Every organism is made of cells. Their size is limited by geometry, their parts have specialised jobs, and a microscope lets you measure them.
🏭 Organelles and cell types
The nucleus holds the DNA. Mitochondria release energy by aerobic respiration. Ribosomes make proteins, the endoplasmic reticulum and Golgi apparatus process and package them, and lysosomes break down waste. Plant cells also have a cell wall, a large vacuole and chloroplasts. Prokaryotes (bacteria) have no membrane-bound nucleus; eukaryotes (animals, plants, fungi, protists) do.
mitochondrion → aerobic respiration (ATP)
chloroplast → photosynthesis (plants and algae)
ribosome → protein synthesis
nucleus → DNA, controls the cell
prokaryote: DNA free in the cytoplasm, no nucleus
eukaryote: DNA inside a membrane-bound nucleus
🖼️ See it
Plant cells add a wall, chloroplasts and a large vacuole to the animal cell toolkit.
📏 Magnification and surface area : volume
Magnification = image size ÷ actual size. Convert to the same units first: 1 mm = 1000 µm. As a cell grows, its volume rises faster than its surface area, so the surface area : volume ratio falls. A small cell exchanges materials with its surroundings more efficiently.
image 5 mm = 5000 µm, actual 50 µm
magnification = 5000 / 50 = 100×
cube side 1 cm: SA = 6 × 1² = 6, V = 1, ratio 6 : 1
cube side 2 cm: SA = 6 × 2² = 24, V = 8, ratio 3 : 1
cube side 3 cm: SA = 6 × 3² = 54, V = 27, ratio 2 : 1
Bigger means a smaller ratio. This is why large organisms need transport systems and folded exchange surfaces.
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Respiration
Which organelle is the main site of aerobic respiration?
The powerhouse.
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Prokaryotes
What do prokaryotic cells lack?
Pro = before the nucleus.
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Plant cells
Which structure is found in plant cells but not in animal cells?
Think of photosynthesis.
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Growing cells
As a cell grows larger, its surface area : volume ratio:
Compare the 1 cm and 3 cm cubes.
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A drawing of a cell is 5 mm (5000 µm) long. The cell is actually 50 µm long.
What is the magnification?
Enter an exact answer, without rounding.
×
Magnification = image size ÷ actual size.
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A cube-shaped cell has sides of 2 cm.
What is its surface area : volume ratio (the first number, as x : 1)?
Enter an exact answer, without rounding.
SA = 6 × side².
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A cell viewed at ×400 has an image length of 20 mm (20 000 µm).
What is the actual length of the cell, in µm?
Enter an exact answer, without rounding.
µm
Actual size = image size ÷ magnification.
📝 Lesson quiz · 5 questions
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Question 1 of 5
Which organelle makes proteins?
Question 2 of 5
Which structure gives a plant cell its fixed shape?
Question 3 of 5
Lysosomes contain enzymes that:
Question 4 of 5
Why can small cells exchange materials efficiently?
Question 5 of 5
1 mm equals:
Lesson 2 · Genetics
🧬 Inheritance & Punnett Squares
Offspring inherit one allele of each gene from each parent. Punnett squares let you predict the chances of each outcome.
🔲 Monohybrid crosses
An allele is a version of a gene. The genotype is the allele combination (AA, Aa, aa); the phenotype is the visible trait. A dominant allele shows when present; a recessive one shows only as aa. Homozygous means two identical alleles, heterozygous two different ones.
Aa × Aa
A a
A AA Aa
a Aa aa
genotypes 1 AA : 2 Aa : 1 aa
phenotypes 3 dominant : 1 recessive (75% : 25%)
Aa × aa (test cross): 50% Aa, 50% aa
Probabilities are chances, not guarantees. Each child is an independent event.
🖼️ See it
Each box shows one equally likely combination of alleles.
