Photosynthesis is a process in which the energy in sunlight is stored in the chemical bonds of glucose (C6H12O6) for later use.
Carbon dioxide is reduced to glucose and water is oxidized. Oxidation is the loss of an electron or hydrogen atom. Reduction is the gain of an electron or hydrogen atom. Oxidation reactions release energy and reduction reactions store energy in chemical bonds.
What is Cellular Respiration?
Cellular respiration allows organisms to use (release) the energy stored in glucose. The energy in glucose is first used to produce ATP. Cells use ATP to supply their energy needs. Cellular respiration is therefore a ...view middle of the document...
Two hydrogen atoms are removed by NAD+ forming 2 NADH (see diagram).
Additional phosphorylation results in intermediate 3-carbon molecules with 2 phosphate groups.
Four ATP are produced by substrate-level phosphorylation. Recall that substrate-level phosphorylation is the production of ATP using energy from other high-energy compounds but without the use of the electron transport system in the mitochondria.
The net yield of ATP in glycolysis is 2 for each glucose molecule (2 are used but 4 are produced).
Some bacteria have alternative energy-producing reactions. Two of these are the pentose phosphate pathway and the Entner-Doudoroff pathway.
Formation of Acetyl CoA
Pyruvate produced by glycolysis (see above) enters the mitochondrion and is converted to acetyl CoA by the reaction below. The remainder of the reactions of cellular respiration occur in the mitochondrion.
pyruvate (C3) ® acetyl CoA (C2) + CO2
During this step, NADH is produced from NAD+ + 2H (oxidation).
This step must occur twice for each glucose molecule because each glucose molecule produces two pyruvate molecules in glycolysis (above).
The two-carbon compound produced is attached to Coenzyme A to produce acetyl CoA.
The Krebs cycle can be summarized by either of the diagrams below. The diagram below occurs twice, once for each acetyl CoA.
When acetyl CoA attaches to a C4 molecule in the Krebs cycle, the Coenzyme A is released.
Two acetyl CoA molecules are consumed to produce 4 CO2, 2ATP, 6 NADH and 2 FADH2. The ATP molecules are produced by substrate-level phosphorylation.
The diagram below also summarizes the Krebs Cycle.
Electron Transport and Oxidative Phosphorylation
The inner membrane forms folds called cristae. These folds contain the carriers of the electron transport system.
Acetyl CoA formation and the Krebs cycle occur in the inner space called the matrix.
The space outside the inner membrane is the intermembrane space. The electron transport system pumps hydrogen ions (H+) into this space for oxidative phosphorylation.
The electron transport system is found in the mitochondrion and chloroplast of eucaryotes and in the plasma membrane of procaryotes. It consists of a series of carrier molecules which pass electrons from a high-energy compound to a final low-energy electron acceptor. Energy is released during these oxidation-reduction reactions to produce ATP.
The discussion below applies to the mitochondria of eucaryotes.
NADH or FADH2 bring electrons to the electron transport system in the mitochondria.
The system contains membrane-bound electron carriers that pass electrons from one to another. When a carrier reduces another, some of the energy that is released as a result of that reduction is used to pump hydrogen ions across the membrane into the intermembrane space. The remaining energy is used to reduce the next carrier.
As a result of the electron transport...