How do cells release the energy stored in glucose to power the work of life?
Explain how cellular respiration releases energy from glucose to make ATP, compare aerobic and anaerobic respiration, and relate respiration to the role of the mitochondria (NYSSLS LS1, energy and matter; structure and function).
A NYSSLS-level answer on cellular respiration for the New York Life Science: Biology Regents: how glucose is broken down to release energy as ATP, the equation, the role of mitochondria, and the difference between aerobic and anaerobic respiration.
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What this topic is asking
NYSSLS LS1 wants you to explain how cells release the energy stored in glucose. The Life Science: Biology Regents pairs respiration with photosynthesis (the inputs and outputs are mirror images) and often sets it in a context such as exercise. The crosscutting concept is energy and matter: glucose is broken down and its chemical energy transferred to ATP.
What respiration does
The energy released is captured in ATP, the cell's energy currency (see ATP and cellular energy). Every living cell respires continuously, because every cell needs ATP for its work.
Aerobic respiration
Aerobic respiration uses oxygen and breaks glucose down completely. The word equation is: glucose + oxygen gives carbon dioxide + water (+ energy). As a balanced equation:
Because the glucose is fully broken down, aerobic respiration releases a large amount of energy as ATP. Most of it occurs in the mitochondria, whose folded inner membranes provide a large surface area for the reactions, an example of structure fitting function. Cells with high energy demand, such as muscle and nerve cells, have many mitochondria.
Anaerobic respiration
During hard exercise, muscles may not get enough oxygen and switch to anaerobic respiration, building up lactic acid (which causes fatigue and aching). Afterwards, the body keeps breathing hard to take in extra oxygen to break down the lactic acid, the oxygen debt. Fermentation by yeast is used to make bread rise and to brew, which the exam sometimes uses as a context.
Respiration and photosynthesis
The products of respiration (carbon dioxide and water) are exactly the reactants of photosynthesis, and the products of photosynthesis (glucose and oxygen) are the reactants of respiration. The two processes cycle carbon, oxygen and hydrogen between organisms and the environment while energy flows one way from sunlight. The ecosystem-scale version of this is in the cycling of energy and matter in cells and energy flow and matter cycling.
Try this
Q1. Write the word equation for aerobic respiration. [2]
- Cue. Glucose + oxygen gives carbon dioxide + water (+ energy).
Q2. Explain why anaerobic respiration releases less energy than aerobic respiration. [2]
- Cue. Without oxygen, glucose is only partly broken down, so much less of its stored chemical energy is released as ATP.
Exam-style practice questions
Practice questions written in the style of NYSED exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Regents (Life Science sample, 2024)3 marksA runner sprints hard and her leg muscles begin to ache as lactic acid builds up. (a) Name the type of respiration occurring in her muscles when oxygen runs short. (b) State why this type of respiration releases less energy than aerobic respiration. (c) Explain why she continues to breathe heavily for some time after stopping.Show worked answer →
A 3-point constructed-response item assessing energy and matter and cause and effect.
(a) 1 point: anaerobic respiration (lactic acid fermentation).
(b) 1 point: anaerobic respiration only partly breaks down glucose (no oxygen to complete it), so much less energy (ATP) is released than in aerobic respiration, which fully breaks glucose down.
(c) 1 point: she keeps breathing hard to take in extra oxygen to break down the lactic acid that built up (repaying the oxygen debt).
Markers reward "incomplete breakdown means less ATP" and the oxygen-debt explanation.
Regents (Life Science CR, 2025)2 marksCellular respiration and photosynthesis are often described as opposite processes. (a) State the reactants of aerobic respiration. (b) Explain how the products of respiration relate to the reactants of photosynthesis.Show worked answer →
A 2-point item linking respiration and photosynthesis.
(a) 1 point: glucose and oxygen.
(b) 1 point: respiration produces carbon dioxide and water, which are exactly the reactants photosynthesis uses; so the two processes cycle carbon, oxygen and hydrogen between organisms and the environment.
Markers reward recognizing that the products of one are the reactants of the other.
Related dot points
- Explain how photosynthesis converts light energy, carbon dioxide and water into glucose and oxygen, identify where it occurs, and analyze how limiting factors affect its rate (NYSSLS LS1, energy and matter; analyzing data).
A NYSSLS-level answer on photosynthesis for the New York Life Science: Biology Regents: the inputs and outputs, the role of chloroplasts and chlorophyll, the word and balanced equations, and how light, carbon dioxide and temperature limit the rate.
- Explain how cells use ATP as their energy currency, how energy is released when ATP is broken down, and how this links to photosynthesis and respiration (NYSSLS LS1, energy and matter; systems and system models).
A NYSSLS-level answer on cellular energy for the New York Life Science: Biology Regents: ATP as the cell's energy currency, how energy is released and stored, and how photosynthesis and respiration supply the energy cells use.
- Explain how enzymes act as biological catalysts, how the active site and substrate fit, and how temperature and pH affect enzyme activity (NYSSLS LS1, structure and function; analyzing data).
A NYSSLS-level answer on enzymes for the New York Life Science: Biology Regents: how enzymes lower activation energy, the active site and substrate fit, and how temperature and pH change the rate of enzyme-controlled reactions.
- Explain how photosynthesis and respiration together cycle carbon and oxygen while energy flows one way, and trace atoms of matter through these processes (NYSSLS LS1, energy and matter; systems and system models).
A NYSSLS-level answer on the cycling of matter and the flow of energy for the New York Life Science: Biology Regents: how photosynthesis and respiration link, why matter is conserved and cycles while energy flows one way, and how to trace atoms through living systems.
- Explain how the circulatory, respiratory and digestive systems work together to transport materials, exchange gases and provide nutrients to cells, maintaining the internal environment (NYSSLS LS1, systems and system models; energy and matter).
A NYSSLS-level answer on the supply systems for the New York Life Science: Biology Regents: how the circulatory, respiratory and digestive systems transport materials, exchange gases and provide nutrients, and how they cooperate to maintain the internal environment.
Sources & how we know this
- New York State P-12 Science Learning Standards (Life Science) — New York State Education Department (2016)
- Educator Guide to the Regents Examination in Life Science: Biology — New York State Education Department (2025)