One handout per topic, in plain English. Read the handout before the textbook, not after it — each one takes about five minutes and is designed to make the idea land first, so the formal version has somewhere to stick.
19 handoutsCambridge O LevelPrintableFree to copy and share
Every living thing is built from cells, and each part of a cell has one job it is shaped to do.
Picture itA factory has an office, a power room, a packing department and a wall around the outside. A cell has exactly the same divisions of labour — nucleus, mitochondria, Golgi, membrane — just at a scale you need a microscope to see.
The parts and what they do
Nucleus: holds the DNA and controls the cell. Cytoplasm: where reactions happen. Cell membrane: controls what enters and leaves. Mitochondria: release energy by respiration. Ribosomes: build proteins. Plants add three more: a cell wall for support, chloroplasts for photosynthesis, and a large vacuole for storage and turgor.
Plant and animal cells differ in three things
Only plants have a cell wall, chloroplasts and a large permanent vacuole. Everything else is shared. If a question asks you to compare, name those three and you have the marks.
Shape follows function
A red blood cell has no nucleus, so there is more room for haemoglobin. A root hair cell has a long extension, so there is more surface area to absorb water. A nerve cell is long and thin, so it can carry signals a long way. Whenever you see an odd shape, ask what job it makes easier.
Cells build up into bodies
Cells → tissues → organs → organ systems → organism. A tissue is a group of similar cells doing one job; an organ combines tissues; a system combines organs.
The bit that catches people outBacteria are cells too, but they have no nucleus — their DNA floats loose in the cytoplasm. They also have no mitochondria or chloroplasts. Calling the bacterial DNA a nucleus is a common and costly slip.
The grown-up words
What it means
What it is called
Note
Controls the cell, contains DNA
nucleus
Absent in bacteria and red blood cells
Releases energy from glucose
mitochondria
Many in active cells
Controls what enters and leaves
cell membrane
Partially permeable
Rigid outer layer of a plant cell
cell wall
Made of cellulose
Where photosynthesis happens
chloroplast
Contains chlorophyll
Group of similar cells doing one job
tissue
Check you have got it
Why do muscle cells contain far more mitochondria than fat cells?
Muscle cells respire much faster because contraction needs a lot of energy, and mitochondria are where respiration releases it.
Name three structures found in a plant cell but not an animal cell.
Cell wall, chloroplasts and a large permanent vacuole.
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Topic 2
Classification
Biologists sort living things into groups by shared features, so that a name tells you what something is like.
Picture itIf someone says they saw a 'mammal', you already know it has fur, warm blood and feeds its young on milk — without ever seeing it. That is what classification buys you: a name that carries information.
The system runs from broad to specific
Kingdom, phylum, class, order, family, genus, species. Each level is a smaller, more closely related group. Every organism gets a two-word name from the last two levels — Homo sapiens, Panthera leo. That is the binomial system, and it is used worldwide so scientists in different languages mean the same organism.
A species is defined by breeding
Members of one species can breed together to produce fertile offspring. A horse and a donkey can breed, but the mule they produce is sterile — so they are different species.
The five kingdoms
Animals, plants, fungi, protoctists and prokaryotes. Learn one or two defining features of each: animals feed on others and move; plants photosynthesise and have cell walls; fungi have cell walls of chitin and feed by absorbing; prokaryotes have no nucleus.
Vertebrate groups you must know
Fish, amphibians, reptiles, birds, mammals — distinguished by skin covering, how they reproduce, and whether they control their own body temperature. Learn one unmistakable feature each: scales and gills, moist skin, dry scaly skin, feathers, fur and milk.
The bit that catches people outModern classification increasingly uses DNA sequences, not just appearance. Two animals can look similar because they live the same way rather than because they are related — a dolphin looks like a fish but is a mammal. DNA settles the question that looks alone cannot.
The grown-up words
What it means
What it is called
Note
Two-word naming system
binomial nomenclature
Genus then species
Group that can breed to give fertile young
species
The mule test
Broadest classification group
kingdom
Five of them
Animal with a backbone
vertebrate
Five classes
Key based on paired either/or questions
dichotomous key
Used to identify organisms
Similar because of shared ancestry
related
DNA is the modern evidence
Check you have got it
A horse and a donkey produce a mule, which cannot reproduce. Are they the same species?
No. Members of one species must produce fertile offspring, and the mule is sterile.
Why is DNA a better guide to relationships than body shape?
Unrelated organisms can evolve similar shapes for similar lifestyles, but DNA sequence similarity reflects actual shared ancestry.
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Topic 3
Movement into and out of cells
Substances get in and out of cells in three ways, and only one of them costs the cell any energy.
Picture itPut a raisin in water and it swells. Put a grape in salty water and it shrivels. Nobody pumped anything — water moved on its own, down a gradient, through the skin. That is osmosis doing exactly what it always does.
Diffusion: spreading out on their own
Particles move randomly, so they spread from where there are many to where there are few — from high to low concentration. No energy is needed; it happens by itself. Oxygen enters your blood in the lungs this way.
Osmosis is diffusion of water, through a membrane
Water moves from a dilute solution (lots of water) to a concentrated one (less water), through a partially permeable membrane. The membrane lets water through but blocks the dissolved solute, so only the water can even out.
Active transport goes uphill and costs energy
Sometimes a cell needs to take in a substance that is already more concentrated inside — against the gradient. That needs carrier proteins and energy from respiration. Root hair cells absorb minerals this way, which is why they are packed with mitochondria.
