How do we see inside a cell? And how do we put a number on how well an enzyme works?
🔍 Microscopy
Magnification = image size ÷ actual size. Resolution is the ability to tell two close points apart. Electron microscopes have far greater resolution than light microscopes because electrons have a much shorter wavelength.
light microscope: resolution about 200 nm
transmission electron microscope: about 0.5 nm
scanning electron microscope: 3D surfaces
image 25 mm, actual 50 µm
M = 25 000 µm ÷ 50 µm = ×500
⚗️ Vmax and Km
Vmax is the maximum rate, when all active sites are occupied. The Michaelis–Menten constant Km is the substrate concentration giving half of Vmax. A low Km means high affinity. A competitive inhibitor raises Km but leaves Vmax unchanged; a non-competitive inhibitor lowers Vmax.
Immobilised enzymes can be reused, and they stay more stable to temperature and pH.
✏️ Quick check · 1 of 6
Resolution
Resolution is:
Telling points apart.
✏️ Quick check · 2 of 6
Electron
Electron microscopes have higher resolution because electrons have:
Compare with light.
✏️ Quick check · 3 of 6
Km
A low Km means the enzyme has:
Binds easily.
✏️ Quick check · 4 of 6
Inhibitor
A competitive inhibitor:
It competes for the site.
✍️ Type your answer · 5 of 6
An image is 25 mm long and the actual length is 50 µm (1 mm = 1000 µm).
What is the magnification?
Enter an exact answer, without rounding.
Use the same units.
✍️ Type your answer · 6 of 6
A mitochondrion is 10 000 times magnified, and its image is 20 mm long.
What is its actual length, in µm?
Enter an exact answer, without rounding.
µm
Actual = image ÷ M.
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
Vmax occurs when:
Question 2 of 5
A scanning electron microscope mainly shows:
Question 3 of 5
Immobilised enzymes are useful because they:
Question 4 of 5
Ribosomes can be seen with a:
Question 5 of 5
A non-competitive inhibitor:
Lesson 2 · Transport
🫀 The Heart & Haemoglobin
Your heart sets its own rhythm, and haemoglobin knows exactly when to let go of oxygen. Together they deliver oxygen just where it is needed.
💓 The cardiac cycle
The sinoatrial node (SAN) starts each beat. The wave spreads over the atria, pauses at the atrioventricular node (AVN), then runs through the Purkyne tissue so the ventricles contract from the base upward.
atrial systole: atria contract, AV valves open
ventricular systole: ventricles contract, AV valves shut, semilunar valves open
diastole: heart relaxes and fills
valves open and close because of pressure differences
🩸 Oxygen and CO₂
Haemoglobin binds oxygen with positive cooperativity, which gives an S-shaped dissociation curve. In the Bohr shift, high CO₂ moves the curve to the right, so haemoglobin releases more oxygen in respiring tissue. Most CO₂ travels as hydrogencarbonate ions, formed with carbonic anhydrase in red blood cells.
Heart rate is 70 beats per minute and stroke volume is 80 cm³.
What is the cardiac output, in cm³ per minute?
Enter an exact answer, without rounding.
Rate × volume.
✍️ Type your answer · 6 of 6
A heart beats 75 times per minute.
How long is one cardiac cycle, in seconds?
Enter an exact answer, without rounding.
s
Divide 60 by the rate.
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
Atrioventricular valves close when:
Question 2 of 5
Fetal haemoglobin has an oxygen dissociation curve:
Question 3 of 5
Carbonic anhydrase is found in:
Question 4 of 5
The chloride shift balances:
Question 5 of 5
The S-shape of the curve is caused by:
Lesson 3 · Homeostasis
💧 The Kidney & Homeostasis
Your kidneys filter about 180 litres of fluid a day and return nearly all of it. The nephron is a beautifully precise sorting machine.
🧬 The nephron
In ultrafiltration, high blood pressure in the glomerulus forces small molecules into Bowman’s capsule. In selective reabsorption, the proximal convoluted tubule takes back all glucose, most ions and much water. The loop of Henle builds a salt gradient in the medulla.
Osmoreceptors in the hypothalamus detect low water potential. The posterior pituitary releases ADH, which makes the collecting duct more permeable by adding aquaporins, so more water is reabsorbed. This is negative feedback. Blood glucose is controlled in the same way by insulin and glucagon.
Stomata are homeostatic too: in dry conditions, the plant hormone abscisic acid makes guard cells close the stomata.
✏️ Quick check · 1 of 6
Filter
Which is NOT normally filtered into the nephron?
Too large.
✏️ Quick check · 2 of 6
Reabsorb
Glucose is normally reabsorbed in the:
First part of the tubule.
✏️ Quick check · 3 of 6
ADH
ADH increases water reabsorption by:
Water channels.
✏️ Quick check · 4 of 6
Gland
ADH is released from the:
Below the hypothalamus.
✍️ Type your answer · 5 of 6
The kidneys filter 180 litres a day and produce 1.5 litres of urine.
How many litres are reabsorbed each day?
Enter an exact answer, without rounding.
litres
Subtract.
✍️ Type your answer · 6 of 6
Glomerular filtrate is formed at 125 cm³ per minute.
