Living things are built from four families of large molecules. Each one is a chain or cluster of small building blocks joined by a repeating reaction.
🧱 Building blocks and polymers
Small units (monomers) join into large molecules (polymers) by condensation, which releases one water molecule per bond. Hydrolysis breaks the bond again by adding water.
monomer polymer bond
monosaccharide polysaccharide glycosidic
amino acid polypeptide peptide
nucleotide nucleic acid phosphodiester
A chain of n monomers has n − 1 bonds,
so it released n − 1 water molecules.
Lipids are not true polymers. A triglyceride is one glycerol joined to three fatty acids by ester bonds.
🖼️ See it
Each group is built from smaller units joined into long chains.
⚡ Carbohydrates, lipids and proteins
Carbohydrates store and supply energy (glucose, starch in plants, glycogen in animals) and give structure (cellulose). Lipids store more energy per gram and form membranes. Proteins have 20 different amino acids and do most of the work in the cell: enzymes, transport, hormones and structure.
Energy per gram
carbohydrate 17 kJ
protein 17 kJ
fat 37 kJ
10 g of fat → 10 · 37 = 370 kJ
🧪 Worked examples and biochemical tests
Counting bonds. Joining n monomers in one chain makes n − 1 bonds. If the molecule is made of several chains, subtract one for each chain.
A protein has 120 amino acids in 2 polypeptide chains
peptide bonds = 120 − 2 = 118
water molecules released = 118
Hydrolysing it completely needs 118 water molecules
Energy content. Multiply each mass by its energy value and add the results.
A serving has 12 g fat, 30 g carbohydrate and 8 g protein
fat: 12 × 37 = 444 kJ
carbohydrate: 30 × 17 = 510 kJ
protein: 8 × 17 = 136 kJ
total = 444 + 510 + 136 = 1090 kJ
Food tests. Always say what you add, what you do, and what you see.
starch iodine solution yellow-brown → blue-black
reducing sugar Benedict’s, heat blue → green → orange → brick-red
non-reducing boil with acid, neutralise,
then Benedict’s brick-red only after the acid step
protein biuret reagent blue → purple (lilac)
lipid ethanol, then water clear → cloudy white emulsion
Why the acid step? Sucrose is not a reducing sugar. Acid hydrolyses the glycosidic bond to give glucose and fructose, which are reducing sugars. The result must be neutralised first, because Benedict’s reagent only works in alkaline conditions.
🎯 Structure and function: exam focus
Examiners reward answers that link a structural feature to a function.
starch (amylose) α-glucose, coiled helix → compact store
amylopectin/glycogen branched → many ends, so enzymes release glucose quickly
cellulose β-glucose, straight chains with H bonds between them
→ microfibrils, strong and rigid cell wall
triglyceride long hydrocarbon tails → high energy per gram,
insoluble so no effect on water potential
Common mistakes. Cellulose is made from β-glucose, not α-glucose. The bond between amino acids is a peptide bond and between sugars a glycosidic bond; do not mix them up. Saturated fatty acids have no C=C double bonds.
Command words. “State” needs a short fact. “Explain” needs a reason using because/so. “Compare” needs a similarity or difference stated for both things.
✏️ Quick check · 1 of 11
Monosaccharide
Which of these is a monosaccharide?
It has only one sugar unit.
✏️ Quick check · 2 of 11
Peptide bond
Amino acids are joined to each other by:
Think of the word polypeptide.
✏️ Quick check · 3 of 11
Triglyceride
A triglyceride is made of:
The prefix tri- means three.
✍️ Type your answer · 4 of 11
A polypeptide chain contains 50 amino acids.
How many peptide bonds does it have?
Enter an exact answer, without rounding.
One fewer bond than units.
✍️ Type your answer · 5 of 11
Ten glucose molecules are joined into one chain by condensation.
How many water molecules are released?
Enter an exact answer, without rounding.
Count the bonds, not the molecules.
✍️ Type your answer · 6 of 11
A snack contains 10 g of fat (37 kJ per gram).
How much energy is stored in the fat, in kJ?
Enter an exact answer, without rounding.
kJ
energy = mass · energy per gram.
✍️ Type your answer · 7 of 11
Each of the 20 amino acids can be placed in either position of a dipeptide.
How many different dipeptides are possible?
Enter an exact answer, without rounding.
Multiply the choices for each position.
✍️ Type your answer · 8 of 11
A protein is made of 120 amino acids arranged in 2 polypeptide chains.
How many peptide bonds does it contain?
Enter an exact answer, without rounding.
Each chain has one fewer bond than amino acids.
✍️ Type your answer · 9 of 11
A portion of food contains 12 g of fat, 30 g of carbohydrate and 8 g of protein. Fat gives 37 kJ per gram; carbohydrate and protein give 17 kJ per gram each.
What is the total energy content, in kJ?
Enter an exact answer, without rounding.
kJ
Work out each nutrient separately, then add.
✏️ Quick check · 10 of 11
Testing for sucrose
A solution gives no change with Benedict’s reagent. After boiling with dilute acid, neutralising and testing again, a brick-red precipitate forms. This shows the solution contained:
Which sugar only reacts after hydrolysis?
✏️ Quick check · 11 of 11
Cellulose
Which feature of cellulose makes it suitable for a plant cell wall?
Think strength, not energy storage.
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
Which polysaccharide is the main carbohydrate store in animals?
Question 2 of 5
Hydrolysis is a reaction that:
Question 3 of 5
Why do lipids store more energy per gram than carbohydrates?
Question 4 of 5
The monomer of a protein is the:
Question 5 of 5
Cellulose is important to plants because it:
Lesson 2 · Cell biology
🧫 Membranes & Transport
Every cell is wrapped in a membrane that decides what comes in and what goes out. Substances cross it passively down a gradient, or actively using energy.
