4 higher-level sections hidden.
Educerie · IB Diploma · Biology
Theme C Interaction and interdependence · C3.1 Integration of body systems
What you must be able to do
| You must be able to | Level | What it looks like in the exam |
|---|---|---|
| Explain system integration and the hierarchy of cells, tissues, organs and systems, with emergent properties | SL, HL | "Outline what is meant by integration" (2 marks) |
| Distinguish nervous and hormonal signalling; give examples of the blood transporting materials between organs | SL, HL | "Distinguish between…" (3 to 4 marks) |
| Describe the roles of the brain, spinal cord, sensory and motor neurons; conscious and unconscious processes | SL, HL | Paper 1A; 2 to 3-mark short answer |
| Describe a nerve in transverse section | SL, HL | Identify structures on a micrograph or drawing |
| Explain the pain reflex arc | SL, HL | "Outline the pain reflex arc" (4 marks), often with a diagram |
| Outline the role of the cerebellum | SL, HL | Paper 1A, 1 mark |
| Explain melatonin and circadian rhythms | SL, HL | Graph of secretion over a day, or 2 to 3 marks |
| Explain how epinephrine prepares the body for vigorous activity | SL, HL | "Explain the effects of epinephrine" (3 to 4 marks) |
| Outline control of the endocrine system by the hypothalamus and pituitary | SL, HL | 2 to 3-mark short answer |
| Explain feedback control of heart rate and of ventilation rate | SL, HL | Section B, 5 to 8 marks; Paper 1B data |
| Outline control of peristalsis by the CNS and the enteric nervous system | SL, HL | 2-mark short answer |
| Observe and measure tropic responses in seedlings; distinguish qualitative and quantitative data | HL only | Paper 1B: evaluate a tropism method (2 to 3 marks) |
| Explain positive phototropism, auxin efflux carriers and how auxin promotes cell growth | HL only | "Explain how a shoot bends towards light" (4 to 5 marks) |
| Outline phytohormones, auxin–cytokinin interactions, and ethylene's positive feedback in ripening | HL only | 2 to 3-mark short answer |
Before you start
You need the neuron, the synapse and the action potential from C2.2, and the idea of a hormone and its target cell from C2.1 if you are HL. You also need the heart and blood vessels from B3.2 and ventilation from B3.1, because this subtopic is about how they are controlled.
1The idea in one paragraph
A body is thousands of specialist parts that must act as one. The brain decides, the heart pumps, the lungs ventilate, the muscles pull, and none of that helps unless each does the right amount at the right moment. Integration is the coordination that makes it happen, and animals do it three ways: nerves for fast, targeted messages; hormones in the blood for slower, widespread, lasting ones; and the blood itself, which carries materials and heat from organ to organ. Much of the control is automatic and runs by negative feedback: a receptor detects a change, a control centre responds, and the response cancels the change. Plants have no nerves, so they integrate with hormones alone.
2System integration and the hierarchy of subsystems
Integration is the coordination of the parts of a system so that together they perform an overall function. Any living system needs it: parts that each do their own job, uncoordinated, do not add up to a working whole.
A multicellular animal is built as a hierarchy of subsystems: cells form tissues, tissues form organs, organs form body systems, and body systems form the organism. Figure 1 lays out the levels.
At each level, integration produces emergent properties: abilities the whole has that none of its parts has. A single muscle fibre can contract, but it cannot sprint. Take a cheetah. To catch a gazelle it must see the prey and judge distance (nervous system), contract its leg muscles hard and in the right sequence (muscular and skeletal systems), deliver oxygen and glucose to those muscles as fast as they use them (circulatory and respiratory systems), and release glucose from its liver (endocrine system). Being an effective predator is not a property of any one of those systems. It emerges only when they are integrated.
