DP Biology SL D3.3 Homeostasis

DP Biology SL D3.3 Homeostasis - Full notes

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DP Biology SL | D3.3 Homeostasis

DP BIOLOGY · THEME D: CONTINUITY AND CHANGE

D3.3

Homeostasis

Full notes for SL · IB-style

Contents

1	What homeostasis is
2	Negative feedback
3	Control of blood glucose
4	Diabetes
5	Thermoregulation
6	Hormones of thermoregulation and the role of the hypothalamus
7	Homeostasis and negative feedback
8	Blood glucose regulation
9	Diabetes
10	Thermoregulation in mammals
11	Vasodilation and vasoconstriction
12	Endotherms and ectotherms
13	The kidney and osmoregulation (introduction)
14	Thyroxine and metabolic rate
15	Leptin and appetite
16	Melatonin and sleep (link)
17	Positive feedback
18	Mixed review: worked problems
19	Additional exam-style practice
20	Interpreting glucose tolerance tests
21	Homeostasis and exercise
22	Common misconceptions and links
23	Exam-style question with markscheme
24	Check your understanding
25	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
define homeostasis and name variables that are kept within narrow limits
explain negative feedback
explain the control of blood glucose by insulin and glucagon
compare type 1 and type 2 diabetes
explain thermoregulation, including the roles of the hypothalamus, skin, muscles and brown adipose tissue
explain the role of thyroxin

This booklet covers the SL content of this subtopic. Additional Higher Level material is not included.

Guiding questions

How are constant internal conditions maintained in the body? What happens when homeostasis fails?

How this topic is examined

D3.3 is examined through feedback diagrams, blood glucose and temperature graphs, and comparisons of type 1 and type 2 diabetes.

Every homeostatic explanation needs: the change detected, the receptor, the control centre, the effector, and the response that reverses the change.

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 What homeostasis is

Keeping the internal environment within narrow limits.

Think about it

Outside temperatures can range from −30 °C to +45 °C, yet your core body temperature stays within about half a degree of 37 °C. How?

Homeostasis is the maintenance of the internal environment of an organism within narrow limits, despite changes in the external environment or in activity. The internal environment includes the blood and tissue fluid surrounding cells.

Variables kept within narrow limits in humans include body temperature (about 37 °C), blood glucose concentration (about 4 to 8 mmol dm⁻³), blood pH (about 7.35 to 7.45), water and solute concentration of blood (osmotic balance), and CO₂ concentration.

Homeostasis is essential because enzymes work best within narrow ranges of temperature and pH, and cells are damaged if the solute concentration of tissue fluid changes too much (D2.3).

2 Negative feedback

A change triggers a response that reverses it.

Homeostasis works mainly by negative feedback: when a variable moves away from its set point, the change is detected, and responses are triggered that reverse the change, returning the variable to the set point. When the variable returns to normal, the corrective response stops.

A feedback system has: receptors that detect the change; a control centre (often in the brain, such as the hypothalamus) that compares the value with the set point and coordinates the response; and effectors (muscles or glands) that bring about the response. Signals travel by nerves or hormones. Because responses overshoot slightly, the variable fluctuates around the set point rather than staying perfectly constant.

Positive feedback amplifies a change instead of reversing it, as in childbirth (D3.1); it is rare in homeostasis.

Figure 1. Negative feedback.

Quick check

1. Define homeostasis.

2. What is negative feedback?

Answers: 1. maintaining internal conditions within narrow limits 2. a response that reverses a change from the set point

3 Control of blood glucose

Insulin lowers, glucagon raises.

Blood glucose concentration is monitored by cells in the islets of Langerhans in the pancreas. Glucose is needed by all cells for respiration, but high concentrations damage blood vessels and draw water out of cells.

