Week 11 · Lesson 1 Photosynthesis (B): light-independent reactions and the equation, starch, factors that affect the rate, vertical farming · 3 lessons
1
The light-independent stage and the equation
Science Prep 2 Term 1, Unit 3: pp. 70–74
GoalDescribe the light-independent stage and write the overall equation of photosynthesis.
New words
light-independent reactions · reactions in the stroma that build glucose using products of the light stageglucose · the sugar, C₆H₁₂O₆, made in photosynthesisby-product · a substance made along with the main product, such as oxygen
Explanation
The light-independent reactions happen in the stroma. They are called “light-independent” because they do not use light directly, but they do not run on their own at night: they need the products of the light stage. In this stage, the hydrogen made in the light stage joins carbon dioxide from the air, with the help of energy and enzymes, to make glucose. This is a simplification of many small steps, and we say it just once. The overall equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, with light and chlorophyll on the arrow. Light energy is stored as chemical energy in the glucose. Oxygen is released as a by-product, not as a waste. We can check the atoms: left, 6 C, 12 H and 18 O; right, 6 C, 12 H and 18 O. The equation is balanced and mass is conserved. A leaf takes in carbon dioxide through tiny pores and loses oxygen the same way.
6CO2 + 6H2Olight, chlorophyll→C6H12O6 + 6O2
Worked examples
Count the atoms. Left side: 6 CO₂ gives 6 C and 12 O; 6 H₂O gives 12 H and 6 O. Total 6 C, 12 H, 18 O. Right side: C₆H₁₂O₆ gives 6 C, 12 H, 6 O; 6 O₂ gives 12 O. Total 6 C, 12 H, 18 O. They match.
If a plant makes 1 glucose molecule, how many CO₂ molecules did it use? Six, because the equation has 6 CO₂ for each C₆H₁₂O₆. And how many O₂ molecules were released? Also six.
Name the energy change: in photosynthesis, light energy is changed into chemical energy stored in glucose. Later, the plant or an animal can release that energy by respiration.
Class activity
Bead model. Use beads of three colours for C, H and O. Build 6 CO₂ molecules and 6 H₂O molecules. Break all the bonds. Rearrange the atoms into one glucose molecule (6 C, 12 H, 6 O in a ring and chain) and 6 O₂ molecules. Count that every bead is used: 6 C, 12 H, 18 O. Write which bead colour is the by-product gas.
