Notes For Homeostasis

Control, coordination and homeostasis Homeostasis literally means “same state” and it refers to the process of keeping the internal body environment in a steady state the hormone system and the nervous system is dedicated to homeostasis • All homeostatic mechanisms use negative feedback to maintain a constant value (called the set point) • Negative feedback means that whenever a change occurs in a system, the change automatically causes a corrective mechanism to start, which reverses the original change and brings the system back to normal Excretion and homeostasis • Excretion means the removal of metabolic waste products from cells. • Carbon dioxide and urea are the common wastes • Lung excrete carbon dioxide • Kidney excrete urea • Urea is produced in liver through deamination (removing nitrogen atom from excessive amino acids) Amino acid metabolism takes place in the liver 1. Deamination • In this reaction an amino group is removed from an amino acid to form ammonia and a keto acid. 2. Urea Synthesis In this reaction ammonia is converted to urea, ready for excretion by the kidney • The kidney remove urea and other toxic wastes from the blood, forming a dilute solution called urine in the process.

Kidney makes urine in two stage : • Ultrafiltration (involves filtering small molecules out of blood into the renal capsule) • Reabsorption (involves taking back any useful molecules from the fluid in the nephron as it flows along)

1. Renal capsule – Ultrafiltration • The renal artery splits into numerous arterioles, each feeding a nephron. • The arteriole splits into numerous capillaries, which form a knot called a glomerulus. • The glomerulus is enclosed by the renal capsule (Bowman’s capsule) • The arteriole leading into the glomerulus (the afferent arteriole) is wider than the one leading out (the efferent arteriole) • so there is high blood pressure in the capillaries of the glomerulus • This pressure forces plasma out of the blood by ultrafiltration. • • • • • •

Both the capillary walls and the capsule walls are formed from a single layer of flattened endothelial cells with gaps between them Endothelial cells of renal capsule forms podocytes (cell with tiny projections, with gaps) at the sides facing the glomerulus; other regions consists of squamous epithelial cells Basement membrane (network of collagen & glycoproteins) act as a filter all molecules with a molecular mass of <70000 are squeezed out of the blood to form a filtrate in the renal capsule Only blood cells and large proteins remain in the blood The glomerular filtrate is identical to blood plasma but do not contain large plasma proteins

Factors affecting glomerular filtration rate The rate at which fluid seeps from the blood in the glomerular capillaries into the renal capsule • Differences in water potential • Blood pressure is relatively high in glomerulus (difference in width of afferent arteriole and efferent arteriole)à raise water potential of blood plasma • But concentration of solutes is higher in blood plasma as well • Overall, the effect of differences in pressure outweighs the effect of the differences in solute concentration • Overall, the water potential of blood plasma in glomerulus is higher than the water potential of the fluid in renal capsule • Water continue to move down water potential gradient 2. Proximal Convoluted Tubule – selective reabsorption • • • •

The proximal convoluted tubule is the longest (14mm) and widest (60µm) part of the nephron. It is lined with epithelial cells containing microvilli and numerous mitochondria In this part of the nephron over 80% of the filtrate is reabsorbed into the tissue fluid and then to the blood. This ensures that all the “useful” materials that were filtered out of the blood (such as glucose and amino acids) are now returned to the blood. All glucose, all amino acids and 85% of mineral ions are reabsorbed by active transport from the filtrate to the tissue fluid. They then diffuse into the blood capillaries.

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Small proteins are reabsorbed by pinocytosis, digested, and the amino acids diffuse into the blood. 80% of the water is reabsorbed to the blood by osmosis. Surprisingly, some urea is reabsorbed to the blood by diffusion. Urea is a small, uncharged molecule, so it can pass through membranes by lipid diffusion and there isn’t much the kidney can do about it. Since this is a passive process, urea diffuses down its concentration gradient until the concentrations of urea in the filtrate and blood are equal. So in each pass through the kidneys half the urea is removed from the blood and half remains in the blood.

