Cells must take in nutrients and remove waste through their membranes. Three processes move substances across them.
🌫️ Diffusion and osmosis
Diffusion is the net movement of particles from a region of higher concentration to lower concentration, down the concentration gradient. It needs no energy. Osmosis is the diffusion of water through a partially permeable membrane, from a dilute solution (high water potential) to a concentrated one (low water potential).
oxygen in the lungs → blood (diffusion)
carbon dioxide in blood → lungs (diffusion)
potato chip in pure water: gains water, gets heavier and firmer
potato chip in strong salt solution: loses water, gets lighter and floppy
🖼️ See it
Particles drift down a gradient for free; moving them up a gradient costs energy.
🔋 Active transport and surface area
Active transport moves particles against the concentration gradient, from low to high, using energy from respiration and carrier proteins. Root hairs use it to take up mineral ions. The rate of diffusion rises with a steeper gradient, higher temperature and a larger surface area to volume ratio.
cube of side 2 cm: surface area = 6 × 4 = 24 cm², volume = 8 cm³
SA : V = 24 / 8 = 3 : 1
cube of side 4 cm: surface area = 6 × 16 = 96 cm², volume = 64 cm³
SA : V = 96 / 64 = 1.5 : 1
smaller objects have a larger ratio
Diffusion is passive; active transport costs energy. Ask which direction the particles move relative to the gradient.
🧪 Worked examples: osmosis practical and percentage change
Potato cylinders in sugar solutions. Cut identical cylinders, weigh them, leave them in different concentrations for a fixed time, blot dry and reweigh. Use percentage change in mass, because the starting masses differ slightly.
start mass 2.50 g, final mass 2.20 g
change = 2.20 − 2.50 = −0.30 g
percentage change = −0.30 ÷ 2.50 × 100 = −12%
(negative: the potato lost water, so the solution was more concentrated than the cells)
Finding the concentration with no net movement. Plot percentage change against concentration. Where the line crosses 0%, there is no net osmosis, so the solution matches the water potential of the potato cells.
Surface area to volume. A cube of side L has surface area 6L² and volume L³, so the ratio is 6 ÷ L. Doubling the side halves the ratio. This is why large animals need gas exchange surfaces, such as lungs, and a transport system.
cube of side 3 cm
SA = 6 × 9 = 54 cm² V = 27 cm³ SA : V = 2 : 1
Cells in solutions. An animal cell in pure water takes in water and may burst. A plant cell in pure water becomes turgid, because the cell wall stops it bursting. In a strong solution, a plant cell becomes flaccid and then plasmolysed, as the membrane pulls away from the wall.
🎯 Exam focus: wording the explanation
Marks for osmosis go to precise language. Always mention the partially permeable membrane and the direction of water movement in terms of water potential.
weak answer: “water moves into the cell because the cell is more concentrated”
better answer: “water moves by osmosis, through the partially permeable membrane,
from a solution with a higher water potential (dilute)
to one with a lower water potential (concentrated)”
Common mistakes. Saying that “water diffuses to where there is less water” without naming the membrane. Using “concentration of water” loosely. Confusing osmosis with diffusion of solute particles. Forgetting that active transport needs energy from respiration, so cells with lots of mitochondria (such as root hair cells) can do it. Forgetting to blot the potato dry before weighing.
Control variables. Same cylinder size, same volume of solution, same time, same temperature. Change only the concentration.
✏️ Quick check · 1 of 11
Diffusion
Diffusion is the movement of particles from:
Down the gradient.
✏️ Quick check · 2 of 11
Osmosis
Osmosis is the movement of:
It involves water.
✏️ Quick check · 3 of 11
Active transport
Active transport needs:
Uphill movement.
✏️ Quick check · 4 of 11
Potato in salt
A potato chip placed in concentrated salt solution will:
Water follows the more concentrated side.
✍️ Type your answer · 5 of 11
A cube has sides of 2 cm.
What is its total surface area, in cm²?
Enter an exact answer, without rounding.
cm²
A cube has 6 equal faces.
