DNA is the master instruction manual for every living cell, written in a code of just four letters.
🔬 The double helix & the genetic code
DNA is a double helix made of two twisted strands held together by base pairs: Adenine always pairs with Thymine (A-T), and Cytosine always pairs with Guanine (C-G). A gene is a segment of DNA that codes for a specific protein.
Protein synthesis has 2 main steps: transcription (DNA's code is copied into RNA) and translation (RNA's code is used to build a protein).
Strand 1Strand 2Base pair rungs (A-T, C-G)
📝 Worked examples
Example 1 · Complementary strand
Pair A–T and C–G. For the strand A T G C:
Strand 1: A T G C
Strand 2: T A C G
Example 2 · Transcription
In RNA, thymine (T) is replaced by uracil (U). DNA template T A C G G A is copied into mRNA:
DNA: T A C G G A
mRNA: A U G C C U
Example 3 · Translation
Every group of 3 bases (a codon) codes for one amino acid. AUG is the start codon. A 12-base mRNA codes for 12 ÷ 3 = 4 amino acids.
📌 Where does it happen?
Transcription
DNA → mRNA, in the nucleus.
Translation
mRNA → protein, at the ribosome in the cytoplasm.
Central dogma: DNA → RNA → protein.
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Base pairing: Adenine
In DNA base pairing, Adenine (A) always pairs with:
Remember the pairs as “Apple in the Tree”.
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Base pairing: Cytosine
In DNA base pairing, Cytosine (C) always pairs with:
Remember the pairs as “Car in the Garage”.
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Defining a gene
A gene is best described as:
Genes are short stretches of a chromosome.
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DNA is built from 4 types of bases: A, T, C, and G.
How many types of bases does DNA have?
Enter an exact answer, without rounding.
Count the letters A, T, C and G.
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A DNA strand has 20 bases, and 8 of them are Adenine (A). Since A always pairs with T, the complementary strand has the same number of T bases opposite those A's.
How many T bases pair with those 8 A's?
Enter an exact answer, without rounding.
Each A has exactly one T opposite it.
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Protein synthesis has 2 main steps: transcription and translation.
How many main steps does protein synthesis have?
Enter an exact answer, without rounding.
Transcription and translation.
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RNA base
Which base is found in RNA but not in DNA?
It starts with the letter U.
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Where?
Translation (building a protein) takes place at the:
Ribosomes make proteins.
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An mRNA strand is 9 bases long.
How many codons does it contain?
Enter an exact answer, without rounding.
A codon is 3 bases.
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In a double-stranded DNA molecule there are 30 adenine bases.
How many thymine bases are there?
Enter an exact answer, without rounding.
Base pairing: A pairs with T.
📝 Lesson quiz · 5 questions
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Question 1 of 5
What is the complementary DNA strand of A T T G C?
Question 2 of 5
What mRNA is made from the DNA template T A C G G A?
Question 3 of 5
How many amino acids does a 15-base mRNA code for?
Question 4 of 5
Which molecule carries the code from DNA to the ribosome?
Question 5 of 5
How many chromosomes are in a normal human body cell?
Lesson 2 · Genetics
🧪 Gene Mutations
DNA copying isn't always perfect, and environmental factors can alter the code too — these changes, called mutations, can range from harmless to life-changing.
🧪 Changes in the code
A mutation is a change in a gene's DNA sequence. Mutations can be caused by errors during DNA replication, or by environmental factors like radiation or certain chemicals. Effects of mutations fall into 3 categories: silent (no noticeable effect), harmful (can disrupt protein function or cause disease), and beneficial (rarely, can provide an advantage).
Original basesChanged base (C → G)
📝 Worked examples
Example 1 · Types of mutation
Original: ATG CAT GGA
Substitution: ATG CCT GGA (one base swapped)
Insertion: ATG CAAT GGA (extra base added)
Deletion: ATG CT GGA (one base lost)
Example 2 · Frameshift (using words)
Imagine the codons as three-letter words: THE CAT SAW THE DOG. Delete the first E:
THC ATS AWT HED OG ← everything after the deletion is scrambled
Insertions and deletions shift the reading frame, so they usually do more damage than a single substitution.
