Use a model to illustrate how cellular respiration breaks the bonds of glucose and oxygen to release energy, and relate it to photosynthesis (Louisiana Student Standards for Science, High School Biology, HS-LS1-7).

What this topic is asking

Louisiana's LS1 standards (HS-LS1-7) ask you to model how cellular respiration breaks the bonds of glucose and oxygen to release energy, and to relate it to photosynthesis (the connection HS-LS1-8 makes). For LEAP 2025 Biology you should know the reactants and products, the role of mitochondria and ATP, the difference between aerobic and anaerobic respiration, and why respiration and photosynthesis are opposites that cycle matter. The test often pairs the two processes in one item.

What cellular respiration is

Every living cell respires, all the time, because every cell needs a constant supply of usable energy (ATP) to grow, move, transport substances, and build molecules. Respiration is how the energy captured by photosynthesis is finally released for use.

Aerobic respiration: with oxygen

In a word equation: glucose + oxygen becomes carbon dioxide + water, releasing energy. The balanced chemical equation is:

Compare this with photosynthesis ($6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$): respiration is essentially the reverse.

Anaerobic respiration: without oxygen

When oxygen is in short supply, cells switch to anaerobic respiration (fermentation), which releases energy from glucose without oxygen. It produces much less energy per glucose than aerobic respiration. The products depend on the organism:

• In animal muscle cells during hard exercise: glucose becomes lactic acid (plus a little energy), which builds up and causes muscle fatigue.
• In yeast (used in baking and brewing): glucose becomes alcohol and carbon dioxide (plus a little energy), a process called alcoholic fermentation.

The link to photosynthesis

The standard pairs respiration with photosynthesis because they are complementary:

• Photosynthesis stores energy (light to chemical) and uses carbon dioxide and water to make glucose and oxygen.
• Respiration releases that energy and uses glucose and oxygen to make carbon dioxide and water.

So the products of one are the reactants of the other. Together they cycle matter (carbon, hydrogen, oxygen) between organisms and the environment and move energy from sunlight into the bodies of living things, the LS1-LS2 connection.

Try this

Q1. Write the word equation for aerobic cellular respiration. [2]

• Cue. Glucose + oxygen becomes carbon dioxide + water, releasing energy.

Q2. State one difference between aerobic and anaerobic respiration. [1]

• Cue. Aerobic respiration uses oxygen and releases much more energy; anaerobic respiration occurs without oxygen and releases less energy (and makes lactic acid or alcohol and carbon dioxide).