Organisms Responding to Changes - Missestruch 2021 PDF

ORGANISMS RESPOND TO CHANGES IN THEIR Key Terms INTERNAL & EXTERNAL ENVIRONMENTS 3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Stimulus Tropism Responses...

ORGANISMS RESPOND TO CHANGES IN THEIR Key Terms INTERNAL & EXTERNAL ENVIRONMENTS 3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Stimulus Tropism Responses for survival MISSESTRUCH 2021 Indoleacetic acid (IAA) A stimulus is a detectable change in the environment. These Elongation changes can be detected by cells, which are called receptors. Phototropism Organisms increase their chance of survival by responding to stimuli via different response mechanisms. Responses in flowering plants Tropism describes the response of plants to stimuli via growth. Tropisms can be positive (growing towards a stimulus) or negative (growing away from a stimulus). Plants respond to light, gravity and water. Tropisms are controlled by specific growth factors and one key example is indoleacetic acid (IAA). IAA is a type of auxin and can control cell elongation in shoots and inhibit the growth of cells in the roots. It is made in the tip of the roots and shoots but can diffuse to other cells. Phototropisms Shoots need light for the LDR in photosynthesis which is why plants grow and then bend towards the light. This is controlled by IAA. This is positive phototropism. Shoot tip cells produce IAA, which causes cell elongation in shoots, and this IAA diffuses to other cells. If there is unilateral light, the IAA will diffuse towards the shaded side of the shoot resulting in a higher concentration of IAA there. The IAA causes the cells on the shaded side to elongate more and this causes the plant to bend towards the light source. Roots do not require light as they do not photosynthesis, and are more able to anchor the plant if they are deep in the soil away from light. In roots, a high concentration of IAA inhibits cell elongation, causing roots cells to elongate more on the lighter side and so the root bends away from light. This is negative phototropism. 1t MISSESTRUCH 2021 ORGANISMS RESPOND TO CHANGES IN THEIR Key Terms INTERNAL & EXTERNAL ENVIRONMENTS 3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Gravitropism Taxes Gravitropism MISSESTRUCH 2021 Kinesis In the shoots, IAA will diffuse from the upper side to the lower Phototaxis side of a shoot in response to gravity. If a plant is vertical, Chemotaxis this causes the plant cells to elongate and the plant grows upwards. If a plant is on its side, it will cause the shoot to bend upwards. This is negative gravitropism. In roots, the IAA moves to the lower side of roots in response to gravity so that the upper side elongates more and the root bends down towards gravity and anchors the plant in. This is positive gravitropism. Taxes and kinesis These are two simple responses that keep organisms within the favourable conditions of their environment (light, moisture, chemicals). Taxes This is a simple response in which an organism will move its entire body towards a favourable stimulus or away from an unfavourable stimulus. When an organism moves towards a stimulus this is known as positive taxis, and when an organism moves away it is described as negative taxis. Earthworms will show negative phototaxis, meaning they move away from light. They will move towards dark environments, such as in the soil, to help them avoid dehydration, predators and to locate food. Bacteria can show positive chemotaxis, as they move towards certain chemicals to aid survival. Kinesis This is when an organism changes its speed of movement and its rate it changes direction. If an organism moves from an area where there are beneficial stimuli to an area with harmful stimuli, its response will be to increase the rate it changes direction to return to the favourable conditions quickly. If it is surrounded by negative stimuli, the rate of turning decreases to keep it moving in a relatively straight line to increase the chances of it finding a new location with favourable conditions. Woodlice respond to water by inhabiting damp areas to prevent excessive water loss across their surface. If a woodlouse crossed a border from a damp to a dry area, the response would be to turn rapidly to increase the probability that it will end up back in the damp area. If the woodlouse was in a completely dry area, the turning rate would decrease so that it would move in a straight line to increase the chances of finding a new damp area. 2 MISSESTRUCH 2021 ORGANISMS RESPOND TO CHANGES IN THEIR Key Terms INTERNAL & EXTERNAL ENVIRONMENTS 3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Central nervous system Neurones MISSESTRUCH 2021 Simple reflex Pacinian corpuscle More complex organisms have a nervous system. A Pressure receptor detecting a stimulus triggers the following response: Channel proteins Stimulus --> Receptor -->Coordinator-->Effector-->Response The nervous system is made up of the peripheral and central nervous system. The PNS includes the receptors, sensory and motor neurones, whilst the CNS is the coordination centres such as the brain and spine. Receptors Each receptor responds only to specific stimuli and this stimulation of a receptor leads to the establishment of a generator potential which can cause a response. Three receptors you must know are: 1. Pacinian corpuscle 2. Rods 3. Cones The Pacinian corpuscle This receptor responds to pressure changes. These receptors occur deep in the skin, mainly in fingers and feet. It consists of a single sensory neurone wrapped with layers of tissue separated by gel. The sensory neurone in the Pacinian corpuscle has special channel proteins in its plasma membrane. 