Chemical equations in biology

To figure out how photosynthesis and aerobic respiration relate, we should place their balanced overall equations next to each other and compare the left side (reactants) and right side (products). In your syllabus, this sits in Metabolism, under Photosynthesis and Cell respiration.

During photosynthesis, plants (and other autotrophs) use light to build glucose from carbon dioxide and water, releasing oxygen. The overall equation is:

${\text{6CO}}_2+{\text{6H}}_2\text{O}+\text{light energy}\to {\text{C}}_6{\text{H}}_{12}{\text{O}}_6+{\text{6O}}_2$

So the reactants are carbon dioxide and water (plus light energy), and the products are glucose and oxygen.

In aerobic respiration, cells use oxygen to break down glucose and transfer the stored chemical energy into ATP, producing carbon dioxide and water. The overall equation is:

${\text{C}}_6{\text{H}}_{12}{\text{O}}_6+{\text{6O}}_2\to {\text{6CO}}_2+{\text{6H}}_2\text{O}+\text{ATP (energy)}$

Here, the reactants are glucose and oxygen, and the products are carbon dioxide and water.

Using the two equations to check each statement:

• Option A doesn’t work because the processes are not the same direction—one stores energy in glucose while the other releases energy from glucose.
• Option B is wrong because oxygen is used up in respiration, but in photosynthesis oxygen is formed and released.
• Option C is wrong because both reactions involve the same elements (C, H, and O); they are just rearranged into different molecules.

Comparing the two equations shows a loop: photosynthesis produces glucose and oxygen, which are then used in aerobic respiration. Respiration produces carbon dioxide and water, which are then used again in photosynthesis. Therefore, the outputs of one process function as the inputs for the other.

To compare the two processes properly, start by looking at their overall chemical equations from the Metabolism section of your syllabus.

Photosynthesis: $6C{O}_2+6H_{2}O+\text{light energy}\to C_6{H}_{12}{O}_6+6O_2$

Aerobic Respiration: $C_6{H}_{12}{O}_6+6O_2\to 6C{O}_2+6H_{2}O+\text{chemical energy (ATP)}$

You can already see that molecules on the reactant side of one equation show up on the product side of the other.

Option A claims the equations are completely the same. They involve the same substances, but the overall change goes in opposite directions. Photosynthesis builds larger energy-rich molecules (anabolic), while respiration breaks them down to release energy (catabolic).

Option B says oxygen is a major reactant in both. The equations show oxygen ($O_2$) is required for aerobic respiration, but photosynthesis produces oxygen as an output. During daylight, plants release oxygen into the air.

Option C suggests the elements involved are different. That’s not correct: both processes use the same elements—Carbon (C), Hydrogen (H), and Oxygen (O). This shared set of elements is why matter can be recycled through living systems.

After ruling out the other choices, the remaining idea is that the two equations are complementary. Photosynthesis makes glucose and oxygen that cells can use for respiration, and respiration returns carbon dioxide and water that plants need for photosynthesis.

In the carbon and oxygen cycles, producers (autotrophs such as plants) convert inorganic substances into organic molecules using light. The overall equation for photosynthesis is:

$6C{O}_2+6H_{2}O+\text{light energy}\to C_6{H}_{12}{O}_6+6O_2$

This means carbon dioxide and water are the reactants, and glucose and oxygen are the products.

To access the energy stored in food molecules, organisms carry out aerobic respiration. The overall equation is:

$C_6{H}_{12}{O}_6+6O_2\to 6C{O}_2+6H_{2}O+\text{ATP (energy)}$

Here, glucose and oxygen are the reactants, while carbon dioxide and water are the products.

Looking at both equations together shows a neat recycling pattern. The glucose and oxygen made by photosynthesis become the materials organisms use in respiration. The carbon dioxide and water produced during respiration then become the starting materials for photosynthesis again.

This is one way ecosystems keep matter moving and reused.

• Option A is incorrect because one process builds energy-storing molecules and the other breaks them down.
• Option B is incorrect because oxygen is only a reactant in respiration; it is a product of photosynthesis.
• Option C is incorrect because both rely on the same elements: Carbon ($C$), Hydrogen ($H$), and Oxygen ($O$).

