🧠 Respiration

This lesson explains how plants release usable energy and why respiration management matters for growth, storage, and post-harvest quality.

Overview of Cellular Respiration

Cellular respiration is the process by which organisms break down glucose and other organic molecules to release energy in the form of ATP (adenosine triphosphate). Unlike photosynthesis, respiration occurs in all living cells continuously. The overall equation for aerobic respiration is:

C6H12O6 + 6O2 --> 6CO2 + 6H2O + Energy (ATP)

Respiration is essential for plant growth, nutrient uptake, seed germination, and fruit ripening. Understanding respiration helps in managing post-harvest losses, seed storage, and crop productivity.

Glycolysis

Glycolysis (from Greek: glycos = sugar, lysis = splitting) is the first stage of respiration, occurring in the cytoplasm of the cell. It does not require oxygen and is therefore common to both aerobic and anaerobic respiration. In glycolysis:

• One molecule of glucose (6-carbon) is broken down into two molecules of pyruvate (3-carbon)
• Net gain: 2 ATP molecules and 2 NADH molecules
• The process involves 10 enzymatic steps, discovered by Embden, Meyerhof, and Parnas (hence also called the EMP pathway)

Glycolysis provides the starting substrate for further energy extraction in subsequent stages.

Krebs Cycle (Citric Acid Cycle)

Under aerobic conditions, pyruvate enters the mitochondria and is first converted to Acetyl CoA (2-carbon) by pyruvate dehydrogenase, releasing one CO2 and one NADH per pyruvate. Acetyl CoA then enters the Krebs cycle (discovered by Hans Krebs), also called the Tricarboxylic Acid (TCA) cycle:

• Acetyl CoA (2C) combines with oxaloacetate (4C) to form citrate (6C)
• Through a series of 8 enzymatic reactions, citrate is progressively oxidized back to oxaloacetate
• Per turn of the cycle: 3 NADH, 1 FADH2, 1 GTP (equivalent to ATP), and 2 CO2 are produced
• Since each glucose produces 2 Acetyl CoA molecules, the Krebs cycle turns twice per glucose

Electron Transport Chain (ETC) and Oxidative Phosphorylation

The ETC is the final stage of aerobic respiration, located on the inner mitochondrial membrane. NADH and FADH2 produced in earlier stages donate their electrons to a series of protein complexes (Complex I, II, III, and IV). As electrons pass through these carriers, energy is released and used to pump H+ ions (protons) across the inner membrane, creating a proton gradient. This gradient drives ATP synthase to produce ATP through chemiosmosis (similar to the process in chloroplasts). At the end of the chain, electrons combine with oxygen and H+ to form water. The total ATP yield from one glucose molecule is approximately 36--38 ATP (2 from glycolysis, 2 from Krebs cycle, 32--34 from ETC).

Aerobic vs. Anaerobic Respiration

• Aerobic respiration: Requires oxygen, occurs in mitochondria, produces 36--38 ATP per glucose, and is the primary energy pathway in most plant tissues
• Anaerobic respiration (fermentation): Occurs without oxygen. Alcoholic fermentation (in yeast and some plant tissues under waterlogged conditions) converts pyruvate to ethanol and CO2, yielding only 2 ATP. Lactic acid fermentation converts pyruvate to lactate. In agriculture, anaerobic conditions in waterlogged soils can damage roots; rice paddies rely on specialized aerenchyma tissue to deliver oxygen to submerged roots. Post-harvest, controlling respiration rates through cold storage and controlled atmosphere (low O2, high CO2) extends the shelf life of fruits, vegetables, and grains.

Summary Cheat Sheet

• Respiration releases ATP from glucose for cellular work.
• Glycolysis occurs in cytoplasm and yields 2 ATP net per glucose.
• Krebs cycle and ETC in mitochondria drive most ATP production.
• Aerobic respiration is efficient; anaerobic pathways yield far less ATP.
• Respiration control is key in waterlogging tolerance and post-harvest storage.