Topic 14 - Homeostasis Flashcards
Why do organisms have control systems?
To keep their internal conditions near constant to function efficiently. HOMEOSTASIS
What are the physiological factors controlled in homeostasis in mammals? (6 things)
- Core body temperature
- Metabolic waster, especially carbon dioxide and urea
- Blood pH
- Blood glucose concentration
- Water potential of blood
- Concentration in blood or respiratory gases, oxygen and carbon dioxide
What does the phrase ‘internal environment’ refer to?
Refers to all the conditions in the body in which cells function
What is the immediate environment of a cell?
Tissue fluid
How do homeostatic mechanisms work?
By controlling the composition of the blood which controls the composition of tissue fluid
How do homeostatic mechanisms work?
By controlling the composition of the blood which controls the composition of tissue fluid
What are the 4 features of tissue fluid that influence cell activities?
- Temperature
- Water Potential
- Blood Glucose Concentration
- pH
How does temperature affect cell activities?
If it’s too high, the proteins and enzymes denature and if it’s too low, it results in slow metabolic reactions
How does water potential affect cell activites?
If it’s too high, water enters the cell, causing it to swell and burst.
If it’s too low, water moves out of the cell by osmosis causing metabolic reactions to slow down or stop
How does blood glucose concentration affect cell activites?
A decrease in concentration causes respiration to stop or slow
An increase in concentration causes water to move out of the cell by osmosis
How does pH affect cell activity?
The cytoplasm pH is around 6.5 to 7
If the pH is outside the range the enzymes will function less efficiently and could denature
What is the most common type of control mechanism used in living organisms to maintain homeostatic balance?
Negative Feedback Loop
What are examples of effectors?
Muscles and Glands
What do receptors do?
Detect a stimulus
What are the 2 types of receptors?
Receptors detecting internal and external stimuli
What do receptors send information to?
Send information about changes they detect in the nervous system to the central control in the brain or spinal cord
What is considered an input in the negative feedback system?
Sensory information
What does the central control instruct?
Instructs the effector to carry out an action or an output
What are actions in the negative feedback loop considered as?
Corrective actions
How does continuous monitoring of a factor in the body by receptors take place and what does this mean?
A steady stream of information is being sent to the control centre so continuous adjustments to the output can be made
So the factor always fluctuates around the set point
Why do homeostatic mechanisms involve the negative feedback?
So it minimises the difference between the actual value of the factor and set point. The set point is never exactly constant as it fluctuates.
The range depends on various factors such as age, sex, time of dat etc.
What are the 2 coordination systems?
The nervous system and the endocrine system
How does the nervous system work?
By electrical impulses that are transmitted along neurons
How does the endocrine system work?
By hormones (chemical messengers) that travel in the blood with long distance signalling
Give an example of a positive feedback loop
You breathe in air with a very high carbon dioxide content
This results in a high level of carbon dioxide in the blood
This is sensed by the CO2 receptors
So breathing rate increases
An increase breathing rate causes an increase in carbon dioxide and the simulation of the receptors continues
What do metabolic reactions produce that can’t be used?
Unwanted or toxic substances
How is metabolic waste removed?
By excretion
What does metabolic waste mostly consist of?
CO2 and urea
What is CO2 produced by?
By aerobically respiring cells
List 4 bullet points about carbon dioxide as an excretory product
- Waste CO2 transported from respiring cells to lungs, in the bloodstream
- Gas exchange takes place within the lungs
- CO2 diffuses from the blood into the alveoli
- CO2 is excreted in the air breathed out
Where and how is urea produced?
In the liver, from excess amino acids
List 3 bullet points about urea as an excretory product
- Transported from the liver to the kidneys, in a solution in the blood plasma
- Kidneys remove the urea from the blood and excrete it in dissolved water
- The solution is called urea
When does deamination take place?
When more protein is eaten than needed and the excess can’t be stored in the body.
The extra amino acids when excess protein is in the body, gives useful energy. To use the energy, what does the liver do?
