Cellular Respiration
Learn how cells break down glucose with oxygen to release energy, why cellular respiration is not the same as breathing, and why plants do it too.
What you'll do in this lesson
A voice-first session with the Crimsora tutor on Cellular Respiration, then targeted practice and FRQs — with the tutor adapting to where you get stuck.
What this lesson covers
Every second, the cells in your body are quietly running a chemical reaction that keeps you alive. It is not dramatic to watch, but without it your muscles could not twitch, your brain could not think, and your body could not stay warm. That reaction is cellular respiration: the process cells use to take the energy stored inside a glucose molecule and move it into a form the cell can actually spend.
In this lesson you will learn the reactants and products of cellular respiration, where in the cell it happens, and how to write and read its chemical equation. You will also sort out two ideas that get tangled together constantly — breathing and cellular respiration are not the same thing — and you will find out why a maple tree, which makes its own sugar, still has to break that sugar back down every day and night.
In this lesson you will learn the reactants and products of cellular respiration, where in the cell it happens, and how to write and read its chemical equation. You will also sort out two ideas that get tangled together constantly — breathing and cellular respiration are not the same thing — and you will find out why a maple tree, which makes its own sugar, still has to break that sugar back down every day and night.
What Cellular Respiration Is and Where It Happens
Cellular respiration is the chemical process in which cells break down glucose using oxygen and release the energy that was stored in the glucose's chemical bonds. The overall reaction looks like this:Read it out loud in words: one glucose molecule plus six oxygen molecules react to produce six carbon dioxide molecules, six water molecules, and usable energy.
The reactants — the things used up — are glucose and oxygen. The products — the things made — are carbon dioxide, water, and energy. Notice that energy is written as a product but it is not a molecule. Energy is not matter, so it does not have a chemical formula and it does not get counted when you balance the equation. Most of the released energy is captured in a molecule called ATP, which cells spend the way you spend money; the rest leaves as heat, which is one reason your body stays around 37 degrees Celsius.
Most of this process takes place inside organelles called mitochondria. Cells that need lots of energy, like heart muscle cells, are packed with mitochondria. That is why mitochondria are nicknamed the powerhouse of the cell — but be precise about the nickname. Mitochondria do not create energy out of nothing. The energy was already in the glucose, originally captured from sunlight during photosynthesis. Mitochondria only transfer it into a form the cell can use. Energy is never created or destroyed, only moved and changed in form.
The reactants — the things used up — are glucose and oxygen. The products — the things made — are carbon dioxide, water, and energy. Notice that energy is written as a product but it is not a molecule. Energy is not matter, so it does not have a chemical formula and it does not get counted when you balance the equation. Most of the released energy is captured in a molecule called ATP, which cells spend the way you spend money; the rest leaves as heat, which is one reason your body stays around 37 degrees Celsius.
Most of this process takes place inside organelles called mitochondria. Cells that need lots of energy, like heart muscle cells, are packed with mitochondria. That is why mitochondria are nicknamed the powerhouse of the cell — but be precise about the nickname. Mitochondria do not create energy out of nothing. The energy was already in the glucose, originally captured from sunlight during photosynthesis. Mitochondria only transfer it into a form the cell can use. Energy is never created or destroyed, only moved and changed in form.
Breathing Is Not Cellular Respiration
This is the single biggest mix-up in the whole unit, partly because the word "respiration" shows up in both. Breathing (scientists call it ventilation) is a physical process performed by an organ system: your ribs and diaphragm move, air flows into your lungs, and gases are exchanged with your blood. Cellular respiration is a chemical process performed inside individual cells, in the mitochondria.
The two are related, and that is exactly why they get confused. Breathing is the delivery service; cellular respiration is the customer.
A useful test: an earthworm has no lungs and never breathes, yet every one of its cells performs cellular respiration, taking in oxygen straight through its moist skin. A tree does not breathe either. So breathing cannot be required for cellular respiration.
Where students go wrong is answering "the lungs" when asked where cellular respiration happens, or saying "cellular respiration is how we breathe." A complete answer connects them correctly: breathing brings oxygen into the body and carries carbon dioxide out, but the actual energy-releasing reaction happens in the cells. If you hold your breath, your cells keep doing cellular respiration for a while — they just run short on oxygen, which is why you feel the urge to breathe again.
