Homeostasis in Mammals PDF

## Homeostasis ### 14.1 Homeostasis in mammals On these pages you will learn to: - Discuss the importance of homeostasis in mammals and explain the principles of homeostasis in terms of internal and external stimuli, receptors, central control, coordination systems, effectors (muscles and glands)...

## Homeostasis ### 14.1 Homeostasis in mammals On these pages you will learn to: - Discuss the importance of homeostasis in mammals and explain the principles of homeostasis in terms of internal and external stimuli, receptors, central control, coordination systems, effectors (muscles and glands) - Define the term 'negative feedback' and explain how it is involved in homeostatic mechanisms - Outline the roles of the endocrine system in coordinating homeostatic mechanisms **As species of organisms evolved from simple, single-celled organisms into complex, multicellular ones, these organisms evolved to perform a specialist function.** With specialisation in one function came the loss of the ability to perform other functions. Different groups of cells each carried out their own function, and this made the cells dependent on each other. Cells specialising in reproduction, for example, depend on different groups of cells for their survival: one group to obtain oxygen for their respiration, another to provide glucose and another to remove their waste products. This means that the more complex, multicellular organisms have a division of labour that is seen in tissues, organs and organ systems. These different functional systems must be coordinated if they are to perform efficiently. **There are two forms of coordination in most multicellular animals: nervous and endocrine.** The nervous system allows rapid communication between specific parts of an organism. The endocrine system usually provides a slower, less specific form of communication. Both systems need to work together. The increased complexity of multicellular organisms meant the development of an internal environment at the same time. This internal environment is made up of extracellular fluids that bathe each cell, supplying nutrients and removing wastes. By maintaining this fluid at levels which suit the cells, the cells are protected from the changes that affect the external environment and so give the organism a degree of independence. **What is homeostasis?** Homeostasis is the **maintenance of a constant state**. More specifically, it refers to the internal environment of organisms and involves maintaining the chemical make-up, volume and other features of blood and tissue fluid within narrow limits, sometimes called **normal ranges**. Homeostasis ensures that the cells of the body are in an environment that meets their needs and allows them to function normally despite external changes. This does not mean that there are no changes - on the contrary, there are continuous fluctuations brought about by variations in internal and external conditions. These changes, however, occur around a set point. Homeostasis is the ability to return to that set point and so maintain organisms in a balanced equilibrium. **The importance of homeostasis** Homeostasis is essential for the proper functioning of organisms because: - **The enzymes that control the biochemical reactions within cells, and other proteins such as membrane channel proteins, are sensitive to changes in pH and temperature**. Any change to these factors reduces the efficiency of enzymes or may even prevent them working altogether, e.g. may denature them. Changes to membrane proteins may mean that substances cannot be transported into or out of cells. - **Changes to the water potential of the blood and tissue fluids may cause cells to shrink and expand** (even to bursting point) owing to water leaving or entering by osmosis. In both situations the cells cannot operate normally. ### 14.2 Excretion and kidney structure On these pages you will learn to: - Describe the deamination of amino acids and outline the formation of urea in the urea cycle - Describe the gross structure of the kidney **Excretion is the removal of the waste products of metabolism from the body.** This is distinct from elimination (egestion), which is the removal of substances such as dietary fibre that have never been involved in the metabolic activities of cells. **Excretory substances** An adult human produces about 500 dm³ of carbon dioxide and 400 cm³ of water each day as a result of respiration. Other excretory products include bile pigments and mineral salts as well as the nitrogenous excretory product urea CO(NH₂)₂. **Urea is used as the nitrogenous excretory product of organisms that have some access to water, but not in large volumes, e.g. animals living on land, such as mammals. Urea is produced in the liver from excess amino acids in three stages:** - **Amino groups (NH₂) are removed from the amino acids in a process called deamination and made into ammonia.