Use a model to explain how cellular respiration releases energy from glucose as ATP, and how it relates to photosynthesis in cycling matter and energy (Tennessee Academic Standards for Science, Biology I, BIO1.LS1).

What this topic is asking

The Tennessee LS1 standards ask you to model how cellular respiration releases the energy stored in glucose so the cell can use it, and how respiration relates to photosynthesis. For the Biology I EOC that means knowing the overall equation, that aerobic respiration happens in the mitochondria and produces ATP, what happens without oxygen (fermentation), and the cause-and-effect relationship between respiration and photosynthesis in cycling matter and energy. The respiration equation and the "reverse of photosynthesis" idea are high-frequency items.

The overall reaction

Notice that this is the photosynthesis equation written backward. Anchoring that comparison is one of the best ways to remember both: photosynthesis stores energy by building glucose; respiration releases that energy by breaking glucose down.

ATP: the energy currency

The energy released from glucose is not used directly; it is transferred to molecules of ATP (adenosine triphosphate), the cell's usable energy currency. When a cell needs energy, it breaks a bond in ATP to release it. So the point of respiration is to convert the chemical energy in glucose into the form (ATP) the cell can actually spend on movement, active transport, building molecules, and other work. Energy is transformed, not created: chemical energy in glucose becomes chemical energy in ATP, with some lost as heat.

Where it happens: the mitochondrion

Most aerobic respiration occurs in the mitochondrion. Its folded inner membrane gives a large surface area for the reactions, and cells with high energy demands (muscle, nerve) contain many mitochondria. This is the link back to the organelles standard: a high mitochondrion count signals a high energy demand. A small first step of respiration (glycolysis) happens in the cytoplasm, but the bulk of the ATP comes from the mitochondrion when oxygen is present.

Without oxygen: fermentation

The EOC may contrast the two: with oxygen, a cell uses aerobic respiration and gets a lot of ATP; without oxygen, it falls back on fermentation, which is quick but yields little ATP. This is why muscles can work briefly without enough oxygen but tire and ache as lactic acid accumulates.

The link to photosynthesis

Because respiration and photosynthesis are near-reverses, their inputs and outputs interlock. Respiration's products (carbon dioxide and water) are photosynthesis's reactants; photosynthesis's products (glucose and oxygen) are respiration's reactants. Together they cycle carbon and oxygen between organisms and the atmosphere and pass the Sun's energy through living things. Plants do both: they photosynthesize in the light and respire all the time.

Try this

Q1. Write the overall equation for aerobic respiration in words, naming the reactants and products. [2]

• Cue. Glucose plus oxygen produce carbon dioxide plus water, releasing energy (ATP).

Q2. Explain why a cell that does a lot of active transport tends to contain many mitochondria. [2]

• Cue. Active transport requires energy (ATP); mitochondria are the site of aerobic respiration that produces ATP, so a cell needing a lot of energy has many mitochondria to supply it.