Why Ana Is Out of Breath
Ana runs 800 meters as fast as she can. Afterward she gasps for air and her heart pounds, even though she has stopped.
Where in her body is all that extra oxygen going?
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Science · Grade 7 Science
Chapter 1: Cells: The Unit of Life
Every part is there for something.
Ana runs 800 meters as fast as she can. Afterward she gasps for air and her heart pounds, even though she has stopped.
Where in her body is all that extra oxygen going?
Make your guess to turn the page.
Still to come: 10 pages · Words to Know · What We Learned · 4 questions · Explain It in Writing · Ask and Wonder
The idea this lesson is working on — Children picture a cell as a bag of liquid with a dot in it, because that is how a cell looks at low power. The book follows a real demand, a runner out of breath, inward to the organelle that meets it, and answers "they are just bigger" with a percentage, which already allows for size. Ask: if muscle cells were only bigger, what share of mitochondria would you expect? The idea to reach is that the parts, and how many of each, follow the job.
Factory analogies have limits — Comparing a cell to a factory helps at first and misleads later: nothing in a cell is directing anything. The parts work because of chemistry, not because of management. The book uses "system" instead, with matter in and out across the membrane and energy changing form inside, which carries on into photosynthesis and body systems.
About the numbers — Mitochondria make up roughly 30–40% of the volume of a heart muscle cell and about 20% of a liver cell; mature red blood cells have none. Oxygen use of about 0.2 L a minute at rest and about 2 L a minute running hard is realistic for a fit young teenager. Oxygen and carbon dioxide cross the membrane by diffusion; glucose enters through transporter proteins, the "doorways" of the book.