The two are related, and that is exactly why they get confused. Breathing is the delivery service; cellular respiration is the customer.
| Breathing | Cellular respiration | |
|---|---|---|
| Type of process | Physical (movement of air) | Chemical (bonds broken and formed) |
| Where | Lungs, airways, diaphragm | Inside cells, mainly mitochondria |
| What happens | Air moves in and out; gases exchange | Glucose plus oxygen release energy |
| Who does it | Animals with lungs | Every living cell — plants, animals, fungi, bacteria |
Where students go wrong is answering "the lungs" when asked where cellular respiration happens, or saying "cellular respiration is how we breathe." A complete answer connects them correctly: breathing brings oxygen into the body and carries carbon dioxide out, but the actual energy-releasing reaction happens in the cells. If you hold your breath, your cells keep doing cellular respiration for a while — they just run short on oxygen, which is why you feel the urge to breathe again.
Why Plants Do It Too
Plants make their own glucose through photosynthesis, so students often assume plants do not need cellular respiration. They absolutely do. Making sugar and using sugar are two different jobs. Photosynthesis stores energy in glucose; cellular respiration is the only way a plant cell can get that energy back out to build cell walls, move minerals up from the roots, open and close stomata, and grow. A plant that could not perform cellular respiration would starve while surrounded by its own food.
Compare the two processes side by side:
Read the top two rows again — the reactants of one are the products of the other. The processes are roughly opposite, which is why they fit together in a cycle across an ecosystem.
Timing matters. During a bright day, a leaf performs photosynthesis faster than cellular respiration, so overall it takes in carbon dioxide and gives off oxygen. At night photosynthesis stops completely, but cellular respiration keeps running, so the plant takes in oxygen and gives off carbon dioxide, just like you do. A plant does not "switch" between the two; respiration never stops. It is just hidden during the day because photosynthesis outpaces it.
Compare the two processes side by side:
| Photosynthesis | Cellular respiration | |
|---|---|---|
| Reactants | Carbon dioxide and water | Glucose and oxygen |
| Products | Glucose and oxygen | Carbon dioxide, water, energy |
| Energy | Stores light energy in sugar | Releases stored energy from sugar |
| Organelle | Chloroplast | Mitochondrion |
| When | Only in light | All the time, day and night |
| Who | Plants, algae, some bacteria | Nearly all living things |
Timing matters. During a bright day, a leaf performs photosynthesis faster than cellular respiration, so overall it takes in carbon dioxide and gives off oxygen. At night photosynthesis stops completely, but cellular respiration keeps running, so the plant takes in oxygen and gives off carbon dioxide, just like you do. A plant does not "switch" between the two; respiration never stops. It is just hidden during the day because photosynthesis outpaces it.
Tracking Atoms and Energy Through the Reaction
Chemical equations follow a strict rule: atoms are never created or destroyed, only rearranged. Count them in cellular respiration.
On the reactant side, one glucose () brings 6 carbon, 12 hydrogen, and 6 oxygen atoms, and brings 12 more oxygen atoms — 18 oxygen atoms total. On the product side, contains 6 carbon and 12 oxygen atoms, and contains 12 hydrogen and 6 oxygen atoms. Carbon: 6 and 6. Hydrogen: 12 and 12. Oxygen: 18 and 18. Balanced.
This counting has a real meaning for your body. The carbon atoms you exhale as carbon dioxide right now came from food you ate. Those atoms were rearranged in your cells, not burned away. Some of the water your cells produce becomes part of the water in your blood.
Energy behaves differently from matter. Matter cycles — the same carbon atoms move from air to plant to animal to air again. Energy flows one way and gradually leaves as heat. That heat is real: a room full of people warms up because every one of those bodies is running cellular respiration.
A common wrong answer is to write energy on the reactant side, as if the cell must add energy to break down glucose. Cellular respiration is energy-releasing overall, so energy goes on the product side. Photosynthesis is the reverse: light energy goes in with the reactants. If you can remember which side energy belongs on for each process, you can reconstruct both equations from scratch.
