Biology by Bradford
Biology by Bradford · AP Biology · Unit 2 · CED 2.8–2.10

Active & Bulk Transport

Gradients are stored energy. This lesson is about what it costs to build one, what a cell buys with it, and where the compartments came from in the first place.

AP · Unit 2CED 2.8–2.10Interactive lesson
Concept 01 · CED 2.8

Paying to go uphill

Everything in the previous lesson was free, because it ran downhill. The moment a cell wants a substance where there is already plenty of it, someone has to pay.

The definition

Against the gradient

Low concentration to high. Never spontaneous, so it always has an energy source attached.

Primary

ATP at the protein

The pump hydrolyses ATP itself and is phosphorylated by it. The Na⁺/K⁺ ATPase is the standard example.

Secondary

Spending a gradient

Let one ion fall downhill to drag another uphill. No ATP here — but ATP built the gradient earlier.

PASSIVEACTIVEDOWNHILL · NO ATPATPUPHILL · ATP SPENTTHE GRADIENT DECIDES WHICH ONE YOU ARE LOOKING AT
The molecule is the same and the protein may be the same. The only thing that distinguishes active from passive transport is which way the gradient points — and therefore whether energy has to be supplied to make the journey happen.
Diagram · interactive

The pump that never stops

Tap each part. This one protein can account for a quarter of the energy a resting cell spends.

OUTSIDECYTOSOLNa / K ATPaseNaNaNa3 SODIUM IN FROM CYTOSOLKK2 POTASSIUMATPP3 OUT · 2 INnet −1 insideTAP A PART
Diagram

Three out, two in

Tap the pump, the sodium ions, the potassium ions, the ATP, or the charge summary.

Signature interactive · CED 2.8

One full cycle

Six steps. Watch when the ATP arrives and what it actually does — it is a chemical modification, not a shove.

Concept 02 · CED 2.8

When the cargo is too big

No pore is wide enough for a bacterium. For anything on that scale the membrane itself has to move.

Concept 03 · CED 2.9–2.10

Where the compartments came from

Internal membranes let a cell run chemistry that would otherwise be mutually destructive. Two of those compartments appear to have arrived as lodgers.

The benefit

Separate conditions

pH 5 inside a lysosome, a proton gradient across a mitochondrial membrane, calcium stored in the smooth ER.

The cost

Maintenance

Every one of those differences leaks. Pumps run continuously to hold them, which is where much of a cell's ATP goes.

The origin

Endosymbiosis

Mitochondria and chloroplasts look like captured prokaryotes — and still behave like them in five telling ways. Remember them as MAD DR.

MITOCHONDRIONFREE-LIVING BACTERIUMTWO MEMBRANES, CRISTAEONE MEMBRANE, CIRCULAR DNAEVIDENCEWHY IT POINTS TO A BACTERIAL ANCESTORMDouble membraneInner membrane is the ancestor’s, outer is the host’sAAntibiotic sensitivityDrugs that hit bacterial ribosomes disrupt mitochondria tooDDNA, circularIts own chromosome, circular and not wound on histonesDDivides by binary fissionReplicates on its own schedule, not with the cell cycleRRibosomes are 70SProkaryotic size, not the 80S of the eukaryotic cytosolMAD DR · FIVE LINES OF EVIDENCE, ONE EXPLANATION
Five independent observations, and the mnemonic runs down the left: Membranes, Antibiotics, DNA, Division, Ribosomes. Each would be odd on its own; together they are hard to explain any other way. The antibiotic line is the one students most often miss and the one examiners like best — drugs such as tetracycline target the 70S ribosomes of bacteria, and they disrupt mitochondria for exactly the same reason, which is also why some antibiotics carry side effects in the tissues that respire hardest.
Live model · particles

The pump against the leak

Two pumps, running continuously. Sodium and potassium cannot cross unaided — but both leak a little, which is exactly why the pumping never stops.

Concept map · interactive

How it all hangs together

Tap a node to light up its connections. The pink dashed link closes the loop back to the first lesson of the unit.

so it needsdefinesexamplecreatespowersalso drivesusesshuttlekeeps them differentAgainst gradientATPActive transportNa / K pumpIon gradientSecondary transportBulk transportVesiclesCompartments
Concept map

Tap a node

Everything here costs energy, directly or indirectly. That is the difference from the last lesson.

Retrieval · drag and drop

Fill the gaps

Drag a term into a gap, or tap a term and then tap a gap. Two terms belong nowhere.

active transportATPagainstelectrogenicsecondaryendocytosisexocytosisendosymbiosis osmosissaturation

Moving a substance from a low to a high concentration requires , powered directly or indirectly by . The sodium–potassium pump moves both ions their gradients, and because it exports three positive charges for every two it imports it is described as . When a cell uses the gradient that pump created to haul glucose inward, that is active transport. Material too large to cross the membrane enters by and leaves by . The double membranes and circular DNA of mitochondria are evidence for .

Check yourself · AP-style

Six questions

Single best answer. You get the reasoning as soon as you commit.