When blood glucose rises (after a meal), beta cells secrete insulin. Insulin binds to receptors on target cells, especially liver and muscle cells. It increases the uptake of glucose by cells (by adding GLUT4 transporters to membranes, C2.1), stimulates the conversion of glucose to glycogen for storage in the liver and muscles (glycogenesis), and increases respiration of glucose. Blood glucose falls.

When blood glucose falls (during fasting or exercise), alpha cells secrete glucagon. Glucagon acts mainly on the liver, stimulating the breakdown of glycogen into glucose (glycogenolysis) and the production of glucose from other substances. Glucose is released into the blood, and the concentration rises.

Insulin and glucagon are antagonistic hormones: they have opposite effects, and together keep blood glucose near the set point.

	Insulin	Glucagon
Secreted by	beta cells of pancreas	alpha cells of pancreas
Stimulus	high blood glucose	low blood glucose
Main targets	liver, muscle, fat cells	liver
Effect	glucose uptake; glycogen formed	glycogen broken down to glucose
Result	blood glucose falls	blood glucose rises

Figure 2. Blood glucose and insulin after a meal.

Common mistake

Confusing glycogen, glucagon and glucose. Glucagon is the hormone; glycogen is the storage polysaccharide; glucose is the sugar.

Quick check

1. Which cells secrete insulin?

2. What does glucagon do?

Answers: 1. beta cells in islets of Langerhans 2. stimulates the liver to break down glycogen into glucose

4 Diabetes

When glucose control fails.

Diabetes mellitus is a condition in which blood glucose is not controlled properly, so it remains too high (hyperglycaemia). Symptoms include glucose in the urine, thirst, frequent urination, tiredness and weight loss. Over time, high blood glucose damages blood vessels, nerves, kidneys and eyes.

Type 1 diabetes is an autoimmune disease: the immune system destroys the beta cells, so little or no insulin is produced. It usually begins in childhood. It is treated with insulin injections (or pumps), matched to diet and activity.

Type 2 diabetes occurs when target cells become less responsive to insulin (insulin resistance), even though insulin is produced. It usually develops in adults and is strongly linked to obesity, physical inactivity, diet high in sugars and fats, and genetic factors. It is treated with diet changes, exercise, weight loss and drugs that increase insulin sensitivity; insulin may be needed later. Type 2 accounts for about 90% of cases and is increasing worldwide.

	Type 1	Type 2
Cause	autoimmune destruction of beta cells	insulin resistance of target cells
Insulin production	little or none	normal at first
Typical onset	childhood	adulthood
Risk factors	genetic, autoimmune	obesity, inactivity, diet, genetics
Treatment	insulin injections	diet, exercise, drugs; sometimes insulin

Quick check

1. Distinguish type 1 and type 2 diabetes.

2. Give two risk factors for type 2 diabetes.

Answers: 1. type 1: no insulin (beta cells destroyed); type 2: cells resistant to insulin 2. obesity, inactivity, poor diet

5 Thermoregulation

Keeping core temperature near 37 °C.

Humans are endotherms: they generate heat from metabolism and keep their core temperature constant. Temperature is monitored by thermoreceptors in the skin (peripheral) and in the hypothalamus (core). The hypothalamus is the control centre.

When body temperature rises: vasodilation: arterioles supplying skin capillaries widen, so more blood flows near the skin surface and more heat is lost by radiation; sweating: sweat glands secrete sweat, which evaporates, removing heat (latent heat of vaporization, A1.1); hairs lie flat (hair erector muscles relax); metabolic rate may decrease.

When body temperature falls: vasoconstriction: skin arterioles narrow, reducing blood flow to the skin and heat loss; shivering: rapid involuntary contractions of skeletal muscles generate heat by respiration; uncoupled respiration in brown adipose tissue (especially in babies): mitochondria release energy as heat instead of making ATP; hair erector muscles contract, raising hairs to trap an insulating layer of air (less effective in humans); behavioural responses such as putting on clothes.