The liver removes the amine groups in a process called deamination
Explain deamination
- Takes place in liver cells
- The amine group (NH2) of the amino acid is removed, together with an extra hydrogen atom
- They both combine to make ammonia (NH3)
- Keto acid is what remains
- Keto acid may enter the Krebs cycle and be respired
- Or it can be converted to glucose
- Or it can be converted to glycogen/fat for storage
How does ammonia affect aquatic animals?
Very soluble and highly toxic compound so it diffuses out from the blood and dissolves in the water around the animal
How does ammonia affect terrestrial animals?
Increases the pH in the cytoplasm and interferes with metabolic processes
How is the damage that ammonia causes prevented?
By converting ammonia to urea which is less soluble and less toxic
What is urea?
The main nitrogenous excretory product of humans
Apart from urea, what are the other nitrogenous excretory products?
Creatinine and uric acid
Where is creatine made and what is it made from?
In the liver, from amino acids
Where is creatine used?
Much of it is used in muscles, in the form of creatinine phosphate which acts as an energy store
Some can also be converted to creatinine and excreted
What is uric acid made from?
Made from the breakdown of purines from nucleotides
What does urea diffuse into from liver cells?
Into the blood plasma
Why does urea need to be excreted?
Otherwise the concentration in the blood builds up and becomes dangerous
When is urea filtered out and excreted?
As blood passes through the kidneys
What does each kidney receive blood from?
From the renal artery
The ureter carries urine from where to where?
The kidney to the bladder
Where does the urethra carry urine to?
Outside the body
Sketch and label a diagram of the excretory system
See 14H 14.2 Notes
What is the kidney made of?
It’s made of thousands of tubules called nephrons and many blood vessels
What surrounds the glomerulus?
The Bowman’s capsule
What is the gomerulus?
A network of capillaries
What structures of nephrons are in the cortex of a kidney?
Glomerulus and capsules
Sketch and label a diagram of a kidney
Check 14H 14.2 Notes
What structures are closely associated with nephrons?
Blood vessels
Each glomerulus is supplied with blood that flows from what?
A brach of the renal artery through an afferent arteriole
At the end of the glomerulus the capillaries rejoin to form what?
An efferent arteriole
Blood flows through the efferent arteriole into what?
A network of capillaries running closely alongside the rest of the nephron and the CD
Blood from the network of capillaries through the nephron flows into venules that empty into what?
A branch of the renal vein
Sketch and label a cross section of the nephron
Check 14H 14.2 Notes
In how many stages does the kidney make urine?
2
What are the names of the 2 stages in which the kidney makes urine?
- Ultrafiltration
2. Selective reabsorption
What is ultrafiltration?
Filtering small molecules including urea out of the blood and into the Bowman’s capsule to form filtrate
The filtrate flows along the nephron to the CD
What is selective reabsorption?
A process that takes back any useful molecules from filtrate as it flows along the nephron
Sketch and label a diagram of the process of ultrafiltration
Check 14H 14.2 Notes
Describe the process of ultrafiltration
- Blood in the glomerular capillaries are separated from the lumen of the Bowman’s capsule by 2 cell layers and a basement membrane
FIRST CELL LAYER: Endothelium of capillary
Each endothelial cell is perforated by many tiny membrane-lined circular holes around 60 to 80nm in diameter
The basement membrane is made of collagen and glycoproteins
SECOND CELL LAYER: Made of epithelial cells that make up the inner lining of the Bowman’s capsule
Cells can have many tiny finger-like projections with gaps between them called podocytes that are wrapped around the capillary
What is the glomerular filtration rate?
The rate at which a fluid filters from blood in the glomerular capillaries into the Bowman’s capsule
What is the average glomerular filtration rate in humans?
125cm^3 per minute
What are the substances int he glomerular filtrate? (8 things)
Water Plasma Proteins Amino Acids Glucose Urea Uric Acid Creatinine Inorganic Ions (Mainly Sodium, Potassium and Chlorine)
What factors can influence glomerular filtration rate? (3 things)
- Water potential gradient between the plasma in the glomerular capillaries and the filtrate in the Bowman’s capsule
- Concentration of solutes in the blood plasma in the capillaries
- Blood pressure
How can the water potential between the plasma in the glomerular capillaries and filtrate in the Bowman’s capsule increase or decrease?