3Nerves, hormones and blood
Animal organs are linked in three ways. The first two send messages; the guide asks you to distinguish them.
| Nervous system | Endocrine system | |
|---|---|---|
| Signal | electrical impulses along neurons, chemical across synapses | hormones secreted into the blood |
| Route | along specific nerve fibres to specific cells | in the blood, to every part of the body |
| Speed | very fast: milliseconds | slower: seconds to hours |
| Duration of effect | usually brief | usually longer lasting |
| Targets | precise: particular muscle fibres or gland cells | widespread: any cell with the receptor |
| Example | pulling a hand from a hot object | epinephrine preparing many organs for activity |
The third link is the transport of materials and energy by the blood. It is not a signal, but without it the organs could not depend on each other at all. Examples to use:
- Oxygen from the lungs to respiring muscles, and carbon dioxide back to the lungs.
- Glucose and amino acids from the small intestine to the liver in the hepatic portal vein, and on to every tissue.
- Urea from the liver, where it is made, to the kidneys, where it is excreted.
- Heat from active muscles and the liver to the skin, where it can be lost: blood carries energy as well as matter.
4The brain, the spinal cord and the neurons that connect them
The central nervous system (CNS) is the brain and the spinal cord. Figure 2 shows information flowing in, being integrated, and flowing out.
The brain is the central information integration organ. It receives impulses from many sensory inputs at once (eyes, ears, skin, muscles, internal receptors), combines them, and decides on a response. You see a ball, hear a shout and feel the ground under you, and the brain combines all three before you move. The brain also stores information from past experience, which is learning and memory, so the response can be better next time.
The spinal cord is an integrating centre for unconscious processes. A conscious process is one you are aware of and can control voluntarily, such as deciding to pick up a pen; conscious processes happen in the cerebral hemispheres. An unconscious process happens without awareness or decision, such as a reflex or the adjustment of your posture while you stand. Many of these are integrated in the spinal cord without the brain being involved at all.
Input: sensory neurons. Sensory neurons carry impulses from receptor cells to the CNS: to the spinal cord, and through it up to the cerebral hemispheres.
Output: motor neurons. Instructions from the cerebral hemispheres travel down the spinal cord and leave it in motor neurons, which carry impulses to muscles and stimulate them to contract. Muscles and glands are the effectors.
5Nerves
A nerve is not a neuron. It is a bundle of nerve fibres, often thousands, wrapped together, and most nerves contain the fibres of both sensory and motor neurons, carrying impulses in opposite directions side by side. Figure 3 is a nerve cut across.
In a transverse section you should recognise: the tough protective sheath of connective tissue around the whole nerve; the fibres grouped into bundles, each with its own thinner sheath; myelinated fibres, which appear as rings, each a small axon inside a circle of myelin; unmyelinated fibres, which appear as small dots with no ring; and small blood vessels supplying the nerve.
6The pain reflex arc
A reflex is a rapid, involuntary response to a stimulus. The pathway it takes is a reflex arc. The guide's example is the pain reflex that pulls your hand away from something hot, and Figure 4 traces it.
- Receptor. A free sensory nerve ending in the skin of the hand, a pain receptor, is stimulated by the heat and generates impulses.
- Sensory neuron. Impulses travel along the sensory neuron, whose cell body sits in a swelling outside the spinal cord (the dorsal root ganglion), and enter the spinal cord.
- Interneuron. In the grey matter of the spinal cord the sensory neuron synapses with a single interneuron (also called a relay neuron).
- Motor neuron. The interneuron synapses with a motor neuron, which carries impulses out of the spinal cord.
- Effector. The motor neuron stimulates a skeletal muscle in the arm to contract, and the hand is pulled away.
The whole arc passes through two synapses in the spinal cord, and the brain is not in the decision. That is why it is fast, and why it is involuntary. The brain is informed a fraction of a second later, when the pain is felt, by which time the hand has already moved. Grey matter is the inner, butterfly-shaped region of cell bodies and synapses; white matter around it is myelinated fibres running up and down the cord.
7The cerebellum
The cerebellum sits at the back of the brain, below the cerebral hemispheres. It coordinates the contraction of skeletal muscles so that movements are smooth, accurate and correctly timed, and it maintains balance and posture. It does not decide to move; the cerebral hemispheres do that. It makes the movement work: when you reach for a cup, the cerebellum adjusts the force and timing of dozens of muscles so your hand arrives at the cup rather than knocking it over. Skills such as riding a bike depend heavily on it.
8Melatonin, sleep and circadian rhythms
A circadian rhythm is a cycle in the body's activity that repeats roughly every 24 hours. The cycle of sleeping and waking is the obvious one. Part of its control is the hormone melatonin, secreted by the pineal gland in the brain. Figure 5 shows its daily pattern.
Melatonin secretion is low during daylight and rises in the evening as it gets dark, peaks in the middle of the night, and falls again towards morning. The pattern is set by light: information from light-sensitive cells in the retina reaches the hypothalamus, which inhibits the pineal gland while it is light. Rising melatonin in the evening promotes sleepiness and helps set the timing of sleep, and its fall towards morning helps waking. Because the rhythm tracks light, jet lag is the melatonin cycle still running on home time; a few days of daylight in the new time zone reset it.
9Epinephrine and vigorous activity
When you are threatened, excited or about to exert yourself, the adrenal glands (on top of the kidneys) secrete epinephrine (adrenaline) into the blood. Within seconds it reaches almost every organ, and its effects together prepare the body for intense muscle contraction:
- Heart rate and stroke volume increase, so more blood, carrying oxygen and glucose, reaches the muscles each minute.
- Bronchioles widen and ventilation increases, so more oxygen enters the blood.
- The liver breaks down glycogen to glucose and releases it, raising blood glucose, fuel for respiration in muscle.
- Arterioles to skeletal muscles dilate, while those to the gut and skin constrict, sending blood where it is needed now.
- Mental alertness rises and the pupils dilate.
Every effect serves the same goal: more ATP in skeletal muscle, faster. Epinephrine is a good example of a hormone's widespread effect, one chemical acting on many target organs at once.
10The hypothalamus and the pituitary gland
The endocrine system has a control centre. The hypothalamus, a small region at the base of the brain, receives nerve impulses from the rest of the brain and monitors the blood directly. It controls the pituitary gland below it, partly by secreting releasing hormones into small blood vessels that lead to the pituitary, and partly through nerve fibres. The pituitary then secretes hormones that travel in the blood to other glands and organs. Figure 6 shows the arrangement.
Because so many glands take their orders from the pituitary, and the pituitary from the hypothalamus, this is where the nervous and endocrine systems meet: nerve signals in the brain become hormone signals in the blood. The chains are usually regulated by negative feedback. In the thyroid chain, for example, the pituitary secretes TSH, which makes the thyroid secrete thyroxine; high thyroxine inhibits both the hypothalamus and the pituitary, so less TSH is secreted and thyroxine falls back. The guide asks only for a general understanding; you do not need the differences between the front and back parts of the pituitary.
11Feedback control of heart rate
Heart rate and the volume pumped per beat (stroke volume) change constantly to match the body's needs, and they are controlled by negative feedback through the brainstem. Figure 7 sets out the loop.
Receptors, and where they are.
- Baroreceptors monitor blood pressure. They are stretch receptors in the walls of the aorta (in the aortic arch) and the carotid arteries in the neck.
- Chemoreceptors monitor blood pH and the concentrations of oxygen and carbon dioxide. They are in small bodies in the walls of the aorta and the carotid arteries, and in the brainstem (the medulla).
Coordination: the medulla. Impulses from these receptors travel along sensory neurons to the medulla (medulla oblongata), in the brainstem. The medulla integrates them and sends impulses along two sets of nerves to the heart's pacemaker, the sinoatrial (SA) node:
- Sympathetic nerves increase the heart rate and the force of contraction, so stroke volume rises.
- The vagus nerve (parasympathetic) decreases the heart rate.
Two examples. During exercise, muscles produce more carbon dioxide, blood pH falls and oxygen falls; chemoreceptors detect this, the medulla increases sympathetic stimulation, heart rate and stroke volume rise, and the extra blood flow removes CO₂ faster. If blood pressure rises too high, baroreceptors are stretched more; the medulla increases vagus stimulation, heart rate falls, and pressure falls back. In each case the response reverses the change: negative feedback.
12Feedback control of ventilation rate
Why the pH of blood changes. Carbon dioxide from respiration dissolves in blood plasma and reacts with water to form carbonic acid, which releases hydrogen ions:
The more a body respires, the more CO₂ it makes and the lower the blood pH falls. During intense exercise, lactic acid from anaerobic respiration lowers the pH further. Figure 8 follows what happens next.
- Chemoreceptors in the brainstem detect the fall in pH (chemoreceptors in the aorta and carotid arteries contribute too).
- The respiratory centre in the medulla increases the rate of impulses along nerves to the diaphragm and the intercostal muscles.
- These muscles contract more often and more strongly: ventilation rate and depth increase.
- CO₂ is exhaled faster, its concentration in the blood falls, and blood pH rises back towards normal. The stimulus is removed: negative feedback.
The main signal for breathing is carbon dioxide, detected as a fall in blood pH, not a lack of oxygen.
13Peristalsis: the CNS and the enteric nervous system
Food is moved along the gut by peristalsis: waves of contraction of the circular muscle in the gut wall, just behind the food, pushing it onwards. Who controls it depends on where the food is. Figure 9 shows the division.
- Swallowing is started voluntarily, under the control of the CNS: you decide when to swallow.
- From the oesophagus to the end of the large intestine, peristalsis is involuntary, controlled by the enteric nervous system (ENS), a network of neurons in the gut wall itself. The ENS coordinates the contractions so material passes through the gut at the right rate, even without instructions from the brain.
- Egestion of faeces is again under voluntary control by the CNS.
The ENS is a striking case of integration: a local nervous system, contained entirely in one organ system, that runs its own coordination.
14HLPhototropism and phytohormones
SL students can skip to section 18.
Plants have no nervous system, yet they respond to their surroundings and coordinate root and shoot growth. They do it with phytohormones, plant hormones: signalling chemicals that control growth, development and responses to stimuli. Plants use a variety of them, including auxin, cytokinins, gibberellins, abscisic acid and ethylene, each with several roles.
A tropism is a directional growth response to a directional stimulus. Positive phototropism is the growth of a plant shoot towards light that comes from one side (lateral light). A seedling on a windowsill bends towards the window. The advantage is obvious: leaves held in brighter light photosynthesise more.
15HLHow auxin makes a shoot bend
Auxin is made mainly in the shoot tip and moves down the shoot, cell by cell. The key to phototropism is that it can be moved sideways too. Figure 10 shows how.
Auxin efflux carriers. Auxin can diffuse freely into plant cells but not out of them. To leave, it must be pumped out by auxin efflux carriers, proteins in the plasma membrane. A cell can position these carriers on just one side. If all the cells in a tissue concentrate their carriers on the same side, auxin is actively transported from cell to cell in one direction and becomes concentrated in one part of the plant. This is how a plant keeps concentration gradients of a phytohormone. In a shoot lit from one side, carriers are repositioned so auxin is moved towards the shaded side.
Auxin promotes cell growth. In the cells of the shoot, auxin promotes the secretion of hydrogen ions by proton pumps into the apoplast, the cell walls and spaces outside the plasma membrane. The cell wall is acidified, which loosens the cross-links between cellulose molecules. The wall can now stretch, and turgor pressure from water entering the cell makes the cell elongate.
Putting it together. More auxin on the shaded side means more acid, looser walls and faster elongation there than on the lit side. A shoot that grows faster on one side than the other curves, towards the slower side: towards the light. The concentration gradient of auxin causes the difference in growth rate that produces the bend.