Response to cold	Response to heat
vasoconstriction of skin arterioles	vasodilation of skin arterioles
shivering	sweating
brown adipose tissue: uncoupled respiration	reduced metabolic rate
hairs raised	hairs flat
thyroxin increases metabolic rate	behaviour: seek shade

Common mistake

Saying blood vessels 'move closer to the skin' during vasodilation. The vessels do not move; arterioles widen, so more blood flows through skin capillaries.

Quick check

1. What is vasodilation and how does it cool the body?

2. How does brown adipose tissue generate heat?

Answers: 1. widening of skin arterioles; more blood near surface; more heat lost 2. uncoupled respiration releases heat instead of making ATP

6 Hormones of thermoregulation and the role of the hypothalamus

Thyroxin adjusts the metabolic rate.

The hypothalamus also controls longer-term responses through hormones. When it is cold, the hypothalamus stimulates the pituitary to release TSH, which stimulates the thyroid gland to secrete thyroxin. Thyroxin increases the metabolic rate of most body cells, generating more heat. In warm conditions, thyroxin secretion falls.

Thyroxin needs iodine for its synthesis; iodine deficiency can cause goitre. Thyroxin also regulates growth and development.

Figure 3. Core temperature and sweat rate during exercise.

Quick check

1. What does thyroxin do?

2. Where is the thermoregulation control centre?

Answers: 1. increases metabolic rate and heat production 2. hypothalamus

7 Homeostasis and negative feedback

Keeping conditions stable.

Homeostasis maintains the internal environment within narrow limits despite external changes: body temperature, blood glucose, blood pH, water potential. Negative feedback: a change from the set point is detected by receptors; the coordinator triggers effectors that reverse the change.

WORKED EXAMPLE 1 Feedback loop (standard)

Outline the components of a negative feedback loop.

Solution

Receptor detects change; coordinator (brain/gland) compares with set pointR1

Effector produces a response that reverses the changeR1

8 Blood glucose regulation

Insulin and glucagon.

Beta cells in the pancreatic islets secrete insulin when blood glucose rises; insulin increases glucose uptake by cells and conversion to glycogen in liver and muscle. Alpha cells secrete glucagon when glucose falls; glucagon stimulates glycogen breakdown in the liver.

WORKED EXAMPLE 2 After a meal (exam level)

Explain how blood glucose returns to normal after a carbohydrate-rich meal.

Solution

Rise detected by β cells, which secrete insulinR1

Cells take up glucose; liver and muscles convert it to glycogenR1

Blood glucose falls; insulin secretion decreases (negative feedback)R1

9 Diabetes

Types 1 and 2.

Feature	Type 1	Type 2
cause	autoimmune destruction of β cells	cells become resistant to insulin
onset	usually childhood	usually adulthood; linked to obesity
treatment	insulin injections	diet, exercise, drugs; sometimes insulin

WORKED EXAMPLE 3 Symptoms (standard)

Explain why untreated diabetes causes glucose in the urine.

Solution

Blood glucose very highR1

Kidneys cannot reabsorb it allR1

In the real world

Over 500 million adults worldwide have diabetes, most of them type 2; lifestyle changes can prevent or delay many cases.

10 Thermoregulation in mammals

Maintaining 37 °C.

Thermoreceptors in the skin and hypothalamus detect temperature. When too hot: vasodilation of skin arterioles, sweating, hairs flatten, reduced metabolism. When too cold: vasoconstriction, shivering, hairs raised, increased metabolic rate (thyroxine), brown fat thermogenesis.

WORKED EXAMPLE 4 Cold (exam level)

Explain three responses to a fall in body temperature.

Solution

Vasoconstriction reduces blood flow near skin surface, less heat lossR1

Shivering: muscle contraction generates heatR1

Increased metabolism (thyroxine); hairs raisedR1

11 Vasodilation and vasoconstriction

Controlling heat loss.

Arterioles supplying skin capillaries dilate (vasodilation) to increase blood flow and heat loss by radiation, or constrict (vasoconstriction) to reduce it. Shunt vessels divert blood away from the surface in the cold.

