Regulation and Regulation and controlcontrol

HomeostasisHomeostasis

Ability to maintain internal conditions with Ability to maintain internal conditions with narrow limits. narrow limits.

Important limits include Important limits include pH and temperature for optimal enzyme pH and temperature for optimal enzyme

activityactivity Water potential optimal for cellsWater potential optimal for cells

Principles of homeostasisPrinciples of homeostasis

Detectors monitor the output of a system – Detectors monitor the output of a system – sensory receptorssensory receptors

Effectors then make small adjustments to Effectors then make small adjustments to return the system to it’s normal state. Eg. return the system to it’s normal state. Eg. Muscles and glands.Muscles and glands.

Negative feedback – self-adjusting system Negative feedback – self-adjusting system or corrective mechanism. Eg. A decrease or corrective mechanism. Eg. A decrease in body temperature causes the body to in body temperature causes the body to generate more heat to return to normal.generate more heat to return to normal.

Negative feedbackNegative feedback

Cholecystokinin- A hormone that stimulates digestion (released in duodenum).

Vagal afferent- A part of the Vagus Nerve, that sends impulses to the brain telling it that food is being digested

Satiety Centre in Medulla- The Medulla is a part of the brain. After a period of stimulation, it removes the feeling of hunger, so you stop eating.

Blue arrows are the negative feedback process

Positive FeedbackPositive Feedback

Positive feedback is the body's mechanism to Positive feedback is the body's mechanism to enhance a output needed to maintain enhance a output needed to maintain homeostasis. homeostasis.

Positive feedback mechanisms push levels outPositive feedback mechanisms push levels outof normal ranges. of normal ranges.

It is rarely used by body because of the risk of the It is rarely used by body because of the risk of the increased stimuli becoming out of control. increased stimuli becoming out of control.

An example of positive feedback is the release of An example of positive feedback is the release of oxytocin to increase and keep the contractions oxytocin to increase and keep the contractions of child birth happening as long as needed for of child birth happening as long as needed for the child's birth. the child's birth.

ExcretionExcretion

Elimination of metabolic waste, excess Elimination of metabolic waste, excess substances and toxic substances.substances and toxic substances.

Two main substances that the body needs Two main substances that the body needs to excrete:to excrete: Carbon dioxide – through lungsCarbon dioxide – through lungs Urea – produced in the liver from deamination Urea – produced in the liver from deamination

of excess amino acids. Removed by kidneys.of excess amino acids. Removed by kidneys.

KidneyKidney

Structure:Structure:

The nephronThe nephron

The The Bowman's capsuleBowman's capsule is a cup-like sac at the  is a cup-like sac at the beginning of the tubular component of beginning of the tubular component of a nephron. a nephron.

A glomerulus is enclosed in the sac. A glomerulus is enclosed in the sac. The basement membrane separates the The basement membrane separates the

glomerulus from the capsule.glomerulus from the capsule. The Bowman’s capsule has special cells called The Bowman’s capsule has special cells called

podocytes with numerous foot-like processes podocytes with numerous foot-like processes that grip on to the basement membrane.that grip on to the basement membrane.

Ultrafiltration: the formation of the Ultrafiltration: the formation of the glomerular filtrateglomerular filtrate

The process of filtration of the blood in the The process of filtration of the blood in the Bowman's capsule is ultrafiltration Bowman's capsule is ultrafiltration 

The normal rate of filtration is 125 ml/min, The normal rate of filtration is 125 ml/min, equivalent to 80 times the daily blood volume.equivalent to 80 times the daily blood volume.

Blood enters the glomerulus at high pressure Blood enters the glomerulus at high pressure forcing water and some solutes through the forcing water and some solutes through the holes in the endothelium of the blood vessels, holes in the endothelium of the blood vessels, then through the basement membrane and then then through the basement membrane and then through the layer of podocytes into the capsular through the layer of podocytes into the capsular space.space.

The endothelium and podocytes act as coarse The endothelium and podocytes act as coarse filters but it is the basement membrane that is filters but it is the basement membrane that is the fine filter.the fine filter.

Each podocyte is Each podocyte is specially shaped so specially shaped so that, when the cells fit that, when the cells fit together, they create together, they create slits which form part of slits which form part of the blood filter. They the blood filter. They can also regulate the can also regulate the slit size, to control slit size, to control filtration.filtration.

Any small molecules Any small molecules such as water, such as water, glucose, salt, amino glucose, salt, amino acids and urea pass acids and urea pass freely into Bowman’s freely into Bowman’s space, but cells, space, but cells, platelets and large platelets and large proteins do not.proteins do not.

Selective ReabsorptionSelective Reabsorption

Glucose is reabsorbed into the blood by Glucose is reabsorbed into the blood by active transport in the proximal convoluted active transport in the proximal convoluted tubule.tubule.

Proximal convoluted tubule is lined with a Proximal convoluted tubule is lined with a single layer of epithelial cells which have a single layer of epithelial cells which have a microvilli border to increase surface area.microvilli border to increase surface area.

Capillaries surround the proximal Capillaries surround the proximal convoluted tubule allowing efficient convoluted tubule allowing efficient transport of substances.transport of substances.

A basement membrane covers the outside of the tubule and it contains protein carriers. Other substances are also reabsorbed:amino acidsvitaminshormoneswatersodium ions

OsmoregulationOsmoregulation

Osmoregulation is the process which Osmoregulation is the process which regulates the concentration and osmotic regulates the concentration and osmotic pressure of blood by regulating the water pressure of blood by regulating the water contents of blood plasma. contents of blood plasma.

It is an important process as excessive It is an important process as excessive loss of water may cause dehydration loss of water may cause dehydration whereas excess of water intake may dilute whereas excess of water intake may dilute the body fluids.the body fluids.

It is an example of negative feedback. It is an example of negative feedback. The receptors monitor the amount of water in The receptors monitor the amount of water in

the body and the kidney is the effector.the body and the kidney is the effector.

Water is mainly reabsorbed through the Water is mainly reabsorbed through the collecting tubules. collecting tubules.

The permeability of the wall of the distal The permeability of the wall of the distal convoluted tubule and collecting tubules is convoluted tubule and collecting tubules is controlled by anti - diuretic hormone or controlled by anti - diuretic hormone or ADH which is released from the posterior ADH which is released from the posterior lobe of the pituitary gland. lobe of the pituitary gland.

Excess water in body fluidsExcess water in body fluids

Excess of water in the body fluids signals to Excess of water in the body fluids signals to posterior pituitary to stop the release of ADH. posterior pituitary to stop the release of ADH.

Deficiency of this hormone lowers the Deficiency of this hormone lowers the permeability of the cells of the distal convoluted permeability of the cells of the distal convoluted tubule and the collecting duct, decreasing the tubule and the collecting duct, decreasing the reabsorption of Na+ from the filtrate. reabsorption of Na+ from the filtrate.

More filtration combined with less reabsorption More filtration combined with less reabsorption of water produces abundant dilute urine and this of water produces abundant dilute urine and this brings down the volume of body fluids to normal.brings down the volume of body fluids to normal.

Shortage of water in body fluidsShortage of water in body fluids

The rate of ultra filtration decreases due to The rate of ultra filtration decreases due to decreased blood volume and low hydrostatic decreased blood volume and low hydrostatic pressure of blood in the glomerular capillaries. pressure of blood in the glomerular capillaries.

Rate of reabsorption of water is increased by Rate of reabsorption of water is increased by increasing the permeability of the wall of the increasing the permeability of the wall of the distal convoluted tubule and collecting tubules distal convoluted tubule and collecting tubules due to increased release of ADH from the due to increased release of ADH from the posterior pituitary. posterior pituitary.

Less ultrafiltration and more reabsorption Less ultrafiltration and more reabsorption produce small amount of hypertonic urine which produce small amount of hypertonic urine which increases body fluid volume to normal.increases body fluid volume to normal.

