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Cell transport flow chart answer key
A membrane-transport flow chart is an efficient way to classify how substances cross the plasma membrane. The key decisions are: (1) particle size and whether vesicles are needed, (2) movement relative to a concentration or electrochemical gradient, and (3) whether transport proteins or cellular energy are required.
Completed flow chart
Answer key in words (decision-by-decision)
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Very large cargo or bulk fluid?
- Yes → Vesicular transport (membrane-bound vesicles).
- No → Non-vesicular transport (directly across the membrane via lipid bilayer and/or proteins).
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If non-vesicular: Is it water movement?
- Yes → Osmosis (net water movement down water potential; often through aquaporins).
- No → evaluate movement relative to a gradient.
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Down concentration/electrochemical gradient only?
- Yes → Passive transport (no ATP required for the transport step).
- No → Active transport (requires energy input to move against a gradient).
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If passive: Transport protein required?
- No → Simple diffusion (typically small nonpolar molecules such as O2, CO2).
- Yes → Facilitated diffusion (channels or carriers; common for ions and polar solutes).
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If active: ATP used directly?
- Yes → Primary active transport (ATP hydrolysis drives pumping, e.g., Na+/K+-ATPase).
- No → Secondary active transport (coupled transport using an ion gradient; symport or antiport).
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If vesicular: Direction?
- Into the cell → Endocytosis (phagocytosis, pinocytosis, receptor-mediated endocytosis).
- Out of the cell → Exocytosis (secretion/export to the extracellular space).
Compact answer-key table
| Endpoint in the flow chart | Defining features | Typical examples |
|---|---|---|
| Simple diffusion | Down concentration gradient; no protein required; depends on lipid solubility and gradient magnitude | O2, CO2, small hydrophobic molecules |
| Facilitated diffusion | Down gradient; requires channel or carrier; selective and saturable (carriers) | Ion channels (K+, Na+), glucose transporters |
| Osmosis | Net water movement; driven by differences in solute concentration/water potential; often via aquaporins | Water movement in hypotonic/hypertonic conditions |
| Primary active transport | Moves against gradient; energy directly from ATP hydrolysis | Na+/K+-ATPase, Ca2+ pumps |
| Secondary active transport | Moves one solute against its gradient by coupling to another solute moving down its gradient | Na+-glucose symport; Na+/H+ antiport |
| Endocytosis | Vesicles bring material into cell; requires energy and cytoskeletal/membrane remodeling | Phagocytosis of particles; receptor-mediated uptake of specific ligands |
| Exocytosis | Vesicles fuse with membrane to release contents outside; requires energy and membrane fusion proteins | Neurotransmitter release; secretion of enzymes/hormones |
Gradient note (for ions)
For ions, “down gradient” refers to the electrochemical gradient. A common way to express the driving force is:
\[ \Delta \mu = RT \ln\!\left(\frac{C_2}{C_1}\right) + zF\Delta \psi \]
Passive transport proceeds when \(\Delta \mu\) favors movement; active transport is required when movement is forced opposite the electrochemical driving force.
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