On the reactant side, one glucose () brings 6 carbon, 12 hydrogen, and 6 oxygen atoms, and brings 12 more oxygen atoms — 18 oxygen atoms total. On the product side, contains 6 carbon and 12 oxygen atoms, and contains 12 hydrogen and 6 oxygen atoms. Carbon: 6 and 6. Hydrogen: 12 and 12. Oxygen: 18 and 18. Balanced.
This counting has a real meaning for your body. The carbon atoms you exhale as carbon dioxide right now came from food you ate. Those atoms were rearranged in your cells, not burned away. Some of the water your cells produce becomes part of the water in your blood.
Energy behaves differently from matter. Matter cycles — the same carbon atoms move from air to plant to animal to air again. Energy flows one way and gradually leaves as heat. That heat is real: a room full of people warms up because every one of those bodies is running cellular respiration.
A common wrong answer is to write energy on the reactant side, as if the cell must add energy to break down glucose. Cellular respiration is energy-releasing overall, so energy goes on the product side. Photosynthesis is the reverse: light energy goes in with the reactants. If you can remember which side energy belongs on for each process, you can reconstruct both equations from scratch.
Key terms
- Cellular respiration.
- The chemical process in which cells use glucose and oxygen to release stored energy, producing carbon dioxide and water as waste products.
- Glucose.
- A sugar with the formula that stores chemical energy; it is the main fuel broken down in cellular respiration.
- Mitochondrion.
- The organelle where most of cellular respiration takes place; cells with high energy needs contain many of them. Plural: mitochondria.
- ATP.
- The small energy-carrying molecule that cells make during cellular respiration and spend to power activities like movement and building molecules.
- Reactants.
- The substances used up in a chemical reaction, written on the left of the arrow. For cellular respiration these are glucose and oxygen.
- Products.
- The substances formed by a chemical reaction, written on the right of the arrow. For cellular respiration these are carbon dioxide, water, and released energy.
- Breathing (ventilation).
- The physical movement of air into and out of the lungs; it delivers oxygen to and removes carbon dioxide from the body, but it is not the chemical reaction itself.
- Chemical energy.
- Energy stored in the bonds of molecules such as glucose, which can be released and transferred when those bonds are rearranged.
Worked example
A student places a small potted plant inside a sealed clear box with a carbon dioxide sensor. The box sits under a bright lamp for two hours, and the sensor shows carbon dioxide levels falling. The student then turns off the lamp and covers the box with a black cloth. Over the next two hours, carbon dioxide levels rise. Explain both results using photosynthesis and cellular respiration, and state whether the plant stopped respiring in the light.
Step 1: Identify which processes could be running. Plant cells can do both photosynthesis (in chloroplasts, only in light) and cellular respiration (in mitochondria, all the time).
Step 2: Match each process to the gas it affects. Photosynthesis uses carbon dioxide as a reactant, so it lowers the carbon dioxide level. Cellular respiration produces carbon dioxide, so it raises the level.
Step 3: Analyze the lighted period. Under the lamp, both processes run at once. The sensor shows carbon dioxide falling, so photosynthesis must be removing carbon dioxide faster than respiration is adding it. The net change is a decrease.
Step 4: Analyze the dark period. With no light, photosynthesis stops — no light energy means no way to build glucose. Cellular respiration continues because the plant still needs energy to stay alive. Now nothing is removing carbon dioxide while respiration keeps producing it, so the level rises.
Step 5: Answer the specific question. No, the plant did not stop respiring in the light. Cellular respiration ran the entire four hours. It was simply masked during the lighted period because photosynthesis was faster. The falling sensor reading shows the net result of two processes happening together, not a single process acting alone.
Step 2: Match each process to the gas it affects. Photosynthesis uses carbon dioxide as a reactant, so it lowers the carbon dioxide level. Cellular respiration produces carbon dioxide, so it raises the level.
Step 3: Analyze the lighted period. Under the lamp, both processes run at once. The sensor shows carbon dioxide falling, so photosynthesis must be removing carbon dioxide faster than respiration is adding it. The net change is a decrease.
Step 4: Analyze the dark period. With no light, photosynthesis stops — no light energy means no way to build glucose. Cellular respiration continues because the plant still needs energy to stay alive. Now nothing is removing carbon dioxide while respiration keeps producing it, so the level rises.
