Citric Acid Cycle & Calvin Cycle

The Citric Acid Cycle, also known as the Krebs Cycle, is crucial for cellular respiration, converting acetyl-CoA into carbon dioxide and capturing energy in the form of ATP. In contrast, the Calvin Cycle occurs in the stroma of chloroplasts, utilizing ATP and NADPH to convert carbon dioxide into glucose, thereby playing a key role in photosynthesis.

Citric Acid Cycle

• The Citric Acid Cycle (Krebs Cycle) is vital for cellular respiration, turning acetyl-CoA into carbon dioxide and energy.

• Start: Acetyl-CoA (2-carbon) combines with oxaloacetate (4-carbon) to form citrate (6-carbon).

• Steps & Energy Production (per acetyl-CoA):

  • Citrate changes to isocitrate.

  • Isocitrate is converted to $\alpha$-ketoglutarate: releases one $CO_2$ and produces one NADH.

  • $\alpha$-ketoglutarate is converted to succinyl-CoA: releases another $CO_2$ and produces a second NADH.

  • Succinyl-CoA becomes succinate: generates one GTP (which converts to ATP).

  • Succinate is oxidized to fumarate: yields one FADH2.

  • Fumarate becomes malate.

  • Malate is oxidized back to oxaloacetate: produces a third NADH, restarting the cycle.

• Overall Output per turn: Two $CO_2$ molecules, three NADH, one FADH2, and one GTP (or ATP).

• Contrast: The Calvin Cycle, in chloroplasts, uses ATP and NADPH to convert $CO_2$ into glucose for photosynthesis.

Calvin Cycle

The Calvin Cycle, also known as the light-independent reactions, is crucial for photosynthesis, converting carbon dioxide into glucose.

• Location: Occurs in the stroma of chloroplasts.

• Purpose: Uses energy from ATP and NADPH (produced during the light-dependent reactions) to fix carbon from $CO_2$ into organic molecules.

• Key Stages (per cycle yielding one G3P):

  • Carbon Fixation: Three molecules of $CO_2$ combine with three 5-carbon RuBP molecules, catalyzed by RuBisCO, to form three unstable 6-carbon intermediates, which immediately split into six 3-carbon 3-PGA molecules.

  • Reduction: Six 3-PGA molecules are converted into six 3-carbon G3P molecules. This step uses six ATP and six NADPH molecules.

  • Regeneration: Five of the six G3P molecules are used to regenerate three RuBP molecules, requiring three more ATP molecules. The remaining one G3P molecule is the net product.

• Overall Inputs (to produce one G3P): 3 $CO_2$, 9 ATP, 6 NADPH.

• Overall Output (per cycle): One G3P (glyceraldehyde-3-phosphate) molecule, which can be used to synthesize glucose and other carbohydrates.

• Contrast: The Citric Acid Cycle (Krebs Cycle) is vital for cellular respiration, turning acetyl-CoA into carbon dioxide and energy carriers like NADH, FADH2, and ATP.