Reabsorption in the proximal convoluted tubule • Basal membrane of the cells lining the proximal tubule actively transport sodium ions out of the cell • The sodium ions are carried away by the blood • Concentration of sodium ions inside the cell decrease • Created a concentration gradient to allow diffusion of ions from the fluid in the lumen into the cells • But need transporter proteins • Transporter proteins transport ions with glucose or amino acids simultaneously • The concentration gradient of sodium provides enough energy to pull in glucose/ amino acid molecules à active transport of glucose and amino acids • As substances move, water follows by osmosis

3. Loop of Henle – Formation of a Salt Bath The function of the loop of Henle is to create high concentration of salts in the tissue fluid of medulla (kidney) à to reabsorb water • Loop of Henle conserve water in the body rather than lost in urine

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The first part of the loop – descending limb The second part of the loop – ascending limb

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The loop of Henle create concentration gradient by actively pumping sodium and chloride ions out of the filtrate into the tissue fluid in the ascending limb (2nd part of the loop) The walls of the upper parts of the ascending limb is impermeable to water; this it the part where active pumping of salts happen This raises the concentration of sodium and chloride ions in the tissue fluid of medulla causes water to leave descending limb The first part of the loop (the descending limb) is permeable to ions and water, but water leaves by osmosis. This makes the filtrate more concentrated as it descends. The longer the loop, the more concentrated towards the bottom of the loop as the concentrated fluid now turns to flow up the ascending limb, sodium and chloride ions leave readily down concentration gradient As the fluid continues up the ascending limb, it becomes gradually less concentrated Having the two limbs running side by side, the fluid flowing down in one and up in one à enables the maximum concentration to be built up both inside and outside the tube at the bottom of the loop This mechanism is known as a counter-current multiplier

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4. Distal Convoluted tubule – Homeostasis and Secretion • It is relatively short and has a brush border (i.e. microvilli) with numerous membrane pumps for active transport. • In the distal convoluted tubule sodium ions are actively transported from the filtrate into tissue fluid • Potassium ions are transported into the tubule

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Secretion of substances occurs here and it is regulated by hormones, so this is the homeostatic part of the kidney. Substances secreted include H+ (for pH homeostasis), K+ (for salt homeostasis), ethanol, toxins, drugs and other “foreign” substances.

5. Collecting Duct • As the collecting duct passes through the hypertonic salt bath in the medulla, water leaves the filtrate by osmosis • So, concentrating the urine and conserving water. • The water leaves through special water channels in the cell membrane called aquaporins. • These aquaporin channels can be controlled by the hormone ADH, so allowing the amount of water in the urine to be controlled. More ADH opens the channels, so more water is conserved in body.

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Control of water content (osmoregulation) • The amount of water in the blood is constantly monitored by cells, osmoreceptors, within the hypothalamus • Probably differences in water content of the blood causes water to move into or out of osmoreceptor cells à triggers stimulation of nerve cells in the hypothalamus • The nerve cells produce antidiuretic hormone (ADH) • ADH – made up of 9 amino acids • When the nerve cells are stimulated by osmoreceptor cells, action potentials (impulse) travel down them • Causes ADH to be released from their endings into the blood in capillaries in the posterior pituitary gland

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ADH acts on the plasma membranes of the cells making up the walls of the collecting ducts

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It causes these membranes to be more permeable to water by increasing the number of waterpermeable channels in the plasma membrane ADH molecule is picked up by a receptor on the plasma membrane Activates an enzyme inside the cell Inside the cell are ready-made vesicles with membrane full of water-permeable channels The activation of the enzyme by ADH causes the vesicle to move and fuse with plasma membrane of the cell à increase number of water-permeable channels on the plasma membrane These channels transport solute-free water back into blood as the fluid flows down through the collecting duct, water is free to move out of the tubule into tissue fluid (high concentration of salts in medulla) The fluid in the collecting duct loses water and becomes more concentrated The secretion of ADH caused the increased reabsorption of water into the blood The amount of urine which is formed is smaller Urine formed is more concentrated

Hormonal communication • In general, the endocrine system is in charge of body processes that happen slowly, such as cell growth • The foundations of the endocrine system are the hormones and glands • A gland is a group of cells which produces and secretes one or more substances • Endocrine glands contain secretory cells which pass their secretions directly into the blood • As the body's chemical messengers, hormones transfer information and instructions from one set of cells to another Hormones • Usually relatively small molecules • Transported in the blood

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Concentrations of hormones in human blood are very small Most endocrine glands can secrete hormones very quickly when an appropriate stimulus arrives Many hormones have a very short life in the body They are broken down by enzymes in the blood or in the cells, or lost in the urine Each hormone acts on particular target cells Target cells contain receptors on the outer surface of plasma membrane, causing a response by the cell