3 MISSESTRUCH 2021 ORGANISMS RESPOND TO CHANGES IN THEIR Key Terms INTERNAL & EXTERNAL ENVIRONMENTS 3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Stretch-mediated sodium channel Generator potential The Pacinian corpuscle MISSESTRUCH 2021 Photoreceptors The plasma membranes contain channel proteins that allow ion Rods transportation and the membranes surrounding the sensory Cones neurones have stretch-mediated sodium channels. These channels will open and allow sodium ions to diffuse into the sensory neurone only when they are stretched and deformed. In the resting state, sodium ion channels are too narrow for sodium ions to diffuse into the sensory neurone. Therefore, the resting potential is maintained. When pressure is applied, the stretch-mediated sodium ion channels are deformed and widen. This allows sodium ions to diffuse in which leads to the establishment of a generator potential. The human retina The retina contains two types of photoreceptors: rods and cones. Rods, so-called because of their shape, cannot distinguish between different wavelengths of light and process images in black and white. Rods can, however, detect light of very low intensity though because many rod cells connect to one sensory neurone (retinal convergence). To create the generator potential, the pigment of rod cells (rhodopsin) must be broken down by light energy. There is enough energy from low- intensity light to cause the breakdown. Enough pigment has to be broken down for the threshold to be met in the bipolar cell. This threshold can be reached even in low light because so many rod cells are connected to a single bipolar cell, this is an example of spatial summation. 4 MISSESTRUCH 2021 ORGANISMS RESPOND TO CHANGES IN THEIR Key Terms INTERNAL & EXTERNAL ENVIRONMENTS 3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Visual acuity Summation However, this retinal convergence means MISSESTRUCH 2021 that the brain cannot distinguish between the separate sources Iodopsin of light that stimulated it. Two light sources close together Retinal convergence cannot be seen as separate so rod cells give low (poor) visual Fovea acuity. Cone cells There are three types of cone cells that contain different types of iodopsin pigment (red, green and blue) which all absorb different wavelengths of light. Depending on the proportion of each cone cell that is stimulated, we perceive coloured images. Iodopsin is only broken down if there is high light intensity, so action potentials can only be generated with enough light. Only one cone cell connects to a bipolar cell. Therefore, no spatial summation occurs and cones can only respond to high light intensity, which is why we can’t see colour when it is dark. As each cone cell is connected to one bipolar cell, the brain can distinguish between the separate sources of light that were detected. So, cone cells give high visual acuity. The distribution of rods and cones The distribution in the retina is uneven. Light is focused by the lens on the part of the retina opposite the pupil (the fovea) which will receive the highest intensity of light. Therefore, most cone cells are located near the fovea as they only respond to high light intensities and rod cells are located further away as these can respond at lower light intensities. 5 MISSESTRUCH 2021 ORGANISMS RESPOND TO CHANGES IN THEIR Key Terms INTERNAL & EXTERNAL ENVIRONMENTS 3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Wave of depolarisation Non-conductive layer Control of the cardiac cycle MISSESTRUCH 2021 Depolarisation Cardiac muscle is myogenic, meaning it contracts of its own Cardiac muscle accord, but the rate of contraction is controlled by the wave of Repolarise electrical activity. This involves the following structures: The sinoatrial node (SAN) is located in the right atrium and is known as the pacemaker. The atrioventricular node (AVN) is located near the border of the right and left ventricle within the atria. The bundle of His runs through the septum. The Purkyne fibres in the walls of the ventricles. The SAN will release a wave of depolarisation across the atria, causing it to contract. The AVN will release another wave of depolarisation when the first reaches it. There is a non-conductive layer between the atria and ventricles that prevents the wave of depolarisation from travelling down to the ventricles. Instead, the bundle of His, running through the septum can conduct and pass the wave of depolarisation down the septum and the Purkyne fibres in the walls of the ventricles. As a result, the apex and then walls of the ventricles contract. There is a short delay before this happens, whilst the AVN transmits the second wave of depolarisation. This allows enough time for the atria to pump all the blood into the ventricles. Finally, the cells repolarise, and the cardiac muscle relaxes. 