metabolic waste

Exchange and Transport of Wastes

· Metabolism is the name given to all of the chemical reactions that take place in an organism.

· Some of these reactions produce waste materials/products that are toxic.

· If they built up in the organism, they would poison it.

· The removal of these waste materials is called excretion.

· Organisms that feed, take in/ingest food molecules.

· The food molecules are then digested, absorbed and assimilated.

· The food is very unlikely to contain the exact amount, or mixture of food molecules that the organism requires at the time.

· This is true for the amino acids produced by the digestion of proteins.

· Animals can not store amino acids, or proteins.

· They use the amino acids they need at the time and the rest are broken down.

· This produces an organic acid, which can be used in respiration (to provide energy), and a waste product.

· The waste product comes from the amine group of the amino acids.

· The amine group contains nitrogen.

· This leads to the idea of a nitrogenous waste product as an end product of nitrogen metabolism.

· Nitrogenous waste products are toxic if they build up in the organism.

· The main forms in which nitrogen is excreted are;

· ammonia,

· uric acid,

· urea.

· Ammonia. Ammonia is highly toxic and very soluble in water.

Humans convert ammonia into less toxic molecules.

· Mammals excrete urea.

· This is less toxic than ammonia

· It is excreted in the urine, with some loss of water.

You should know that useful substances that are in excess may be eliminated from the organism to maintain a constant internal environment.

· Organisms try to maintain a constant internal environment - homeostasis.

· This may require the elimination/removal of some useful substances, if they are in excess.

· This is not excretion, since these are not waste products of metabolism.

· For example, if there is too much water uptake by someone, then their body fluids may start to become too dilute.

· This would affect the concentrations of vital substances in the body and water potentials.

· The excess water would be removed from the body by increasing the volume of urine eliminated.

· Similarly, there might be a surplus of certain mineral ions in the blood - sodium, or potassium, for example.

· Again, the body can alter the amounts of these ions eliminated in the urine, to bring their concentrations back to normal.

You should understand how excess amino acids are deaminated into ammonia.

· Amino acids absorbed after digestion pass to the liver.

· Some are used in protein synthesis.

· Some are turned into other amino acids by transamination. (See section 3.5.2)

· Any surplus amino acids can not be stored.

· They are deaminated.

· The amine group is removed from the amino acid, to form ammonia and an organic acid.

· The organic acid can be used as a substrate for respiration - it goes into Kreb’s cycle.

· The ammonia can be used to make urea.

Note. This is synoptic with sections 1.1 and 1.5 of By01.

You should understand the ornithine cycle in the mammalian liver which results in the formation of urea.

· As soon as ammonia is formed by deamination, it is involved in reactions which prevent it building up and damaging cells.

· Ammonia can be taken into the ornithine cycle, leading to the production of urea.

· Ammonia reacts with a molecule of ornithine, to produce citrulline.

· Citrulline reacts with another ammonia molecule, to form arginine.

· Arginine then reacts with water, to form urea and re-form ornithine.

· The ornithine can react with more ammonia.

You should know the structure of a mammalian kidney and of a nephron.

· The nephron/kidney tubule is the functional unit of the kidney.

· The Bowman’s capsule is the cup-shaped end of the nephron.

· It surrounds a knot of capillaries, the glomerulus.

· The Bowman’s capsule is connected to the proximal convoluted (part of the) tubule.

· This is connected to the Loop of Henle.

· This connects to the distal convoluted (part of the) tubule.

· This connects to the collecting duct.

· Many tubules are connected to each collecting duct.

· The tubule is surrounded by capillaries coming from the glomerulus.

· A kidney is made up mainly of nephrons/kidney tubules and their blood vessels.

· The arrangement of these structures is orderly and produces distinct layers in the kidney.

· The cortex is made up of the glomeruli, Bowman’s capsules, proximal and distal tubules.

· The medulla is made up of the Loops of Henle and collecting ducts.

· The pelvis is the space into which the collecting ducts deliver urine.

· The ureter takes urine from the pelvis to the bladder.

You should understand the process of selective reabsorption which occurs in the proximal convoluted tubule of the nephron.

You should understand the mechanisms which produce a gradient of ions across the medulla.

Note. The treatment of kidney function in this section is deliberately simplified. Many candidates get themselves into a terrible mess trying to include half-remembered and poorly understood details of kidney function. It is pointless going into much detail, since even experts disagree about aspects of how the kidney works!

Formation of Filtrate

· Blood enters the kidney under very high pressure, along the renal artery.

· This branches rapidly, delivering blood under high pressure into the glomerulus.

· The walls of the capillaries here have small pores/holes.

· The pores allow much of the liquid part of the blood to pass through.

· Blood cells and blood proteins are too large to pass through.

· The liquid that leaves the blood is called the filtrate: because it has been filtered through the walls of the glomerulus.

· This filtering is often called ultrafiltration, because materials are forced into the filtrate due to the high pressure involved.

· The filtrate enters the Bowman’s capsule, the first part of the nephron.

· This filtrate contains urea which the body needs to excrete.

· It also contains many substances which the body can not afford to lose, such as water, mineral salts and glucose.

· These substances have to be reabsorbed.

Proximal convoluted tubule.

· Many of the sodium and potassium ions are actively transported out of the filtrate and back into the blood.

· They are transported by specific transmembrane protein channels/pumps.

· Glucose is actively transported out of the filtrate and back into the blood.

· There are specific transmembrane proteins which have receptor sites for glucose and transport it across the cell membranes of the cells lining the tubule.

· There should be no glucose left/lost in the filtrate/urine.

· The reabsorption of sodium, potassium and glucose, plus the presence of the blood proteins makes the water potential of the blood plasma in the capillaries surrounding the proximal tubule more negative/lower.

· This causes water to leave the filtrate by osmosis - most of the water reabsorbed from the filtrate is reabsorbed here (about 65%).

· Urea is not reabsorbed.

· There are many places in the tubules where active transport takes place.

· This requires ATP from respiration.

· The kidneys have a very high rate of respiration and need a good blood supply to provide oxygen and glucose.

Loop of Henle.

· Here more water and sodium ions are reabsorbed.

· By the time the filtrate leaves the Loop, about 85% of its original water content has been reabsorbed: together with all of the glucose and most of the sodium and potassium.

· The remaining 15% of water would still be too much for the body to lose.

· Almost all of it is reabsorbed by the distal convoluted tubule and collecting duct.

· It is the proportion of this last 15% that is reabsorbed that the body regulates and thus regulates the water content of the body.

· If the body is short of water, then more is reabsorbed. Less is reabsorbed if the body has too much water.

· The remaining filtrate contains quite a high concentration of urea.

· This gives it a very negative/low water potential and the more water that is reabsorbed, the more negative/lower the water potential becomes.

· To get water to leave the filtrate by osmosis, there has to be a more negative/lower water potential in the tissue fluid surrounding the distal tubule and especially the collecting duct.

You should note that the Loop of Henle and the collecting duct are found in the medulla of the kidney - they are neighbours.

· The cells in the ascending (going up) part of the Loop of Henle actively pump chloride/sodium ions out of the filtrate and into the tissue fluid surrounding the Loop.

· This tissue fluid also surrounds the collecting ducts.

· The pumping produces a very high concentration of sodium chloride in the tissue fluids.

· This gives a water potential which is more negative/lower than that of the filtrate in the distal tubule and collecting duct.

· The result is that water can leave the filtrate by osmosis.

· The longer the Loop of Henle, the more sodium/chloride pumps there are.

· The more pumps there are, the more sodium chloride in the tissue fluid and the more negative its water potential.

· This allows for more water reabsorption.

· It is not surprising that desert animals tend to have long Loops of Henle, to conserve water.

You should understand the role of ADH in the control of reabsorption of water by the distal convoluted tubule and the collecting duct.

· The amount of water reabsorbed by the distal tubule and collecting duct is controlled by the hormone ADH.

· If the body is short of water, the concentration of the blood plasma will rise.

· This stimulus is detected by receptors which send information to the hypothalamus in the brain.

· This instructs the pituitary gland (effector) to release more ADH .

· This travels in the blood to the kidneys.

· A rise in ADH makes the cells lining the distal tubule and collecting duct more permeable to water (a response).

· This allows more water to leave the filtrate by osmosis.

· A smaller volume of more concentrated urine is produced.

· This saves water and helps to return the concentration of the blood plasma to normal.

· This would cause a fall in ADH release - an example of negative feedback.

· Obviously, if there is too much water in the blood plasma, then less ADH is released, less water is reabsorbed and a larger volume of more dilute urine is produced.