Plant and animal cells react differently to water
A plant cell in pure water swells until the rigid wall stops it — turgid, which is what holds a plant upright. In concentrated solution it loses water and the contents shrink away from the wall — plasmolysed. An animal cell has no wall, so in pure water it bursts and in concentrated solution it shrivels.
The bit that catches people outOsmosis is not 'water moving to where there is more salt' as a purposeful act. Water moves because it is more concentrated on one side. Always describe it as water moving from a dilute to a concentrated solution — or, better, from high to low water concentration.
The grown-up words
What it means
What it is called
Note
High to low concentration, no energy
diffusion
Gases and dissolved substances
Diffusion of water through a membrane
osmosis
Dilute to concentrated solution
Against the gradient, uses energy
active transport
Needs carrier proteins and respiration
Lets some substances through, not others
partially permeable
The cell membrane
Swollen and firm plant cell
turgid
What keeps a plant standing
Contents pulled away from the wall
plasmolysed
Plant cell in concentrated solution
Check you have got it
Why does a wilted plant recover when watered?
Water enters the cells by osmosis, making them turgid again, and turgor pressure holds the plant upright.
Root hair cells contain many mitochondria. What does that tell you about how they absorb minerals?
They use active transport, which requires energy from respiration — and mitochondria release that energy.
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Topic 4
Biological molecules
Four families of molecule build every living thing, and each is made by joining small units into long chains.
Picture itLego bricks come in a few shapes, but what you build depends entirely on which bricks and in what order. Life uses the same trick: about twenty amino acids, joined in different orders, make every protein in every organism on Earth.
Carbohydrates: energy, fast
Made of carbon, hydrogen and oxygen. Small units are simple sugars like glucose; join many and you get starch (plant storage), glycogen (animal storage) or cellulose (plant cell walls). Same building block, different arrangements, completely different jobs.
Fats: energy, dense
Made from glycerol and fatty acids. They store more energy per gram than carbohydrate, which is why the body uses them for long-term storage, and they also insulate and protect organs.
Proteins: everything else
Built from amino acids joined in a chain, which then folds into a specific shape. That shape is the function — enzymes, antibodies, haemoglobin, muscle fibres and hormones are all proteins. Proteins contain nitrogen, which carbohydrates and fats do not.
The food tests
Starch: iodine solution, yellow-brown to blue-black. Reducing sugar: Benedict's solution, heat, blue to brick-red. Protein: biuret, blue to purple. Fat: ethanol emulsion test, cloudy white. These come up in every practical paper.
The bit that catches people outA negative Benedict's test does not mean 'no sugar'. It means no reducing sugar. Sucrose is a sugar and gives a negative result until it is broken down first. Writing 'no sugar present' throws away the mark.
The grown-up words
What it means
What it is called
Note
Simple sugar, the body's main fuel
glucose
Tested with Benedict's
Plant energy store
starch
Tested with iodine
Animal energy store
glycogen
Stored in liver and muscle
Building block of protein
amino acid
About twenty kinds
Contains nitrogen as well as C, H and O
protein
Tested with biuret
Energy store with the most per gram
fat
Tested with ethanol emulsion
Check you have got it
Which food group contains nitrogen, and how does that help you identify it?
Protein. Carbohydrates and fats contain only carbon, hydrogen and oxygen.
A student tests a solution with Benedict's and it stays blue. Can they conclude there is no sugar?
No — only that there is no reducing sugar. Sucrose would need to be hydrolysed first.
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Topic 5
Enzymes
Enzymes are protein catalysts that make reactions fast enough for life — and their power comes entirely from their shape.
Picture itDigesting a meal without enzymes would take weeks. With them it takes hours. Nothing about the chemistry changed; a molecule shaped to fit simply made the reaction vastly easier to start.
Shape is everything
Each enzyme has an active site shaped to fit one particular substrate, like a key in a lock. That is why enzymes are specific — amylase digests starch and nothing else.
They lower the energy barrier
Reactions need a push to get started. An enzyme provides a route that needs a much smaller push, so far more collisions succeed. The enzyme itself is unchanged and can be used again immediately.
Temperature: up, then off a cliff
Warming speeds things up because molecules collide more often. But above the optimum the protein's shape is destroyed — denatured — the active site no longer fits, and the reaction stops. This is permanent; cooling does not fix it.
pH has an optimum too
Each enzyme works best at a particular pH and is denatured outside it. Stomach protease works at about pH 2; enzymes in the small intestine work at about pH 8. That is why bile neutralises stomach acid before intestinal digestion.
The bit that catches people outEnzymes are never 'killed'. They are denatured — a protein is not alive. And it is not that the enzyme 'stops working because it is too hot'; it is that the active site changes shape so the substrate no longer fits. Say that and the mark is yours.
The grown-up words
What it means
What it is called
Note
The pocket the substrate fits into
active site
Shape decides specificity
The molecule an enzyme acts on
substrate
One enzyme, one substrate
Shape destroyed by heat or pH
denatured
Permanent — not 'killed'
Speeds a reaction without being used up
catalyst
Enzymes are biological catalysts
Temperature or pH where it works best
optimum
Different for each enzyme
Energy needed to start a reaction
activation energy
Enzymes lower it
Check you have got it
Why does boiling stop an enzyme working permanently, but freezing does not?
Boiling denatures the enzyme — the active site's shape is destroyed irreversibly. Freezing only slows the molecules down, and warming restores activity.
Amylase does not digest protein. Why not?
Its active site is shaped only for starch. Protein molecules do not fit, so no reaction occurs.
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Topic 6
Plant nutrition
Plants build their own food out of air and water, using light as the power source — and everything else on Earth eats because of it.
Picture itA tree weighing several tonnes grew from a seed in soil that lost almost no mass. Nearly all of that wood came out of the air, as carbon dioxide, assembled using sunlight. It is one of the strangest true facts in biology.
The reaction
carbon dioxide + water → glucose + oxygen, using light energy trapped by chlorophyll in chloroplasts. The oxygen is a by-product — the plant's purpose is the glucose.
Three things can limit the rate
Light intensity, carbon dioxide concentration and temperature. Whichever is in shortest supply is the limiting factor. Increase it and the rate rises; increase the others and nothing happens. That is why greenhouses add CO₂ and heat as well as light.
Leaves are built for the job
Broad and flat for a large surface area, and thin so gases diffuse quickly. A waxy cuticle stops water loss. Palisade cells packed with chloroplasts sit near the top, where the light is. Air spaces in the spongy layer let gases move around inside. Stomata underneath, opened and closed by guard cells, let carbon dioxide in.
Minerals matter too
Photosynthesis makes carbohydrate, but plants need more. Nitrate for making amino acids and proteins — without it, growth is stunted and leaves yellow. Magnesium for making chlorophyll — without it, leaves also yellow. Both deficiencies look similar, so read the question carefully.
The bit that catches people outPlants respire all the time, including in daylight. They only photosynthesise when there is light. During the day photosynthesis usually outpaces respiration so there is a net oxygen output — but respiration never stops.
The grown-up words
What it means
What it is called
Note
Green pigment that traps light
chlorophyll
Held in chloroplasts
The factor in shortest supply
limiting factor
Light, CO₂ or temperature
Pore in the leaf for gas exchange
stoma
Opened by guard cells
Packed with chloroplasts, near the top
palisade mesophyll
Where most photosynthesis happens
Needed to make amino acids
nitrate
Deficiency stunts growth
Needed to make chlorophyll
magnesium
Deficiency yellows the leaves
Check you have got it
A greenhouse already has bright light and warmth. What else could a grower add to increase growth, and why?
Carbon dioxide — if light and temperature are plentiful, CO₂ becomes the limiting factor.
Why must a plant be kept in the dark before a starch test?
So it uses up the starch already stored, and any starch found afterwards must have been made during the experiment.
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Topic 7
Transport in flowering plants
Plants have two separate plumbing systems: one carries water up, the other carries food both ways.
Picture itA tall tree lifts water a hundred metres with no pump and no moving parts. It works by evaporation at the top pulling on an unbroken thread of water, all the way down to the roots.
Two tissues, two jobs
Xylem carries water and dissolved minerals upwards only, in dead hollow tubes with thick walls. Phloem carries dissolved sugars up or down to wherever they are needed, in living tubes with sieve plates.
Water's journey
Soil → root hair cell by osmosis → across the root → up the xylem → into the leaf → evaporates from the surfaces and diffuses out through the stomata. That last step is transpiration, and it is what pulls the whole column up.
What changes the rate of transpiration
It goes up with light (stomata open), temperature (faster evaporation), and wind (moist air blown away). It goes down in high humidity, because the air outside is already damp so there is less of a gradient.
Root hair cells are shaped for absorption
A long thin extension gives an enormous surface area for taking in water and minerals — the same design principle as villi in the gut and alveoli in the lungs.
The bit that catches people outTranspiration is not a waste process the plant would rather avoid. It pulls water up, delivers dissolved minerals, and cools the leaves. The plant does lose water — but the loss is the price of a transport system that needs no energy.
The grown-up words
What it means
What it is called
Note
Carries water up, dead hollow tubes
xylem
Thick lignified walls
Carries sugars both ways, living tubes
phloem
Sieve plates
Loss of water vapour from the leaves
transpiration
Pulls the water column up
Long extension for absorbing water
root hair cell
Large surface area
Moving sugars from leaves to elsewhere
translocation
Happens in the phloem
Water moving up as an unbroken column
transpiration stream
Held together by cohesion
Check you have got it
Why does transpiration slow down on a humid day?
The air outside the leaf is already full of water vapour, so the concentration gradient is smaller and less water diffuses out.
A ring of bark is removed from a tree trunk. Why does sugar build up above the ring?
The phloem is in the bark, so removing it cuts the route sugars take downwards from the leaves.
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Topic 8
Human nutrition
Digestion is the process of breaking food into pieces small enough to cross into your blood.
Picture itA cheese sandwich is useless to your cells as a sandwich. Every molecule in it is far too big to get through a cell membrane. Digestion is simply the business of cutting it into pieces small enough to pass.
A balanced diet has seven components
Carbohydrate for energy, protein for growth and repair, fat for energy and insulation, vitamins, minerals, fibre and water. Learn one deficiency disease each: scurvy from vitamin C, rickets from vitamin D, anaemia from iron.
The journey and who does what
Mouth (amylase starts on starch, teeth increase surface area) → oesophagus (peristalsis) → stomach (protease, acid) → small intestine (most digestion and all absorption) → large intestine (water absorbed) → rectum.
Three enzyme families
Amylase breaks starch into maltose. Protease breaks protein into amino acids. Lipase breaks fats into fatty acids and glycerol. Learn which organ makes each and where each works.
Bile is not an enzyme
It is made in the liver, stored in the gall bladder, and does two things: it neutralises stomach acid so intestinal enzymes can work, and it emulsifies fat into small droplets, increasing surface area for lipase. It digests nothing itself.
Villi make absorption efficient
The small intestine is lined with millions of finger-like villi, each covered in microvilli — an enormous surface area. They are one cell thick with a rich blood supply, so the diffusion distance is short and the gradient stays steep.
The bit that catches people outBile emulsifies fat — it does not digest it. Emulsifying means breaking large droplets into small ones, which increases surface area so lipase can work faster. Writing that bile digests fat loses the mark every time.
The grown-up words
What it means
What it is called
Note
Physically breaking food up
mechanical digestion
Teeth, stomach churning
Enzymes breaking molecules apart
chemical digestion
Amylase, protease, lipase
Muscular waves pushing food along
peristalsis
Throughout the gut
Neutralises acid and emulsifies fat
bile
Made in the liver, no enzymes
Finger-like projection in the small intestine
villus
Huge surface area
Taking digested food into the blood
absorption
Happens in the small intestine
Check you have got it
Why does the small intestine have villi?
They give a very large surface area for absorption, and being one cell thick with a good blood supply they keep the diffusion distance short and the gradient steep.
Why must bile be released before intestinal enzymes can work well?
Stomach acid would denature them. Bile is alkaline and neutralises it, giving the enzymes their optimum pH.
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Topic 9
Human gas exchange
Your lungs exist to put oxygen into your blood and take carbon dioxide out — and every feature of them serves that one exchange.
Picture itSpread the inside of your lungs out flat and it would cover roughly half a tennis court. All that area is folded into your chest for one reason: diffusion is slow, so you need an enormous surface to get enough oxygen fast enough.
The route in
Nose and mouth → trachea → bronchi → bronchioles → alveoli. Rings of cartilage hold the airways open. Ciliated cells sweep mucus, made by goblet cells, up and away from the lungs, carrying trapped dust and bacteria with it.
Alveoli are built for diffusion
Huge total surface area; walls one cell thick and capillary walls one cell thick, so the distance is tiny; a dense capillary network and constant breathing keep a steep concentration gradient; and the moist lining lets gases dissolve.
Breathing is mechanics, not magic
To breathe in, the diaphragm flattens and the ribs move up and out. The chest volume increases, the pressure inside falls below atmospheric, and air rushes in. Breathing out is the reverse — mostly by elastic recoil, which is why it takes no effort at rest.
Exercise and smoking
During exercise, muscles respire faster, so breathing rate and depth increase to supply oxygen and remove carbon dioxide. Smoking damages cilia so mucus collects, destroys alveolar walls reducing surface area (emphysema), and carbon monoxide takes up space on haemoglobin.
The bit that catches people outBreathing and respiration are different things. Breathing is the physical movement of air. Respiration is the chemical release of energy inside every cell. You can hold your breath, but you cannot pause respiration — and using the words interchangeably is a routine lost mark.
The grown-up words
What it means
What it is called
Note
Movement of air in and out
breathing / ventilation
A physical process
Releasing energy inside cells
respiration
A chemical process
Tiny air sac where exchange happens
alveolus
One cell thick
Sheet of muscle below the lungs
diaphragm
Flattens to breathe in
Hairs that sweep mucus upwards
cilia
Damaged by smoking
Keeping a difference in concentration
concentration gradient
Maintained by blood flow and ventilation
Check you have got it
Give three features of an alveolus that speed up gas exchange.
Large surface area, walls one cell thick for a short diffusion distance, and a rich blood supply that maintains a steep concentration gradient.
Why does smoking cause breathlessness?
Alveolar walls are destroyed, reducing surface area for gas exchange, and carbon monoxide occupies haemoglobin that would otherwise carry oxygen.
Edvia Free Resources · Biology 5090 · Topic 9 — free to copy and share
Topic 10
Respiration
Respiration releases the energy stored in glucose — in every cell, all the time, whether you are running or asleep.
Picture itYou are warm right now. That heat is not stored sunshine; it is being released this second, in billions of your cells, by breaking glucose apart. Stop respiring and you go cold within hours.
Aerobic respiration: with oxygen
glucose + oxygen → carbon dioxide + water, releasing a large amount of energy. It happens in the mitochondria, which is why hard-working cells have so many.
Anaerobic respiration: without oxygen
In humans, glucose → lactic acid, releasing much less energy. In yeast, glucose → ethanol + carbon dioxide — which is fermentation, and the basis of bread and brewing.
Why less energy without oxygen
The glucose is only partly broken down. Lactic acid and ethanol still contain a lot of unreleased energy. Aerobic respiration takes glucose all the way to carbon dioxide and water, so it extracts far more.
Oxygen debt explains why you pant after running
During hard exercise you cannot supply oxygen fast enough, so muscles respire anaerobically and lactic acid builds up, causing fatigue. Afterwards you keep breathing hard to take in the extra oxygen needed to break that lactic acid down — the oxygen debt.
The bit that catches people outRespiration is not breathing, and it is not something that only happens in the lungs. It happens in every living cell, including plant cells, twenty-four hours a day. The lungs only supply the oxygen.