How many cm³ is that per hour?
Enter an exact answer, without rounding.
cm³
Multiply by 60.
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
Ultrafiltration is driven by:
Question 2 of 5
The loop of Henle creates a:
Question 3 of 5
After a salty meal, ADH levels:
Question 4 of 5
Insulin causes blood glucose to:
Question 5 of 5
Abscisic acid makes stomata:
Lesson 4 · Control and coordination
⚡ Nerves, Muscle & Plant Hormones
A nerve fires in a millisecond and a muscle shortens as its filaments slide. Plants coordinate too, just far more slowly.
⚡ Action potentials
A neurone’s resting potential is about −70 mV. At threshold, sodium channels open and Na⁺ rushes in (depolarisation). Then K⁺ leaves (repolarisation). The refractory period makes impulses travel one way. Myelin gives saltatory conduction between nodes of Ranvier.
At a cholinergic synapse:
Ca²⁺ enters the presynaptic knob
vesicles release acetylcholine
it binds to receptors, opening Na⁺ channels
acetylcholinesterase breaks it down
💪 Muscle and plant control
In the sliding filament model, Ca²⁺ exposes binding sites on actin. Myosin heads bind, pull and release using ATP, so the sarcomere shortens. In plants, auxin promotes cell elongation by acidifying cell walls, and gibberellins cause stem elongation and seed germination.
in a contraction:
sarcomere: shorter
I band: shorter
H zone: shorter
A band: stays the same length
✏️ Quick check · 1 of 6
Resting
The resting potential of a neurone is about:
Inside is negative.
✏️ Quick check · 2 of 6
Depolarise
Depolarisation is caused by:
Sodium rushes in.
✏️ Quick check · 3 of 6
Myelin
Myelin speeds up an impulse by allowing:
It jumps.
✏️ Quick check · 4 of 6
Muscle
During contraction, the band that stays the same length is the:
Filament lengths do not change.
✍️ Type your answer · 5 of 6
An impulse travels at 100 m/s along an axon 0.5 m long.
How long does it take, in ms?
Enter an exact answer, without rounding.
ms
Time = distance ÷ speed.
✍️ Type your answer · 6 of 6
A neurone rests at −70 mV and peaks at +40 mV.
What is the total change in potential, in mV?
Enter an exact answer, without rounding.
mV
Subtract a negative.
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
At the synapse, vesicle release is triggered by:
Question 2 of 5
The refractory period ensures that impulses:
Question 3 of 5
Acetylcholinesterase:
Question 4 of 5
ATP is used in muscle contraction to:
Question 5 of 5
Gibberellins stimulate:
Lesson 5 · Genes and diversity
🧬 Gene Control & Classification
Every cell has the same genes, yet they do different jobs, because genes get switched on and off. Living things are sorted by the same evidence.
🔛 The lac operon
In E. coli, the lac operon switches on lactose use. Without lactose, a repressor binds the operator and blocks RNA polymerase. Lactose binds the repressor and removes it, so the genes for β-galactosidase are transcribed. In eukaryotes, transcription factors control gene expression, for example in response to gibberellin.
structural genes: lacZ, lacY, lacA
no lactose: repressor bound, genes off
lactose: repressor released, genes on
🌳 Domains and conservation
The three domains are Bacteria, Archaea and Eukarya. The taxonomic ranks run domain, kingdom, phylum, class, order, family, genus, species. Conservation measures include national parks, seed banks, zoos and CITES, which controls trade in endangered species.
Viruses are not in any domain. They are classified by their genetic material: DNA or RNA.
✏️ Quick check · 1 of 6
Repressor
Without lactose, the lac repressor:
It blocks the way.
✏️ Quick check · 2 of 6
Inducer
Lactose switches the operon on by:
It removes the block.
✏️ Quick check · 3 of 6
Domains
Which domain has membrane lipids most different from the other two?
Often extreme.
✏️ Quick check · 4 of 6
Ranks
Which rank is the broadest of these?
Nearest the top.
✍️ Type your answer · 5 of 6
The taxonomic hierarchy runs from domain down to species.
How many ranks does it have in total?
Enter an exact answer, without rounding.
Count them.
✍️ Type your answer · 6 of 6
A seed bank stores 2.4 billion seeds from 1200 species, with equal numbers.
How many million seeds per species?
Enter an exact answer, without rounding.
Divide.
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
Which is NOT in the lac operon system’s structural genes?
Question 2 of 5
Transcription factors:
Question 3 of 5
CITES controls:
Question 4 of 5
Seed banks are useful because they:
Question 5 of 5
Viruses are classified using:
Final Challenge
🏆 10-Question Challenge Quiz
Questions from every topic in this course. Pick an answer for each one, then hit Check My Answers.
0 / 10
💪 Keep practicing — go back through the lessons and retake the quiz!
Q1 · Microscopes & Enzyme Kinetics
Km is the substrate concentration at:
Q2 · Microscopes & Enzyme Kinetics
Magnification =
Q3 · The Heart & Haemoglobin
Purkyne tissue carries the impulse to the:
Q4 · The Heart & Haemoglobin
At a respiring muscle, high CO₂ makes haemoglobin: