- IB DP Biology 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Biology 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Biology 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
- IB DP Biology 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
D3.3 Homeostasis
Homeostasis as a characteristic of life
⮚ External environment can change
e.g. temperature, diet, food availability…
⮚ These changes can affect the internal environment
e.g. temperature, blood pH, glucose blood concentration, hormones…
⮚ Some animals can maintain their internal environment more-or-less constant = Regulators
⮚ Temperature is regulated to stay around the optimum temperature of body
Optimum temperature = temperature at which enzymes work best
– Muscles contraction efficient in a wide range of external temperatures
Nervous system co-ordinates responses precisely
– Even in unfavorable conditions,
Can avoid danger
Can take advantage of non-regulators
e.g. catch and eat them
– Regulators have e advantages on non-regulators
Can adapt to changing environment more efficiently
⮚ Homeostasis = maintenance of a constant internal environment control of internal conditions within set limits
Internal environment = blood and tissue fluid
Tissue fluid delivers nutrients from blood and remove wastes from cells
⮚ Composition and temperature of tissue fluid kept within narrow limits
By controlling composition and temperature of blood within narrow limits
e.g. concentration of glucose in blood
e.g. concentration of salts in blood
e.g. amount of water in blood
e.g. temperature of blood/body
e.g. blood pH
⮚ All these parameters change constantly due to
– body’s activities
– external environment changes
Parameters need to be adjusted constantly
= Homeostasis
Negative feedback loops in homeostasis
Negative feedback = type of control in which the conditions being regulated are brought back to a set value as soon as it is detected that they have deviated from it
Example of the water bath in laboratory
– Variable = temperature
– Set value = for example 37oC
– Detector measures temperature
– Effector increases or decreases temperature
Stimulus A
Stimulus B
1. Temperature rises higher than 37oC
2. Detector detects this higher set value temperature
3. Effector decreases temperature to… slightly lower than 37oC
Response A
Response B
4. Temperature falls lower than 37oC
5. Detector detects this lower than set value temperature
6. Effector increases temperature to… slightly higher than 37oC
1 2 3 4 5 6
1 2 3 4 5 6
…….
The variable oscillates around the set value
Each response is a negative feedback on the stimulus
To bring back the variable to the set value
– Concentration of glucose in blood
Concentration of salts in blood
Amount of water in blood
Temperature of blood/body
Blood pH
Regulation of blood glucose concentration
Reminder
Pancreas is exocrine gland
Digestive enzymes into small intestine
Pancreas = also endocrine gland
Islets of Langerhans
\(\alpha\)-cells produce and secrete Glucagon
\(\beta\)-cells produce and secrete Insulin
Both released into blood
Symptoms of diabetes type I and II
Causes and treatments of diabetes type I and II
Causes
\(\beta\)-cells destroyed auto-immune disease
Cells “resist/insensitive” to insulin
Insulin receptors and/or Glucose transporters deficient Genetics, Age, obesity, lack of exercise
Treatment
– Regular injections of insulin Into bloodstream
“insulin-dependent”
– Exercise
– Balanced diet (less sugar and fats)
The glucose tolerance test
Test to diagnose diabetes
⮚ Patient drinks concentrated glucose solution
⮚ Monitor blood glucose concentration
⮚ Measure how long taken to clear glucose from blood
Thermoregulation
⮚ Normal human body temperature varies between 35.8 degree C and 37.7 degree C
⮚ Below 34 degree C or above 40 o C = Danger
⮚ Below 34 degree C, enzymes do not work efficiently enough
Core organs may fail to sustain life
⮚ Above 40 degree C , enzymes in core organs start to denature
Core organs may fail to sustain life
⮚ Core temperature only varies by 1 or 2 degree C thanks to homeostasis
⮚ When alive, mammals maintain temperature of body relatively constant
– Use heat generated by cell respiration
– Generate extra heat by shivering when cold