** - **The remainder of the amino acid can be respired to give ATP**. - **The ammonia is converted to urea by the addition of carbon dioxide in a pathway called the ornithine cycle.** ATP is required for this process. **Other excretory substances** - **Ammonia (NH₃)** is the easiest product to form from the amino groups (NH₂) produced when amino acids are oxidised. Its production requires no ATP and it is very soluble in water and so is easily dissolved and washed out of the body. It is, however, extremely poisonous - 800 times more so than carbon dioxide. Only organisms such as freshwater fish with access to large volumes of water are therefore able to use ammonia as their nitrogenous excretory product. - **Uric acid** is almost insoluble in water and cannot diffuse into cells, making it hardly poisonous at all. However, it takes seven ATP molecules to produce it. As almost no water is needed for its removal, it is used by organisms living in very dry conditions. As it is low in mass when stored it is also used by flying organisms. Animals such as birds and reptiles that lay eggs have an additional reason for using it - to remove their nitrogenous waste. As the young develop within the egg, their wastes cannot be removed and so anything more toxic than uric acid would kill them. ### 14.3 The structure of the nephron On these pages you will learn to: - Describe the detailed structure of the nephron with its associated blood vessels using photomicrographs and electron micrographs - Describe the gross structure of the kidney **The nephron is the functional unit of the kidney**. It is a narrow tube, closed at one end, with two twisted regions separated by a long hairpin loop. Each nephron is made up of a: - **Renal (Bowman's) capsule** - the closed end at the start of the nephron. It is cup-shaped and contains within it a mass of blood capillaries known as the glomerulus. Its inner layer is made up of specialised cells called podocytes. - **Proximal (first) convoluted tubule** - a series of loops surrounded by blood capillaries. Its walls are made of cuboidal epithelial cells with microvilli. - **Loop of Henlé** - a long, hairpin loop that extends from the cortex into the medulla of the kidney and back again. It is surrounded by blood capillaries. - **Distal (second) convoluted tubule** - a series of loops surrounded by blood capillaries. Its walls are made of cuboidal epithelial cells, but it is surrounded by fewer capillaries than the proximal tubule. - **Collecting duct** - a tube into which a number of distal convoluted tubules empty. It is lined by cuboidal epithelial cells and becomes increasingly wide as it empties into the pelvis of the kidney. **Associated with each nephron are a number of blood vessels:** - **Afferent arteriole** - a tiny vessel that is a branch of the renal artery and supplies the nephron with blood. The afferent arteriole enters the renal capsule of the nephron where it forms the - **Glomerulus** - a many-branched knot of capillaries from which fluid is forced out of the blood. - **Efferent arteriole** - a tiny vessel that leaves the renal capsule. It has a smaller diameter than the afferent arteriole, which causes an increase in blood pressure within the glomerulus. The efferent arteriole carries blood away from the renal capsule and later branches to form the - **Peritubular capillaries** - a concentrated network of capillaries that surrounds the proximal convoluted tubule, the loop of Henlé and the distal convoluted tubule and from where they reabsorb mineral salts, glucose and water. The peritubular capillaries merge together into venules (tiny veins) that in turn merge together to form the renal vein. ### 14.4 Kidney function – ultrafiltration and selective reabsorption On these pages you will learn to: - Describe how the processes of ultrafiltration and selective reabsorption are involved with the formation of urine in the nephron **The functions of the kidney are listed in the table below.** The main one, that of regulating the composition of blood, is carried out by the nephrons in a series of stages - ultrafiltration, selective reabsorption and the reabsorption of water and minerals. | **Function** | **Description** | |---|---| | Regulating the composition of the blood and maintaining a constant water potential by: | Maintaining a constant volume of water, removing wastes such as urea, maintaining the concentration of mineral ions and other substances constant | | Regulating blood pressure | | | Maintaining the body's calcium level | | | Stimulating the production of red blood cells | | **Ultrafiltration** Blood enters the kidney through the renal artery, which branches frequently to give around one million tiny arterioles, each of which enters a renal (Bowman's) capsule of a nephron. This is called the afferent arteriole and it divides to give a complex of capillaries known as the glomerulus. The glomerular capillaries later merge to form