🧬 The fluid mosaic model
The membrane is a phospholipid bilayer with proteins embedded in it. The hydrophilic heads face the water on both sides and the hydrophobic tails face inward, so small non-polar molecules (O₂, CO₂) pass through but ions and large polar molecules cannot.
phospholipids form the barrier
channel proteins let specific ions through
carrier proteins change shape to move a molecule
cholesterol keeps the membrane stable
glycoproteins cell recognition
🖼️ See it
Facilitated diffusion uses channel or carrier proteins but is still passive, because the molecules move down their gradient.
🚚 Ways to cross the membrane
Passive transport needs no ATP because particles move down their concentration gradient. Active transport uses ATP and carrier proteins to move particles against the gradient.
simple diffusion small, non-polar (O₂)
facilitated diffusion through a protein, no ATP
osmosis water, high → low water potential
active transport against gradient, needs ATP
Rate of diffusion ∝ surface area · gradient / thickness
Osmosis in animal cells: in pure water a red blood cell bursts, and in a strong salt solution it shrinks.
🧪 Worked examples: water potential and percentage change
Water potential (ψ, in kPa) is the tendency of water to move. Water moves from a higher (less negative) to a lower (more negative) water potential. Pure water at atmospheric pressure has ψ = 0, the highest possible value.
ψ = ψs + ψp
(solute potential + pressure potential)
A plant cell has ψs = −800 kPa and ψp = +300 kPa
ψcell = −800 + 300 = −500 kPa
Surrounding solution: ψ = −300 kPa
−300 is higher than −500, so water moves INTO the cell
Potato experiments. Use the percentage change in mass so that pieces of different starting size can be compared.
percentage change = (final − initial) ÷ initial × 100
initial 4.0 g, final 3.4 g in a 0.6 mol/dm³ sucrose solution
(3.4 − 4.0) ÷ 4.0 × 100 = −15%
The point where the line on the graph crosses 0% change is
where the solution has the same water potential as the potato tissue
Turgid, flaccid, plasmolysed. A turgid cell has high pressure potential and is firm. A flaccid cell has ψp = 0. In a strongly concentrated solution the membrane pulls away from the wall: the cell is plasmolysed.
🎯 Exam focus: definitions and explanations
Learn the definition of osmosis word for word, because it is asked often:
Osmosis is the net movement of water molecules from a region
of higher water potential to a region of lower water potential,
across a partially permeable membrane.
For an “explain” question, state the gradient, name the route, and say whether energy is needed.
facilitated diffusion: down the gradient, through channel or carrier
proteins, no ATP
active transport: against the gradient, carrier protein, ATP used
co-transport: glucose enters with Na⁺, which moves down its gradient
Common mistakes. Do not say water moves “to where there is more solute” without mentioning water potential. Do not say facilitated diffusion needs energy. Surface area and thickness change the rate of diffusion but not the direction.
Graph tip. In a “rate against concentration difference” graph, a straight line shows simple diffusion. A line that levels off means carrier or channel proteins are all in use.
✏️ Quick check · 1 of 11
Active transport
Which process requires ATP?
Look for the one that goes against the gradient.
✏️ Quick check · 2 of 11
Red blood cell
A red blood cell is placed in distilled water. What happens?
Water moves toward the more concentrated solution.
✏️ Quick check · 3 of 11
Facilitated diffusion
Facilitated diffusion uses:
It is still passive.
✍️ Type your answer · 4 of 11
A cube-shaped cell has sides of 2 cm. Its surface area is 24 cm² and its volume is 8 cm³.
What is the surface area to volume ratio?
Enter an exact answer, without rounding.
Divide surface area by volume.
✍️ Type your answer · 5 of 11
A cube-shaped cell has sides of 4 cm. Its surface area is 96 cm² and its volume is 64 cm³.
What is the surface area to volume ratio?
Enter an exact answer, without rounding.
Divide surface area by volume.
✍️ Type your answer · 6 of 11
A potato cylinder has a mass of 5.0 g. After soaking in salt water it has a mass of 4.0 g.
What is the percentage decrease in mass?
Enter an exact answer, without rounding.
%
Change divided by original, times 100.
✍️ Type your answer · 7 of 11
Fick’s law: the diffusion rate is inversely proportional to the thickness of the exchange surface.
If the surface becomes half as thick, by what factor does the rate change?
Enter an exact answer, without rounding.
Inverse proportion.
✍️ Type your answer · 8 of 11
A plant cell has a solute potential of −800 kPa and a water potential of −500 kPa.
What is its pressure potential, in kPa?
Enter an exact answer, without rounding.
kPa
Rearrange ψ = ψs + ψp.
✍️ Type your answer · 9 of 11
A potato cylinder has a mass of 4.0 g. After 30 minutes in a concentrated sucrose solution its mass is 3.4 g.
By what percentage has the mass fallen?
Enter an exact answer, without rounding.
%
Percentage change = change ÷ original × 100.
✏️ Quick check · 10 of 11
Direction of water movement
A cell has a water potential of −500 kPa and is placed in a solution with a water potential of −300 kPa. What happens?
Water moves from higher to lower water potential.
✏️ Quick check · 11 of 11
Increasing diffusion rate
Which change would give the fastest rate of diffusion across an exchange surface?
Use Fick’s law.
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
The hydrophobic tails of a phospholipid face:
Question 2 of 5
Osmosis is the net movement of:
Question 3 of 5
Which feature increases the rate of diffusion?
Question 4 of 5
Cholesterol in the membrane:
Question 5 of 5
Why are root hair cells long and thin?
Lesson 3 · Metabolism
🔑 Enzymes
Enzymes are protein catalysts that make the reactions of life fast enough to matter. Their shape is their function, so anything that changes the shape changes the rate.