16HLObserving and measuring tropic responses
Tropisms can be investigated with seedlings, for example grown in a box with a slit that lets light in from one side.
Qualitative data are descriptions and drawings: a labelled diagram of the seedlings at intervals, showing which way and how much they have bent. Quantitative data are measurements with numbers and units: most usefully the angle of curvature, the angle between the vertical and the direction of the shoot tip, measured with a protractor from a photograph. Figure 11 shows the kind of result you might get.
The limits on the measurement are part of what is examined.
- Precision is how fine and how repeatable the measurement is. A protractor read by eye on a curved stem gives perhaps ±2° to ±5°. Measuring on enlarged photographs, or with image software, improves it.
- Accuracy is how close the measurement is to the true value. It depends on measuring from the same reference line every time: always from the vertical, always to the same point on the tip.
- Reliability is whether the result can be trusted as typical. It comes from large samples (the more seedlings, the better the mean), repeats, controls (seedlings lit from above, which should not bend), and keeping other variables constant: light intensity, temperature, seedling age and water.
The bars in Figure 11 show variation between seedlings. Where bars overlap, as they nearly do between 3 and 4 hours, the difference between those two means may not be significant.
17HLAuxin with cytokinin, and ethylene
Auxin and cytokinin integrate root and shoot growth. Root tips produce cytokinin, which is transported up to the shoots. Shoot tips produce auxin, which is transported down to the roots. Each hormone helps the other part of the plant grow: cytokinin promotes growth in the shoot, auxin promotes the formation of roots. The result, shown in Figure 12(a), is that the root system and the shoot system grow in step. A plant with a large root system sends more cytokinin to its shoots; a plant with a large shoot sends more auxin to its roots. Neither part can outgrow the other for long.
Ethylene and fruit ripening: positive feedback. Ethylene (IUPAC name ethene) is a gas that stimulates the changes of ripening: the fruit softens, starch is converted to sugar, and the colour changes. Ripening, in turn, stimulates the fruit to produce more ethylene. This is positive feedback, shown in Figure 12(b): the more a fruit ripens, the more ethylene it makes, and the faster it ripens. Because ethylene is a gas, it also diffuses to neighbouring fruits and starts them ripening too.
The benefit is that ripening is rapid and synchronized. The fruits on a plant ripen together, over a short time, which makes a large, conspicuous display for the animals that eat fruit and disperse the seeds. It is also why one ripe banana in a bag speeds the ripening of the rest.
18Where marks are lost
Calling a nerve a neuron. A neuron is one cell. A nerve is a bundle of many nerve fibres, usually from both sensory and motor neurons.
Letting the brain decide a reflex. In the pain reflex the decision is made in the spinal cord, through one interneuron. The brain is told afterwards. "The brain sends a message to the muscle" loses the mark.
Mixing up the cerebellum and the cerebral hemispheres. The cerebral hemispheres decide to move and handle conscious thought; the cerebellum coordinates the movement and keeps balance.
Saying oxygen drives breathing. The main stimulus is a rise in carbon dioxide, detected as a fall in blood pH by chemoreceptors in the brainstem.
Putting the baroreceptors in the heart. They are in the walls of the aorta and the carotid arteries. The medulla, not the heart, coordinates the response.
Saying melatonin is made in darkness by the eye. It is secreted by the pineal gland in the brain. Light, detected by the eye, inhibits its secretion.
Saying auxin moves towards the light (HL). Auxin is moved to the shaded side, and the shaded side grows faster, which bends the shoot towards the light.
Calling fruit ripening negative feedback (HL). Ethylene causes ripening and ripening causes more ethylene: the response amplifies the stimulus, which is positive feedback.
19Draw it right
- Reflex arc: receptor in the skin, sensory neuron entering the spinal cord, one interneuron in the grey matter, motor neuron leaving, skeletal muscle. Arrows show the direction of the impulse. Label grey and white matter.