🔀 Beyond simple dominance
In codominance both alleles show (AB blood group). In incomplete dominance the heterozygote is a blend (red × white gives pink). A dihybrid cross follows two genes; two heterozygotes give a 9 : 3 : 3 : 1 ratio from 16 combinations. Sex-linked genes lie on the X chromosome, so recessive conditions such as colour blindness are more common in males (XY).
dihybrid AaBb × AaBb: 4 × 4 = 16 combinations, ratio 9 : 3 : 3 : 1
carrier mother X^B X^b × normal father X^B Y
daughters: all unaffected (half are carriers)
sons: 50% X^B Y (normal), 50% X^b Y (colour-blind)
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Genotype
Aa is a:
The alleles are different.
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Recessive trait
When does a recessive trait show in the phenotype?
Both alleles must be recessive.
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Incomplete dominance
Red flowers crossed with white flowers give pink flowers. This is:
A blend.
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Test cross
A test cross is between an unknown dominant-trait individual and:
The recessive reveals everything.
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Two heterozygous parents (Aa × Aa) have offspring.
What percentage are expected to be aa?
Enter an exact answer, without rounding.
%
Draw the Punnett square.
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Cross Aa × aa.
What percentage of offspring are expected to be heterozygous (Aa)?
Enter an exact answer, without rounding.
%
Draw the Punnett square.
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A cross of two Aa parents produces 64 offspring.
About how many are expected to show the dominant phenotype?
Enter an exact answer, without rounding.
75% show the dominant trait.
📝 Lesson quiz · 5 questions
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Question 1 of 5
A dihybrid cross of two heterozygotes gives a phenotype ratio of:
Question 2 of 5
Why is colour blindness more common in males?
Question 3 of 5
AB blood group is an example of:
Question 4 of 5
A heterozygous genotype has:
Question 5 of 5
Which statement about probability is correct?
Lesson 3 · Evolution
🦎 Evolution & Populations
Populations evolve when the frequencies of their alleles change over generations. Natural selection is the best-known cause.
🌿 Natural selection and speciation
Natural selection needs four things: variation in a population, traits that are inherited, more offspring born than can survive, and differential survival and reproduction. Over time, helpful traits become more common. A new species can arise when populations become reproductively isolated, often by a geographic barrier (allopatric speciation). Evidence for evolution includes fossils, homologous structures, similar embryos and DNA comparisons.
peppered moth: dark moths survive better on sooty trees
→ dark allele becomes more common (selection)
homologous structures: human arm, bat wing, whale flipper
(same bones, different uses → common ancestor)
antibiotic resistance: resistant bacteria survive and reproduce
🖼️ See it
Repeated over many generations, this process changes the whole population.
🧮 Allele frequencies: Hardy–Weinberg
If a population is not evolving, allele frequencies stay constant. With two alleles, p = frequency of the dominant allele and q = frequency of the recessive allele, and p + q = 1. Genotype frequencies are p² + 2pq + q² = 1. Start from the recessive phenotype, since only aa shows as q².
Genetic drift and gene flow also change allele frequencies, especially in small populations.
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Needed for selection
Which is essential for natural selection to act?
Variation.
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Homologous
The human arm and a bat wing are:
Same bones.
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Speciation
A river splits a squirrel population in two. Over time they cannot interbreed. This is:
Geographic barrier.
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Antibiotics
Antibiotic resistance spreads because:
Survival of the resistant.
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In a population, q² = 0.09 (the recessive phenotype frequency).
What is q?
Enter an exact answer, without rounding.
Take the square root.
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In a population, q = 0.3.
What is p?
Enter an exact answer, without rounding.
p + q = 1.
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In a population, p = 0.8 and q = 0.2.
What percentage are carriers (2pq)?
Enter an exact answer, without rounding.
%
Compute 2pq, then convert.
📝 Lesson quiz · 5 questions
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Question 1 of 5
Fitness in biology means:
Question 2 of 5
Genetic drift has the biggest effect in:
Question 3 of 5
Which is evidence from molecular biology?
Question 4 of 5
In the Hardy–Weinberg equation, 2pq represents the:
Question 5 of 5
Mutations are important to evolution because they:
Lesson 4 · Ecology
🌍 Ecology & Energy Flow
Ecologists measure how energy moves through food chains and how populations change. The numbers show why food chains are short.
