Respiration in Plants

What is respiration?

Cellular respiration means breaking the C–C bonds of food inside cells to release energy. The energy is stored in ATP, the cell's energy coin. The food that is broken is the respiratory substrate, usually glucose, but fats, proteins and organic acids can be used too. Energy is released step by step, not all at once, so the cell can catch it.

Do plants breathe? Gas exchange

Plants have no lungs, but every living cell respires. Gas exchange happens by simple diffusion through:

• stomata in leaves,
• lenticels in woody stems and roots.

This is enough because (1) each part handles its own gases, (2) plants need far less gas exchange than animals, (3) most living cells are close to the surface (the inside of stems is dead wood), and (4) in daylight the O₂ made by photosynthesis is available inside the leaf.

Glycolysis: splitting sugar

Glycolysis (also called the EMP pathway) happens in the cytoplasm of every living cell and needs no O₂. Glucose (6C) is broken into two pyruvic acid (3C) molecules in about ten enzyme steps.

• Spend: 2 ATP to activate glucose (glucose → glucose-6-phosphate; fructose-6-phosphate → fructose-1,6-bisphosphate).
• Split: fructose-1,6-bisphosphate breaks into two 3C sugars (PGAL).
• Earn: 4 ATP made directly, and 2 NADH formed when PGAL is oxidised.

Net: 2 ATP + 2 NADH + 2 pyruvate per glucose. In plants sucrose is first split into glucose and fructose by invertase.

Fermentation: life without oxygen

When O₂ is missing, cells keep glycolysis going by fermentation.

• Alcoholic fermentation (yeast): pyruvate → acetaldehyde + CO₂ → ethanol. Enzymes: pyruvic acid decarboxylase and alcohol dehydrogenase.
• Lactic acid fermentation (some bacteria, our muscles in hard exercise): pyruvate → lactic acid by lactate dehydrogenase.

In both, NADH is turned back into NAD⁺, so glycolysis can continue. No extra ATP is made; less than 7% of the energy of glucose is released. Yeast dies when alcohol goes above about 13%.

Link reaction and the Krebs (TCA) cycle

With O₂, pyruvate enters the mitochondrial matrix.

Link reaction

Pyruvate + CoA + NAD⁺ → acetyl CoA + CO₂ + NADH (enzyme: pyruvate dehydrogenase).

TCA cycle (citric acid cycle)

1. Acetyl CoA (2C) + oxaloacetic acid (OAA, 4C) + water → citric acid (6C), by citrate synthase. Citric acid is a tricarboxylic acid, hence TCA.
2. Citrate → isocitrate → two steps that each release CO₂ → α-ketoglutarate (5C) → succinyl CoA (4C).
3. Succinyl CoA → succinate gives one GTP (= 1 ATP), by substrate-level phosphorylation.
4. Succinate → fumarate → malate → OAA, making FADH₂ and more NADH. OAA is ready to pick up the next acetyl CoA.

Per turn: 2 CO₂, 3 NADH, 1 FADH₂, 1 GTP. One glucose gives two pyruvates, so the cycle turns twice.

Electron transport system and oxidative phosphorylation

The NADH and FADH₂ must give their electrons away. This happens on the inner mitochondrial membrane.

1. Complex I (NADH dehydrogenase) takes electrons from NADH.
2. FADH₂ gives electrons to complex II.
3. Electrons pass to ubiquinone → complex III (cytochrome bc₁) → cytochrome c → complex IV (cytochrome c oxidase).
4. At the end O₂ takes the electrons and H⁺ and forms water. O₂ is the final electron acceptor.

As electrons move, H⁺ is pumped into the space between the two membranes. H⁺ flows back through ATP synthase (complex V): F₀ is the channel, F₁ is the head that makes ATP. This is chemiosmosis again. Making ATP using the energy of oxidation is called oxidative phosphorylation.

Counting ATP: the balance sheet

Using the textbook rule 1 NADH → 3 ATP and 1 FADH₂ → 2 ATP:

This 38 is a best-case figure. It assumes the path runs in a fixed order, NADH from glycolysis enters the mitochondrion without loss, and nothing is taken out for other uses. Real cells usually get less. Fermentation gives only 2 ATP.

The amphibolic pathway

Breaking down is catabolism; building up is anabolism. The respiratory pathway does both:

• Fats → glycerol (enters as PGAL) + fatty acids (become acetyl CoA).
• Proteins → amino acids, which enter as pyruvate, acetyl CoA or Krebs acids.
• When the cell needs to build fatty acids, acetyl CoA is taken out of the path; when it needs amino acids, Krebs acids are taken out.

Because it serves both breakdown and synthesis, it is called amphibolic (amphi = both), not only catabolic.

Respiratory quotient (RQ)

RQ = volume of CO₂ given out ÷ volume of O₂ taken in. It tells which food is being burnt.

• Carbohydrate: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. RQ = 6/6 = 1.
• Fat (tripalmitin): 2C₅₁H₉₈O₆ + 145O₂ → 102CO₂ + 98H₂O. RQ = 102/145 ≈ 0.7. Fats have little oxygen, so they need more O₂.
• Protein: about 0.9.
• Organic acids (e.g. malic acid, oxygen-rich): RQ more than 1.
• Anaerobic respiration: CO₂ but no O₂, so RQ is infinite.

Try it: yeast balloon

1. Put 2 spoons of sugar and 1 spoon of dry yeast in half a bottle of warm water.
2. Stretch a balloon over the mouth. Keep one bottle warm and one in the fridge.
3. Predict which balloon grows more. After an hour, check. The warm one blows up: yeast ferments sugar and gives CO₂.
4. In the 3D free play, pick each food and guess its RQ before reading the number.

Key formulas and definitions

• C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
• Glycolysis net: 2 ATP + 2 NADH + 2 pyruvate
• Link: pyruvate + CoA + NAD⁺ → acetyl CoA + CO₂ + NADH
• Krebs per turn: 2 CO₂ + 3 NADH + 1 FADH₂ + 1 GTP
• 1 NADH → 3 ATP · 1 FADH₂ → 2 ATP
• Aerobic total = 38 ATP per glucose (maximum)
• RQ = CO₂ evolved ÷ O₂ consumed

Worked examples

Common mistakes

• Saying glycolysis needs O₂. It happens with or without O₂, in the cytoplasm.
• Counting 4 ATP as the gain of glycolysis. 4 are made but 2 were spent, so net is 2.
• Thinking O₂ is used in the Krebs cycle. O₂ is used only at the end of the ETS as the final electron acceptor.
• Writing RQ as O₂ ÷ CO₂. It is CO₂ given out divided by O₂ taken in.