6 MISSESTRUCH 2021 ORGANISMS RESPOND TO CHANGES IN THEIR Key Terms INTERNAL & EXTERNAL ENVIRONMENTS 3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Medulla oblongata Autonomic nervous system Control of the heart rate MISSESTRUCH 2021 Sympathetic nervous system The medulla oblongata in the brain controls the heart rate, via Parasympathetic nervous system the autonomic nervous system. Chemoreceptors There are two parts: a centre linked to the sinoatrial node that increases heart rate via the sympathetic nervous system and another that decreases heart rate via the parasympathetic nervous system. The heart rate changes in response to pH and blood pressure and these stimuli are detected by chemoreceptors and pressure receptors (baroreceptors) in the aorta and carotid artery. Response to Pressure If the blood pressure is too high, this can cause damage to the walls of the arteries and it is important to put mechanisms in place to reduce the blood pressure. If the blood pressure is too low, there may be an insufficient supply of oxygenated blood to respiring cells and the removal of waste. Response to pH The pH of the blood will decrease during times of high respiratory rate, due to the production of carbon dioxide or lactic acid. Excess acid must be removed from the blood rapidly to prevent enzymes from denaturing. This is achieved by increasing the heart rate, so carbon dioxide can diffuse out into the alveoli more rapidly to be removed. Response sequence for high blood pressure 1. Stimulus- Increased pressure 2. Receptor – Pressure receptors in the wall of the aorta and carotid artery are stretched if high blood pressure. 3. Coordination: More electrical impulses sent to the medulla oblongata and then impulses sent via the parasympathetic nervous system to SAN to decrease the frequency of electrical impulses. 4. Effector- Cardiac muscle – SAN tissues 5. Response – Reduced heart rate. 7 MISSESTRUCH 2021 ORGANISMS RESPOND TO CHANGES IN THEIR INTERNAL & EXTERNAL ENVIRONMENTS 3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Response sequence low blood pressure MISSESTRUCH 2021 1. Stimulus- Decreased pressure 2. Receptor – Pressure receptors in the wall of the aorta and carotid artery are stretched less if low blood pressure. 3. Coordination: More electrical impulses sent to the medulla oblongata and then impulses sent via the sympathetic nervous system to SAN to increase the frequency of electrical impulses. 4. Effector- Cardiac muscle – SAN tissues 5. Response – Increased heart rate. Response sequence for low pH 1. Stimulus- Decreased pH 2. Receptor – Chemoreceptor in the wall of the aorta and carotid artery. 3. Coordination: More electrical impulses sent to the medulla oblongata and then impulses sent via the sympathetic nervous system to SAN to increase the frequency of electrical impulses. 4. Effector- Cardiac muscle – SAN tissues 5. Response – Increased heart rate to deliver blood to the heart more rapidly to remove carbon dioxide. Key points Essay Links: The Pacinian corpuscle links to diffusion Organisms are more likely to survive if they and protein channels. can respond to stimuli. The chemoreceptors triggering a Some simple responses are tropisms in response to low blood pH links to plants, taxes, kinesis and reflexes. The Pacinian corpuscle is a receptor that enzymes denaturing. detects pressure changes in the skin. The pressure receptors triggering a Rods and cones are photoreceptors found in response to high blood pressure links to the human retina. the structure and function of arteries, as The SAN is the natural pacemaker. The SAN, these can be damaged by high blood AVN, Bundle of his and the Purkyne fibres pressure. control the cardiac cycle. The diffusion of IAA through plants links to Chemoreceptors and pressure receptors transport across membranes. can detect changes in blood pH and pressure and trigger a response to alter the heart rate. 8 MISSESTRUCH 2021 NERVOUS COORDINATION 3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Key Terms Cell body Dendrites Myelinated motor neurone MISSESTRUCH 2021 Schwann cells The cell body of the neurone contains the organelles Axon found in a typical animal cell, including the nucleus. Myelin sheath Proteins and neurotransmitter chemicals are made here. Dendrites carry the action potentials to surrounding cells. The axon is the conductive, long fibre that carries the nervous impulse along the motor neurone. Schwann cells wrap around the axon to form the myelin sheath, which is a lipid and therefore does not allow charged ions to pass through it. There are gaps between the myelin sheath, called nodes of Ranvier. Resting Potential When a neurone is not conducting an impulse, there is a difference between the electrical charge inside and outside of the neurone. This is known as the resting potential. There are more positive ions (Na+ and K+ ), outside compared to inside. Therefore, the inside of the neurone is comparatively more negative at -70mV. The resting potential is maintained by a sodium- potassium pump, involving active transport and therefore