Example 3 · Not all mutations are harmful
Some change nothing (silent). Rarely, a mutation is beneficial, and these are the raw material for evolution.
📌 Who inherits it?
Somatic mutations (in body cells) affect only that person and are not passed to children.
Germ-line mutations (in eggs or sperm) can be inherited.
Mutagens that raise the mutation rate include UV light, X-rays and some chemicals.
Sickle cell disease is caused by a single-base substitution in the haemoglobin gene.
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Defining a mutation
A mutation is best described as:
Mutate means to change.
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Causes of mutations
Mutations can be caused by:
Copying mistakes, plus things like radiation or chemicals.
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Mutations with no effect
A mutation with no noticeable effect on the organism is called:
It makes no difference you can hear or see.
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A DNA sequence of 10 bases has 1 base changed by a mutation.
How many bases remain unchanged?
Enter an exact answer, without rounding.
Take 1 from 10.
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Frameshift
Which change is most likely to cause a frameshift?
Which change shifts the reading frame?
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Inherited?
A skin cell mutation caused by too much sun is:
Is the mutation in a sex cell?
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A gene has 12 bases. One extra base is inserted.
How many bases are there now?
Enter an exact answer, without rounding.
Add the inserted base.
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A 15-base gene loses 3 bases (a whole codon).
How many complete codons are left?
Enter an exact answer, without rounding.
Subtract, then divide by 3.
📝 Lesson quiz · 5 questions
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Question 1 of 5
A substitution mutation is when:
Question 2 of 5
Which mutations can be passed to offspring?
Question 3 of 5
Which of these is a common mutagen?
Question 4 of 5
A frameshift mutation affects:
Question 5 of 5
Mutations are important for evolution because they:
Lesson 3 · Cells
🔄 Cell Division: Mitosis vs. Meiosis
Cells divide in two very different ways depending on the job — growing and repairing the body, or creating brand-new sex cells for reproduction.
🔄 Two ways to divide
Mitosis produces 2 genetically identical daughter cells, each with the full set of chromosomes — used for growth and repair. Meiosis produces 4 daughter cells (gametes), each with half the number of chromosomes — used for sexual reproduction. Human body cells have 46 chromosomes (23 pairs); human gametes have 23.
Meiosis makes sex cells with half the chromosomes.
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Purpose of mitosis
Mitosis is mainly used for:
Think of what your body needs when you grow or heal a cut.
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A human body cell has a full set of chromosomes.
How many chromosomes does a human body cell have?
Enter an exact answer, without rounding.
It is 23 pairs.
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A human gamete has half the chromosomes of a body cell.
How many chromosomes does a human gamete have?
Enter an exact answer, without rounding.
Half of 46.
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Meiosis produces multiple daughter cells from 1 starting cell.
How many daughter cells does meiosis produce?
Enter an exact answer, without rounding.
Meiosis divides twice: 1 → 2 → ?
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Purpose
Which process repairs a cut on your skin?
Which one makes identical body cells?
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Gametes
Meiosis produces cells that are:
Think of eggs and sperm.
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A cell undergoes mitosis 3 times in a row.
How many cells are there at the end?
Enter an exact answer, without rounding.
Each round doubles the number.
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A human body cell has 46 chromosomes.
How many chromosomes are in a human egg cell?
Enter an exact answer, without rounding.
Half of 46.
📝 Lesson quiz · 5 questions
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Question 1 of 5
How many cells does one meiosis start produce?
Question 2 of 5
A cell with 8 chromosomes divides by mitosis. How many chromosomes does each daughter cell have?
Question 3 of 5
What increases genetic variation during meiosis?
Question 4 of 5
Haploid (n) means a cell has:
Question 5 of 5
Which of these is NOT done by mitosis?
Lesson 4 · Reproduction
🌸 Sexual vs. Asexual Reproduction
Living things reproduce in two fundamentally different ways, and the choice between them has a huge effect on how much genetic variety shows up in the next generation.
🌸 One parent or two?
Asexual reproduction requires only 1 parent, and offspring are genetically identical clones of that parent. Sexual reproduction requires 2 parents, and offspring are genetically unique — a mix of both parents' genes. This genetic variation from sexual reproduction is important because it gives natural selection more traits to act on.
