How do cells release the energy stored in glucose to power their activities?
Identify cellular respiration as the process that releases energy from glucose, describe its reactants and products, and distinguish aerobic respiration from fermentation (TEKS Biology, Reporting Category 4; energy and matter; cause and effect).
A TEKS-level answer on cellular respiration for the Texas STAAR Biology EOC: the reactants and products, the role of mitochondria and ATP, the overall equation, and the difference between aerobic respiration and fermentation.
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What this topic is asking
The Biology TEKS ask you to identify cellular respiration as the process that releases energy from glucose, describe its reactants and products, and tell apart aerobic respiration from fermentation. For STAAR Reporting Category 4 you need the inputs and outputs, the role of the mitochondria and ATP, and what happens when oxygen is short. This is an energy and matter and cause-and-effect topic, almost always paired with photosynthesis.
What cellular respiration does
All living cells respire, all the time, because every activity (movement, building molecules, active transport) needs energy. Respiration is how the chemical energy locked in glucose by photosynthesis is released so the cell can use it.
Reactants, products, and the equation
The balanced equation is:
Notice this is the reverse of photosynthesis: respiration uses the glucose and oxygen that photosynthesis made, and releases the carbon dioxide and water that photosynthesis uses. The energy released is captured as ATP, the cell's usable energy currency.
Aerobic respiration versus fermentation
When oxygen is available, cells use aerobic respiration, which releases a large amount of energy per glucose. When oxygen is short (for example, in a muscle during intense exercise), cells switch to fermentation, also called anaerobic respiration:
- In animal (muscle) cells, fermentation produces lactic acid and a small amount of energy.
- In yeast and some microbes, fermentation produces alcohol (ethanol) and carbon dioxide, the basis of baking and brewing.
The key point STAAR tests: fermentation releases less energy per glucose than aerobic respiration, because it does not fully break down the glucose, but it lets the cell keep producing some energy without oxygen.
Where respiration fits in the system
Respiration connects to the body and the ecosystem. In the body, the respiratory system brings in the oxygen and the circulatory system delivers it to cells, while carbon dioxide is carried back out, an example of body systems working together (see human body systems). In an ecosystem, respiration returns carbon dioxide to the air, part of the cycling of matter. This is the energy and matter theme linking the cell to the whole organism and beyond.
Try this
Q1. State the reactants and products of aerobic cellular respiration. [2]
- Cue. Reactants: glucose and oxygen. Products: carbon dioxide, water, and energy (ATP).
Q2. Explain why fermentation is useful to a muscle cell during intense exercise even though it releases less energy. [2]
- Cue. When oxygen runs low, fermentation lets the cell keep releasing some energy without oxygen, so it can keep working briefly.
Exam-style practice questions
Practice questions written in the style of TEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
STAAR Biology (2023 released style)1 marksWhat are the reactants of aerobic cellular respiration? (A) Glucose and oxygen. (B) Carbon dioxide and water. (C) Glucose and carbon dioxide. (D) Oxygen and water.Show worked answer →
A 1-point multiple-choice item on the inputs of respiration.
The correct answer is A. Aerobic respiration uses glucose and oxygen to release energy (ATP), producing carbon dioxide and water. B lists the products, and C and D mix reactants and products. Respiration is essentially the reverse of photosynthesis.
Aerobic respiration: glucose plus oxygen gives carbon dioxide plus water plus energy.
STAAR Biology (2024 SCR style)2 marksA muscle cell working hard during intense exercise runs low on oxygen but still needs energy. Explain how the cell can release some energy without oxygen, and name the process. Support your answer with reasoning.Show worked answer →
A 2-point short constructed response on fermentation.
Full credit (2 points): when oxygen runs low, the cell switches to fermentation (anaerobic respiration), which releases some energy from glucose without oxygen; in muscle cells this produces lactic acid. It provides less energy per glucose than aerobic respiration but lets the cell keep going briefly when oxygen is limited.
Partial credit (1 point): names fermentation or anaerobic respiration without explaining that it releases energy without oxygen (and gives less energy). The science is scored.
Related dot points
- Describe the reactants, products, and energy transformation of photosynthesis, and explain its role in capturing light energy as chemical energy in glucose (TEKS Biology, Reporting Category 4; energy and matter; cause and effect).
A TEKS-level answer on photosynthesis for the Texas STAAR Biology EOC: the reactants and products, the role of light and chlorophyll in chloroplasts, the energy transformation from light to chemical energy, and the overall word and balanced equation.
- Compare the reactants, products, and energy flow of photosynthesis and cellular respiration, and explain how they form a connected cycle of energy and matter (TEKS Biology, Reporting Category 4; energy and matter; systems and system models).
A TEKS-level answer comparing photosynthesis and cellular respiration for the Texas STAAR Biology EOC: how their reactants and products mirror each other, the contrast in energy flow, and how together they cycle energy and matter.
- Identify the four major classes of biological macromolecules and their functions, and explain how enzymes act as biological catalysts affected by temperature and pH (TEKS Biology, Reporting Category 4; structure and function; cause and effect).
A TEKS-level answer on biomolecules and enzymes for the Texas STAAR Biology EOC: carbohydrates, lipids, proteins, and nucleic acids and their functions, and how enzymes catalyze reactions and are affected by temperature and pH.
- Investigate and explain how the major human body systems interact to carry out vital functions and maintain the organism (TEKS Biology, Reporting Category 4; systems and system models; structure and function).
A TEKS-level answer on human body systems for the Texas STAAR Biology EOC: the functions of the major organ systems and, above all, how systems such as the digestive, circulatory, respiratory, and nervous systems work together.
- Describe how matter cycles through ecosystems, including the carbon, nitrogen, and water cycles, and explain the role of decomposers in returning nutrients (TEKS Biology, Reporting Category 5; energy and matter; systems and system models).
A TEKS-level answer on biogeochemical cycles for the Texas STAAR Biology EOC: how carbon, nitrogen, and water cycle through ecosystems, the role of decomposers, and why matter cycles while energy flows one way.
Sources & how we know this
- Texas Essential Knowledge and Skills for Science (Biology) — Texas Education Agency (2024)
- STAAR Biology Assessed Curriculum — Texas Education Agency (2024)