GCSE/iGCSE Biology - Year 1 Week 12
- Sallyann Clark
- 19 hours ago
- 8 min read
WEEK 12: How do Leaves Breathe?
How Does a Plant Breathe Without Lungs?
The Big Question This Week
"If a plant needs to let carbon dioxide in to survive, how does it stop itself from drying out in the process?"
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 11, go back and read what you wrote.
Before You Begin — Predict!
Write down whether you think these are true or false, and why:
"A leaf's surface is basically solid, gases can't really pass in and out of it directly."
"Plants only need to exchange gases during the day, because that's when photosynthesis happens."
Hold onto your predictions. We will return to them at the end of the week.
Story of the Week
Turn a leaf over and look closely, really closely, ideally under a microscope, at its underside, and you'll find something that looks like it belongs in a science fiction film: thousands of tiny mouth-shaped pores, opening and closing, each one flanked by a pair of curved, bean-shaped cells that control exactly how wide it gapes. These pores are called stomata (singular: stoma, from the Greek word for "mouth"), and they are how a plant exchanges gases with the world around it, no lungs, no diaphragm, no ribcage required.
Through these pores, carbon dioxide diffuses in for photosynthesis, and oxygen produced by photosynthesis diffuses back out. But stomata create an unavoidable dilemma for a plant. The same opening that lets carbon dioxide in also lets water vapour escape, in a process called transpiration. Keep the stomata open all day to maximise carbon dioxide intake, and the plant risks drying out. Keep them shut to conserve water, and photosynthesis grinds to a halt for lack of raw material. Every plant on Earth is constantly balancing this trade-off, opening and closing millions of tiny pores in response to light, humidity, and water availability.
This week we look closely at leaf structure, how gases actually get where they need to go inside a leaf, and a scientific puzzle that stumped botanists for decades: how does a tree manage to pull water dozens of metres straight up against gravity, with nothing resembling a pump?
Here is a diagram of the structure of a leaf for you to refer back to if needed.
Video Session One
While you watch, look out for:
The three main types of plant tissue described: dermal, vascular, and ground tissue, and roughly what each one does
What stomata and guard cells actually are, and how they're described in this video
The difference between xylem and phloem, and which one carries water up from the roots
Draw a simple cross-section of a leaf in your journal, labelling the upper epidermis, palisade mesophyll, spongy mesophyll, and lower epidermis with stomata. Add a note on which layer contains the most chloroplasts and why that makes sense.
Video Session Two
While you watch, look out for:
How plants transport water and nutrients around their bodies (roughly 2–5 minutes into the video)
How plants get rid of waste products, and why this is a different challenge for a plant than for an animal
Any mention of how plants can influence conditions around them, beyond their own individual bodies
⚠️ Common Misconception Alert
Many students assume that stomata are only found on leaves and are only ever open during daylight. In fact, most plants do close their stomata at night to conserve water, since photosynthesis can't happen without light anyway — but stomata are also found on green stems in many plants, and some specially adapted plants (particularly those living in very hot, dry conditions) actually open their stomata mainly at night and store the carbon dioxide chemically until daylight, to minimise water loss during the hottest, driest part of the day.
Here is a good place to stop if you need to.
Reading Assignment
Booklet Readings 21 and 22 accompany this week, going into more detail on how guard cells actually open and close, and the story of a scientific puzzle about water and gravity that took decades to properly solve.
Work through the presentation to consolidate leaf structure.
A good place to stop is once the video ends, if you need to.
Hands-On Activity — Counting Stomata Using a Leaf Peel
What you need: A leaf from a houseplant or garden plant (something with a reasonably tough, waxy leaf works well, such as an ivy or a rubber plant leaf), clear nail varnish, clear sticky tape, a microscope slide (or a clean piece of glass or clear plastic), and a microscope if you have access to one — otherwise a strong magnifying glass will still show useful detail.
What to do:
Paint a thin, even layer of clear nail varnish onto the underside of the leaf, covering an area roughly 2 cm across. Allow it to dry completely (this can take 10–15 minutes).
Once dry, carefully peel the varnish film away from the leaf using a piece of sticky tape, pressing the tape onto the dried varnish and lifting gently.
Stick the tape (varnish side down) onto a microscope slide or clear surface.
Examine your peel under the microscope (or magnifying glass), and try to count the number of stomata visible in one field of view.
Watch Launchpad Learning - Investigating Stomata
If you have access to a leaf from a different plant species, or a leaf from the same plant grown in a different location (shady versus sunny, for example), repeat the process and compare stomata counts. In your journal, sketch what you observed and write two sentences about what you noticed regarding stomata density, and, if you compared two leaves, what might explain any difference.
