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Mechanisms of Transport

▶︎  Watch it animatedinteractive step-through · ~3 min · optional

Diffusion and channels move small molecules across the membrane one at a time, but a cell also needs to import a whole bacterium, engulf a droplet of fluid, or release a burst of hormone all at once. That material is far too large — or far too much — to slip through a protein pore. So the cell moves it in bulk, wrapped inside a piece of membrane. In endocytosis the membrane folds inward, pinches off, and carries cargo in as a vesicle. In exocytosis a vesicle fuses with the membrane from the inside and dumps its contents out. This is bulk transport: material crosses the boundary packaged in membrane, never dissolved through it.

Because the cell has to bend, pinch, and fuse whole sheets of membrane, bulk transport always costs energy — it is an active process powered by ATP, regardless of which way the cargo is heading. That is the point that separates it from the transport that came before: a channel or diffusion follows the concentration gradient for free, but wrapping a payload in a vesicle is work the cell must pay for every time. Read the direction from the mechanism: membrane folding in means import, a vesicle fusing out means export, and both are the cell spending ATP to move what no pore could ever carry.

2.9 Mechanisms of Transport UNIT 2 2.9 MECHANISMS OF TRANSPORT Cells move materials across the plasma membrane by passive, active, and bulk (vesicle) transport. BULK TRANSPORT Large amounts moved inside vesicles · uses ATP ENDOCYTOSIS — INTO THE CELL ↓ PHAGOCYTOSIS “cell eating” Engulfs solids: bacteria, debris. Uses pseudopodia. PINOCYTOSIS “cell drinking” Takes in extracellular fluid & dissolved solutes. RECEPTOR-MEDIATED specific uptake Ligands bind receptors → coated vesicle forms. EXOCYTOSIS — OUT OF THE CELL ↑ Vesicles fuse with the membrane and secrete their contents. PASSIVE No ATP · down gradient SIMPLE DIFFUSION FACILITATED DIFF. OSMOSIS O₂, CO₂ cross bilayer channels / carriers: glucose, ions water via aquaporins ACTIVE ATP · against gradient PRIMARY SECONDARY pump uses ATP directly e.g. Na⁺/K⁺ pump uses ion gradient set by a pump (indirect ATP) e.g. Na⁺/glucose symport HOW THEY COMPARE MECHANISM ATP? DIRECTION VESICLE? EXAMPLE Simple diffusion No Down gradient No O₂, CO₂, steroids Facilitated diffusion No Down gradient No Glucose, ions Osmosis No Down (water) No Water via aquaporins Primary active Yes Against gradient No Na⁺/K⁺ pump Secondary active Yes* Against gradient No Na⁺/glucose symport Endocytosis Yes Into cell Yes Phago-, pino-, RME REAL-WORLD EXAMPLES Immune cells phagocytose pathogens Neurons release neurotransmitter (exo) Kidney tubules reabsorb solutes (active) Intestine Na⁺/glucose symport uptake Glands secrete peptide hormones (exo) Passive moves substances DOWN the gradient with no ATP; active moves AGAINST it using ATP. Endocytosis = materials IN, exocytosis = OUT.

Fold the membrane inward to engulf cargo as a vesicle, or fuse a vesicle outward to release it — and watch the ATP cost each move demands. Endocytosis on one side, exocytosis on the other, and the energy that both require.

Bulk Transport · Open the full sandbox →

The common mistake here is treating bulk transport like the protein-mediated transport students just learned: assuming that endocytosis and exocytosis move cargo through channels or carrier proteins, and that moving "with the gradient" makes it free. Neither holds. The cargo never passes through a protein — it rides inside a membrane-bound vesicle — and bending and fusing that membrane costs ATP no matter which direction the material moves. Every scenario in this topic asks the same thing: say why vesicle packaging, and not a pore, is what carries the load, and why that packaging is never free.

The work

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Lesson
Mechanisms of Transport ›

Cells move material too large for any pore by wrapping it in membrane — endocytosis to bring it in, exocytosis to send it out, ATP spent either way. The lesson walks the ways students confuse bulk transport with protein channels and assume it is free, then closes with a ten-scenario applet: read the direction from the mechanism and say why vesicle packaging is what carries the load.

Skill check · 10 scenarios
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Diagnostic
10-item topic check ›

Ten items on bulk transport (U2-BIO16): reading import versus export from membrane folding or vesicle fusion, recognizing that cargo rides inside a vesicle rather than through a protein, and catching the assumption that moving with the gradient makes it free. Take it cold to surface which links are still broken, or after the lesson to confirm they hold.

Not started · 10 items · ~15 min
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Targeted Practice
Drill a single misconception ›

Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears it for now and moves you to the next. Two in a row is a checkpoint, not proof: if the error resurfaces later, the misconception comes back.

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