✍️ Type your answer · 6 of 11
A cube has sides of 2 cm.
What is its volume, in cm³?
Enter an exact answer, without rounding.
cm³
Side cubed.
✍️ Type your answer · 7 of 11
A cube has a surface area of 24 cm² and a volume of 8 cm³.
What is the surface area to volume ratio, as a single number?
Enter an exact answer, without rounding.
Divide the area by the volume.
✍️ Type your answer · 8 of 11
A potato cylinder has a mass of 2.50 g at the start and 2.20 g after being left in a sugar solution.
What is the percentage change in mass? Give the size of the change as a positive number.
Enter an exact answer, without rounding.
%
Divide the change by the starting mass.
✍️ Type your answer · 9 of 11
A cube has sides of 3 cm.
What is its surface area to volume ratio, as a single number?
Enter an exact answer, without rounding.
Use 6L² and L³.
✍️ Type your answer · 10 of 11
A potato cylinder gains 0.40 g from a start mass of 4.00 g.
What is the percentage increase in mass?
Enter an exact answer, without rounding.
%
Change divided by starting mass.
✏️ Quick check · 11 of 11
Plant cells
A plant cell is placed in pure water. What happens?
Think about the cell wall.
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
Which process needs energy?
Question 2 of 5
Which change will speed up diffusion?
Question 3 of 5
Small cells exchange materials efficiently because they have a:
Question 4 of 5
Root hairs take up mineral ions from dilute soil water by:
Question 5 of 5
Red blood cells placed in pure water will:
Lesson 2 · Human nutrition
🍞 Digestion & Enzymes
Food molecules are too large to be absorbed. Enzymes in the digestive system break them into small, soluble molecules.
✂️ Enzymes and digestion
Enzymes are biological catalysts made of protein. Each fits one substrate (the lock-and-key model). Amylase breaks down starch into sugars, protease breaks proteins into amino acids, and lipase breaks fats into fatty acids and glycerol. Bile from the liver is not an enzyme; it emulsifies fat to give lipase a larger surface area.
Food passes through the gut in this order; the small intestine absorbs most nutrients.
🌡️ Conditions and the gut
Enzyme activity rises with temperature up to an optimum (about 37 °C for human enzymes); above this the enzyme is denatured because its shape changes. Each enzyme also has an optimum pH: pepsin in the acidic stomach works best near pH 2, while pancreatic enzymes work best in the alkaline small intestine. The small intestine absorbs digested food through villi, which give it a very large surface area.
mouth: amylase starts digesting starch
stomach: acid + protease digest protein
small intestine: most digestion + absorption
large intestine: absorbs water
Denaturing is permanent. Cooling a denatured enzyme does not bring its shape back.
🧪 Worked examples: reading enzyme graphs and tests
Food tests. Learn the reagent, the conditions and the positive result for each.
starch iodine solution yellow-brown → blue-black
reducing sugar Benedict’s solution, heat blue → green → orange → brick red
protein biuret (NaOH + copper sulfate) blue → purple/lilac
fat ethanol, then add water cloudy white emulsion
Rate of an enzyme reaction. Rate = amount of product (or substrate used) ÷ time. A common practical is the time taken for starch to disappear: rate ∝ 1 ÷ time.
amylase at 20 °C: starch gone in 120 s rate = 1 ÷ 120 = 0.0083 per s
amylase at 37 °C: starch gone in 30 s rate = 1 ÷ 30 = 0.033 per s
the rate at 37 °C is 4 times faster
Reading the temperature curve. The rate rises with temperature as particles gain energy, collide more often and with more energy. The peak is the optimum. After that the rate drops sharply, because the active site changes shape (denaturation).
Digestive juices. Saliva (amylase), gastric juice (protease and hydrochloric acid), pancreatic juice (amylase, protease, lipase), bile (emulsifies fat; made in the liver, stored in the gall bladder).
