Cellular Respiration: Aerobic and Anaerobic

1Overview

A sprinter crossing the finish line is gasping for air, muscles burning. A loaf of bread rises in a warm kitchen because of microscopic fungi. A bacterium in deep mud, where oxygen never arrives, grows happily. All three are running the same basic chemical program: pulling energy out of food molecules and storing it in a form that cells can spend. The details differ with the availability of oxygen, and these differences explain a surprising amount of biology, from athletic fatigue to the history of Earth's atmosphere.

2ATP: the energy currency of the cell

Cells cannot use the energy in a glucose molecule directly. Burning glucose in one burst would waste most of it as heat. Instead, cells release the energy in small steps and store it in ATP (adenosine triphosphate). ATP works like a rechargeable battery: when a phosphate group is removed, energy is released to power a task such as muscle contraction or building a protein, leaving ADP. Cellular respiration recharges ADP back to ATP by adding a phosphate.

Nearly all living things regenerate ATP from the breakdown of glucose or similar sugars with the formula C₆H₁₂O₆. Two helper molecules shuttle electrons during this process: NAD⁺ is reduced to NADH, and FAD is reduced to FADH₂. These electron carriers collect energy in the early steps and deliver it to the final stage.

3The overall equation and the three stages

In aerobic cellular respiration, glucose is broken down using oxygen, and the products are carbon dioxide, water and energy captured as ATP:

C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + energy (ATP and heat)

This summary equation hides a long series of small reactions in three main stages:

1. Glycolysis, in the cytoplasm.
2. Pyruvate oxidation and the citric acid cycle, in the mitochondrial matrix (in eukaryotes).
3. Oxidative phosphorylation: the electron transport chain and chemiosmosis, in the inner mitochondrial membrane.

Why release energy in so many small steps?

A waterfall can be used to turn a single large wheel, but a long staircase of small drops can run many small machines. Respiration follows the staircase model. Each step releases a manageable amount of energy that can be captured, and the cell avoids wasting all of it at once as heat. The step-wise design also allows regulation. For example, one early glycolysis enzyme, phosphofructokinase, is slowed when ATP is already plentiful, so the cell does not break down glucose faster than it needs to. Supply follows demand.

Stage 1: glycolysis

Glycolysis ("sugar splitting") occurs in the cytoplasm of both prokaryotic and eukaryotic cells. It does not use oxygen directly, so it is anaerobic. One six-carbon glucose is converted into two three-carbon molecules of pyruvate in ten steps.

• Energy investment (steps 1 to 5): 2 ATP are spent to prepare and split the glucose.
• Energy payoff (steps 6 to 10): 4 ATP and 2 NADH are produced.
• Net result per glucose: 2 pyruvate, a net gain of 2 ATP (4 made minus 2 spent), and 2 NADH.

Stage 2: pyruvate oxidation and the citric acid cycle

If oxygen is available, pyruvate enters the mitochondrion. In the matrix:

1. Each pyruvate loses a carbon as CO₂ and becomes acetyl CoA, reducing NAD⁺ to NADH. (For one glucose: 2 CO₂ and 2 NADH.)
2. Acetyl CoA joins a four-carbon molecule, oxaloacetate, to form citrate. A cycle of eight reactions then regenerates oxaloacetate, which is why it is called the citric acid cycle or Krebs cycle.
3. Each turn releases 2 CO₂ and produces 3 NADH, 1 FADH₂ and 1 ATP (or GTP). Two turns occur per glucose.

Stage 3: the electron transport chain and chemiosmosis

The final stage generates about 90% of the ATP made from glucose. It takes place in the inner mitochondrial membrane of eukaryotes (and in the plasma membrane of prokaryotes).

1. NADH and FADH₂ deliver their electrons to a series of protein complexes (I to IV) in the electron transport chain.
2. As electrons pass along the chain, energy is used to pump hydrogen ions (H⁺) from the matrix into the intermembrane space, building up a gradient.
3. Oxygen is the final electron acceptor. It picks up electrons and then two H⁺, forming water. This is the only step in glucose breakdown that directly uses atmospheric oxygen.
4. H⁺ ions can flow back into the matrix only through the protein ATP synthase. Like water turning a mill wheel, this flow turns part of the enzyme and drives the addition of a phosphate to ADP, forming ATP. This is chemiosmosis.

4Respiration without oxygen

When oxygen is absent, the electron transport chain stops. Cells then have two options.

Anaerobic respiration

Some prokaryotes run an electron transport chain using a final electron acceptor other than oxygen. Methanogens (archaea) reduce carbon dioxide to methane and live in soil and in the digestive tracts of ruminant animals. Sulfate-reducing bacteria reduce sulfate to hydrogen sulfide.

Fermentation

Fermentation does not use an electron transport chain. It uses an organic molecule as the final electron acceptor. Its role is not to make extra ATP but to regenerate NAD⁺ from NADH so glycolysis can keep running. Glycolysis still yields 2 ATP per glucose, and that is all.

5Respiration in prokaryotes and in eukaryotes

Prokaryotes have no mitochondria, yet many carry out aerobic respiration. Glycolysis occurs in the cytoplasm as in eukaryotes, and oxidative phosphorylation occurs in the plasma membrane instead of an inner mitochondrial membrane. This fits the endosymbiotic story from the cell structure lesson: mitochondria resemble bacteria in part because they inherited bacterial respiration. It also shows that the key requirement is a membrane that can hold a proton gradient, not an organelle by itself.

6Investigating respiration

The Curricular Competencies encourage planning and conducting investigations. Respiration lends itself to simple experiments:

• Yeast and sugar. Yeast in a sugar solution releases carbon dioxide. Collecting the gas (in a balloon, or by displacement of water) gives a measurable response variable.
• Temperature. Running the same yeast mixture at different temperatures allows a graph of gas produced against temperature. Controls should include the same quantities of yeast and sugar, and repeat trials improve reliability.
• Different sugars or no sugar. A control with no sugar shows how much gas is produced from the yeast's own reserves.

7Aerobic and anaerobic organisms: single-celled and multi-celled

Life began in a world without free oxygen. Earth's atmosphere was anoxic for roughly its first two billion years, so only anaerobic organisms could live. Photosynthesis by cyanobacteria later added oxygen, which made more efficient oxygen-using pathways possible. Today, living things fall along a spectrum:

Prokaryotes illustrate the full range, because they are the most abundant life in nearly every habitat, including oxygen-free ones. Many bacteria and archaea are anaerobic and play major roles in the carbon and nitrogen cycles. Multi-celled organisms such as animals are mostly aerobic, but their tissues can still switch temporarily to fermentation, as in working muscle.

The next lesson shows the reverse transformation: photosynthesis uses light energy to build glucose from carbon dioxide and water, and releases the oxygen that cellular respiration uses. The two processes together drive the flow of carbon and energy through ecosystems. See the article on ecosystems.

8Summary

• ATP is the energy currency; respiration converts the chemical energy in glucose into ATP, with some lost as heat.
• Glycolysis (cytoplasm) makes 2 pyruvate, net 2 ATP and 2 NADH.
• Pyruvate oxidation and the citric acid cycle (matrix) release all six carbons as CO₂ and load electron carriers.
• The electron transport chain and ATP synthase make about 90% of the ATP; oxygen is the final acceptor and forms water.
• Without oxygen, cells use fermentation (lactic acid or alcohol) to regenerate NAD⁺, or some prokaryotes use anaerobic respiration.
• Organisms may be aerobic, obligate anaerobes or facultative anaerobes.

?Quick check

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