Describe the reactants, products, and energy transformation of photosynthesis, and explain its role in capturing light energy as chemical energy in glucose (TEKS Biology, Reporting Category 4; energy and matter; cause and effect).

What this topic is asking

The Biology TEKS ask you to describe the reactants, products, and energy transformation of photosynthesis. For STAAR Reporting Category 4 you need the inputs and outputs, where it happens, and the central idea that photosynthesis captures light energy as chemical energy in glucose. This is an energy and matter and cause-and-effect topic, and it is almost always paired with cellular respiration.

What photosynthesis does

The point of photosynthesis is to capture energy. Light energy is not something a cell can use directly for most of its work, so plants convert it into the chemical energy of glucose, a stable, usable store. That glucose then feeds the plant (through respiration) and, when the plant is eaten, feeds the rest of the food chain.

Reactants, products, and the equation

The balanced chemical equation is:

So six molecules of carbon dioxide and six of water are converted into one molecule of glucose and six of oxygen. The oxygen released is a by-product, and it is the source of most of the oxygen in the air.

The energy transformation

The central idea is the energy transformation: photosynthesis converts light energy into chemical energy stored in the bonds of glucose. This is why photosynthesis is essential to nearly all life, it is the main way that the Sun's energy is captured and made available to living things. The chemical energy in glucose is later released by cellular respiration.

Because photosynthesis depends on light, its rate changes with conditions: more light (up to a point), more carbon dioxide, and a suitable temperature all increase the rate, while darkness stops it. STAAR often gives data showing how a factor affects the rate and asks you to explain the pattern.

Why it matters to the whole system

Photosynthesis links the cell to the entire living world. It is the entry point of energy into food chains: producers capture light energy as glucose, and that energy then flows to consumers (see energy flow and food webs). It also moves carbon out of the air and into living things, a key step in the cycling of matter. This is the energy and matter theme on the largest scale.

Try this

Q1. State the reactants and products of photosynthesis. [2]

• Cue. Reactants: carbon dioxide and water. Products: glucose and oxygen.

Q2. State the energy transformation that takes place in photosynthesis. [1]

• Cue. Light energy is converted into chemical energy stored in glucose.