3.5 Cellular Respiration

Keywords

English Term	中文翻译	Definition & Explanation
Cellular Respiration	细胞呼吸	A set of metabolic reactions that extract energy from biological macromolecules to synthesize $\ce{ATP}$.
Cristae	嵴	The foldings of the inner mitochondrial membrane that increase surface area for the electron transport chain.
Glycolysis	糖酵解	A biochemical pathway that breaks down glucose into pyruvate, releasing $\ce{ATP}$ and $\ce{NADH}$. Occurs in the cytosol.
Krebs Cycle	克雷布斯循环	Also known as the Citric Acid Cycle. It extracts high-energy electrons ($\ce{NADH}$, $\ce{FADH2}$) and releases $\ce{CO2}$. Occurs in the mitochondrial matrix.
Oxidative Phosphorylation	氧化磷酸化	The production of $\ce{ATP}$ using energy derived from the redox reactions of the ETC and chemiosmosis.
Terminal Electron Acceptor	最终电子受体	The final molecule that accepts electrons at the end of the ETC. In aerobic respiration, it is Oxygen ($\ce{O2}$).
Fermentation	发酵	An anaerobic pathway that allows glycolysis to proceed by regenerating $\ce{NAD+}$, producing alcohol or lactic acid.

1. Mitochondria Anatomy: Structure Dictates Function

To understand how aerobic cellular respiration works, we first need to look at where it happens. The mitochondrion has a highly specialized double-membrane structure.

Outer Membrane: Smooth and highly permeable.
Inner Membrane & Cristae: The inner membrane is highly folded into structures called cristae. This folding greatly increases the surface area, allowing the mitochondrion to pack in thousands of copies of the Electron Transport Chain (ETC) and ATP synthase proteins, thereby maximizing $\ce{ATP}$ production.
Intermembrane Space: The narrow space between the inner and outer membranes. (This is where protons will be pumped to create a gradient).
Mitochondrial Matrix: The dense fluid enclosed by the inner membrane. (This is where the Krebs cycle occurs).

(Placeholder: A cross-section of a mitochondrion highlighting the cristae and the two distinct compartments: matrix and intermembrane space.)

2. The Four Stages of Aerobic Cellular Respiration

Aerobic cellular respiration uses biological macromolecules (like glucose) and oxygen to synthesize large amounts of $\ce{ATP}$.

Navigating the 'Alphabet Soup': The Uber Analogy

In cellular respiration, you will see many chemical acronyms like $\ce{NAD+}$, $\ce{NADH}$, $\ce{FAD}$, and $\ce{FADH2}$. The AP Exam does not require you to know their chemical structures. Just remember their function using the "Uber Analogy":

Glucose & Pyruvate: These are the "gold mines" filled with valuable passengers: High-energy electrons.
$\ce{NAD+}$ and $\ce{FAD}$: Think of these as empty Uber cars. They cruise around the cell looking for passengers.
$\ce{NADH}$ and $\ce{FADH2}$: These are the loaded Uber cars. They have picked up high-energy electrons and are speeding toward the final destination (the ETC) to drop them off. Once they drop off the electrons, they become empty again ($\ce{NAD+}$/$\ce{FAD}$) and go back to pick up more.

Extension: Wait, what about $\ce{NADPH}$? (Click to expand)

It is very easy to confuse $\ce{NAD+}$/$\ce{NADH}$ (used in respiration) with $\ce{NADP+}$/$\ce{NADPH}$ (used in photosynthesis).

$\ce{NADH}$ is used in Catabolic reactions (breaking things down, like Cellular Respiration). It carries electrons to the ETC to make $\ce{ATP}$.
$\ce{NADPH}$ is used in Anabolic reactions (building things up, like the Calvin Cycle in Photosynthesis). The extra "P" can be thought of as standing for "Photosynthesis" (though chemically it stands for a Phosphate group). It provides the reducing power to build sugars.

AP Exam Exclusion Statement: Metabolic Pathways

Specific steps, names of enzymes, and exact intermediates of Glycolysis and the Krebs cycle are beyond the scope of the AP Exam. Focus on the flow of energy and electrons!

