How do cells release the energy stored in glucose to power life?
Explain cellular respiration as the release and transformation of stored energy: glucose and oxygen are broken down in mitochondria to release energy (ATP), with carbon dioxide and water as products, and compare aerobic respiration with fermentation (Virginia 2018 Biology SOL BIO.2.e).
A SOL-level answer on cellular respiration for the Virginia Biology EOC: aerobic respiration in mitochondria, the reactants and products, ATP as the energy currency, and how fermentation releases energy without oxygen.
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What this topic is asking
Virginia Biology SOL standard BIO.2.e pairs photosynthesis with cellular respiration: the process that releases the energy stored in glucose so cells can use it. The Biology EOC expects you to know the reactants and products, where respiration happens, that the usable energy is captured as ATP, and how fermentation lets cells release energy without oxygen. Respiration is the mirror image of photosynthesis, and seeing that link is the key to this part of the standard.
What cellular respiration does
Every living cell respires, all the time, because every life process, growth, movement, active transport, building molecules, needs a constant supply of energy. Glucose is the fuel; ATP is the spendable currency the cell actually uses.
Aerobic respiration: the equation and the site
This equation is the reverse of photosynthesis, which is the single most useful thing to notice: the products of one are the reactants of the other. Cells with high energy needs (such as muscle cells) have many mitochondria, which is a structure-and-function point the EOC may ask about.
ATP: the energy currency
The energy released from glucose is not used directly; it is first stored in ATP (adenosine triphosphate). ATP is like a rechargeable battery: when the cell needs energy, ATP is broken down (releasing energy for work), and respiration recharges it. Describing ATP as the cell's energy currency captures the idea that respiration "earns" ATP and cellular processes "spend" it. The EOC expects you to know that the point of respiration is to make ATP.
Photosynthesis and respiration: a connected cycle
Photosynthesis and respiration form a cycle of energy and matter. Photosynthesis uses carbon dioxide and water and light to store energy in glucose and releases oxygen. Respiration uses glucose and oxygen to release that energy and returns carbon dioxide and water. The oxygen and carbon dioxide cycle between them, and the energy that entered as light leaves as heat and work. Plants do both processes; animals do only respiration and depend on producers for glucose and oxygen.
Fermentation: respiration without oxygen
When oxygen is in short supply, cells switch to fermentation (anaerobic respiration), which releases energy from glucose without oxygen but yields far less ATP per glucose. There are two common types:
- Lactic acid fermentation (in animal muscle and some bacteria) produces lactic acid. It lets muscles keep working briefly during intense exercise when oxygen cannot be delivered fast enough; the build-up of lactic acid contributes to muscle fatigue.
- Alcoholic fermentation (in yeast) produces ethanol and carbon dioxide. It is used in baking (the carbon dioxide makes bread rise) and brewing.
The trade-off is speed and independence from oxygen versus a much lower energy yield.
Try this
Q1. State the reactants and products of aerobic cellular respiration. [2]
- Cue. Reactants: glucose and oxygen. Products: carbon dioxide and water (plus energy as ATP).
Q2. Explain how the equation for respiration relates to the equation for photosynthesis. [2]
- Cue. They are reverses of each other: the products of photosynthesis (glucose and oxygen) are the reactants of respiration, and the products of respiration (carbon dioxide and water) are the reactants of photosynthesis.
Exam-style practice questions
Practice questions written in the style of VDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
VA Biology SOL (2023 released style)1 marksIn which organelle does most aerobic cellular respiration take place? (A) the nucleus. (B) the chloroplast. (C) the mitochondrion. (D) the ribosome.Show worked answer →
A 1-point multiple-choice item on the site of respiration.
The correct answer is C. The mitochondrion is the site of aerobic respiration, where glucose is broken down with oxygen to release energy as ATP. The nucleus stores DNA, the chloroplast carries out photosynthesis, and the ribosome makes proteins.
The test rewards knowing the mitochondrion as the site of aerobic respiration.
VA Biology SOL (2024 released style)2 marksA runner's muscles begin to produce lactic acid during a sprint. (a) Name the process occurring when oxygen runs short. (b) Explain why this process is used even though it releases less energy than aerobic respiration.Show worked answer →
A 2-point item comparing fermentation with aerobic respiration.
(a) 1 point: (lactic acid) fermentation, also called anaerobic respiration.
(b) 1 point: it does not require oxygen, so it can release some energy quickly when oxygen cannot be supplied fast enough to the muscles; this keeps the cells supplied with ATP during intense activity even though the yield per glucose is much lower.
Markers reward naming fermentation and explaining that it works without oxygen to provide energy when oxygen is limited.
Related dot points
- Explain photosynthesis as the capture, transformation, and storage of energy: light energy and the reactants carbon dioxide and water are converted in chloroplasts into glucose and oxygen (Virginia 2018 Biology SOL BIO.2.e).
A SOL-level answer on photosynthesis for the Virginia Biology EOC: the reactants and products, the role of chlorophyll and chloroplasts, the energy transformation from light to chemical energy, and the factors that limit the rate.
- Describe the four classes of biological macromolecules (carbohydrates, lipids, proteins, and nucleic acids), their monomers, and their roles in maintaining life processes (Virginia 2018 Biology SOL BIO.2.b).
A SOL-level answer on biological macromolecules for the Virginia Biology EOC: carbohydrates, lipids, proteins, and nucleic acids, their monomers and functions, and how dehydration synthesis and hydrolysis build and break them.
- Explain that enzymes are protein catalysts with specific functions: they lower activation energy, act on specific substrates at an active site, and are affected by temperature, pH, and concentration (Virginia 2018 Biology SOL BIO.2.c, BIO.2.d).
A SOL-level answer on enzymes for the Virginia Biology EOC: enzymes as protein catalysts, activation energy, the active site and specificity, and how temperature, pH, and concentration affect enzyme activity, including denaturation.
- Identify the major cell organelles and relate each structure to its function, showing how organelles work together to support life processes (Virginia 2018 Biology SOL BIO.3.a).
A SOL-level answer on organelles for the Virginia Biology EOC: the nucleus, mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus, chloroplasts, vacuoles, and cell wall, and how structure relates to function.
- Explain how energy flows through ecosystems through food chains, food webs, and trophic levels, including the roles of producers, consumers, and decomposers and the ten percent rule (Virginia 2018 Biology SOL BIO.8.b).
A SOL-level answer on energy flow for the Virginia Biology EOC: producers, consumers, and decomposers; food chains, food webs, and trophic levels; energy pyramids and the ten percent rule; and why energy flows one way while matter cycles, with worked calculations.
Sources & how we know this
- 2018 Science Standards of Learning (Biology) — Virginia Department of Education (2018)
- SOL Practice Items (All Subjects) — Virginia Department of Education (2024)