Step 5: Answer the specific question. No, the plant did not stop respiring in the light. Cellular respiration ran the entire four hours. It was simply masked during the lighted period because photosynthesis was faster. The falling sensor reading shows the net result of two processes happening together, not a single process acting alone.
Practice questions
Which statement correctly describes the relationship between breathing and cellular respiration?
- Breathing and cellular respiration are two names for the same process.
- Breathing supplies oxygen to the body, while cellular respiration uses that oxygen inside cells to release energy from glucose.
- Cellular respiration happens in the lungs, and breathing happens in the mitochondria.
- Only animals with lungs perform cellular respiration.
Answer: Breathing supplies oxygen to the body, while cellular respiration uses that oxygen inside cells to release energy from glucose.
Breathing is a physical process that moves air in and out of the lungs; cellular respiration is a chemical reaction inside mitochondria. Breathing is the delivery system that gets oxygen to cells and removes the carbon dioxide they produce. The third choice reverses the locations, and the last choice is disproved by earthworms, trees, and bacteria, none of which have lungs but all of which respire at the cellular level.
A classmate says, "Plants don't need cellular respiration because photosynthesis already gives them energy." Explain why this statement is incorrect, and describe what a plant cell would be missing if its mitochondria stopped working.
Answer: Photosynthesis only stores energy in glucose; it does not make that energy usable. Cellular respiration is the process that releases the stored energy from glucose so the cell can use it, so plants need both. Without working mitochondria, a plant cell could still make sugar but could not release energy from it to grow, transport minerals, or build new structures, and the cell would die surrounded by unusable food.
The key distinction is storing versus releasing energy. Photosynthesis captures light energy and locks it into the chemical bonds of glucose. That energy is not in a form the cell can spend. Cellular respiration breaks glucose down and transfers the energy into ATP, which powers cell activities. Because plants run cellular respiration around the clock, they take in oxygen and release carbon dioxide at night, when photosynthesis is not happening to mask it.
Write the balanced chemical equation for cellular respiration, then explain why energy appears on the product side rather than the reactant side.
Answer: . Energy is a product because cellular respiration releases energy that was already stored in the bonds of glucose rather than requiring energy to be added overall.
Check the atoms: 6 carbon, 12 hydrogen, and 18 oxygen on each side. Energy is not counted in balancing because it is not matter and has no chemical formula. Placing energy on the product side signals an energy-releasing reaction. Compare this with photosynthesis, where light energy is listed with the reactants because that reaction stores energy instead of releasing it.
FAQ
- Do plants perform cellular respiration only at night?
- No. Plants perform cellular respiration constantly, day and night. During the day it is hidden because photosynthesis runs faster and uses up more carbon dioxide than respiration produces, so the plant appears to only take in carbon dioxide and give off oxygen. At night photosynthesis stops, leaving respiration as the only gas-exchanging process, so the plant takes in oxygen and releases carbon dioxide.
- Is cellular respiration the same as burning fuel?
- They are chemically similar but happen very differently. Both combine a fuel with oxygen and release carbon dioxide, water, and energy. Burning releases all the energy at once as heat and light. Cellular respiration releases the energy in many small controlled steps, capturing most of it in ATP so cells can use it, with only some lost as heat. That control is why your cells do not catch fire.
- What happens if a cell does not get enough oxygen?
- Cells can partially break down glucose without oxygen through a backup process called fermentation, but it releases far less energy from each glucose molecule. Your muscle cells switch to this during hard sprinting, which produces lactic acid and contributes to that burning, tired feeling. Cells that need steady energy, like brain cells, are damaged quickly when oxygen runs out.
- Why do I get warm when I exercise?
- Exercising muscles demand more energy, so their mitochondria speed up cellular respiration. Not all of the energy released from glucose ends up in ATP — a large portion leaves as heat. More respiration means more heat, so your body temperature rises and you sweat to cool down. The heat is direct evidence that this chemical reaction is happening inside you.
Learn this with a teacher, not a page
The Crimsora tutor teaches Cellular Respiration live — explaining on a whiteboard, asking you questions, and adapting to where you get stuck.