Homeostasis and Excretion

• Homeostasis is the maintenance of a relatively constant internal environment for cells.
• Proper homeostasis ensures optimal conditions for enzyme action and cell function.
• Tissue fluid activity is affected by temperature, water potential, glucose concentration, and pH.

Negative Feedback Mechanism

• A change in a physiological factor beyond the set point serves as a stimulus.
• Stimuli, internal or external, are detected by receptors in the body.
• Receptors transmit impulses or information about the change via the nervous system to the central control, referred to as input.
• The central nervous system then directs an effector to act, called the output.
• Output example: insulin secretion with high glucose concentration or muscle contraction.
• The output reaction, or corrective action, corrects or maintains the detected change, bringing the physiological factor back to its set point.

Excretion

• Excretion is the removal of metabolic waste products, mainly carbon dioxide and urea.
• Carbon dioxide (CO2) is produced during aerobic respiration.
• Urea is produced in the liver from excess amino acids and then removed as urine.

Deamination

• Deamination is the breakdown of excess amino acids in the liver.
• An amino acid loses -2H + H2O to become a keto acid, which can be respired or converted to glucose or fat.
• The removed NH2 group combines with an extra Hydrogen (H).
• Ammonia (NH3) combines with carbon dioxide (CO2) to produce urea.

Structure of the Kidney

• Fibrous Capsule: Outer protective layer of the kidney.
• Cortex: Outer region of the kidney containing glomeruli and convoluted tubules.
• Medulla: Inner region of the kidney containing the loops of Henle and collecting ducts.
• Renal Pelvis: Funnel-shaped structure that collects urine from the kidney.
• Renal Artery & Renal Vein: Blood vessels supplying and draining the kidney.
• Ureter: Tube that carries urine from the kidney to the bladder.
• Glomerulus: Network of capillaries where filtration occurs.
• Bowman's Capsule: Cup-shaped structure surrounding the glomerulus.
• Proximal Convoluted Tubule (PCT): Portion of the nephron responsible for reabsorption.
• Loop of Henle: Section of the nephron that conserves water and salts.
• Distal Convoluted Tubule (DCT): Portion of the nephron involved in solute and water balance.
• Collecting Duct: Tube that collects urine from multiple nephrons.

Urine Formation

• Urine formation occurs in two steps: ultrafiltration and selective reabsorption.

Ultrafiltration

• Afferent arterioles carry blood to the Bowman's capsule.
• Efferent arterioles carry blood away from the Bowman's capsule.
• The renal artery (afferent and efferent arterioles) forms the glomerulus, which sits within the Bowman's capsule.
• The diameter of the renal artery narrows at the glomerulus, increasing blood pressure.
• Hydrostatic pressure pushes molecules (60-80nm) out of the capillary endothelium into Bowman's capsule, forming the filtrate.
• The filtrate passes through the basement membrane, which filters out large protein molecules (69000 Mr), RBCs, WBCs, and platelets using a network of collagen and glycoprotein.
• Filtrate reaches the epithelium of Bowman's Capsule composed of podocyte cells with finger-like projections and gaps in between to filter into the lumen.
• This filtrate is now called glomerular filtrate.
• Glomerular filtrate: Amino acids, water, glucose, urea, and inorganic ions (mainly Na+, K+, and Cl-).
• Glomerular Filtration Rate: 125cm3 min-1.
• Ultrafiltration occurs due to water potential differences between plasma in glomerular capillaries and filtrate in Bowman's capsule.
• Pressure increases the water potential of blood plasma in glomerular capillaries, causing water to move into Bowman's capsule.
• High solute concentration in the blood capillary makes water move from the Bowman's capsule into the glomerular capillaries.
• Overall, the pressure gradient is the key factor for water transfer from blood into Bowman's capsule.