ATP. The pump moves 2 K + ions in and 3 Na+ ions out. This creates an electrochemical gradient and results in K + diffusing out and Na + diffusing in. However, because the membrane is more permeable to K+ , more K+ moved out resulting in the -70mV. 9 MISSESTRUCH 2021 NERVOUS COORDINATION 3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Key Terms Nervous impulse Stimulus MISSESTRUCH 2021 Action potential Voltage-gated channels An action potential is when the neurone’s voltage increases Depolarisation beyond a set point from the resting potential. This generates a Hyperpolarisation nervous impulse. An increase in voltage, or depolarisation, is due to the neurone membrane becoming more permeable to Na+. Once an action potential is generated, it moves along the axon like a Mexican wave. A stimulus provides the energy needed for the sodium voltage-gated channels in the axon membrane to open. This causes Na + to diffuse in, which increases the positivity inside of the axon. This causes more voltage-gated channels to open, so even more Na + diffuse in. When a threshold of +40mV is reached inside the axon, the voltage-gated sodium channels close and instead voltage- gated potassium ion channels open. This results in potassium ions diffusing out, and the axon becomes negative again which repolarises the axon. Temporarily, the axon becomes more negative than the -70mV and is hyperpolarised. The potassium ion gates will now close and the sodium- potassium pump restores normal activity to reform the resting potential. 10 MISSESTRUCH 2021 NERVOUS COORDINATION 3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Key Terms -55mV threshold All-or-nothing principle MISSESTRUCH 2021 All-or-nothing principle Refractory period If the depolarisation does not exceed the -55 mV threshold, Discrete impulses then an action potential is not produced (nothing). Voltage Any stimulus that does trigger depolarisation to -55mV will always peak at the same maximum voltage (all). Instead, bigger stimuli increase the frequency of action potentials. This is the all-or-nothing principle and it is important, as it makes sure that animals only respond to large enough stimuli, rather than responding to every slight change in the environment which would overwhelm them. Refractory period After an action potential has been generated, the membrane enters a refractory period when it can’t be stimulated, because sodium channels are recovering and can’t be opened. This is important for three reasons: 1. It ensures that discrete impulses are produced, meaning that an action potential cannot be generated immediately after another one, which makes sure that each is separate. 2. It ensures that action potentials travel in one direction. This stops the action potential from spreading out in two directions which would prevent a response. 3. It limits the number of impulse transmissions. This is important to prevent overreaction to a stimulus and therefore overwhelming the senses. 11 MISSESTRUCH 2021 NERVOUS COORDINATION 3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Key Terms Nodes of Ranvier Saltatory conduction The speed of conductance MISSESTRUCH 2021 Refractory period There are three factors that affect speed: Depolarisation 1.Myelination and saltatory conduction; Hyperpolarisation 2.Axon diameter; 3.Temperature. Saltatory conduction There are gaps between the myelin sheath, called nodes of Ranvier. The action potential jumps from node to node (saltatory conduction), which means the action potential travels along the axon faster as it doesn’t have to generate an action potential along the entire length, just at the nodes of Ranvier. Axon diameter With a wider diameter, the speed of conductance increases. A wider diameter means that there is less leakage of ions and therefore action potentials travel faster. Temperature A higher temperature increases the speed of conductance for two reasons: 1.The ions diffuse faster 2.The enzymes involved in respiration work faster. Therefore, more ATP for active transport in the + + Na /K pump. 12 MISSESTRUCH 2021  NERVOUS COORDINATION 3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Key Terms Cholinergic synapse Neurotransmitter Synapse MISSESTRUCH 2021 Synaptic knob Synapses are the gaps between the end of the axon of one Synaptic cleft neurone and the dendrite of another one. Here, the action Acetylcholine potential is transmitted as neurotransmitters that diffuse across the synapse. The function of a synapse 1. An action potential arrives at the synaptic knob. Depolarisation of the synaptic knob leads to the opening of Ca2+ channels and Ca2+ diffuses into the synaptic knob. 2. Vesicles containing neurotransmitters move towards and fuse with the presynaptic membrane. The neurotransmitters are released into the synaptic cleft. 3. The neurotransmitter diffuses down a concentration gradient across the synaptic cleft, to the post-synaptic membrane; the neurotransmitter binds by the complementarity of shape to receptors on the surface of the post-synaptic membrane. 4. Na+ ion channels on the post-synaptic membrane open and Na+ diffuse in; if enough neurotransmitter binds, and enough Na + diffuse in to raise the membrane potential above the -55mV threshold, then the post-synaptic neurone becomes depolarised. 