GCSE/iGCSE Biology- Year 1 Week 5
Updated: Aug 28
WEEK 5: The Kingdom of the Small
Bacteria, Viruses, and the Question of What Counts as Alive
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 something can infect you, reproduce, and even evolve, but isn't made of cells and can't survive on its own, is it actually alive?"
Write this in your learning journal as a title before you begin. You wrote this question down at the end of Week 4, go back and read what you predicted then, if you wrote anything. Don't change it yet.
Print out and use these cards to warm up and help you remember the organelles and their functions.
Before You Begin — Predict!
Write down whether you think these are true or false, and why:
"Bacteria are basically just tiny, simple versions of the animal and plant cells you learned about last week."
"A virus is a kind of very small, very simple bacterium."
Hold onto your predictions. We will return to them at the end of the week.
Story of the Week
Right up until 1977, biologists thought they had life's family tree pretty well sorted. There were eukaryotes, cells with a nucleus, like the ones you spent all of last week exploring. And there were prokaryotes, cells without one, which everybody lumped together under a single label: bacteria.
Then an American microbiologist named Carl Woese did something almost nobody else had thought to try. Instead of comparing what organisms looked like under a microscope, he compared their genetic sequences directly. What he found split the "bacteria" category clean in two. Some of these nucleus-free cells were bacteria in the way we usually mean the word. But others turned out to be so genetically different from bacteria, and, in some ways, so strangely closer to us, that Woese proposed they belonged in an entirely separate group: the Archaea. Many archaea live in places that would kill almost anything else on Earth: boiling volcanic vents, saturated salt lakes, the crushing dark of the deep sea floor.
Today biologists recognise three great domains of life: Bacteria, Archaea, and Eukarya (that's you, every animal, every plant, every fungus). All three share the same basic chemistry of life you met in Week 3. But only two of the three domains build their cells around a nucleus.
And then there's a fourth category that doesn't sit inside any domain at all, because most biologists don't consider it truly alive. This week we meet both: the astonishingly successful, nucleus-free world of bacteria, and the stranger, more contested world of viruses.
Video Session One
Crash Course Biology #38 — Bacterial DNA and Genetics
While you watch, look out for:
The structural differences between a prokaryotic (bacterial) cell and the eukaryotic cells you studied last week, particularly what a bacterial cell is missing
What a plasmid is, and how bacteria trade genetic material between each other
Why bacteria are not simply "germs", how scientists have put bacterial DNA to work producing something as important as human insulin.
Draw a simple labelled diagram of a bacterial cell in your journal: cell wall, cell membrane, cytoplasm, circular DNA (in the nucleoid region, since there's no nucleus), plasmids, and ribosomes. Compare it side by side with the animal cell diagram you drew last week.
A good place to stop is once the video ends, if you need to.
Video Session Two
While you watch, look out for:
What a virus is actually built from, and why the phrase "genes in a box" is such a good description
How a virus gets a living cell to make copies of it, since a virus cannot reproduce by itself
The basic idea of how a vaccine works (we'll come back to this topic properly and in much more detail in Week 20, so a general sense is all you need for now).
⚠️ Common Misconception Alert
It's easy to assume "bacteria = bad" and "virus = a very small bacterium," but both ideas are wrong. The overwhelming majority of bacteria on and inside your body are harmless or actively helpful, you could not digest food properly without them. And a virus is not a tiny bacterium at all. Bacteria are living cells that can grow and divide on their own. A virus has no cell structure whatsoever, no cytoplasm, no ribosomes, nothing that can carry out a chemical reaction by itself. It is, essentially, a scrap of genetic instructions wrapped in a protein coat, and it is completely helpless until it finds a living cell to hijack.
Reading Assignment
Read stories 8 and 9.
Here is a good place to stop if you need to.
Hands-On Activity — The Three-Circle Comparison
What you need: Paper, a pencil, and three overlapping circles drawn in the style of a Venn diagram (or three separate boxes if you'd rather compare side by side).
What to do:
Label the three sections Animal Cell, Bacterial Cell, and Virus. For each one, sort the following features into the correct section (some features belong to more than one, so they go in the overlaps):
Has a nucleus · has a cell membrane · has a cell wall · has ribosomes · has cytoplasm · genetic material is DNA or RNA · can reproduce without help from another organism · is generally agreed to be "alive"
When you've finished sorting, write two or three sentences in your journal explaining why biologists still argue about whether to call viruses "alive."