Asexual (1 parent)
Sexual (2 parents)
Asexual: offspring are identical clonesSexual: offspring have a mix of both parents' genes
📝 Worked examples
Example 1 · Asexual (one parent)
Bacteria divide by binary fission. If one divides every 20 minutes:
0 min: 1
20 min: 2
40 min: 4
60 min: 8
Example 2 · Other asexual methods
Hydra grow buds. Strawberry plants send out runners. Starfish can regenerate from a broken arm.
Example 3 · Sexual (two parents)
egg (n) + sperm (n) → zygote (2n) → embryo
⚖️ Compare
Asexual
One parent, fast, offspring are clones. No variation, so a single disease can wipe out a population.
Sexual
Two parents, slower, offspring are different. More variation helps populations adapt.
Clones are genetically identical to the parent, apart from rare mutations.
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Parents needed for asexual reproduction
Asexual reproduction requires:
A- means without: no partner is needed.
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Offspring from sexual reproduction
Offspring produced by sexual reproduction are:
Each parent gives half the genes, in a new mix.
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Genetic variation
Which type of reproduction produces more genetic variation?
Mixing two parents’ genes makes more differences.
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Asexual reproduction needs only 1 parent.
How many parents does asexual reproduction require?
Enter an exact answer, without rounding.
No partner is needed.
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Sexual reproduction needs 2 parents.
How many parents does sexual reproduction require?
Enter an exact answer, without rounding.
A mother and a father.
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Offspring from asexual reproduction are exact clones of their single parent.
What percent of DNA do they share with that parent?
Enter an exact answer, without rounding.
%
Clones are exact copies.
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Asexual example
Which is an example of asexual reproduction?
Which needs only one parent?
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Why variation?
What is the main advantage of sexual reproduction?
Think about what a changing environment needs.
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Bacteria double every 30 minutes, starting with 1 cell.
How many cells are there after 2 hours?
Enter an exact answer, without rounding.
2 hours contains four 30-minute periods.
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A strawberry plant makes a new plant through runners.
How many parents are needed?
Enter an exact answer, without rounding.
Asexual reproduction needs just one parent.
📝 Lesson quiz · 5 questions
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Question 1 of 5
Which method of reproduction produces genetically identical offspring?
Question 2 of 5
Which organisms commonly reproduce by binary fission?
Gregor Mendel discovered that traits pass from parents to offspring in predictable patterns, which we can map out using a simple grid.
🥚 Genotype, phenotype & the Punnett square
Genotype is an organism's genetic makeup (like Bb). Phenotype is the physical trait that genotype produces (like brown eyes). An organism with two identical alleles (like BB or bb) is called homozygous, while an organism with two different alleles (like Bb) is called heterozygous. A Punnett square predicts the possible genotypes of offspring from a cross. For a Bb × Bb cross: 1 BB, 2 Bb, 1 bb — a 3:1 ratio of dominant to recessive phenotypes.
Shows dominant trait (BB or Bb)Shows recessive trait (bb)
📝 Worked examples
Example 1 · Heterozygous × heterozygous
In pea plants, tall (T) is dominant over short (t). Cross Tt × Tt:
T t
T TT Tt
t Tt tt
Genotypes: 1 TT : 2 Tt : 1 tt
Phenotypes: 3 tall : 1 short (75% tall)
Example 2 · Test cross
Tt × tt → Tt, Tt, tt, tt
50% tall, 50% short
Example 3 · Terms
Genotype = the alleles (TT, Tt, tt). Phenotype = the visible trait (tall or short). Homozygous = two identical alleles. Heterozygous = two different alleles.
📌 Mendel's idea
Each parent has two alleles for a trait but passes only one to each offspring (the law of segregation). A dominant allele hides the recessive one in a heterozygote.
Probability has no memory. If two Tt parents have three tall children, the chance that the next child is short is still 25%.
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Defining genotype
Genotype refers to:
Geno- is about genes: the letters.
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Defining phenotype
Phenotype refers to:
Pheno- is about what you can see.
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Purpose of a Punnett square
A Punnett square is used to:
It is a grid that shows every possible combination.