Loop of HenleLoop of Henle

The loop of Henle plays an important role.The loop of Henle plays an important role. It functions as a countercurrent exchange It functions as a countercurrent exchange

mechanism that increases the solute mechanism that increases the solute concentration in the medulla.concentration in the medulla.

An osmotic gradient is set up to help An osmotic gradient is set up to help withdraw water from the collecting duct if withdraw water from the collecting duct if circumstances require it.circumstances require it.

The descending limbs both thick and thin The descending limbs both thick and thin have low permeability to ions and urea, while have low permeability to ions and urea, while being highly permeable to water. The loop being highly permeable to water. The loop has a sharp bend in the renal medulla going has a sharp bend in the renal medulla going from descending to ascending thin limb.from descending to ascending thin limb.

Thin ascending limb of loop of HenleThin ascending limb of loop of Henle The thin ascending limb is not permeable to The thin ascending limb is not permeable to

water, but it is permeable to ions.water, but it is permeable to ions. Thick ascending limb of loop of HenleThick ascending limb of loop of Henle

Sodium (Na+), potassium (K+) Sodium (Na+), potassium (K+) and chloride (Cl-) ions are reabsorbed from and chloride (Cl-) ions are reabsorbed from the urine by active transportthe urine by active transport

Loop of HenleLoop of Henle

Communication systemsCommunication systems

Sensitvity ≡ ability to respond appropriately to Sensitvity ≡ ability to respond appropriately to external and internal stimuliexternal and internal stimuli

Stimuli are detected by receptors in sense Stimuli are detected by receptors in sense organs, and the organs that respond are called organs, and the organs that respond are called effectors.effectors.

The body needs to be coordinated in its actions The body needs to be coordinated in its actions so one part of the body must be able to send so one part of the body must be able to send information to another part..information to another part..

Nervous SystemNervous System

Made of neurones which use electrical nerve Made of neurones which use electrical nerve impulses.impulses.

The central nervous system of vertebrates The central nervous system of vertebrates contains the brain, spinal cord, and retina.contains the brain, spinal cord, and retina.

The peripheral nervous system consists of The peripheral nervous system consists of sensory neurons, clusters of neurons sensory neurons, clusters of neurons called ganglia, and nerves connecting them to called ganglia, and nerves connecting them to each other and to the central nervous system. each other and to the central nervous system. 

There are three types of neurones:There are three types of neurones: Sensory neurone (sensor)Sensory neurone (sensor) Relay neurone (connector)Relay neurone (connector) Motor neurone (effector)Motor neurone (effector)

Sensory receptorsSensory receptors

A sensory receptor receives information A sensory receptor receives information from the world and relates it to your from the world and relates it to your nervous system.nervous system.

activated when they are bent, squished, or activated when they are bent, squished, or disturbed in some way.  Others are disturbed in some way.  Others are activated by chemicals or temperature or activated by chemicals or temperature or light.  light. 

Physiologists refer to receptors Physiologists refer to receptors as as selective transducersselective transducers..

They are called transducers because they They are called transducers because they 'convert' the energy contained in the 'convert' the energy contained in the stimulus into another form of energy, stimulus into another form of energy, specifically into some sort of membrane specifically into some sort of membrane potential. potential.

They are selective because they are They are selective because they are highly specific (selective) with respect to highly specific (selective) with respect to the type of stimulus it responds to. the type of stimulus it responds to.

When you press on a Pacinian Corpuscle (sensitive to When you press on a Pacinian Corpuscle (sensitive to pressure), you deform the lamellae and cause them to pressure), you deform the lamellae and cause them to press on the tip of the sensory neuron. press on the tip of the sensory neuron.

That, in turn, physically deforms the neuron's plasma That, in turn, physically deforms the neuron's plasma membrane and makes it 'leaky' to sodium ions (Na+)membrane and makes it 'leaky' to sodium ions (Na+)

The increased positive charge inside the axon is called a The increased positive charge inside the axon is called a receptor potential.receptor potential.

