Use a model to describe how cellular respiration releases the chemical energy in glucose as ATP, comparing aerobic respiration with anaerobic respiration and fermentation (Ohio's Learning Standards for Science, Biology, B.C.2).

What this topic is asking

Ohio standard B.C.2 says cells "carry on specific functions that sustain life," and releasing usable energy from food is the most fundamental. Ohio's Biology EOC turns cellular respiration into items on the reactants and products, the site (the mitochondrion), the role of ATP, and the difference between aerobic and anaerobic respiration. The crosscutting idea is energy and matter: respiration converts the chemical energy in glucose into ATP, the usable energy currency of the cell. The topic pairs naturally with photosynthesis as the reverse reaction.

The overall reaction

Read the equation as an energy story in reverse to photosynthesis. The energy-rich bonds of glucose are broken down using oxygen, and the released energy is captured in ATP that the cell can spend on its activities (movement, building molecules, active transport). Carbon dioxide and water are released as waste.

ATP: the energy currency

The point of respiration is not the carbon dioxide and water; it is the ATP. Cells cannot use the energy in glucose directly, so they transfer it to ATP, a small molecule that powers reactions when needed. Whenever the EOC asks what respiration is "for," the answer is to make ATP (usable energy), not simply to break down glucose.

Where it happens: the mitochondrion

Aerobic respiration takes place mainly in the mitochondrion, which is why cells with a high energy demand (muscle, kidney, nerve) contain so many. The mitochondrion's folded inner membrane gives a large surface area for the energy-releasing reactions, an example of the structure and function theme. (The first steps of glucose breakdown begin in the cytoplasm before the rest is completed in the mitochondrion, but at EOC level "mitochondrion" is the site to name.)

Aerobic versus anaerobic respiration

The key contrast on the EOC is whether oxygen is available.

• Aerobic respiration uses oxygen, fully breaks down glucose to carbon dioxide and water, and releases a large amount of energy (many ATP per glucose).
• Anaerobic respiration (fermentation) runs without oxygen and releases a small amount of energy (few ATP per glucose), because glucose is only partly broken down.

Anaerobic respiration takes two common forms:

• In human muscle (lactic acid fermentation): when exercise outpaces the oxygen supply, muscle cells make lactic acid, which builds up and contributes to fatigue and soreness.
• In yeast and some microbes (alcoholic fermentation): glucose is converted to ethanol and carbon dioxide, the basis of baking and brewing.

The link to photosynthesis

Respiration and photosynthesis are mirror images, and Ohio's standards expect you to connect them.

• Photosynthesis: $CO_2 + H_2O \to$ glucose $+ O_2$ (stores energy).
• Respiration: glucose $+ O_2 \to CO_2 + H_2O$ (releases energy).

The products of one are the reactants of the other, so the two processes cycle carbon and energy between organisms and the environment. This is the cellular basis of the carbon cycle examined in the cycling of matter.

Try this

Q1. Write the word equation for aerobic cellular respiration and name the organelle where it mainly occurs. [3]

• Cue. Glucose + oxygen gives carbon dioxide + water + energy (ATP); it mainly occurs in the mitochondrion.

Q2. State two ways anaerobic respiration differs from aerobic respiration. [2]

• Cue. Anaerobic respiration needs no oxygen and releases much less energy (ATP) per glucose; it also makes a product such as lactic acid or ethanol rather than only carbon dioxide and water.