🔒 How enzymes work
An enzyme has an active site with a shape matching its substrate. In the induced fit model the site changes shape slightly as the substrate binds. Forming the enzyme–substrate complex lowers the activation energy, so the reaction goes faster.
E + S → ES complex → E + P
Rate = amount of product / time
24 cm³ in 8 s → 24 / 8 = 3 cm³/s
The enzyme is not used up, so a small amount can convert a lot of substrate.
🖼️ See it
Rate rises with temperature until the enzyme’s shape is damaged, then it drops sharply.
🌡️ What changes the rate
Temperature: the rate rises as particles collide more often, up to the optimum. Above it the bonds holding the shape break and the enzyme denatures. pH has an optimum too. Substrate concentration: the rate rises until every active site is busy, then levels off.
Competitive inhibitor binds the active site
→ more substrate reduces the effect
Non-competitive inhibitor binds elsewhere, changes the shape
→ more substrate does not help
pH is logarithmic. A difference of 1 pH unit is a factor of 10 in H⁺ concentration.
🧪 Worked examples: rates, Q₁₀ and dilutions
Rate from a time. When you time how long a reaction takes to finish, the rate is the reciprocal: rate = 1 ÷ time.
amylase breaks down starch; the iodine colour disappears in 40 s
rate = 1 ÷ 40 = 0.025 s⁻¹
at a higher temperature it takes 20 s
rate = 1 ÷ 20 = 0.05 s⁻¹ (twice as fast)
Temperature coefficient. Q₁₀ compares the rates 10 °C apart.
Q₁₀ = rate at (T + 10) ÷ rate at T
rate 6 units at 25 °C and 12 units at 35 °C
Q₁₀ = 12 ÷ 6 = 2
Most enzymes have Q₁₀ ≈ 2 below the optimum:
the rate doubles for every 10 °C rise
Making a dilution. Use C₁V₁ = C₂V₂, or work out the ratio.
Make 50 cm³ of 0.5% enzyme solution from a 2% stock
factor = 2 ÷ 0.5 = 4 times weaker
stock needed = 50 ÷ 4 = 12.5 cm³
water needed = 50 − 12.5 = 37.5 cm³
Control variables. In a rate investigation say what you change (independent variable), what you measure (dependent variable) and what you keep the same: pH (use a buffer), temperature (water bath), substrate and enzyme volumes.
🎯 Exam focus: describing and explaining graphs
For any rate graph give the pattern and the reason, in that order.
Temperature graph
rising to the optimum: more kinetic energy, more collisions,
more enzyme–substrate complexes
above the optimum: bonds holding the tertiary structure break,
the active site loses its shape,
the substrate no longer fits (denatured)
Substrate graph
levels off: all active sites are occupied, so enzyme
concentration is the limiting factor
Common mistakes. Enzymes are not “killed” or “alive”: they are denatured. Enzymes are not “used up” by the reaction. A very low temperature does not denature an enzyme; it only slows it, and the activity returns on warming.
Inhibitors on a graph. A competitive inhibitor lowers the rate at low substrate concentration, but at high substrate concentration the rate reaches the same maximum. A non-competitive inhibitor lowers the maximum rate itself.
✏️ Quick check · 1 of 11
Active site
The active site of an enzyme is:
Think of where the substrate fits.
✏️ Quick check · 2 of 11
Denaturation
Above the optimum temperature an enzyme loses activity because:
Shape decides function.
✏️ Quick check · 3 of 11
Competitive inhibitor
The effect of a competitive inhibitor can be reduced by:
It competes with the substrate.
✍️ Type your answer · 4 of 11
An enzyme releases 24 cm³ of oxygen in 8 seconds.
What is the rate, in cm³/s?
Enter an exact answer, without rounding.
cm³/s
rate = volume / time.
✍️ Type your answer · 5 of 11
An enzyme produces 12 cm³ of gas in 4 minutes.
What is the rate, in cm³/min?
Enter an exact answer, without rounding.
cm³/min
rate = volume / time.
✍️ Type your answer · 6 of 11
At 20 °C the rate is 4 units. It doubles for every 10 °C rise (below the optimum).
What is the rate at 30 °C?
Enter an exact answer, without rounding.
One doubling.
✍️ Type your answer · 7 of 11
Pepsin works best in the stomach at pH 2.
How many times more acidic is pH 2 than pH 4?
Enter an exact answer, without rounding.
Two pH units apart.
✍️ Type your answer · 8 of 11
The blue-black colour of starch with iodine disappears after 40 s when amylase is added.
What is the rate of reaction, in s⁻¹? (rate = 1 ÷ time)
Enter an exact answer, without rounding.
s⁻¹
Rate is the reciprocal of the time.
✍️ Type your answer · 9 of 11
An enzyme reaction has a rate of 6 units at 25 °C and 12 units at 35 °C.
What is the temperature coefficient Q₁₀?
Enter an exact answer, without rounding.
Divide the higher rate by the lower rate.
✍️ Type your answer · 10 of 11
You need 50 cm³ of a 0.5% enzyme solution and you have a 2% stock solution.
What volume of stock solution is needed, in cm³?
Enter an exact answer, without rounding.
cm³
Work out the dilution factor first.
✏️ Quick check · 11 of 11
Substrate graph
The rate of an enzyme reaction stops increasing as the substrate concentration is raised further. The best explanation is that:
What is fully used at the plateau?
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
Enzymes speed up reactions by:
Question 2 of 5
At high substrate concentration the rate stops increasing because:
Question 3 of 5
A non-competitive inhibitor:
Question 4 of 5
Enzymes are usually specific because:
Question 5 of 5
Which statement about enzymes is correct?
Lesson 4 · Energy
🔥 Cellular Respiration
Cells release the energy stored in glucose and use it to make ATP, the energy currency that powers nearly everything a cell does.