the efferent arteriole, which then sub-divides again into capillaries (the peritubular capillaries), which wind their way around the various tubules of the nephron before combining to form the renal vein (see extension). The walls of the glomerular capillaries are made up of endothelial cells with pores between them. As the diameter of the afferent arteriole is greater than that of the efferent arteriole, there is a build up of hydrostatic pressure within the glomerulus. As a result, water, glucose, mineral ions and other substances up to a relative molecular mass of up to 68000 are squeezed out of the capillary to form the glomerular filtrate. The movement of this filtrate out of the glomerulus is resisted by the: - capillary endothelium - basement membrane of the epithelial layer of the renal (Bowman's) capsule - epithelial cells of the renal (Bowman's) capsule - the hydrostatic pressure of the fluid in the renal capsule space (the intracapsular pressure) - the low water potential of the blood in the glomerulus. This total resistance would be sufficient to prevent filtrate leaving the glomerular capillaries, but there are some modifications to reduce this barrier to the flow of filtrate: - The inner layer of the renal (Bowman's) capsule is made up of highly specialised cells called podocytes. These cells, which are illustrated in the image, are lifted off the surface membrane on little 'feet' ('podo' = feet). This allows filtrate to pass beneath them and through gaps between their branches. Filtrate passes between these cells rather than through them. - The endothelium of the glomerular capillaries has spaces up to 100 nm wide between its cells. Again, fluid can therefore pass between, rather than through, these cells. As a result, the hydrostatic pressure of the blood in the glomerulus is high enough to overcome the resistance and so filtrate passes from the blood into the renal capsule. This filtration under pressure is known as ultrafiltration. The filtrate has much the same composition as blood plasma, with the exception of the plasma proteins which are too large to pass across the basement membrane. Many of the substances in the 125cm³ of filtrate passing out of blood each minute are extremely useful to the body and need to be reabsorbed. ### 14.5 Kidney function – loop of Henlé and reabsorption of water The loop of Henlé is a hairpin-shaped tubule that extends into the medulla of the kidney. It is responsible for creating the conditions in the surrounding interstitial fluid that lead to the reabsorption of water from the distal convoluted tubule and the collecting duct. This results in concentrating the urine so that it has a lower water potential than the blood. The concentration of the urine produced is directly related to the length of the loop of Henlé. It is short in mammals whose habitats are in or by water (e.g. beavers) and long in those whose habitats are dry regions (e.g. kangaroo rat). The loop of Henlé has two regions: - The descending limb, which is narrow, with thin walls that are highly permeable to water. - The ascending limb, which after a short distance is wider, with thick walls that are impermeable to water. The **loop of Henlé acts as a counter-current multiplier**. To understand how this works it is necessary to consider the following sequence of events using the image, to which the numbers refer. 1. Sodium and chloride ions are actively pumped out of the ascending limb of the loop of Henlé using ATP provided by the many mitochondria in the cells of its wall. 2. This creates a low water potential (high ion concentration) in the region of the medulla between the two limbs (called the interstitial region). In normal circumstances water would pass out of the ascending limb by osmosis. However, the thick walls are almost impermeable to water and so very little, if any, escapes. 3. The walls of the descending limb are, however, very permeable to water and as the cells have many membrane protein channels known as aquaporins it passes out of the filtrate, by osmosis, into the interstitial space. This water enters the blood capillaries in this region by osmosis and is carried away. 4. The filtrate progressively loses water in this way as it moves down the descending limb lowering its water potential. It reaches its minimum water potential at the tip of the hairpin. 5. At the base of the ascending limb, sodium and chloride ions diffuse out of the filtrate and as it moves up the ascending limb these ions are also actively pumped out (see point 1) and therefore the filtrate develops a progressively higher water potential. 6. In the interstitial space between the ascending limb and the collecting duct there is a gradient of water potential with the highest water potential in the cortex and an increasingly lower water potential the further into the medulla one goes (see Topic 14.6). 