How do cells release the energy stored in glucose, and what is the difference between aerobic and anaerobic respiration?
Use a model to illustrate how cellular respiration breaks the bonds of glucose and oxygen to release energy as ATP, and compare aerobic respiration with anaerobic respiration and fermentation (MA STE HS-LS1-7, HS-LS2-3).
A standard-level answer on cellular respiration for the Massachusetts High School Biology MCAS: how glucose and oxygen are broken down to release energy as ATP, the reactants and products, and the difference between aerobic respiration and fermentation under HS-LS1-7.
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What this topic is asking
The Massachusetts STE framework standard HS-LS1-7 asks you to use a model to show that cellular respiration breaks the bonds of food and oxygen molecules and forms new bonds, resulting in a net transfer of energy. On the High School Biology MCAS, respiration questions check that you can write the equation, name where it happens, explain that it makes ATP, and compare aerobic respiration with anaerobic respiration (fermentation). The crosscutting concept is energy and matter: respiration both releases energy and rearranges atoms.
What cellular respiration does
The point of respiration is not to destroy glucose but to release its energy in a usable form. The energy released is used to attach phosphate groups to ADP, making ATP (covered in ATP and energy in cells). Every living cell respires, including plant cells, and it happens continuously, day and night.
Aerobic respiration
Aerobic respiration uses oxygen and is the main way cells release energy. Most of it occurs in the mitochondria. The balanced equation is:
In words: glucose plus oxygen produces carbon dioxide plus water, releasing energy that is captured in ATP. Because oxygen lets the cell break glucose down completely, aerobic respiration releases a large amount of energy from each glucose molecule. This is why cells with high energy demands have many mitochondria.
Anaerobic respiration and fermentation
When oxygen is in short supply, cells switch to anaerobic respiration, also called fermentation. Without oxygen, glucose is only partly broken down, so much less energy is released.
- In animal cells (and human muscle): glucose is converted to lactic acid. This lets the muscle keep producing some ATP during hard exercise, but the lactic acid builds up and contributes to muscle fatigue.
- In yeast and some microbes: glucose is converted to alcohol (ethanol) and carbon dioxide. This is the basis of brewing and bread-making.
The key MCAS comparison is energy yield: aerobic respiration releases much more energy per glucose than anaerobic respiration, because aerobic respiration breaks glucose down fully while anaerobic respiration does not.
Energy and matter in respiration
Like photosynthesis, respiration is both an energy transformation and a rearrangement of matter. The chemical energy in glucose is transferred to ATP (and some is lost as heat). The atoms in glucose and oxygen are rearranged into carbon dioxide and water; none are created or destroyed, which is why the equation balances. Comparing respiration with photosynthesis side by side is so useful that it has its own page: comparing photosynthesis and respiration.
Try this
Q1. State the reactants and products of aerobic respiration. [2]
- Cue. Reactants: glucose and oxygen. Products: carbon dioxide and water (plus energy for ATP).
Q2. Explain why anaerobic respiration releases less energy than aerobic respiration. [2]
- Cue. Without oxygen, glucose is only partly broken down, so most of its energy stays in the products; aerobic respiration breaks glucose down fully and releases much more.
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 cell carries out aerobic cellular respiration. (a) Write a word equation for aerobic respiration. (b) Name the organelle where most of it happens. (c) Explain what the cell does with the energy released.Show worked answer →
A 3-point item on energy and matter with the practice of developing and using models.
(a) 1 point: glucose plus oxygen produces carbon dioxide plus water (and releases energy).
(b) 1 point: the mitochondria.
(c) 1 point: the released energy is used to make ATP, which the cell then uses for work such as movement, active transport, and building molecules. Markers reward linking the energy to ATP.
HS Biology MCAS (style)3 marksDuring hard exercise, a muscle cell runs short of oxygen. (a) Name the type of respiration the cell switches to. (b) Name the product that builds up and causes muscle fatigue. (c) Explain why anaerobic respiration releases far less energy than aerobic respiration.Show worked answer →
A 3-point item on cause and effect.
(a) 1 point: anaerobic respiration (lactic acid fermentation).
(b) 1 point: lactic acid (lactate).
(c) 1 point: without oxygen, glucose is only partly broken down, so most of its chemical energy remains locked in the products; aerobic respiration fully breaks glucose down and releases much more energy. Markers reward the idea that glucose is only partly broken down without oxygen.
Related dot points
- 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.
- Compare photosynthesis and cellular respiration as linked processes, contrasting their reactants, products, energy changes, and locations, and explain how together they cycle matter and transfer energy (MA STE HS-LS1-5, HS-LS1-7, energy and matter).
A standard-level answer comparing photosynthesis and cellular respiration for the Massachusetts High School Biology MCAS: their opposite reactants and products, where each happens, the energy changes, and how they link as an energy and matter cycle.
- Explain how cells capture, store, and release energy, the role of ATP as the cell's usable energy currency, and how energy transformations obey the conservation of energy (MA STE HS-LS1-7 supporting, energy and matter).
A standard-level answer on ATP and cellular energy for the Massachusetts High School Biology MCAS: why ATP is the usable energy currency, how it stores and releases energy, and how energy transformations conserve energy under HS-LS1.
- Develop a model of the role of photosynthesis and cellular respiration in cycling carbon, and explain how cells combine atoms from sugars into amino acids and other large carbon-based molecules (MA STE HS-LS1-6, HS-LS2-5).
A standard-level answer on carbon cycling and matter in organisms for the Massachusetts High School Biology MCAS: how photosynthesis and respiration move carbon, and how cells build amino acids and large molecules from sugars under HS-LS1-6 and HS-LS2-5.
- Describe the structures and functions of the major organelles in plant and animal cells, distinguish prokaryotic from eukaryotic cells, and relate cell structure to function (MA STE HS-LS1).
A standard-level answer on cell structure and function for the Massachusetts High School Biology MCAS: the major organelles and their jobs, plant versus animal cells, prokaryotes versus eukaryotes, and how structure suits function under HS-LS1.
Sources & how we know this
- Massachusetts Science and Technology/Engineering Curriculum Framework (2016) — Massachusetts Department of Elementary and Secondary Education (2016)
- Science and Technology/Engineering (STE) Test Design and Development — Massachusetts Department of Elementary and Secondary Education (2024)