🔋 The stages of aerobic respiration
Overall: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + energy (about 30 ATP per glucose).
stage where main output
glycolysis cytoplasm 2 pyruvate, net 2 ATP
link reaction mitochondrial matrix acetyl CoA, CO₂
Krebs cycle mitochondrial matrix CO₂, reduced carriers
electron transport inner membrane (cristae) most of the ATP
oxygen is the final acceptor, forming water
Without oxygen the electron transport chain stops, so the cell relies on glycolysis alone.
🖼️ See it
The electron transport chain makes the bulk of the roughly 30 ATP per glucose.
🍺 Anaerobic respiration and RQ
Anaerobic respiration only gets the net 2 ATP from glycolysis. Animals make lactate; yeast makes ethanol and CO₂.
The respiratory quotient
RQ = CO₂ produced / O₂ used
glucose: 6 CO₂ / 6 O₂ = 1.0
fat: about 0.7
CO₂ 140 mL, O₂ 200 mL → RQ = 140 / 200 = 0.7
RQ tells you the fuel. A value near 0.7 means fat is being used, and about 1.0 means carbohydrate.
🧪 Worked examples: respirometers and efficiency
Measuring oxygen uptake. In a respirometer, soda lime absorbs the CO₂, so the movement of the liquid shows the volume of O₂ used.
liquid moves 3.0 cm³ in 10 minutes
rate of oxygen uptake = 3.0 ÷ 10 = 0.3 cm³ per minute
per gram of tissue: if the sample is 2.0 g
0.3 ÷ 2.0 = 0.15 cm³ per minute per gram
Efficiency of respiration. Compare the energy captured in ATP with the energy in the glucose.
Use 1 mol glucose = 3000 kJ and 1 mol ATP = 30 kJ (stored)
30 ATP × 30 kJ = 900 kJ captured
efficiency = 900 ÷ 3000 × 100 = 30%
The other 70% is released as heat, which keeps mammals warm
What each stage gives. Keep a mental table: where, what goes in, what comes out.
glycolysis cytoplasm glucose → 2 pyruvate, net 2 ATP, 2 reduced NAD
link reaction matrix pyruvate → acetyl CoA + CO₂ + reduced NAD
Krebs cycle matrix acetyl CoA → 2 CO₂, reduced NAD and FAD, 1 ATP
oxidative phos. cristae reduced NAD/FAD + O₂ → water, most of the ATP
Role of NAD and FAD. They are coenzymes that pick up hydrogen. The reduced forms carry the hydrogen to the electron transport chain, where its energy is used to make ATP.
🎯 Exam focus: wording that gains marks
Be precise about energy: it is transferred, not made. ATP is the immediate energy source, not a long-term store.
Why is anaerobic respiration far less efficient?
• glucose is only partly broken down (to lactate or ethanol)
• much of the energy remains in those products
• only glycolysis makes ATP: net 2 per glucose, not about 30
Common mistakes. Respiration is a chemical process in every living cell; it is not the same as breathing. Oxygen is needed at the very end, as the final electron acceptor, not at the start. Do not write that CO₂ is produced in the electron transport chain; it comes from the link reaction and the Krebs cycle.
Lactate and exercise. During hard exercise muscles use oxygen faster than it is supplied, so pyruvate is converted to lactate. Afterwards the extra oxygen taken in (the oxygen debt) lets the liver convert lactate back to pyruvate or glucose.
✏️ Quick check · 1 of 11
Glycolysis site
Glycolysis takes place in the:
It happens before the mitochondria are involved.
✏️ Quick check · 2 of 11
Final electron acceptor
What is the final electron acceptor in the electron transport chain?
It is why we breathe.
✏️ Quick check · 3 of 11
Yeast
When yeast respires anaerobically it produces:
Think of bread and beer.
✍️ Type your answer · 4 of 11
One glucose molecule goes through complete aerobic respiration.
How many CO₂ molecules are released?
Enter an exact answer, without rounding.
Glucose has six carbons.
✍️ Type your answer · 5 of 11
Use the equation C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O.
How many O₂ molecules are needed per glucose?
Enter an exact answer, without rounding.
Read the balanced equation.
✍️ Type your answer · 6 of 11
A person produces 140 mL of CO₂ and uses 200 mL of O₂.
What is the respiratory quotient?
Enter an exact answer, without rounding.
RQ = CO₂ / O₂.
✍️ Type your answer · 7 of 11
Aerobic respiration gives about 30 ATP per glucose and anaerobic gives 2.
How many times more ATP does aerobic respiration make?
Enter an exact answer, without rounding.
Divide aerobic by anaerobic.
✍️ Type your answer · 8 of 11
In a respirometer containing soda lime, the liquid in the capillary tube moves 3.0 cm³ in 10 minutes.
What is the rate of oxygen uptake, in cm³ per minute?
Enter an exact answer, without rounding.
cm³/min
Volume divided by time.
✍️ Type your answer · 9 of 11
Assume 1 mol of glucose releases 3000 kJ and respiration makes 30 mol of ATP, each storing 30 kJ.
What is the efficiency of respiration, as a percentage?
Enter an exact answer, without rounding.
%
Energy captured ÷ energy available × 100.
✏️ Quick check · 10 of 11
The link reaction
What happens to pyruvate in the link reaction?
Which product enters the Krebs cycle?
✏️ Quick check · 11 of 11
Efficiency
Why does anaerobic respiration make much less ATP per glucose than aerobic respiration?
Which stages need oxygen?
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
Most ATP in aerobic respiration is made in the:
Question 2 of 5
The net ATP yield of glycolysis is:
Question 3 of 5
In human muscles during hard exercise, pyruvate is converted to:
Question 4 of 5
Which structure is the site of the Krebs cycle?
