Explain how photosynthesis and respiration together cycle carbon and oxygen while energy flows one way, and trace atoms of matter through these processes (NYSSLS LS1, energy and matter; systems and system models).

What this topic is asking

NYSSLS LS1 links photosynthesis and respiration into one big picture: together they cycle matter (carbon and oxygen) between organisms and the environment, while energy flows one way from the Sun. The Life Science: Biology Regents tests this with the crosscutting concept of energy and matter, often asking you to trace atoms or to explain why energy must keep entering while matter does not.

Two linked processes

The reactants of photosynthesis are the products of respiration, and vice versa. This is easiest to see side by side:

• Photosynthesis: carbon dioxide + water, with light energy, gives glucose + oxygen.
• Respiration: glucose + oxygen gives carbon dioxide + water (+ usable energy as ATP).

Because they are mirror images, the carbon, oxygen and hydrogen atoms pass back and forth between living things and the environment. This is the cellular basis of the carbon cycle covered at ecosystem scale in energy flow and matter cycling.

Matter cycles and is conserved

A classic exam point follows from this: the mass a plant gains as it grows comes mostly from carbon dioxide in the air, not from the soil, because photosynthesis builds the carbon of glucose (and everything made from it) out of carbon dioxide. The water and minerals from the soil matter too, but the bulk of the added dry mass is captured carbon.

Energy flows one way

In contrast to matter, energy does not cycle. Light energy is captured by photosynthesis and stored in glucose; respiration transfers it to ATP; the cell's work transfers it onward, and at every step some is lost as heat. Heat cannot be recaptured for biological work, so energy flows one way through the system and must be continually replaced by sunlight. This is why ecosystems depend on a constant input of light, while they can recycle their matter.

Tracing atoms and energy

The exam likes "follow the atom" questions. To answer them, use the two equations and the conservation rule: identify where an atom enters (for example carbon in carbon dioxide), which process incorporates it (photosynthesis), where it goes (glucose, then body tissue, then back to carbon dioxide by respiration or decomposition). For energy, trace the form changes (light to chemical to heat) and note that it is lost as heat, so it does not return.

Try this

Q1. Explain why the dry mass a plant gains as it grows comes mostly from the air. [2]

• Cue. Photosynthesis builds the carbon of glucose (and the molecules made from it) from carbon dioxide in the air, so most added mass is captured carbon, not soil material.

Q2. Explain why energy must keep entering an ecosystem but matter does not. [2]

• Cue. Energy is lost as heat at each transfer and cannot be reused, so it must be replaced by sunlight; matter is conserved and cycles between organisms and the environment.