On this page:
Introduction What Is a Word Equation? What Is a Chemical Equation? Equation 1: Photosynthesis Equation 2: Aerobic Respiration Equation 3: Anaerobic Respiration in Animals Equation 4: Anaerobic Respiration in Yeast Comparing the Key Equations Photosynthesis and Respiration Relationship How to Balance an Equation Common Mistakes to Avoid Why Equations Matter in Biology
Science is built on precision. A vague description of a chemical process is useful for a general understanding, but scientists need something more exact. They need a way to state precisely what goes in, what comes out, and in what quantities.
That is exactly what equations provide. Word equations name the substances involved. Chemical equations go further, showing the actual molecules, their formulas, and the exact numbers involved. Learning to read, write, and balance these equations is a fundamental skill in biology and chemistry.
A word equation describes a chemical reaction using the names of the reactants and products written out in full.
The arrow means "produces" or "yields." Everything to the left of the arrow goes in. Everything to the right comes out.
Word equations are useful for understanding what is happening in general terms, but they do not show the quantities of each substance involved.
A chemical equation uses chemical symbols and formulas to represent a reaction, showing the actual molecules involved and the numbers of each.
Where A and B are reactants, C and D are products, and a, b, c, and d are the numbers of molecules involved (called coefficients).
A balanced chemical equation has the same number of atoms of each element on both sides of the arrow. This is because atoms are neither created nor destroyed in chemical reactions. They are simply rearranged.
Producers capture light energy and use it to build glucose from carbon dioxide and water. Oxygen is released as a byproduct.
This reaction requires light energy and chlorophyll.
| Symbol | Name | Role |
|---|---|---|
| CO₂ | Carbon dioxide | Reactant, absorbed from the air through stomata |
| H₂O | Water | Reactant, absorbed from soil through roots |
| C₆H₁₂O₆ | Glucose | Product stores chemical energy |
| O₂ | Oxygen | Product, released as a byproduct |
| 6 (coefficient) | Six molecules | Balances atoms on both sides |
| Element | Left side | Right side | Balanced? |
|---|---|---|---|
| Carbon (C) | 6 (from 6CO₂) | 6 (from C₆H₁₂O₆) | Yes |
| Hydrogen (H) | 12 (from 6H₂O) | 12 (from C₆H₁₂O₆) | Yes |
| Oxygen (O) | 18 (12 from 6CO₂ + 6 from 6H₂O) | 18 (6 from C₆H₁₂O₆ + 12 from 6O₂) | Yes |
Conditions Required: This reaction requires light energy (from the Sun), chlorophyll (to absorb the light), and suitable temperature for enzyme activity. These conditions are often written above or below the arrow in the equation.
Living cells break down glucose using oxygen to release energy stored in ATP. Carbon dioxide and water are released as byproducts.
| Symbol | Name | Role |
|---|---|---|
| C₆H₁₂O₆ | Glucose | Reactant, energy source |
| O₂ | Oxygen | Reactant, accepts electrons at the end of the ETC |
| CO₂ | Carbon dioxide | Product, released through the lungs or stomata |
| H₂O | Water | Product, released as a byproduct |
| ATP | Adenosine triphosphate | Product, usable cellular energy |
| Element | Left side | Right side | Balanced? |
|---|---|---|---|
| Carbon (C) | 6 (from C₆H₁₂O₆) | 6 (from 6CO₂) | Yes |
| Hydrogen (H) | 12 (from C₆H₁₂O₆) | 12 (from 6H₂O) | Yes |
| Oxygen (O) | 18 (6 from C₆H₁₂O₆ + 12 from 6O₂) | 18 (12 from 6CO₂ + 6 from 6H₂O) | Yes |
When oxygen is insufficient, animal cells break down glucose through glycolysis only, producing lactic acid. Much less ATP is produced.
| Element | Left side | Right side | Balanced? |
|---|---|---|---|
| Carbon (C) | 6 | 6 (from 2 x C₃H₆O₃) | Yes |
| Hydrogen (H) | 12 | 12 (from 2 x C₃H₆O₃) | Yes |
| Oxygen (O) | 6 | 6 (from 2 x C₃H₆O₃) | Yes |
When oxygen is unavailable, yeast and plant cells ferment glucose, producing ethanol and carbon dioxide. This is alcoholic fermentation.
| Element | Left side | Right side | Balanced? |
|---|---|---|---|
| Carbon (C) | 6 | 6 (4 from 2C₂H₅OH + 2 from 2CO₂) | Yes |
| Hydrogen (H) | 12 | 12 (from 2C₂H₅OH) | Yes |
| Oxygen (O) | 6 | 6 (2 from 2C₂H₅OH + 4 from 2CO₂) | Yes |
| Process | Reactants | Products | Oxygen Needed | ATP Yield |
|---|---|---|---|---|
| Photosynthesis | CO₂ + H₂O | Glucose + O₂ | No (produces it) | Not applicable |
| Aerobic respiration | Glucose + O₂ | CO₂ + H₂O | Yes | 36-38 |
| Anaerobic (animals) | Glucose | Lactic acid | No | 2 |
| Anaerobic (yeast) | Glucose | Ethanol + CO₂ | No | 2 |
The chemical equation for photosynthesis and the chemical equation for aerobic respiration are the exact reverse of each other.
This is not a coincidence. Photosynthesis builds glucose and stores energy in its chemical bonds. Aerobic respiration breaks glucose down and releases the stored energy as ATP.
Together, they form the basis of the carbon cycle. Carbon enters living systems through photosynthesis, passes through food chains as organisms eat each other, and returns to the atmosphere through respiration and decomposition.
When given an unbalanced equation, follow these steps.
Important rule: Coefficients multiply everything in the formula. In 6H₂O, there are 6 x 2 = 12 hydrogen atoms and 6 x 1 = 6 oxygen atoms.
Equations do more than satisfy a requirement to memorize formulas. They tell the complete story of a biological process in the most precise language available.
The equation for photosynthesis tells us exactly what raw materials are needed and in what proportions. It explains why plants need sunlight and why they absorb carbon dioxide. It tells us where oxygen in the atmosphere comes from.
The equation for aerobic respiration explains why humans and other aerobic organisms need to breathe oxygen. It explains why we exhale carbon dioxide. It shows the precise relationship between the food we eat and the energy our cells use.
Reading and understanding these equations is not just a chemistry skill. It is a window into understanding life itself at the most fundamental and precise level possible.