🎯 Exam focus: explaining denaturation and absorption
Explain denaturation in three steps: (1) the high temperature (or wrong pH) breaks the bonds holding the enzyme in shape, (2) the active site changes shape, (3) the substrate no longer fits, so the reaction stops.
villi adaptations
• one cell thick wall → short diffusion distance
• rich blood supply → keeps the concentration gradient steep
• large surface area from the folded lining → faster absorption
• lacteal → absorbs fatty acids and glycerol
Common mistakes. Saying that enzymes are “killed” (they were never alive; they are denatured). Writing that bile digests fat (it only emulsifies it). Saying that low temperature denatures an enzyme: it only slows it, and the enzyme works again on warming. Forgetting that the stomach is acidic and the small intestine alkaline.
Absorption versus digestion. Digestion breaks large molecules into small soluble ones. Absorption moves the small molecules into the blood.
✏️ Quick check · 1 of 11
Enzyme
An enzyme is a:
It is a protein that speeds reactions.
✏️ Quick check · 2 of 11
Amylase
Amylase digests:
Think of sugar.
✏️ Quick check · 3 of 11
Bile
Bile helps digestion by:
It increases the surface area.
✏️ Quick check · 4 of 11
Denaturing
If an enzyme becomes denatured, it:
Shape controls function.
✍️ Type your answer · 5 of 11
Amylase works best at 37 °C. A student heats it to 80 °C.
How many degrees above its optimum temperature is this?
Enter an exact answer, without rounding.
°C
Subtract the optimum.
✍️ Type your answer · 6 of 11
A food sample contains 12 g of protein and 8 g of fat.
What is the total mass of protein and fat, in g?
Enter an exact answer, without rounding.
g
Add the two masses.
✍️ Type your answer · 7 of 11
An enzyme digests 24 g of starch in 6 minutes at a steady rate.
How many grams are digested per minute?
Enter an exact answer, without rounding.
g/min
Mass ÷ time.
✍️ Type your answer · 8 of 11
Starch disappears in 30 s with amylase at 37 °C and in 120 s at 20 °C.
How many times faster is the reaction at 37 °C?
Enter an exact answer, without rounding.
Divide the two times.
✏️ Quick check · 9 of 11
Food test
A food gives a brick-red precipitate when heated with Benedict’s solution. What does it contain?
Name the reagent’s target.
✏️ Quick check · 10 of 11
Cold enzymes
An enzyme solution is cooled to 0 °C. What happens to the enzyme?
Compare cold and heat.
✏️ Quick check · 11 of 11
Bile
What is the function of bile?
Is it an enzyme?
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
Protease breaks protein down into:
Question 2 of 5
Most absorption of digested food occurs in the:
Question 3 of 5
The stomach contains acid because:
Question 4 of 5
Villi are adapted for absorption by having:
Question 5 of 5
Cooking an egg changes its protein shape permanently. This is like:
Lesson 3 · Human biology
❤️ Heart, Blood & Circulation
The circulatory system delivers oxygen and nutrients to every cell and carries away waste.
🫀 The double circulation
The heart is a muscular pump with four chambers. The right side sends deoxygenated blood to the lungs; the left side sends oxygenated blood around the body, so the left ventricle has the thicker wall. Arteries carry blood away from the heart at high pressure; veins return it, with valves; capillaries are one cell thick, so substances can diffuse across them.
body → vena cava → right atrium → right ventricle
→ pulmonary artery → lungs (gains O₂)
→ pulmonary vein → left atrium → left ventricle
→ aorta → body
🖼️ See it
Blood passes through the heart twice per circuit: to the lungs to pick up oxygen, then to the body.
🩸 Blood and heart rate
Blood contains red cells (haemoglobin carries oxygen, no nucleus), white cells (fight pathogens), platelets (clotting) and plasma (carries dissolved substances). Cardiac output = heart rate × stroke volume. Exercise raises the heart rate to deliver more oxygen to the muscles.
Arteries carry blood away, veins return it. Not all arteries carry oxygenated blood: the pulmonary artery does not.
