Unit 1: Energy, Water, and the Chemistry Underneath Biology
The thermodynamics, equilibrium, and water chemistry a cell runs on before any biology happens at all.
9 topics · 34 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Energy and Entropy in Living Systems
- How it worksHow does a cell use the entropy it dumps into its surroundings to build order inside itself without breaking the second law?
- Common mistakeWhy doesn't a growing, self-organizing cell violate the second law of thermodynamics?
- Going furtherWhat happens to the orderly structure inside a cell the moment its energy supply is cut off?
Your turn1 of 2
A cell runs a metabolic reaction that dumps 33 kilojoules of heat into its surroundings at a constant body temperature of 310 kelvin. Find the entropy increase of the surroundings in joules per kelvin.Gibbs Free Energy and Whether a Reaction Runs
- How it worksHow do a reaction's change in enthalpy and change in entropy combine into one number that predicts which way it runs?
- Worked exampleFind whether a reaction with delta H of negative 20 kilojoules per mole and delta S of negative 60 joules per mole-kelvin is spontaneous at 310 kelvin.
- Common mistakeWhy doesn't a negative delta G tell you anything about how fast a reaction actually happens?
- Going furtherWhat happens to a reaction's spontaneity once rising temperature flips the sign of delta G?
Your turn1 of 2
A reaction has a delta H of -16 kilojoules per mole and a delta S of 16 joules per mole-kelvin. Find delta G in kilojoules per mole at a temperature of 313 kelvin.Coupled Reactions and ATP-Driven Chemistry
- How it worksHow does hydrolyzing ATP get chemically linked to a reaction that would never run on its own?
- How it worksWalk me through how adding the free energies of two coupled reactions can turn an overall positive delta G negative.
- Worked exampleFind the overall delta G of a reaction with delta G of plus 25 kilojoules per mole when it is coupled to ATP hydrolysis releasing negative 30.5 kilojoules per mole.
- Going furtherWhat happens to a coupled reaction's favorability if the ATP-hydrolysis step driving it gets blocked?
Your turn1 of 2
A biosynthetic step has a delta G of plus 26 kilojoules per mole on its own. The cell couples it to ATP hydrolysis, which releases -28 kilojoules per mole. Find the overall delta G of the coupled reaction.Chemical Equilibrium and Le Chatelier's Principle in the Cell
- How it worksHow does constantly hauling away a reaction's product keep a cell's chemistry pushed away from equilibrium?
- Worked exampleFind the equilibrium constant for a reaction with delta G standard of negative 11.4 kilojoules per mole at 298 kelvin.
- Going furtherWhat happens to a reaction's equilibrium position when a cell keeps pumping in fresh substrate and removing product?
- Common mistakeWhy isn't reaching chemical equilibrium actually something a living cell wants to do?
Your turn1 of 2
A reaction has a standard delta G of negative 12 kilojoules per mole at a temperature of 301 kelvin. Find its equilibrium constant.Water's Structure and Its Hydrogen-Bond Network
- How it worksHow does oxygen's stronger pull on shared electrons give a single water molecule a positive end and a negative end?
- Going furtherWhat happens to water's hydrogen-bond network in the instant a charged ion drops into it?
- Common mistakeWhy isn't a hydrogen bond just a weaker version of the covalent bonds holding a water molecule together?
Your turn1 of 2
A student learns that a hydrogen bond forms between a slightly positive hydrogen on one water molecule and a slightly negative oxygen on a neighboring one, and concludes that a hydrogen bond must just be a weaker version of the same covalent bond holding each water molecule's own hydrogen and oxygen together. Why is this comparison wrong?The Hydrophobic Effect as an Entropy Story
- How it worksHow does forcing water molecules into a rigid cage around an oily molecule cost the system entropy?
- How it worksWalk me through how oil droplets merging into one blob frees more water molecules to move randomly than staying separated does.
- Common mistakeWhy isn't the hydrophobic effect actually water molecules disliking or repelling oil?
- Going furtherWhat happens to the strength of the hydrophobic effect as the temperature rises?
- Ties togetherHow does the same entropy-driven hydrophobic effect that clumps oil droplets also fold a protein's core?
Your turn1 of 2
Someone watches oil droplets in water merge into a single larger blob and explains it by saying water molecules dislike the oil and physically push it away. What is the actual thermodynamic explanation for why the droplets merge?Acids, Bases, and pH Inside the Cell
- How it worksHow does the concentration of free hydrogen ions in a solution get compressed into the pH scale?
- Worked exampleFind the pH of a solution with a hydrogen ion concentration of 3.2 times 10 to the negative 5 molar.
- Common mistakeWhy isn't going from pH 6 to pH 7 a small, one-unit change in how acidic something is?
Your turn1 of 2
Find the pH of a solution with a hydrogen ion concentration of 7.09 times 10 to the negative 6 molar.pKa and the Henderson-Hasselbalch Equation
- How it worksHow does the ratio of an acid's dissociated form to its protonated form shift as pH moves away from its pKa?
- Worked exampleFind the pH of a buffer made from acetic acid with a pKa of 4.76 when the acetate-to-acetic-acid ratio is 3 to 1.
- Worked exampleFind the ratio of a weak acid's protonated to unprotonated form at a pH exactly 2 units below its pKa.
- Going furtherWhat happens to a weak acid's charge state as the surrounding pH sweeps from far below to far above its pKa?
Your turn1 of 2
A buffer is made from a weak acid with a pKa of 5.13 when the ratio of its base form to its acid form is 3.96 to 1. Find the pH of the buffer.Buffers and How Cells Hold Their pH Steady
- How it worksHow does a mix of a weak acid and its conjugate base soak up added hydrogen ions without much pH change?
- Worked exampleFind the ratio of bicarbonate to carbonic acid needed to hold blood pH at 7.4, given a pKa of 6.1.
- Going furtherWhat happens to a buffer's ability to resist pH change once you push far past its pKa in either direction?
- Ties togetherHow does the bicarbonate buffer system in blood use the same acid-base chemistry as a buffer mixed on a lab bench?
Your turn1 of 2
Find the ratio of bicarbonate to carbonic acid needed to hold blood at pH 7.487, given a pKa of 6.219.