How a Plant Cell Squeezes Energy Out of a Single Glucose Molecule

Photosynthesis stores energy in sugar, but a root cell deep in the soil, a germinating seed or a flower bud opening at night cannot use that sugar directly. Every living plant cell breaks food down to release energy as ATP, day and night.

This lesson covers the types of respiration and glycolysis, the Krebs cycle and electron transport, fermentation and amphibolic pathways, and the respiratory quotient.

Respiration is aerobic when oxygen is used to break food down completely to carbon dioxide and water, and anaerobic when food is broken down incompletely without oxygen; both begin with glycolysis, which splits one glucose into two pyruvic acid molecules in the cytoplasm.

Types of respiration:

- Aerobic — needs oxygen; complete breakdown; large ATP yield; in most plant and animal cells
- Anaerobic — without oxygen; incomplete breakdown to ethanol or lactic acid; small ATP yield; in yeast and some bacteria

Glycolysis:

- Takes place in the cytoplasm of all living cells
- Glucose is phosphorylated twice, using 2 ATP, and split into two 3-carbon molecules of glyceraldehyde-3-phosphate
- Each is oxidised, forming NADH, and converted through several steps into pyruvic acid
- 4 ATP are made directly, so the net gain is 2 ATP and 2 NADH per glucose

An everyday example. Dosa batter rising overnight begins with glycolysis in its microbes, which break sugars down to pyruvic acid before fermenting them.

The substance. Glycolysis needs no oxygen — it is the shared first stage of aerobic and anaerobic respiration, and oxygen decides only what happens to pyruvic acid afterwards.

**In the mitochondrial matrix, pyruvic acid becomes acetyl CoA and is fully oxidised in the Krebs cycle, releasing carbon dioxide and loading NADH and $\mathrm{FAD}{\mathrm{H}}_2$; the electron transport system on the inner mitochondrial membrane then passes their electrons to oxygen, making most of the cell's ATP.

Link reaction. Pyruvic acid is converted to acetyl CoA, releasing carbon dioxide and forming NADH, by the enzyme pyruvate dehydrogenase.

Krebs cycle:

- Acetyl CoA (2 carbons) joins oxaloacetic acid (4 carbons) to form citric acid (6 carbons)
- Each turn yields 3 NADH, 1** $\mathrm{FAD}{\mathrm{H}}_2$ and 1 GTP
- Oxaloacetic acid is regenerated, and the cycle turns twice per glucose

Electron transport system:

- Complex I accepts electrons from NADH, and Complex II from $\mathrm{FAD}{\mathrm{H}}_2$
- Electrons pass through ubiquinone, Complex III, cytochrome c and Complex IV to oxygen, forming water
- Each NADH yields about 3 ATP and each $\mathrm{FAD}{\mathrm{H}}_2$ about 2 ATP

ATP from one glucose:

- Glycolysis: 2 ATP + 2 NADH (6 ATP) = 8
- Link reaction: 2 NADH = 6
- Krebs cycle, two turns: 6 NADH (18) + 2 $\mathrm{FAD}{\mathrm{H}}_2$ (4) + 2 GTP (2) = 24
- Total: $8+6+24=38$ ATP

An everyday example. A heap of germinating moong seeds on wet cloth feels slightly warm because respiration releases energy, some of it as heat.

The substance. The figure of 38 ATP is a theoretical maximum — real cells make fewer, because some energy is lost and NADH from glycolysis must be moved into the mitochondrion.

Fermentation is the incomplete breakdown of pyruvic acid without oxygen to ethanol or lactic acid, oxidative phosphorylation is ATP synthesis powered by electron transport through a proton gradient, and an amphibolic pathway is one that serves both breakdown and synthesis.

Fermentation:

- Alcoholic — in yeast, pyruvic acid becomes ethanol and carbon dioxide
- Lactic acid — in some bacteria and in muscle cells short of oxygen, pyruvic acid is reduced to lactic acid
- Only the 2 ATP of glycolysis are gained; NADH is reoxidised to NAD so glycolysis can continue

Oxidative phosphorylation:

- Electron transport pumps hydrogen ions into the space between the two mitochondrial membranes
- The ions flow back through ATP synthase, with its ${\mathrm{F}}_0$ channel and ${\mathrm{F}}_1$ head, making ATP
- Oxygen is the final electron acceptor; without it, the whole chain stops

Amphibolic pathway:

- Respiration breaks food down, but its intermediates are also withdrawn to build new molecules
- Fats enter as glycerol or acetyl CoA; proteins enter as amino acids at pyruvic acid, acetyl CoA or the Krebs cycle

An everyday example. Tired, burning leg muscles during a fast sprint in a school race are working partly by lactic acid fermentation, because oxygen cannot reach them quickly enough.

The substance. Respiration is not purely a breakdown process — because the same pathway supplies building blocks, it is better described as amphibolic than catabolic.

The respiratory quotient (RQ) is the ratio of the volume of carbon dioxide released to the volume of oxygen consumed during respiration, and it depends on the substrate being respired.

$$\mathrm{RQ}=\frac{\text{volume of carbon dioxide evolved}}{\text{volume of oxygen consumed}}$$

Carbohydrates. For glucose,

$${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{O}}_2\to 6\mathrm{C}{\mathrm{O}}_2+6{\mathrm{H}}_2\mathrm{O},\mathrm{RQ}=\frac{6}{6}=1$$

Fats. For tripalmitin,

$$2{\mathrm{C}}_{51}{\mathrm{H}}_{98}{\mathrm{O}}_6+145{\mathrm{O}}_2\to 102\mathrm{C}{\mathrm{O}}_2+98{\mathrm{H}}_2\mathrm{O},\mathrm{RQ}=\frac{102}{145}\approx 0.7$$

Proteins. The RQ is about 0.9.

Other cases:

- Organic acids such as malic acid give an RQ above 1, because they are already partly oxidised and need less oxygen

An everyday example. Germinating groundnut seeds, which store oil, show an RQ well below 1, while germinating wheat grains, rich in starch, show an RQ close to 1.

The substance. Fats release more energy per gram but need more oxygen — their molecules contain little oxygen, so more must be taken in, which is exactly why their RQ is below 1.