DP Biology HL C3.1 Integration of Body Systems

DP Biology HL C3.1 Integration of Body Systems - Full notes

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DP Biology HL | C3.1 Integration of Body Systems

DP BIOLOGY · THEME C: INTERACTION AND INTERDEPENDENCE

C3.1

Integration of Body Systems

Full notes for HL · IB-style

Contents

1	Integration and hierarchy in organisms
2	The nervous and endocrine systems compared
3	The brain as the integrating centre
4	Receptors, effectors and reflex arcs
5	The cerebellum, melatonin and the control of heart rate
6	Feedback control of the heart and breathing
7	Integration in plants: tropisms and auxin (HL only)
8	Integration: why systems must cooperate
9	Nervous versus endocrine coordination
10	The brain as a central integrating organ
11	The spinal cord and reflexes
12	Voluntary and involuntary responses
13	The autonomic nervous system
14	Control of heart rate
15	Control of ventilation rate
16	Hormones from the hypothalamus and pituitary
17	Melatonin and circadian rhythms
18	Blood as a transport and integrating system
19	Integration in plants
20	Mixed review: worked problems
21	Additional exam-style practice
22	Feedback in integration
23	Practical: measuring reaction time
24	Systems integration in exercise
25	Common misconceptions and links
26	Exam-style question with markscheme
27	Check your understanding
28	Summary and topic map

These are full teaching notes. Read each section in order, try the Think about it question before you read on, and do every Quick check before moving to the next section.

Before you start

Rate yourself on each statement now, and again after you've worked through the notes. Shade R (not yet), A (getting there) or G (confident).

I can…	R	A	G
explain emergent properties and the hierarchy of organization
compare nervous and endocrine signalling
explain the role of the brain in integration
explain reflex arcs with the pain withdrawal reflex
explain the roles of the cerebellum and melatonin
explain how the medulla controls heart rate and ventilation rate
explain phototropism and the role of auxin
explain cell-to-cell signalling by plant hormones

Sections marked HL only (Section 7) are Additional Higher Level content. Every other section is common to SL and HL.

Guiding questions

How are the activities of the different systems of an organism coordinated? How do nerves and hormones work together?

How this topic is examined

C3.1 is examined through reflex arc diagrams, comparisons of nervous and hormonal control, and data on heart rate or breathing during exercise. HL questions include auxin experiments and tropisms.

Answers should identify the receptor, the integrating centre, the effector and the response in each control system.

Command term	What the examiner wants
State / Identify	a brief answer with no explanation needed
Outline	a brief account or summary
Describe	a detailed account, including trends or features, without reasons
Explain	a detailed account including causes, reasons or mechanisms
Distinguish / Compare and contrast	give the differences (and similarities), point by point
Calculate	a numerical answer with working and units
Deduce / Suggest	reach a conclusion from data; propose a reasonable explanation
Evaluate / Discuss	weigh strengths and limitations and reach a judgement

How to use these notes

Each section teaches one part of the syllabus in full. You'll see these boxes throughout; each has one job:

Box	What it's for
Think about it	a question to get you thinking before a section; the answer comes in the section
Key point	the one idea you must not forget
Common mistake	an error examiners see every year, and how to avoid it
In the real world	where the biology shows up outside the classroom
Try this	something quick you can do yourself to test the idea
Worked example	full solutions with IB-style marks (M1 method, A1 answer, R1 reasoning)
Quick check	short questions to test yourself at the end of each section

1 Integration and hierarchy in organisms

The whole is more than the sum of its parts.

Think about it

Your heart, lungs and muscles all speed up when you run. Who or what tells each of them to change?

Multicellular organisms are organized in a hierarchy: cells form tissues, tissues form organs, and organs work together in organ systems. At each level, new properties emerge that the parts alone do not have: a single heart muscle cell cannot pump blood, but the heart can. These are emergent properties.

For an organism to function, its systems must be integrated: coordinated so that they work together. Integration requires communication between parts of the body. Animals use two systems: the nervous system and the endocrine (hormonal) system. Integration also depends on the circulatory system, which transports hormones, oxygen and nutrients to every part.

2 The nervous and endocrine systems compared

Fast and precise, or slow and widespread.

The nervous system uses electrical impulses carried by neurons and chemical transmission at synapses. Signals are fast (milliseconds), precise (sent to specific cells) and usually short-lived.

The endocrine system uses hormones, chemical messengers secreted by endocrine glands into the blood, which carries them throughout the body. Only target cells with the right receptors respond. Hormonal signals are slower (seconds to hours), widespread, and often long-lasting.