5. The neurotransmitter is degraded and released from the receptor; the Na+ channels close and the post-synaptic neurone can re-establish resting potential; the neurotransmitter is transported back into the presynaptic neuron where it is recycled. At a cholinergic synapse, the neurotransmitter is acetylcholine. The enzyme that degrades the neurotransmitter down is acetylcholinesterase. This breaks the acetylcholine into choline and acetate to be recycled in the pre-synaptic neurone. 13 MISSESTRUCH 2021 NERVOUS COORDINATION 3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Key Terms Unidirectional Spatial summation Unidirectional MISSESTRUCH 2021 Temporal summation The impulse can only travel in one direction across the synapse. Inhibitory synapse This is because the neurotransmitter is only released from the Chloride ions pre-synaptic neurone, and therefore only diffuses from here to the post-synaptic neurone. Secondly, there are only receptors for the neurotransmitter on the post-synaptic neurone. Summation Summation is the rapid build-up of neurotransmitters in the synapse to help generate an action potential by two methods; spatial or temporal summation. This is needed because some action potentials do not result in sufficient concentrations of neurotransmitters being released to generate a new action potential. Spatial summation: many different neurones collectively trigger a new action potential by combining the neurotransmitter they release to exceed the threshold value. Temporal summation: One neurone releases neurotransmitters repeatedly over a short period of time in order to exceed the threshold value. Inhibitory synapses An inhibitory synapse causes chloride ions to move into the postsynaptic neurone and potassium ions to move out. The combined effect of negative ions moving in and positive ions moving out makes the membrane potential increase to -80mV (hyperpolarisation) and therefore an action potential is highly unlikely. 14 MISSESTRUCH 2021 NERVOUS COORDINATION 3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Key Terms Neuromuscular junction Excitatory MISSESTRUCH 2021 Neuromuscular junction Inhibitory Skeletal muscle This is a synapse that occurs Antagonistic pairs between a motor neurone and a muscle and is very similar to a synaptic junction. Below is a comparison table.. Skeletal muscles Muscles work in antagonistic pairs against an incompressible skeleton to create movement. This can be automatic as part of a reflex response or controlled by conscious thought. Below is the ultrastructure of a myofibril. Myofibrils are made up of fused cells that share nuclei and cytoplasm (sarcoplasm) and there is a high number of mitochondria. Muscle fibres are made up of millions of myofibrils which collectively bring about the force to cause movement. 15 MISSESTRUCH 2021 NERVOUS COORDINATION 3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Key Terms Myofibril Sarcomere Sarcomere MISSESTRUCH 2021 Myosin Actin Myofibrils are made up of two key types of protein, myosin and actin, that collectively forms a sarcomere. Sliding filament theory Sliding filament theory When an action potential reaches a muscle, it stimulates a response. Calcium ions enter and cause the protein tropomyosin, which blocks binding sites for the myosin head of the actin, to move and uncover the binding sites. Whilst ADP is attached to the myosin head, it can bind to the binding site on the actin to form a cross-bridge. The angle created in this cross-bridge creates tension and as a result, the actin filament is pulled and slides along the myosin. In doing so, the ADP molecule is released. An ATP molecule can then bind to the myosin head and causes it to change shape slightly and as a result, it detaches from the actin. Within the sarcoplasm, there is the enzyme ATPase, which is activated by the calcium ions, to hydrolyse the ATP on the myosin head into ADP and releases enough energy for the myosin head to return to its original position. This entire process repeats continually whilst the calcium ions remain in high concentrations in the sarcoplasm and therefore whilst the muscle remains stimulated by the nervous system. 16 MISSESTRUCH 2021 NERVOUS COORDINATION 3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Key Terms ATP Phosphocreatine The role of ATP MISSESTRUCH 2021 Myoglobin Active muscles require a high concentration of ATP. Slow-twitch Fast-twitch In times when aerobic respiration cannot create enough ATP to meet this demand, anaerobic respiration occurs. The chemical phosphocreatine, which is stored in muscles, combats this by providing phosphate to regenerate ATP from ADP. Fast and slow-twitch muscle fibres There are two different types of skeletal muscle, fast and slow-twitch, which you need to know the properties of. The table below summarises these. 