2.4 Respiration

Cellular respiration breaks down organic molecules to release energy as ATP, fueling everything from cell division to nutrient transport. This process is central to plant physiology because even though plants make their own food through photosynthesis, they still need respiration to actually use that food for energy.

The three main stages are glycolysis, the Krebs cycle, and the electron transport chain. Together, they extract energy from glucose step by step, producing ATP, NADH, and $FADH_2$ while releasing $CO_2$ as a byproduct.

Cellular respiration overview

Cellular respiration is a metabolic process that breaks down organic molecules (primarily glucose) to release energy in the form of ATP. Every living plant cell relies on it to power growth, development, and maintenance.

Aerobic vs anaerobic respiration

• Aerobic respiration requires oxygen and yields up to about 30–32 ATP per glucose molecule (the theoretical maximum of 38 is rarely achieved in practice due to energy costs of transporting molecules across membranes).
• Anaerobic respiration (fermentation) occurs without oxygen and produces only 2 ATP per glucose molecule.
• Aerobic respiration is far more efficient, but anaerobic pathways keep cells alive when oxygen is scarce, such as in waterlogged soils.

Importance of cellular respiration

• Energy production: Generates ATP to power energy-demanding processes like cell division, active transport, and growth.
• Carbon skeletons: Intermediates from respiration serve as building blocks for synthesizing amino acids, nucleotides, and other compounds.
• Redox balance: Helps maintain cellular homeostasis by cycling reducing equivalents like NADH back to $NAD^+$.

Glycolysis

Glycolysis is the first stage of cellular respiration and takes place in the cytosol. It splits one six-carbon glucose molecule into two three-carbon pyruvate molecules through a series of enzyme-catalyzed reactions. Because it doesn't require oxygen, glycolysis occurs under both aerobic and anaerobic conditions.

Glycolysis steps

Glycolysis has ten reactions, but you can think of it in two phases:

Energy investment phase (uses 2 ATP):

1. Glucose is phosphorylated by hexokinase, using one ATP to form glucose-6-phosphate.
2. Glucose-6-phosphate is rearranged to fructose-6-phosphate.
3. Fructose-6-phosphate is phosphorylated by phosphofructokinase (PFK), using a second ATP to form fructose-1,6-bisphosphate.
4. Fructose-1,6-bisphosphate is split into two three-carbon molecules: glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP). DHAP is converted to G3P, so from here everything happens twice.

Energy payoff phase (produces 4 ATP and 2 NADH):

1. G3P is oxidized by glyceraldehyde-3-phosphate dehydrogenase, reducing $NAD^+$ to NADH and producing 1,3-bisphosphoglycerate.
2. 1,3-bisphosphoglycerate transfers a phosphate to ADP, producing ATP and 3-phosphoglycerate (via phosphoglycerate kinase).
3. 3-phosphoglycerate is rearranged to 2-phosphoglycerate.
4. 2-phosphoglycerate loses water (via enolase) to form phosphoenolpyruvate (PEP).
5. PEP transfers its phosphate to ADP (via pyruvate kinase), producing ATP and pyruvate.

Glycolysis products

Per one glucose molecule:

• 2 pyruvate molecules
• 2 ATP (net gain: 4 produced minus 2 invested)
• 2 NADH

Glycolysis regulation

The cell controls glycolysis mainly through three enzymes:

• Hexokinase: inhibited by its own product, glucose-6-phosphate.
• Phosphofructokinase (PFK): the main regulatory enzyme. High ATP inhibits it (signaling plenty of energy), while high ADP or AMP activates it (signaling energy is needed). Citrate also inhibits PFK.
• Pyruvate kinase: inhibited by ATP.

This feedback system ensures the cell only breaks down glucose as fast as it needs energy.

Krebs cycle

The Krebs cycle (also called the citric acid cycle or TCA cycle) is the second stage of respiration. It takes place in the mitochondrial matrix and completes the oxidation of fuel molecules, generating most of the $NADH$ and $FADH_2$ that will drive ATP production in the next stage.

Before the cycle begins, pyruvate from glycolysis is transported into the mitochondrion and converted to acetyl-CoA by the pyruvate dehydrogenase complex. This step releases one $CO_2$ and produces one NADH per pyruvate.

Krebs cycle steps

1. Acetyl-CoA + oxaloacetate → citrate (catalyzed by citrate synthase).
2. Citrate → isocitrate (catalyzed by aconitase).
3. Isocitrate → α-ketoglutarate (catalyzed by isocitrate dehydrogenase). Produces NADH and releases $CO_2$.
4. α-ketoglutarate → succinyl-CoA (catalyzed by α-ketoglutarate dehydrogenase complex). Produces NADH and releases $CO_2$.
5. Succinyl-CoA → succinate (catalyzed by succinyl-CoA synthetase). Produces ATP (or GTP).
6. Succinate → fumarate (catalyzed by succinate dehydrogenase). Reduces FAD to $FADH_2$.
7. Fumarate → malate (catalyzed by fumarase).
8. Malate → oxaloacetate (catalyzed by malate dehydrogenase). Produces NADH.