- Nerve in section: outer protective sheath, bundles, myelinated fibres as rings, unmyelinated fibres as dots.
- Feedback loops (heart rate, ventilation): in order, receptor → sensory neurons → medulla → motor or autonomic nerves → effector → change reversed, with the loop closed by an arrow. Name the receptor location and the effector.
- Melatonin graph: time of day on x, concentration on y; low in daylight, rising after dusk, peak in the night. Shade or mark the dark period.
- Phototropism (HL): light arrow from one side, auxin arrow to the shaded side, longer cells on the shaded side, shoot bending towards the light.
- Tropism data (HL): time or treatment on x, mean angle of curvature (°) on y, error bars, a control series.
20Try it
Marks in brackets. Answers and marker's notes are at the end.
Q1. Where are baroreceptors that monitor blood pressure located? 1 mark
A. In the medulla
B. In the walls of the aorta and the carotid arteries
C. In the sinoatrial node
D. In the cerebellum
Q2. Distinguish between the ways in which the nervous system and the endocrine system send messages. 3 marks
Q3. Outline the pain reflex arc that withdraws the hand from a hot object. 4 marks
Q4. A volunteer at rest breathed air with different concentrations of carbon dioxide. Their ventilation rate was measured. (Invented data.)
| CO₂ in inhaled air (%) | 0.04 | 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|---|---|
| Ventilation rate (dm³ min⁻¹) | 7 | 8 | 11 | 17 | 26 | 38 |
(a) Calculate the percentage increase in ventilation rate between 0.04% and 3% CO₂. 1 mark
(b) Explain how breathing air with more carbon dioxide causes the ventilation rate to rise. 3 marks
(c) Suggest why ventilation rises only a little between 0.04% and 1% CO₂. 1 mark
Q5 (HL). Groups of 10 seedlings were lit from one side. One group had been treated with a chemical that blocks auxin efflux carriers. (Invented data.)
| Time (h) | 0 | 2 | 4 | 6 |
|---|---|---|---|---|
| Mean angle of curvature, untreated (°) | 0 | 12 | 27 | 35 |
| Mean angle of curvature, treated (°) | 0 | 2 | 3 | 4 |
(a) Calculate the mean rate of curvature of the untreated seedlings over the first 4 hours. 1 mark
(b) Explain the difference between the two groups. 3 marks
(c) Suggest one way to improve the precision of the angle measurements. 1 mark
Q6 (HL). Explain the role of positive feedback in the ripening of fruit. 3 marks
21In one breath
Integration is coordination that lets the parts of a system act as one; a body is a hierarchy (cells, tissues, organs, systems, organism), and integration gives emergent properties, like a cheetah's hunting. Organs are linked by nerves (fast, brief, targeted), hormones (slow, lasting, widespread) and blood (oxygen, glucose, urea, heat). The brain combines many inputs and learns and remembers; the cerebral hemispheres handle conscious processes, the spinal cord unconscious ones; sensory neurons carry impulses in from receptors, motor neurons carry them out to muscles; a nerve is a bundle of both, myelinated and unmyelinated, in a sheath. Pain reflex: free nerve ending, sensory neuron, one interneuron in grey matter, motor neuron, skeletal muscle, with no brain decision. The cerebellum coordinates movement and balance. The pineal gland secretes melatonin in darkness, setting the sleep–wake cycle. Epinephrine from the adrenal glands raises heart rate, ventilation and blood glucose for intense muscle work. The hypothalamus controls the pituitary, which controls other glands, with negative feedback. Baroreceptors and chemoreceptors in the aorta and carotid arteries report to the medulla, which speeds the heart through sympathetic nerves and slows it through the vagus. More CO₂ lowers blood pH; brainstem chemoreceptors make the medulla drive the diaphragm and intercostals harder. Swallowing and egestion are voluntary (CNS); peristalsis between is run by the enteric nervous system. HL: phytohormones integrate plants; efflux carriers move auxin to the shaded side, where it acidifies cell walls, loosens cellulose cross-links and lengthens cells, so shoots bend to the light; measure curvature as an angle, with large samples and controls; root cytokinin and shoot auxin keep growth in step; ethylene and ripening drive each other in positive feedback.