Common mistake

Saying that capillaries move closer to the skin surface. Capillaries do not move; blood flow through them changes.

Quick check

1. Why does the skin look flushed when hot?

Answers: 1. vasodilation increases blood flow near the surface

12 Endotherms and ectotherms

Two strategies.

Endotherms (birds and mammals) generate heat metabolically and regulate a constant body temperature. Ectotherms (reptiles, fish, amphibians) rely on external heat and regulate temperature by behaviour (basking, seeking shade).

Quick check

1. Give one advantage of being an ectotherm.

Answers: 1. needs much less food energy

13 The kidney and osmoregulation (introduction)

Balancing water.

The kidneys filter blood, reabsorb useful substances and water, and excrete waste (urea) in urine. Osmoreceptors in the hypothalamus detect blood water potential; ADH from the posterior pituitary increases water reabsorption in the collecting ducts by inserting aquaporins.

WORKED EXAMPLE 5 Dehydration (exam level)

Explain how the body responds to dehydration.

Solution

Osmoreceptors detect low water potential; ADH releasedR1

More aquaporins in collecting duct; more water reabsorbedR1

Small volume of concentrated urineR1

14 Thyroxine and metabolic rate

Long-term control.

Thyroxine from the thyroid gland increases metabolic rate and heat production. Its release is controlled by TRH (hypothalamus) and TSH (pituitary), with negative feedback. Iodine is needed to make thyroxine.

Quick check

1. Why does iodine deficiency cause a goitre?

Answers: 1. low thyroxine, so TSH stays high, stimulating thyroid enlargement

15 Leptin and appetite

Hormonal control of food intake.

Leptin is secreted by adipose tissue and acts on the hypothalamus to reduce appetite. In some obese people, the brain becomes resistant to leptin. Mice lacking the leptin gene become obese.

Quick check

1. Explain why leptin injections do not help most obese people.

Answers: 1. they are leptin-resistant; receptors do not respond

16 Melatonin and sleep (link)

Circadian control.

Melatonin from the pineal gland rises at night, promoting sleep. It helps synchronize body rhythms with day and night. Melatonin supplements are used for jet lag.

Quick check

1. Which gland secretes melatonin?

Answers: 1. pineal gland

17 Positive feedback

Amplifying a change.

Positive feedback amplifies a change rather than reversing it: during childbirth, stretch of the cervix stimulates oxytocin release, which increases contractions, which stretch the cervix further, until birth ends the loop.

Quick check

1. Why must positive feedback loops have an end point?

Answers: 1. otherwise the change would continue indefinitely

18 Mixed review: worked problems

Bringing the ideas together.

WORKED EXAMPLE 6 Mixed problem (exam level)

Compare the control of blood glucose and body temperature.

Solution

Both use negative feedback around a set pointR1

Glucose: hormonal (insulin, glucagon) from pancreasR1

Temperature: nervous and hormonal, controlled by hypothalamusR1

19 Additional exam-style practice

Paper 1 and Paper 2.

WORKED EXAMPLE 7 Paper 1 question (standard)

Which cells secrete glucagon? A. α cells B. β cells C. liver cells D. adrenal cells

Solution

AA1

WORKED EXAMPLE 8 Paper 2 question (exam level)

Distinguish between type 1 and type 2 diabetes.

Solution

Type 1: no insulin produced (β cells destroyed)R1

Type 2: insulin produced but target cells resistantR1

20 Interpreting glucose tolerance tests

Data analysis.

In a glucose tolerance test, a person drinks glucose solution after fasting, and blood glucose is measured over two hours. In diabetic people, glucose rises higher and returns to normal more slowly.

Figure 4. Glucose tolerance test results.

WORKED EXAMPLE 9 Interpreting (exam level)

Compare the two curves.

Solution

Diabetic: higher fasting level and peakR1

Slower return: insulin absent or ineffectiveR1

21 Homeostasis and exercise

Many systems at once.

