Use a model to explain how photosynthesis transforms light energy into the chemical energy of sugars, using carbon dioxide and water (Tennessee Academic Standards for Science, Biology I, BIO1.LS1).

What this topic is asking

The Tennessee LS1 standards ask you to model how photosynthesis captures light energy and stores it as chemical energy in sugar. For the Biology I EOC that means knowing the overall equation (reactants and products), where it happens (the chloroplast, using chlorophyll), what form the energy takes before and after, and how photosynthesis links to cellular respiration in the cycling of matter and energy. Items often ask you to read the equation in the right direction or to predict the effect of changing light.

The overall reaction

The single most common EOC error is reading this equation backward, so anchor it: carbon dioxide and water in, glucose and oxygen out. The plant uses the glucose for its own energy (through respiration) and as a building block for other molecules such as cellulose and starch.

Where it happens: the chloroplast

Photosynthesis takes place in the chloroplast, an organelle found in plant cells and algae. The chloroplast contains the green pigment chlorophyll, which absorbs light energy (mostly red and blue light, reflecting green, which is why plants look green). The captured light energy powers the reactions that join carbon dioxide and water into glucose. A leaf is well suited to this job: it is broad and flat to catch light, and its cells are full of chloroplasts.

Energy is transformed, not created

A key idea for the three-dimensional standards is energy and matter: photosynthesis does not create energy, it transforms it. Light energy (from the Sun) is converted into the chemical energy stored in the bonds of glucose. That stored energy is later released by cellular respiration to power the cell. So photosynthesis is the step that brings the Sun's energy into the living world, where it can flow through food chains.

The link to cellular respiration

This connection matters across the exam. The oxygen a plant releases in photosynthesis is used by animals (and the plant itself) in respiration; the carbon dioxide animals release in respiration is used by plants in photosynthesis. This coupling is developed in the photosynthesis and respiration link and in the cycling of matter.

Try this

Q1. Write the overall equation for photosynthesis in words, naming the reactants, the products, and the energy source. [3]

• Cue. Carbon dioxide plus water, using light energy, produce glucose plus oxygen.

Q2. Explain what happens to the light energy a plant absorbs during photosynthesis. [2]

• Cue. It is transformed into chemical energy stored in the bonds of glucose; energy is converted from one form to another, not created.