Energy in Living Things — Science

  1. Grade 10 — ATP & the Flow of Energy
  2. Grade 10 — Photosynthesis

    Energy in living things — photosynthesis is the biology face of the crosscutting Energy concept.

  3. Grade 10 — Cellular Respiration
  4. Grade 10 — Comparing Photosynthesis & Respiration

Apply it in a new setting

Problem 1

The overall equation for photosynthesis is 6 CO_2 + 6 H_2O + light → C_{6}H_{12}O_{6} + 6 O_2. If a plant cell takes in 48 molecules of CO_2 and uses them completely, how many molecules of glucose (C_{6}H_{12}O_{6}) can it build?
Hints

Look at the ratio of CO_2 to glucose in the equation.

It takes 6 CO_2 to build 1 glucose.

Divide 48 by 6.

Answer and explanation

8 molecules

The equation shows that 6 CO_2 are needed to make 1 glucose.

48 ÷ 6 = 8.

The cell can build 8 molecules of glucose.

Problem 2

On a sunny afternoon a leaf runs photosynthesis and cellular respiration at the same time. In one hour its chloroplasts take in (fix) 90 molecules of CO_2 by photosynthesis, while its mitochondria release 24 molecules of CO_2 by respiration. What is the NET number of CO_2 molecules the leaf removes from the air that hour?
Hints

Photosynthesis takes CO_2 in; respiration puts CO_2 back out.

Net removed = CO_2 taken in − CO_2 released.

Subtract 24 from 90.

Answer and explanation

66 molecules

Photosynthesis removes 90 CO_2 from the air, while respiration returns 24 CO_2 to it.

Net removed = 90 − 24 = 66.

The leaf nets 66 molecules of CO_2 removed from the air.

Problem 3

The overall equation for aerobic cellular respiration is C_{6}H_{12}O_{6} + 6 O_2 → 6 CO_2 + 6 H_2O + energy (ATP). If a muscle cell completely breaks down 5 molecules of glucose, how many molecules of O_2 does it consume?
Hints

Find the ratio of O_2 to glucose in the equation.

Each glucose requires 6 O_2.

Multiply 5 by 6.

Answer and explanation

30 molecules

The equation shows that 6 O_2 are consumed per glucose.

5 × 6 = 30.

The cell consumes 30 molecules of O_2.

Problem 4

A tree seedling weighing 2 kg grows over 20 years into a tree weighing 400 kg, while the soil around it loses only a tiny fraction of a kilogram. Where does the vast majority of the tree's added mass come from?
  1. From minerals and solid matter that the roots pulled up out of the soil.
  2. From the water the tree absorbed, which turned directly into wood.
  3. From carbon atoms in CO_2 taken from the air and built into sugars during photosynthesis.
  4. From sunlight, which was converted directly into physical mass.
Hints

The soil lost almost no mass, so the material did not mostly come from the soil.

Photosynthesis pulls carbon out of a gas in the air.

Wood is built mainly from carbon-based molecules — where did that carbon come from?

Answer and explanation

From carbon atoms in CO_2 taken from the air and built into sugars during photosynthesis.

The soil lost almost no mass, which rules it out as the main source.

Photosynthesis takes CO_2 from the air and fixes its carbon into glucose, which builds the tree's tissues.

Most of the added mass comes from CO_2 in the air — the correct answer is C.

Problem 5

A student notices that photosynthesis and cellular respiration look like mirror images of each other. Which statement most accurately describes how they are related?
  1. The products of photosynthesis (glucose and O_2) are the reactants of respiration, and the products of respiration (CO_2 and H_2O) are the reactants of photosynthesis.
  2. Both processes build glucose, so both take in CO_2 and release O_2.
  3. Photosynthesis happens only in animals and respiration happens only in plants.
  4. Both processes release energy by breaking glucose down.
Hints

Write out what each process takes in and what it gives off.

Photosynthesis: CO_2 + H_2O → glucose + O_2. Respiration runs essentially in reverse.

Compare the reactants of one process with the products of the other.

Answer and explanation

The products of photosynthesis (glucose and O_2) are the reactants of respiration, and the products of respiration (CO_2 and H_2O) are the reactants of photosynthesis.

Photosynthesis uses CO_2 and H_2O to make glucose and O_2, storing energy.

Respiration uses glucose and O_2 to make CO_2 and H_2O, releasing energy — the reverse pattern.

The outputs of one are the inputs of the other — the correct answer is A.

Problem 6

A student claims: 'Plants only photosynthesize and never carry out cellular respiration — respiration is the animals' job.' Which response is correct?
  1. Correct — plants make their own food, so they never need respiration.
  2. Plants respire only, and it is actually the animals that photosynthesize.
  3. Plants photosynthesize during the day and then stop making or using any energy at night.
  4. Plants carry out cellular respiration all the time to release usable energy from glucose; they also photosynthesize, but only when light is available.
Hints

Photosynthesis stores energy in glucose; how does a cell get that energy back out in usable form?

Every living cell, plant or animal, needs ATP from respiration.

Does a plant stop needing energy once the sun goes down?

Answer and explanation

Plants carry out cellular respiration all the time to release usable energy from glucose; they also photosynthesize, but only when light is available.

Photosynthesis stores energy in glucose, but cells still need respiration to release it as usable ATP.

Plants respire continuously, day and night, and photosynthesize only when light is available.

So plants do both — the correct answer is D.

Problem 7

Cyanide is a poison that blocks the electron transport chain in the mitochondria — the final stage of aerobic respiration, where most ATP is produced. What would happen to a cell exposed to cyanide?
  1. It would make even more ATP than usual to make up for the blockage.
  2. Its ATP production would fall sharply, starving the cell of usable energy even if glucose and oxygen are still present.
  3. It would be unaffected, because cells draw energy directly from glucose rather than from ATP.
  4. It would switch to photosynthesis to keep its ATP supply going.
Hints

Most of a cell's ATP is produced by the electron transport chain.

If that final stage is blocked, what happens to the ATP supply?

ATP is the cell's usable energy currency, so losing it starves the cell.

Answer and explanation

Its ATP production would fall sharply, starving the cell of usable energy even if glucose and oxygen are still present.

The electron transport chain produces the bulk of the ATP in aerobic respiration.

Blocking it with cyanide makes ATP production collapse, even when glucose and O_2 are available.

The cell is starved of usable energy — the correct answer is B.

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