🧪 Worked examples: cardiac output and heart rate data
Cardiac output = heart rate × stroke volume.
at rest: 70 beats/min × 70 mL = 4900 mL/min
exercise: 150 beats/min × 100 mL = 15 000 mL/min = 15 L/min
increase = 15 000 ÷ 4900 ≈ 3.1 times
Recovery time. After exercise the heart rate stays high for a time, to pay back the oxygen debt and remove lactic acid. Fitter people have a lower resting heart rate and recover faster.
Pressure changes in the heart. When the ventricles contract, the pressure rises above that in the arteries, so the semilunar valves open and blood is pumped out. When they relax, the pressure falls, the semilunar valves close and the atrioventricular valves open to let the ventricles fill.
heart rate 75 beats/min
time for one beat = 60 ÷ 75 = 0.8 s
Heart disease. A blocked coronary artery starves heart muscle of oxygen and glucose, causing a heart attack. Risk factors include smoking, a diet high in saturated fat, high blood pressure and lack of exercise.
🎯 Exam focus: describing and explaining circulation
Describe a route by naming every structure in order, including both valves where the question asks for them. For example: vena cava → right atrium → tricuspid valve → right ventricle → semilunar valve → pulmonary artery.
artery thick muscular wall, elastic fibres, narrow lumen (high pressure)
vein thin wall, wide lumen, valves (low pressure)
capillary one cell thick (short diffusion distance), narrow, very many
Common mistakes. Calling the arteries “the vessels with oxygenated blood”: the pulmonary artery carries deoxygenated blood. Drawing the left and right sides the wrong way round (the diagram is viewed as if you face the person). Forgetting that the heart is supplied by its own coronary arteries. Giving a heart rate answer per 15 s rather than per minute.
Thicker left ventricle. It pumps blood around the whole body at a higher pressure than the right ventricle pumps to the nearby lungs.
✏️ Quick check · 1 of 11
Vessels
Blood flows away from the heart in:
A for away.
✏️ Quick check · 2 of 11
Capillaries
Capillaries are adapted for exchange because their walls are:
Thin walls.
✏️ Quick check · 3 of 11
Red cells
Red blood cells carry oxygen using:
A red pigment.
✏️ Quick check · 4 of 11
Left ventricle
The left ventricle has a thicker wall than the right because it:
Compare how far the blood travels.
✍️ Type your answer · 5 of 11
A person’s pulse is 20 beats in 15 seconds.
What is the heart rate, in beats per minute?
Enter an exact answer, without rounding.
beats/min
There are four 15 s periods in a minute.
✍️ Type your answer · 6 of 11
Heart rate is 70 beats/min and each beat pumps 70 mL.
What is the cardiac output, in mL/min?
Enter an exact answer, without rounding.
mL/min
Rate × volume per beat.
✍️ Type your answer · 7 of 11
A heart beats 60 times per minute.
How many times does it beat in 10 minutes?
Enter an exact answer, without rounding.
Multiply the rate by the time.
✍️ Type your answer · 8 of 11
At rest, a person has a heart rate of 70 beats per minute and a stroke volume of 70 mL.
What is the cardiac output, in mL per minute?
Enter an exact answer, without rounding.
mL/min
Heart rate × stroke volume.
✍️ Type your answer · 9 of 11
During exercise, the heart rate is 150 beats per minute and the stroke volume is 100 mL.
What is the cardiac output, in litres per minute?
Enter an exact answer, without rounding.
L/min
Convert mL to litres at the end.
✍️ Type your answer · 10 of 11
A heart beats 75 times in one minute.
How long does one beat last, in seconds?
Enter an exact answer, without rounding.
s
Divide 60 s by the number of beats.
✏️ Quick check · 11 of 11
Blood vessels
Which feature allows capillaries to exchange substances quickly?
Think of the distance.
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
Which blood vessel carries blood from the heart to the lungs?