Motor NeuroneMotor Neurone Cell body lies within brain or spinal cord.Cell body lies within brain or spinal cord. Dendrites conduct impulses towards the cell Dendrites conduct impulses towards the cell

body.body. Axon conducts impulse away from cell body to Axon conducts impulse away from cell body to

the effector.the effector.

Schwann cells wrap themselves around Schwann cells wrap themselves around the axon.the axon.

This forms an enclosing sheath called the This forms an enclosing sheath called the myelin sheath.myelin sheath.

The small space between the Schwann The small space between the Schwann cells is called the node of Ranvier.cells is called the node of Ranvier.

They occur every 1-3mm in human nerves They occur every 1-3mm in human nerves and are about 2-3um wide.and are about 2-3um wide.

The Schwann cells and nodes of Ranvier The Schwann cells and nodes of Ranvier affect the speed of the impulse.affect the speed of the impulse.

Sensory NeuroneSensory Neurone

Origin of a membrane Origin of a membrane potentialpotential

If the smaller ions are If the smaller ions are able to diffuse through able to diffuse through the membrane but the the membrane but the larger ions cannot, a larger ions cannot, a potential difference will potential difference will develop between the develop between the two solutions. two solutions.

Nervous impulsesNervous impulses

The normal potential difference between the inner and outer parts of nerve cells is about –70 mv. Transmissin of a nerve impulse is initiated by a reduction of this potential difference to about –20 mv.

Action PotentialsAction Potentials

Action potentials are generated by special types Action potentials are generated by special types of voltage-gated ion channels embedded in a of voltage-gated ion channels embedded in a cell's plasma membrane. cell's plasma membrane. 

These channels are shut when the membrane These channels are shut when the membrane potential is near the resting potential of the cell, potential is near the resting potential of the cell, but they rapidly begin to open if the membrane but they rapidly begin to open if the membrane potential increases to a precisely defined potential increases to a precisely defined threshold value. threshold value.

When the channels open, they allow an inward When the channels open, they allow an inward flow of sodium ions, which changes the flow of sodium ions, which changes the electrochemical gradient, which in turn produces electrochemical gradient, which in turn produces a further rise in the membrane potential. a further rise in the membrane potential.

This then causes more channels to open, This then causes more channels to open, producing a greater electric current, and so on. producing a greater electric current, and so on.

The process proceeds explosively until all of the The process proceeds explosively until all of the available ion channels are open, resulting in a available ion channels are open, resulting in a large upswing in the membrane potential. large upswing in the membrane potential.

The rapid influx of sodium ions causes the The rapid influx of sodium ions causes the polarity of the plasma membrane to reverse, and polarity of the plasma membrane to reverse, and the ion channels then rapidly inactivate. the ion channels then rapidly inactivate.

As the sodium channels close, sodium ions can As the sodium channels close, sodium ions can no longer enter the neuron, and they are actively no longer enter the neuron, and they are actively transported out of the plasma membrane.  transported out of the plasma membrane. 

Potassium channels are then activated, Potassium channels are then activated, and there is an outward current of and there is an outward current of potassium ions, returning the potassium ions, returning the electrochemical gradient to the resting electrochemical gradient to the resting state. state.

After an action potential has occurred, After an action potential has occurred, there is a transient negative shift, called there is a transient negative shift, called the refractory period, due to additional the refractory period, due to additional potassium currents.potassium currents.

This is the mechanism that prevents an This is the mechanism that prevents an action potential from traveling back the action potential from traveling back the way it just came.way it just came.

SynapsesSynapses

Information from one neuron flows to another Information from one neuron flows to another neuron across a neuron across a synapsesynapse. .

