Use a model to describe how photosynthesis converts light energy into the chemical energy of glucose, and identify its reactants, products, and site (Ohio's Learning Standards for Science, Biology, B.C.2).

What this topic is asking

Ohio standard B.C.2 says cells "carry on specific functions that sustain life," and photosynthesis is the process that captures the energy almost all life depends on. Ohio's Biology EOC turns this into items on the reactants and products, the site (the chloroplast), the two stages, and the link between photosynthesis and cellular respiration. The crosscutting idea is energy and matter: photosynthesis converts light energy into the chemical energy stored in glucose. Many items pair the topic with a graph of rate against a limiting factor.

The overall reaction

Read the equation as an energy story. Light energy from the Sun is captured and stored in the chemical bonds of glucose, a stable, energy-rich sugar. Oxygen is released as a by-product, which is the source of the oxygen in Earth's atmosphere. The raw materials are simple and abundant: carbon dioxide from the air and water from the soil.

Where it happens: the chloroplast

Photosynthesis occurs in the chloroplast, an organelle found in plant cells and some protists. The chloroplast contains stacks of membranes (thylakoids) holding the green pigment chlorophyll, which absorbs light (mainly red and blue, reflecting green, which is why leaves look green). The structure suits the function: the large internal membrane area gives chlorophyll room to capture as much light as possible, an example of the structure and function theme.

The two stages

The EOC expects you to know that photosynthesis happens in two linked stages.

• Light-dependent reactions. In the thylakoid membranes, chlorophyll captures light energy. Water is split, releasing oxygen as a by-product, and the captured energy is stored in energy carriers.
• Light-independent reactions (the Calvin cycle). In the stroma (the fluid around the thylakoids), the energy carriers power the building of glucose from carbon dioxide. This stage does not use light directly but depends on the products of the light-dependent stage.

At the EOC level you are not expected to recall every molecule, but you should know that light is captured first (releasing oxygen) and glucose is built second (fixing carbon dioxide).

Limiting factors

Because photosynthesis is the EOC's favorite graph topic, know the idea of a limiting factor: the rate is set by whatever is in shortest supply. As you raise light intensity, the rate rises, then levels off once another factor (carbon dioxide concentration or temperature) becomes limiting. Reading such a graph, the rising part means the named factor is limiting; the flat part means something else is.

Why it matters for ecosystems

Photosynthesis is the entry point of energy into almost every food web. Producers (plants, algae) use it to convert light into the chemical energy that consumers then obtain by eating. This is why Ohio's Diversity and Interdependence strand (B.DI) traces energy from the Sun, to producers, to consumers, an idea picked up in energy flow and food webs.

Try this

Q1. Write the word equation for photosynthesis and name the organelle where it occurs. [3]

• Cue. Carbon dioxide + water, with light energy, gives glucose + oxygen; it occurs in the chloroplast.

Q2. State the form of energy that enters photosynthesis and the form in which it is stored. [2]

• Cue. Light energy enters and is stored as chemical energy in the bonds of glucose.