7. The collecting duct is permeable to water and so, as the filtrate moves down it, water passes out of it by osmosis. This water passes by osmosis into the blood vessels that occupy this space, and is carried away (see Topic 14.6). 8. As water passes out of the filtrate its water potential is lowered. However, the water potential is also lowered in the interstitial space and so water continues to move out by osmosis down the whole length of the collecting duct. The counter-current multiplier ensures that there is always a water potential gradient drawing water out of the tubule. The water that passes out of the collecting duct by osmosis does so through aquaporins (water channels). The hormone ADH (Topic 14.6) can alter the number of these channels and so control water loss. By the time the filtrate, now called urine, leaves the collecting duct on its way to the bladder, it has lost most of its water and so it has a lower water potential than the blood. **The distal (second) convoluted tubule** The cells that make up the walls of the distal (second) convoluted tubule have microvilli and many mitochondria that allow them to reabsorb material rapidly from the filtrate, by either diffusion or active transport. The main role of the distal tubule is to make final adjustments to the water and salts that are reabsorbed and to control the pH of the blood by selecting which ions to reabsorb. To achieve this, the permeability of its walls becomes altered under the influence of various hormones (Topic 14.6). A summary of the processes taking place in the nephron is given in the image. **Counter-current multiplier** When two liquids flow in opposite directions past one another, the exchange of substances (or heat) between them is greater than if they flowed in the same direction next to each other. In the case of the loop of Henlé, the counter-current flow means that the filtrate in the collecting duct with a lower water potential meets interstitial fluid that has an even lower water potential. This means that, although the water potential gradient between the collecting duct and interstitial fluid is small, it exists for the whole length of the collecting duct. There is therefore a steady flow of water into the interstitial fluid, so that around 80% of the water enters the interstitial fluid and hence the blood. If the two flows were in the same direction (parallel) less of the water would enter the blood. ### 14.6 Control of water and solute concentration of the blood On these pages you will learn to: - Describe the roles of the hypothalamus, posterior pituitary, ADH and collecting ducts in osmoregulation The quantity of water and salts we take in varies from day to day, as does the quantity we lose. The table below shows the daily balance between loss and gain of salts and water for a typical human. The blood, however, needs to have a constant volume of water and concentration of salts to avoid osmotic disruption to cells. The homeostatic control of water and solute concentrations in the blood is achieved by hormones that act on the distal (second) convoluted tubule and the collecting duct. | **Water** | **Volume of water / cm³ day^-1** | |---|---| | Water gain | Water loss | | Diet | 2300 | Urine | 1500 | | Metabolism, e.g. respiration | 200 | Expired Air | 400 | | | | Evaporation from skin | 350 | | | | Faeces | 150 | | | | Sweat | 100 | | **TOTAL** | **2500** | **TOTAL** | **2500** | | **Salt** | **Mass of salt/gday^-1** | | Salt gain | Salt loss | | Diet | 10.50 | Urine | 10.00 | | | | Faeces | 0.25 | | | | Sweat | 0.25 | | **TOTAL** | **10.50** | **TOTAL ** | **10.50** | As a result of facilitated diffusion and active transport by cells in the ascending limb of the loop of Henlé, sodium and chloride ions are concentrated in the interstitial fluid surrounding the distal convoluted tubule and the collecting duct. There is a gradient of water potential within this interestial region, with the highest water potential in the cortex and the lowest in the medulla region closest to the renal pelvis (see Topic 14.5, Extension, for further details). Some urea passes out of the filtrate in the collecting duct into the interstitial region, so further increasing the concentration of solutes and decreasing the water potential. At all points, the interstitial region has a lower water potential than the filtrate passing down the collecting duct. The **water potential of the blood is determined by the balance of water and salts within it**. A rise in solute concentration lowers its water potential. This may be caused by: - too little water being consumed - much sweating occurring - large amounts of salt being taken in (ingested). The body responds to this decrease in water potential as follows: - **Sensory cells called osmoreceptors in the hypothalamus of the brain detect the decrease in water potential.