Bioenergetics & Metabolic Foundations

BCH 100 — Introductory Biochemistry · Dr. Radi

build Jul 17 · 19:00 · CC BY-NC-SA 4.0 · owned figures (RDKit / matplotlib / PyMOL)
Dr. Radi

By the end of this unit, you can…

  • Interpret ΔG (with ΔH, ΔS) to judge spontaneity, and explain reaction coupling
  • Describe ATP as the energy currency — its bonds, the ATP/ADP cycle, and food-energy connection
  • Explain cofactors, coenzymes, and the electron carriers NADH, FADH₂, and CoA (and their vitamin origins)
  • Map the stages of catabolism and anabolism and the hormones that regulate them
Dr. Radi

Today's route 🗺️

  1. Bioenergetics — Free Energy & Coupling
  2. ATP — The Cell's Energy Currency
  3. Coenzymes, Cofactors & Electron Carriers
  4. The Stages of Metabolism
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1 · Bioenergetics — Free Energy & Coupling

"Use ΔG to tell which reactions run on their own — and how the cell pays for the ones that don't."

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Does a reaction go? Ask ΔG

Whether a reaction runs downhill is set by its free energy: ΔG = ΔH − TΔS — the heat released (ΔH) minus the disorder created (TΔS). ΔG < 0 is exergonic and goes on its own; ΔG > 0 is endergonic and needs a push.

ΔG says nothing about speed — that's the enzyme's job. It only says which way is downhill.
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Coupling: let ATP pay for it

Cells run uphill reactions constantly — they just pay for them. Bolt an endergonic reaction onto the big exergonic hydrolysis of ATP, and because they share a step the ΔG°′ values add up. Uphill + a bigger downhill = net downhill.

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2 · ATP — The Cell's Energy Currency

"Meet ATP — why its phosphate bonds store energy, how the ATP/ADP cycle spends it, and where the Calories on a label go."

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ATP: adenine, ribose, and three phosphates

ATP is adenosine with a triphosphate tail. The energy lives in the two phosphoanhydride bonds linking the phosphates — cram three negative phosphates together and they repel, so snapping one off releases a lot of energy.

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The ATP ⇌ ADP cycle

ATP is a rechargeable battery, not a storage tank. Hydrolyzing it to ADP + Pi releases −30.5 kJ/mol to power muscle, biosynthesis, and transport; catabolism (and the electron transport chain) recharges it. A cell turns over its body weight in ATP each day.

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Where the Calories go

The Calories on a food label are just fuel — the energy your body harvests from fat, carbs, and protein and banks as ATP. That "230 Cal" is about 960 kJ (1 Cal = 4.184 kJ) of chemical energy headed for the ATP cycle.

Nutrition Facts label: U.S. FDA, public domain (Wikimedia Commons)
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3 · Coenzymes, Cofactors & Electron Carriers

"Meet the enzyme's helpers — metal cofactors, organic coenzymes, and the NADH / FADH₂ / CoA that shuttle electrons and groups."

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Cofactors vs coenzymes

Many enzymes can't work alone — they need a non-protein helper. Cofactors are inorganic (metal ions like Zn²⁺, Mg²⁺, Fe²⁺); coenzymes are small organic molecules, often built from vitamins. Both sit in the active site and make catalysis happen.

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NAD⁺ and FAD: the electron taxis

Catabolism strips electrons off fuel, and NAD⁺ and FAD catch them — becoming NADH and FADH₂. The reduced, "loaded" forms carry that energy to the electron transport chain, where it's cashed in to make ATP.

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Coenzyme A: the acyl carrier

Coenzyme A carries acyl groups on a reactive –SH end. Load it with a 2-carbon acetyl group and you get acetyl-CoA — the high-energy thioester that feeds carbons into the TCA cycle.

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Why you need your vitamins

Most coenzymes are built from water-soluble vitamins — niacin → NAD⁺, riboflavin → FAD, pantothenate → CoA. That's why a vitamin deficiency doesn't break one reaction; it stalls whole pathways (think pellagra, beriberi).

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4 · The Stages of Metabolism

"Zoom out to the whole map — catabolism vs anabolism, the funnel down to acetyl-CoA, and the hormones that flip the switch."

Dr. Radi

Two directions: catabolism & anabolism

Metabolism runs both ways. Catabolism breaks big molecules down, oxidizing them to release energy (making ATP and NADH). Anabolism runs the other way — building big molecules from small ones, spending ATP and NADPH to do it.

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Everything funnels to acetyl-CoA

Catabolism runs in three stages: (1) digestion chops proteins, carbs, and fats into building blocks; (2) those converge on one hub, acetyl-CoA; (3) the TCA cycle + electron transport chain oxidize it fully to CO₂ + H₂O + lots of ATP.

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Where it happens: the mitochondrion

Much of this runs inside the mitochondrion — you can see its folded inner membrane (cristae) in this real electron micrograph. Those folds are studded with the electron transport chain, while the matrix they enclose runs the TCA cycle — together turning fuel into ATP.

TEM: Louisa Howard, public domain (Wikimedia Commons)
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Hormones flip the switch

Your body decides store or spend with hormones. Insulin (fed) says store — anabolic. Glucagon (fasting) says release — catabolic. Epinephrine (stress) says mobilize now — the fight-or-flight surge of glucose and fat.

Dr. Radi

Can you…?

  • ☐ interpret ΔG (with ΔH, ΔS) to judge spontaneity, and explain reaction coupling?
  • ☐ describe ATP as the energy currency — its bonds, the ATP/ADP cycle, and food-energy connection?
  • ☐ explain cofactors, coenzymes, and the electron carriers NADH, FADH₂, and CoA (and their vitamin origins)?
  • ☐ map the stages of catabolism and anabolism and the hormones that regulate them?

If any box stays empty, the practice site has a drill for it. 🧪

Dr. Radi