The synapse is a small gap separating neurons. The synapse is a small gap separating neurons. The synapse consists of:The synapse consists of:

a a presynaptic endingpresynaptic ending that contains  that contains neurotransmitters, mitochondria and other cell neurotransmitters, mitochondria and other cell organelles,organelles,

a a postsynaptic endingpostsynaptic ending that contains receptor sites  that contains receptor sites for neurotransmitters and,for neurotransmitters and,

a a synaptic cleftsynaptic cleft or space between the presynaptic  or space between the presynaptic and postsynaptic endings.  It is about 20nm wide.and postsynaptic endings.  It is about 20nm wide.

Action potentials Action potentials cannotcannot cross the synaptic cleft cross the synaptic cleft Nerve impulse is carried by neurotransmitters.Nerve impulse is carried by neurotransmitters.

How the impulse is transmitted across How the impulse is transmitted across the synaptic cleftthe synaptic cleft

action potential reaches the presynaptic action potential reaches the presynaptic terminalterminal

↓ ↓ voltage-gated Ca2+ channels open voltage-gated Ca2+ channels open  ↓ ↓ influx of Ca2+influx of Ca2+ ↓ ↓ synaptic vesicles fuse with membrane synaptic vesicles fuse with membrane

(exocytosis)(exocytosis) ↓ ↓

  neurotransmitters are released into synaptic neurotransmitters are released into synaptic cleft and diffuse to postsynaptic terminalcleft and diffuse to postsynaptic terminal

↓ ↓ neurotransmitter binds to neuroreceptor on neurotransmitter binds to neuroreceptor on

postsynaptic membranepostsynaptic membrane ↓ ↓ causes Na+ channels to open, and Na+ flows causes Na+ channels to open, and Na+ flows

into postsynaptic membraneinto postsynaptic membrane ↓ ↓ if threshold is reached then action potential is if threshold is reached then action potential is

initiatedinitiated ↓ ↓ neurotransmitter is broken down by specific neurotransmitter is broken down by specific

enzymes in the synaptic cleft enzymes in the synaptic cleft

So why bother? Why have gaps So why bother? Why have gaps in the nerves?in the nerves?    

Flow of impulses is in one direction only because the Flow of impulses is in one direction only because the vesicles containing the transmitter are only in the vesicles containing the transmitter are only in the presynaptic membrane and the receptor molecules are presynaptic membrane and the receptor molecules are only on the postsynaptic membrane.only on the postsynaptic membrane.

Allow integration, e.g. an impulse travelling down a Allow integration, e.g. an impulse travelling down a neurone may reach a synapse which has several post neurone may reach a synapse which has several post synaptic neurones, all going to different locations.  synaptic neurones, all going to different locations.  

Allow the ‘filtering out’ of continual unnecessary or Allow the ‘filtering out’ of continual unnecessary or unimportant background stimuli.  If a neurone is unimportant background stimuli.  If a neurone is constantly stimulated (e.g. clothes touching the skin) the constantly stimulated (e.g. clothes touching the skin) the synapse will not be able to renew its supply of synapse will not be able to renew its supply of transmitter fast enough to continue passing the impulse transmitter fast enough to continue passing the impulse across the cleft.   This ‘fatigue’ places an upper limit on across the cleft.   This ‘fatigue’ places an upper limit on the frequency of depolarisation.the frequency of depolarisation.

NeurotransmittersNeurotransmitters

Acetylcholine Acetylcholine  - released by all motor neurones, - released by all motor neurones,

activating skeletal musclesactivating skeletal muscles - involved in the parasympathetic nervous - involved in the parasympathetic nervous

system (relaxing responses)system (relaxing responses) - cholinergic synapses- cholinergic synapses

Noradrenaline Noradrenaline  - involved in the sympathetic nervous - involved in the sympathetic nervous

system ('fight or flight' responsessystem ('fight or flight' responses - adrenergic synapses- adrenergic synapses

Hormonal communicationHormonal communication

Hormones are secreted into the blood by Hormones are secreted into the blood by endocrine glands.endocrine glands.