Because the substances produced in photosynthesis are used up in respiration, and the substances produced in respiration are used up in photosynthesis, the best description is that each process supplies the other with its reactants.

In biology and chemistry, equations are a quick way to show what changes during a reaction. The usual format places one set of substances before the arrow and another set after it:

$\text{Left Side}\to \text{Right Side}$

The task is asking for the official name used for the substances located on the left of that arrow.

The substances that are present at the beginning of a reaction are called reactants. They are the materials that will be changed during the reaction, so they are written on the left side.

$\text{Reactants}\to \text{Products}$

We can rule out the other terms by what they mean:

• Products are what the reaction makes, so they belong on the right side.
• Catalysts/Enzymes help a reaction happen faster but are not used up as the main starting substances; they’re often shown above the arrow instead of on the left.

Because the question is asking for the name of the starting substances placed on the left side of the arrow, the correct term is reactants.

In biology and chemistry, balanced equations show a change from what you begin with to what you end with. The general pattern is:

$\text{Starting Materials}\to \text{Final Substances}$

The arrow points from the beginning substances to the ending substances, so we need the specific vocabulary for what is on the left.

Products are the substances created by the reaction. Since they are the outcome, they are written on the right side of the arrow.

Because the question is about the left side, this option doesn’t fit.

Catalysts (including biological catalysts called enzymes) make reactions go faster without being used up.

They may be involved, but they are not the main substances that get converted into new substances. In equations, they are commonly written above the arrow, not as the key items on the left. So both can be removed.

After eliminating the other choices, reactants remains. This word is used because these substances are the ones that “react” together at the start, matching the idea of the starting materials on the left side.

$\text{Reactants}\to \text{Products}$

Start by focusing on what goes in and what comes out of the reaction:

$\text{lactose}\to \text{glucose}+\text{galactose}$

There is one starting substance (lactose) and it becomes two different substances (glucose and galactose). That pattern shows a larger molecule being split into smaller molecules, which is a breakdown (decomposition) reaction in digestion.

Carbohydrates can be grouped by how many sugar units they contain:

• Lactose is a disaccharide (two sugar units linked together).
• Glucose and galactose are monosaccharides (single sugar units).

Because the equation shows a disaccharide changing into monosaccharides, it represents breaking a more complex carbohydrate into simpler ones.

Two choices describe cellular respiration: using oxygen to release energy, or producing carbon dioxide and water.

But the given equation does not include oxygen as a reactant, and it does not show carbon dioxide or water as products. So those respiration-based options cannot match this word equation.

A synthesis reaction would combine smaller molecules to make a bigger one, so it would look like:

$\text{glucose}+\text{galactose}\to \text{lactose}$

Instead, the arrow shows lactose being split. Also, the context is digestion, which is about breaking food molecules into smaller parts. So the synthesis option is not correct.

This reaction shows lactose being digested into its two component monosaccharides (glucose and galactose). That is best described as splitting a complex sugar into simpler sugars.

Look at the word equation first:

$\text{lactose}\to \text{glucose}+\text{galactose}$

One substance (lactose) is converted into two substances (glucose and galactose). This indicates that a bigger molecule is being separated into smaller molecules, which fits the idea of digestion in the Processes topic.

Option A claims oxygen is used to release energy. If that were the case, oxygen would need to appear as a reactant on the left side of the arrow. Since only lactose is shown as the reactant, this option is eliminated.

Option C describes making carbon dioxide and water, which is the outcome of aerobic respiration. The products in the equation are glucose and galactose instead, so this option is eliminated.

Option D is a “build-up” (synthesis) idea, where small molecules combine to form a larger one. The equation shows the opposite: one molecule splitting into two. So this option is eliminated.

With the other options removed, Option B fits the evidence:

1. Lactose is a larger sugar (a disaccharide).
2. Glucose and galactose are smaller sugars (monosaccharides).
3. The arrow shows a breakdown step, which is what digestion does.

Chemical formulas and equations use different kinds of numbers for different meanings. Here, the question is asking for the term used for the number 6 that shows there are six molecules of CO₂ involved.