Question 2 of 5
Valves in veins:
Question 3 of 5
White blood cells mainly:
Question 4 of 5
During exercise, the heart rate rises to:
Question 5 of 5
Platelets help to:
Lesson 4 · Plant biology
🌿 Plant Transport & Transpiration
Plants have no heart. Instead, water is pulled up from the roots by evaporation from the leaves, and sugars travel to where they are needed.
🚰 Xylem, phloem and the transpiration stream
Xylem vessels carry water and minerals up from the roots; their walls are strengthened with lignin. Phloem carries dissolved sugars (translocation) up and down the plant. Transpiration is the loss of water vapour from the leaves through the stomata. Water evaporates from the leaf, pulling a continuous column of water up the xylem.
root hair → root cortex → xylem → stem → leaf
→ evaporates from the leaf surface
→ diffuses out through the stomata
xylem: water up phloem: sugar up and down
🖼️ See it
Evaporation from the leaves pulls a continuous stream of water up the plant.
☀️ Factors that affect transpiration
Transpiration is faster in hot, dry, windy and bright conditions, because water evaporates faster and the water vapour is carried away. Humid air slows it down. Guard cells open and close the stomata; they close at night or when the plant is short of water, which reduces water loss.
a leaf loses 12 mL of water in 4 hours
rate = 12 / 4 = 3 mL per hour
water lost = rate × time = 3 × 6 = 18 mL in 6 hours
Plants lose water so that they can take in carbon dioxide. Open stomata allow both gas exchange and water loss.
🧪 Worked examples: the potometer and stomatal counts
Potometer. It measures the uptake of water by a cut shoot by tracking an air bubble in a capillary tube. Uptake is almost equal to the water lost by transpiration.
the bubble moves 24 mm in 6 minutes
rate = 24 ÷ 6 = 4 mm per minute
volume = distance × cross-sectional area of the tube (πr²)
r = 0.5 mm: area = 3.14 × 0.25 = 0.785 mm²
volume per minute = 4 × 0.785 = 3.14 mm³
Stomatal density. Count the stomata in a known area of the leaf under the microscope.
45 stomata counted in a field of view of area 0.5 mm²
density = 45 ÷ 0.5 = 90 stomata per mm²
Comparing two conditions. Always compare numbers with a calculation, such as “twice as fast” or “a 50% increase”.
in still air: 3 mm/min with a fan: 6 mm/min
6 ÷ 3 = 2, so wind doubled the rate
Wilting. If water loss from leaves exceeds uptake, the cells lose turgor, the plant becomes flaccid and wilts. Closing the stomata reduces the loss.
🎯 Exam focus: xylem, phloem and the evidence
xylem dead cells with no end walls, lignin rings, carry water and mineral ions UP
phloem living cells with sieve plates and companion cells, carry sucrose UP and DOWN
Explain the transpiration stream in order: water evaporates from the mesophyll cells into the air spaces, diffuses out through the stomata, creating a pull that draws a continuous column of water up the xylem from the roots. Water molecules stick together (cohesion), which keeps the column unbroken.
Common mistakes. Saying that phloem carries water. Saying that plants “breathe in through the roots”. Mixing up transpiration (loss of water vapour) with translocation (movement of sugars). Forgetting a control in the potometer: the apparatus must be airtight and the shoot cut under water.
Why do plants need stomata at all? For carbon dioxide to enter for photosynthesis. Water loss is the unavoidable cost.
✏️ Quick check · 1 of 11
Xylem
Xylem vessels transport:
Think x for water up.
✏️ Quick check · 2 of 11
Phloem
Phloem tubes transport:
Sucrose in the plant.
✏️ Quick check · 3 of 11
Stomata
Water vapour leaves a leaf mainly through the:
Pores on the underside.
✏️ Quick check · 4 of 11
Fast transpiration
Transpiration is fastest on a:
Think about drying clothes.
✍️ Type your answer · 5 of 11
A leaf loses 12 mL of water in 4 hours at a steady rate.
What is the rate of water loss, in mL per hour?
Enter an exact answer, without rounding.
mL/h
Water lost ÷ time.