(Placeholder: Overview diagram showing glucose entering glycolysis in the cytosol, pyruvate moving into the mitochondrial matrix for oxidation and the Krebs Cycle, and the ETC embedded in the inner membrane.)

Stage 1: Glycolysis

Location: Cytosol (outside the mitochondrion).
What happens: A 6-carbon glucose is split into two 3-carbon molecules called pyruvate.
Energy extracted: A small amount of $\ce{ATP}$ is made directly, and some "empty Ubers" pick up electrons to become $\ce{NADH}$.

Stage 2: Pyruvate Oxidation

Location: As pyruvate enters the mitochondrial matrix.
What happens: Pyruvate is oxidized. Carbon is broken off and released as $\ce{CO2}$. More electrons are transferred to create more $\ce{NADH}$.

Stage 3: The Krebs Cycle (Citric Acid Cycle)

Location: Mitochondrial Matrix.
What happens: The remnants of the glucose molecule are completely dismantled. More $\ce{CO2}$ is released as a waste product. A tiny amount of $\ce{ATP}$ is synthesized.
The Main Goal: To extract as many high-energy electrons as possible! A massive fleet of Ubers ($\ce{NADH}$ and $\ce{FADH2}$) is loaded up and sent to the next stage.

Stage 4: Oxidative Phosphorylation (ETC and Chemiosmosis)

Location: Inner mitochondrial membrane.
The Electron Transport Chain (ETC): The loaded Ubers ($\ce{NADH}$ and $\ce{FADH2}$) arrive and drop off their electrons. As electrons pass through the ETC proteins, their energy is used to pump protons ($\ce{H+}$) from the matrix into the intermembrane space.
- The Result: The intermembrane space becomes highly acidic (low pH), while the matrix has a lower proton concentration (higher pH).
Terminal Electron Acceptor: At the very end of the ETC, the electrons must go somewhere. Oxygen ($\ce{O2}$) waits at the end of the line, grabs the electrons and some protons, and forms Water ($\ce{H2O}$).
Chemiosmosis: The protons ($\ce{H+}$) trapped in the intermembrane space rush back into the matrix down their concentration gradient through the enzyme ATP synthase. This flow acts like water turning a turbine, driving the massive synthesis of $\ce{ATP}$.

Biological Application: Decoupling and Heat Generation

In certain endothermic organisms (like hibernating bears or human infants with "brown fat"), oxidative phosphorylation can be intentionally decoupled from electron transport. Specific uncoupling proteins allow protons to leak back across the membrane without passing through ATP synthase. Instead of making $\ce{ATP}$, the energy from the proton gradient is released as heat to regulate body temperature.

(Placeholder: Diagram showing $\ce{NADH}$ dropping off electrons, protons being pumped into the intermembrane space creating a gradient, oxygen accepting electrons to make water, and protons flowing through ATP synthase to generate $\ce{ATP}$.)

3. Anaerobic Respiration and Fermentation

What happens if there is no oxygen ($\ce{O2}$) available to act as the terminal electron acceptor? The ETC backs up, and oxidative phosphorylation stops.

Anaerobic Respiration: Some prokaryotes live in environments without oxygen. They still use an ETC, but they use molecules other than oxygen (like sulfate or nitrate) as the terminal electron acceptor.
Fermentation: In many organisms (including humans), if oxygen is absent, the cell relies on fermentation.
- The Problem: Glycolysis can still make a tiny bit of $\ce{ATP}$ without oxygen, but it requires empty $\ce{NAD+}$ (empty Ubers) to work. Without the ETC to empty the $\ce{NADH}$, the cell runs out of $\ce{NAD+}$, and everything stops.
- The Solution: Fermentation pathways simply take the electrons from $\ce{NADH}$ and dump them onto an organic molecule (like pyruvate). This regenerates $\ce{NAD+}$, allowing glycolysis to continue running.
- Byproducts: Depending on the organism, this process produces organic molecules such as alcohol (ethanol in yeast) or lactic acid (in human muscle cells).

(Placeholder: Diagram emphasizing that the entire point of fermentation is the loop that turns $\ce{NADH}$ back into $\ce{NAD+}$ so Glycolysis can keep going.)

Quiz

Source: Campbell Biology Practice Test - Chapter 9 (Cellular Respiration and Fermentation)