– Control heat lost through skin carefully
⮚ Humans: core temperature (inner body temperature) just below 37 degreeC
The core of the body contains the vital organs
e.g. heart, kidneys, lungs, liver…
⮚ If outside temperature is low, the core temperature is kept just below 37 degree C
Outer parts of the body may be colder
Difference between ECTOTHERMS and ENDOTHERMS
⮚ When alive, mammals maintain temperature of body relatively constant
⮚ Body temperature fluctuates in a 24-hour period
Increases during exercise
Decreases during sleep
Heat production in the body
⮚ Main source of heat = biochemical reactions that generate heat
Especially cell respiration
⮚ Amount of heat produced varies between organs
Liver extremely active in metabolism
But most of reactions in liver require heat = endergonic reactions
– Little heat lost by liver to blood
Liver is neutral thermally
⮚ Heat is then distributed in whole body by blood circulation
⮚ At rest: 70% of body heat comes from internal organs
Especially heart, kidneys, lungs and brain
Body mass is 90% skeleton, muscles and skin
But produce only 30% of body heat
⮚ During exercise: skeletal muscles work intensely
Need lots of energy to contract
Intense cell respiration
Generate a lot of heat
⮚ During arduous exercise such as marathon
Body temperature can rise to dangerous levels
Occasionally leading to death
Skin structure
Actors of thermoregulation in the skin
Sweat
⮚ In warm conditions, sweat is secreted by sweat glands
Makes a layer of sweat on skin
⮚ Sweat mostly made of water
Salts \((Na^{+}, Cl^{-} , Mg^{2+})\)
Some metabolic wastes, including urea
⮚ Odorless (?)
⮚ Colorless
⮚ pH 4.5 – 5.5
Role of sweat in thermoregulation
⮚ Once sweat is on surface of skin,
Water from sweat “steals” heat from body by conduction
⮚ Water has a high heat of evaporation
– Much heat is needed to evaporate water on skin
– Cooling effect on the body
⮚ Evaporation influenced by
– humidity
– air movements
Higher humidity and Absence of air movement Slows down evaporation
Lower humidity and Strong air movements Speeds up evaporation
Role of hairs in thermoregulation
Role of capillaries in thermoregulation
Role of skeletal muscles in thermoregulation
⮚ In cold conditions, skeletal muscles contract rapidly = shivering
– Need energy to contract
– Perform much cell respiration
Cell respiration releases heat
– Shivering releases much heat
Control mechanisms of thermoregulation
⮚ Thermoregulation control performed by hypothalamus in brain
⮚ Specific region of hypothalamus dedicated to thermoregulation = thermoregulation centre
Divided into “heat loss centre” and “heat gain centre”
⮚ Neurons sensitive to temperature detect changes in temperature of blood flowing through brain
+ Receive information from temperature receptors in skin and internal organs via sensory neurons
⮚ Hypothalamus communicates with rest of body for responses via the autonomous nervous system
“autonomous”: unconscious control
Heat loss and Heat gain are normally in balance
If imbalanced, homeostasis corrects the body temperature
Heat loss
- Vasodilation
- Sweating
Heat gain
- Shivering
- Vasoconstriction
Thyroxin, metabolic rate and body temperature
⮚ Thyroxin produced and secreted by thyroid gland
⮚ Contains four atoms of Iodine
– Diet has to contain Iodine for normal production
⮚ Targets almost all cells of the body
⮚ Increases metabolic rate of cells to produce energy
– Increased activity in mitochondria = Respiration
More oxygen consumed
More \(CO_2\) , Water and ATP produced
More HEAT produced
Increased protein synthesis needed
Effects of thyroxin deficiency = Hypothyroidism
The kidneys and osmoregulation (HL only)
⮚ In mammals, regulation of
– body temperature
– blood glucose concentration
– blood and tissue fluid ions concentration
– blood and tissue fluid amount
Osmoregulation: To control movements of water from one region to another in the body
⮚ Detectors = specialized cells in brain or other organs
⮚ Effectors = organs
e.g. skin, liver, kidneys
⮚ Information passed between detectors and effectors through
– neurons of nervous system
– hormones of the endocrine system
– both
⮚ Outcome = internal environment very precisely regulated