Question 5 of 5
ATP is useful as an energy currency because it:
Lesson 5 · Energy
🌿 Photosynthesis
Plants, algae and some bacteria capture light energy and use it to build glucose from carbon dioxide and water. Almost every food chain starts here.
light-dependent thylakoid membranes
light splits water (photolysis) → O₂, H⁺, electrons
makes ATP and NADPH
Calvin cycle (light-independent) stroma
rubisco fixes CO₂ onto RuBP
uses ATP and NADPH to make sugar
The oxygen released comes from the water, not from the carbon dioxide.
🖼️ See it
The light reactions supply the energy carriers that power the Calvin cycle.
📊 Limiting factors
The rate of photosynthesis is set by the factor in shortest supply: light intensity, CO₂ concentration or temperature. Raising a factor that is not limiting changes nothing.
Light intensity ∝ 1 / d²
move the lamp from 10 cm to 20 cm
→ d doubles → intensity falls to 1/4
Rate = 45 bubbles / 3 min = 15 per minute
Why leaves are green: chlorophyll absorbs red and blue light and reflects green.
🧪 Worked examples: chromatography and net photosynthesis
Separating pigments. Paper chromatography separates the pigments in a leaf extract. The Rf value identifies each pigment, because it is the same in the same solvent.
Rf = distance moved by the pigment ÷ distance moved by the solvent front
chlorophyll a moved 3.2 cm; solvent front moved 8.0 cm
Rf = 3.2 ÷ 8.0 = 0.4
A pigment can never have an Rf above 1.
Mark the pencil start line and the solvent front straight away.
Gross and net photosynthesis. A leaf also respires, so the gas exchange you measure is the net result.
gross photosynthesis = net photosynthesis + respiration
net CO₂ uptake 12 µmol per minute; respiration releases 5 µmol per minute
gross = 12 + 5 = 17 µmol per minute
Compensation point: light intensity where photosynthesis = respiration,
so there is no net gas exchange
Action and absorption spectra. The absorption spectrum shows which wavelengths pigments absorb (peaks in blue, about 430–450 nm, and red, about 650–680 nm). The action spectrum shows the rate of photosynthesis at each wavelength and closely matches those peaks.
🎯 Exam focus: the two stages
Examiners expect each stage to be described with its location, its inputs and its outputs.
Common mistakes. The light-independent stage does not happen “in the dark”: it needs ATP and reduced NADP from the light-dependent stage, so it stops soon after the light is switched off. Plants respire all the time, not only at night. The oxygen comes from water, not from CO₂.
Limiting factor questions. Look at the graph and state which factor is limiting on each section. Where the line is rising, the factor on the x-axis is limiting. Where it is flat, another factor (CO₂, temperature or light) is limiting.
✏️ Quick check · 1 of 11
Light-dependent site
The light-dependent reactions occur in the:
Where is chlorophyll?
✏️ Quick check · 2 of 11
Source of oxygen
The oxygen released in photosynthesis comes from:
It happens in the light-dependent stage.
✏️ Quick check · 3 of 11
Rubisco
Which enzyme fixes carbon dioxide in the Calvin cycle?
It is the most abundant enzyme on Earth.
✍️ Type your answer · 4 of 11
Use the equation 6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂.
How many O₂ molecules are released per glucose made?
Enter an exact answer, without rounding.
Read the balanced equation.
✍️ Type your answer · 5 of 11
A pondweed releases 45 bubbles of oxygen in 3 minutes.
How many bubbles is that per minute?
Enter an exact answer, without rounding.
rate = count / time.
✍️ Type your answer · 6 of 11
Light intensity is proportional to 1/d². A lamp is moved from 10 cm to 20 cm away.
What fraction of the original intensity is left?
Enter an exact answer, without rounding.
Square the factor.
✍️ Type your answer · 7 of 11
Each glucose needs 6 carbon atoms and each CO₂ supplies one.
How many CO₂ molecules are fixed to make one glucose?
Enter an exact answer, without rounding.
Count the carbons.
✍️ Type your answer · 8 of 11
On a chromatogram, chlorophyll a travelled 3.2 cm and the solvent front travelled 8.0 cm.
What is the Rf value of chlorophyll a?
Enter an exact answer, without rounding.
Distance of pigment ÷ distance of solvent.
✍️ Type your answer · 9 of 11
A leaf takes up 12 µmol of CO₂ per minute in the light. In the dark the same leaf releases 5 µmol of CO₂ per minute by respiration.
What is the rate of gross photosynthesis, in µmol of CO₂ per minute?
Enter an exact answer, without rounding.
µmol/min
Photosynthesis must also cover the CO₂ released by respiration.
✏️ Quick check · 10 of 11
Compensation point
At the light compensation point:
Think net gas exchange of zero.
✏️ Quick check · 11 of 11
Calvin cycle
What is the role of ATP and reduced NADP in the light-independent stage?
These come from the light-dependent stage.
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
A plant limited by CO₂ concentration is given brighter light. The rate of photosynthesis will:
Question 2 of 5
The products of the light-dependent stage that are used in the Calvin cycle are:
Question 3 of 5
Chlorophyll absorbs mostly:
Question 4 of 5
The Calvin cycle takes place in the:
Question 5 of 5
Which is a product of photosynthesis?
Lesson 6 · Genetics
🧬 DNA Structure & Replication
DNA stores the instructions for building every protein. Its double-helix structure explains both how it holds information and how it is copied.
🪜 The double helix
DNA is two antiparallel strands of nucleotides (sugar, phosphate and a base) twisted into a helix. The bases pair by complementary base pairing: A with T (2 hydrogen bonds) and C with G (3 hydrogen bonds).
A = T and C ≡ G
If 20% of the bases are A, then 20% are T
A + T = 40%, so G + C = 60%, so G = 30%
Segment: 10 A–T pairs and 5 G–C pairs
hydrogen bonds = 10·2 + 5·3 = 35
Chargaff’s rule: in double-stranded DNA the amount of A equals T and C equals G.