Endocrine ≡ internal secretionEndocrine ≡ internal secretion Secrete directly into the blood stream Secrete directly into the blood stream

therefore no ducts. therefore no ducts. (opposite is exocrine which use ducts to (opposite is exocrine which use ducts to

carry their secretions to specific places eg. carry their secretions to specific places eg. Salivary gland and sweat gland.)Salivary gland and sweat gland.)

PancreasPancreas

An excocrine and an endocrine glandAn excocrine and an endocrine gland Excocrine – digestive enzymes into the Excocrine – digestive enzymes into the

small intestinesmall intestine Endocrine – secretes insulin and glucagon Endocrine – secretes insulin and glucagon

into the blood stream.into the blood stream.

StructureStructure

Made of islet of Made of islet of Langerhans. Langerhans.

Look like islands in a Look like islands in a sea of uniform tissue.sea of uniform tissue.

Made of alpha and Made of alpha and beta cells.beta cells.

Alpha secrete Alpha secrete glucagon and beta glucagon and beta secrete insulin.secrete insulin.

FunctionFunction

The pancreas regulates blood glucose The pancreas regulates blood glucose concentration using negative feedback concentration using negative feedback mechanisms.mechanisms.

Blood glucose lower than the norm Blood glucose lower than the norm glucagon is produced and insulin is glucagon is produced and insulin is inhibited.inhibited.

Blood glucose higher than the norm insulin Blood glucose higher than the norm insulin is produced and glucagon is inhibited.is produced and glucagon is inhibited.

Effect of insulinEffect of insulin

51 amino acid chain.51 amino acid chain. Insulin molecules bind to an exposed Insulin molecules bind to an exposed

glycoprotein receptor on the cell surface glycoprotein receptor on the cell surface membranes.membranes.

The cell surface membrane then becoes more The cell surface membrane then becoes more permeable to glucose and the following occurs:permeable to glucose and the following occurs:

1.1. Increased uptake of glucose into the cellsIncreased uptake of glucose into the cells2.2. Increased rate of cellular respirationIncreased rate of cellular respiration3.3. Increased rate of conversion of glucose to fat in Increased rate of conversion of glucose to fat in

adipose cellsadipose cells4.4. Increased rate of conversion of glucose to glycogen Increased rate of conversion of glucose to glycogen

in liver and muscle cells. (glycogenesis)in liver and muscle cells. (glycogenesis)

Effect of GlucagonEffect of Glucagon

29 amino acid chain.29 amino acid chain. Glucagon activates phosphorylase in the Glucagon activates phosphorylase in the

liver and it catalyses the breakdown of liver and it catalyses the breakdown of glycogen into glucose. (glycogenolysis)glycogen into glucose. (glycogenolysis)

It also stimulates glucose to be produced It also stimulates glucose to be produced from non-carbohydrate source such as from non-carbohydrate source such as amino acids. (gluconeogenesis)amino acids. (gluconeogenesis)

DefinitionsDefinitions

Medical DictionaryMedical Dictionary osmotic shock osmotic shock n. n. 

 The rupture of bacterial or other cells in a solution follow The rupture of bacterial or other cells in a solution following a ing a sudden reduction in osmotic pressure; it is sometimes insudden reduction in osmotic pressure; it is sometimes induced to duced to release cellular components for biochemical analysis.release cellular components for biochemical analysis.

Osmotic pressure n.Osmotic pressure n. The pressure exerted by the flow of water through a The pressure exerted by the flow of water through a

semipermeable membrane separating two solutions with semipermeable membrane separating two solutions with different concentrations of solute. often used to express different concentrations of solute. often used to express the concentration of the solution the concentration of the solution

Communication systems in Communication systems in flowering plantsflowering plants

Plant growth substances are chemicals Plant growth substances are chemicals that occur naturally in plants.that occur naturally in plants.

