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Photosynthesis and Respiration

Biology · Photosynthesis and Respiration

Cellular Respiration

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Essential Question

How do cells release the energy stored in glucose?

Every cell in your body is using energy right now, even while you sit still. That energy comes from glucose and oxygen. In this lesson, you’ll see how cellular respiration releases the energy stored in glucose, where it happens in the cell, and how much of that energy a cell is able to capture.

Video lesson coming soon. In the meantime, read through the lesson below.

Cellular Respiration Releases Energy From Glucose

  • Inputs: glucose (C6H12O6) and O2
  • Outputs: CO2, H2O, and energy
  • Energy is captured in ATP; the rest is heat
  • Done by almost all organisms, including plants
Three boxes joined by arrows. Inputs: glucose and 6 oxygen. Cellular respiration in cells. Outputs: 6 carbon dioxide, 6 water, and energy as ATP and heat. Below is the equation C6H12O6 + 6O2 gives 6CO2 + 6H2O + energy. A note says the bonds of glucose and oxygen break, new bonds form in carbon dioxide and water, and the energy released is used to make ATP.

Cellular respiration is the chemical process that releases the energy stored in food molecules such as glucose: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy. The bonds of glucose and oxygen are broken, and new bonds form in carbon dioxide and water. The new bonds hold less chemical energy than the old ones, so the difference is released. Cells capture part of that energy in ATP, the molecule that powers cell work (Unit 5), and the rest is released as heat. It is the reverse of the photosynthesis equation, as you saw in Unit 1.

Where Cellular Respiration Happens

  • Glycolysis: in the cytoplasm
  • The rest: in the mitochondria
  • Mitochondria have two membranes
  • The inner membrane is folded into cristae
A mitochondrion cut open. A points to the outer membrane, B to the inner membrane folded into cristae, C to the matrix in the middle, and D to the space between the two membranes.

In plants, animals, fungi, and other cells with a nucleus, the first step of respiration happens in the cytoplasm and the later steps happen in the mitochondria. A mitochondrion has an outer membrane and a much-folded inner membrane. The folds are called cristae, and they give the inner membrane a large surface area. The fluid inside is the matrix. Bacteria have no mitochondria, but many of them also respire, using their cytoplasm and cell membrane in place of mitochondria. Plant cells have mitochondria as well as chloroplasts, so plants respire all the time.

How Much Energy Does the Cell Capture?

  • About 32% of the energy goes into ATP (model)
  • About 68% is released as heat
  • Heat helps keep body temperature steady
  • ATP powers movement, growth, and active transport
Bar showing where the energy from one glucose goes, in model values: about 32 percent captured in ATP, about 30 ATP, and about 68 percent released as heat.

Cells are not perfect energy converters. In a common model, one glucose makes about 30 ATP. Since each mole of ATP stores about 30.5 kJ, and a mole of glucose releases about 2,870 kJ, about 32% of the energy ends up in ATP and about 68% is released as heat. The heat is not wasted in warm-blooded animals, because it helps keep the body warm. The ATP is used for muscle contraction, building molecules, and active transport (Unit 6), and it is later broken down to ADP and phosphate to release its energy.

Breathing Is Not Cellular Respiration

  • Breathing: moves air in and out of the lungs
  • Blood carries O2 to cells and CO2 away
  • Cellular respiration: the chemical process in cells
  • They are linked, but they are not the same
Flow of gases between breathing in the lungs, blood circulation, and body cells. Oxygen moves from the lungs through the blood to body cells, and carbon dioxide moves back the other way. An arrow from body cells goes down to cellular respiration in mitochondria. A note says breathing moves gases between the air and the blood, and respiration is the chemical process that happens inside cells.

In everyday speech, people say they are “breathing hard” or “short of respiration,” but breathing and cellular respiration are different. Breathing moves air in and out of the lungs. The blood then carries O2 from the lungs to the body cells and carries CO2 back. Cellular respiration is the chemical process inside the cells that uses that O2 and makes that CO2. This is why you breathe faster when you exercise: your cells are using more O2 and making more CO2.

Evidence: Germinating Peas Use Oxygen

  • A respirometer measures O2 used
  • Germinating peas use O2 steadily
  • Warmer peas use it faster
  • Dry peas use almost none
Graph of oxygen used in milliliters against time from 0 to 20 minutes. Germinating peas at 30 degrees Celsius rise to 2.8 mL, germinating peas at 20 degrees rise to 1.6 mL, and dry peas at 30 degrees stay near 0.1 mL.