Steroid hormones are lipid soluble; receptors are in the cytoplasm Control of blood glucose • The human body wants blood glucose (blood sugar) maintained in a very narrow range. • Insulin and glucagon are the hormones which make this happen • Both insulin and glucagon are secreted from the pancreas, and thus are referred to as pancreatic endocrine hormones • Insulin and glucagon are hormones secreted in response to blood sugar levels, but act in opposite fashion • Insulin produced by β cells while glucagon produced by α cells (islet of Langerhans)

Insulin affects many cells, especially those in the liver and muscles Effects : • An increased absorption of glucose from the blood into the cells • An increase in the rate of use of glucose in respiration • An increase in the rate at which glucose is converted into the storage polysaccharide glycogen All these processes take glucose out of the blood • Lowering the blood glucose level • A drop in blood glucose concentration is detected by β cells and α cells • α cells response by secreting glucagon; β cells response by stopping the secretion of insulin •

The lack of insulin puts a stop to the increased uptake and usage of glucose by liver and muscle; uptake occur only at normal rate The presence of glucagon affects • The breakdown of glycogen to glucose • The use of fatty acids instead of glucose as the main fuel in respiration • The production of glucose from other compounds such as fats As a result, the liver releases glucose into the blood • The blood flows around • At pancreas, α cells and β cells sense the raised glucose levels • Switch off glucagon secretion; switching on insulin secretion diabetes

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Diabetes is a disease caused by a failure of glucose homeostasis In insulin-dependent diabetes (also known as type 1 or early-onset diabetes) there is a severe insulin deficiency due to autoimmune killing of β cells (possibly due to a virus) or deficiency in the gene coding for the production of insulin This form of diabetes usually begins very early in life In non insulin-dependent diabetes (also known as type 2 or late-onset diabetes) insulin is produced, but the insulin receptors in the target cells don’t work (the liver and muscle cells do not respond properly to it) This form of diabetes usually begins relatively late in life and is often associated with obesity In both cases there is a very high blood glucose concentration after a meal, so the active transport pumps in the proximal convoluted tubule of the kidney can’t reabsorb it all from the kidney filtrate much of the glucose is excreted in urine (diabetes mellitus means “sweet fountain”)

This leads to the symptoms of diabetes: • high thirst (water and salts follow glucose to be passed out in the urine) • poor vision due to osmotic loss of water from the eye lens • tiredness due to loss of glucose in urine and poor uptake of glucose by liver and muscle cells. • In insulin-dependent diabetes, regular injections of insulin, with a carefully controlled diet, are used to keep blood glucose levels near normal • In non-insulin dependent diabetes, insulin injections are not normally needed. Control is by diet alone Biotechnology in insulin production Advantages of using genetically engineered human insulin : • Insulin produced is cheaper than that obtained from animals • Pure • More rapid response • Less chance of an immune response to the insulin • Effective in people who have developed a tolerance for animal-derived insulin • More acceptable to people who feel it is unethical to use pig or cattle insulin Nervous communication Nervous system : • consists of neurones or nerve cells • transmit impulses • response is faster ; more precise Structure of mammalian neurone • The neurone is the functional unit of the nervous system • Cell body lies within spinal cord and the brain • Many thin cytoplasmic processes extend from the cell body • Dendrons or dendrites à conduct impulses towards the cell body • Axon à longer, conduct impulses away from the cell body

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Within the cytoplasm of an axon, all of the usual organelles are present Large numbers of mitochondria and vesicles containing neurotransmitter are found at the tip of terminal branch of axon In some neurone, Schwann cells spirals themselves around the axon all along its length The enclosing sheath of many layers of plasma membranes of Schwann cell is called the myelin sheath (made largely of lipid, with some proteins) Schwann cells serve as supportive, nutritive, and service facilities for neurons The gap between Schwann cells is known as the node of Ranvier and serves as points along the neurone for generating a signal (Signals jumping from node to node travel faster than signals traveling along the surface of the axon)

Myelin serves as insulation and as an aid to efficient conduction of signals on neurone • The axon of sensory and motor neurone run through the body in bundles, forming nerves • These axons are arranged in buldles surrounded by perineurium • Blood vessels lie between and within the bundles • The whole structure is surrounded by a tough outer covering called the epineurium

Types of neurone • Sensory neurone • Intermediate neurone • Motor neurone Reflex arc • Is the pathway along which impulses are carried from a receptor to an effector, without involving “conscious” regions of the brain • The effector responds to the stimulus before there is any voluntary response à reflex action • It is a fast, automatic response to stimulus • Very useful way of responding to danger signals

