Compare photosynthesis and cellular respiration and explain how they cycle matter and energy between organisms and the environment (North Carolina Standard Course of Study, Biology, LS.Bio.3).

What this topic is asking

North Carolina LS.Bio.3 (linking to the ecosystems strand) asks you to compare photosynthesis and cellular respiration and explain how together they cycle matter and direct the flow of energy. For the Biology EOC you need to see that the two processes are roughly opposite reactions, that the gases and matter cycle between them, and that energy flows in one direction from sunlight to ATP and ultimately to heat. This is a favorite for model and diagram items.

The two processes side by side

The clearest way to see the link is to write the equations together.

• Photosynthesis: $6CO_2 + 6H_2O \xrightarrow{\text{light}} C_6H_{12}O_6 + 6O_2$
• Respiration: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$

Reading them together, the products of photosynthesis (glucose and oxygen) are the reactants of respiration, and the products of respiration (carbon dioxide and water) are the reactants of photosynthesis. The two are not identical (photosynthesis stores energy from light; respiration releases energy as ATP), but they are mirror images in terms of the matter involved.

Matter cycles, energy flows

This distinction explains a sealed terrarium: the plants can keep cycling carbon dioxide and oxygen between photosynthesis and respiration indefinitely (matter is conserved), but only because light keeps entering to supply energy. Cut off the light, and energy stops flowing in, so the system runs down even though the atoms are still there.

Where each process occurs

A common point of confusion is which cells do which process. Photosynthesis occurs only in cells with chloroplasts (plants, algae, and some bacteria). Respiration occurs in all living cells, including plant cells, because every cell needs ATP. So a plant in the light does both: it photosynthesises in its chloroplasts and respires in its mitochondria at the same time. In bright light a plant produces more oxygen than it uses; in the dark it only respires, taking in oxygen and releasing carbon dioxide.

Try this

Q1. State how the reactants and products of photosynthesis and respiration are related. [2]

• Cue. The products of photosynthesis (glucose and oxygen) are the reactants of respiration, and vice versa, so the two are reverse processes.

Q2. Explain the statement "matter cycles but energy flows." [2]

• Cue. Atoms are reused between the two processes (matter cycles), but energy enters as sunlight, is passed along, and is lost as heat (energy flows one way and is not recycled).