Biology: photosynthesis and cellular respiration (grade 9) · lesson

Photosynthesis builds sugar

💡 Photosynthesis uses light energy to build sugar from carbon dioxide and water, and it gives off oxygen.

In the chloroplasts, light energy drives photosynthesis: carbon dioxide + water → sugar (glucose) + oxygen. The oxygen escapes as a gas. That is why a water plant in light gives off bubbles.

Sunlight supplies energy, not atoms. The carbon atoms in sugar come from carbon dioxide in the air. The plant uses that sugar to build wood, leaves, and roots. So most of a tree's mass came from the air, not the soil.

🧠 Let's work it out

To find where the atoms in wood came from, list what a plant takes in: carbon dioxide, water, and a few minerals.
Only one of them is rich in carbon.
Energy (light) can't become matter, so it is not the answer.

What do you think comes next?

chlorophyll: the green pigment in chloroplasts that absorbs light
glucose: a simple sugar, C₆H₁₂O₆

🔗 Khan Academy: Energy and matter in biological systems · Khan Academy (free) ↗

Cellular respiration releases energy

💡 Cellular respiration breaks down sugar with oxygen, releasing carbon dioxide, water, and usable energy, day and night.

In the mitochondria, aerobic (oxygen-using) respiration runs: glucose + 6 oxygen → 6 carbon dioxide + 6 water, plus energy stored in ATP. It is almost the reverse of photosynthesis.

Plants do both. Photosynthesis needs light and stops in the dark. Respiration never stops, because cells always need energy for jobs like building proteins. A plant sealed in a dark jar slowly uses up the oxygen in the jar.

The numbers in front of each formula tell how many molecules react. One glucose needs 6 oxygen molecules, so 3 glucose need 18.

🧠 Let's work it out

Sort each substance: is it used up (a reactant) or made (a product)?
Glucose and oxygen go in.
Carbon dioxide and water come out.
Then ask whether the process needs light.

What do you think comes next?

aerobic: using oxygen
reactant: a substance used up in a reaction
product: a substance made by a reaction

The path of energy

💡 Energy flows from sunlight into sugar and then into ATP that powers a cell's work.

Chloroplasts capture light and store its energy in the bonds of sugar. An animal eats the plant and digests the sugar. Its mitochondria break the sugar down and make ATP. Muscles use ATP directly to move.

At each step, energy changes form, but it is not created from nothing. It always traces back to the Sun.

🧠 Let's work it out

Follow one packet of energy.
Where does it enter? (light in a leaf)
Where is it stored? (sugar)
How does it reach a muscle? (eaten, then broken down into ATP).

What do you think comes next?

food energy: chemical energy stored in the bonds of molecules like sugar

Testing one variable at a time

💡 A fair test changes one variable, measures one result, and keeps everything else the same.

The independent variable is the one you change on purpose, such as lamp distance. The dependent variable is what you measure, such as bubbles per minute. Controlled variables are kept the same, such as water temperature and the kind of plant.

If two things change at once, you can't tell which one caused the result. Repeating trials and averaging makes results more reliable.

Also check the measuring tool. Counting bubbles treats a big bubble and a small bubble the same. Collecting the gas and measuring its volume is more accurate.

🧠 Let's work it out

Leo's data: 30 bubbles per minute at 10 cm, 18 at 20 cm, 6 at 40 cm.

The closer the lamp, the faster the bubbling.
Since temperature was held near 22 °C, light is the best explanation.
The drop from 10 cm to 40 cm is 30 − 6 = 24.

What do you think comes next?

independent variable: what the scientist changes on purpose
dependent variable: what the scientist measures
controlled variable: a condition kept the same in every trial