🖼️ See it
Each new double helix keeps one old strand: replication is semi-conservative.
📑 Semi-conservative replication
Before a cell divides, each DNA molecule is copied. Helicase unwinds and unzips the helix, and DNA polymerase adds nucleotides to each exposed template strand. Each new molecule has one old and one new strand, which is why it is called semi-conservative.
1 molecule → 2 → 4 → 8
after n rounds: 2ⁿ molecules
Meselson–Stahl: heavy ¹⁵N DNA moved to ¹⁴N
after 2 rounds 2 of 4 molecules still hold some ¹⁵N
🧪 Worked examples: base counts and DNA length
Chargaff’s rule. A = T and C = G, so the percentages of A + T + C + G always total 100%.
A DNA sample has 2400 bases and 22% are cytosine
C = 22% → G = 22%
A + T = 100 − 44 = 56%, so A = 28% and T = 28%
adenine = 28% × 2400 = 672 bases
Length of a gene. The distance between base pairs along the helix is 0.34 nm.
2000 base pairs × 0.34 nm = 680 nm
Human DNA is about 3.2 × 10⁹ base pairs in one set of chromosomes
length = 3.2 × 10⁹ × 0.34 nm ≈ 1.1 m, packed into a nucleus of
about 6 µm across by coiling around histone proteins
Bonds in DNA. Distinguish the two types clearly.
phosphodiester bond joins neighbouring nucleotides along ONE strand
(strong covalent bond, sugar–phosphate backbone)
hydrogen bonds join the bases ACROSS the two strands
(weak individually, strong together, easy to unzip)
Why weak bonds matter. Hydrogen bonds break easily, so helicase can unzip the strands for replication and transcription, but the many hydrogen bonds together keep the molecule stable.
🎯 Exam focus: replication and structure
When asked how the structure of DNA suits its functions, give three features:
stable sugar–phosphate backbone, many hydrogen bonds,
bases protected inside the helix
stores information the sequence of bases forms the genetic code
copies accurately complementary base pairing, so each strand is
a template (semi-conservative replication)
Replication in order
1 helicase breaks the hydrogen bonds and unwinds the helix
2 each strand acts as a template
3 free nucleotides pair with the exposed bases (A–T, C–G)
4 DNA polymerase joins them with phosphodiester bonds
5 two molecules form, each with one old and one new strand
Common mistakes. Do not say the strands are joined across by “covalent bonds” (they are hydrogen bonds). DNA polymerase joins nucleotides; helicase only unwinds. Replication happens in the S phase of interphase, before mitosis, not during it. Adenine pairs with thymine in DNA, never with uracil.
✏️ Quick check · 1 of 11
Base pairing
Adenine pairs with:
A–T and C–G.
✏️ Quick check · 2 of 11
Semi-conservative
Semi-conservative replication means that each new DNA molecule:
Semi means half.
✏️ Quick check · 3 of 11
Helicase
The enzyme that unwinds the DNA double helix is:
Think of the word helix.
✍️ Type your answer · 4 of 11
In a sample of double-stranded DNA, 20% of the bases are adenine.
What percentage are guanine?
Enter an exact answer, without rounding.
%
A = T and G = C.
✍️ Type your answer · 5 of 11
A double-stranded DNA molecule contains 400 nucleotides in total.
How many base pairs is that?
Enter an exact answer, without rounding.
One pair is two nucleotides.
✍️ Type your answer · 6 of 11
A segment has 10 A–T pairs (2 hydrogen bonds each) and 5 G–C pairs (3 each).
How many hydrogen bonds hold it together?
Enter an exact answer, without rounding.
Add the bonds from each type of pair.
✍️ Type your answer · 7 of 11
One DNA molecule is replicated three times.
How many DNA molecules are there at the end?
Enter an exact answer, without rounding.
It doubles each round.
✍️ Type your answer · 8 of 11
A sample of DNA contains 2400 bases. 22% of the bases are cytosine.
How many adenine bases does it contain?
Enter an exact answer, without rounding.
Use A = T and C = G, then share what is left.
✍️ Type your answer · 9 of 11
A gene is 2000 base pairs long, and neighbouring base pairs are 0.34 nm apart.
What is the length of the gene, in nm?
Enter an exact answer, without rounding.
nm
Multiply the number of base pairs by the spacing.
✏️ Quick check · 10 of 11
Backbone bond
Which bond joins two neighbouring nucleotides on the same DNA strand?
It is part of the sugar–phosphate backbone.
✏️ Quick check · 11 of 11
DNA polymerase
What is the function of DNA polymerase in replication?
Polymerase means “makes a polymer”.
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
The sugar in DNA is:
Question 2 of 5
Which bases are pyrimidines?
Question 3 of 5
DNA polymerase:
Question 4 of 5
Why is G–C rich DNA harder to separate by heating than A–T rich DNA?
Question 5 of 5
The two strands of DNA run:
Lesson 7 · Genetics
🧵 Protein Synthesis
A gene is a recipe for a polypeptide. The cell first copies the recipe into mRNA, then reads it three bases at a time to assemble the protein.
✍️ Transcription
In the nucleus RNA polymerase reads the template strand of a gene and builds a complementary strand of mRNA. RNA uses uracil (U) instead of thymine, so A pairs with U.
DNA template 3′–T A C G G T–5′
mRNA 5′–A U G C C A–3′
Each group of 3 bases = 1 codon
🖼️ See it
DNA → RNA → protein is the central dogma of molecular biology.
🏭 Translation
At a ribosome, tRNA molecules bring amino acids. Each tRNA has an anticodon that pairs with an mRNA codon. The start codon is AUG (methionine), and UAA, UAG and UGA are stop codons.