The grown-up words
What it means
What it is called
Note
With oxygen, in mitochondria
aerobic respiration
Releases the most energy
Without oxygen
anaerobic respiration
Much less energy released
Made in muscles without oxygen
lactic acid
Causes fatigue
Yeast making ethanol and CO₂
fermentation
Used in bread and brewing
Extra oxygen needed afterwards
oxygen debt
Why you pant after sprinting
Where aerobic respiration happens
mitochondria
Numerous in active cells
Check you have got it
Why does anaerobic respiration release less energy than aerobic respiration?
The glucose is only partially broken down, so the products still contain a lot of unreleased chemical energy.
Why do you keep breathing heavily for several minutes after a sprint?
To repay the oxygen debt — the extra oxygen is needed to break down the lactic acid built up in the muscles.
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Topic 11
Transport in humans
A double circulation, a four-chambered pump, and blood that carries almost everything the body needs to move.
Picture itYour heart beats about 100,000 times a day without a single conscious instruction. It pushes blood through roughly 100,000 km of vessels — enough to circle the Earth twice — and it does this for decades without a service.
Double circulation, and why it is better
Blood passes through the heart twice per circuit: once to the lungs at low pressure, once to the body at high pressure. A single circulation would have to send blood to the body at whatever pressure survived the lungs — much lower, so delivery would be slower.
Vessels match their jobs
Arteries: thick muscular elastic walls, narrow lumen, carry blood away from the heart at high pressure. Capillaries: one cell thick, for exchange. Veins: thin walls, wide lumen, low pressure, and valves to stop backflow.
Four components of blood
Red cells carry oxygen using haemoglobin — no nucleus, biconcave for surface area. White cells fight disease by engulfing microbes or making antibodies. Platelets clot the blood. Plasma carries everything dissolved: carbon dioxide, food, hormones, urea, heat.
The heart's own design
Left side pumps to the body, so its wall is much thicker than the right, which only pumps to the lungs. Valves between atria and ventricles, and at the exits, stop backflow. Coronary arteries supply the heart muscle itself — blocking them causes a heart attack.
The bit that catches people outArteries carry blood away from the heart and veins carry it towards the heart — that is the definition, not 'arteries carry oxygenated blood'. The pulmonary artery carries deoxygenated blood to the lungs, and the pulmonary vein carries oxygenated blood back.
The grown-up words
What it means
What it is called
Note
Carries blood away from the heart
artery
Thick wall, high pressure
One cell thick, site of exchange
capillary
Short diffusion distance
Carries blood towards the heart
vein
Valves prevent backflow
Oxygen-carrying protein
haemoglobin
In red blood cells
Liquid part of blood
plasma
Carries CO₂, food, hormones, urea, heat
Blood passing through the heart twice
double circulation
Allows high body pressure
Check you have got it
Why is the wall of the left ventricle thicker than that of the right?
It pumps blood all round the body at high pressure, while the right only pumps to the nearby lungs at low pressure.
Why do veins need valves but arteries do not?
Blood in veins is at low pressure and could flow backwards; arterial blood is pushed strongly by the heart.
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Topic 12
Disease and immunity
Pathogens cause disease, and your body has layers of defence — some you are born with, some you build.
Picture itVaccination works by giving your immune system a rehearsal. It meets a harmless version of the enemy, makes the weapons, and remembers how. When the real thing arrives, the response is so fast you never feel ill.
Pathogens and how they spread
Bacteria, viruses, fungi and protoctists can all cause disease. They spread through air, water, food, direct contact, body fluids or animal vectors — a mosquito carrying malaria is a vector.
First line of defence: keeping them out
Skin as a barrier, mucus and cilia trapping and sweeping, stomach acid killing swallowed microbes, tears containing enzymes, and blood clotting to seal wounds. None of this targets a specific pathogen — it just blocks everything.
Second line: the specific immune response
Phagocytes engulf and digest pathogens non-specifically. Lymphocytes make antibodies, each shaped to fit one specific antigen, which clump pathogens together and mark them for destruction.
Memory is what makes immunity
After an infection, memory cells remain. Meeting the same pathogen again triggers a much faster and larger antibody response, so you do not get ill. Vaccination creates that memory deliberately, using a weakened or dead pathogen.
Antibiotics work on bacteria only
They damage bacterial structures like the cell wall. Viruses have no such structures and reproduce inside your own cells, so antibiotics do nothing. Overuse causes resistance: random mutations make some bacteria survive, and they are the ones that reproduce.
The bit that catches people outAntibiotic resistance does not happen because bacteria 'get used to' the drug. Resistant bacteria appear by random mutation before the antibiotic arrives. The antibiotic then kills the rest and leaves the resistant ones to multiply. It is selection, not adaptation.
The grown-up words
What it means
What it is called
Note
Organism that causes disease
pathogen
Bacteria, virus, fungus, protoctist
Animal that carries a pathogen
vector
Mosquito for malaria
Cell that engulfs pathogens
phagocyte
Non-specific
Protein that binds one specific antigen
antibody
Made by lymphocytes
Molecule the immune system recognises
antigen
On the pathogen's surface
Weakened pathogen given to build memory
vaccine
Creates active immunity
Check you have got it
Why do antibiotics not work against a cold?
A cold is caused by a virus. Antibiotics target bacterial structures, which viruses do not have.
Why does a second infection with the same pathogen usually cause no symptoms?
Memory cells from the first infection produce antibodies far faster and in greater quantity, destroying the pathogen before it can multiply.
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Topic 13
Excretion
Your body must get rid of the toxic waste it makes itself — and the kidneys do it while carefully keeping what is still useful.
