GCSE/iGCSE Biology - Year 1 Week 9
- Sallyann Clark
- 2 days ago
- 8 min read
The Original Solar Power
Where Does a Plant's Mass Actually Come From?
If you want to use the templates to help you record your work, download them now and print them out. They may not fit perfectly with every week, and you may need extra pages to record some answers, especially when you need to draw a diagram or graph.
The Big Question This Week
"Where does a plant actually get its mass from, the soil, the air, or somewhere else?"
Write this in your learning journal as a title before you begin. Don't look
anything up yet — just think. If you prepared this question at the end of Week 8, go back and read what you wrote.
Before You Begin — Predict!
Write down whether you think these are true or false, and why:
"A tree's wood is made mostly from minerals and nutrients it sucks up out of the soil through its roots."
"Photosynthesis only matters for plants, animals don't really depend on it."
Hold onto your predictions. We will return to them at the end of the week.
Story of the Week
Somewhere on Earth, right now, something green is quietly performing the single most important chemical reaction on the planet. It's not happening in a lab or a factory. It's happening in a chloroplast, a tiny, green-tinted structure inside a plant cell, and it is, without exaggeration, the reaction that feeds almost every living thing on Earth, directly or indirectly.
Photosynthesis takes three humble, ordinary ingredients, carbon dioxide from the air, water from the soil, and energy from sunlight, and turns them into glucose, a sugar the plant can use for energy and for building its own body, plus oxygen as a by-product. That oxygen, released almost as an afterthought, happens to be the exact gas every animal on Earth, including you, needs to breathe.
Every mouthful of food you have ever eaten either came from a plant, or came from something that ate a plant, or ate something that ate something that ate a plant. Trace any food chain back far enough and you land, always, on a green leaf catching sunlight.
This week we look at exactly how that reaction works: what goes in, what comes out, and what factors can speed it up or slow it down. We'll also meet a very confused 17th-century scientist who was convinced he had solved the mystery of plant growth completely, and had actually only found half the answer.
Video Session One
While you watch, look out for:
The word equation for photosynthesis, and what each reactant and product actually is
The difference between the light-dependent reactions and the light-independent reactions (the Calvin cycle), and roughly where each happens inside the chloroplast
Why photosynthesis is described as the reverse of cellular respiration in terms of its overall equation
Draw the word equation for photosynthesis in your journal: carbon dioxide + water → glucose + oxygen (with light energy written above the arrow). Label where each reactant comes from and where each product goes.
Video Session Two
While you watch, look out for:
Which organisms other than plants can also carry out photosynthesis
How carbon dioxide actually gets into a leaf, and the name of the pore it enters through
How the products of photosynthesis link directly back to what you learned about cellular respiration and glucose in earlier weeks
⚠️ Common Misconception Alert
A very common mistake is thinking that plants only do photosynthesis and don't need to respire. Plants do both: they photosynthesise (only in the light, and only in cells with chloroplasts) and they respire constantly, day and night, in every living cell, just like animals do. During the day, photosynthesis usually happens faster than respiration, so a plant appears to be a net producer of oxygen and consumer of carbon dioxide, but the respiration never stops. So during the night, when plants stop photosythesising they produce more carbon dioxide than oxygen.
Here is a good place to stop if you need to.
Reading Assignment
Booklet Readings 14 and 15 accompany this week and go into the limiting factors in more detail, plus a very different piece of photosynthesis history from this week's Brilliant Mistake.
Hands-On Activity — Required Practical: Light Intensity and Rate of Photosynthesis
What you need: A sprig of pondweed (Elodea or Cabomba, available from an aquarium shop), a large beaker or clear jar, water with a small amount of sodium bicarbonate dissolved in it (to provide extra carbon dioxide), a bright lamp, a ruler, and a stopwatch.
What to do:
Place the pondweed cut-end up in the beaker of water so that bubbles of gas released from the cut stem can be counted or collected.
Position the lamp at a set distance from the beaker — start at 10 cm.
Allow one minute for the plant to adjust, then count the number of bubbles released in one minute. Repeat twice more at the same distance and calculate an average.
Move the lamp further away in stages (20 cm, 30 cm, 40 cm, 50 cm) and repeat the bubble count at each distance.
Record your results in a table: distance from lamp, and average bubbles per minute.
In your journal, plot a graph of bubbles per minute (y-axis) against distance from the lamp (x-axis).
Describe the pattern you see, and explain it in terms of light intensity as a limiting factor for photosynthesis.
If you have time, research the inverse square law and consider plotting bubbles per minute against 1 ÷ distance² instead — real scientists use this relationship because light intensity does not decrease in a simple straight line as distance increases.
You can watch the whole required practical with all the relevant information on Freesciencelessons.