** - It is thought that, when the water potential of the blood is low, water is lost from these osmoreceptor cells by osmosis. - Owing to this water loss the osmoreceptor cells shrink, a change that stimulates the neurosecretory cells in the hypothalamus to produce a hormone called antidiuretic hormone (ADH). - ADH passes along the neurones (nerve cells) to the posterior pituitary gland, from where it is secreted into the capillaries. - ADH passes in the blood to the kidney, where it increases the permeability to water of the cell surface membrane of the cells that make up the walls of the distal (second) convoluted tubule and the collecting duct. - Receptors on the cell surface membrane of these cells bind to ADH molecules, activating a second messenger system within the cell (cyclic AMP - Topic 14.7). This results in the activation of a protein kinase, an enzyme that adds phosphate groups to other proteins to activate them. - The action of protein kinase causes vesicles within the cell to move to, and fuse with, its cell surface membrane. (ADH binding also leads to an increase in transcription of the gene coding for the aquaporin protein, increasing the number of available aquaporins.) - When the vesicles fuse with the cell surface membrane, the number of aquaporins in the membrane increases greatly, making the cell surface membrane much more permeable to water. - As it is a small molecule, some urea can cross the phospholipid bilayer of the membrane. ADH binding also leads to an increase in membrane transport proteins for urea, so that the collecting duct becomes more permeable to urea and some will leave the filtrate to further decreases the water potential of the interstitial region. - The combined effect is that more water leaves the collecting duct by osmosis down a water potential gradient and re-enters the blood. - As the reabsorbed water came from the blood in the first place, this will not, in itself, increase the water potential of the blood, but merely prevent it from decreasing any further. Therefore the osmoreceptors also stimulate the thirst centre of the brain, to encourage the individual to seek out and drink more water. - The osmoreceptors in the hypothalamus detect the increase in water potential and ADH secretion from the posterior pituitary is reduced. - The decrease in ADH concentration in the blood will lead to a decreased permeability of the collecting duct to water and urea so that the permeability returns to its former state. This is an example of homeostasis and the principle of negative feedback (Topic 14.1). ### 14.7 Hormones and the endocrine glands On these pages you will learn to: - Outline the role of cyclic AMP as a second messenger with reference to the stimulation of liver cells by adrenaline and glucagon - Describe the three main stages of cell signalling in the control of blood glucose by adrenaline as follows: - hormone-receptor interaction at the cell surface - formation of cyclic AMP, which binds to kinase proteins - an enzyme cascade involving activation of enzymes by phosphorylation to amplify the signal **Mammals possess two main coordinating systems - the nervous system that communicates rapidly, and the endocrine system that usually does so more slowly**. Both systems interact in order to maintain a constant internal environment, at the same time being responsive to a varying external environment. Both systems also use chemical messengers - the endocrine system exclusively so, and the nervous system through the use of neurotransmitters in chemical synapses. The chemical messenger adrenaline may act both as a hormone and a neurotransmitter. **Hormones** A hormone is a regulating chemical produced and secreted by an endocrine gland and is carried in the blood to the cells, tissues or organ on which it acts - known as the target cell (target tissue, target organ) - that have complementary receptors on their cell surface membranes or their internal membranes. Hormones may differ chemically from one another, but they share many common characteristics. Some hormones have their action at the cell surface and other hormones are able to enter the cell to have an effect. **Hormones are:** - effective in very small quantities, but often have widespread and permanent effects - normally relatively small molecules - often proteins or polypeptides, although some are steroids - transported by the blood system - produced by endocrine glands. **Mechanisms of hormone action** One mechanism of hormone action is called the cyclic AMP second messenger system. An example of this system occurs in the stimulation of liver cells by the hormones adrenaline and glucagon, leading to the conversion of glycogen to glucose. The process involving adrenaline is shown in the image, and has three stages: 1. Adrenaline binds to its complementary receptor on the cell surface membrane of a liver cell. Binding activates a membrane protein, termed a G protein. 2. The G protein activates another membrane protein, an enzyme called adenylate cyclase. The activated enzyme converts ATP to cyclic AMP (the second messenger). Cyclic AMP binds to and activates a kinase protein. 