This is a key skill for writing chemical equations, found in the Processes part of the MYP Biology syllabus under Word and chemical equations.

A denominator is a maths word for the lower value in a fraction (for example, the 5 in 3/5). Chemical equations don’t use “denominator” to name a whole number that shows how many molecules are present.

A subscript is the small number written down low in a formula, like the 2 in CO₂. That number tells you how many atoms of an element are in a single molecule, not how many molecules you have altogether.

A superscript is written up high, such as the − in Cl⁻. In chemistry it is mainly used to show ionic charge, not the count of molecules in an equation.

By remembering standard chemical notation, the large number written before a chemical formula is called a coefficient. So six molecules of carbon dioxide are written as 6CO₂.

You are being asked to name the number 6 when it is written in front of a chemical formula like CO₂ to show how many molecules there are. This fits the topic Processes: Word and chemical equations.

A denominator is the bottom of a fraction (like the 2 in 1/2). Even though fractions can appear in some advanced balancing, that term does not describe a whole number placed before a formula to count molecules, so it can be eliminated.

A subscript is the small lower number in a formula, such as the 2 in CO₂. It indicates atoms per molecule, not the total number of molecules (six), so it should be eliminated.

A superscript is written above the line (for example, Ca²⁺). It is mainly used to show ionic charge, not the number of molecules present, so it can be eliminated.

After eliminating the other choices, the correct term is coefficient. In chemical equations, a coefficient is the big number written in front of a formula. For instance, in photosynthesis:

$6{\text{CO}}_2+6{\text{H}}_2\text{O}\to {\text{C}}_6{\text{H}}_{12}{\text{O}}_6+6{\text{O}}_2$

The 6 placed before CO₂ tells you there are six molecules of carbon dioxide.

In biology and chemistry, we often describe processes using word equations, like those for photosynthesis or respiration. These equations follow agreed rules so we can quickly see how substances are involved in the process. This fits the topic Processes: Word and chemical equations.

When one substance becomes another in a word equation, that change is shown with an arrow ($\to$), not a plus sign. The arrow can be read as “produces” or “forms,” so a plus sign is not the symbol for conversion.

If a reaction can happen in both directions, we use a reversible symbol such as double arrows ($\rightleftharpoons$). The plus sign does not communicate direction or reversibility, so this choice does not match the meaning of $+$.

Enzymes act as catalysts: they speed up reactions without being used up. In word equations, an enzyme name is usually written above or below the arrow, rather than being listed as a reactant with a plus sign.

By convention, the plus sign ($+$) separates substances that are on the same side of the equation.

• On the left of the arrow, it means those substances are reacting together.
• On the right of the arrow, it means those substances are formed at the same time.

Example (aerobic respiration): $\text{Glucose}+\text{Oxygen}\to \text{Carbon Dioxide}+\text{Water}+\text{Energy}$

The question is asking specifically about the meaning of the plus sign ($+$) in a biological word equation. In biology and chemistry, different symbols have different jobs, so we can solve this by comparing $+$ to the symbols used for other meanings.

The symbol that shows one set of substances changing into another is the arrow ($\to$). For instance, in aerobic respiration:

$\text{Glucose}+\text{Oxygen}\to \text{Carbon Dioxide}+\text{Water}$

Here, the arrow shows what is produced, so the plus sign cannot be the “turns into” symbol. Therefore, we eliminate this option.

A reaction that can go both forward and backward is shown with a double arrow ($\rightleftharpoons$ or $\leftrightarrow$). Since $+$ never indicates direction, we can rule out the reversible-reaction option.

Enzymes are not written as reactants separated by a plus sign in word equations. Instead, the enzyme is typically shown above or below the arrow (for example, $\stackrel{\text{enzyme}}{\to }$), because it helps the reaction happen but isn’t used up.

After removing options that describe the arrow, reversible arrows, and enzyme notation, the remaining meaning fits standard equation conventions. The plus sign ($+$) works like the word “and.”

• On the reactant side, it means substances are used together.
• On the product side, it means substances are made together.