⮚ Excretion = removal from organisms of toxic materials
– waste products of metabolism that the body itself produced
e.g. carbon dioxide, urea
– substances in excess of body’s requirements
e.g. salts in excess
– hormones that are not needed anymore
⮚ Common misconception
Egestion is not excretion
Faeces contain food that body could not digest
Faeces are not made by the body
Except bilirubin
⮚ Four main excretory products in humans
1. Carbon dioxide
2. Urea
3. Excess salts
4. Water
⮚ Three main excreting organs in humans
1. Lungs
2. Kidney
3. Skin
⮚ One additional organ involved
Liver
⮚ Kidneys filter the blood
⮚ Kidneys excrete
– urea formed by liver from excess amino acids from blood
– excess salts
– excess water
⮚ Kidneys produce urine
Urine contains water, urea and excess salts
The urinary system
Structure of kidneys
⮚ Three main parts:
– the cortex
– the medulla
– the pelvis
⮚ Cortex and medulla contains nephrons
Basic unit of kidney
⮚ Each nephron begins in the cortex, loops down into the medulla, back into the cortex, and then goes down again through the medulla to the pelvis
⮚ Nephrons branch in to form collecting ducts
⮚ Collecting ducts branch in to form the ureters in pelvis
⮚ The ureters carry urine made by nephrons to the bladder
Each kidney contains 350.000 nephrons
Total 700.000 nephrons in human body
⮚ Blood comes in from aorta through renal arteries
⮚ Blood is cleaned by nephrons
⮚ Metabolic wastes + excess water = urine in ureters
⮚ Cleaned blood in renal veins into vena cava
⮚ Kidneys consist of microscopic structures called nephrons or Kidney tubules
⮚ A nephron is made of :
– Glomerulus: a network of capillaries
– Renal capsule: cup-like structure surrounding the glomerulus
– Renal tubule: hollow, from Bowman’s capsule to collecting duct
⮚ A collecting duct collects urine from several nephrons
Collecting ducts branch in to form the ureter
⮚ Renal artery that brings blood branches out into arterioles
⮚ Arterioles branch out into Glomerulus
⮚ Capillaries out of renal capsule wraps around renal tubule
⮚ Capillaries branch in to join renal vein
⮚ Renal vein carries blood out of the kidney back to heart via vena cava
Function of a kidney
Ultrafiltration
⮚ Arteriole to glomerulus: diameter bigger than arteriole from glomerulus
High pressure in capillaries of glomerulus
Part of blood plasma leaks through capillary walls into renal capsule
⮚ What leaks out depends on
1. Size of pores of capillary walls
and
2. Size of substances
– Water, salts, urea, amino acids and glucose: smaller than pores
– Leak out of capillaries into renal capsule
Only some of blood plasma’s water leaks out
– Proteins, RBCs, WBCs: bigger than pores
– Stay in blood plasma
Selective reabsorption
⮚ Filtrate from glomerulus trickles down renal tubule
⮚ On the way to collecting duct, capillaries surrounding renal tubule
– absorb back substances that body needs
– do not absorb back other substances
“Selective reabsorption”
Reabsorbed:
– 100% Glucose
– 100% amino acids
– Most water
– Salts that are needed
Not reabsorbed:
– Urea and uric acid
– Ammonia
– Excess salts
Salts will be at the correct concentrations in the blood after reabsorption
What has not been reabsorbed = Urine
Urine and urination
⮚Water content of urine depends on needs of body
⮚Measures how much water in blood
– Dehydrated:
reabsorption water increases,
volume urine decreases,
concentration solutes in urine increases
– Too much water in blood:
reabsorption water decreases,
volume urine increases,
concentration solutes in urine decreases
Reabsorption of water in osmoregulation
⮚ Blood concentration is monitored by osmoreceptors in the hypothalamus
⮚ If the concentration rises the posterior pituitary releases ADH
⮚ ADH makes the collecting duct walls more permeable
⮚ More water is reabsorbed from the filtrate as the ducts pass through the salty tissues of the medulla
⮚Bladder can expand and hold about 400 \(cm^3\) of urine
⮚ If band of circular muscle = sphincter contracted,
Urine kept in bladder
⮚ When sphincter relaxes, muscles of bladder wall contract
Urine expelled through urethra
⮚ Adults can control sphincter
Babies cannot
After 3 years old, most children can
Changes in blood supply to organs (HL only)