Water potential decreases in the presence of solutes
Water potential increases due to high pressures
Inside the capillaries in the glomerulus, why is the blood pressure relatively high?
Because the diameter of the afferent arteriole is greater than the efferent arteriole
This causes pressure inside the glomerulus
It raises the water potential of blood plasma above the contents of the Bowman’s capsule
Inside the capillaries in the glomerulus, why is the blood pressure relatively high?
Because the diameter of the afferent arteriole is greater than the efferent arteriole
This causes pressure inside the glomerulus
It raises the water potential of blood plasma above the contents of the Bowman’s capsule
Why is the concentration of solutes in the blood plasma in the capillaries greater than the concentration of solutes in the filtrate in the Bowman’s capsule?
Most contents of the blood plasma filter through the basement membrane and into the capsule
Except plasma proteins because they’re too big
This makes the water potential int he blood capillaries less than the filtrate in the Bowman’s capsule
Why does water move down the water potential gradient from the blood into the capsule as the blood flows through the glomerulus?
Because the effect of the difference in pressure outweighs the effect of the difference in solute concentration so the water potential of blood plasma in the glomerulus is greater than the filtrate in the capsule
Where does selective reabsorption mainly take place?
In the proximal convoluted tubule (PCT)
What is the lining of the PCT made of?
A single layer of cuboidal epithelial cells
What are the adaptations of the cell for selective reabsorption? (4 things)
- Microvilli of surface of the lumen or luminal membrane to increase SA
- Many co-transporter proteins in the luminal membrane
- Tight junctions holding adjacent cells together firmly so the fluid can’t pass between the cell and all of it has to go through the cell
- Many mitochondria to give energy for NaK pump proteins in the basal membranes of the cells
Sketch and label a diagram explaining selective reabsorption
Check 14H 14.2 Notes
Explain how blood capillaries help with selective reabsorption in the PCT
Blood capillaries are very close to the outer surface of the tubule
The blood in the capillaries directly from the glomerulus have much less plasma as it has lost a lot of water, ions, solutes etc.
Explain how the basal membrane helps with selective reabsorption in the PCT
The basal membrane of the cells lining the PCT are closest to the blood capillaries
NaK pumps in these membranes move Na ions out of cells
Na ions are carried away in the blood
This decreases the concentration of Na ions in cells
So Na ions in the filtrate diffuse down the concentration gradient through the luminal membranes
And they enter though the co-transporter proteins in the membrane
What does the passive movement of Na ions result in?
Gives energy to move glucose into cell even against the concentration gradient
This is indirect or secondary active transport
Because ATP is used in pumping Na ions, it’s not used for moving solutes
Once inside the cell, the glucose diffuses down its concentration gradient through the transport membrane in the basal membrane, into the blood
What is the glomerular filtrate like after selective reabsorption in the PCT?
All the glucose in the glomerular filtrate transported out of the PCT into the blood
Normally no glucose is left in the filtrate
SO no glucose in urine
Na and Cl and amino acids and vitamins are reabsorbed
Fill out the table in 14H 14.2 Notes for selective reabsoprtion
Check 14H 14.2 Notes
What does the removal of solutes from the filtrate in the PCT cause?
Increases the water potential of the filtrate and lowers the water potential of the blood
Creates a steep water potential gradient
Water is reabsorbed into the blood by osmosis
Recall the important points of urea reabsorption in the PCT
CSM slightly permeable to urea
Its concentration in the filtrate is greater than the concentration in the capillaries
So it diffuses through the cells of the PCT into the blood
How is reabsorption of water in the Loop of Henle possible?
The loop of Henle creates a very high concentration of sodium and chlorine in tissue fluid in the medulla
This is partly achieved by active transport by the cells of a thick region of the ascending loop
It can now reabsorb water and prevent dehydration
What happens when the filtrate leaves the loop of Henle?
The filtrate continues through the DCT which runs into the medulla again
Passes through regions with a high solute concentration and low water potential
So water moves out of the CD by osmosis and is reabsorbed
Until the water potential of the urine is equal to the water potential of the tissue fluid in the medulla
May be the water potential is slightly greater than the water potential of the blood but this is controlled by ADH
What does a thicker medulla result in?
