Lesson 11 of 11 · 12 min
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Must-know facts
19 facts
- 1Respiration: oxidative breaking of C–C bonds inside the cell, releasing energy trapped as ATP, the energy currency.
- 2Plants exchange gases through stomata and lenticels; they have no specialised respiratory organs.
- 3Glycolysis = EMP pathway (Embden, Meyerhof, Parnas); cytoplasm; all living organisms; ten reactions; glucose → 2 pyruvic acid.
- 4Invertase: sucrose → glucose + fructose. Hexokinase: glucose → glucose-6-phosphate.
- 5Glycolysis ATP used: glucose → G-6-P and F-6-P → F-1,6-bisP. ATP made: BPGA → PGA and PEP → pyruvic acid.
- 6Only NADH step of glycolysis: PGAL → BPGA. Per glucose: net 2 ATP, 2 NADH.
- 7Alcoholic fermentation (yeast): pyruvic acid decarboxylase + alcohol dehydrogenase → CO₂ + ethanol. Lactic acid: lactate dehydrogenase.
- 8Fermentation releases less than 7% of glucose energy; yeast dies at about 13% alcohol.
- 9Link reaction (matrix): pyruvic dehydrogenase, NAD⁺, CoA, Mg²⁺; pyruvic acid → acetyl CoA + CO₂ + NADH.
- 10Krebs' cycle (matrix): acetyl CoA + OAA + H₂O → citric acid, by citrate synthase.
- 11Krebs' cycle per turn: 3 NADH, 1 FADH₂, 1 GTP (→ ATP, substrate-level), 2 CO₂.
- 12ETS on inner membrane: complex I NADH dehydrogenase, II (FADH₂), III cytochrome bc₁, IV cytochrome c oxidase (cyt a, a₃, 2 copper centres), V ATP synthase.
- 13Cytochrome c: small mobile protein on the outer surface of the inner membrane, between complexes III and IV.
- 14NADH → 3 ATP; FADH₂ → 2 ATP. O₂ is the final hydrogen acceptor.
- 15ATP synthase: F₁ headpiece (catalytic site), F₀ proton channel; 4 H⁺ per ATP from intermembrane space to matrix.
- 16Net gain in aerobic respiration: 38 ATP per glucose; fermentation: 2 ATP.
- 17Entry points: fatty acids → acetyl CoA; glycerol → PGAL; amino acids (after deamination) → pyruvate, acetyl CoA or Krebs' cycle.
- 18Respiratory pathway is amphibolic (both catabolic and anabolic).
- 19RQ = CO₂ evolved / O₂ consumed: carbohydrate 1.0, fat (tripalmitin) 0.7, protein about 0.9.
Common traps
Where marks are lost
Writing the glycolysis gain as 4 ATP per glucose.
4 ATP are made directly, but 2 were spent at the start, so the net gain is 2 ATP (plus 2 NADH).
Placing Krebs' cycle on the inner mitochondrial membrane.
The link reaction and Krebs' cycle run in the matrix; only the ETS and ATP synthase sit on the inner membrane.
Thinking glycolysis needs O₂ or happens in mitochondria.
Glycolysis is anaerobic, runs in the cytoplasm, and is shared by all living organisms.
Believing O₂ is used all through aerobic respiration.
O₂ appears only at the end of the ETS as the final hydrogen acceptor, yet without it the whole chain stops.
Counting GTP made in Krebs' cycle as oxidative phosphorylation.
Succinyl-CoA → succinic acid gives GTP by substrate-level phosphorylation; the ATP from the ETS is oxidative phosphorylation.
Swapping the yields: 2 ATP per NADH and 3 per FADH₂.
NADH enters at complex I and gives 3 ATP; FADH₂ enters later, via complex II, and gives 2 ATP.
Giving fats an RQ above 1 because they hold more energy.
Fats are poor in oxygen, so they need more O₂ than the CO₂ they give: tripalmitin 102/145 = 0.7.
Calling respiration purely catabolic.
Intermediates such as acetyl CoA are drawn off to build fatty acids and amino acids, so the pathway is amphibolic.
Key terms
17 terms
- Respiratory substrate
- Any compound oxidised in respiration to release energy; usually carbohydrate.
- ATP
- The molecule that stores the energy released in respiration and gives it up where work is done: the cell's energy currency.
- Lenticel
- An opening in the bark of woody stems through which gases reach the living cells beneath.
- Glycolysis
- The ten-step breakdown of glucose to two pyruvic acids in the cytoplasm, without O₂.
- EMP pathway
- Glycolysis, named after Embden, Meyerhof and Parnas.
- Facultative anaerobe
- An organism that can live with or without oxygen.
- Obligate anaerobe
- An organism that must have anaerobic conditions.
- Fermentation
- Anaerobic incomplete oxidation of glucose, ending in ethanol and CO₂ or in lactic acid, with a net gain of 2 ATP.
- Oxidative decarboxylation
- Removal of CO₂ together with hydrogen, as when pyruvic acid becomes acetyl CoA.
- Acetyl CoA
- The 2-carbon acetyl group on coenzyme A that feeds Krebs' cycle.
- Substrate-level phosphorylation
- ATP (or GTP) made directly in a pathway reaction, without the ETS.
- Ubiquinone
- The carrier in the inner membrane that takes electrons from complexes I and II to complex III.
- Cytochrome c
- A small mobile protein carrying electrons from complex III to complex IV.
- Oxidative phosphorylation
- ATP synthesis driven by the energy of oxidation-reduction in the ETS.
- F₀ and F₁
- The proton channel in the inner membrane and the headpiece that makes ATP, together forming ATP synthase.
- Amphibolic pathway
- A pathway used both for breaking molecules down and for building them up.
- Respiratory quotient
- Volume of CO₂ evolved divided by volume of O₂ consumed in respiration.