Explain how photosynthesis converts light energy, carbon dioxide and water into glucose and oxygen, identify where it occurs, and analyze how limiting factors affect its rate (NYSSLS LS1, energy and matter; analyzing data).

What this topic is asking

NYSSLS LS1 wants you to explain how photosynthesis brings energy and matter into living systems: plants use light to build glucose. On the Life Science: Biology Regents this very often comes as a cluster with data from an investigation (for example oxygen bubbles from a water plant) where you read a rate and explain it using limiting factors.

The inputs and outputs

The word equation is: carbon dioxide + water, in the presence of light energy and chlorophyll, gives glucose + oxygen. As a balanced symbol equation:

The carbon dioxide enters from the air, the water from the soil, and the light from the Sun. The glucose can be used at once in respiration, stored as starch, or built into other molecules; the oxygen is released as a by-product.

Where photosynthesis happens

Photosynthesis takes place in chloroplasts, organelles found mainly in leaf cells. Chloroplasts contain the green pigment chlorophyll, which absorbs light energy (mostly red and blue light, reflecting green, which is why leaves look green). The structure of the leaf suits the function: a broad, flat shape to capture light, many chloroplasts near the surface, and pores (stomata) to let carbon dioxide in and oxygen out.

Energy and matter

Photosynthesis is where the crosscutting concept of energy and matter begins for most ecosystems. Light energy is captured and stored as chemical energy in glucose, and the matter (carbon from carbon dioxide) is built into the plant. Animals later obtain both the energy and the matter by eating plants. This is why photosynthesis is so important beyond plants: it supplies the food and the oxygen that most other life depends on.

Limiting factors

This is the key to most photosynthesis data questions. If a graph of rate against light intensity rises and then plateaus, the plateau means light is no longer limiting and something else (carbon dioxide or temperature) now is. Temperature acts through enzymes: too cold and the reactions are slow, too hot and the enzymes denature (see enzymes and metabolism).

Try this

Q1. Write the word equation for photosynthesis. [2]

• Cue. Carbon dioxide + water, in the presence of light (and chlorophyll), gives glucose + oxygen.

Q2. Explain why increasing light intensity eventually has no further effect on the rate of photosynthesis. [2]

• Cue. Beyond a certain point another factor (carbon dioxide or temperature) becomes the limiting factor, so light is no longer what holds back the rate.