Written for this site from the class slides, with textbook page references for each statement. The chapter is on pages 412–424 of the course textbook. Image credits are on slide 17.
Oxidation is loss of electrons; because a hydrogen atom carries an electron, removing hydrogen (dehydrogenation) oxidizes a substance. Respiration oxidizes glucose step by step. NAD accepts the hydrogen and is reduced, carrying it to the electron transport chain. Oxidation and reduction always occur together (redox).
In the cytoplasm, glucose is phosphorylated by 2 ATP, split into two 3C sugars, oxidized (2 NAD reduced) and converted to two pyruvate, making 4 ATP by substrate-level phosphorylation. Net yield: 2 ATP and 2 reduced NAD. Each step has its own enzyme.
Without oxygen, reduced NAD cannot be oxidized by the electron transport chain. In human muscle, pyruvate accepts its hydrogen and becomes lactate, regenerating NAD+ so that glycolysis can continue. The net yield is only 2 ATP per glucose.
Yeast uses the same glycolysis, but regenerates NAD by converting pyruvate to ethanol and carbon dioxide. In baking, carbon dioxide makes dough rise and the ethanol evaporates in the oven; in brewing, ethanol is the product wanted.
In the mitochondrial matrix, pyruvate is decarboxylated (CO2 released) and oxidized (NAD reduced). The 2C acetyl group joins coenzyme A as acetyl-CoA. Fatty acids from lipids are also cut into acetyl groups that enter the same way.
The acetyl group joins oxaloacetate (4C) to form citrate (6C). Two decarboxylations and four oxidations (dehydrogenations) regenerate oxaloacetate. Each turn gives 2 CO2, 3 reduced NAD, 1 reduced FAD and 1 ATP; the cycle turns twice per glucose.
Reduced NAD from glycolysis, the link reaction and the Krebs cycle passes a pair of electrons to the first carrier of the chain in the inner membrane, becoming NAD+ again. This transfers its energy to the chain.
As electrons pass from carrier to carrier they release energy in small steps, which is used to pump protons from the matrix into the small intermembrane space. The inner membrane is impermeable to protons, so a steep gradient builds up.
Protons flow back into the matrix through ATP synthase. The flow turns part of the enzyme and provides energy to join ADP and phosphate: oxidative phosphorylation. This makes most of the ATP from aerobic respiration.
At the end of the chain, oxygen accepts electrons and protons from the matrix, forming metabolic water. This keeps electrons flowing; without oxygen the chain stops and only glycolysis can produce ATP.
Lipids give about twice as much energy per gram as carbohydrates, because they contain less oxygen and more hydrogen and carbon to oxidize. Fatty acids enter respiration as acetyl-CoA, not through glycolysis, so lipids can only be respired aerobically; carbohydrates can also be used anaerobically.
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A printable worksheet covering all eleven statements, from quick recall to data questions and a longer answer. It has 40 marks and takes about 40 minutes, or you can do one section at a time.
Your teacher has the mark scheme.
You will need: Yeast suspension, glucose solution, dilute methylene blue, test tubes, water bath, timer
Safety: Eye protection. Methylene blue stains skin and clothes.
Carry this out with your teacher's supervision and follow your school's lab rules.
Peter Mitchell's chemiosmotic theory of 1961 was doubted by many. It later gained strong support, and he received a Nobel Prize in 1978.
Researchers had been searching for a chemical intermediate. Consider how an expected answer can make a different one harder to accept.
Summarised in our own words. Use each code to find the exact wording in your textbook or the IB subject guide.
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