Photosynthesis & Cellular Respiration — Practice · Grade 7 Science

Chapter: Cells, Transport & Cellular Energy

What this practice covers

By the end of Photosynthesis & Cellular Respiration you should be able to:

8 questions, written answer, multiple choice and numeric, each with a worked explanation.

Vocabulary used: photosynthesis, cellular respiration, reactant, product, net change.

Problem 1

Where does much of a growing plant’s carbon come from?
Answer and explanation

Carbon dioxide taken from the surroundings supplies carbon used in organic molecules; light supplies energy rather than carbon matter.

Problem 2

Why does a root without chloroplasts still need sugar?
Answer and explanation

It uses organic material and releases usable energy through cellular processes such as respiration.

Problem 3

Does falling carbon dioxide around a lit plant prove it stopped respiring?
Answer and explanation

No. Photosynthetic uptake can exceed respiration while both continue; the measurement is a net change.

Problem 4

Why is “energy cycles just like carbon” misleading?
Answer and explanation

Carbon atoms can be reused in cycles, while usable energy flows through processes and ultimately dissipates as thermal energy.

Problem 5

Illustrative sealed-chamber data for a short study: carbon dioxide reading begins at 400 units, falls to 350 under light, and rises to 370 during a later dark interval. Readings share one calibrated unit; plant condition and temperature are otherwise stated comparable. Calculate both changes and explain them through net processes without claiming either process was observed directly.
Answer and explanation

The light interval shows a 50-unit decrease, consistent with net photosynthetic uptake exceeding respiratory release. The dark interval shows a 20-unit increase, consistent with respiration without the same light-driven uptake. These net gas measurements do not separately measure each process; leaks, other organisms, and instrument conditions would need checking in an actual investigation.

Light change: 350−400=−50 units.

Dark change: 370−350=+20 units.

Compare photosynthetic uptake and respiratory release under the model.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 6

A student says plants use respiration only at night. Correct the claim.
Answer and explanation

Plants respire in light and darkness. Photosynthesis can occur in suitable light at the same time, so a net gas change must account for both processes.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 7

Which organism can both photosynthesize and respire?
  1. A typical green plant
  2. Only an animal
  3. A rock
  4. A scale bar
Answer and explanation

A typical green plant

Green plants can perform both; the processes have different roles.

Problem 8

A gas reading changes from 300 to 260 units. Find the signed change.
Answer and explanation

-40 units

Final minus initial: 260−300=−40 units.

Browse all courses