More concentrated urine
The cells lining the ascending limb have deep infolds so they have many NaK pumps
The cytoplasm has many mitochondria which produce ATP for this
Describe reabsorprtion in the first part of the DCT
Functions like the ascending loop of Henle
Describe reabsorprtion in the second part of the DCT
Functions like the CD
How is the concentration of ions in the blood regulated?
In the DCT and CD, Na ions are actively pumped from the fluid in the tubule into the tissue fluid then into the blood
K ions are actively transported into the tubule
The rate at which ions move in and out of the fluid varies and this helps regulate the concentration of these ions in the blood
What are sodium and potassium ions important for?
In the conduction of nerve impulses
Look at the 2 graphs at the bottom of 14H 14.2 Notes
Check 14H 14.2 Notes
What structures in the body are involved in osmoregulation?
The hypothalamus, posterior pituitary gland, and the kidneys
The water potential of the blood is constantly monitored by specialised sensory neurons in the hypothalamus called what?
Osmoreceptors
What happens when the water potential of the blood is below the set point?
- Nerve impulses are sent along the neurons
- They terminate in the posterior pituitary gland
- The impulses stimulate the release of ADH
- Molecules of ADH enter the blood in the capillaries
- They are carried all over the body
What is ADH?
A peptide hormone made of 9 amino acids
What does ADH do?
It decreases water loss in urine and makes the kidneys reabsorb as much water as possible
What is diuresis?
The production of dilute urine
What does ADH stop?
Stops the production of dilute urine
What are the target cells for ADH?
The cells of the CD
What does ADH act on?
Acts on the luminal membranes of the CD
What does the ADH do to the luminal membranes of the CD?
Makes them more permeable to water
How does ADH increase the luminal membrane’s permeability to water?
By increasing the number of aquaporins in the luminal membranes of the CD cells
What do cells have ready-made in their membranes?
Aquaporins
What happens from when the ADH binds to the luminal membrane to change the permeability?
- ADH molecules bind to receptor proteins which stimulate the production of cyclic AMP
- cAMP = second messenger
- cAMP activates a signalling cascade
- Which results in the phosphorylation of aquaporin molecules
- This results in the activation of aquaporin molecules
- The vesicles move towards the luminal membrane and fuse with it
- This increases the permeability of the membrane to water
Once the permeability of the membrane has been changed to allow more water to go through, what happens?
- The fluid flows through the CD
- Water molecules move through the aquaporins, out of the tubule, into the tissue fluid
- Because the tissue fluid in the medulla has a low water potential and the fluid in the CD has a high water potential, the fluid in the CD loses the water to become more concentrated.
- A small volume of concentrated urine flows from the kidneys through the ureter into the bladder
What happens when the water potential of the blood is too high, above the set point?
- The osmoreceptors in the hypothalamus are no longer stimulated
- The neurons in the posterior pituitary gland stop secreting ADH
- So there isn’t any stimulus from ADH
- The aquaporins move out of the CSM of the CD and back into the cytoplasm as part of the vesicles
- The CD cells are impermeable to water
What happens when the water potential of the blood is too high, above the set point?
- The osmoreceptors in the hypothalamus are no longer stimulated
- The neurons in the posterior pituitary gland stop secreting ADH
- So there isn’t any stimulus from ADH
- The aquaporins move out of the CSM of the CD and back into the cytoplasm as part of the vesicles
- The CD cells are impermeable to water
What happens when the CD cells are impermeable to water?
- The fluid flows down the CD without losing any water
- Dilute urine collects in the renal pelvis and flows down the ureter to the bladder
- Produces large volume of dilute urine
Do CD cells respond immediately to a decrease in ADH secretion by the posterior pituitary gland? So what happens?
No
It takes time for ADH that’s already in the blood to be broken down.
Approximately half is destroyed every 15-20 minutes
Do CD cells respond immediately to a decrease in ADH secretion by the posterior pituitary gland? So what happens?
No
It takes time for ADH that’s already in the blood to be broken down.
Approximately half is destroyed every 15-20 minutes
What happens once ADH stops arriving at the CD cells?