4 bases, 3 per codon → 4³ = 64 codons
3 are stop codons → 61 code for amino acids
20 amino acids → the code is degenerate (several codons per amino acid)
mRNA codon AUG → tRNA anticodon UAC
30 coding bases → 10 amino acids
100 amino acids → 3·100 + 3 (stop) = 303 bases
A mutation that changes a codon to another for the same amino acid is silent. Deleting one base shifts the whole reading frame (frameshift).
🧪 Worked example: from gene to polypeptide
Follow the information in four steps: template → mRNA → codons → amino acids.
DNA template strand 3′ TAC CGA TTT ACT 5′
mRNA (transcribed) 5′ AUG GCU AAA UGA 3′
codon table: AUG = methionine (start) GCU = alanine
AAA = lysine UGA = stop
polypeptide: Met – Ala – Lys (then the ribosome stops)
Pairing rule for transcription. Every base on the template pairs with its complement, but T on the DNA template pairs with A, and A on the template pairs with U in the mRNA.
Mutations. Compare the new sequence codon by codon.
GCU → GCC still alanine silent (no change)
GAG → GUG glutamic acid → valine missense (sickle cell)
UAU → UAA tyrosine → stop nonsense (short protein)
insert or delete 1 base every later codon changes frameshift
Eukaryote processing. The pre-mRNA contains introns (non-coding) and exons (coding). Introns are removed and the exons joined (splicing) before the mRNA leaves through a nuclear pore.
template strand the DNA strand that is read; the mRNA matches the OTHER
(coding) strand, with U for T
codon 3 bases on mRNA
anticodon 3 bases on tRNA, complementary to the codon
gene a sequence of DNA coding for a polypeptide
universal code nearly all organisms use the same codons
degenerate code more than one codon for most amino acids
Common mistakes. mRNA has codons; tRNA has anticodons. mRNA contains uracil, never thymine. Proteins are made at ribosomes in the cytoplasm, not in the nucleus. The mRNA passes through a nuclear pore; it does not cross the nuclear envelope itself.
Why a frameshift is worse. Adding or removing one base shifts the reading frame, so every codon after the change is altered. A substitution can change at most one amino acid. Adding or removing three bases removes or adds one amino acid but keeps the rest of the frame.
✏️ Quick check · 1 of 11
Transcription site
Transcription happens in the:
Where is the DNA?
✏️ Quick check · 2 of 11
Template to mRNA
A DNA template strand reads 3′–TAC–5′. The mRNA codon formed is:
Remember that RNA has U instead of T.
✏️ Quick check · 3 of 11
Anticodon
The tRNA anticodon that pairs with the mRNA codon AUG is:
Pair each base of the codon.
✍️ Type your answer · 4 of 11
A reading frame has 30 nucleotides, all coding for amino acids (no stop codon).
How many amino acids are in the polypeptide?
Enter an exact answer, without rounding.
Three bases per codon.
✍️ Type your answer · 5 of 11
A polypeptide has 100 amino acids and its mRNA ends with a stop codon.
How many mRNA bases are in the coding region including the stop codon?
Enter an exact answer, without rounding.
Do not forget the stop codon.
✍️ Type your answer · 6 of 11
There are 4 different bases and a codon is 3 bases long.
How many different codons are possible?
Enter an exact answer, without rounding.
4 choices for each of 3 positions.
✍️ Type your answer · 7 of 11
Of the 64 codons, 3 are stop codons.
How many codons code for an amino acid?
Enter an exact answer, without rounding.
Subtract the stop codons.
✍️ Type your answer · 8 of 11
The coding region of an mRNA, including the stop codon, is 1200 nucleotides long.
How many amino acids are in the polypeptide?
Enter an exact answer, without rounding.
Divide by 3, then remove the stop codon.
✍️ Type your answer · 9 of 11
A pre-mRNA transcript is 1500 nucleotides long. The introns total 600 nucleotides.
How long is the mature mRNA, in nucleotides?
Enter an exact answer, without rounding.
nucleotides
Only exons remain after splicing.
✏️ Quick check · 10 of 11
Silent mutation
A DNA change alters the mRNA codon from GAG to GAA, and both code for glutamic acid. This mutation is:
Is the amino acid changed?
✏️ Quick check · 11 of 11
Frameshift
Why does the insertion of one nucleotide in a gene usually cause more damage than a substitution?
Think about groups of three.
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
RNA differs from DNA because it:
Question 2 of 5
The genetic code is described as degenerate because:
Question 3 of 5
Where does translation take place?
Question 4 of 5
A mutation that changes the codon CCU to CCC, still coding for proline, is:
Question 5 of 5
The role of tRNA is to:
Lesson 8 · Genetics
✂️ Cell Cycle, Mitosis & Meiosis
Cells divide to grow and repair (mitosis) and to make gametes for sexual reproduction (meiosis). Both begin with copied chromosomes, but they end very differently.
🔁 The cell cycle and mitosis
The cycle has interphase (G₁ growth, S DNA replication, G₂ preparation) and M phase (mitosis then cytokinesis). Mitosis has four stages: prophase, metaphase, anaphase, telophase. It makes two genetically identical diploid cells.
Human body cell 2n = 46
after mitosis: two cells, each 46 chromosomes
Mitotic index = cells in mitosis / total cells · 100
12 of 200 cells → 6%
doubling time 20 min → in 2 hours (6 cycles)
1 cell → 2⁶ = 64 cells
🖼️ See it
Interphase is G1, S and G2; the cell only divides in the M phase.
🎲 Meiosis and variation
Meiosis has two divisions and makes four haploid cells that are genetically different. In meiosis I homologous chromosomes separate, and in meiosis II the chromatids separate.
