Atlas MCAT Course
Chemical Foundations → Acids & Bases
Acids & Bases
Proton control in biological systems.
01The idea
Enzymes are proteins. Proteins are made of amino acids. Amino acids gain and lose protons.
That means pH controls protein behavior, and therefore controls metabolism, signaling, and physiology.
THREAD · Structure → function
If you can track where protons go, you can predict enzyme activity, binding interactions, and structural stability.
02Build the model
ABrønsted–Lowry: the only definition that matters here
- Acid: proton donor
- Base: proton acceptor
This definition works universally in water, cells, and enzyme active sites.
BConjugate acid–base pairs
Whenever an acid donates a proton, it becomes its conjugate base. Whenever a base accepts a proton, it becomes its conjugate acid. These pairs are inseparable.
CStrength is about willingness, not labels
C.1Acid strength
Acid strength reflects how easily a molecule gives up a proton.
| Strong acids | Weak acids | |
|---|---|---|
| Donate protons | Readily | Reluctantly |
| Conjugate base | Weak | Stronger |
C.2Base strength
Base strength reflects how strongly a molecule attracts a proton: strong bases readily accept protons; weak bases accept protons less readily.
DpKa: the proton tipping point
pKa is the pH at which a molecule is 50% protonated and 50% deprotonated.
- pH < pKa → protonated form dominates
- pH > pKa → deprotonated form dominates
This single rule explains amino acid charge states, enzyme activity optima, and binding behavior. No math required.
EAcid–base chemistry in proteins
Many amino acid side chains can donate or accept protons and change charge with pH.
Changing protonation state can:
- Alter ionic interactions
- Disrupt hydrogen bonds
- Change protein shape
- Kill enzyme activity
FWhy water matters (but isn’t special)
Water can donate a proton (becoming OH⁻) or accept one (becoming H₃O⁺). But in cells, most acid–base reactions occur within proteins, not free solution.
03See the connections
The lesson names these links itself. Read the map left to right.
Proton transfer
- Acid: donor
- Base: acceptor
- Conjugate pairs, in equilibrium
Decided by
- Strength (willingness to donate)
- pH compared with pKa
In proteins
- Side-chain charge states
- Ionic and hydrogen-bond interactions
- Shape and enzyme activity
Next: buffers, Henderson–Hasselbalch, and physiological pH control.
04Reason with it
Four questions. Answer in your head first, then choose. You’ll see the reasoning either way.
Think first.
A side chain has a pKa of 7. The pH is 5. Which form dominates?
Think first.
A strong acid has what kind of conjugate base?
Think first.
The pH in an active site rises above the pKa of a key catalytic side chain. What can happen?
Think first.
What does calling an acid “strong” describe?
05MCAT lens
The MCAT emphasizes
Directional reasoning (protonated vs deprotonated)
Functional consequences of pH change
Acid–base logic applied to biology
You are rarely asked to calculate pH or memorize Ka values. You are often asked to predict what happens when pH shifts.
06Common traps
Trap 1: Strong acids are dangerous because they are acidic
Misconception
Strength reflects dissociation, not harm.
Correct model
Strength reflects proton donation tendency.
Trap 2: pKa is just a constant to memorize
Misconception
It’s a decision boundary.
Correct model
Compare pH to pKa.
Trap 3: Acid–base chemistry stops once metabolism starts
Misconception
It underlies all enzyme activity.
Correct model
Metabolism depends on proton control.
07Dependency check
Can you reconstruct this concept without looking at the page? You should now be able to reason through:
What makes something an acid or base
Check against 02 · A: proton donor or proton acceptor.
How conjugate pairs behave
Check against 02 · B and C: inseparable, reversible, and inversely related in strength.
What pKa tells you biologically
Check against 02 · D: which form dominates at a given pH.
Why pH affects enzyme activity
Check against 02 · E: side chains change charge, breaking interactions and changing shape.
If something feels unclear, the missing idea is usually thisBiological acid–base chemistry is proton bookkeeping.
08Remember
REMEMBER · if you keep only a few things
- Acid donates a proton; base accepts one. Conjugate pairs are inseparable.
- Strong acid ↔ weak conjugate base.
- pKa = pH at 50% protonated. pH below pKa: protonated wins. Above: deprotonated wins.
- Side chains are acids and bases, so enzymes are pH-sensitive.
- Strength ≠ harm ≠ concentration.
- pH effects are local before they are global.
09Forward hook
Acid–base chemistry explains how individual groups gain or lose protons. The next question is:
How do biological systems resist changes in pH?
That leads directly to buffers, Henderson–Hasselbalch, and physiological pH control.
Next lessonBuffers→