Answers
Q1. B. The medulla (A) is where the information is processed, not where it is detected. B only.
Q2. The nervous system sends electrical impulses along neurons, whereas the endocrine system secretes hormones that travel in the blood. Nervous messages are faster, whereas hormonal messages are slower. Nervous messages go to specific target cells, whereas hormones reach all cells and act on any with the right receptor, so effects are widespread. Nervous effects are usually short-lived, whereas hormonal effects are longer lasting. 1 per valid paired difference, up to 3. Separate descriptions without comparison are capped at 2.
Q3. A free nerve ending in the skin of the hand acts as a pain receptor and is stimulated by the heat. Impulses pass along a sensory neuron into the spinal cord. In the grey matter the sensory neuron synapses with an interneuron, which synapses with a motor neuron. The motor neuron carries impulses to a skeletal muscle (the effector) in the arm, which contracts and withdraws the hand. The response is involuntary; the brain is not involved in the decision. 1 each for receptor (free nerve ending), sensory neuron to spinal cord, interneuron in grey matter, motor neuron to skeletal muscle which contracts. Stating that the brain decides loses one mark.
Q4. (a) (17 − 7) ÷ 7 × 100 = 143%. value, accept 142.9%. (b) Breathing more CO₂ raises the CO₂ concentration of the blood. CO₂ reacts with water to form carbonic acid, releasing H⁺ ions, so blood pH falls. Chemoreceptors in the brainstem detect the fall in pH. The medulla sends more impulses to the diaphragm and intercostal muscles, which contract more often and more strongly, increasing ventilation rate. 1 for CO₂ lowering blood pH, 1 for detection by chemoreceptors (brainstem), 1 for medulla increasing impulses to the diaphragm and intercostals. (c) At 1% the rise in blood CO₂ is small, so the pH changes little; the body's buffers or a small increase in ventilation removes the extra CO₂ easily, so the response is small. any reasoned suggestion linked to a small change in pH.
Q5 (HL). (a) 27 ÷ 4 = 6.75° per hour (accept 6.8° h⁻¹). value with unit. (b) Untreated seedlings bent strongly towards the light (35° after 6 hours) while treated seedlings barely bent (4°). In the untreated shoots efflux carriers moved auxin to the shaded side, where it caused more cell elongation, so the shoot curved. With the carriers blocked, auxin could not be concentrated on the shaded side, so both sides grew at about the same rate and there was little curvature. 1 for a comparison using the data, 1 for efflux carriers moving auxin to the shaded side in the untreated group, 1 for the treated group being unable to set up the gradient, so no difference in growth rate. (c) Measure the angle from photographs using image software, or enlarge the photographs before measuring; always measure from the vertical to the same point on the tip. any valid method that reduces the measurement uncertainty. "Use more seedlings" improves reliability, not precision, and scores 0.
Q6 (HL). Ethylene stimulates the changes of ripening in fruit, such as softening and the conversion of starch to sugar. Ripening fruit in turn produce more ethylene, so ripening accelerates: this is positive feedback, because the response increases the stimulus. Ethylene is a gas and diffuses to nearby fruit, so ripening spreads and is synchronized, making the fruits ripe at the same time, which attracts seed-dispersing animals. 1 for ethylene stimulating ripening, 1 for ripening stimulating ethylene production (the loop), 1 for the benefit: rapid, synchronized ripening.
Educerie · written from the published IB Diploma Programme Biology guide, first assessment 2025, section C3.1 Integration of body systems. Original text, examples and questions. Diagrams drawn by Educerie. Last reviewed 25 September 2026.
Check your understanding
The main ideas of this note. Tick each one you could do now, in an exam, without looking back up. Anything you cannot tick yet is the part to read again.