GCSE/iGCSE Biology - Year 1 Week 3
Updated: Aug 28
What Is Every Living Thing Actually Made Of?
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
"If you had to build a living thing from scratch using only chemistry, where would you start?"
Write this in your learning journal as a title before you begin. Don't look anything up yet, just think. You already made a guess about this at the end of Week 2. Go back and read what you wrote. Does it still feel right?
Before You Begin — Predict!
Write down whether you think these are true or false, and why:
"Living things are made of completely different chemicals to the non-living world."
"Water is basically just there to keep you hydrated — it doesn't really do anything else important in your cells."
Hold onto your predictions. We will return to them at the end of the week.
Story of the Week
Every living thing you have ever seen, every oak tree, every jellyfish, every human being reading this sentence, is built from the same tiny toolkit of atoms. Strip away the fur, the bark, the shell, the skin, and you are left with almost the same six elements, arranged and rearranged in endless combinations: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. Biologists sometimes call this team CHNOPS, and carbon is the captain.
Why carbon? Because a single carbon atom can form four strong bonds at once, and it can bond to other carbon atoms to build chains, rings, and branching structures almost without limit, it is a very social atom! No other common element does this as well. That property is the entire reason biology is even possible. Without carbon's flexibility, there would be no proteins folding into precise 3D shapes, no DNA spiralling into a double helix, no fats storing energy for a hibernating bear.
And then there is water, the quiet, unglamorous molecule that gets far less credit than it deserves. Water is not just the stuff you drink. It is the solvent that every chemical reaction inside you happens in, the reason a water strider can walk on a pond's surface, the reason ice floats instead of sinking and killing every fish beneath it in winter. Life on Earth did not just happen to use water. Life on Earth could not exist without it.
This week we go smaller than cells, smaller than organelles, all the way down to the atoms and molecules that build every living thing, and we find out that biology, underneath it all, is chemistry with ambition.
Video Session One
While you watch, look out for:
Why carbon is described as the "atomic captain" of life's chemistry
The four major classes of biomolecules and what each one is built from
What a polymer is, and the difference between building one up (dehydration reaction) and breaking one down (hydrolysis)
Draw a simple table in your journal with four columns headed Carbohydrates, Lipids, Proteins, and Nucleic Acids. Under each, note the small building-block units they're made from (e.g. proteins are built from amino acids) and one example of each from the video.
A good place to stop is once the video ends, if you need to.
Video Session Two
While you watch, look out for:
Why water is described as a polar molecule, and what that means for hydrogen bonding
At least two of water's unusual properties (e.g. why ice floats, why water is such a good solvent)
What the pH scale actually measures, and why your body works hard to keep its internal pH within a narrow range
⚠️ Common Misconception Alert
Many students think water is chemically "boring" because it's so familiar. In fact, almost none of water's properties are what you'd predict from two hydrogen atoms and one oxygen atom stuck together. Its polarity, its hydrogen bonding, and its unusual solid-is-less-dense-than-liquid behaviour make it one of the strangest and most important molecules in the universe. Nothing about water is ordinary; it just seems that way because you're surrounded by it.
Reading Assignment
Story 4 and 5 in the Biology Readings.
Here is a good place to stop if you need to.
Hands-On Activity — Build a Molecule
What you need: Modelling clay or playdough in at least two colours, toothpicks or matchsticks, or a molecular-model building kit if you have one. Paper and coloured pens work too if you'd rather draw.
What to do:
Build (or draw) a simple carbon skeleton — one carbon atom bonded to four others, showing carbon's ability to form four bonds.
Now build a simple representation of a water molecule: one oxygen atom bonded to two hydrogen atoms, at an angle (not a straight line — this bent shape is why water is polar).
Label which end of your water molecule is slightly negative (near the oxygen) and which end is slightly positive (near the hydrogens).
In your journal, write two sentences explaining why this bent, lopsided shape gives water its unusual "sticky" properties (hydrogen bonding).
The Brilliant Mistake — Stories Science Gets Wrong
This Week's Story: The Search for "Vital Force"
For much of the 1700s and early 1800s, chemists believed that molecules made by living things, what we now call organic molecules, could only ever be produced by a mysterious "vital force" found in living organisms. No chemist, they believed, could ever create one of these molecules from ordinary, non-living chemicals in a laboratory.
In 1828, a German chemist named Friedrich Wöhler was trying to make an entirely different compound and accidentally produced urea, a molecule found in urine, and very much a product of living things, by heating two simple, non-living chemicals together. There was no vital force involved. Just ordinary chemistry.
Wöhler's accident quietly ended the idea of vital force and helped found the entire field of organic chemistry, the study of carbon-based compounds, which is, not coincidentally, the same chemistry that underpins every biological molecule you learned about this week.
Journal Prompt:
Why do you think "vital force" felt like a satisfying explanation to scientists at the time?
Wöhler wasn't even trying to disprove vital force when he made his discovery. What does this tell us about how scientific progress sometimes happens?
Can you connect this story back to what you learned in Week 1 about theories being overturned by evidence?
Scientist Portrait
Friedrich Wöhler (1800–1882)
Add Friedrich Wöhler to your Scientist journal. Draw or print a portrait. Beneath it, write: "He tried to make one thing and accidentally proved that life's chemistry is just chemistry."
Return to Your Predictions
Go back to the two statements from the start of the week:
"Living things are made of completely different chemicals to the non-living world."
"Water is basically just there to keep you hydrated — it doesn't really do anything else important in your cells."
Rewrite each one now, using what you've learned this week.
The Big Question — Final Answer
Return to this week's big question:
"If you had to build a living thing from scratch using only chemistry, where would you start?"
Write a paragraph in your learning journal. A strong answer will mention carbon's ability to form chains and rings, at least one of the four biomolecule classes, and why water is essential rather than incidental.
Practice Questions — 10 Question Set
Name the six elements that make up the vast majority of living matter (CHNOPS).
Why is carbon particularly suited to forming the "backbone" of biological molecules?
Name the four major classes of biomolecules found in living things.
What is a polymer? Give one biological example.
What is the difference between a dehydration reaction and hydrolysis?
Why is water described as a polar molecule?
Give two properties of water that make it essential for life.
What does the pH scale measure, and why does the human body need to regulate its internal pH?
Friedrich Wöhler accidentally made urea from non-living chemicals in 1828. Why was this significant for the history of biology and chemistry?
A student says, "Living things are made of magic chemicals that don't exist anywhere else." Using this week's learning, explain why this statement is incorrect.
Week 3 Checklist
Before moving to Week 4, tick off each item:
Watched Crash Course Biology #20: Carbon & Biological Molecules
Completed the four-column biomolecule table in your journal
Watched Crash Course Biology #21: The Unexpected Truth About Water
Completed the Build a Molecule activity
Read the Wöhler "vital force" story and answered the journal prompts
Added Friedrich Wöhler 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 4 Preview
Next week we zoom out and meet the cell properly, the basic unit of life itself. We'll look at the differences between plant, animal, fungal, and single-celled protoctist life, and start building the vocabulary you'll use for the rest of this course: nucleus, cytoplasm, membrane, and more.
Prepare by writing this question in your journal: "If someone shrank you down to the size of a single cell, what do you think you'd actually see?"
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