, how many ions are moved per cycle and where?
Endocytosis (Bulk Intake) Inward Vesicle
Cell membrane invaginates to engulf large particles (phagocytosis) or droplets (pinocytosis).
Name a cellular/immune example:
Exocytosis (Bulk Secretion) Outward Fusion
Internal Golgi vesicles fuse with the membrane to release hormones, enzymes, or signals.
Name an endocrine/nerve example:
5
Master Comparison Matrix: Transport Mechanisms
Complete each cell precisely
| Mechanism | ATP Needed? | Direction of Movement | Membrane Protein? | Key Molecular Examples |
|---|
| Simple Diffusion | | | | |
| Facilitated Diffusion | | | | |
| Osmosis | | | | |
| Active Transport (Pump) | | | | |
| Bulk Transport (Vesicles) | | | | |
6
Clinical & Biological Case Studies
Applied Inquiry
Case A: The Emergency Room IV Mistake
A patient with severe dehydration is mistakenly infused with pure distilled water instead of isotonic 0.9% saline.
Explain what will happen to the patient's red blood cells and why:
Case B: Oral Rehydration Solution (ORS)
Cholera bacteria cause extreme dehydration. World Health Organization ORS packets blend salt (\(Na^+\)) and glucose.
How does intestinal transport of \(Na^+\) and glucose trigger osmotic reabsorption?