✍️ Type your answer · 6 of 11
A shoot loses water at 3 mL per hour.
How much water does it lose in 6 hours, in mL?
Enter an exact answer, without rounding.
mL
Rate × time.
✍️ Type your answer · 7 of 11
A potometer bubble moves 15 mm in 5 minutes.
What is the rate, in mm per minute?
Enter an exact answer, without rounding.
mm/min
Distance ÷ time.
✍️ Type your answer · 8 of 11
In a potometer, the air bubble moves 24 mm in 6 minutes.
What is the rate, in mm per minute?
Enter an exact answer, without rounding.
mm/min
Distance divided by time.
✍️ Type your answer · 9 of 11
A microscope field of view has an area of 0.5 mm² and shows 45 stomata.
What is the stomatal density, in stomata per mm²?
Enter an exact answer, without rounding.
Count divided by area.
✍️ Type your answer · 10 of 11
A shoot takes up water at 3 mm per minute in still air and 6 mm per minute in wind.
How many times faster is the uptake in the wind?
Enter an exact answer, without rounding.
Divide the new rate by the old one.
✏️ Quick check · 11 of 11
Transport tissues
Which tissue carries sucrose from the leaves to the roots?
Xylem moves water.
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
Guard cells control the opening of the:
Question 2 of 5
Lignin in xylem vessels:
Question 3 of 5
Which would slow transpiration?
Question 4 of 5
Wilting happens when a plant:
Question 5 of 5
The transpiration stream moves water:
Lesson 5 · Coordination
🧠 Nervous System & Hormones
Organisms detect changes and respond. Nerves send fast electrical signals; hormones send slower chemical messages in the blood.
⚡ Nerves and reflexes
A stimulus is detected by a receptor, which sends an impulse along a sensory neurone to the central nervous system (brain and spinal cord). A relay neurone passes it on to a motor neurone, which makes an effector (a muscle or gland) respond. A reflex is fast and automatic because it skips the conscious part of the brain. Neurones meet at a synapse, where a chemical crosses the gap.
hot pan → receptor in skin
→ sensory neurone → spinal cord (relay)
→ motor neurone → muscle in arm
→ hand pulls away
stimulus → receptor → coordinator → effector → response
🖼️ See it
A reflex skips the conscious brain, so the response is fast and automatic.
🧪 Hormones and the endocrine system
Hormones are chemicals made by glands and carried in the blood to target organs. Their effects are slower but last longer than nerve signals. Insulin from the pancreas lowers blood glucose; adrenaline prepares the body for “fight or flight”; ADH controls water balance. In diabetes, blood glucose is not controlled properly.
blood glucose rises after a meal
→ pancreas releases insulin
→ liver and muscles take up glucose
→ blood glucose falls (negative feedback)
Nerves are fast and short-lived; hormones are slow and long-lasting. That is the key comparison to remember.
🧪 Worked examples: reaction time and glucose control
Ruler-drop test. A ruler is dropped and caught, and the distance is converted into a time using d = ½gt². With g = 10 m/s²: t = √(d ÷ 5).
ruler caught after falling 0.20 m
t = √(0.20 ÷ 5) = √0.04 = 0.2 s
(this includes the time for the signal to travel: eye → brain → hand)
Reading a mean from repeated trials. Ignore anomalous results, then average the rest.
trials: 0.21 s, 0.19 s, 0.20 s, 0.55 s (distracted)
ignore 0.55 s; mean = (0.21 + 0.19 + 0.20) ÷ 3 = 0.20 s
Control of blood glucose. After a meal the glucose level rises, the pancreas releases insulin, and the liver turns glucose into glycogen. When glucose falls, the pancreas releases glucagon and the liver converts glycogen back to glucose. This is negative feedback.
Type 1 and type 2 diabetes. In type 1 the pancreas makes too little insulin, and treatment is insulin injections and diet. In type 2 the body’s cells respond poorly to insulin, and treatment is diet and exercise.