🔺 Trophic levels and the 10% rule
Energy enters an ecosystem through producers and passes to primary, secondary and tertiary consumers. Only about 10% of the energy at one level reaches the next; the rest is used in respiration, lost as heat or left in waste. So energy pyramids narrow quickly and food chains rarely have more than four or five links.
producers 10 000 kJ
primary consumers 1 000 kJ (10%)
secondary consumers 100 kJ
tertiary consumers 10 kJ
efficiency = energy passed on / energy received × 100
🖼️ See it
Only about 10% of the energy moves up each level, so food chains rarely have more than four or five links.
📈 Populations and sampling
Populations grow by births and immigration and shrink by deaths and emigration. They level off at the carrying capacity. To estimate the size of a mobile population, use mark–recapture: N = (M × n) ÷ m, where M animals were marked, n are caught in the second sample and m of them are marked.
marked M = 50; second catch n = 40, of which marked m = 10
N = 50 × 40 / 10 = 200
300 mice in 15 hectares → density = 300 / 15 = 20 per hectare
births 120, deaths 80, no migration (population 2000)
change = +40 → 40 / 2000 = 2% growth
The carbon and nitrogen cycles recycle matter, but energy flows one way and is lost as heat.
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Energy loss
Where does most of the energy at each trophic level go?
Think of the 10% rule.
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Short chains
Why do food chains rarely have more than five links?
The energy runs out.
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Carrying capacity
The carrying capacity of an environment is:
The maximum.
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Producers
Producers are organisms that:
The start of the chain.
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Producers store 10 000 kJ. Assume 10% is passed on at each step.
How much energy reaches the secondary consumers, in kJ?
Enter an exact answer, without rounding.
kJ
Apply 10% twice.
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A mark–recapture study: 50 animals marked, a second catch of 40 contains 10 marked animals.
What is the estimated population size?
Enter an exact answer, without rounding.
N = M × n / m.
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A population of 2000 has 120 births and 80 deaths, with no migration.
What is the percentage growth?
Enter an exact answer, without rounding.
%
Net change ÷ starting size.
📝 Lesson quiz · 5 questions
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Decomposers are important because they:
Question 2 of 5
Nitrogen-fixing bacteria:
Question 3 of 5
Which action reduces biodiversity?
Question 4 of 5
A biotic factor is:
Question 5 of 5
When a population reaches carrying capacity, its growth:
Lesson 5 · Human biology
🧠 Control & Homeostasis
The body keeps conditions steady using nerves and hormones, often through negative feedback.
⚡ The nervous system
A stimulus is detected by a receptor, an impulse travels along a sensory neuron to the central nervous system (brain and spinal cord), then along a motor neuron to an effector (muscle or gland). A reflex arc is a fast, automatic response that passes through the spinal cord without waiting for the brain. Neurons meet at synapses, where chemicals called neurotransmitters carry the signal.
stimulus → receptor → sensory neuron → CNS → motor neuron → effector → response
impulse speed 100 m/s, path length 1.5 m
time = 1.5 / 100 = 0.015 s
🖼️ See it
The response reverses the change, which is what “negative” means.
🩸 Hormones and feedback
Hormones are chemical messengers released into the blood by endocrine glands. They act more slowly than nerves but their effects last longer. In negative feedback, a change triggers a response that reverses it. After a meal, blood glucose rises, so the pancreas releases insulin and cells take up glucose; when it falls, glucagon releases glucose from the liver. In type 1 diabetes the body makes too little insulin.
glucose too high → insulin → cells and liver store glucose → level falls
glucose too low → glucagon → liver releases glucose → level rises
body temperature 37 °C; fever of 39 °C = +2 °C above normal
cardiac output = heart rate × stroke volume
72 beats/min × 70 mL = 5040 mL/min
A nerve impulse travels at 100 m/s along a path of 1.5 m.
How long does it take, in s?
Enter an exact answer, without rounding.
s
time = distance ÷ speed.
