Cellular Respiration and Energy Transfer
Sofia sprints down the soccer field, and within seconds she is breathing hard. Her muscle cells are demanding energy right now. Where does that energy come from, and why does she breathe out more carbon dioxide than when she is sitting on the bench? By the end of this page you will be able to answer both questions.
The equation for cellular respiration
Your cells cannot use the energy in a sandwich directly. They first break down glucose, a sugar, and transfer its stored chemical energy to a molecule called ATP. This process is aerobic cellular respiration ("aerobic" means it uses oxygen).
The overall equation is:
$$C_6H_{12}O_6 + 6\,O_2 \rightarrow 6\,CO_2 + 6\,H_2O + \text{energy (ATP)}$$-
Reactants: glucose and oxygen.
-
Products: carbon dioxide, water, and energy (some stored in ATP, the rest released as heat).
Where does it happen? Respiration starts in the cytoplasm, but most ATP is made in the mitochondria, the "power plants" of eukaryotic cells. You do not need the detailed steps (glycolysis, Krebs cycle) for this course. Focus on the inputs, the outputs, and the energy transfer.
Write the balanced chemical equation for aerobic cellular respiration using glucose, and identify the reactants and the products.
Write the balanced chemical equation for aerobic cellular respiration using glucose, and identify the reactants and the products.
Name the organelle where most ATP is produced during aerobic cellular respiration in eukaryotic cells. Then state what ATP is used for in a cell, in one sentence.
Name the organelle where most ATP is produced during aerobic cellular respiration in eukaryotic cells. Then state what ATP is used for in a cell, in one sentence.
Matter is conserved
Look at the equation again. Glucose seems to disappear, but nothing is destroyed. The atoms are rearranged into new molecules. To check, count the atoms on each side.
The same atoms enter and leave, so no atoms are created or destroyed. This is the law of conservation of matter. It also answers a nice question: when you exhale, the carbon in your $CO_2$ came from the glucose in your food.
Energy is also conserved, but it changes form. Glucose stores chemical energy. Respiration converts it into chemical energy in ATP and into thermal energy (heat), which is why you feel warm when you exercise.
In the equation $C_6H_{12}O_6 + 6\,O_2 \rightarrow 6\,CO_2 + 6\,H_2O$ + energy, count the atoms of carbon, hydrogen, and oxygen on each side. Use your counts to explain…
In the equation $C_6H_{12}O_6 + 6\,O_2 \rightarrow 6\,CO_2 + 6\,H_2O$ + energy, count the atoms of carbon, hydrogen, and oxygen on each side. Use your counts to explain how this equation shows that matter is conserved.
Sofia exercises hard during soccer practice and her breathing rate increases. a) Explain how a faster breathing rate helps her cells carry out cellular respiration. b)…
Sofia exercises hard during soccer practice and her breathing rate increases.
- a) Explain how a faster breathing rate helps her cells carry out cellular respiration.
- b) Explain where the carbon atoms in the carbon dioxide she exhales came from.
- c) Describe the form of energy stored in glucose and the forms it is converted to during respiration.
Respiration and photosynthesis: opposite processes
Photosynthesis is respiration's mirror image:
$$6\,CO_2 + 6\,H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6\,O_2$$The products of one process are the reactants of the other.
| Feature | Photosynthesis | Cellular respiration |
|---|---|---|
| Reactants | $CO_2$, $H_2O$ | glucose, $O_2$ |
| Products | glucose, $O_2$ | $CO_2$, $H_2O$ |
| Energy | Light energy is stored as chemical energy | Chemical energy is released and transferred to ATP and heat |
| Location (plant cell) | Chloroplasts | Mitochondria (and cytoplasm) |
| Occurs in | Plants, algae, some bacteria | Nearly all living things |
Plants do both. They make glucose in chloroplasts, then break some of it down in their mitochondria. Animals only do respiration, so they depend on photosynthesizers for glucose and oxygen.
Matter cycles between the two processes. Energy does not: it enters as light, passes through organisms as chemical energy, and leaves as heat.
Which two substances are reactants in photosynthesis, and which two are products? Say how these relate to the products and reactants of cellular respiration.
Which two substances are reactants in photosynthesis, and which two are products? Say how these relate to the products and reactants of cellular respiration.
Compare photosynthesis and cellular respiration by completing these points in a short paragraph or table: the energy source, where each occurs in a plant cell, the…
Compare photosynthesis and cellular respiration by completing these points in a short paragraph or table: the energy source, where each occurs in a plant cell, the direction of energy flow, and how the products of one process become the reactants of the other.
Enzymes and respiration in experiments
Respiration is not one reaction but many, and each step is sped up by an enzyme. Enzymes are proteins, so heat can denature them. A denatured enzyme loses its shape, and the reactions stop.
Scientists often measure respiration by tracking $CO_2$ released (or $O_2$ used). A faster rate means more active cells.
A good experiment needs a control (for example, dry seeds, or boiled seeds, treated the same way). Keep other variables the same, such as temperature, mass of seeds, and time. Change only one independent variable.
Putting it together: a sealed jar
Picture a sealed jar with a plant and a mouse in bright light. The mouse respires: it uses $O_2$ and releases $CO_2$. The plant photosynthesizes (uses $CO_2$, releases $O_2$) and also respires. In light, the plant makes more $O_2$ than it uses, so both survive.
In the dark, the plant cannot photosynthesize. Both organisms respire, $O_2$ falls, $CO_2$ rises, and eventually both would die.
Matter is conserved because the jar is sealed and atoms just cycle between $CO_2$, $H_2O$, glucose, and $O_2$. Energy is different: light enters from outside and heat leaves. Chemical energy is not recycled.
A sealed jar contains a living green plant and a small mouse. The jar sits in bright light for several days, and both organisms survive. a) Explain how the plant and the…
A sealed jar contains a living green plant and a small mouse. The jar sits in bright light for several days, and both organisms survive.
- a) Explain how the plant and the mouse each affect the concentrations of oxygen and carbon dioxide in the jar.
- b) Predict what would happen if the jar were placed in complete darkness for several days, and justify your prediction using the equations for both processes.
- c) Explain how matter is conserved in the jar while energy is not recycled in the same way. Identify where the energy enters the system and where it leaves.
A student measures the carbon dioxide released by germinating pea seeds and by non-germinating (dry) pea seeds at 25 °C using a sensor. After 20 minutes the germinating…
A student measures the carbon dioxide released by germinating pea seeds and by non-germinating (dry) pea seeds at 25 °C using a sensor. After 20 minutes the germinating seeds released 0.80 mg of CO₂ per gram of seeds, and the dry seeds released 0.02 mg per gram.
- a) Calculate how many times greater the CO₂ release was in the germinating seeds.
- b) Explain the difference using cellular respiration and the role of enzymes.
- c) Predict the effect on CO₂ release if the germinating seeds were boiled before the test, and explain why.
- d) Describe a control for this experiment and identify two variables that should be held constant.
Back to the start: Sofia's muscle cells break down glucose with oxygen to make ATP. Breathing faster brings in more $O_2$ and removes the extra $CO_2$ her cells produce.
Summary
-
Aerobic cellular respiration: $C_6H_{12}O_6 + 6\,O_2 \rightarrow 6\,CO_2 + 6\,H_2O$ + energy (ATP).
-
Most ATP is made in the mitochondria. Cells use ATP for work such as muscle contraction and active transport.
-
Matter is conserved: the same atoms (6 C, 12 H, 18 O) are rearranged. Energy changes from chemical energy to ATP and heat.
-
Photosynthesis and respiration are opposite processes: each one's products are the other's reactants. Matter cycles, but energy flows through.
-
Enzymes drive respiration, so heat or other conditions that denature them lower $CO_2$ release.