D3.3Theme D — Continuity and change
Homeostasis — IB Biology SL Revision Notes & Topical Questions
5 min read 11 parts Lesson 9 of 14 3 quizzes
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Lesson summary
D3.3 Homeostasis — IB BIOLOGY SL notes: Explain homeostasis by negative feedback, the control of blood glucose, diabetes and thermoregulation.Homeostasis = keeping conditions inside the body within preset limits, giving the best conditions for enzymes and cellsRegulated variables…
- Negative feedback loop
- Coordination
- Positive feedback
- Normal level
- Long-term damage
- Hypothalamus
Part 1 of 9Homeostasis and negative feedback
Homeostasis
Homeostasis keeping conditions inside the body within preset limits, giving the best conditions for enzymes and cells
- Regulated variables are core body temperature, blood pH, blood glucose concentration, water balance and the osmotic concentration of the blood.
- Sensory cells detect changes inside and outside the body.
Negative feedback loop
- Negative feedback loop most homeostatic mechanisms use it to return a value to a set point
- A receptor detects the change.
- A coordination system, the brain and nervous system, passes the information on.
- An effector, a muscle or gland, brings about a response that reverses the change.
- The factor is monitored continuously, so a rise causes a fall and a fall causes a rise.
- The variable fluctuates near the set point.
Coordination
- Coordination the nervous and endocrine systems carry signals to effectors
Positive feedback
- Positive feedback it amplifies a change and is rare in homeostasis, as with oxytocin in childbirth
Part 2 of 9Control of blood glucose
Why glucose is controlled
- Glucose is needed for respiration, so a low level starves cells, especially neurones.
- Glucose is soluble, so a high level makes the blood too concentrated and draws water out of cells by osmosis.
- A dangerously high level after a meal can damage organs.
Normal level
- Normal level fasting blood glucose is about 4-6 mmol dm-3
Pancreas
- Pancreas it has exocrine tissue for digestive enzymes and endocrine tissue called the islets of Langerhans
- The α cells secrete glucagon.
- The β cells secrete insulin.
Hormones
- Hormones insulin and glucagon are antagonistic, travel in the blood and act only on cells with matching receptors
Part 3 of 9Insulin and glucagon
| Feature | Insulin | Glucagon |
|---|---|---|
| Secreting cell | β cells | α cells |
| Stimulus | High blood glucose | Low blood glucose |
| Main targets | Liver, muscle and adipose tissue | Liver |
| Effect on glycogen | Glucose to glycogen (glycogenesis) | Glycogen to glucose (glycogenolysis) |
| Effect on uptake | More glucose taken into cells | Glucose released into the blood |
| Net result | Blood glucose falls | Blood glucose rises |
Insulin
- Insulin channels open in the cell surface membranes and glucose enters the cells by facilitated diffusion
- It also raises the rate of respiration and converts glucose to fatty acids for fat storage.
Glucagon
- Glucagon released after fasting or exercise
- It activates enzymes that hydrolyse glycogen in liver and muscle cells.
- The glucose leaves the liver into the blood.
Dominance
- Dominance glucagon dominates between meals and in exercise, and insulin dominates after meals
1
Glucose is absorbed and the blood glucose rises above the set point.
2
Pancreatic β cells detect the rise.
3
The β cells secrete insulin into the blood.
4
Liver and muscle cells take in glucose and store it as glycogen.
5
Blood glucose falls back towards the set point.
6
Insulin secretion decreases as the stimulus is removed.
- Spelling
- glucagon is the hormone and glycogen is the storage polysaccharide
Part 4 of 9Diabetes
Diabetes
Diabetes a condition in which homeostatic control of blood glucose has failed or deteriorated
- Glucose stays in the blood rather than entering cells, so respiration falls and the person is fatigued.
- The kidneys cannot filter out the excess glucose, so it appears in the urine.
- The kidneys make large volumes of urine, leading to dehydration and thirst.
- Weight loss can also occur.
Long-term damage
- Long-term damage high glucose can harm blood vessels, kidneys, eyes and nerves
Part 5 of 9Type 1 and type 2 diabetes
Type 1 and type 2 diabetes
Type 1
- Type 1 the pancreas fails to make enough insulin
- It normally starts in childhood as an autoimmune response in which the immune system destroys the β cells.
- It is treated with blood glucose monitoring, insulin injections and a modified diet.
Type 2
- Type 2 more common and usually in older adults
- The pancreas still makes insulin but the cell receptors are fewer or no longer respond, which is insulin resistance.
- The pancreas secretes more insulin until it can no longer compensate.
- Glucose uptake is reduced, so blood glucose stays high.