The Brilliant Mistake — Stories Science Gets Wrong
This Week's Story: The Doctor Who Was Right Too Soon
In the 1840s, a Hungarian doctor named Ignaz Semmelweis worked at a Viennese hospital with two maternity wards. In one ward, staffed by midwives, roughly 1 in 50 mothers died shortly after giving birth from a condition called childbed fever. In the other ward, staffed by doctors and medical students, the death rate was five to ten times higher. Nobody could explain why the ward with more highly trained staff had a far worse survival rate.
Semmelweis noticed something the other doctors had missed: the medical students in the worse-performing ward often came straight from performing autopsies to delivering babies, without washing their hands in between. He had no concept of "germs", that idea was still decades away, but he suspected something invisible was being carried from the dead to the living on the doctors' hands.
Semmelweis ordered doctors on his ward to wash their hands in a chlorinated solution before every delivery. The death rate on his ward immediately dropped by more than ninety percent.
You might expect this to have made Semmelweis famous. Instead, the medical establishment largely rejected and mocked him. Handwashing had no accepted scientific explanation yet, doctors were offended by the implication that they themselves were the source of infection, and Semmelweis's evidence, though overwhelming, arrived before anyone understood why it worked. He died in 1865, in an asylum, having never seen his idea widely accepted. It was only after Louis Pasteur and Robert Koch established germ theory properly, years later, that Semmelweis was recognised as having been right all along.
Journal Prompt:
Semmelweis had strong evidence but no accepted explanation for why it worked. Why do you think that made it so easy for other doctors to dismiss him?
What does this story suggest about the difference between having the right answer and being able to persuade other people that you have the right answer?
Can you connect this to Robert Hooke's story from Week 4, or the vital force story from Week 3, is there a pattern in how new scientific ideas tend to get treated?
Scientist Portrait
Carl Woese (1928–2012)
Add Carl Woese to your Scientist journal. Draw or print a portrait. Beneath it, write: "He looked past what cells looked like, compared their genes instead, and discovered an entire domain of life nobody knew was there."
Return to Your Predictions
Go back to the two statements from the start of the week and think about how you would now answer them:
"Bacteria are basically just tiny, simple versions of the animal and plant cells you learned about last week."
"A virus is a kind of very small, very simple bacterium."
Complete the worksheet below:
The Big Question — Final Answer
Return to this week's big question:
"If something can infect you, reproduce, and even evolve, but isn't made of cells and can't survive on its own, is it actually alive?"
Write a paragraph in your learning journal. There is genuinely no single agreed answer among scientists, so a strong response won't just pick a side, it will explain the reasoning on both sides, using what you now know about the differences between cells and viruses.
Practice Questions — 10 Question Set
Name three structures found in a bacterial cell.
Name two structures found in an animal cell but not in a bacterial cell.
What is a plasmid, and why is it useful to scientists?
Explain why a virus cannot reproduce without a host cell.
Give two reasons why "all bacteria are harmful" is a misconception.
What are the three domains of life, and which domains lack a nucleus?
What discovery led Carl Woese to propose the domain Archaea?
What did Ignaz Semmelweis observe about the two maternity wards, and what solution did he propose?
Why was Semmelweis's idea rejected by most doctors at the time, even though his evidence was strong?
A student says, "Viruses are just very small bacteria." Using this week's learning, explain why this is incorrect.
Week 5 Checklist
Before moving to Week 6, tick off each item:
Watched Crash Course Biology #38: We're Full of Bacteria!
Drawn and labelled a bacterial cell diagram
Watched Crash Course Biology #39: Viruses & Vaccines
Completed the Three-Circle Comparison activity
Read the Ignaz Semmelweis story and answered the journal prompts
Added Carl Woese 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 6 Preview
Next week we return to the eukaryotic cell, but this time we ask a much stranger question. Every single cell in your body was produced by one fertilised egg cell dividing over and over again, and every one of those cells carries an identical copy of your DNA. So how does a skin cell end up completely different from a nerve cell, or a red blood cell, if they're all working from exactly the same genetic instructions?
Prepare by writing this question in your journal:
"If every cell in your body has the exact same DNA, how does a skin cell end up different from a nerve cell?"
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