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8.1 Cell respiration
AHL
Overview of cellular energy metabolism
• Electron-rich food molecules synthesized by plants are used by plants themselves, and by animals and other eukaryotes.
• The electrons are removed from fuel substances, such as sugars, and donated to other molecules such as oxygen that act as electron acceptors.
• In the process, some energy of the electrons is released and used to drive the synthesis of ATP.
Oxidation and Reduction• Oxidation involves the loss of electrons from
an element, frequently gaining oxygen or losing hydrogen
• Reduction involves a gain of electron, frequently involves losing oxygen or gaining hydrogen.
OIL RIGOxidation Reduction Is IsLoss Gain
Cellular respiration includes both the reactions that transfer electrons from organic molecules to oxygen and the reactions the make ATP.
In this overall reaction, electrons and protons are transferred from glucose to oxygen, forming water, and the carbons left after the transfer are released as carbon dioxide.
Glycolisis
• Sugars such as glucose are partially oxidized and broken down into smaller molecules.
• It starts with the six-carbon sugar glucose and produces two molecules of the three-carbon organic substance pyruvate in 10 sequential, ezyme-catalyzed reactions.
• Pyruvate still contains many electrons that can be removed by oxidation
The first series of oxidative reactions that remove electrons from cellular fuel molecules
Glycolisis: a)Phosphorylation
• Outline: Oxidation of Glucose (6 carbons) to two Pyruvate (3 carbons) is coupled to the reduction of ADP to ATP
• Begins with phosphorylation = the adding of two phosphates
• Glucose 1, 6 diphosphate
The phosphate groups allow a stronger interaction between the hexose and its enzyme
An energy-requiring reaction
Glycolisis: b)Lysis
• Involves the breaking of the hexose diphosphate into two triose phosphate molecules.
• The triose phosphate is an intermediate in many biochemical reactions.
The subsequent energy-releasing reaction
Glycolisis: b)Lysis
• Electrons are removed from the phosphorylated glucose derivatives
• 4 ATP are produced, giving a net gain of 2 ATP
• The removed electrons are accepted by the electron carrier molecule NAD+
NAD+ = Nicotinamide Adenine Dinucleotide (NAD+ when oxidized, NADH when reduced)
The subsequent energy-releasing reaction
c) OxidationThe major oxidation of glycolysis removes 2 electrons and 2 protons from G3P.
Both electrons and one proton (H+) are picked up by NAD+ to form NADH. The other proton is released into the cytosol.
Each Triose phosphate is oxidized to a 3 carbon molecule called Pyruvate
Right before oxidation, one other phosphate is added to each of the G3P molecules
Each triose phosphate adds a phosphate to ADP reducing this to ATP (called substrate level phosphorylation reaction)
Link reaction: oxidative decarboxylation