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In a Bb × Bb cross, the Punnett square has 4 boxes: 1 BB, 2 Bb, 1 bb.
Out of the 4 boxes, how many show BB?
Enter an exact answer, without rounding.
Count the boxes with two capital Bs.
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In a Bb × Bb cross, the Punnett square has 4 boxes: 1 BB, 2 Bb, 1 bb.
Out of the 4 boxes, how many show Bb?
Enter an exact answer, without rounding.
Count the boxes with one capital and one small b.
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In a Bb × Bb cross, the Punnett square has 4 boxes: 1 BB, 2 Bb, 1 bb.
Out of the 4 boxes, how many show bb (recessive)?
Enter an exact answer, without rounding.
Count the boxes with two small bs.
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Genotype name
The genotype Tt is called:
Hetero means different.
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Phenotype
A phenotype is:
Phenotype ~ physical.
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Cross Bb × Bb (brown B is dominant, blue b is recessive).
What percentage of offspring are bb?
Enter an exact answer, without rounding.
%
Draw a 2 × 2 Punnett square.
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Cross Bb × bb.
What percentage of offspring are Bb?
Enter an exact answer, without rounding.
%
Each box in the square has equal probability.
📝 Lesson quiz · 5 questions
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Question 1 of 5
TT × tt gives offspring that are:
Question 2 of 5
What is the phenotype ratio from Tt × Tt?
Question 3 of 5
The genotype tt is:
Question 4 of 5
Two Tt parents have 8 offspring. About how many are expected to be tt?
Question 5 of 5
The law of segregation says that:
Lesson 6 · Evolution
🌿 Evidence for Evolution
Scientists didn't just guess that species change over time — the evidence is written directly into rock layers and into the bones of living animals.
🌿 Fossils and homologous structures
The fossil record shows how species have changed over long periods of time, with older fossils generally found in deeper rock layers. Homologous structures are body parts with a similar underlying structure in different species (like the human arm, whale flipper, and bat wing), providing strong evidence of a shared common ancestor.
Upper arm boneForearm bonesFinger bones
📝 Worked examples
Example 1 · Homologous structures
The human arm, whale flipper and bat wing look different but share the same bone pattern. This shows they came from a common ancestor.
Example 2 · Analogous structures
A bird wing and an insect wing do the same job but have different structures. They evolved separately.
Example 3 · Radiometric dating
Carbon-14 has a half-life of 5730 years: every 5730 years, half is left.
After 1 half-life: 1/2
After 2 half-lives: 1/4
After 3 half-lives: 1/8
📌 Lines of evidence
Fossils: deeper layers are generally older.
Homologous structures: similar bones, different jobs.
Vestigial structures: reduced and no longer needed (e.g. the human tailbone).
Biogeography: related species live near each other.
Observed evolution: e.g. bacteria becoming resistant to antibiotics.
Remember: evolution happens to populations over generations, not to individuals during their lifetime.
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What the fossil record shows
The fossil record provides evidence of:
Fossils from different times show how living things looked then.
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What homologous structures indicate
Homologous structures, like the human arm and whale flipper, indicate:
Similar bones in different animals suggest they share an ancestor.
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Identifying a homologous pair
Which of these is an example of a homologous structure comparison used in this lesson?
The pair used in this lesson comes from two different species.
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In a rock formation with 5 layers, older fossils form in deeper layers, so the oldest fossil is in the bottommost layer, layer 5 (counting from the top as layer 1).
Which layer number contains the oldest fossil?
Enter an exact answer, without rounding.
Older layers are deeper down.
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Homologous
A human arm and a whale flipper share the same basic bones. They are:
Same structure, different function.
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Vestigial
Which is an example of a vestigial structure?
Reduced and no longer needed.
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A fossil has 1/4 of its original carbon-14 left (half-life 5730 years).
How many half-lives have passed?
Enter an exact answer, without rounding.
Halve it until you reach 1/4.
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A sample starts with 80 g of a radioactive isotope.
How many grams remain after 3 half-lives?
Enter an exact answer, without rounding.
g
Halve the amount three times.
