GCSE · Biology · Edexcel · Spec 1BI0
Human physical and chemical defences
Before your body works out what a pathogen even is, walls, traps and acid are already waiting for it. Let's follow one breathed-in germ.
Follow the pathogen
A pathogen has just been breathed in. Tap each stage to see what is going on and why it matters.
Biology · First defences
Sort the defences
Pick a defence, then pick where it belongs. Start with what it does to a pathogen, then switch the rule to physical or chemical.
What does this defence do to a pathogen?
Still to sort
Keeps it out (0)
A barrier the pathogen can't get through.
Where the line is: Keeping out stops pathogens getting in. Trapping deals with the ones that are already close.
Traps or removes it (0)
Catches the pathogen, or carries it away.
Where the line is: A trapped or removed pathogen is not dead yet. Only the next group kills.
Kills it (0)
Destroys the pathogen.
Where the line is: If it destroys the pathogen it belongs here, whether it is in tears, saliva or the stomach.
Eight defences, two different rules. Do they line up?
Repairing the barrier
Reason it through
A cut breaks the skin. How does the body put the barrier back?
First link · your turn
What does a cut do to the skin barrier?
Predict, then check
Think back to the journey from the trachea to the stomach. What if one stage broke?
Smoking can damage the cilia in the trachea. What happens to the mucus and the pathogens trapped in it?
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
How the body keeps pathogens out, traps them, or kills them, without needing to recognise them
What you need to know
- Your body's first defences are non-specific: they work against all pathogens, not just particular ones.
- Physical defences keep pathogens out, trap them or move them on. Chemical defences kill them.
Have a goA defence kills the pathogen it meets. Is it physical or chemical?
Chemical.
Chemical defences kill pathogens, while physical ones keep them out, trap them or move them on.
- Skin is a physical barrier that covers nearly the whole body.
- If skin is cut, platelets stick together to form a clot, which seals the wound and forms a scab.
Have a goYour friend insists a scab is just crusty leftovers that do nothing. Give them one reason to think again.
The scab comes from the clot, which seals the wound so it keeps pathogens out.
It feels like a mess left behind, but it is the sealed barrier, and a sealed wound has no gap for pathogens to use.
- In the nose, tiny hairs and mucus trap dirt and dust that may carry pathogens. The mucus can be blown out.
- Tears contain lysozymes, enzymes that can kill pathogens. Saliva has lysozymes too.
- Goblet cells lining the trachea and bronchi make mucus that traps dirt, dust and pathogens.
- Ciliated cells have tiny finger-like cilia that beat rhythmically, moving the mucus up to the throat to be swallowed.
Have a goGoblet cells and cilia work as a team in the airways. Which one makes the mucus, and which one moves it?
Goblet cells make it. Cilia move it.
The two jobs are easy to swap, but making mucus and moving mucus belong to different cells.
- Stomach acid is hydrochloric acid, a chemical defence that kills pathogens in food and in swallowed mucus.
- Smoking can damage cilia. Then mucus isn't moved out, pathogens are held in the trachea, and lung infections become more likely.
The big picture
Your body's first defences don't need to recognise a pathogen. They work against all of them (they're non-specific). Physical defences like skin, blood clots, nose hairs, mucus and cilia keep pathogens out or trap and move them on. Chemical defences like lysozymes and stomach acid kill them. In the airways the two team up: cilia beat pathogen-laden mucus up to the throat, it's swallowed, and stomach acid finishes the job.
Key points
Worked example
Problem
A pathogen is on a piece of food that goes into your mouth. Describe how the body’s defences could deal with it from there.
⚠ Watch out
Thinking that swallowed mucus lets pathogens in, or that each defence is aimed at one particular pathogen. Swallowed mucus carries pathogens to stomach acid, which kills them, and these defences work against all pathogens.
Memory hook
Out, trap, kill. Skin and clots keep pathogens out. Hairs, mucus and cilia trap them and move them on. Lysozymes and stomach acid kill them.
Check yourself
Without looking back: pick one physical defence and one chemical defence. For each, say where in the body it is and what it does to a pathogen.
Flashcards
(11)What does non-specific mean for the body’s first defences?
What kind of defence is the skin?
What do platelets do when the skin is cut?
What do tiny hairs and mucus in the nose do?
What is the job of goblet cells in the trachea and bronchi?
What are cilia, and what do they do?
Where do the pathogens in swallowed mucus end up?
What acid is in the stomach, and what does it do?
What are lysozymes, and where are they found?
Why are smokers more likely to get lung infections?
In one phrase, what do physical and chemical defences each do?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
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