17 MISSESTRUCH 2021 NERVOUS COORDINATION 3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE MISSESTRUCH 2021 Key points The -70mV resting potential of a membrane is maintained by the Na+/K+ pump and the subsequent diffusion of sodium and potassium ions. An influx of sodium ions results in depolarisation. If the -55mV threshold is met, the action potential is always generated (all-or-nothing principle). The cholinergic synapse is a gap between neurones where acetylcholine neurotransmitter diffuses through to help initiate an action potential in the post-synaptic neurone. Synapses can be excitatory or inhibitory. Skeletal muscles work as antagonistic pairs. Actin, myosin, calcium ions and ATP are all required for a myofibril to contract. Essay Links: The diffusion of neurotransmitter at a synapse links to transport across membranes. The Na+/K+ pump that maintains the resting potential links to co-transport. The role of ATP in muscle contractions links to biological molecules, demonstrating the importance of ATP. The lipid in myelin sheaths links to biological molecules, demonstrating the role of a lipid. Muscle contraction links to aerobic and anaerobic respiration. The movement of ions across the axon membrane links to the phospholipid bilayer structure. The role of the protein carriers and channels in an action potential demonstrates the importance of proteins and transport across membranes. 18 MISSESTRUCH 2021 HOMEOSTASIS 3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Key Terms Kinetic energy Denature Homeostasis MISSESTRUCH 2021 Water potential Homeostasis is the maintenance of a constant internal Negative feedback environment via physiological control systems. These control Pancreas systems keep temperature, blood pH, blood glucose and water potential within set limits. Importance of Homeostasis If body temperature is too low there will be insufficient kinetic energy for enzyme-controlled reactions, and if body temperature is too high then enzymes will denature. Either way, metabolic reactions could slow to the point that cells die. Alterations in blood pH will also result in enzymes denaturing. Glucose is needed for respiration, so a lack of glucose in the blood could result in cell death. If blood glucose levels are too high, then this will lower the water potential of the blood and water will leave surrounding cells by osmosis and prevent normal cell function. If the water potential of the blood is too high, water will move into cells by osmosis and can cause them to burst (lyse). Negative Feedback Negative feedback is when there is a deviation from the normal values and restorative systems are put in place to return this back to the original level. This involves the nervous system and often hormones. Control of blood glucose Blood glucose will increase following ingestion of food or drink containing carbohydrates and will fall following exercise or if you have not eaten. The pancreas detects changes in the blood glucose levels and contains endocrine cells in the Islets of Langerhans which release the hormones insulin and glucagon to bring blood glucose levels back to normal. Adrenaline is released by the adrenal glands if your body anticipates danger and results in more glucose being released from stores of glycogen in the liver. 19 MISSESTRUCH 2021 HOMEOSTASIS 3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Key Terms Beta cells Alpha cells Negative feedback example MISSESTRUCH 2021 Islets of Langerhans Blood glucose control is an example of homeostasis and Glycogen negative feedback. The diagram below shows the negative Insulin and glucagon feedback loop of how blood glucose is controlled. a Key terms for blood glucose explained Glycogenesis (genesis means to make) This is the process of when excess glucose is converted to glycogen when blood glucose is higher than normal. This occurs in the liver. Glycogenolysis (lysis means to breakdown) This is the breakdown of glycogen back into glucose in the liver. This occurs when blood glucose is lower than normal. Gluconeogenesis (amino acids to glucose) This is the process of creating glucose from non- carbohydrate stores in the liver. This will occur if all glycogen has already been hydrolysed back into glucose and your body still needs more glucose. The above processes are controlled by the three hormones insulin, adrenaline and glucagon. 20 MISSESTRUCH 2021 HOMEOSTASIS 3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Key Terms Receptors Protein carriers Action of insulin MISSESTRUCH 2021 Activating enzymes Beta cells in the Islets of Langerhans detect if blood glucose is Second messenger model too high and will secrete insulin. Insulin will decrease blood glucose in the following ways: cAMP 1. Attaching to receptors on the surfaces of target cells. This changes the tertiary structure of the channel proteins resulting in more glucose being absorbed by facilitated diffusion. 2. More protein carriers are incorporated into cell membranes so that more glucose is absorbed from the blood into cells. 3. Activating enzymes involved in the conversion of glucose to glycogen. This results in glycogenesis in the liver. Action of glucagon Alpha cells in the Islets of Langerhans detect if blood glucose is too low and will secrete glucagon. Glucagon will increase blood glucose in the following ways: 1. Attaching to receptors on the surfaces of target cells (liver cells). 2. When glucagon binds it causes a protein to be activated into adenylate cyclase which catalyses the conversion of ATP into a molecule called cyclic AMP (cAMP). cAMP activates an enzyme, protein kinase, that can hydrolyse glycogen into glucose. 