Here is a good place for a break.
The Brilliant Mistake — Stories Science Gets Wrong
This Week's Story: The Willow Tree That Fooled a Scientist for Five Years
In the early 1600s, a Flemish scientist named Jan Baptist van Helmont set out to answer a question that had puzzled people for centuries: where does a growing plant actually get its extra mass from?
Van Helmont designed what was, for its time, a remarkably careful experiment. He planted a young willow tree, weighing exactly 5 pounds, into a pot containing exactly 200 pounds of dried soil. He covered the soil to keep out dust and debris, and for five years he watered the willow with nothing but rainwater, changing nothing else.
After five years, van Helmont dug up the tree and weighed everything again. The willow tree now weighed 169 pounds, an increase of 164 pounds. The soil, meanwhile, had lost almost nothing: only about 2 ounces. Van Helmont's conclusion was logical, careful, and almost entirely wrong: since the soil had barely changed, he reasoned, the tree's enormous new mass must have come from the only other thing he had added — water.
Van Helmont wasn't foolish. His experiment was genuinely well-designed for its era, and his conclusion that soil alone could not explain a plant's growth was correct and important; it overturned an older belief that plants were built entirely "from" the soil, the way a mineral might dissolve into it. But van Helmont had no way of detecting or even imagining that gases from the air might have mass and might be absorbed by a plant. The idea that ordinary air contained a specific gas, carbon dioxide, that a plant could pull in through its leaves and build into solid wood wouldn't be understood for another 150 years, until scientists like Joseph Priestley and Jan Ingenhousz began working out the role of gases and light in plant growth in the 1770s.
Journal Prompt:
Van Helmont's experiment was careful and quantitative, yet his conclusion was still wrong. What does this tell us about the limits of even a "good" experiment if the right tools or concepts don't yet exist?
If you had been in the room with van Helmont in the 1640s, what question might you have asked him that could have nudged him toward the missing piece of the puzzle?
This is now the third time this course has met a scientist whose careful, honest work still led to an incomplete or wrong conclusion (think back to earlier weeks). Why might "being careful" not be enough on its own?
Scientist Portrait
Jan Ingenhousz (1730–1799)
Add Jan Ingenhousz to your Scientist journal. Draw or print a portrait. Beneath it, write: "He showed that plants need light, not just water, to do their strange and vital trick, and finally finished the puzzle van Helmont started."
Return to Your Predictions
Go back to the two statements from the start of the week:
"A tree's wood is made mostly from minerals and nutrients it sucks up out of the soil through its roots."
"Photosynthesis only matters for plants, animals don't really depend on it."
Rewrite each one now, using what you've learned this week.
The Big Question — Final Answer
Return to this week's big question:
"Where does a plant actually get its mass from, the soil, the air, or somewhere else?"
Write a paragraph in your learning journal. A strong answer will mention carbon dioxide from the air, water from the soil, and explain that most of a plant's dry mass actually comes from the air, not the ground.
Practice Questions — 10 Question Set
Write the word equation for photosynthesis.
Name the organelle in a plant cell where photosynthesis takes place.
Name three factors that can limit the rate of photosynthesis.
In the pondweed practical, why does moving the lamp further away reduce the rate of gas production?
Explain, in terms of gases entering and leaving the leaf, why photosynthesis and respiration are described as roughly opposite processes.
What is meant by a limiting factor?
Explain why increasing carbon dioxide concentration might not increase the rate of photosynthesis if light intensity is very low.
Van Helmont concluded that a willow tree's extra mass came entirely from water. What did his experiment get right, and what crucial factor did it miss?
Why do plants need to carry out respiration as well as photosynthesis?
Name the pore in a leaf through which carbon dioxide enters, and the specialised cells that control how open or closed it is.
Week 9 Checklist
Before moving to Week 10, tick off each item:
Watched Crash Course Biology #28: Photosynthesis
Drawn and labelled the word equation for photosynthesis
Watched Amoeba Sisters: Photosynthesis (UPDATED)
Completed the pondweed light intensity practical and plotted your results
Read the van Helmont willow tree story and answered the journal prompts
Added Jan Ingenhousz to your Scientist journal
Rewritten your two predictions from the start of the week
Answered the 10 practice questions
Written a final answer to the Big Question
Looking Ahead — Week 10 Preview
Next week we follow food on its journey through the human body, from the moment it enters your mouth to the moment your body absorbs what it needs. We'll meet the enzymes that break food down, the organs that do the work, and a scientist whose case study patient gave him an extraordinary window straight into a living human stomach.
Prepare by writing this question in your journal: "What actually happens to a piece of bread between the moment you swallow it and the moment your body can use it for energy?"
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