Approximately 10-15 minutes for the aqauporins to be removed from the CSM into the cytoplasm until they are needed again
What can brain cells only respire?
Glucose
What is the control of blood glucose carried out by?
2 hormones secreted by the endocrine tissue in the pancreas
The endocrine tissue in the pancreas consists of a group of cells called what?
The islets of langerhans
What two cells do the islets of langerhans have and what do they secrete?
alpha cells - secrete glucagon
beta cells - secrete insulin
What do alpha and beta cells act as?
The receptors and central control for controlling blood glucose concentration in the blood
What do insulin and glucagon do overall?
Coordinate the actions of the effectors
What happens when the blood glucose concentration is too high, above the set point?
- alpha and beta cells detect the increase in concentration
- alpha cells stop secreting glucagon
- beta cells secrete insulin into blood plasma
- insulin is carried to all parts of the body
- insulin binds to the receptor in the CSM
- stimulates cells to increase the rate at which they absorb glucose from the blood and convert to glycogen. Or increase the use of glucose in respiration
- vesicles with GLUT 4 proteins move to the CSM and fuse with it
- GLUT 4 proteins facilitate movement of glucose into the cell
- insulin stimulates the activation of glucokinase - phosphorylates glucose
- traps glucose so it can’t pass through the glucose transporters
- insulin stimulates the activation of enzyme phosphofructokinase and glycogen synthase
- catalyses the addition of glucose molecules to glycogen
- this is glycogenesis
What happens when the blood glucose concentration is too high, above the set point?
- alpha and beta cells detect the increase in concentration
- alpha cells stop secreting glucagon
- beta cells secrete insulin into blood plasma
- insulin is carried to all parts of the body
- insulin binds to the receptor in the CSM
- stimulates cells to increase the rate at which they absorb glucose from the blood and convert to glycogen. Or increase the use of glucose in respiration
- vesicles with GLUT 4 proteins move to the CSM and fuse with it
- GLUT 4 proteins facilitate movement of glucose into the cell
- insulin stimulates the activation of glucokinase - phosphorylates glucose
- traps glucose so it can’t pass through the glucose transporters
- insulin stimulates the activation of enzyme phosphofructokinase and glycogen synthase
- catalyses the addition of glucose molecules to glycogen
- this is glycogenesis
What happens when the blood glucose concentration is too high, above the set point?
- alpha and beta cells detect the increase in concentration
- alpha cells stop secreting glucagon
- beta cells secrete insulin into blood plasma
- insulin is carried to all parts of the body
- insulin binds to the receptor in the CSM
- stimulates cells to increase the rate at which they absorb glucose from the blood and convert to glycogen. Or increase the use of glucose in respiration
- vesicles with GLUT 4 proteins move to the CSM and fuse with it
- GLUT 4 proteins facilitate movement of glucose into the cell
- insulin stimulates the activation of glucokinase - phosphorylates glucose
- traps glucose so it can’t pass through the glucose transporters
- insulin stimulates the activation of enzyme phosphofructokinase and glycogen synthase
- catalyses the addition of glucose molecules to glycogen
- this is glycogenesis
How does glucose enter the cells?
By facilitated diffusion through the GLUT transporter proteins
What cells have GLUT 4 proteins?
Muscle cells
Where are GLUT proteins kept?
In the cytoplasm
What type of GLUT proteins do brain cells have?
GLUT 1
What type of GLUT proteins do liver cells have?
GLUT 2
What GLUT proteins are always in the CSM?
GLUT 1 and 2. Their distribution isn’t altered by insulin
What is glycogen and where is it found?
It’s the short term energy store found in liver and muscle cells
Are there glucogen receptors in muscle cells?
No
What happens when the blood glucose concentration in the blood is too low, below the set point?
