2.67 understand the origin of carbon dioxide and oxygen as waste products of metabolism and their loss from the stomata of a leaf
Plants excrete carbon dioxide as it is produced as a waste product of respiration, as well as oxygen as it is a waste product of photosynthesis. They both leave the leaf through the stomata by diffusion in gas exchange.
Humans:
2.68 recall that the lungs, kidneys and skin are organs of excretion
The lungs excrete carbon dioxide, the kidneys excrete urea and other waste products in the blood to be passed through the urinary tract, and the skin excretes urea and salts in sweat.
2.69 understand how the kidney carries out its roles of excretion and osmoregulation
Excretion
Urea is a chemical produced in the liver from excess amino acids (deamination), and is toxic to the body. It is released from the liver into the blood, and the blood then flows through the renal artery into the kidneys. Inside the kidneys, there are many nephrons (filtration units within the kidney). The blood flows through capillaries into the glomerulus, where high pressure forces water, urea, salts and glucose into the bowmans capsule and through the proximal convoluted tubule. Here, glucose is selectively reabsorbed into the bloodstream from the glomerular filtrate by active transport. Salts are reabsorbed, but only just enough, no excess. Once the glomerular filtrate has travelled through the proixmal convoluted tubule, the loop of Henle, and the distal convoluted tubule, it enters the collecting duct as urine and is transported to the bladder, where it is held until it is excreted.
Osmoregulation
ADH (anti-diuretic hormone) is released from the pituitary gland, as stimulated by the hypothalamus in the brain. If the hypothalamus detects a rise in water content, the pituitary gland will release less ADH, but if it detects a fall in water content, the pituitary gland will release more ADH. More ADH makes the tubules more permeable, so more water is reabsorbed, and the opposite is also true.
2.70 describe the structure of the urinary system, including the kidneys, ureters, bladder and urethra
Blood is filtered to remove waste products in the kidneys, and leaves through the collecting duct into the ureter, which transports urine to the bladder. Urine is expelled through the urethra.
2.71 describe the structure of a nephron, to include Bowman’s capsule and glomerulus, convoluted tubules, loop of Henlé and collecting duct
The nephron is a series of tubules and blood vessels. The glomerulus is inside the bowman's capsule, and this is where ultrafiltration occurs. The filtrate travels through the proximal convoluted tubule, then the loop of Henle, then the collecting duct.
2.72 describe ultrafiltration in the Bowman’s capsule and the composition of the glomerular filtrate
Blood is pushed through the glomerulus at a high pressure, causing smaller molecules to be forced through into the bowman's capsule. Larger particles (cells etc.) remain in the blood as they are too large to fit through. Glomerular filtrate is therefore made up of water, glucose, urea and salts.
2.73 understand that water is reabsorbed into the blood from the collecting duct
Water is reabsorbed from the glomerular filtrate throughout the whole series of tubules, including the collecting duct, except for the second half of the loop of Henle, where only salts are reabsorbed.
2.74 understand that selective reabsorption of glucose occurs at the proximal convoluted tubule
Glucose is a very valuable nutrient required for respiration, so is selectively reabsorbed in the proximal convoluted tubule. This happens through active transport, because while this requires respiration to occur, the glucose saved means that it is worth it.
2.75 describe the role of ADH in regulating the water content of the blood
Excess of water detected by hypothalamus > Pituitary gland stimulated to produce less ADH > less ADH travels through the blood into the nephron > Tubules become less permeable > Less water is reabsorbed > Normal water content level achieved
Deficit of water detected by hypothalamus > Pituitary gland stimulated to produce more ADH > more ADH travels through the blood to the nephron > Tubules become more permeable > More water is reabsorbed > Normal water content level achieved
2.76 understand that urine contains water, urea and salts.
Urine is the substance excreted by the kidneys. It contains urea, a toxic substance from broken down amino acids, excess salts, and excess water. It should not contain glucose or proteins, which are indications of illness.
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