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How does photosynthesis convert light energy into stored chemical energy?

Use models to describe how photosynthesis converts light energy into stored chemical energy in glucose (North Carolina Standard Course of Study, Biology, LS.Bio.3).

A standard-level answer on photosynthesis for the North Carolina Biology EOC: the reactants, products, and equation, the role of the chloroplast and chlorophyll, the two stages, and the factors that affect the rate.

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  1. What this topic is asking
  2. The reactants, products, and equation
  3. Where it happens: chloroplast and chlorophyll
  4. The two stages
  5. Factors that affect the rate
  6. Try this

What this topic is asking

North Carolina LS.Bio.3 asks you to model how photosynthesis converts light energy into stored chemical energy. For the Biology EOC you need the reactants and products, the equation, the chloroplast and chlorophyll as the place and pigment, the two stages (light-dependent and light-independent), and the factors that limit the rate. Many items give you a graph of rate against light, carbon dioxide, or temperature.

The reactants, products, and equation

The balanced word and symbol equations are worth memorizing:

6CO2+6H2O→lightC6H12O6+6O26CO_2 + 6H_2O \xrightarrow{\text{light}} C_6H_{12}O_6 + 6O_2

In words: carbon dioxide plus water, using light energy, produces glucose plus oxygen. The reactants are carbon dioxide and water; the products are glucose (the stored energy) and oxygen (released as a by-product, the source of the oxygen we breathe). The EOC often tests whether you can tell reactants from products, so fix the direction: carbon dioxide and water go in, glucose and oxygen come out.

Where it happens: chloroplast and chlorophyll

Photosynthesis takes place in the chloroplast, the organelle found in plant and algal cells. Inside the chloroplast, the green pigment chlorophyll absorbs light energy, mainly red and blue light, and reflects green (which is why plants look green). The chloroplast has stacked membranes called thylakoids (where light is captured) surrounded by a fluid called the stroma (where glucose is built). This is a structure-and-function link: the folded thylakoid membranes give a large surface area for capturing light.

The two stages

Factors that affect the rate

Three factors limit the rate of photosynthesis, and the EOC commonly shows them as graphs.

  • Light intensity. More light increases the rate, because light supplies the energy, until another factor becomes limiting and the graph levels off.
  • Carbon dioxide concentration. More carbon dioxide increases the rate (it is a reactant), again until another factor limits it.
  • Temperature. Photosynthesis depends on enzymes, so the rate rises with temperature to an optimum, then falls as the enzymes denature. Near freezing, the rate is very low because the enzymes work slowly.

At any moment, the factor in shortest supply is the limiting factor: increasing it raises the rate, while increasing the others does not until the limiting factor is relieved.

Try this

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

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

Q2. State where oxygen comes from in photosynthesis. [1]

  • Cue. From the splitting of water in the light-dependent reactions.

Exam-style practice questions

Practice questions written in the style of NCDPI exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

NC Biology EOC (style)1 marksWhich are the reactants of photosynthesis? (A) Glucose and oxygen. (B) Carbon dioxide and water. (C) Oxygen and water. (D) Glucose and carbon dioxide.
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A 1-point item on the photosynthesis equation.

The correct answer is B. Photosynthesis uses carbon dioxide and water, with light energy, to produce glucose and oxygen. A and D list glucose, which is a product, and C lists oxygen, which is also a product.

Reactants in: carbon dioxide and water; products out: glucose and oxygen.

NC Biology EOC (style)2 marksA plant is given more light, then the light is kept bright but the temperature is lowered close to freezing. (a) Predict the effect of more light on the rate of photosynthesis. (b) Explain why the cold lowers the rate even with bright light.
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A 2-point item on limiting factors.

(a) 1 point: more light increases the rate of photosynthesis (up to a point), because light provides the energy for the reactions.
(b) 1 point: photosynthesis depends on enzymes, and near freezing the enzymes work very slowly, so temperature becomes the limiting factor and the rate falls despite the bright light.

Markers reward the light effect and a temperature-as-limiting-factor explanation tied to enzymes.

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