The two systems work together. The hypothalamus in the brain links them: it receives nervous input and controls the pituitary gland, which secretes hormones that control many other glands.

Feature	Nervous system	Endocrine system
Signal	electrical impulses and neurotransmitters	hormones
Route	neurons	bloodstream
Speed	very fast	slower
Target	specific cells	any cells with receptors
Duration	short	often longer
Example	withdrawal reflex	control of blood glucose

Quick check

1. Give three differences between nervous and endocrine signalling.

2. Which brain structure links the two systems?

Answers: 1. speed, route, duration, target 2. hypothalamus

3 The brain as the integrating centre

The brain processes information and coordinates responses.

The brain is the central integrating organ. It receives information from receptors, processes it (often by comparing it with stored memories), and sends signals to effectors. Different regions have different roles: the cerebrum (thinking, voluntary movement, memory, language), the cerebellum (coordination of movement and balance), the medulla oblongata (automatic functions such as heart rate and breathing), the hypothalamus (homeostasis, link to the endocrine system) and the pituitary gland (hormone secretion).

The spinal cord relays impulses between the brain and the body and is also an integrating centre for simple reflexes.

Brain region	Main role
Cerebrum	conscious thought, voluntary movement, learning, memory
Cerebellum	coordination, balance, fine control of movement
Medulla oblongata	heart rate, breathing, swallowing
Hypothalamus	homeostasis; controls pituitary
Pituitary gland	secretes hormones (ADH, FSH, LH, growth hormone)

4 Receptors, effectors and reflex arcs

The fastest responses bypass conscious thought.

Receptors detect stimuli (changes in the environment): photoreceptors detect light, chemoreceptors detect chemicals, thermoreceptors detect temperature, mechanoreceptors detect pressure, and pain receptors (nociceptors) detect damage. Effectors carry out responses: muscles contract and glands secrete.

A reflex is a rapid, automatic, involuntary response to a stimulus. The pathway is a reflex arc. In the pain withdrawal reflex, touching something hot stimulates pain receptors in the skin; a sensory neuron carries impulses to the spinal cord; there it synapses with a relay neuron (interneuron) in the grey matter; the relay neuron synapses with a motor neuron, which carries impulses to the effector, a muscle (such as the biceps), which contracts and pulls the hand away. The impulse also travels to the brain, so you become aware of the pain, but only after the reflex has happened.

Reflexes are fast because they involve few synapses and do not need conscious processing in the brain. They protect the body from damage.

Figure 1. The pain withdrawal reflex arc.

Quick check

1. List the components of a reflex arc.

2. Why are reflexes fast?

Answers: 1. receptor, sensory neuron, relay neuron, motor neuron, effector 2. few synapses; no conscious processing

5 The cerebellum, melatonin and the control of heart rate

Examples of integration in action.

The cerebellum coordinates skeletal muscle contractions to produce smooth, precise, balanced movements. It compares the intended movement (from the cerebrum) with information from receptors in muscles, joints and the inner ear, and adjusts motor commands. Damage to the cerebellum causes clumsy, uncoordinated movements.

Melatonin is a hormone secreted by the pineal gland, mainly at night. Light detected by the retina inhibits melatonin secretion, so levels rise in the evening and fall in the morning. Melatonin helps regulate circadian rhythms, the roughly 24-hour cycles of sleep and wakefulness. Jet lag occurs when circadian rhythms are out of step with local time; melatonin is sometimes used to help reset them.

Figure 2. Melatonin is secreted mainly at night.

6 Feedback control of the heart and breathing

The medulla adjusts heart rate and breathing to demand.

During exercise, muscles need more oxygen and produce more carbon dioxide. Chemoreceptors in the aorta, carotid arteries and medulla detect a rise in blood CO₂ (which lowers blood pH). They send impulses to the medulla oblongata, which responds in two ways.

Ventilation: the medulla increases the rate and depth of breathing by sending impulses to the diaphragm and intercostal muscles.

Heart rate: the heart's pacemaker (sinoatrial node) sets the basic rate, but the medulla adjusts it through nerves: sympathetic nerves increase heart rate; parasympathetic nerves (the vagus nerve) decrease it. The hormone adrenaline from the adrenal glands also increases heart rate in stressful situations.

As CO₂ levels return to normal, the stimulus decreases and rates fall: this is negative feedback. Baroreceptors detecting blood pressure provide further feedback.

Figure 3. Heart rate and breathing rate before, during and after exercise.

Quick check

1. What do chemoreceptors detect during exercise?

2. Which nerves increase heart rate?

Answers: 1. rise in CO₂ / fall in pH 2. sympathetic nerves

7 Integration in plants: tropisms and auxin (HL only)

Plants coordinate growth using hormones.

