Explain photosynthesis as the capture, transformation, and storage of energy: light energy and the reactants carbon dioxide and water are converted in chloroplasts into glucose and oxygen (Virginia 2018 Biology SOL BIO.2.e).

What this topic is asking

Virginia Biology SOL standard BIO.2.e asks you to understand photosynthesis as part of the capture, storage, transformation, and flow of energy. Photosynthesis is how plants and other producers convert light energy into the chemical energy stored in glucose, supplying the energy that almost all life depends on. The Biology EOC expects you to know the reactants and products, where it happens, the energy transformation, and the factors that limit the rate, often shown as a graph.

What photosynthesis does

This is the entry point of energy into almost every food web. The glucose made in photosynthesis stores the captured energy, and that energy is later released by cellular respiration to power life processes. Without photosynthesis, there would be no food and little oxygen.

Where it happens: chloroplasts and chlorophyll

Photosynthesis takes place in the chloroplasts, organelles found in plant cells and algae. Chloroplasts contain the green pigment chlorophyll, which absorbs light energy (mostly red and blue light, reflecting green, which is why plants look green). The absorbed light energy drives the reactions that build glucose. The presence of chloroplasts is one of the features that distinguishes a plant cell from an animal cell.

The reactants, products, and equation

Notice the symmetry with respiration, which runs the opposite way. The carbon in carbon dioxide ends up in glucose (carbon fixation), the oxygen released comes from splitting water, and the energy that was in light is now stored in glucose. The EOC often asks you to identify reactants and products from this equation or to interpret a diagram of gases moving in and out of a leaf.

The energy transformation

Photosynthesis is best understood as energy capture and storage. Light is a form of energy that cells cannot use directly to do work, so chlorophyll captures it and the chloroplast uses it to build glucose, locking the energy into chemical bonds. That stored chemical energy can then be released, in the same cell or in any organism that eats the plant, by cellular respiration. This is why photosynthesis and respiration are described together as the way energy flows through living systems.

Factors that limit the rate

The rate of photosynthesis depends on three main limiting factors:

• Light intensity. More light gives a faster rate, up to a point where another factor becomes limiting.
• Carbon dioxide concentration. More carbon dioxide raises the rate, again up to a limit.
• Temperature. A higher temperature speeds the enzyme-controlled reactions up to an optimum; too high, and the enzymes denature.

On a graph, the rate rises then plateaus: the plateau means that whatever was being increased is no longer limiting, and a different factor now caps the rate. This is the same limiting-factor logic the EOC tests with population and enzyme graphs.

Try this

Q1. State the energy transformation that takes place in photosynthesis. [1]

• Cue. Light (radiant) energy is transformed into chemical energy stored in glucose.

Q2. A farmer adds carbon dioxide to a greenhouse and the plants grow faster. Explain why, using the idea of limiting factors. [2]

• Cue. Carbon dioxide was a limiting factor; increasing it allows a faster rate of photosynthesis, so more glucose is made and the plants grow faster, until another factor becomes limiting.