The Krebs cycle is a process that is absent in cellular respiration in anaerobic organisms but present in aerobic organisms. The Calvin cycle is a process that doesn't require oxygen.
The Krebs cycle cannot take place in the absence of oxygen, although oxygen is not directly involved in the cycle. The oxygen is required for the electron transport chain, involved in the four reduction reactions of the cycle.
The Calvin Cycle uses one molecule of glyceraldehyde. The six left over oxygen atoms are released into the atmosphere where they are available for use in respiration.
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