📝 Lesson quiz · 5 questions
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Question 1 of 5
Fossils in deeper rock layers are generally:
Question 2 of 5
Homologous structures provide evidence of:
Question 3 of 5
Vestigial structures are:
Question 4 of 5
A bird wing and an insect wing are:
Question 5 of 5
Which is the best example of evolution observed in a lab?
Lesson 7 · Evolution
🔭 Embryology & DNA Evidence for Evolution
Two more powerful clues support evolution beyond fossils and bones — how similar embryos look in early development, and how similar species' DNA actually is.
🔭 Embryos and genetic code
Embryology compares the early-stage embryos of different species, which often look strikingly similar (for example, all vertebrate embryos briefly develop gill-like structures), suggesting a shared ancestry. DNA comparisons show that more closely related species have more similar DNA sequences — humans and chimpanzees, for instance, share about 98% of their DNA, supporting a very close evolutionary relationship.
Embryology
DNA Comparison
Species that are more closely related share more similar DNA and more similar early embryos.
📝 Worked examples
Example 1 · Embryos
Early fish, chicken and human embryos all have pharyngeal (gill) pouches and a tail. Such similarity suggests a shared ancestry.
Example 2 · Comparing DNA / proteins
Differences in the protein cytochrome c compared with humans:
Each branch point is a common ancestor. Species that branch off closer together are more closely related.
📌 DNA is strong evidence
All living things use the same genetic code, which points to a single origin of life. The more similar two species' DNA, the more recently they shared a common ancestor.
Similarity ≠ "descended from": humans did not come from chimpanzees. Both share a recent common ancestor.
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Defining embryology
Comparing the early embryos of different species is called:
An embryo is an organism at a very early stage of development.
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DNA and relatedness
Species that are more closely related tend to have:
Close relatives share more of their genetic code.
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New lines of evidence
This lesson adds which 2 new lines of evidence to the fossil record and homologous structures already covered?
One looks at early development; the other looks at genes.
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Humans and chimpanzees share about 98% of their DNA.
How many percentage points is that away from a full 100% match?
Enter an exact answer, without rounding.
Take 98 from 100.
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Two species share 95% DNA similarity.
By how many percentage points do they differ?
Enter an exact answer, without rounding.
Take 95 from 100.
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Closest relative
Which pair would you expect to have the most similar DNA?
Which is the closest relative?
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Why embryos?
Similar early embryos in different vertebrates suggest they:
What does similarity imply?
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Two species share 96% of their DNA.
What percentage differs?
Enter an exact answer, without rounding.
%
Subtract from 100.
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Cytochrome c differs from the human version by 11 amino acids in dogs and by 1 in rhesus monkeys.
How many more differences does the dog have than the rhesus monkey?
Enter an exact answer, without rounding.
Subtract.
📝 Lesson quiz · 5 questions
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Question 1 of 5
What does a branch point on a cladogram represent?
Question 2 of 5
Species with more similar DNA shared a common ancestor:
Question 3 of 5
Which is strong molecular evidence for evolution?
Question 4 of 5
Gill-like pouches in human embryos suggest:
Question 5 of 5
Which is the best evidence of how closely two species are related?
Evolution doesn't happen randomly — it's driven by environmental pressures that determine which traits help organisms survive and reproduce.
🦋 Selection pressure in action
Natural selection: organisms with traits well-suited to their environment survive and reproduce more, passing those traits to the next generation. An adaptation is a trait that improves an organism's chances of survival. Selective pressure is any environmental factor (like a predator, climate, or food scarcity) that favors certain traits over others.
Light bark (before pollution)
Dark bark (after pollution)
Camouflaged moths survive and reproduce more — shifting the population over generations.
📝 Worked examples
Example 1 · Peppered moths
Before factories, light moths were hard to see on pale tree bark. Soot darkened the trees, so dark moths were better camouflaged and survived more often. Their dark-colour allele became more common.
Example 2 · Four ingredients
1. Variation — individuals differ
2. Inheritance — traits are passed on
3. Selection — some survive and reproduce more
4. Time — frequencies change over generations
Example 3 · Antibiotic resistance
A few bacteria happen to resist an antibiotic. The drug kills the others, so the resistant ones multiply and the population becomes resistant.
📌 Types of adaptation
Structural: a cactus's spines, a bird's beak shape.