3. Activating enzymes involved in the conversion of glycerol and amino acids into glucose. 21 MISSESTRUCH 2021 HOMEOSTASIS 3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Key Terms Glucagon Adenylate cyclase The second messenger model MISSESTRUCH 2021 Protein kinase Second messenger model This is demonstrated below in the action of glucagon, but adrenaline also results in this second messenger model. cAMP Action of adrenaline If blood glucose is too low the adrenal glands will also secrete adrenaline. Adrenaline will increase blood glucose in the following ways: 1. Adrenaline attaches to receptors on the surfaces of target cells. This causes a protein (G protein) to be activated and to convert ATP into cAMP. 2. cAMP activates an enzyme that can hydrolyse glycogen into glucose. 3. This is known as the second messenger model of adrenaline and glucagon action because the process results in the formation of cAMP, which acts as a second messenger. 22 MISSESTRUCH 2021  HOMEOSTASIS 3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Key Terms Collecting duct Glomerular filtrate Diabetes MISSESTRUCH 2021 Proximal convoluted tubule Diabetes is a disease when blood glucose concentration cannot Loop of Henle be controlled naturally. Distal convoluted tubule Type I diabetes is due to the body being unable to produce insulin, it starts in childhood and could be the result of an autoimmune disease where the beta cells are attacked. Treatment involves an injection of insulin. Type II diabetes is due to receptors on the target cells losing their responsiveness to insulin, which usually develops in adults because of obesity and poor diet. It is controlled by regulating the intake of carbohydrates, increasing exercise and insulin injections. Osmoregulation and the nephron Osmoregulation is the process of controlling the water potential of the blood. The nephron is the structure in the kidney where the blood is filtered, and useful substances are reabsorbed into the blood. Nephron Structure The Bowman’s (renal) capsule - Ultrafiltration occurs here, as the afferent arteriole (entering the glomerulus) is wider than the efferent arteriole (leaving the glomerulus) creating high hydrostatic pressure. Small molecules and water are forced out of the capillaries into the renal capsule creating the glomerular filtrate. Large proteins and blood cells remain in the blood. Proximal convoluted tubule (PCT) - The walls are made of microvilli epithelial cells to provide a large surface area for the diffusion of glucose into the cells from the PCT. Glucose is then actively transported out of the cells into the intercellular space to create a concentration gradient. Glucose can then diffuse into the blood again. Loop of Henle- Sodium ions are actively transported out of the ascending limb into the medulla to create a low water potential. Water moves out of the descending limb and out of the distal convoluted tubule and collecting duct by osmosis due to this water potential gradient. 23 MISSESTRUCH 2021  HOMEOSTASIS 3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Key Terms Water potential ADH Osmoregulation MISSESTRUCH 2021 Hypothalamus Blood with too low a water potential (hypertonic) could be Posterior pituitary gland due to loss of water from sweating, not drinking enough water Osmoreceptors and lots of ions in food and drink. The corrective mechanism is that more water is reabsorbed by osmosis into the blood from the tubules of the nephrons. This means the urine is more concentrated as less water is lost in the urine. Blood with too high a water potential (hypotonic) could be due to drinking too much water and a lack of ions in the diet. The corrective mechanisms are that less water is reabsorbed by osmosis into the blood from the tubules of the nephrons. This means the urine is more dilute and more water is lost in urine. The role of the hypothalamus and the posterior pituitary gland Changes in the water potential of the blood are detected by osmoreceptors found in the hypothalamus. The hypothalamus is also where the antidiuretic hormone (ADH) is produced. ADH then moves to the posterior pituitary gland and from here it is released into capillaries and into the blood. ADH travels through the blood to its target organ, the kidney. If the water potential of the blood is too low water leaves the osmoreceptors by osmosis and they shrivel. This stimulates the hypothalamus to produce more of the hormone ADH. If the water potential of the blood is too high, water enters the osmoreceptors by osmosis. This stimulates the hypothalamus to produce less ADH. ADH ADH When ADH reaches the kidney, it causes an increase in the permeability of the walls of the collecting duct and distal convoluted tubule to water. This means more water leaves the nephron and is reabsorbed into the blood, so urine is more concentrated. 