HL only

This section is Additional Higher Level content. SL students do not need it.

Plants have no nervous system, but they coordinate responses using plant hormones (phytohormones). A tropism is a directional growth response to a directional stimulus. Phototropism is growth towards (positive) or away from light; shoots grow towards light, maximizing photosynthesis. Gravitropism is growth in response to gravity: roots grow down, shoots up.

Auxin (indole-3-acetic acid) is produced in the shoot tip and moves down the shoot. When light comes from one side, auxin is transported to the shaded side by auxin efflux carriers (PIN proteins) in the cell membranes, which can be repositioned. The higher auxin concentration on the shaded side causes cells there to elongate more, so the shoot bends towards the light.

Auxin promotes cell elongation by activating proton pumps that move H⁺ into the cell walls. The acidic conditions activate proteins (expansins) that loosen the bonds between cellulose microfibrils, so the wall stretches as the cell takes up water. Auxin also affects gene expression.

Other plant hormones include gibberellins (stem growth, germination), cytokinins (cell division) and ethylene (fruit ripening). Cytokinins and auxin interact in shoot and root growth. Ethylene is used commercially to ripen fruit, and its production is an example of positive feedback: ripening fruit releases ethylene, which speeds further ripening.

Figure 4. Phototropism.

Quick check

1. Define tropism.

2. How does auxin cause bending towards light?

3. Which hormone ripens fruit?

Answers: 1. directional growth response to a directional stimulus 2. accumulates on shaded side, causing more elongation there 3. ethylene

8 Integration: why systems must cooperate

Emergent properties.

Organisms function as integrated wholes: cells form tissues, tissues form organs, organs form systems, and the systems interact. Emergent properties (such as consciousness or thermoregulation) arise from interactions that individual parts cannot produce alone.

WORKED EXAMPLE 1 Emergent property (standard)

Define an emergent property and give an example.

Solution

A property of a whole that is not present in its partsR1

Example: heartbeat rhythm from interacting cardiac cells; consciousnessA1

9 Nervous versus endocrine coordination

Two communication systems.

Feature	Nervous system	Endocrine system
signal	electrical impulses and neurotransmitters	hormones in blood
speed	very fast	slower
duration	short-lived	long-lasting
target	specific cells (via neurons)	any cell with receptors
example	reflex withdrawal	growth, blood glucose

10 The brain as a central integrating organ

Processing information.

The brain receives information from receptors, processes it, stores memories and sends signals to effectors. The cerebral hemispheres handle conscious thought and voluntary movement; the cerebellum coordinates movement and balance; the medulla controls heart rate and breathing; the hypothalamus links the nervous and endocrine systems.

WORKED EXAMPLE 2 Brain regions (standard)

State the functions of the cerebellum and medulla oblongata.

Solution

Cerebellum: coordination and balanceA1

Medulla: autonomic control of heart rate and ventilationA1

11 The spinal cord and reflexes

Unconscious processing.

The spinal cord integrates unconscious reflexes. Grey matter (cell bodies, relay neurons) is central; white matter (myelinated axons) surrounds it. Sensory neurons enter by the dorsal root; motor neurons leave by the ventral root.

WORKED EXAMPLE 3 Pain reflex (exam level)

Outline the pain withdrawal reflex.

Solution

Nociceptor stimulated; sensory neuron to spinal cordR1

Relay neuron in grey matter; motor neuron to muscleR1

Muscle contracts, withdrawing the limbR1

12 Voluntary and involuntary responses

Cerebral or not.

Voluntary actions are initiated in the cerebral hemispheres (motor cortex), such as picking up a pen. Involuntary actions (reflexes, heart rate, digestion) are controlled by the spinal cord, medulla and autonomic nervous system.

Quick check

1. Is breathing voluntary or involuntary?

Answers: 1. normally involuntary (medulla), but can be voluntarily controlled

13 The autonomic nervous system

Sympathetic and parasympathetic.

The autonomic nervous system controls involuntary processes. The sympathetic division prepares for action (increases heart rate, dilates pupils, inhibits digestion); the parasympathetic division promotes rest (slows heart, stimulates digestion). They act antagonistically.

WORKED EXAMPLE 4 Antagonism (standard)

Describe the effects of sympathetic and parasympathetic stimulation on heart rate.

Solution

Sympathetic increases heart rateA1

Parasympathetic (vagus nerve) decreases itA1

14 Control of heart rate

Medulla and SA node.

