BIOL1019 Chap.10 Metabolism and Cellular Respiration
Metabolism and Cellular Respiration
Cellular respiration oxidises glucose to make ATP, the cell's energy currency, and the course gives the total yield as 36 ATP per glucose.
This chapter follows its four stages in order: glycolysis in the cytosol, which uses 2 ATP and makes 4 for a net gain of 2 along with 2 pyruvate and 2 NADH; the conversion of pyruvate to acetyl CoA in the mitochondrial matrix; the Krebs cycle, which releases carbon dioxide and loads NADH and FADH2; and the electron transport chain with chemiosmosis, where oxygen accepts electrons and ATP synthase makes most of the ATP.
It tracks the six carbons of glucose to six carbon dioxide and explains what fermentation achieves when oxygen runs out. Metabolism opens the material for Test 2. The lectorial checks it with multiple choice questions that hinge on single words, such as protons or electrons and inner or outer membrane, and with extended answers on what each stage achieves and why.
What this chapter covers
- 01
Catabolism, anabolism and ATP
- 02
Oxidation, reduction, NAD and FAD
- 03
Glycolysis and its yield
- 04
Acetyl CoA and the Krebs cycle
- 05
Electron transport and chemiosmosis
- 06
Substrate-level and oxidative phosphorylation
- 07
Fermentation and lactate
- 08
Fuel from fats and proteins
Worked example · free
Counting carbons from glucose to carbon dioxide
- +1Glycolysis splits glucose into two three-carbon pyruvate: 2 x 3 = 6 carbons, none released.
- +1Two pyruvate become two two-carbon acetyl CoA: 2 x 2 = 4 carbons remain and 2 CO2 are released.
- +1The Krebs cycle releases 2 CO2 per acetyl CoA, so 2 x 2 = 4 CO2; in total 2 + 4 = 6 CO2.
Key terms
- Glycolysis
- The cytosolic breakdown of one glucose into two pyruvate, with a net gain of two ATP and two NADH.
- Krebs cycle
- A cycle of about eight reactions in the mitochondrial matrix that oxidises acetyl CoA, releasing carbon dioxide.
- Chemiosmosis
- The flow of protons back through ATP synthase, driving the joining of ADP and phosphate into ATP.
- Substrate-level phosphorylation
- Direct transfer of a phosphate group from a substrate molecule to ADP, as in glycolysis.
- Fermentation
- Conversion of pyruvate to lactate or ethanol that regenerates NAD+ so glycolysis can continue without oxygen.
- ATP synthase
- The inner mitochondrial membrane enzyme that uses the proton flow to join ADP and phosphate into ATP.
Metabolism and Cellular Respiration FAQ
Why does glycolysis happen outside the mitochondria?
The lectorial answer is that glucose is too large to enter the mitochondrion, so glycolysis runs in the cytoplasm and splits it into two pyruvate first. The pyruvate then moves into the mitochondrial matrix, where it is oxidised to acetyl CoA and enters the Krebs cycle when oxygen is available.
Where does most of the ATP come from?
Glycolysis gives a net 2 ATP and the Krebs cycle 1 ATP per turn, so very few ATP are made before the electron transport chain. Most of the course's total of 36 comes from oxidative phosphorylation, in which the chain pumps protons into the intermembrane space and their flow back through ATP synthase makes ATP.
What is the point of turning pyruvate into lactate?
Without oxygen as the final acceptor, NADH cannot be reoxidised by the electron transport chain, so glycolysis would stop for lack of NAD+. Converting pyruvate to lactate in the cytoplasm turns NADH back into NAD+, so glycolysis keeps producing ATP, a small yield that still beats none.
What do oxidation and reduction mean in respiration?
Oxidation is the removal of electrons, often as hydrogen atoms, which lowers a molecule's potential energy; reduction is the addition of electrons, which raises it. In respiration glucose is oxidised and the carriers NAD+ and FAD are reduced to NADH and FADH2, which deliver the electrons to the chain where oxygen is finally reduced.
Exam move
Draw one diagram of a cell with the cytosol, matrix, inner membrane and intermembrane space, and place each stage of respiration in its location with its inputs and outputs. Practise the carbon count and the ATP count until you can write both from memory. Then prepare for the lectorial's multiple choice traps by stating, in one line each, what passes through ATP synthase and which membrane holds the chain.
Close by answering the lectorial's extended questions in your own words: the purpose of glycolysis, the fate of electrons from the Krebs cycle, the role of the chain, and the point of glycolysis during fermentation.
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