The Brilliant Mistake — Stories Science Gets Wrong
This Week's Story: The Tree That Shouldn't Be Able to Drink
For much of the 1800s, botanists faced a genuine physical puzzle: how does water get from a tree's roots all the way to leaves at the top of a trunk that might be 50 metres tall or more?
One popular explanation was root pressure, the idea that roots actively pump water upward, pushing it up through the trunk rather like a garden hose connected to a tap. Root pressure is real and measurable in many plants, particularly early in the growing season, and for smaller plants it can genuinely account for a good deal of upward water movement.
The trouble is the numbers simply don't work for tall trees. Physicists calculated that root pressure alone could push water only a few metres upward at most, nowhere near enough to explain how a giant redwood or oak tree gets water to leaves 30, 50, even over 100 metres above the ground. For decades, this left a real gap in botanical understanding. Something else had to be doing most of the work, but nobody had proven what.
The breakthrough came in 1894, when the Irish scientists Henry Horatio Dixon and John Joly proposed what became known as the cohesion-tension theory. Their idea was elegant. Water molecules are strongly attracted to each other through hydrogen bonding (you may remember this property of water from Week 3), giving water a kind of internal "stickiness," or cohesion. As water evaporates from the surface of leaf cells during transpiration, it creates a tension, a pulling force, that travels continuously down through connected water molecules inside the xylem, all the way to the roots, rather like pulling one end of a long, unbroken chain. Water doesn't need to be pushed up from below at all, Dixon and Joly argued, as it's being pulled up from above, by the constant evaporation of water at the top.
The theory faced serious scepticism for years, since it required the water column inside the narrow xylem tubes to withstand enormous tension without breaking or forming air bubbles that would interrupt the chain, a physically demanding claim that took further decades of careful experimental work by other scientists to fully confirm. Today, cohesion-tension theory is the accepted explanation for how the tallest trees on Earth manage to move water against gravity, using no muscles, no pump, and no moving parts at all.
Journal Prompt:
Root pressure wasn't a foolish idea — it genuinely does happen in plants. Why might a real, correct observation still lead to an incorrect overall conclusion if it's assumed to be the whole explanation?
Dixon and Joly's theory depended on a property of water you already met back in Week 3. Can you explain, in your own words, how hydrogen bonding makes cohesion-tension theory possible?
This theory faced serious scientific scepticism for years before being widely accepted, even though it was eventually shown to be correct. Why might a genuinely correct idea still take a long time to be accepted by the scientific community?
Scientist Portrait
Henry Horatio Dixon (1869–1953)
Add Henry Horatio Dixon to your Scientist journal. Draw or print a portrait. Beneath it, write: "He showed that trees don't push water up from their roots — they pull it up from their leaves, one sticky water molecule at a time."
Return to Your Predictions
Go back to the two statements from the start of the week:
"A leaf's surface is basically solid, gases can't really pass in and out of it directly."
"Plants only need to exchange gases during the day, because that's when photosynthesis happens."
Rewrite each one now, using what you've learned this week.
The Big Question — Final Answer
Return to this week's big question:
"If a plant needs to let carbon dioxide in to survive, how does it stop itself from drying out in the process?"
Write a paragraph in your learning journal. A strong answer will mention stomata, guard cells, and the trade-off between gas exchange and water loss.
Practice Questions — 10 Question Set
Name the pores found on the underside of a leaf that allow gas exchange.
What is the role of guard cells?
Define transpiration.
Name two environmental factors that affect the rate of transpiration.
Explain why most stomata are found on the lower surface of a leaf rather than the upper surface.
In the leaf peel practical, what does a higher number of stomata per field of view suggest about a leaf?
Explain, using the term water potential, why water moves out of a leaf cell into the air spaces inside a leaf during transpiration.
What did the root pressure theory correctly explain, and where did it fall short?
Explain, in your own words, how cohesion-tension theory accounts for water movement in a very tall tree.
Name the plant tissue responsible for transporting water from roots to leaves.
Week 12 Checklist
Before moving to Week 13, tick off each item:
Watched Crash Course Botany: What Do These Creepy Plant Mouths Do?
Drawn and labelled a cross-section of a leaf
Watched Crash Course Biology #42: Plants Are Hardcore
Completed the leaf peel practical and recorded your observations
Read the root pressure/cohesion-tension theory story and answered the journal prompts
Added Henry Horatio Dixon 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 13 Preview
Next week we turn to human gas exchange, following air on its journey deep into the lungs, meeting the tiny structures where oxygen and carbon dioxide actually cross into and out of the blood, and looking honestly at what smoking does to this delicate system.
Prepare by writing this question in your journal: "What do you think happens, structurally, inside a smoker's lungs that doesn't happen inside a non-smoker's?"
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