Biology by Bradford
Biology by Bradford · AP Biology · Unit 3.3

Cellular Energy

Order costs something. Every structure a cell maintains is paid for continuously, in a currency it holds only seconds of.

AP · U3.3CED 3.3Interactive lesson
Concept 01 · CED 3.3

A currency, not a savings account

Cells do not run on glucose. They run on ATP, and they hold barely any of it, regenerating the entire pool many times a minute.

The store

Glucose

Dense, stable and transportable. Excellent for keeping energy; useless for spending it directly.

The currency

ATP

Small, soluble and immediately spendable. A typical cell turns over its own mass in ATP every day.

The rate

Seconds of supply

You hold only a few seconds' worth at any moment. Stop making it and the shortage is immediate.

ATPADP + PiHYDROLYSISreleases energyEXERGONICSYNTHESISrequires energyENDERGONIC−30.5 kJPER MOLEA CYCLE, NOT A STORE · TURNOVER IS MEASURED IN SECONDS
The reason a cell keeps so little ATP is that it does not need to. Hydrolysis and resynthesis run continuously in a tight loop, so ATP behaves like cash moving through a till rather than money in a vault. The same phosphate group may be attached and removed thousands of times a day.
Diagram · interactive

Inside the molecule

Tap each part. The instability is the feature, not a flaw, and it comes from charge, not from the bond itself.

ADENINEA PURINE BASERIBOSEPENTOSE SUGARPPTHE BOND THAT MATTERSPTERMINALTHREE NEGATIVE CHARGESCROWDED TOGETHERTAP A PART
Diagram

Adenine, ribose, three phosphates

Tap the base, the sugar, any of the three phosphates, the terminal bond, or the charge panel.

Signature interactive · CED 3.3

Paying for an uphill reaction

Five steps that turn an impossible reaction into a spontaneous one, without breaking any thermodynamics.

Concept 02 · CED 3.3

Which way will it go?

One sign decides everything. But it decides direction only: never speed, and never whether the reaction will happen this century.

ΔG < 0

Exergonic

Products hold less free energy than reactants. Energy is released and the reaction can proceed unaided.

ΔG > 0

Endergonic

Products hold more. Energy must be supplied from outside, and no enzyme can substitute for it.

The caveat

Spontaneous ≠ fast

Glucose oxidising in air is spontaneous, yet sugar keeps for years. Direction and rate are separate questions.

EXERGONICENDERGONICΔG < 0RELEASES ENERGYREACTANTSPRODUCTSΔG > 0REQUIRES ENERGYPRODUCTSREACTANTSSPONTANEOUS DOES NOT MEAN FAST
The dashed curve between the levels is the activation energy, and it exists in both diagrams. That is exactly why spontaneous does not mean fast, and why exergonic reactions in cells still need enzymes to happen at a useful rate.
Concept map · interactive

How it all hangs together

Tap a node to light up its connections. The pink dashed link is the rule that makes ATP necessary at all.

captured asreleased bymakeshydrolysedpowersoxidation ispays forneedsdrivesSunlightGlucoseRespirationATPHydrolysisCellular workExergonicCouplingEndergonic
Concept map

Tap a node

Follow the left column downward: sunlight to glucose to ATP to work.

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.

ATPexergonicendergoniccouplingphosphorylationhydrolysisspontaneousfree energy denaturationosmosis

The immediate energy currency of the cell is . A reaction with a negative change in is described as and is , whereas a reaction with a positive value is and will not proceed alone. Cells solve this by energy , in which the of ATP pays for the uphill reaction. Transferring the terminal phosphate group onto another molecule is called .

Check yourself · AP-style

Six questions

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