Nerve impulse ? • Very brief changes in the distribution of electrical charges across the plasma membrane of neurone • Caused by very rapid movement of Na+ and K+ into and out of axon • The movement of these ions is affected by their ability to pass through the Cell membrane, their Concentration Inside and Out of the Cell, and Their Charge • Neurones have an electrical charge different from the extracellular fluid that surrounds them • A difference in electrical charge between two locations is called a POTENTIAL. Resting potential • A Nerve Cell has ELECTRICAL POTENTIAL across its cell membrane because of a difference in the number of Positively and Negatively Charged IONS on each side of the Cell Membrane

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The Electrical Potential is due to PROTEINS in the neurone known as Sodium-Potassium Pumps that move 3 Sodium ions (Na+) OUT of the Cell and Actively Pump 2 Potassium ions (K+) INTO the Cell The result is the Cytoplasm of the neurone contains MORE K+ IONS and FEWER Na+ IONS than the surrounding medium K+ ions can leak out across the membrane more easily than Na+ ions can leak in The Net Result of the leakage of positively charged ions out of the cell is a Negative Charge on the INSIDE of the neuron's Cell Membrane The Charge Difference is known as the RESTING POTENTIAL of the Neuron's Cell Membrane The electrical potential of the inside of the axon is -65mV compare to the outside As a result of its Resting Potential, the neurone is said to be POLARIZED A neurone maintains this polarization until it is stimulated A STIMULUS is a change in the environment that may be of sufficient strength to initiate an impulse. The ability of a neurone to respond to a Stimulus and Convert it into a nerve impulse is known as EXCITABILITY

Action potential • Nerve Impulse causes a movement of ions across the cell membrane of a neurone… Similar to a ripple passing along the surface of a pond. • The cell membrane of a neurone contains thousands of tiny molecules known as GATES à channels in the plasma membrane that allows Sodium and Potassium ions to pass through • They open and close depending on the electrical potential across membrane (voltage-gated channels) • Generally the Gates on a neurone are CLOSED • A Nerve Impulse STARTS when pressure or other sensory inputs, disturbs a neurone's plasma membrane, causing Sodium Gates to OPEN • At the beginning of an impulse, the Sodium Gates OPEN, allowing positively charged Na+ ions to flow into the cell (higher concentration outside the axon) • The INSIDE of the membrane temporarily becomes MORE POSITIVE than the OUTSIDE. • The Membrane is now said to be DEPOLARIZED: the charge inside the axon changes from negative to positive as sodium ions enter the interior of axon • The inside of axon continues to build up positive charge until it reaches a potential of +40mV • As the impulse passes, the Potassium Gates OPEN, sodium channels close, allowing positively charged K+ ions to FLOW OUT • REPOLARIZED: the inside of the axon resumes a negative charge • The membrane is now said to be REPOLARIZED. Once again NEGATIVELY Charged on the INSIDE and POSITIVELY Charged on the OUTSIDE. • Too many potassium ions leave the axon, the potential difference across membrane becomes even more negative than the normal resting potential • Channel for potassium is close, sodium-potassium pump act by restoring the resting potential • Action Potential is the rapid, fleeting change in potential difference across the membrane (impulse) à The DEPOLARIZATION and REPOLARIZATION

Steps in an Action Potential • At rest the outside of the membrane is more positive than the inside. • Na+ moves inside the cell causing an action potential, the influx of positive sodium ions makes the inside of the membrane more positive than the outside. • K+ ions flow out of the cell, restoring the resting potential net charges. • Sodium ions are pumped out of the cell and potassium ions are pumped into the cell, restoring the original distribution of ions. Transmission of action potentials • An action potential at any point in an axon’s plasma membrane triggers the production of an action potential in the membrane on either side of it • In practice, action potential only travel in one direction What makes the impulse travel ? • Na+ ions rushed into the axon are attracted sideways toward the negatively charged are inside the axon ahead of the action potential • A similar effect occurs outside the axon where Na+ are attracted in the opposite direction • This depolarises the membrane ahead of the action potential causing the Na+ channels to open and generate an action potential • The action potential begins at one spot on the membrane, but spreads to adjacent areas of the membrane, propagating the message along the length of the cell membrane. • After passage of the action potential, there is a brief period, the refractory period, during which the membrane cannot be stimulated. • This prevents the message from being transmitted backward along the membrane Speed of conduction • Myelin speeds up the rate at which action potential travel by insulating the axon membrane • Na+ and K+ cannot flow through myelin sheath • Depolarisation can only occur at nodes of Ranvier • Thus action potential jump from one node to the other (1-3 mm) à saltatory conduction • Large diameter axon transmit action potential faster than the smaller ones What starts off an action potential ? • A cell which responds to stimulus by initiating action potential à receptor cell