Sources of variation
1. crossing over (prophase I) swaps parts of homologues
2. independent assortment: 2ⁿ combinations
n = 4 pairs → 2⁴ = 16
Human gamete: n = 23 chromosomes
Non-disjunction → e.g. 47 chromosomes (trisomy 21)
Mitosis keeps the chromosome number; meiosis halves it. That is why fertilisation restores 46.
🧪 Worked examples: counting chromosomes and timing mitosis
Chromosomes, chromatids and DNA. After the S phase each chromosome has two identical sister chromatids joined at the centromere. The number of chromosomes is counted by the centromeres.
Human cell, 2n = 46
G1: 46 chromosomes, 46 DNA molecules
after S phase (G2, prophase, metaphase): still 46 chromosomes,
but 92 chromatids (92 DNA molecules)
anaphase: sister chromatids separate → 92 chromosomes briefly
telophase: two nuclei, each with 46 chromosomes
How long does each stage last? The proportion of cells seen in a stage matches the proportion of the cycle spent in it.
cell cycle length 24 h; 8% of cells are in mitosis
duration of mitosis = 0.08 × 24 = 1.92 hours
Variation from independent assortment. With n pairs of homologous chromosomes, a parent can make 2ⁿ different gametes (before crossing over).
2n = 10 → n = 5 pairs → 2⁵ = 32 gamete types
human 2n = 46 → n = 23 → 2²³ = 8 388 608 gamete types
Two parents: 8 388 608² ≈ 7 × 10¹³ possible zygotes
Checkpoints. The cell cycle is controlled at checkpoints (G₁, G₂ and during mitosis). Cancer happens when mutations in genes that control the cycle let cells divide without control.
🎯 Exam focus: comparing mitosis and meiosis
mitosis meiosis
divisions 1 2
daughter cells 2 4
chromosome no. same as parent (2n) half of parent (n)
genetically identical different (variation)
homologues pair no yes, in prophase I
crossing over no yes
function growth, repair, making gametes
asexual reproduction
Common mistakes. In meiosis I the homologous chromosomes separate; in meiosis II the chromatids separate. Chromosomes line up at the equator in metaphase, not anaphase. Cytokinesis is the division of the cytoplasm and is separate from mitosis. Do not write that “the cell has half as much DNA” after S phase: it has twice as much.
Drawing tip. Label the centromere, the spindle fibres and the position of the chromosomes. In an exam drawing of anaphase, V-shaped chromatids are pulled to opposite poles with the centromere leading.
✏️ Quick check · 1 of 11
S phase
During the S phase of interphase the cell:
S is for synthesis.
✏️ Quick check · 2 of 11
Crossing over
Crossing over occurs during:
Think of meiosis I.
✏️ Quick check · 3 of 11
Mitosis outcome
Mitosis produces:
Identical copies.
✍️ Type your answer · 4 of 11
A human body cell has 46 chromosomes and undergoes meiosis.
How many chromosomes are in each gamete?
Enter an exact answer, without rounding.
Gametes are haploid.
✍️ Type your answer · 5 of 11
One diploid cell goes through meiosis.
How many cells are produced?
Enter an exact answer, without rounding.
Two divisions.
✍️ Type your answer · 6 of 11
An organism has 4 pairs of chromosomes (n = 4). Ignore crossing over.
How many different gametes are possible by independent assortment?
Enter an exact answer, without rounding.
2 raised to the number of pairs.
✍️ Type your answer · 7 of 11
Of 200 root-tip cells counted, 12 are in mitosis.
What is the mitotic index, in percent?
Enter an exact answer, without rounding.
%
Divide by the total cells and multiply by 100.
✍️ Type your answer · 8 of 11
The cell cycle of a root-tip cell lasts 24 hours. In a stained slide, 8% of the cells are in mitosis.
How long does mitosis last, in hours?
Enter an exact answer, without rounding.
hours
Multiply the proportion by the cycle time.
✍️ Type your answer · 9 of 11
A plant cell has 14 chromosomes (2n = 14). It has completed the S phase and is now in prophase.
How many chromatids does it contain?
Enter an exact answer, without rounding.
Each chromosome is now made of two chromatids.
✍️ Type your answer · 10 of 11
An organism has 5 pairs of homologous chromosomes (2n = 10).
How many different gametes could it make by independent assortment alone?
Enter an exact answer, without rounding.
Use 2ⁿ, where n is the number of pairs.
✏️ Quick check · 11 of 11
Anaphase
A cell with 2n = 4 is in anaphase of mitosis. How many chromosomes does it contain?
Count the chromatids that have just separated.
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
In which stage of mitosis do chromosomes line up at the equator?
Question 2 of 5
Cytokinesis is the division of the:
Question 3 of 5
Down syndrome is usually caused by:
Question 4 of 5
Meiosis produces genetic variation by:
Question 5 of 5
Uncontrolled mitosis can lead to:
Final Challenge
🏆 16-Question Challenge Quiz
Questions from every topic in this course. Pick an answer for each one, then hit Check My Answers.
0 / 16
💪 Keep practicing — go back through the lessons and retake the quiz!
Q1 · Biomolecules
The bond that joins two amino acids is a:
Q2 · Biomolecules
Cellulose is important to plants because it:
Q3 · Membranes & Transport
Oxygen enters a cell mainly by:
Q4 · Membranes & Transport
Why are root hair cells long and thin?
Q5 · Enzymes
Most enzymes are:
Q6 · Enzymes
Which statement about enzymes is correct?
Q7 · Cellular Respiration
Which stage of respiration needs no oxygen?
Q8 · Cellular Respiration
ATP is useful as an energy currency because it:
Q9 · Photosynthesis
The compensation point is the light intensity where:
Q10 · Photosynthesis
Which is a product of photosynthesis?
Q11 · DNA Structure & Replication
Which pair of bases is joined by three hydrogen bonds?