Selective Reabsorption

• Important substances from the glomerular filtrate are reabsorbed into the blood as filtrate passes through the nephron.
• Glucose reabsorption: Proximal convoluted tubule (PCT).
• Water and salts reabsorption: Loop of Henle and collecting duct.
• PCT lining consists of epithelial cells adapted for reabsorption.
• Adaptation of PCT: Microvilli to increase surface area
• Adaptation of PCT: Co-transporter proteins for solute transport.
• Adaptation of PCT: High number of mitochondria to provide ATP for protein pumps.
• Adaptation of PCT: Tightly packed cells to prevent filtration.
• Blood capillaries are close to the outer surface of the proximal convoluted tubule.
• Basal membranes of proximal convoluted tubule epithelial cells are closest to the blood capillaries.
• Sodium/Potassium Pumps use ATP to actively pump sodium (Na+) ions out of the epithelial cell and into the blood.
• Lower Na+ concentration in the epithelial cell causes Na+ ions in the filtrate to diffuse into the cell through the luminal membranes.
• Na+ ions do not diffuse freely but use co-transporters and transport another solute (e.g., aa/glucose).
• After the solute is inside the epithelial cells, they diffuse down their concentration gradients and enter the blood via transport proteins in the basal membranes.
• The movement of solutes from the proximal convoluted tubule into capillaries raises water potential and causes water to move into the blood by osmosis.

Osmoregulation

• Osmoregulation: Blood and tissue fluid by controlling water content and ion concentration (Na+).
• Osmoreceptors: Hypothalamus detects decrease in water potential of the blood, triggering nerve impulses to the posterior pituitary gland, stimulating ADH release.
• ADH enters the blood and increases water reabsorption in the kidneys by reducing water loss in urine.

Effect of ADH in the Kidney

• Water reabsorption occurs by osmosis.
• This reabsorption occurs at the collecting ducts.
• ADH increases the number of aquaporins in the luminal membranes of the collecting duct cells.
• Collecting duct cells contain vesicles containing aquaporins.
• ADH binds to receptor proteins in the cell membrane of cells lining the collecting duct.
• The binding leads to the production of cyclic AMP (cAMP), which is a secondary messenger, activating a signalling cascade to phosphorylate aquaporin molecules.
• Activation of aquaporin causes the vesicle to move towards the luminal membrane and fuse with it.
• Fusion releases aquaporin, creating a water-permeable channel allowing water from the filtrate to move down its water potential gradient into the blood.
• Water reabsorption prevents excessive water loss.
• Low volume, high salt concentration urine is created.
• In the case of excess water, osmoreceptors in the hypothalamus detect the increase and stop ADH secretion.
• Without ADH, aquaporins are removed.
• Collecting duct cells become impermeable to water.
• Producing large volume, low salt concentration urine.

Control of Blood Glucose Concentration

• Blood glucose concentration is hormonally controlled by endocrine tissue in the pancreas.
• Tissue is made of groups of cells known as the islets of Langerhans.
• Islets of Langerhans cell types: Alpha (α) cells secrete glucagon.
• Islets of Langerhans cell types: Beta (β) cells secrete insulin.

Increase in Blood Glucose Concentration

• Alpha and beta cells detect an increase in glucose concentration.
• Alpha cells stop glucagon secretion.
• Beta cells secrete insulin into the blood.
• Insulin causes muscle and liver cells to increase glucose absorption.
• Insulin causes muscle and liver cells to increase glucose respiration rate.
• Insulin causes muscle and liver cells to increase conversion from glucose to glycogen.
• Insulin cannot directly pass through the membrane.
• Insulin binds to a receptor on the specific cell.
• Vesicles containing GLUT proteins move toward cell membrane and fuse with it.
• GLUT proteins helps glucose enter the cell by converting and respiring it.
• GLUT proteins are specific to cell type.
• Brain cells: GLUT 1
• Liver cells: GLUT 2
• Muscle cells: GLUT 4
• Insulin stimulates glucokinase to phosphorylate glucose.
• Glucokinase: Enzyme that phosphorylates glucose.
• Phosphorylated glucose cannot pass out of cell by GLUT proteins.
• Insulin stimulates enzymes phosphofructokinase and glycogen synthase.
• These enzymes allow glucose molecules to form 1,4 glycosidic bonds to produce a polysaccharide glycogen molecule – Glycogenesis.
• Glycogen provides short-term energy and can be converted for glucose.
• Increase in size of glycogen granules in liver and muscle cells.