• Receptor cells convert different forms of energy into nerve impulses / action potential Example : Pacinian corpuscle • Receptor in the skin sensing pressure • When pressure is applied, the nerve ending deforms (change in its structure / shape) • Causes Na+ and K+ channels to open in the cell membrane • Na+ flows in; K+ flows out à depolarises the membrane • Increases positive charge in the axon is called a receptor potential • The greater the pressure, more channels will be opened à increases the receptor potential • Below a certain threshold, the pressure stimuli only causes local depolarisation, not action potential • Above the threshold, action potentials are initiated • As pressure increases, the action potentials are produced more frequently à strong impulse How does the brain know the nature of stimulus ? • Based on the position of the sensory neurone bringing the information Action potentials do not change in size as they travel (+40mV inside all the way) The speed of axon potential transmission is always the same Difference in intensity of impulse is their frequency and the number of neurones carrying the impulse Strong stimulus

Weak stimulus

Freq. of action potential

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Fewer

Number of stimulated

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fewer

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synapses • The site where two neurones meet, with a gap (20mm) à synaptic cleft Mechanism of synaptic transmission • Action potential cannot jump over synapse • The signal needs to be passed on by chemicals à transmitter substance

Mechanism of synaptic transmission • Action potential arriving at presynaptic neurone causes it to release transmitter substance into the cleft • The transmitter substance diffuse across the cleft in less than a milisecond • This may set up an action potential in the plasma membrane of postsynaptic neurone • There are more than 40 different transmitter substances • Noradrenaline and acetylcholine are found throughout the nervous system • Dopamine and glutamic acid occur only in brain • Synapses which uses acetylcholine (ACh) à Cholinergic synapse How a cholinergic synapse work ? • Synthesis of acetylcholine – Ach is made from choline and acetate from acetylcoenzyme A; Ach stored in vesicles until it is needed • Impulse arrival – when an action potential arrives at the synaptic terminal, Ca2+ channels open; Ca2+ ions diffuse into the presynaptic terminal besides Na+ ions • Release of ACh – the influx of Ca2+ ions cause the vesicles to fuse with the presynaptic membrane, releasing ACh into the synaptic cleft through exocytosis • Each vesicle contains up to 10000 molecules of ACh • • • • •

Ach binding to receptors – ACh diffuses across the synaptic cleft in less than 0.5s and binds to Ach receptors in the postsynaptic membrane Part of the receptor protein has a complementary shape to part of ACh Changes in shape causes the opening of Na+ ion channels which results in depolarisation and the production of an action potential in the postsynaptic neurone Breakdown and recycling of ACh – enzyme acetylcholinesterase is attached to the postsynaptic membrane and breaks down Ach Much of the choline is taken up through the presynaptic membrane and used to make acetylcholine once more

Effects of other chemicals at synapse • nicotine (cigarette smoke)- part of its molecule is similar in shape to acetylcholine and will fit into the ACh receptors on postsynaptic membrane

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initiates action potentials; but nicotine is not rapidly broken down by enzymes à remains in the receptors ; accumulation of nicotine can be fatal nicotine activates nerves to release dopamine à causing pleasure; people will be addicted Receipt and processing of signals from nicotine are critical to muscle control and movement, automatic control of bodily organs to maintain survival, and brain functions such as mood, thinking, and many forms of sensory perception

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botulinum toxin produced by bacteria Clostridium botulinum (grown in contaminated canned food) acts at the presynaptic membrane • it prevents the release of acetylcholine • By inhibiting acetylcholine release, the toxin interferes with nerve impulses and causes paralysis of muscles in botulism • However, botulinum is used to treat muscles problem Eg the muscles of the eyelids contract permanently; these people cannot open their eyes • Use botulinum toxin to block the release of Ach in the nerves à causing the muscles to relax; can raised the eyelids • • •

