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How do plants transform light energy into the chemical energy stored in sugars, and what raw materials and products are involved?

Use a model to illustrate how photosynthesis transforms light energy into stored chemical energy in sugars, including the reactants, products, and the role of chlorophyll (MA STE HS-LS1-5).

A standard-level answer on photosynthesis for the Massachusetts High School Biology MCAS: how light energy becomes chemical energy in sugars, the reactants and products, the role of chlorophyll and chloroplasts, and limiting factors under HS-LS1-5.

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  1. What this topic is asking
  2. What photosynthesis does
  3. Where it happens, and the role of chlorophyll
  4. The energy and matter transformation
  5. Limiting factors and rate
  6. Try this

What this topic is asking

The Massachusetts STE framework standard HS-LS1-5 asks you to use a model to illustrate how photosynthesis transforms light energy into stored chemical energy. The key word is model: on the High School Biology MCAS you are often given a diagram of a chloroplast, an equation, or a graph and asked to identify the inputs and outputs, describe the energy transformation, or explain how a factor changes the rate. The crosscutting concept is energy and matter, because photosynthesis both transforms energy and rearranges matter (carbon, hydrogen, oxygen) into sugar.

What photosynthesis does

The overall reaction is written as a balanced equation:

6 CO2+6 H2O→light energyC6H12O6+6 O26\,CO_2 + 6\,H_2O \xrightarrow{\text{light energy}} C_6H_{12}O_6 + 6\,O_2

In words: six carbon dioxide molecules plus six water molecules, using light energy captured by chlorophyll, produce one glucose molecule and six oxygen molecules. The glucose stores the energy; the oxygen is released as a by-product. This single reaction is why plants are called producers: they make their own food and, in doing so, supply energy and oxygen to nearly all other life.

Where it happens, and the role of chlorophyll

Photosynthesis takes place in the chloroplasts, organelles found in plant and algal cells. Chloroplasts contain the green pigment chlorophyll, which absorbs light (mostly red and blue wavelengths) and reflects green, which is why leaves look green. The captured light energy powers the reactions that build glucose. Because chloroplasts are needed, only cells that have them, such as leaf cells, can photosynthesize, which is one reason cell structure (covered in cell structure and function) matters here.

The energy and matter transformation

Two things happen at once in photosynthesis, and the MCAS wants both:

  • Energy is transformed. Light energy is captured and stored as chemical energy in glucose. The energy is not created; it is converted from one form (light) to another (chemical).
  • Matter is rearranged. The carbon, hydrogen, and oxygen atoms in carbon dioxide and water are rearranged into glucose and oxygen. No atoms are created or destroyed; they are reorganized, which is why the equation is balanced.

This dual view, energy transformed and matter rearranged, is exactly the energy and matter crosscutting concept, and it sets up the comparison with respiration.

Limiting factors and rate

The rate of photosynthesis depends on three main limiting factors: light intensity, carbon dioxide concentration, and temperature. A limiting factor is the one in shortest supply, the one currently capping the rate. On a graph of rate against light intensity, the rate rises while light is limiting, then levels off when some other factor (carbon dioxide or temperature) becomes limiting. Reading these graphs and naming the limiting factor is a common MCAS task.

Try this

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

  • Cue. Reactants: carbon dioxide and water. Products: glucose and oxygen (light energy is the input).

Q2. Explain the role of chlorophyll in photosynthesis. [2]

  • Cue. Chlorophyll is the green pigment in chloroplasts that absorbs light energy, which powers the reactions that build glucose.

Exam-style practice questions

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

HS Biology MCAS (style)3 marksA plant is kept in bright light. (a) Write a word equation for photosynthesis. (b) State the form of energy the plant takes in and the form it stores. (c) Explain why photosynthesis is important for animals as well as plants.
Show worked answer →

A 3-point item on energy and matter with the practice of developing and using models.

(a) 1 point: carbon dioxide plus water, using light energy, produces glucose plus oxygen.
(b) 1 point: the plant takes in light (radiant) energy and stores it as chemical energy in glucose.
(c) 1 point: animals depend on photosynthesis because it makes the food (chemical energy) they eat and releases the oxygen they need for respiration. Markers reward both food and oxygen.

HS Biology MCAS (style)3 marksA graph shows the rate of photosynthesis rising as light intensity increases, then leveling off at high light intensity. (a) Explain why the rate rises at low light intensity. (b) Explain why the rate levels off at high light intensity.
Show worked answer →

A 3-point item on analyzing and interpreting data.

(a) Up to 2 points: at low light intensity, light is the limiting factor, so adding more light gives the plant more energy and the rate of photosynthesis increases (1 point for more light energy, 1 point for light being limiting).
(b) 1 point: at high light intensity the rate levels off because some other factor (such as carbon dioxide concentration or temperature) has become the limiting factor, so more light no longer increases the rate. Markers reward naming a different limiting factor.

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