✍️ Type your answer · 6 of 7
Normal body temperature is 37 °C. A patient has a temperature of 39 °C.
By how many °C is it above normal?
Enter an exact answer, without rounding.
°C
Subtract.
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A heart beats 72 times per minute and pumps 70 mL per beat.
What is the cardiac output, in mL/min?
Enter an exact answer, without rounding.
mL/min
Cardiac output = heart rate × stroke volume.
📝 Lesson quiz · 5 questions
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Question 1 of 5
Which hormone raises blood glucose?
Question 2 of 5
Which gland releases insulin?
Question 3 of 5
Which is part of the central nervous system?
Question 4 of 5
When body temperature rises, the body responds by:
Question 5 of 5
A synapse is:
Lesson 6 · Health & technology
💉 Immunity & Biotechnology
The immune system fights pathogens and remembers them. Biotechnology now lets us copy, cut and read DNA.
🛡️ Defence against disease
Pathogens include bacteria, viruses, fungi and protists. First lines of defence are physical and chemical: skin, mucus and stomach acid. Phagocytes engulf invaders. B lymphocytes make antibodies specific to an antigen, and T lymphocytes help and kill. Memory cells give faster protection next time, which is how vaccines work. Antibiotics kill bacteria, not viruses.
Memory cells are why a second infection with the same pathogen is usually milder.
🧪 DNA technology
PCR copies a section of DNA: each cycle doubles the number of copies, so after n cycles one molecule becomes 2ⁿ. In gel electrophoresis fragments move through a gel under an electric field; smaller fragments travel farther. CRISPR-Cas9 uses a guide RNA to cut DNA at a chosen sequence so genes can be edited. Genetic modification inserts a gene from one organism into another, as in insulin-producing bacteria.
PCR from 1 molecule:
5 cycles → 2⁵ = 32 copies
10 cycles → 2¹⁰ = 1024 copies
electrophoresis: small fragments are near the bottom of the gel
Science and ethics: gene editing raises questions about safety, fairness and consent that science alone cannot answer.
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Vaccines
Vaccines protect because they:
Think of memory.
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Antibiotics
Antibiotics are effective against:
Not a flu medicine.
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Electrophoresis
In gel electrophoresis, which DNA fragments travel farthest?
Small ones slip through.
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CRISPR
CRISPR-Cas9 is used to:
Gene editing.
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PCR starts with 1 DNA molecule.
How many copies are there after 5 cycles?
Enter an exact answer, without rounding.
Double each cycle.
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A disease has R₀ = 4. The herd immunity threshold is 1 − 1/R₀.
What percentage of the population must be immune?
Enter an exact answer, without rounding.
%
Compute 1 − 1/4.
✍️ Type your answer · 7 of 7
A class of 40 students has 85% vaccinated.
How many students are vaccinated?
Enter an exact answer, without rounding.
students
Find 85% of 40.
📝 Lesson quiz · 5 questions
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Question 1 of 5
Antibodies are made by:
Question 2 of 5
An antigen is:
Question 3 of 5
Why is a second infection often milder?
Question 4 of 5
Genetically modified bacteria can be used to make:
Question 5 of 5
PCR is used to:
Final Challenge
🏆 12-Question Challenge Quiz
Questions from every topic in this course. Pick an answer for each one, then hit Check My Answers.
0 / 12
💪 Keep practicing — go back through the lessons and retake the quiz!
Q1 · Cells, Organelles & Microscopy
A cell with DNA inside a membrane-bound nucleus is:
Q2 · Cells, Organelles & Microscopy
1 mm equals:
Q3 · Inheritance & Punnett Squares
Two carriers of a recessive disease (Aa × Aa) have a chance of an affected child of:
Q4 · Inheritance & Punnett Squares
Which statement about probability is correct?
Q5 · Evolution & Populations
Which describes natural selection correctly?
Q6 · Evolution & Populations
Mutations are important to evolution because they:
Q7 · Ecology & Energy Flow
The flow of energy through an ecosystem is:
Q8 · Ecology & Energy Flow
When a population reaches carrying capacity, its growth: