Compare photosynthesis and cellular respiration as linked processes, contrasting their reactants, products, energy changes, and locations, and explain how together they cycle matter and transfer energy (MA STE HS-LS1-5, HS-LS1-7, energy and matter).

What this topic is asking

The Massachusetts STE framework wants you to see photosynthesis and cellular respiration not as two separate facts to memorize but as linked, opposite processes that together cycle matter and transfer energy. On the High School Biology MCAS, this is tested with a model or diagram showing the two processes connected, and you are asked to match their reactants and products, contrast where and when they happen, and explain why matter cycles but energy flows one way. This page pulls together photosynthesis and cellular respiration into one comparison.

The two processes side by side

The fastest way to hold this topic is a comparison:

• Photosynthesis: reactants carbon dioxide and water; products glucose and oxygen; energy input is light, stored as chemical energy; happens in chloroplasts (plants and algae); needs light, so it occurs in the light.
• Cellular respiration: reactants glucose and oxygen; products carbon dioxide and water; energy is released from glucose and transferred to ATP (some lost as heat); happens mainly in mitochondria (all living cells); occurs all the time, day and night.

Notice the symmetry. The equation for photosynthesis read backward is essentially the equation for aerobic respiration. The two processes use each other's outputs.

How the products and reactants link

This is the relationship the MCAS asks about most:

• The products of photosynthesis (glucose and oxygen) are the reactants of respiration.
• The products of respiration (carbon dioxide and water) are the reactants of photosynthesis.

So the gases and atoms cycle between the two processes: the oxygen a plant releases is breathed in and used in respiration; the carbon dioxide an animal breathes out is taken up by plants in photosynthesis. This loop is the engine of the carbon cycle covered in carbon cycling and matter in organisms.

Matter cycles, but energy flows one way

Here is the deepest idea, and the one that separates strong answers. Matter cycles, but energy flows one way:

• The atoms (carbon, hydrogen, oxygen) pass back and forth between photosynthesis and respiration endlessly, so matter is recycled.
• Energy does not recycle. Light energy is captured and stored as chemical energy in glucose, then released during respiration and partly lost as heat. Heat cannot be captured back into glucose, so a continuous input of light energy is needed to keep the cycle going.

This is why ecosystems need a constant energy source (the Sun) but can reuse their matter, a principle you meet again in energy flow in ecosystems.

A common point of confusion: plants do both

Plants photosynthesize and respire. In the light, a plant usually photosynthesizes faster than it respires, so it has a net intake of carbon dioxide and a net release of oxygen. In the dark, photosynthesis stops but respiration continues, so the plant has a net release of carbon dioxide. The MCAS often probes this by asking why a plant releases oxygen overall during the day: because photosynthesis outpaces respiration.

Try this

Q1. State how the products of photosynthesis relate to the reactants of respiration. [1]

• Cue. The products of photosynthesis (glucose and oxygen) are the reactants of respiration.

Q2. Explain why matter cycles between the two processes but energy does not. [2]

• Cue. The atoms pass back and forth and are reused, but energy is captured as light, released as heat during respiration, and cannot be reused, so it flows one way.