🎯 Exam focus: comparing nerves and hormones
nervous system endocrine system
transmission electrical impulses chemical hormones in the blood
speed very fast slower
duration short longer lasting
target specific muscles/glands target organs with receptors
For a reflex arc, give the full pathway: stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response. State that reflexes are fast, automatic and protect the body from harm.
Common mistakes. Saying that hormones are “released by the brain”: most are made in glands such as the pancreas, adrenal glands and pituitary. Mixing up sensory and motor neurones. Describing a reflex as controlled by the brain (the spinal cord coordinates many reflexes). Forgetting to name the effector.
Adrenaline. It increases heart rate, widens the pupils and releases glucose, to prepare the body for fight or flight.
✏️ Quick check · 1 of 11
Receptor
A receptor is a cell that:
It senses.
✏️ Quick check · 2 of 11
Effector
An effector is:
It produces the response.
✏️ Quick check · 3 of 11
Reflex
A reflex action is:
No thinking required.
✏️ Quick check · 4 of 11
Hormone transport
Hormones travel around the body in the:
The circulatory system.
✍️ Type your answer · 5 of 11
A reflex impulse travels 0.9 m along neurones at 90 m/s.
How long does it take, in s?
Enter an exact answer, without rounding.
s
time = distance ÷ speed.
✍️ Type your answer · 6 of 11
A nerve impulse travels at 100 m/s along a 1.5 m pathway.
How long does it take, in ms?
Enter an exact answer, without rounding.
ms
Find the time in seconds, then × 1000.
✍️ Type your answer · 7 of 11
A student’s reaction time is 0.25 s.
How many times could they react in 10 s at this rate?
Enter an exact answer, without rounding.
Divide the total time by the time per reaction.
✍️ Type your answer · 8 of 11
A ruler is caught after falling 0.20 m. Use d = 5t² (with g = 10 m/s²).
What is the reaction time, in seconds?
Enter an exact answer, without rounding.
s
Rearrange d = 5t² for t.
✍️ Type your answer · 9 of 11
Four reaction times are 0.21 s, 0.19 s, 0.20 s and an anomalous 0.55 s.
What is the mean of the three valid results, in seconds?
Enter an exact answer, without rounding.
s
Leave out the anomalous result.
✏️ Quick check · 10 of 11
Glucose control
After a meal, blood glucose rises. What does the pancreas release?
Which hormone lowers glucose?
✏️ Quick check · 11 of 11
Reflex arc
In a reflex arc, which neurone carries the impulse to the effector?
Effector means output.
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
Which gland releases insulin?
Question 2 of 5
The gap between two neurones is a:
Question 3 of 5
Adrenaline causes:
Question 4 of 5
Which is the correct order for a reflex arc?
Question 5 of 5
Compared with nerves, hormones act:
Lesson 6 · Ecology
🌍 Biodiversity & Conservation
Biodiversity is the variety of life. Measuring it and protecting it depends on good sampling and an understanding of human impact.
🔍 Sampling an ecosystem
You cannot count every organism, so ecologists sample. A quadrat (a square frame) is placed at random positions to estimate the population of plants or slow animals. A transect line shows how species change across a gradient such as shade or moisture. Larger samples and more repeats give more reliable estimates.
field area = 400 m²
10 quadrats of 1 m² contain 30 daisies in total
mean = 30 / 10 = 3 per m²
population estimate = 3 × 400 = 1200 daisies
🖼️ See it
Most species decline has several causes acting at once.
🛡️ Biodiversity and threats
A habitat with many different species is more stable. Human activities that threaten biodiversity include deforestation, pollution, overhunting, climate change and introduced invasive species. Conservation includes protected areas, captive breeding, seed banks, sustainable farming and fishing and laws against hunting. A food chain loses about 90% of the energy at each step.
Remember random sampling. Throwing a quadrat over your shoulder avoids choosing the best-looking patch.