- It is treated with medication, a low-carbohydrate diet and exercise.
| Feature | Type 1 | Type 2 | Healthy person |
|---|---|---|---|
| Cause | Immune system destroys β cells | Insulin receptors less sensitive | Normal secretion and response |
| Insulin level | Very low | Normal or high at first | Rises after meals |
| Usual onset | Childhood | Adulthood, increasingly younger | Not applicable |
| Risk factors | Genetic plus unknown triggers | Obesity, low activity, diet and genes | Not applicable |
| Treatment | Insulin injections or pump | Diet, exercise and drugs | None needed |
| Prevention | Not currently preventable | Often preventable by lifestyle | Not applicable |
- Obesity
- a major risk factor, as over-production of insulin in response to a high-carbohydrate diet triggers insulin resistance
Part 6 of 9Thermoregulation control
Thermoregulation
Thermoregulation control of the internal body temperature by negative feedback
- Enzymes work best at about 37 °C.
- Lower temperatures slow reactions because molecules have less kinetic energy.
- Higher temperatures denature enzymes.
Receptors
- Peripheral thermoreceptors in the skin detect external temperature, with receptors for both heat and cold.
- Receptors in the hypothalamus monitor the temperature of the blood.
Hypothalamus
- Hypothalamus compares the temperature with the set point and sends nerve impulses and hormonal signals
Effectors
- Effectors control heat loss at the skin and the generation of heat by metabolism
Endotherms
- Endotherms mammals and birds keep a constant temperature
- They use physiological mechanisms such as shivering and altered metabolism.
- They use behaviour such as sheltering in a burrow or sunbathing.
Part 7 of 9Responses to heat
Vasodilation
- Vasodilation the muscles in the arteriole walls relax, so more blood flows through the skin capillaries
- More heat is lost by radiation from the skin.
Sweating
- Sweating evaporation uses heat energy from the body, which cools the skin
Hairs lie flat
- Hairs lie flat the hair erector muscles relax, so there is no insulating layer of air and heat can leave
Muscle
- Muscle it stays relaxed and brown fat is less active
Arterioles
- Arterioles vasodilation and vasoconstriction are caused by muscles in arterioles, not capillaries
Part 8 of 9Responses to cold
Vasoconstriction
- Vasoconstriction the arteriole muscles contract, so blood is diverted through deeper shunt vessels
- Less heat is lost by radiation.
Hairs stand up
- Hairs stand up the hair erector muscles contract and trap an insulating layer of air, though humans have little hair so this is less effective
Shivering
- Shivering rapid muscle contraction makes heat that warms the blood and raises the core temperature
Brown adipose tissue
- Brown adipose tissue uncoupled respiration releases all the energy as heat instead of making ATP
- It gives non-shivering thermogenesis, mainly in newborn infants.
White adipose tissue
- White adipose tissue a lipid layer under the skin and around organs that insulates
Thyroxine
- Thyroxine the hypothalamus releases thyrotropin-releasing hormone,
- which makes the pituitary release thyroid-stimulating hormone,
- so the thyroid releases thyroxine that raises the basal metabolic rate
1
Cold air lowers the skin temperature.
2
Skin thermoreceptors send impulses to the hypothalamus.
3
The hypothalamus compares the input with the set point.
4
Nerve signals cause vasoconstriction and shivering.
5
Heat production rises and heat loss falls.
6
Core temperature returns to about 37 °C and the responses stop.
| Mechanism | Body too cold | Body too hot |
|---|---|---|
| Skin arterioles | Vasoconstriction | Vasodilation |
| Muscles | Shivering releases heat | Relaxed |
| Sweat glands | Little sweat | Sweating, with cooling by evaporation |
| Hair erector muscles | Contract, so hairs stand up | Relax, so hairs lie flat |
| Brown fat | Uncoupled respiration releases heat | Less active |
Part 9 of 9More from the specification
1Thermoregulatory negative feedback loops
- Thermoregulatory negative feedback loops hypothalamic thermoreceptors detect core temperature shifts, activating physiological negative feedback loops such as vasodilation and sweating.
2Internal environment
- The tissue fluid surrounding cells, kept stable by the blood.
Key terms
| Term | Meaning |
|---|---|
| Homeostasis | The maintenance of internal conditions within tolerable limits. |
| Negative feedback | A control process in which a change is detected and reversed to return a value to its set point. |
| Insulin | A hormone from β cells that lowers blood glucose. |
| Glucagon | A hormone from α cells that raises blood glucose. |
| Insulin resistance | The failure of cells to respond to insulin, as in type 2 diabetes. |
| Thermoregulation | The control of internal body temperature. |
| Vasodilation | The widening of arterioles in the skin that increases heat loss. |
| Endotherm | An animal that maintains a constant internal body temperature. |
Avoid these mistakes
1
Saying negative feedback amplifies a change — it reverses the change
2
Confusing glucagon with glycogen — glucagon is the hormone and glycogen is the stored polysaccharide
3
Saying type 2 diabetes is caused by lack of insulin — the cells have become resistant to insulin
4
Saying capillaries dilate to lose heat — the arterioles do, because capillaries have no muscle
Marked practiceQuizzes for this lesson
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