Oxaloacetate is regenerated at the end, which is why it's a cycle.

Krebs cycle products

Per turn of the cycle (remember, each glucose produces 2 acetyl-CoA, so the cycle turns twice):

• 3 NADH
• 1 $FADH_2$
• 1 ATP (or GTP)
• 2 $CO_2$

Krebs cycle regulation

The cycle is regulated by substrate availability and feedback from energy carriers:

• Citrate synthase: inhibited by high ATP and NADH.
• Isocitrate dehydrogenase: stimulated by ADP, inhibited by NADH and ATP.
• α-ketoglutarate dehydrogenase: inhibited by NADH and succinyl-CoA, stimulated by $Ca^{2+}$.

When the cell has plenty of ATP and NADH, the cycle slows down. When energy demand rises, it speeds up.

Electron transport chain

The electron transport chain (ETC) is the final stage of aerobic respiration and produces the vast majority of ATP. It consists of protein complexes embedded in the inner mitochondrial membrane that pass electrons from NADH and $FADH_2$ to oxygen.

Electron transport chain components

• Complex I (NADH dehydrogenase): Accepts electrons from NADH, passes them to ubiquinone. Pumps protons ($H^+$) across the membrane.
• Complex II (succinate dehydrogenase): Accepts electrons from $FADH_2$, passes them to ubiquinone. Does not pump protons, which is why $FADH_2$ yields less ATP than NADH.
• Complex III (cytochrome bc1 complex): Transfers electrons from ubiquinone to cytochrome c. Pumps protons.
• Complex IV (cytochrome c oxidase): Transfers electrons from cytochrome c to $O_2$, forming water. Pumps protons.
• ATP synthase (Complex V): Uses the proton gradient to synthesize ATP.

Chemiosmosis in the electron transport chain

As electrons move through Complexes I, III, and IV, protons are pumped from the mitochondrial matrix into the intermembrane space. This builds up a proton gradient (also called the proton-motive force) across the inner membrane, with a higher $H^+$ concentration in the intermembrane space.

That gradient is a form of stored energy. Protons can only flow back into the matrix through ATP synthase, and as they do, they drive ATP production.

ATP synthase role

ATP synthase works like a tiny turbine:

1. Protons flow down their concentration gradient through a channel in ATP synthase.
2. This flow causes part of the enzyme to physically rotate.
3. The rotational energy drives the binding of ADP and inorganic phosphate ($P_i$) to form ATP.

Oxidative phosphorylation process

Oxidative phosphorylation is the term for the coupled process of electron transport and chemiosmotic ATP synthesis. Here's the sequence:

1. NADH and $FADH_2$ (from glycolysis and the Krebs cycle) donate electrons to the ETC.
2. Electrons pass through the complexes, releasing energy at each step.
3. That energy pumps $H^+$ into the intermembrane space.
4. The resulting proton gradient drives ATP synthase to produce ATP.
5. Oxygen serves as the final electron acceptor, combining with electrons and $H^+$ to form water.

Without oxygen to accept electrons at the end, the entire chain backs up and aerobic respiration stops.

Fermentation

Fermentation is an anaerobic process that allows glycolysis to keep running when oxygen is unavailable. The key problem without oxygen is that NADH builds up and $NAD^+$ runs out. Glycolysis needs $NAD^+$ to continue, so fermentation regenerates it by transferring electrons from NADH to pyruvate (or a derivative of pyruvate).

Lactic acid fermentation

• Pyruvate is directly reduced to lactate by lactate dehydrogenase, regenerating $NAD^+$.
• This occurs in plant tissues under anaerobic conditions, such as roots in waterlogged soils or tissues during post-harvest storage.

Alcoholic fermentation

This is a two-step process:

1. Pyruvate is decarboxylated to acetaldehyde by pyruvate decarboxylase, releasing $CO_2$.
2. Acetaldehyde is reduced to ethanol by alcohol dehydrogenase, regenerating $NAD^+$.

Alcoholic fermentation occurs in yeast and in some plant tissues (particularly fruits and seeds) under low-oxygen conditions. It contributes to flavor and aroma development in ripening fruits.

Fermentation in plant cells

Fermentation yields only 2 ATP per glucose (from glycolysis alone), making it far less efficient than aerobic respiration. Plants typically use it as a short-term survival strategy. Prolonged anaerobic conditions can be harmful because ethanol and lactate accumulate and can damage cells.

Respiration in plants

All living plant tissues respire, not just leaves. Roots, stems, seeds, flowers, and fruits all consume $O_2$ and release $CO_2$ continuously.