🧪 Worked examples: mark-release-recapture and diversity
Estimating the size of an animal population. Capture a sample, mark it, release it, and later capture a second sample. The proportion of marked animals in the second sample estimates the proportion in the whole population.
first sample: 40 marked and released
second sample: 30 captured, 6 of them marked
N = (first × second) ÷ marked in second = (40 × 30) ÷ 6 = 200
Assumptions. Marking does not harm or change the animal’s behaviour; the marks stay; the animals mix randomly; there is no migration, birth or death between the samples.
Energy in food chains. About 10% of the energy is passed to the next level. The rest is lost as heat, in waste and in parts that are not eaten.
energy efficiency = energy in the next level ÷ energy in the previous level × 100
10 000 kJ in grass → 1200 kJ in the herbivore
1200 ÷ 10 000 × 100 = 12%
Species diversity. A habitat with many species and a fairly even number of each is more diverse than one dominated by a single species.
🎯 Exam focus: evaluating conservation
For a conservation question, give a method and a reason it works:
• captive breeding increases numbers, to be released later
• seed bank stores genetic diversity as a backup
• protected areas stop habitat loss and hunting
• quotas on fishing let stocks recover
• education and monitoring reduce harmful practices and track success
Common mistakes. Using a quadrat for fast-moving animals (use capture methods instead). Placing quadrats in the best-looking area instead of at random, which introduces bias. Calling extinction “just natural”: today’s rate is far higher because of humans. Forgetting that food chains are short because energy is lost at each level, so there are rarely more than 4 or 5 trophic levels.
Random quadrats. Use two random numbers from a table as coordinates on a grid.
✏️ Quick check · 1 of 11
Quadrat
A quadrat is used to:
A square frame.
✏️ Quick check · 2 of 11
Random sampling
Why are quadrats placed at random?
Fair selection.
✏️ Quick check · 3 of 11
Biodiversity threat
Which is a major threat to biodiversity?
Loss of habitat.
✏️ Quick check · 4 of 11
Conservation
A seed bank helps conservation by:
It is a store.
✍️ Type your answer · 5 of 11
A student counts 30 plants in 10 quadrats of 1 m².
What is the mean number of plants per m²?
Enter an exact answer, without rounding.
per m²
Total ÷ number of quadrats.
✍️ Type your answer · 6 of 11
The mean is 3 plants per m² in a field of 400 m².
What is the estimated population?
Enter an exact answer, without rounding.
Mean × total area.
✍️ Type your answer · 7 of 11
A food chain starts with 10 000 kJ in grass. A herbivore receives 10% of it.
How much energy does the herbivore receive, in kJ?
Enter an exact answer, without rounding.
kJ
Find one tenth.
✍️ Type your answer · 8 of 11
A first sample of 40 fish is marked and released. A second sample has 30 fish, of which 6 are marked.
Estimate the population size.
Enter an exact answer, without rounding.
First sample × second sample ÷ marked recaptured.
✍️ Type your answer · 9 of 11
A grass field produces 10 000 kJ and a herbivore receives 1200 kJ of it.
What is the percentage energy transfer?
Enter an exact answer, without rounding.
%
Next level divided by previous level.
✍️ Type your answer · 10 of 11
A transect has 8 quadrats of 1 m² with 24 plants in total. The area of the field is 500 m².
Estimate the number of plants in the field.
Enter an exact answer, without rounding.
Find the mean per m² first.
✏️ Quick check · 11 of 11
Sampling
Which is the best way to choose quadrat positions for an unbiased sample?
Why is “best-looking” bad?
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
Which is an example of an invasive species problem?
Question 2 of 5
Why does a food chain rarely have more than five links?
Question 3 of 5
A high biodiversity habitat is usually:
Question 4 of 5
A transect is useful for studying:
Question 5 of 5
Sustainable fishing means:
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 · Diffusion, Osmosis & Active Transport
The smell of perfume spreading across a room is an example of:
Q2 · Diffusion, Osmosis & Active Transport
Red blood cells placed in pure water will:
Q3 · Digestion & Enzymes
Which organ produces bile?
Q4 · Digestion & Enzymes
Cooking an egg changes its protein shape permanently. This is like: