2.4.Energy transformation

1 Cellular respiration

Cellular respiration harvests energy from glucose (a catabolic, exergonic process:

C6H12O6+6O2→6CO2+6H2O ΔG=−680 kcal/mol)

Energy is captured as ATP, the cell's currency, synthesized by substrate-level phosphorylation or oxidative phosphorylation.
Enzymes, crucial for energy transformations, lower activation energy ($E_A$).

Glycolysis

• Occurs in the cytoplasm.

• Converts 1 glucose into 2 pyruvate, yielding 2 net ATP and 2 NADH.

Pyruvate oxidation and the Krebs cycle

• In the mitochondrial matrix, 2 pyruvate are converted to 2 acetyl-CoA, producing 2 CO₂ and 2 NADH.

• The Krebs cycle (per glucose) yields 2 ATP (GTP), 4 CO₂, 6 NADH, and 2 FADH₂.

Oxidative phosphorylation and the electron transport chain (ETC)

• NADH and FADH₂ donate electrons to the ETC on the inner mitochondrial membrane.

• Electron flow pumps protons, creating a gradient that drives ATP synthase via chemiosmosis.

• Molecular oxygen ($O_2$) is the final electron acceptor, forming water.

• Total ATP yield per glucose is approximately $30-32\ \text{ATP}$ (malate-aspartate shuttle) or $28-30\ \text{ATP}$ (glycerol phosphate shuttle).

Fermentation and anaerobic respiration

• Occurs when $O_2$ is unavailable, regenerating NAD⁺ for glycolysis.

• Lactic acid fermentation: pyruvate converts to lactate.

• Ethanol fermentation: pyruvate converts to acetaldehyde, then ethanol.

• Yields significantly less ATP than aerobic respiration.

Metabolic shifts and cancer metabolism (Warburg effect)

• Cancer cells exhibit increased glycolysis and lactate production even with $O_2$, supporting rapid cell proliferation with less ATP per glucose.

2 Energy transformation (Photosynthesis)

Photosynthesis converts light energy into chemical energy (an anabolic, endergonic process: 6CO2+6H2O→C6H12O6+6O2).

It occurs in chloroplasts and has two stages:

Stages of photosynthesis

1. Light-dependent reactions:

   • Occur in thylakoid membranes.

   • Light energy splits water ($H_2O$) to release O₂ and drives electron flow through an ETC.

   • Produces ATP and NADPH.

2. Calvin cycle (light-independent reactions):

   • Occurs in the stroma.

   • Uses ATP and NADPH to convert CO₂ into glyceraldehyde-3-phosphate (G3P).

   • Key enzyme: RuBisCO fixes CO₂ to RuBP.

   • For every 3 CO₂ fixed, one G3P is produced, consuming 9 ATP and 6 NADPH.

Chloroplast structure and pigment organization

• Thylakoids (in grana) contain chlorophylls (a and b) and carotenoids, which absorb light.

• Stroma is the fluid where the Calvin cycle occurs.

• Photosystems (I and II) capture light energy and transfer electrons.

Connections and implications

• Photosynthesis and cellular respiration are interdependent, with products of one serving as reactants for the other.

• Metabolic flexibility, like the Warburg effect, highlights cellular adaptability.

• Understanding these processes is vital for bioenergy, agriculture, and medical research.