Aerobic respiration

Q: Name the cell structure where aerobic respiration occurs in plant cells.

Mitochondria (or mitochondrion).

Q: State the balanced chemical equation for aerobic respiration.

$$C_6H_{12}O_6 + 6O_2 rightarrow 6CO_2 + 6H_2O$$

Paper 1Paper 2Paper 3Paper 4Paper 5

What is Aerobic Respiration?

Aerobic respiration is the process by which cells break down nutrient molecules (specifically glucose) in the presence of oxygen to release energy. This energy is not lost as heat alone; it is captured and stored in a molecule called ATP (adenosine triphosphate). ATP acts as the immediate energy currency for all cellular activities, such as muscle contraction, active transport, and protein synthesis.

The word 'aerobic' means 'requiring air' (specifically oxygen). Without oxygen, this specific pathway cannot occur. This process takes place primarily in the mitochondria, which are organelles found in both plant and animal cells.

Building on previous concepts: You may recall that plants perform photosynthesis to make glucose. Aerobic respiration is essentially the reverse process of photosynthesis in terms of inputs and outputs, but it occurs in all living cells (plants and animals) to use that glucose for energy.

Mitochondria

Mitochondria (singular: mitochondrion) are the organelles where aerobic respiration occurs. They are often described as the 'powerhouses' of the cell because they generate most of the cell's supply of ATP.

Word Equation for Aerobic Respiration

The word equation summarizes the reactants and products:

glucose + oxygen → carbon dioxide + water

Reactants: Glucose (the nutrient) and Oxygen (required for the reaction).
Products: Carbon dioxide and Water (waste products that are excreted or used elsewhere).

Balanced Chemical Equation for Aerobic Respiration

The balanced chemical equation uses molecular formulas to show the exact number of molecules involved:

C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O

Variable Definitions:

C_6H_{12}O_6: One molecule of glucose.
6O_2: Six molecules of oxygen gas.
6CO_2: Six molecules of carbon dioxide.
6H_2O: Six molecules of water.

Identifying Reactants and Products

Consider a plant cell during the day. It is performing both photosynthesis and aerobic respiration.

For Aerobic Respiration:
- The cell takes in oxygen (from the air or produced by its own photosynthesis).
- It breaks down glucose (stored starch/sugar).
- It releases carbon dioxide and water.
- It produces ATP for cellular work.
Key Distinction: Even though plants produce oxygen during photosynthesis, they still need to respire to survive. They cannot live on photosynthesis alone; they must break down the glucose made to get usable energy.

⚠︎ Confusing Respiration with Breathing

The Error: Students often use the terms 'respiration' and 'breathing' interchangeably.

The Correct Understanding:

Breathing (Ventilation): A physical process of moving air in and out of the lungs to exchange gases. It is a macroscopic mechanical action.
Respiration: A chemical process occurring inside cells that releases energy from glucose. It is a microscopic biochemical reaction.

Why this matters: Examiners mark down answers that say 'plants do not respire' or confuse the two terms. Plants do respire 24/7, even when they are photosynthesizing.

⚠︎ Unbalanced Chemical Equations

The Error: Writing the chemical equation without balancing the atoms, e.g., C_6H_{12}O_6 + O_2 \rightarrow CO_2 + H_2O.

The Correct Understanding: The Law of Conservation of Mass states that atoms are neither created nor destroyed. You must ensure the number of Carbon (C), Hydrogen (H), and Oxygen (O) atoms is the same on both sides.

Left side: 6 C, 12 H, 8 O (6 from glucose + 12 from oxygen).
Right side: 6 C, 12 H, 18 O (12 from CO2 + 6 from H2O). Wait, let's re-verify the standard balanced equation provided in the syllabus.

Syllabus Standard: The Cambridge syllabus requires you to state the specific balanced equation:
C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O
Check your counts:

C: 6 on left, 6 on right.
H: 12 on left, 12 on right (6 \times 2).
O: 6 + (6 \times 2) = 18 on left. (6 \times 2) + (6 \times 1) = 18 on right.
It is balanced.

Writing the Word Equation Correctly

When to use this tip: When asked to 'state' or 'write' the word equation for aerobic respiration in Papers 1, 2, 3, and 4.

Examiner Acceptance: The markscheme accepts glucose + oxygen → carbon dioxide + water. Note that the order of products (carbon dioxide vs. water) does not matter, but the reactants must be glucose and oxygen.

Reasoning: Examiners look for the specific keywords 'glucose' and 'oxygen' as inputs. Spelling errors in 'glucose' or 'carbon dioxide' may cost marks if they make the word unrecognizable. Use a forward arrow (\rightarrow) to indicate the direction of the reaction.

Example:

Correct: glucose + oxygen → carbon dioxide + water
Incorrect: sugar + air → gas + liquid (Too vague)

Stating the Balanced Chemical Equation

When to use this tip: When asked for the 'balanced chemical equation' in Paper 4 (Supplement) or Paper 2.

Examiner Acceptance: The markscheme strictly requires: C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O. You must include the state symbols if specified in the question, but usually, just the balanced formula is required. Crucially, you must include the coefficient 6 for oxygen, carbon dioxide, and water.

Reasoning: This tests your ability to balance chemical equations. If you write C_6H_{12}O_6 + O_2 \rightarrow CO_2 + H_2O, you will receive zero marks because it is not balanced. The subscript numbers (e.g., the 6 in C_6) are part of the formula and cannot be changed; only the coefficients (the big numbers in front) can be adjusted to balance.

Example:

Correct: C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O
Incorrect: C_6H_{12}O_6 + O_2 \rightarrow 6CO_2 + 6H_2O (Unbalanced oxygen)

Identifying Structures and Processes

Name the cell structure where aerobic respiration occurs in plant cells.

Mitochondria (or mitochondrion).

State the word equation for aerobic respiration.

glucose + oxygen → carbon dioxide + water

State the balanced chemical equation for aerobic respiration.

C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O

Explain why it is incorrect to say that plants only photosynthesize and do not respire.

Plants need energy for cellular processes (like active transport and synthesis) which cannot be obtained from photosynthesis alone. Therefore, they must perform aerobic respiration in their mitochondria to break down glucose and release ATP.