Respiration in leaves

Leaves photosynthesize and respire. During the day, the $CO_2$ released by respiration is immediately reused by photosynthesis, so net gas exchange shows $CO_2$ uptake and $O_2$ release. At night, photosynthesis stops, and respiration becomes the only gas exchange process, so leaves release $CO_2$ and consume $O_2$.

This is why the light compensation point matters: it's the light intensity at which photosynthetic $CO_2$ fixation exactly equals respiratory $CO_2$ release.

Respiration in roots

Roots rely entirely on respiration for energy to support growth, nutrient uptake (especially active transport of ions), and water absorption. They need adequate soil oxygen for aerobic respiration. In waterlogged or compacted soils, oxygen becomes limited, and roots may switch to fermentation. Some wetland plants have evolved aerenchyma (air channels) to transport oxygen from shoots to roots.

Respiration in seeds

Seeds are metabolically active during development and germination. During dry storage, respiration rates are very low, which helps maintain seed viability. As seeds imbibe water during germination, respiration rates increase sharply to fuel cell division and growth of the embryo. Temperature, moisture, and oxygen availability all influence seed respiration rates.

Factors affecting plant respiration

• Temperature: Respiration rates roughly double for every 10°C increase, a relationship known as the $Q_{10}$ effect. At very high temperatures, enzymes denature and respiration drops.
• Oxygen availability: Adequate $O_2$ is needed for aerobic respiration. Low $O_2$ shifts metabolism toward fermentation.
• Water status: Water stress reduces metabolic activity and limits gas exchange, lowering respiration rates.
• Developmental stage: Rapidly growing tissues (meristems, expanding leaves, ripening fruits) respire faster than mature or senescing tissues.

Respiration and photosynthesis

Respiration and photosynthesis are complementary processes. Photosynthesis is anabolic (builds organic molecules using light energy), while respiration is catabolic (breaks down organic molecules to release energy). Plants need both.

Respiration vs photosynthesis

Interplay of respiration and photosynthesis

These two processes form a cycle within the plant:

• Organic compounds (sugars) produced by photosynthesis serve as substrates for respiration.
• $O_2$ released by photosynthesis is used for aerobic respiration.
• $CO_2$ released by respiration can be refixed by photosynthesis during the day.

The balance between the two determines net carbon gain. If photosynthesis exceeds respiration, the plant grows. If respiration exceeds photosynthesis (as in deep shade or high temperatures), the plant loses biomass over time.

Respiration measurement techniques

Researchers measure plant respiration to understand how environmental conditions and genetics affect energy metabolism. The three main approaches target different outputs of respiration.

Oxygen consumption measurement

• An oxygen electrode (Clark-type electrode) measures $O_2$ uptake by plant tissue.
• Tissue is sealed in a closed chamber, and the decline in $O_2$ concentration is recorded over time.
• Respiration rate is calculated from the rate of $O_2$ decrease, normalized to tissue fresh weight or dry weight.

Carbon dioxide production measurement

• An infrared gas analyzer (IRGA) detects $CO_2$ released from plant tissue.
• Tissue is placed in a sealed chamber, and the increase in $CO_2$ concentration is measured over time.
• This method is especially useful for measuring dark respiration in leaves (when photosynthesis is not occurring).

Calorimetry in respiration studies

• A calorimeter measures heat released during respiration.
• Since respiration is exothermic, the heat output is proportional to metabolic rate.
• Calorimetry captures total metabolic heat, including energy from pathways that don't directly consume $O_2$ or produce $CO_2$ (like the alternative oxidase pathway in plants).

Respiration and plant metabolism

Respiration is tightly connected to the rest of plant metabolism. It doesn't just produce ATP; its intermediates feed into biosynthetic pathways throughout the cell.

Respiration and plant growth

Respiration provides the ATP and carbon skeletons needed for cell division, elongation, and differentiation. Rapidly growing tissues like shoot and root meristems have high respiration rates. The relationship is straightforward: more growth demand means more respiration.

Plant scientists distinguish between growth respiration (energy used to build new biomass) and maintenance respiration (energy used to sustain existing cells). As a plant matures, maintenance respiration takes up a larger share of total respiration.

Respiration and stress responses

Under stress conditions like drought, salinity, or extreme temperatures, respiration rates often change. Stress can increase respiration as the plant invests energy in defense and repair processes such as osmotic adjustment, antioxidant synthesis, and heat shock protein production. However, severe stress can also damage mitochondria and reduce respiration capacity.

Respiration and crop yield

Respiration directly affects crop yield because it determines how much of the carbon fixed by photosynthesis is retained as biomass versus lost as $CO_2$. In many crops, 30–60% of daily photosynthetic carbon gain is consumed by respiration. Reducing unnecessary respiratory losses (particularly maintenance respiration) is an active area of crop improvement research, since even small efficiency gains could translate to meaningful yield increases.