"Fitness" means how many offspring survive to reproduce. It does not mean strength or speed.
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Summarising natural selection
Natural selection means organisms with helpful traits:
Survival of the fittest.
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Defining selective pressure
Selective pressure is best described as:
It is a pressure from the environment.
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Defining an adaptation
An adaptation is a trait that:
Adapt means to fit your surroundings.
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In a population of 100 moths, 30% are dark-coloured.
How many moths are dark-coloured?
Enter an exact answer, without rounding.
Find 30% of 100.
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In a population of 100 light-coloured moths, 60 are eaten by predators.
How many light-coloured moths survive?
Enter an exact answer, without rounding.
Take 60 from 100.
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Natural selection
Natural selection acts on:
It needs differences between individuals.
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What is fitness?
In biology, a "fit" individual is one that:
It is about offspring.
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A beetle population has 50 beetles; 20 are green and the rest brown.
What percentage are brown?
Enter an exact answer, without rounding.
%
Brown = 50 − 20 = 30.
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Birds eat 3 in every 4 light moths but only 1 in 4 dark moths on dark bark. Out of 100 dark moths:
How many are expected to be eaten?
Enter an exact answer, without rounding.
Take 1/4 of 100.
📝 Lesson quiz · 5 questions
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Question 1 of 5
Which is NOT needed for natural selection?
Question 2 of 5
Camouflage is an example of a:
Question 3 of 5
Why do bacteria become resistant to antibiotics?
Question 4 of 5
Hibernation is a:
Question 5 of 5
Evolution by natural selection occurs in:
Lesson 9 · Evolution
🐕 Selective Breeding vs. Natural Selection
Humans have been shaping other species for thousands of years using the very same principle nature uses — just with a very different judge deciding which traits win.
🐕 Who's doing the choosing?
Natural selection is driven by environmental survival pressure — nature "selects" which traits help organisms survive and reproduce. Selective breeding (also called artificial selection) is driven by human choice — people deliberately choose which organisms breed based on desired traits, like breeding dogs for size or crops for yield. Both processes work by favoring certain traits over many generations, but the "selector" is completely different.
Natural Selection
Artificial Selection
Natural selection: the environment selectsArtificial selection: humans select
📝 Worked examples
Example 1 · One plant, many foods
Wild mustard was bred by people for different traits:
Big leaves → kale
Large buds → cabbage
Flower buds → broccoli
Swollen stem → kohlrabi
Example 2 · Selecting each generation
A farmer picks the 10% of 200 plants with the biggest fruit to breed from: 20 plants. If average fruit mass rises by 5 g each generation from 50 g, after 4 generations it is 50 + 4 × 5 = 70 g.
Less genetic diversity, inbreeding, inherited health problems (e.g. in some dog breeds).
Same mechanism, different chooser: in both cases, individuals with some traits reproduce more. Only the selecting agent differs.
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Another name for selective breeding
Selective breeding is also known as:
People do the choosing, not nature.
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What drives natural selection
Natural selection is driven by:
Nature decides who survives.
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What drives artificial selection
Artificial selection is driven by:
People pick which animals or plants breed.
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Farmers selectively breed corn for 20 generations to increase yield, then continue for 15 more generations.
How many total generations of selective breeding is that?
Enter an exact answer, without rounding.
Add 20 and 15.
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Who selects?
In selective breeding, who chooses which individuals reproduce?
It is also called artificial selection.
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A risk
A major risk of selective breeding is:
What happens when you breed from only a few?
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A breeder picks the top 10% of 200 plants to breed.
How many plants are chosen?
Enter an exact answer, without rounding.
Multiply 200 by 0.1.
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Average fruit mass starts at 50 g and rises 5 g each generation.
What is the average mass after 4 generations?
Enter an exact answer, without rounding.
g
Add 5 g four times.
📝 Lesson quiz · 5 questions
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Question 1 of 5
Kale, cabbage and broccoli all came from which wild plant?
Question 2 of 5
Natural selection chooses traits that:
Question 3 of 5
Which dog trait problem is linked to selective breeding?
Question 4 of 5
Which is generally faster for producing a particular trait?