24 MISSESTRUCH 2021 HOMEOSTASIS 3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE Key Terms Aquaporins Phosphorylase enzyme Aquaporins MISSESTRUCH 2021 Protein channels There are receptors on the cell membranes of the DCT and Distal convoluted tubules collecting duct to which ADH binds. When bound, it activates a Reabsorbed phosphorylase enzyme in the cells. Phosphorylase causes the vesicles containing aquaporins to fuse with the cell membrane and this results in more embedded aquaporins. Aquaporins are protein channels for water to pass through. With more aquaporins in the cell membrane, more water leaves the DCT and collecting tubule and is reabsorbed into the blood. Negative feedback example Blood water potential control is an example of homeostasis and negative feedback. The diagram below shows the negative feedback loop for this. 25 MISSESTRUCH 2021 HOMEOSTASIS 3.6 STIMULI ARE DETECTED & LEAD TO A RESPONSE MISSESTRUCH 2021 Key points Homeostasis in mammals is maintaining the internal environment within restricted limits through control mechanisms. Negative feedback is the restoration of systems to their original level e.g water potential of blood. Blood glucose levels are detected by alpha and beta cells in the Islets of Langerhans. If blood glucose levels are too high, insulin is released and if they are too low, glucagon is released. Osmoregulation is an example of homeostasis. It is the control of the water potential of the blood. The hypothalamus, posterior pituitary gland and antidiuretic hormone (ADH) all work together to coordinate in osmoregulation. Essay Links: The importance of homeostasis in terms of temperature and pH links to enzyme and protein structure and function. The importance of homeostasis in terms of water potential links to osmosis and cells. The function of the nephron links to active transport, osmosis and diffusion. Blood glucose control links to respiration. The role of ADH and aquaporins links to plasma membranes and channel proteins. 26 MISSESTRUCH 2021 Image Credits https://commons.wikimedia.org/wiki/File:Phototropism_Diagram.svg https://www.needpix.com/photo/1136211/earthworms-the-frogs-perspective https://commons.wikimedia.org/wiki/File:Skin_proprioception.jpg http://learn.neurotechedu.com/the_nervous_system/ https://commons.wikimedia.org/wiki/File:Three_Main_Layers_of_the_Eye.png https://commons.wikimedia.org/wiki/File:1414_Rods_and_Cones.jpg https://commons.wikimedia.org/wiki/File:Cone2.svg https://commons.wikimedia.org/wiki/File:Heart_diagram-en.svg https://commons.wikimedia.org/wiki/File:2032_Automatic_Innervation.jpg https://commons.wikimedia.org/wiki/File:Heart_diagram-en.svg https://commons.wikimedia.org/wiki/File:2032_Automatic_Innervation.jpg https://commons.wikimedia.org/wiki/File:Neuron.svg https://commons.wikimedia.org/wiki/File:Sodium-potassium_pump_and_diffusion.png https://commons.wikimedia.org/wiki/File:Action_potential.svg https://commons.wikimedia.org/wiki/File:Neuron.svg https://commons.wikimedia.org/wiki/File:Sodium-potassium_pump_and_diffusion.png https://en.wikipedia.org/wiki/Myelin https://commons.wikimedia.org/wiki/File:Action_potential_propagation_animation.gif https://commons.wikimedia.org/wiki/File:Action_potential_propagation_in_unmyelinated_axon.gif https://commons.wikimedia.org/wiki/File:Propagation_of_action_potential_along_myelinated_nerve_fiber_en.svg https://commons.wikimedia.org/wiki/File:Cholinergic_synapse.svg https://commons.wikimedia.org/wiki/File:Neuromuscular_junction.svg https://commons.wikimedia.org/wiki/File:Synapse_Illustration2_tweaked.svg https://commons.wikimedia.org/wiki/File:Temporal_summation.JPG https://commons.wikimedia.org/wiki/File:Interneuronalrelations.jpg https://commons.wikimedia.org/wiki/File:Cholinergic_synapse.svg https://commons.wikimedia.org/wiki/File:Neuromuscular_junction.svg https://commons.wikimedia.org/wiki/File:Synapse_Illustration2_tweaked.svg https://commons.wikimedia.org/wiki/File:Temporal_summation.JPG https://commons.wikimedia.org/wiki/File:Interneuronalrelations.jpg https://commons.wikimedia.org/wiki/File:Blausen_0801_SkeletalMuscle.png https://commons.wikimedia.org/wiki/File:Figure_38_04_03.jpg https://courses.lumenlearning.com/cuny-csi-ap-1/chapter/muscular-levels-of-organization/ https://commons.wikimedia.org/wiki/File:Transverse_sections_through_part_of_a_myofibril.jpg https://commons.wikimedia.org/wiki/File:%D7%9E%D7%91%D7%A0%D7%94_%D7%94%D7%9E%D7%95%D7%9C%D7%A7%D7%95%D7%9C%D7%94_-_Sliding_filament.gif https://commons.wikimedia.org/wiki/File:Sarcomere.svg https://commons.wikimedia.org/wiki/File:1008_Skeletal_Muscle_Contraction.jpg https://en.wikipedia.org/wiki/File:Pancreatic-Model-of-Exocrine-and-Endocrine-Function-Locations.jpg https://commons.wikimedia.org/wiki/File:Adrenal_gland_blood_supply.png https://www.needpix.com/photo/1195839/liver-organ-anatomy-free-pictures-free-photos-free-images-royalty-free https://www.flickr.com/photos/163837264@N07/30765227058 https://commons.wikimedia.org/wiki/File:Kidney_Nephron.png https://upload.wikimedia.org/wikipedia/commons/2/2e/2611_Blood_Flow_in_the_Nephron.jpg https://upload.wikimedia.org/wikipedia/commons/1/1c/1806_The_Hypothalamus-Pituitary_Complex.jpg https://commons.wikimedia.org/wiki/File:2709_ADH.jpg https://commons.wikimedia.org/wiki/File:Figure_41_03_04.jpg https://commons.wikimedia.org/wiki/File:2625_Aquaporin_Water_Channel.jpg https://commons.wikimedia.org/wiki/File:2710_Aquaporins-01.jpg MISSESTRUCH 2021