Shanghai Edition Junior High School English Grade 9, Semester 1: The Driving Force of Life: Food and Energy Density

Imagine that food is not merely a filler for your stomach—it's more like a naturally packagedbiological battery. Each bite contains tiny chemical bonds, and these are the ultimate source of energy that powers our breathing, running, and even thinking.

Core Concepts and Biological Logic

Microscopic Energy Storage: Open your textbook to page 94. Under the microscope, plants are not just green tissues—they contain orderly arrangements of starch granules or lipid droplets. These tiny structures serve as 'fuel depots' for future generations or the plant itself.
The Determining Factor in Density: Why are walnuts more filling than cucumbers? The answer lies in fat. Fat molecules have an extremely high carbon-to-hydrogen ratio, resulting in an energy density of about 9 kcal/g—twice that of carbohydrates. This is evolutionary wisdom: carrying the most energy with the lightest weight.
Dilution Effect: While water and fiber are crucial for health, they provide no calories. The high water content in fresh fruits significantly 'dilutes' their energy density—explaining why migratory birds prefer nuts over juicy fruits.

QUESTION 1

When observing potato and rapeseed cell samples under a microscope, what microscopic structures did you identify as specifically storing energy? What are their corresponding chemical components?

Starch granules (starch) and lipid droplets (fat)

Chloroplasts (chlorophyll) and vacuoles (water)

Nuclei (DNA) and mitochondria (energy)

QUESTION 2

As part of your experimental investigation, recall the process used in your lab to measure the energy content of different types of food. How much did the water temperature rise for each sample your group measured? If you could repeat the experiment, would you change the amount of water or food samples used? Why?

Yes; adjusting the amount of water or food mass could reduce heat loss or improve measurement sensitivity.

No; the experimental quantities do not affect the final calculated energy value.

QUESTION 3

Fats contain roughly twice or more energy per unit mass than carbohydrates. What evolutionary significance does this high energy density hold for organisms?

Increasing body size to deter predators

Storing more energy while minimizing body weight, beneficial for long-distance flight or winter survival

Increasing body water content

QUESTION 4

Which of the following components significantly reduces the overall energy density of a food?

Unsaturated fatty acids

Glucose molecules

Water and dietary fiber

QUESTION 5

In the textbook energy measurement experiment, to minimize error, what value do we typically calculate from multiple trials?

Maximum value

Average value

Minimum value