During exercise, homeostatic mechanisms work together: thermoregulation increases sweating; blood glucose is maintained by glucagon and adrenaline; ventilation adjusts CO2; ADH reduces water loss in urine.

Quick check

1. Why does urine output fall during prolonged exercise?

Answers: 1. water lost in sweat; ADH increases water reabsorption

22 Common misconceptions and links

Students often think…	Actually…
glucagon and glycogen are the same	glucagon is a hormone; glycogen a storage polysaccharide
blood vessels move to the skin surface	arterioles dilate or constrict
type 2 diabetics produce no insulin	they are resistant to insulin

How D3.3 connects to the rest of the course

Topic	Connection
C2.1 Signalling (HL)	insulin receptor
C3.1 Integration	hypothalamus and nervous control
B1.1 Carbohydrates	glycogen
A1.1 Water	evaporative cooling
D2.3 Water potential	osmosis

23 Exam-style question with markscheme

Try this under exam conditions (about 18 minutes) before reading the markscheme.

Figure 5. Blood glucose after a meal.

(a) Describe the change in blood glucose after the meal. [2]

(b) Explain how blood glucose returns to normal. [3]

(d) Explain two responses to a fall in body temperature. [4]

Markscheme

Part	Answer	Marks
(a)	rises from 5.0 to 8.5 mmol dm⁻³ at 30 min	A1
	returns to about 5 by 120 min	A1
(b)	beta cells secrete insulin	R1
	cells take up glucose; liver and muscle make glycogen	R1
	negative feedback lowers glucose	R1
(c)	type 1: no insulin, autoimmune	A1
	type 2: insulin resistance	A1
	type 1 childhood, insulin injections; type 2 adult, lifestyle	A1
(d)	vasoconstriction reduces heat loss	R1 R1
	shivering generates heat	R1 R1

Examiner's comments. In (b), weaker answers wrote 'glucagon' for 'glycogen'.

24 Check your understanding

Section A: multiple choice

1. Insulin is secreted by

A. beta cells B. alpha cells C. the liver D. the hypothalamus

2. Glucagon causes

A. glucose uptake by muscle B. glycogen formation C. glycogen breakdown in the liver D. lower blood glucose

3. Type 1 diabetes is caused by

A. insulin resistance B. destruction of beta cells C. too much glucagon D. obesity only

4. Vasodilation

A. reduces heat loss B. causes shivering C. raises metabolic rate D. increases heat loss

5. Thyroxin

A. lowers blood glucose B. increases metabolic rate C. causes sweating D. reduces heat production

6. Negative feedback

A. reverses a change B. amplifies a change C. only occurs in plants D. has no set point

Section B: structured

7. Explain the control of blood glucose by negative feedback. [5]

8. Explain the role of the hypothalamus in thermoregulation. [4]

9. Explain why type 2 diabetes is increasing. [3]

Answers

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

7. Rise detected by pancreas; insulin; uptake, glycogen; fall: glucagon; glycogenolysis; antagonistic hormones.

8. Thermoreceptors; compares with set point; nerves to skin arterioles, sweat glands, muscles; thyroxin via pituitary.

9. Rising obesity, inactivity, diets high in sugar and fat; ageing populations.

25 Summary and topic map

Idea	Remember
Homeostasis	internal conditions within narrow limits
Negative feedback	receptor, control centre, effector; reverses change
Blood glucose	insulin (beta cells) and glucagon (alpha cells)
Diabetes	type 1 autoimmune; type 2 insulin resistance
Thermoregulation	hypothalamus; vasodilation, sweating; vasoconstriction, shivering, brown fat
Thyroxin	raises metabolic rate

Before the exam, make sure you…

•name receptor, control centre, effector and response
•spell glucagon and glycogen correctly
•describe vasodilation accurately
•compare diabetes types in a table

Homeostasis 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 Homeostasis.

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