Organophosphorous insecticides inhibit the action of acetylcholinesterase Allowing Ach to cause continuous production of action potential Some nerve gases act this way

The roles of synapses 1. Synapses ensure one-way transmission of action potential – allows signals to be directed toward specific goal 2.Synapses increase the possible range of actions in response to a stimulus • Synapse allow wider range of behaviour by allowing interconnection of many nerve pathways • Signals from many sources; interpreted at synapse • Stimulatory synapse – action potential arriving here stimulates an action potential • Inhibitory synapse – action potential arriving here cause release of transmitter substances that inhibit production of action potential 3. Synapses are involve in memory and learning • Learning - The acquisition of new information. • Memory - The retention of learned information - the acquisition, storage, and retrieval of information • It is thought that electrical stimulation of neuron can produce either a brief or long-lasting change in synaptic transmission. • With repeated stimulation there is Presynaptic alterations in presynaptic proteins and modification of K+ channels Plant growth regulators • Plants have communication system to coordinate different parts of the body • Most communication depends on chemicals à plant growth regulators / plant hormones • Plant hormones are produced in variety of tissues • Produced in minute quantity • Move through diffusion across cells / carried in phloem sap or xylem vessels; some may remain at the site of synthesis auxin

• Is synthesised in the growing tips of roots and shoots • Transported across cells via active transport; lesser extent in phloem sap • Auxin may be involved in determining apical growth or budding / branching • Presence of apical bud stops lateral buds from growing à apical dominance • Auxin is involved in tropism à growth response in a direction determined by the stimulus Effects of auxin • Cell enlargement - auxin stimulates cell enlargement and stem growth. • Cell division - auxin stimulates cell division in the cambium and, in combination with cytokinin, in tissue culture. • Vascular tissue differentiation - auxin stimulates differentiation of phloem and xylem. • Root initiation - auxin stimulates root initiation on stem cuttings, and also the development of branch roots and the differentiation of roots in tissue culture. • Tropistic responses - auxin mediates the tropistic (bending) response of shoots and roots to gravity and light. • Delayed leaf senescence. • Leaf and fruit abscission - auxin may inhibit or promote (via ethylene) leaf and fruit abscission depending on the timing and position of the source. • Delayed fruit ripening. Gibberellins • Plant hormones that promote elongation including stem extension, seed germination, flowering and fruit growth. • Synthesised in seeds and most plant cells especially in young leaves • Applying active gibberellin to plants (dwarf) can stimulate plants to grow tall • It function by stimulating cell division and cell elongation in stem, causing them grow tall • In some seeds, gibberellins are involved in the control of germination • • •

Gibberellin, made in the embryo, diffuses out to the aleurone layer Aleurone layer (tissue containing protein and lipid stores) In response to gibberellin, the aleurone layer synthesises amylase and the amylase diffuses into the endosperm • Amylase catalyses the hydrolysis of starch to maltose Maltose diffuse into the embryo as an energy source Gibberellin activates the gene coding for amylase production Abscisic acid and stomatal closure • The plant hormone abscisic acid (ABA) is the major player in mediating the adaptation of the plant to stress. • if a plant is subjected to difficult drought condition, ABA is produced in large quantity in all cells that possess chloroplast • In normal condition, an ATP-powered proton pump actively pump H+ out of the guard cell • Lowering of H+ inside the cells opens the potassium channels • K+ moves into guard cell down an electrical gradient • K+ concentration in cell increase à lowers the water potential; water diffuse into guard cell down its water potential gradient

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Turgidity of guard cells increase, stoma opens ABA binds to ABA receptors on plasma membrane of guard cells à inhibiting the proton pump H+ stop pumping out of guard cells K+ channels are not open Water would not diffuse into guard cells Guard cells become flaccid Stoma is closed

Abscisic acid with leaf abscission • Abscission zone forms where the petiole meets the stem • Nearer to the leave à separation layer • Nearer to the stem à protective layer (contain suberin) • During abscission, useful substances are withdrawn from the leaves into the stem • It is related to forming a barrier for substance to transport at the point of attachment (increase accumulation of suberin on the stem side) • The flow of nutrient into the leave is blocked • Decreasing supply of nutrient cause the leaf tissues to soften • Enzymes breakdown the cell walls in the separation layer • Petiole breaks • Protective layer form the scar on the stem Benefits of abscission : • Quick release of mature fruit • Mature fruit selectivity • Money-saved for the grower