3. There follows an enzyme cascade in which enzymes are activated by phosphorylation. This amplifies the first signal as one enzyme molecule can catalyse the phosphorylation of many other enzyme molecules. The last enzyme in the chain catalyses the breakdown of glycogen to glucose. The glucose diffuses out of the liver cell and into the blood, through protein carriers called transporter proteins ### 14.8 Regulation of blood glucose On these pages you will learn to: - Explain how the blood glucose concentration is regulated by negative feedback control mechanisms, with reference to insulin and glucagon Glucose is the main substrate for respiration, providing the source of energy for almost all organisms. It is therefore essential that the blood of mammals contains a relatively constant concentration of glucose for respiration. If it falls too low, the energy supply in cells will be too low and the cells will die - brain cells are especially sensitive in this respect because they can only respire glucose. If the concentration rises too high, it decreases the water potential of the blood and creates osmotic problems that can cause dehydration and be equally dangerous. Homeostatic control (Topic 14.1) of blood glucose concentration is therefore essential. **Blood glucose and variations in its concentration** The normal concentration of blood glucose is 90 mg in each 100cm³ of blood. There are three sources of blood glucose: - Directly from the diet as glucose from the breakdown of other carbohydrates such as starch, maltose, lactose and sucrose. - Breakdown of glycogen (glycogenolysis) from the stores in the liver and muscle cells. A normal liver contains 75-100g of glycogen, made by converting excess glucose from the diet in a process called glycogenesis. - Gluconeogenesis is the production of new glucose, i.e. from sources other than carbohydrate. The liver, for example, can make glucose from glycerol and amino acids. As animals may not eat continuously as their diet varies, their intake of glucose fluctuates. Likewise, glucose is used up at different rates depending on the level of mental and physical activity. With changes in the supply and demand of glucose the concentration of glucose in the blood fluctuates. Three main hormones, insulin, glucagon and adrenaline operate to maintain a constant blood glucose concentration. **Insulin and the ß cells of the pancreas** We say in Topic 14.7 that in the pancreas there are groups of special cells known as the islets of Langerhans. These cells are of two types: larger alpha (a) cells and smaller beta (β) cells. The ẞ cells detect and respond to a rise in blood glucose concentration by secreting the hormone insulin directly into the blood. Insulin is a globular protein made up of 51 amino acids. **Almost all body cells (but not red blood cells) have glycoprotein receptors on their membranes that bind with insulin molecules.** Binding of insulin leads to an increase in membrane permeability and enzyme action so that the blood glucose concentration is lowered in one or more of the following ways: - Cellular respiratory rate is increased, using up more glucose and increasing its uptake by cells. - The rate of conversion of glucose into glycogen (glycogenesis) is increased in the cells of the liver and muscles. - The rate of conversion of glucose to fat in adipose tissue is increased. - The rate of absorption of glucose into cells increases, especially in muscle cells. **Glucagon and the a cells of the pancreas** The a cells of the islets of Langerhans respond to a fall in blood glucose by secreting the hormone glucagon directly into the blood. Only the cells of the liver have receptors that bind to glucagon, so only liver cells respond, by activating the enzyme phosphorylase, which converts glycogen to glucose, and by increasing the conversion of amino acids and glycerol into glucose. The overall effect is therefore to increase the quantity of glucose in the blood and return it to its normal concentration. This increase in the blood glucose concentration causes the a cells to reduce the secretion of glucagon. **Adrenaline and other hormones regulating the blood glucose level** There are at least four other hormones besides glucagon that can increase blood glucose level. The best known of these is adrenaline. At times of excitement or stress, adrenaline is produced by the adrenal glands that lie above the kidneys. It causes the breakdown of glycogen in the liver, raising the blood glucose concentration. If the glycogen supplies in the liver are used up, the adrenal glands produce the hormone cortisol, which causes the liver to convert amino acids and glycerol into glucose. **Hormone interaction in regulating blood sugar** The two hormones, insulin and glucagon, act in opposite directions. Insulin lowers blood glucose concentration, whereas glucagon increases it. The two hormones are said