Calvin Cycle vs Krebs Cycle: A Comprehensive Guide for Biology Students

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The Calvin cycle and the Krebs cycle are two fundamental processes in biology, each playing a crucial role in energy production and carbon fixation. While the Krebs cycle generates energy by breaking down complex molecules, the Calvin cycle utilizes that energy to synthesize complex molecules.

The Calvin Cycle: Harnessing Light Energy for Carbon Fixation

The Calvin cycle, also known as the light-independent reactions of photosynthesis, occurs in the stroma of chloroplasts in plant cells. It takes place in three main stages: carbon fixation, reduction, and regeneration of the starting molecule, ribulose-1,5-bisphosphate (RuBP).

Carbon Fixation

  1. Carbon Dioxide Incorporation: Carbon dioxide (CO2) is combined with RuBP, catalyzed by the enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase), to form an unstable six-carbon compound.
  2. Splitting the Compound: The unstable six-carbon compound immediately splits into two three-carbon molecules, 3-phosphoglycerate (3-PGA).

Reduction

  1. Energy Inputs: ATP and NADPH, produced in the light-dependent reactions of photosynthesis, are used to convert 3-PGA into triose phosphate, a simple sugar.
  2. Energy Conversion: The energy from ATP and NADPH is used to reduce the 3-PGA molecules, converting them into more energy-rich triose phosphate.

Regeneration of RuBP

  1. Triose Phosphate Utilization: Some of the triose phosphate is used to regenerate RuBP, the starting molecule of the Calvin cycle.
  2. Glucose and Other Compounds: The remaining triose phosphate is converted into glucose, other sugars, or precursors for fat and protein synthesis.

The Krebs Cycle: Generating Energy through Oxidation

calvin cycle vs krebs cycle

The Krebs cycle, also known as the citric acid cycle, takes place in the mitochondrial matrix of eukaryotic cells and the cytoplasm of prokaryotic cells. It is a series of redox reactions that generates energy in the form of ATP, FADH2, and NADH.

Acetyl-CoA Formation

  1. Pyruvate Conversion: Pyruvate, produced from glucose in glycolysis, is converted into acetyl-CoA, which enters the Krebs cycle.
  2. Acetyl-CoA Entry: Acetyl-CoA, the starting molecule of the Krebs cycle, is formed from the breakdown of pyruvate.

Citric Acid Formation

  1. Oxaloacetate Combination: Acetyl-CoA combines with oxaloacetate to form citric acid, initiating the Krebs cycle.
  2. Citric Acid Synthesis: The combination of acetyl-CoA and oxaloacetate results in the formation of citric acid, the first intermediate in the Krebs cycle.

Redox Reactions

  1. Energy Release: A series of redox reactions occur within the Krebs cycle, releasing ATP, FADH2, and NADH.
  2. Electron Transport Chain: These energy-rich molecules (ATP, FADH2, and NADH) are then used in the electron transport chain to generate more ATP through oxidative phosphorylation.

Oxaloacetate Regeneration

  1. Cyclic Nature: The Krebs cycle regenerates oxaloacetate, allowing the cycle to continue indefinitely as long as there is a supply of acetyl-CoA.
  2. Continuous Energy Production: The cyclic nature of the Krebs cycle ensures a continuous supply of energy-rich molecules for the electron transport chain.

Biological Specification

Calvin Cycle

The Calvin cycle is specific to photoautotrophs, such as plants and cyanobacteria, which use light energy to produce organic compounds from CO2. This process is essential for the primary production of organic matter in ecosystems, forming the foundation of the food chain.

Krebs Cycle

In contrast, the Krebs cycle is ubiquitous, occurring in all organisms that perform aerobic respiration, from bacteria to humans. It is a crucial component of cellular respiration, providing the energy necessary for various cellular processes.

Key Differences

  1. Energy Source: The Calvin cycle uses light energy to produce complex molecules, while the Krebs cycle generates energy by breaking down complex molecules.
  2. Location: The Calvin cycle occurs in the stroma of chloroplasts, while the Krebs cycle takes place in the mitochondrial matrix or the cytoplasm of prokaryotic cells.
  3. Redox Reactions: The Calvin cycle involves reduction reactions, while the Krebs cycle consists of oxidation reactions.
  4. Carbon Fixation: The Calvin cycle is responsible for carbon fixation, while the Krebs cycle releases CO2.

References

  1. Reddit – What is the difference between the Krebs Cycle and the Calvin Cycle?
  2. CK-12 – How do you distinguish between the Calvin cycle and the Krebs cycle?
  3. Quizlet – Calvin cycle and kreb cycle Flashcards