The SA node sets heart rate. Chemoreceptors (detecting CO2/pH) and baroreceptors (detecting blood pressure) send signals to the cardiovascular centre in the medulla, which adjusts heart rate through sympathetic and parasympathetic nerves. Adrenaline also increases heart rate.

WORKED EXAMPLE 5 Exercise (exam level)

Explain how heart rate increases during exercise.

Solution

Rising CO2 lowers blood pH, detected by chemoreceptorsR1

Medulla sends impulses via sympathetic nerves to SA nodeR1

Adrenaline also increases rateR1

15 Control of ventilation rate

Responding to CO2.

Chemoreceptors in the medulla and in the aorta and carotid arteries detect rising CO2 (falling pH). The respiratory centre increases the rate and depth of breathing. This keeps blood CO2 and pH stable.

Quick check

1. Why is CO2, rather than O2, the main stimulus for breathing?

Answers: 1. CO2 changes blood pH, which is detected sensitively

16 Hormones from the hypothalamus and pituitary

The master control.

The hypothalamus produces releasing hormones that control the anterior pituitary, and makes ADH and oxytocin, which are stored and released by the posterior pituitary. The pituitary controls other glands (thyroid, adrenal, gonads).

WORKED EXAMPLE 6 Link (standard)

Explain why the hypothalamus is said to link the nervous and endocrine systems.

Solution

Receives nerve signalsR1

Releases hormones and controls the pituitaryR1

17 Melatonin and circadian rhythms

Body clock.

The pineal gland secretes melatonin in darkness, promoting sleep. Light detected by the retina inhibits melatonin secretion via the suprachiasmatic nucleus of the hypothalamus. Jet lag occurs when the circadian rhythm is out of step with local time.

In the real world

Shift workers and frequent flyers can have disrupted melatonin rhythms, which are linked to sleep disorders and other health problems.

Quick check

1. Why does exposure to bright screens at night affect sleep?

Answers: 1. light inhibits melatonin secretion

18 Blood as a transport and integrating system

Carrying signals and materials.

Blood distributes nutrients, oxygen, hormones and heat around the body, linking organs. Changes in blood composition (glucose, CO2, water) are detected and corrected by homeostatic mechanisms.

Quick check

1. Give two ways blood integrates body systems.

Answers: 1. transports hormones; distributes heat and nutrients

19 Integration in plants

Plant hormones.

Plants integrate responses through hormones: auxin (phototropism, apical dominance), gibberellins (stem growth, germination), ethylene (fruit ripening), abscisic acid (stomatal closure). Phototropism: auxin moves to the shaded side, causing cell elongation there, so the shoot bends towards light.

WORKED EXAMPLE 7 Phototropism (exam level)

Explain how a shoot bends towards light.

Solution

Auxin redistributed to shaded sideR1

Promotes cell elongation on shaded side; shoot bends towards lightR1

20 Mixed review: worked problems

Bringing the ideas together.

WORKED EXAMPLE 8 Mixed problem (exam level)

Explain how the nervous and endocrine systems work together during a fright.

Solution

Sympathetic nerves stimulate adrenal glandsR1

Adrenaline released; increases heart rate, glucose releaseR1

Rapid nervous response sustained by hormonal responseR1

21 Additional exam-style practice

Paper 1 and Paper 2.

WORKED EXAMPLE 9 Paper 1 question (standard)

Which part of the brain coordinates balance? A. cerebrum B. cerebellum C. medulla D. hypothalamus

Solution

BA1

WORKED EXAMPLE 10 Paper 2 question (exam level)

Compare nervous and hormonal communication.

Solution

Nervous: electrical, fast, short-lived, specificR1

Hormonal: chemical in blood, slower, longer-lasting, widespreadR1

22 Feedback in integration

Negative and positive feedback.

Negative feedback reverses changes to maintain stability (body temperature, blood glucose). Positive feedback amplifies a change (oxytocin during childbirth, blood clotting) and is used when a rapid, complete response is needed.

Quick check

1. Give an example of positive feedback.

Answers: 1. oxytocin release during labour

23 Practical: measuring reaction time

Testing integration.

Reaction times can be measured with a ruler-drop test or online software. Variables such as caffeine, practice, distraction and time of day can be investigated.

WORKED EXAMPLE 11 Ruler drop (standard)

A ruler falls 0.20 m before being caught. Calculate the reaction time (g = 9.8 m s−2).

Solution

t = √(2 × 0.20/9.8)M1

= 0.20 sA1

Try this

Use the ruler-drop test: a partner drops a ruler; catch it and convert the distance fallen to reaction time with t = √(2d/g).

