Do plants breathe?
All living cells need energy, obtained by breaking down food molecules in cellular respiration. Plants respire too, but they have no special respiratory organs. Each part takes care of its own gas exchange, through stomata in leaves and lenticels in stems. Plants manage this because:
each part has a relatively low demand for gases,
most living cells are close to the surface,
leaves release oxygen during photosynthesis that their cells can use.
The main substrate is glucose. Energy is not released in one burst but in small steps, and is trapped as ATP, the cell's energy currency.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
Glycolysis
Glycolysis (glycos = sugar, lysis = splitting), also called the EMP pathway after Embden, Meyerhof and Parnas, takes place in the cytoplasm of all living cells, and is the only process of respiration in anaerobic organisms.
One molecule of glucose (6C) is broken into two molecules of pyruvic acid (3C) in ten steps:
Glucose is phosphorylated to glucose-6-phosphate, using 1 ATP (enzyme: hexokinase), and then rearranged into fructose-6-phosphate.
It is phosphorylated again to fructose-1,6-bisphosphate, using another ATP.
This splits into two 3-carbon sugars (triose phosphates).
Each triose phosphate is oxidised, forming NADH + H⁺, and eventually converted into pyruvic acid, producing ATP directly.
Net gain from one glucose: 2 ATP and 2 NADH. (4 ATP are made, 2 are used.)
Fermentation
Without oxygen, pyruvic acid is converted in different ways:
Organism | Product | Notes |
|---|---|---|
Yeast | ethanol + CO₂ (alcoholic fermentation) | enzymes pyruvic acid decarboxylase and alcohol dehydrogenase |
Some bacteria, and muscle cells short of oxygen | lactic acid | enzyme lactate dehydrogenase; NADH is the reducing agent |
Fermentation releases less than 7% of the energy in glucose, and the alcohol or acid produced is harmful. Yeasts die when the alcohol concentration reaches about 13%.
Aerobic respiration
With oxygen, pyruvate enters the mitochondria and is completely oxidised to CO₂ and water.
The link reaction
Pyruvic acid undergoes oxidative decarboxylation, catalysed by pyruvate dehydrogenase:
pyruvic acid + CoA + NAD⁺ → acetyl CoA + CO₂ + NADH + H⁺
Two molecules of NADH form per glucose.
The Krebs cycle
The tricarboxylic acid (TCA) cycle, described by Hans Krebs, takes place in the mitochondrial matrix.
Acetyl CoA (2C) combines with oxaloacetic acid (4C) and water to form citric acid (6C), enzyme citrate synthase.
Citric acid is converted, through a series of steps with two decarboxylations, back to oxaloacetic acid, which continues the cycle.
Per turn of the cycle (one acetyl CoA): 3 NADH, 1 FADH₂ and 1 GTP (ATP) are formed, and 2 CO₂ released. Since one glucose gives two acetyl CoA, the cycle turns twice per glucose.
The electron transport system and oxidative phosphorylation
NADH and FADH₂ carry electrons to the electron transport system (ETS) in the inner mitochondrial membrane.
Electrons pass through a chain of carriers: complex I (NADH dehydrogenase), ubiquinone, complex III (cytochrome bc₁), cytochrome c, and complex IV (cytochrome c oxidase).
Oxygen is the final electron acceptor; it combines with electrons and protons to form water. Without oxygen, the whole chain stops.
The flow of electrons pumps protons across the membrane, building up a proton gradient. Protons flow back through complex V (ATP synthase), which uses the energy to make ATP.
This is oxidative phosphorylation. Each NADH gives about 3 ATP and each FADH₂ about 2 ATP.
The respiratory balance sheet
The theoretical yield from one glucose molecule, assuming everything flows smoothly:
Stage | Direct ATP | NADH | FADH₂ |
|---|---|---|---|
Glycolysis | 2 | 2 | |
Link reaction | 2 | ||
Krebs cycle (two turns) | 2 | 6 | 2 |
Total: 4 ATP directly + 10 NADH × 3 + 2 FADH₂ × 2 = 4 + 30 + 4 = 38 ATP.
The real figure is lower, because the pathways don't work in isolation, NADH from glycolysis has to be shuttled into mitochondria, and intermediates are drawn off for other uses.
Fermentation | Aerobic respiration | |
|---|---|---|
Breakdown of glucose | partial | complete, to CO₂ and H₂O |
Net ATP per glucose | 2 | up to 38 |
NADH | reoxidised slowly to NAD⁺ | oxidised rapidly through the ETS |
Respiration as an amphibolic pathway
Respiration is not only catabolic. Its intermediates are used to build other molecules: acetyl CoA is used to make fatty acids, and Krebs cycle intermediates give amino acids. Fats and proteins can also be broken down and fed into the pathway at different points. Because it is both catabolic and anabolic, the respiratory pathway is called amphibolic.
Respiratory quotient
RQ = volume of CO₂ evolved ⁄ volume of O₂ consumed
Substrate | RQ |
|---|---|
Carbohydrates | 1.0 (6CO₂ ⁄ 6O₂) |
Fats | less than 1 (e.g. tripalmitin: 102 CO₂ ⁄ 145 O₂ ≈ 0.7) |
Proteins | about 0.9 |
Summary
Glycolysis in the cytoplasm: glucose → 2 pyruvate, net 2 ATP + 2 NADH.
Without oxygen: fermentation to ethanol + CO₂ or lactic acid.
With oxygen: link reaction and Krebs cycle in the matrix, ETS on the inner membrane; O₂ is the final electron acceptor.
Up to 38 ATP per glucose; respiration is amphibolic; RQ is 1 for carbohydrates.