to act antagonistically. The system is self-regulating because the concentration of glucose in the blood determines the quantity of insulin and glucagon produced. In this way the interaction of these two hormones allows highly sensitive control of the blood glucose concentration. The concentration of glucose is, however, not constant, but fluctuates around a set point. This is because of the way negative feedback mechanisms work. Only when the blood glucose concentration falls below the set point is insulin secretion reduced (negative feedback), leading to a rise in blood glucose. In the same way, only when the concentration exceeds the set point is glucagon secretion reduced (negative feedback), causing a fall in the blood glucose concentration. The control of blood glucose level is summarised in the image. ### 14.9 Biosensors On these pages you will learn to: - Explain the principles of operation of dip sticks containing glucose oxidase and peroxidase enzymes, and biosensors that can be used for quantitative measurements of glucose in blood and urine - Explain how urine analysis is used in diagnosis with reference to glucose, protein and ketones **Biosensors are devices that use immobilised biological molecules, such as enzymes or antibodies, or biological systems, such as whole cells, to detect a specific chemical and in most cases, measure the concentration of the chemical**. In its simplest form, a biosensor can be a simple dipstick, but commonly biosensors are now taken to mean those devices that are coupled with microelectronics so that results are rapid, the measurements are extremely accurate, and the chemical can be detected in very small quantities. **Detecting glucose in blood and urine** As explained in Topic 14.8, it is important for healthy functioning that blood glucose concentrations remain at or around a normal concentration of 90mg 100cm³ blood. If concentrations rise too high, glucose is excreted in the urine. Simple dipsticks or more complex biosensors are frequently used by people with diabetes to measure the concentration of glucose in the blood or urine. For people with type I diabetes (a lack of insulin) a check on glucose concentration will help to inform them if they need to adjust their insulin doses. Health care professionals routinely use dipsticks as a quick and reliable method to detect the abnormal presence of glucose in urine. The method of detection of glucose makes use of immobilised enzymes (see Topic 3.4). For both dipsticks and biosensors the same enzymes can be used: - Glucose oxidase catalyses the conversion of glucose to gluconic acid and hydrogen peroxide: Glucose + O₂ gluconic acid + H₂O₂ - Peroxidase catalyses the breakdown of hydrogen peroxide to water and oxygen: 2H₂O₂ 2H₂O + O₂ **Dipsticks to detect glucose** Glucose dipsticks are a convenient way to detect glucose in a sample of urine. They involve a colour change, which could be a change in the intensity of colour, to give a semi-quantitative measure of glucose concentration. The colour change can be compared to a coloured standards chart and an estimate of concentration of glucose can be obtained. The dipsticks are highly specific and detect only glucose. To obtain a colour change, a colourless hydrogen donor is used to react with the oxygen released in the peroxidase reaction shown above. The hydrogen donor acts as a chromogen, changing colour by being oxidised by the oxygen that is given off. 2H₂O₂ + DH₂ → 2H₂O + D **The dipstick is a thin strip of absorbent paper** (some dipsticks are a thin strip of plastic with a paper pad stuck on at one end). At the test end of the strip the chromogen and the enzymes glucose oxidase and peroxidase are added. A thin cellulose membrane covers the area so that only small molecules such as glucose can enter the test area. The end of the dipstick is dipped into the urine sample and after a set time any colour change can be compared to the chart. If glucose is present in the urine, the action of the two enzymes will result in a colour change. **Biosensors to detect glucose** Biosensors are extremely sensitive and accurate. As glucose oxidase is a highly specific enzyme, biosensors used to detect glucose in blood samples are insensitive to other chemicals present. They can also be re-used, so are cost effective and also have the advantage of being small and portable so that they carried by a person and used at any time. In a glucose biosensor, the enzymes glucose oxidase is immobilised onto an inert supporting material to form a biological recognition layer. The layer is separated from the blood sample by a partially permeable membrane that only allows small molecules such as glucose to diffuse through. Glucose molecules present in the blood will bind to the active sites of the glucose oxidase enzymes and the reaction results in the production of gluconic acid and hydrogen peroxide, as shown above. The next part of the biosensor detects