Picture itYour kidneys filter your entire blood supply about sixty times a day. Almost everything filtered out is then taken straight back in. It looks wasteful, and it is actually the most reliable way to catch every last bit of waste.
Excretion is not egestion
Excretion is removing waste your body made — carbon dioxide from respiration, urea from breaking down excess protein. Egestion is passing out undigested food, which never entered your cells at all.
Where urea comes from
You cannot store excess amino acids. The liver breaks them down — deamination — producing urea, which is toxic and must go. The kidneys remove it.
Filter first, then take back what you need
In each nephron, high pressure forces small molecules out of the blood: water, glucose, salts and urea. Big things like proteins and blood cells stay behind. Then selective reabsorption takes back all the glucose, most of the water and the salts needed. What is left is urine.
Water balance is adjustable
If you are dehydrated, more water is reabsorbed and urine is small and concentrated. Drink a lot and less is reabsorbed, giving large volumes of dilute urine. The kidneys are not just a filter; they are a control system.
The bit that catches people outGlucose should never appear in urine. All of it is normally reabsorbed. Finding glucose in urine is a classic sign of diabetes, where blood glucose is so high the kidney cannot reabsorb it all — which is exactly why the test exists.
The grown-up words
What it means
What it is called
Note
Removing waste made by the body
excretion
CO₂ and urea
Passing out undigested food
egestion
Not excretion
Breaking down excess amino acids
deamination
Happens in the liver
Toxic nitrogenous waste
urea
Removed by the kidneys
Forcing small molecules out of the blood
ultrafiltration
In the glomerulus
Taking back glucose and water
selective reabsorption
All glucose is normally recovered
Check you have got it
Why is passing faeces not classed as excretion?
Faeces are undigested food that never entered the body's cells, so they are not waste produced by metabolism.
A urine test finds glucose. What does that suggest?
Blood glucose is abnormally high — typically diabetes — because normally all glucose is reabsorbed.
Edvia Free Resources · Biology 5090 · Topic 13 — free to copy and share
Topic 14
Coordination and control
Two systems keep your body steady and responsive: nerves for fast messages and hormones for slower, longer-lasting ones.
Picture itTouch something hot and your hand pulls back before you consciously feel the pain. The signal never reached your brain first — it took a shortcut through the spinal cord, because a fraction of a second matters when you are burning.
Nerves and hormones compared
Nervous: electrical, very fast, short-lived, precisely targeted. Hormonal: chemical, carried in the blood, slower, longer-lasting, and affects many places at once. Both exist because both kinds of control are needed.
The reflex arc
Receptor → sensory neurone → spinal cord (relay neurone) → motor neurone → effector. It bypasses conscious thought, which is what makes it fast and automatic. The gap between neurones is a synapse, crossed by chemical transmitters.
Homeostasis: keeping conditions steady
Body temperature, blood glucose and water content are all held within narrow limits by negative feedback — a change is detected and a response reverses it. Too hot: sweat, vasodilation. Too cold: shiver, vasoconstriction, hairs raised.
Blood glucose control
The pancreas detects blood glucose. Too high: it releases insulin, which makes the liver convert glucose to glycogen. Too low: it releases glucagon, which reverses that. In type 1 diabetes insulin is not produced, so glucose is not controlled.
The eye adjusts to light
In bright light the circular iris muscles contract and the pupil narrows, protecting the retina. In dim light the radial muscles contract and the pupil widens. Both are reflexes.
The bit that catches people outVasodilation does not mean blood vessels 'move to the surface'. The vessels stay where they are; the ones near the skin simply widen, so more blood flows through them and more heat is lost. Vessels do not travel.
The grown-up words
What it means
What it is called
Note
Fast electrical signalling
nervous system
Short-lived, precise
Slower chemical signalling in the blood
endocrine system
Long-lasting, widespread
Automatic protective response
reflex
Bypasses the conscious brain
Gap between two neurones
synapse
Crossed by chemicals
Keeping internal conditions constant
homeostasis
Runs on negative feedback
Lowers blood glucose
insulin
From the pancreas
Widening of blood vessels near the skin
vasodilation
Increases heat loss
Check you have got it
Why is a reflex action faster than a voluntary one?
The signal travels a short route through the spinal cord without going to the brain for a conscious decision.
How does the body respond to a fall in blood glucose?
The pancreas releases glucagon, which makes the liver convert stored glycogen back into glucose.
Edvia Free Resources · Biology 5090 · Topic 14 — free to copy and share
Topic 15
Coordination and response in plants
Plants cannot move, but they can grow in a chosen direction — and a single hormone does most of the work.
Picture itA seedling on a windowsill bends towards the glass within a day. Nobody turned it. It grew unevenly on purpose, because one side received less light and responded by stretching more.
Tropisms are growth responses with a direction
Phototropism is growth in response to light; gravitropism in response to gravity. Positive means growing towards the stimulus, negative means away. Shoots are positively phototropic and negatively gravitropic; roots are the reverse.
Auxin does it by making cells longer
Auxin is a plant hormone that causes cell elongation. It moves away from light, so it builds up on the shaded side of a shoot. That side's cells grow longer, and the shoot bends towards the light.
Roots respond in the opposite direction
In roots, a high auxin concentration inhibits elongation instead. Auxin gathers on the lower side under gravity, that side grows less, and the root curves downwards. Same hormone, opposite effect — because the tissue responds differently.