The regulate aspects of plant growth and The regulate aspects of plant growth and development:development: Seed formationSeed formation GerminationGermination Plant responses to the environmentPlant responses to the environment

Auxins and apical dominanceAuxins and apical dominance

Auxins are plant hormones. Auxins are plant hormones. The most important auxin produced by plants The most important auxin produced by plants

is is indole-3-acetic acidindole-3-acetic acid ( (IAAIAA). ). A shoot has leaves attached to a stem and the A shoot has leaves attached to a stem and the

apex of the stem has an actively growing apex of the stem has an actively growing apical apical budbud. .

It produces additional nodes and internodes to It produces additional nodes and internodes to add to the length of the shoot. Additional leaves add to the length of the shoot. Additional leaves appear at the nodes as they are produced.appear at the nodes as they are produced.

In some plants, the In some plants, the lateral budlateral bud does not does not grow to form branches due to grow to form branches due to apical apical dominancedominance..

Apical dominance is thought to be caused Apical dominance is thought to be caused by the apical bud producing by the apical bud producing IAAIAA (auxin) in  (auxin) in abundance.abundance.

This auxin is transported from the apical This auxin is transported from the apical bud down to the lateral buds and causes bud down to the lateral buds and causes them to remain dormant. them to remain dormant.

The difference in The difference in response is explained by response is explained by their sensitivity to the their sensitivity to the auxin concentration. auxin concentration.

The lateral buds are The lateral buds are more sensitive to auxin more sensitive to auxin than the apical bud. than the apical bud.

When the apical bud is When the apical bud is removed, the source of removed, the source of IAA is removed. Since IAA is removed. Since the auxin concentration is the auxin concentration is much lower, the lateral much lower, the lateral buds can now grow. buds can now grow.

Thus, decapitating Thus, decapitating (pruning) a shoot will (pruning) a shoot will cause it to branch!cause it to branch!

Gibberellins in stem elongationGibberellins in stem elongation

These are plant hormones that regulate These are plant hormones that regulate growth and influence various growth and influence various developmental processes, including stem developmental processes, including stem elongation, germination, dormancy and elongation, germination, dormancy and flowering.flowering.

They stimulate stem elongation by They stimulate stem elongation by stimulating cell division and cellulose stimulating cell division and cellulose softening to make the cells elongate. softening to make the cells elongate.

Gibberellins in germinationGibberellins in germination

In barley seeds gibberellins stimulate he In barley seeds gibberellins stimulate he synthesis of alpha amylase. synthesis of alpha amylase.

This breaks down starch to produce This breaks down starch to produce sugars for the germinating seed to grow.sugars for the germinating seed to grow.

Abscisic acid and stomataAbscisic acid and stomata

Called the stress hormone.Called the stress hormone. If a plant is subjected to difficult conditions If a plant is subjected to difficult conditions

such as high temperatures ABA is such as high temperatures ABA is secreted.secreted.

Guard cells control the opening and Guard cells control the opening and closing of stomata.closing of stomata.

Each guard cell has a thick cell wall on Each guard cell has a thick cell wall on one side and a thin cell wall on the other.one side and a thin cell wall on the other.

When the guard cell becomes turgid they expand into a curved shape and the stoma opens.When they lose water and become flaccid the guard cells collapse together.

The mechanism:The mechanism: ABA binds to receptors at the surface of the plasma ABA binds to receptors at the surface of the plasma

membrane of the guard cells.membrane of the guard cells. The receptors activate The receptors activate

a rise in pH in the cytosol as Ha rise in pH in the cytosol as H++ ions flow in; ions flow in; transfer of Catransfer of Ca2+2+ from the vacuole to the cytosol. from the vacuole to the cytosol.

These changes stimulateThese changes stimulate the loss of negatively-charged ions, especially the loss of negatively-charged ions, especially

NONO3−3− and Cl and Cl−−, from the cell , from the cell the loss of Kthe loss of K++ from the cell. from the cell.

The loss of these solutes in the cytosol causes water to The loss of these solutes in the cytosol causes water to move out by osmosis and the cell becomes flaccidmove out by osmosis and the cell becomes flaccid

The stomata close.The stomata close.