- alpha and beta cells detect the decrease in concentration
- alpha cells secrete glucagon
- beta cells stop secreting insulin
- the decrease in insulin concentration reduces the rate of uptake of glucose by the liver and muscle cells
- glucagon binds to the different specific receptor molecules in the liver cell CSM
- transduction occurs
- the binding causes a conformational change in the receptor proteins
- this activates the G protein
- this activates the enzyme adenylyl cyclase which is part of the CSM
- adenylyl cyclase catalyses the conversion of ATP to cyclic AMP (2nd messenger)
- molecules of cAMP binds to the protein kinase A enzymes within the cytoplasm and activates them
- these activate phosphorylase kinase enzymes by adding phosphate groups to them
- these activate glycogen phosphorylase by adding phosphate groups to them
- this results in an enzyme cascade that amplifies the original signal from glucagon
- when activated, glycogen phosphorylase catalyses the breakdown of glycogen to glucose - glycogenolysis
- the glucose concentration inside cells increases
- molecules of glucose diffuse out through the GLUT 2 transporter proteins into the blood
What happens when the blood glucose concentration in the blood is too low, below the set point?
- alpha and beta cells detect the decrease in concentration
- alpha cells secrete glucagon
- beta cells stop secreting insulin
- the decrease in insulin concentration reduces the rate of uptake of glucose by the liver and muscle cells
- glucagon binds to the different specific receptor molecules in the liver cell CSM
- transduction occurs
- the binding causes a conformational change in the receptor proteins
- this activates the G protein
- this activates the enzyme adenylyl cyclase which is part of the CSM
- adenylyl cyclase catalyses the conversion of ATP to cyclic AMP (2nd messenger)
- molecules of cAMP binds to the protein kinase A enzymes within the cytoplasm and activates them
- these activate phosphorylase kinase enzymes by adding phosphate groups to them
- these activate glycogen phosphorylase by adding phosphate groups to them
- this results in an enzyme cascade that amplifies the original signal from glucagon
- when activated, glycogen phosphorylase catalyses the breakdown of glycogen to glucose - glycogenolysis
- the glucose concentration inside cells increases
- molecules of glucose diffuse out through the GLUT 2 transporter proteins into the blood
What happens when the blood glucose concentration in the blood is too low, below the set point?
- alpha and beta cells detect the decrease in concentration
- alpha cells secrete glucagon
- beta cells stop secreting insulin
- the decrease in insulin concentration reduces the rate of uptake of glucose by the liver and muscle cells
- glucagon binds to the different specific receptor molecules in the liver cell CSM
- transduction occurs
- the binding causes a conformational change in the receptor proteins
- this activates the G protein
- this activates the enzyme adenylyl cyclase which is part of the CSM
- adenylyl cyclase catalyses the conversion of ATP to cyclic AMP (2nd messenger)
- molecules of cAMP binds to the protein kinase A enzymes within the cytoplasm and activates them
- these activate phosphorylase kinase enzymes by adding phosphate groups to them
- these activate glycogen phosphorylase by adding phosphate groups to them
- this results in an enzyme cascade that amplifies the original signal from glucagon
- when activated, glycogen phosphorylase catalyses the breakdown of glycogen to glucose - glycogenolysis
- the glucose concentration inside cells increases
- molecules of glucose diffuse out through the GLUT 2 transporter proteins into the blood
What happens when the blood glucose concentration in the blood is too low, below the set point?
- alpha and beta cells detect the decrease in concentration
- alpha cells secrete glucagon
- beta cells stop secreting insulin
- the decrease in insulin concentration reduces the rate of uptake of glucose by the liver and muscle cells
- glucagon binds to the different specific receptor molecules in the liver cell CSM
- transduction occurs
- the binding causes a conformational change in the receptor proteins
- this activates the G protein
- this activates the enzyme adenylyl cyclase which is part of the CSM
- adenylyl cyclase catalyses the conversion of ATP to cyclic AMP (2nd messenger)
- molecules of cAMP binds to the protein kinase A enzymes within the cytoplasm and activates them
- these activate phosphorylase kinase enzymes by adding phosphate groups to them
- these activate glycogen phosphorylase by adding phosphate groups to them
- this results in an enzyme cascade that amplifies the original signal from glucagon
- when activated, glycogen phosphorylase catalyses the breakdown of glycogen to glucose - glycogenolysis
- the glucose concentration inside cells increases
- molecules of glucose diffuse out through the GLUT 2 transporter proteins into the blood
What does glucagon also stimulate the formation of?
Formation of glucose from amino acids, fatty acids, glycerol, pyruvate, and lactate.