Question 5 of 5
Selective breeding is also called:
Lesson 10 · Human Body
🫀 Homeostasis & Human Body System Interdependence
Your body constantly works to keep its internal conditions stable, and that balancing act requires several body systems cooperating in real time.
🫀 Feedback loops & teamwork
Homeostasis is the maintenance of stable internal conditions (like body temperature or blood sugar). A negative feedback loop detects a change and triggers a response that reverses it, restoring balance. During exercise, the nervous, endocrine, respiratory, and circulatory systems work together: the nervous system detects increased oxygen demand, the endocrine system releases hormones like adrenaline, the respiratory system increases breathing rate, and the circulatory system increases heart rate to deliver oxygen faster.
A negative feedback loop detects a change, triggers a response, and restores balance.
📝 Worked examples
Example 1 · Temperature control
Normal body temperature is about 37 °C.
Too hot → sweat, blood vessels widen → cools
Too cold → shiver, blood vessels narrow → warms
Example 2 · Blood sugar
After a meal: glucose rises → pancreas releases insulin → cells take up glucose → level falls
Between meals: glucose falls → pancreas releases glucagon → liver releases glucose → level rises
Example 3 · Negative feedback
Homeostasis works mostly through negative feedback: the response reverses the change.
📌 Systems working together
System
Main job
Nervous
Fast electrical signals
Endocrine
Slower chemical signals (hormones)
Circulatory
Transports blood, oxygen, nutrients
Respiratory
Gas exchange in the lungs
Excretory
Removes wastes, balances water
Homeostasis = keeping internal conditions stable even when the outside changes.
✏️ Quick check · 1 of 9
Defining homeostasis
Homeostasis refers to:
Homeo- means same; -stasis means staying still.
✏️ Quick check · 2 of 9
How a negative feedback loop works
A negative feedback loop works by:
Negative feedback pushes things back towards normal.
✏️ Quick check · 3 of 9
Body systems during exercise
During exercise, increased heart rate and breathing rate show cooperation between the:
The heart pumps blood and the lungs breathe air.
✍️ Type your answer · 4 of 9
A resting heart rate is 70 bpm. During exercise, it rises by 50 bpm.
What is the new heart rate, in bpm?
Enter an exact answer, without rounding.
bpm
Add 50 to 70.
✍️ Type your answer · 5 of 9
Normal body temperature is 37°C. A fever raises it by 2°C.
What is the new body temperature, in °C?
Enter an exact answer, without rounding.
°C
Add 2 to 37.
✏️ Quick check · 6 of 9
Homeostasis
Homeostasis is best described as:
Stable internal environment.
✏️ Quick check · 7 of 9
Signal speed
Which body system sends signals the fastest?
Think reflexes.
✍️ Type your answer · 8 of 9
Your body temperature rises to 39 °C. Normal is 37 °C.
How many degrees above normal is it?
Enter an exact answer, without rounding.
°C
Subtract.
✍️ Type your answer · 9 of 9
A resting heart beats 72 times per minute.
How many beats is that in 1 hour?
Enter an exact answer, without rounding.
beats
There are 60 minutes in an hour.
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
Which hormone lowers blood glucose?
Question 2 of 5
Negative feedback:
Question 3 of 5
Shivering helps because it:
Question 4 of 5
Which system exchanges oxygen and carbon dioxide?
Question 5 of 5
Which organ removes waste from blood and balances water?
Lesson 11 · Ecosystems
🌍 Ecosystem Cycles & Human Impact on Biodiversity
Matter constantly cycles through ecosystems in predictable loops — and human activity is increasingly disrupting those cycles and the species that depend on them.
🌍 Cycles under pressure
The carbon cycle moves carbon dioxide between the atmosphere and living things (absorbed by plants via photosynthesis, released via respiration and combustion). The nitrogen cycle relies on bacteria to convert atmospheric nitrogen into forms plants can use. The water cycle moves water through evaporation, condensation, and precipitation. Human impacts on biodiversity include habitat destruction, pollution, and overharvesting, all of which reduce species diversity.
The 3 cycles
Human impacts
📝 Worked examples
Example 1 · The 10% rule
Only about 10% of energy passes to the next trophic level; the rest is lost as heat or used for life processes.