Why it matters to the plant
Growing towards light maximises photosynthesis. Growing roots downwards finds water and anchors the plant. These are not decisions; they are automatic growth responses that happen to be extremely useful.
The bit that catches people outA tropism is a growth response, not a movement. The plant does not lean over — it grows unevenly, permanently. That is why a bent seedling stays bent even after you move the light.
The grown-up words
What it means
What it is called
Note
Growth response to a directional stimulus
tropism
Growth, not movement
Response to light
phototropism
Shoots grow towards it
Response to gravity
gravitropism
Roots grow towards it
Plant hormone causing cell elongation
auxin
Moves away from light
Growing towards the stimulus
positive tropism
Shoot to light
Growing away from the stimulus
negative tropism
Shoot away from gravity
Check you have got it
Why does a shoot bend towards the light?
Auxin moves to the shaded side, where it makes cells elongate more, so that side grows longer and the shoot curves towards the light.
Auxin makes shoots grow but slows roots. Why is that useful?
It means shoots grow up towards light and roots grow down towards water and anchorage, using a single hormone.
Edvia Free Resources · Biology 5090 · Topic 15 — free to copy and share
Topic 16
Development of organisms and continuity of life
Cells divide in two different ways: one to build a body, the other to make sex cells — and the difference decides whether offspring are identical or unique.
Picture itYou started as a single cell. Every cell in your body came from it by division, and they all carry the same DNA. Yet a skin cell and a nerve cell look nothing alike — same instructions, different pages read.
Mitosis: copying exactly
One cell becomes two identical cells with the same number of chromosomes. Used for growth, repair, replacing worn cells, and asexual reproduction. Identical offspring are called clones.
Meiosis: halving and mixing
One cell becomes four cells with half the chromosome number, all genetically different. Used only to make gametes — sperm and egg. Fertilisation then restores the full number.
Why sexual reproduction produces variation
Meiosis shuffles the chromosomes, and which sperm meets which egg is random. So offspring differ from each other and from their parents. That variation is the raw material for evolution.
Sexual and asexual compared
Asexual is fast, needs no partner and preserves a successful set of genes — but a whole population is vulnerable to the same disease. Sexual is slower and needs a partner, but the variation means some individuals are likely to survive a change.
Flowers and human reproduction
In plants, pollination transfers pollen to the stigma; the pollen tube grows down to the ovule for fertilisation. In humans, know the reproductive organs, the menstrual cycle, fertilisation, implantation and the role of the placenta in exchanging materials between mother and fetus.
The bit that catches people outPollination and fertilisation are two different events. Pollination is pollen landing on the stigma. Fertilisation is the male nucleus fusing with the egg nucleus, and it happens later, after the pollen tube has grown down. Using them interchangeably loses marks.
The grown-up words
What it means
What it is called
Note
Division making two identical cells
mitosis
Growth and repair
Division making four different gametes
meiosis
Halves the chromosome number
Genetically identical offspring
clone
From asexual reproduction
Sex cell
gamete
Sperm, egg, pollen
Pollen landing on the stigma
pollination
Before fertilisation
Nuclei fusing
fertilisation
Restores the full chromosome number
Exchanges materials between mother and fetus
placenta
Food, oxygen, waste
Check you have got it
Why must gametes be made by meiosis rather than mitosis?
So each gamete has half the chromosome number, and fertilisation restores the normal number rather than doubling it each generation.
Give one advantage and one disadvantage of asexual reproduction.
Advantage: fast and needs no partner. Disadvantage: no variation, so the whole population is vulnerable to the same disease or change.
Edvia Free Resources · Biology 5090 · Topic 16 — free to copy and share
Topic 17
Inheritance
Features pass from parents to offspring through genes — and simple rules let you predict the outcome.
Picture itTwo brown-eyed parents can have a blue-eyed child. That is not a mistake or a scandal; it is exactly what the rules predict when both parents carry a hidden recessive allele.
The vocabulary is most of the battle
A gene is a length of DNA coding for a characteristic. An allele is a version of that gene. Homozygous means two identical alleles, heterozygous means two different ones. Genotype is the alleles you have; phenotype is what you actually look like.
Dominant hides recessive
A dominant allele shows even if only one copy is present. A recessive allele only shows if both copies are recessive. That is why a recessive condition can skip generations and appear unexpectedly.
Punnett squares do the work
Put one parent's gametes along the top, the other's down the side, and fill in the combinations. Always show parental phenotypes, genotypes, gametes and the ratio — the marks are given line by line, not just for the final answer.
Sex determination and sex linkage
Females are XX, males XY. A father's sperm decides the sex. Some genes sit on the X chromosome, so a male with one faulty copy has no second copy to mask it — which is why colour blindness and haemophilia are far commoner in males.
Variation comes from genes and environment
Continuous variation like height forms a smooth range and is influenced by many genes plus the environment. Discontinuous variation like blood group falls into distinct categories and is controlled by few genes. Mutation — a random change in DNA — is the ultimate source of new alleles.
The bit that catches people outWrite genotypes with consistent letters, using a capital for dominant and the same letter in lower case for recessive — Bb, not Bd. Different letters mean different genes, and mixing them up makes the whole cross unmarkable.
The grown-up words
What it means
What it is called
Note
Length of DNA coding for a characteristic
gene
A version of a gene
allele
One from each parent
Two identical alleles
homozygous
BB or bb
Two different alleles
heterozygous
Bb
The alleles present
genotype
Bb
The characteristic shown
phenotype
Brown eyes
Random change in DNA
mutation
Source of new alleles
Check you have got it
Two heterozygous brown-eyed parents (Bb) have a child. What is the chance of blue eyes (bb)?