This is gluconeogenesis
What other hormone also increase blood glucose concentration?
Adrenaline
How does adrenaline increase blood glucose concentration?
By binding to different receptors on the surface of liver cells that activate the same enzyme cascade activated by glycogen
What does adrenaline also stimulate the breakdown of?
Glycogen stores in muscles during exercise which produces glucose to be used
What is an example of plant cells maintaining a constant, internal environment?
Mesophyll cells needing constant supply of CO2 for photosynthesis
What controls the diffusion of gases in and out of the leaf?
The stomata
What is the stomata essentially?
The apeture between guard cells
What are guard cells?
Highly specialised cells that respond to a wide range of environmental stimuli to control the inner atmosphere of the leaf
Where is the stomata distributed?
Leaves, green stems, and flowers
What part of the leaf has the highest density of stomata?
The lower epidermis
What is each stoma surrounded by?
2 elliptical guard cells
What are guard cells much smaller than?
Much smaller than cells in spongy and palisade mesophyll
Guard cells are metabolically __________
Very active
What are the features in a typical guard cell? (11 things)
- Thick cell wall that faces the air outside the leaf and stomatal pore
- Outer wall has thick waxy cuticle and is often extended into ledges
- The walls facing adjacent epidermal cells are much thinner
- Cellulose microfibrils are arranged into bands around the cell
- Cell walls have no plasmodesmata
- CSM often folded and contains many channel and carrier proteins
- Cytoplasm has high density of chloroplasts and mitochondria
- Chloroplasts have thylakoids and few grana
- Mitochondria have many cristae
- The nucleus is the same size as in mesophyll cells but it occupies a much larger proportion of the cell
- Has several small vacuoles rather than one large vacuole
Why does a guard cell’s nucleus occupy a much larger proportion of the cell than in mesophyll cells even if the size of the nucleus is the same?
The size of guard cells are smaller than mesophyll cells
What do the starch grains in chloroplast do?
They increase in size at night as starch is stored and decrease in size during the day
What do stomata show daily rhythms of?
Opening and closing
Do the rhythms of the stomata persist whether it’s light or dark?
Yes
What do stomata open in response to? (2 things)
- Increase in light intensity
2. Low carbon dioxide concentration in air spaces within the leaf
What do stomata close in response to? (5 things)
- Darkness
- High carbon dioxide concentration within the leaf
- Low humidity
- High temperature
- Water stress
What is water stress?
When the supply of water from the roots is limited or there are high rates of transpiration
How do guard cells gain and lose water?
By osmosis
Guard cells open when _____________
they gain water and become turgid
Guard cells close when ____________
they lose water and become flaccid
How do the stomata open?
- Decrease in water potential of cells
- Decrease brought about by transport proteins in the CSMs
- In response to light, ATP- powered proton pumps in the membrane actively transport hydrogen ions out of guard cells
- Decrease in hydrogen ion concentration causes channel proteins in the CSM to open so potassium ions move into the cell as the decrease in hydrogen ion concentration leaves the inside of the cell negative compared to the outside
- Potassium ions are positive so they move down an electrical gradient toward the negative region. Potassium also diffuses into cells down a concentration gradient. ELECTROCHEMICAL GRADIENT
- Other ions, mainly chloride ions and nitrate, also enter to maintain the electrical balance
- Extra potassium inside the guard cells increases the concentration of the solutes and decreases water potential
- Water potential gradient is established between the outside and inside of the cell so water moves in by osmosis through aquaporins in the membrane and most enters the vacuoles which increase in size
- Turgor pressure of the guard cells increases and stoma opens
- Starch stored in chloroplasts are broken down to form negative malate ions that enter the vacuoles. This also helps maintain the electrical balance and also contribute to decreasing the water potential during opening
Guard cells have unevenly thickened cell walls. Describe this
Wall adjacent to the pore is very thick and wall furthest from the pore is thin
What prevents the expansion of cells in all directions?
Bundles of cellulose microfibrils prevent the expansion of the cell in all directions. Cell increases in length, not in diameter
The 2 ends of the guard cells are joined so how is the pore between these 2 cells opened?