In a respirometer, living tissue is sealed in a tube, and the CO2 it makes is absorbed, so the drop in gas volume shows the O2 used. In the sample data, germinating peas used 2.8 mL of O2 in 20 minutes at 30 °C and 1.6 mL at 20 °C. Dry peas, which are barely alive, used only 0.1 mL. Respiration is faster in the warm peas because enzyme-controlled reactions speed up, and it is faster in the germinating peas because they are growing and need more ATP.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Counting Inputs and Outputs

A cell respires 5 molecules of glucose. Use the balanced equation, C6H12O6 + 6O2 → 6CO2 + 6H2O + energy. (a) How many molecules of O2 are used, and how many of CO2 and H2O are made? (b) In the model, 1 glucose makes about 30 ATP. How many ATP are made? (c) Check that the oxygen atoms balance.

Show the solution
  1. (a) Multiply by 5: O2: 5 × 6 = 30. CO2: 5 × 6 = 30. H2O: 5 × 6 = 30.
  2. (b) ATP: 5 × 30 = 150 ATP (a model value, not an exact count)
  3. (c) Oxygen atoms in: 5 × 6 (glucose) + 30 × 2 (O2) = 90. Out: 30 × 2 (CO2) + 30 × 1 (H2O) = 90.
  4. Answer: 30 O2 used; 30 CO2 and 30 H2O made; about 150 ATP. The oxygen atoms balance (90 in, 90 out).

Example 2: How Much Energy Is Captured?

A mole of glucose releases about 2,870 kJ when it is respired. The cell makes about 30 mol of ATP, and each mole of ATP stores about 30.5 kJ. (a) What percent of the energy is captured in ATP? (b) How much is released as heat? (c) What happens to the heat?

Bar showing where the energy from one glucose goes, in model values: about 32 percent captured in ATP and about 68 percent released as heat.
Show the solution
  1. (a) Energy in ATP: 30 mol × 30.5 kJ/mol = 915 kJ. Percent: 915 kJ ÷ 2,870 kJ × 100 = 32%
  2. (b) Heat: 100% − 32% = 68%, which is about 1,950 kJ
  3. (c) Heat: it spreads into the surroundings. In warm-blooded animals, it also helps keep body temperature steady.
  4. Answer: About 32% is captured in ATP. About 68% (about 1,950 kJ) is released as heat.

Example 3: Respiration in Peas

Use the respirometer data. (a) Find the rate of O2 use for germinating peas at 30 °C and at 20 °C, in mL/min. (b) How many times as fast is respiration at 30 °C? (c) State a claim about the dry peas, with evidence and reasoning.

Graph of oxygen used against time for germinating peas at 30 and 20 degrees Celsius and dry peas, as in the lesson.
Show the solution
  1. (a) Rates: 30 °C: 2.8 mL ÷ 20 min = 0.14 mL/min. 20 °C: 1.6 mL ÷ 20 min = 0.08 mL/min.
  2. (b) Compare: 0.14 ÷ 0.08 = 1.75, so about 1.75 times as fast at 30 °C.
  3. (c) Claim: dry peas carry out almost no cellular respiration.
  4. Evidence: they used only 0.1 mL of O2 in 20 min, about 0.005 mL/min.
  5. Reasoning: dry peas are barely active and are not growing, so they need little ATP. Warm, germinating peas are growing and use much more.
  6. Answer: 0.14 and 0.08 mL/min; 1.75 times as fast. Dry peas respire very little.

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Practice

Complete the respiration equation, label a mitochondrion, read respirometer data, and calculate how much energy goes to ATP and to heat. Try it on your own first, then check your work with the answer key.

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Lesson Slides

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Big Ideas

  • C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
  • Glucose and O2 bonds break; CO2 and H2O bonds form; energy is released
  • About 32% of the energy goes into ATP (model); the rest is heat
  • In cells with a nucleus, glycolysis is in the cytoplasm and the rest is in the mitochondria
  • Breathing is not cellular respiration

Back to the essential question: cellular respiration releases the energy stored in glucose by breaking the bonds of glucose and oxygen and forming new bonds in carbon dioxide and water. The cell captures about a third of that energy in ATP, and the rest becomes heat. In cells with a nucleus, it happens in the cytoplasm and mitochondria. Next, you’ll follow glucose through the three stages of cellular respiration.

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NGSS Alignment

This lesson builds toward these Next Generation Science Standards (NGSS) performance expectations:

  • HS-LS1-7: Use a model to illustrate that cellular respiration is a chemical process whereby the bonds of food molecules and oxygen molecules are broken and the bonds in new compounds are formed resulting in a net transfer of energy.

Disciplinary core ideas: LS1.C Organization for Matter and Energy Flow in Organisms; PS3.D Energy in Chemical Processes

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Next: Stages of Cellular Respiration →