One in four. The Punnett square gives BB, Bb, Bb, bb — so 25% are bb.
Why is colour blindness more common in males?
The gene is on the X chromosome. Males have only one X, so a single faulty allele is not masked, while females would need two.
Edvia Free Resources · Biology 5090 · Topic 17 — free to copy and share
Topic 18
Biotechnology and genetic modification
We can now move a gene from one organism into another — and use microorganisms as tiny factories.
Picture itBefore 1982, insulin for diabetics came from the pancreases of slaughtered pigs and cattle. Today bacteria carrying a human gene make it in vats. Same molecule, no animals, unlimited supply.
Microorganisms are useful because they are fast
Bacteria and yeast reproduce quickly, need only simple food, and can be grown in large vessels under controlled conditions. Yeast makes bread rise and produces alcohol; bacteria make yoghurt; both are used industrially.
Enzymes in industry
Isolated enzymes are used in biological washing powders (breaking down protein and fat stains at low temperature), in fruit juice production (pectinase increases yield), and in making lactose-free milk.
How genetic modification works
Cut the wanted gene out with restriction enzymes, which leave sticky ends. Cut a bacterial plasmid with the same enzyme so the ends match. Join them with ligase. Put the plasmid back into a bacterium, which now makes the human protein as it multiplies.
The arguments on both sides
GM crops can raise yields, resist pests and add nutrients — Golden Rice was engineered to carry vitamin A. Concerns include genes spreading to wild plants, effects on other species, reduced crop diversity, and farmers' dependence on seed companies. A good answer gives both and then judges.
The bit that catches people outA plasmid is a small circular loop of DNA separate from the bacterium's main chromosome. It is the vector — the delivery vehicle — not the gene itself. Calling the plasmid 'the gene' confuses the container with the contents.
The grown-up words
What it means
What it is called
Note
Small circular DNA loop in bacteria
plasmid
Used as a vector
Cuts DNA at a specific sequence
restriction enzyme
Leaves sticky ends
Joins DNA fragments together
ligase
Seals the backbone
DNA combined from two sources
recombinant DNA
The product of GM
Carrier that delivers the gene
vector
Usually a plasmid
Growing microorganisms in large vessels
fermentation
Industrial scale
Check you have got it
Why is the same restriction enzyme used to cut both the gene and the plasmid?
So both have matching sticky ends, which allows them to join together correctly.
Give one benefit and one concern about GM crops.
Benefit: higher yields or added nutrients such as vitamin A in Golden Rice. Concern: transgenes could spread to wild relatives, or crop genetic diversity could fall.
Edvia Free Resources · Biology 5090 · Topic 18 — free to copy and share
Topic 19
Relationships of organisms with one another and with the environment
Energy flows one way through an ecosystem and nutrients cycle round — and human activity disrupts both.
Picture itEvery meal you have ever eaten was powered by sunlight. Trace any food chain back far enough and it starts with a plant catching photons. There is no other significant entry point for energy into life on Earth.
Food chains show energy transfer
Producer → primary consumer → secondary consumer → tertiary consumer. The arrows point in the direction the energy flows, which is why they point from the eaten to the eater. A food web is many chains linked together.
Why chains are short
Only about 10% of energy passes to the next level. The rest is lost as heat from respiration, in movement, and in undigested waste. After four or five levels there is simply not enough energy left to support another.
Nutrients cycle, energy does not
Carbon cycles through photosynthesis, respiration, decomposition and combustion. Energy enters as sunlight and leaves as heat — it never comes back round. That is the single most important structural fact about ecosystems.
Decomposers close the loop
Bacteria and fungi break down dead material and release nutrients back into the soil for plants to reuse. Without them, nutrients would stay locked in dead bodies and life would stop.
Human impact
Deforestation reduces photosynthesis, causes soil erosion and destroys habitats. Burning fossil fuels raises carbon dioxide and drives climate change. Fertiliser run-off causes eutrophication, where algae bloom, block light, then decompose and strip the water of oxygen, killing fish. Overfishing and pollution reduce biodiversity.
The bit that catches people outThe arrows in a food chain point from the organism being eaten to the one eating it, because they show the direction of energy flow — not who eats whom in the order you would say it. Drawing them backwards is a guaranteed lost mark.
The grown-up words
What it means
What it is called
Note
Organism that makes its own food
producer
Always starts the chain
Organism that eats others
consumer
Primary, secondary, tertiary
Breaks down dead material
decomposer
Bacteria and fungi
Feeding level in a chain
trophic level
About 10% of energy passes on
Many food chains linked
food web
More realistic than a single chain
Fertiliser run-off killing river life
eutrophication
Algae bloom then decompose
Variety of living things
biodiversity
Reduced by human activity
Check you have got it
Why are food chains rarely longer than five links?
Only about 10% of the energy passes to each next level, so after a few levels there is not enough energy left to support another population.
Explain how fertiliser run-off can kill fish.
Nutrients cause algae to bloom, which blocks light so plants die. Decomposing bacteria multiply and use up the dissolved oxygen, and the fish suffocate.
Edvia Free Resources · Biology 5090 · Topic 19 — free to copy and share
Like how this is taught?
Every handout starts with the idea in plain English and only then the formal version. That is how every class at Edvia College works — for two full years of Cambridge A Levels.