Decrease in Blood Glucose Concentration

• Alpha and beta cells detect the decrease in glucose concentration.
• Alpha cells secrete glucagon.
• Beta cells stop insulin secretion.
• Glucagon binds to the receptor protein on the cell membrane of liver cells.
• The binding causes a confirmational change in the receptor protein that activates a G-Protein which in turn activates adenylyl cyclase.
• Adenylyl cyclase catalyses the conversion of ATP to cyclic AMP (cAMP), which then acts as a second messenger.
• cAMP binds to protein kinase A enzyme, activating them.
• Active protein kinase then activates phosphorylase kinase, which activates glycogen phosphorylase.
• Active glycogen phosphorylase enzyme catalyses the break down of glycogen to glucose (glycogenolysis).
• This is an example of an enzyme cascade.
• Glucose concentration increases inside the cell which diffuses the glucose out.
• Glucagon stimulates the formation of glucose from amino acids, fatty acids, glycerol, pyruvate and lactate (Gluconeogenesis).

Negative Feedback Control of Blood Glucose

• Blood glucose concentration is regulated by negative feedback control mechanisms.
• Receptors detect if a specific level is too low or too high.
• Information is communicated through the hormonal or nervous system to effectors.
• Effectors react to counteract the change by bringing the level back to normal.
• Alpha and beta cells in the pancreas act as receptors.
• They release glucagon from a cells and insulin from b cells.
• Liver cells act as effectors to glucagon.
• Liver, muscle and fat cells act as effectors to insulin.

Test Strip and Biosensors

• Test strips can be used to test the concentration in urine.
• Two enzymes on a small pad at the end of the test strip: glucose oxidase and peroxidase.
• The pad is immersed in the urine sample if glucose is present:.
• Glucose oxidase catalyses the oxidation of glucose to form gluconic acid and hydrogen peroxide.
• Peroxidase catalyses a reaction between hydrogen peroxide and a colourless chemical in the pad to form a brown compound and water.
• The colour of the pad is compared to a colour chart, representing glucose.
• Urine tests only show whether the blood glucose concentration was above the renal threshold during urine collecting.

Biosensors

• Biosensors show the current blood glucose concentration.
• Biosensors use glucose oxidase only.
• A partially permeable membrane cover the recognition layer and small molecules from the blood can reach immobilised enzymes.
• Glucose oxidase catalyses glucose in the blood sample into gluconic acid and hydrogen peroxide.
• Hydrogen peroxide is oxidised at an electrode, detecting electron transfers.
• Electron flow is proportional to the glucose sample concentration.
• The reading is produced in a matter of seconds.

Homeostasis in Plants

• Plants carry out homeostasis to maintain a constant internal environment.
• Stomata (specifically guard cells) control the diffusion of gases in and out of leaves.
• Stomata control the entry of carbon dioxide into leaves
• Environmental stimuli include light intensity and carbon dioxide concentration which causes the stomata control of the diameter for carbon dioxide intake and minimise water loss by diffusion.

Opening and Closing of the Stomata

• Stomata open and close in a daily rhythm.
• Open during the day and close at the night.

Guard cells have special adaptation

• Each stomata has 2 guard cells surrounding it
• Thick cell wall facing the air outside the leaf.
• Cellulose microfibrils arranged in bands around the cell.
• No plasmodesmata in cell wall.
• Folded cell membrane and contains channel and carrier proteins.
• Cytoplasm of have a high density of chloroplasts and mitochondria.

Mechanism for Opening Stomata

• Water potential decreases
• In response to light the ATP Powered-proton pumps in the cell membrane active transport pumps H+ ions out of the guard cell
• The decrease of H+ ions causes a channel protein to open allowing K+ ions to enter the cell down their electrical gradient towards negative regions
• Electrical Gradient + Concentration Gradient = Electrochemical Gradient
• K+ ions increase solute concentration to decrease water potential.
• Water Potential Gradient
• Water moves into the cell by osmosis through aquaporins to increase the turgor pressure only to get longer opening it.

Mechanism for Closing the Stomata

• Hydrogen H+ ions are moved in the cell for active transport out by proton pump.
• Membrane channels that allow potassium K+ ions (K⁺) to leave open reducing water potential.
• High potential causes the water to leave the cells by osmosis making in now flaccid.

Abscisic Acid & Stomatal Closure

• Abscisic acid hormones (ABA) in the guard cell which acts on its membrane and cell receptors for active transport and inhibit hydrogen proton transfer.
• Calcium (Ca2+) ions enter the cytoplasm of the guard cells through membrane cells.
• They cause the channel proteins to open that transport the chloride ions out of the cell.
• This allows the potassium K+ lons also to leave.
• Water potential goes down, closing the stomata guard cells and reducing the entry of pathogens.