24 Systems integration in exercise

A whole-body example.

During exercise, the nervous system increases heart rate and ventilation; adrenaline mobilizes glucose; muscles use ATP and generate heat; thermoregulation increases sweating and vasodilation; kidneys reduce urine output. Many systems act together.

Quick check

1. Name three systems that change during exercise.

Answers: 1. circulatory, respiratory, nervous (also endocrine, muscular)

25 Common misconceptions and links

Students often think…	Actually…
reflexes are controlled by the brain	simple reflexes are coordinated in the spinal cord
hormones act on all cells	only cells with receptors respond
heart rate is set only by the brain	the SA node sets the rate; the medulla adjusts it
auxin moves to the lit side	auxin moves to the shaded side

How C3.1 connects to the rest of the course

Topic	Connection
C2.2 Neural signalling	neurons and synapses
C2.1 Signalling (HL)	hormones and receptors
D3.3 Homeostasis	negative feedback
B3.1 Gas exchange	ventilation
B3.2 Transport	the heart

26 Exam-style question with markscheme

Try this under exam conditions (about 15 minutes) before reading the markscheme.

Figure 5. Heart and breathing rates before, during and after exercise.

(a) Describe the changes in heart rate during the exercise period. [2]

(b) Explain how the medulla increases heart rate during exercise. [3]

(d) Compare nervous and hormonal communication. [3]

(e) Explain how auxin causes a shoot to grow towards light. [3]

Markscheme

Part	Answer	Marks
(a)	rises from 72 to about 150 per minute	A1
	rapid rise then levels off	A1
(b)	chemoreceptors detect raised CO₂ / lower pH	R1
	impulses to medulla	R1
	sympathetic nerves to SA node increase rate	R1
(c)	receptor, sensory neuron	A1
	relay neuron in spinal cord	A1
	motor neuron, effector muscle	A1
(d)	nervous faster, electrical, neurons	A1
	hormonal slower, blood, widespread	A1
	nervous short-lived; hormonal longer	A1
(e)	auxin transported to shaded side	R1
	promotes cell elongation (H⁺, loosened walls)	R1
	shaded side grows faster, shoot bends	R1

Examiner's comments. In (b), the best answers named the receptor, the integrating centre and the nerve pathway.

27 Check your understanding

Section A: multiple choice

1. Which brain region controls breathing rate?

A. medulla oblongata B. cerebellum C. cerebrum D. pituitary

2. Melatonin is secreted by the

A. pituitary B. thyroid C. pineal gland D. adrenal glands

3. In a reflex arc the relay neuron is in the

A. skin B. spinal cord C. muscle D. cerebellum

4. The cerebellum coordinates

A. heart rate B. hormone secretion C. vision D. movement and balance

5. Hormones travel in

A. neurons B. the blood C. lymph only D. air

6. Sympathetic nerves

A. increase heart rate B. decrease heart rate C. stop breathing D. release melatonin

7. Auxin accumulates on the

A. lit side B. root tip only C. leaves only D. shaded side of a shoot

Section B: structured

8. Explain the pain withdrawal reflex. [5]

9. Explain how breathing rate is controlled during exercise. [4]

10. Outline the role of melatonin. [3]

11. Explain phototropism. [4]

Answers

1. A 2. C 3. B 4. D 5. B 6. A 7. D

8. Pain receptor; sensory neuron; relay neuron in spinal cord; motor neuron; muscle contracts; fast, involuntary.

9. Chemoreceptors detect CO₂/pH; medulla; impulses to diaphragm and intercostals; negative feedback.

10. Pineal gland; secreted at night; light inhibits; regulates circadian rhythm; jet lag.

11. Auxin from tip; PIN carriers move it to shaded side; elongation; bending.

28 Summary and topic map

Idea	Remember
Hierarchy	cells, tissues, organs, systems; emergent properties
Nervous vs endocrine	fast, precise vs slow, widespread
Brain	cerebrum, cerebellum, medulla, hypothalamus, pituitary
Reflex arc	receptor, sensory, relay, motor, effector
Melatonin	pineal; circadian rhythms
Heart and breathing	chemoreceptors, medulla, sympathetic and parasympathetic
HL	tropisms; auxin; PIN carriers; plant hormones

Before the exam, make sure you…

•name receptor, integrator, effector and response
•compare nervous and hormonal control in a table
•draw reflex arcs as flow charts
•use negative feedback in explanations

Integration of Body Systems on one page

Cover this page with a sheet of paper and try to rebuild it from memory. Whatever you can't recall is what to revise next.

Figure 6. Topic map for Integration of Body Systems.

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