that a reaction has occurred and converts this into an electric current. This conversion is carried out by a transducer. In some biosensors the decrease in oxygen can be detected by a platinum oxygen electrode. In others, the production of hydrogen ions (from the gluconic acid produced) can be detected. The final part of the biosensor is the amplification of the electrical signal and the production of a digital reading. The reading is proportional to the reaction that has occurred (for example, proportional to the decrease in oxygen, or the increase in hydrogen ions) so it is proportional to the concentration of glucose in the sample. **Urine analysis** Dipsticks can be used to estimate the quantity of substances in urine. The results provide valuable information about the body's metabolism and enable doctors to make medical diagnoses. Among the substances that can be identified are: - **Proteins**, which can be detected using the albustix test. As proteins are large molecules, they do not normally leave the glomerulus during ultrafiltration in the kidneys. Their presence in urine (proteinuria) indicates that they are being forced out of the glomerulus. This might be due to high blood pressure (hypertension). Damage to kidneys due to high blood pressure, diabetes or infection could also result in protein being present in urine. Urinary tract infections can also lead to the presence of protein in urine. - **Glucose**, which can be detected using a dipstick test such as the Diastix test. Glucose is normally reabsorbed in the proximal convoluted tubules of the kidney. Its presence in urine (glucosuria) could indicate diabetes. - **Ketones**, which are produced when fatty acids, rather than glucose, are being used as a respiratory substrate. Their presence suggests that the supply of glucose is exhausted. This might be the result of starvation or, where a person is diabetic, inadequate control of the condition. ### 14.10 Homeostasis in plants On these pages you will learn to: - Explain that stomata have daily rhythms of opening and closing and also respond to changes in environmental conditions to allow diffusion of carbon dioxide and regulate water loss by transpiration - Describe the structure and function of guard cells and explain the mechanism by which they open and close stomata - Describe the role of abscisic acid in the closure of stomata during times of water stress **Plants, as well as animals, have homeostatic mechanisms to ensure that they maintain a constant level of essential materials for their needs.** Carbon dioxide uptake and water loss are two examples of processes that must be regulated. This regulation is achieved by controlling the opening and closure of structures called stomata. To take in carbon dioxide from the external atmosphere, the stomata must open. However, open stomata mean water loss. Plants overcome this problem by having daily rhythms of opening and closing their stomata. To obtain carbon dioxide, they open in the light, when photosynthesis is occurring. This means that water loss is an inevitable consequence of photosynthesis. They then close their stomata at night, when there is no need for carbon dioxide, and so minimise water loss. Stomata can also respond to changes in the environment to regulate diffusion of carbon dioxide and water vapour loss by transpiration. **Stomata** Stomata are minute pores that occur mainly on the leaves, especially the underside (lower epidermis). Each stoma (singular) is surrounded by a pair of special, kidney-shaped cells called **guard cells**. When a stoma is open, these cells surround a small opening a few micrometres wide called the **stomatal pore**. Unlike other epidermal cells, guard cells have chloroplasts and dense cytoplasm. The inner cell walls of the guard cells are thicker and less elastic than the outer ones. This means that any increase in the volume of the guard cells, for example due to the osmotic intake of water, causes the outer wall to bend more than the inner wall and so widen the stomatal pore. To 'close' the stoma completely, the reverse occurs and the stomatal pore decreases in size until it is no longer present. In this way they can control the rate of gaseous exchange. **The mechanism of stomatal opening** It is known that stomata open and close in response to certain stimuli. For example, they usually open in the light and close in the dark. One suggested mechanism for the opening and closing of stomata is as follows: - a particular stimulus such as light activates ATP synthase, an enzyme that increases the production of ATP by the chloroplasts in the guard cells - these chloroplasts only have photosystem I and no Calvin cycle enzymes, so ATP is produced in cyclic photophosphorylation but not used up in the Calvin cycle - this ATP is therefore available to provide more energy for the active transport of protons (H+) from the guard cells - the reduced H+ concentration and increased