Thin outer walls bend more readily than thick inner walls ad guard cells become curved and bulge into adjoining cells. This opens the pore
How do the stomata close?
- Stomata close when the hydrogen pump proteins stop and potassium ions leave guard cells and enter neighbouring cells
- Malate ions are returned to the chloroplasts to be converted to starch
- The water potential in opposite direction
- So water leaves guard cells so they become flaccid and close the stoma
What happens to water processes in the leaf when stomata close?
Decrease in uptake of carbon dioxide for photosynthesis
Decrease in rate of transpiration
What is transpiration for?
Used for cooling the plant
For maintaining transpiration stream that supplies water and minerals to the leaves
In conditions of water stress, what is produced?
Hormone abisic acid (ABA) is produced in plants to stimulate stomatal closure
Where is ABA found?
In every part of the plant. It’s synthesised in almost all cells that posses chloroplasts or amyloplasts
What are amyloplasts?
Organelles with large starch grains and no chlorophyll
Why is ABA called a stress hormone?
It coordinates response to stress
How does ABA work?
Guard cells have ABA receptors on the CSM
When ABA binds with these, it inhibits proton pumps to stop hydrogen ions being pumped out
ABA stimulates movement of calcium ions into the cytoplasm through the CSM and tonoplasts
What is the tonoplast?
Membrane around the vacuole
After ABA binds to the ABA receptors on the CSM and inhibits proton pumps what happens?
- Calcium ions act as second messengers to activate channel proteins to open that allow negative ions to leave guard cells
- This stimulates the opening of more channel proteins
- Which allows the movement of calcium ions out of cells
- At the same time calcium ions stimulate closure of channel proteins that allow potassium ions to enter
- The loss of ions causes the water potential of cells to increase
- Water passes out by osmosis
- Guard cells become flaccid and stomata close
What is a very common disease to do with not being able to control blood sugar?
Diabetes mellitus
What are the 2 ways to develop diabetes?
Some people develop disease early in life as alpha cells stop producing insulin
Most people develop the disease later in life when their cells fail to respond to insulin
What may indicate that a person has diabetes?
Presence of glucose in urine
For people with diabetes, why is there glucose in the urine?
If blood glucose concentration increases above the renal threshold, not all glucose is reabsorbed from filtrate in the PCT of kidney and some will be present in the urine
Test strips can test various different factors in the urine:
4 things
- pH
- Glucose
- Ketones
- Proteins
How are urine test strips used?
- 2 enzymes are immobilised on a small pad at one end of the stick
- Has a covering pad which is a cellulose membrane that allows small molecules from the blood to reach the enzymes
- The pad is immersed in the urine for a brief time
If urine contains glucose, what happens on the urine test strip?
- Glucose oxidase catalyses a chemical reaction in which glucose is oxidised into gluconic acid
- Hydrogen peroxide is produced
- Peroxidase catalyses the reaction between hydrogen peroxide and a colourless chemical in the pad to form a brown compound
What are the 2 equations for urine test strips when glucose is present?
- glucose + oxygen ———> (glucose oxidase) gluconic acid + hydrogen peroxide
- hydrogen peroxide + chromogen (colourless) ——-> (peroxidase) oxidised chromogen (colourless) + water
Glucose oxidase is specific for _______ so _________
- glucose
2. test gives negative for other sugars such as fructose, lactose, sucrose (enzyme specificity)
What do urine tests show you and what don’t they tell you?
Show if glucose concentration is greater than the renal threshold in the period of time where urine was collecting in the bladder
Urine tests don’t indicate the current blood glucose concentration
What will show you the blood glucose concentration reading if not urine test strips?
Biosensor
How does a biosensor work to get a reading of blood glucose concentration?
- Uses glucose oxidase immobilised on a recognition layer
- Small sample of blood is tested
- Small molecules in plasma passes through the membrane
- Glucose molecules enter active sites of the enzyme that catalyses the reaction to produce gluconic acid and hydrogen peroxide
- Hydrogen peroxide is oxidised at an electrode that detects electron transfers
- Electron flow is proportional to the number of glucose molecules in the blood
- Biosensor amplifies the current which is read by the meter and produces a digital reading for blood glucose concentration within seconds
- Results are stored electronically