You should understand the role of aldosterone in the control of the reabsorption of mineral salts by the nephron.

· The final regulation of sodium and potassium ion reabsorption takes place in the distal convoluted tubule.

· This regulation is controlled by the hormone aldosterone.

· In the absence of aldosterone, all of the potassium ions are reabsorbed and none appears in the urine.

· There is a small loss of sodium ions - about 2% of the sodium ions in the original filtrate.

· When aldosterone is secreted, all of the sodium ions are reabsorbed and some potassium ions are lost with the urine.

· Aldosterone allows the body to regulate the balance of sodium and potassium ions in the blood.

· The intention is to keep the levels of each of these ions in the blood constant.

Note. This is synoptic with sections 1.3, 1.5, 1.8 and 1.9 of By01.

You should understand how hydrogencarbonate and phosphate buffers, together with the activity of the distal tubule, prevent excessive changes in blood pH.

· The kidneys have a homeostatic function in helping to keep the pH of the blood constant.

· The nephrons can excrete hydrogen ions, H⁺, into the filtrate.

· They can also reabsorb hydrogencarbonate ions, HCO₃^- into the blood.

· This ion is always present in the filtrate: it is the form in which most carbon dioxide is transported in the blood.

· Since H⁺ ions lower pH/make more acidic, excreting them will help to prevent acidosis - an increase in the acidity of the blood.

· Reabsorbing hydrogencarbonate ions will produce a similar change in pH.

· You can think of the hydrogencarbonate ions reacting with H⁺, taking them out of the blood plasma.

· The hydrogencarbonate acts as a buffer.

· A buffer resists changes in pH and acts to maintain a constant pH.

· Along the length of the nephron, H⁺ ions pass into the filtrate, in exchange for Na⁺ ions which are reabsorbed into the blood.

· This means that urine is usually slightly acidic, about pH5 to 7.

· The cells of the distal convoluted tubule contain the enzyme carbonic anhydrase.

· This combines water and carbon dioxide in the cell, to make carbonic acid.

· This dissociates/splits into H⁺ and HCO₃^- ions.

· If the blood is becoming acidic, then the H⁺ are pumped into the filtrate and the HCO₃^- diffuse into the blood.

· The HCO₃^- raises the pH of the blood plasma/makes it less acidic.

· If the blood is becoming slightly too alkaline (alkalosis), then the H⁺ go into the blood plasma and lower the pH.

· There is a problem associated with extra pumping of H⁺ into the filtrate.

· There is a limit to the concentration gradient against which the tubule cells can pump hydrogen ions into the filtrate.

· To get more hydrogen ions into the filtrate they have to be buffered.

· This effectively takes them out of solution.

· This buffering is done by phosphates and ammonia.

· The phosphate is present as hydrogenphosphate ions in the filtrate formed in the glomerulus and Bowman’s capsule, it is a normal component of blood plasma.

· The ammonia is produced by the tubule cells, by deamination of amino acids.

Common Mistakes

Not knowing the structure of an amino acid. (Synoptic)

Too much jumbled detail about the ornithine cycle. The detail in these notes is sufficient.

Too much jumbled information about the functioning of the Loop of Henle. The detail in these notes is sufficient.

Getting the confused about what ADH does. It makes the collecting duct more permeable to water, leading to more water being reabsorbed from the urine, producing a smaller volume of more concentrated urine.

Ignoring aldosterone.

Not learning about the role of the kidney in regulating blood pH and the role of buffers. The detail in these notes is sufficient.

Practice Questions

iii)

Kidney tubules are involved in regulating blood pH. The diagram shows some of the reactions involved.

What is the name given to compounds such as Na₂HPO₄ and NaHCO₃ which prevent changes in pH? (1 mark)

Explain how the reactions shown in the diagram help to regulate the acidity of the blood and tubule cell. (3 marks)

(6 lines were allowed for the answer.)

A man drinks coffee because he is thirsty. Caffeine in the coffee will inhibit the release of ADH. Explain how the coffee will affect his thirst. (3 marks)

(4 lines were allowed for the answer.)

Northern Examinations and Assessment Board February ‘96. Question 5.

2.	a)	i)	What is the end product of nitrogen metabolism that is excreted by terrestrial insects? (1 mark)
		ii)	What is the advantage to terrestrial insects of excreting this substance? (1 mark)
		iii)	Explain why fish are able to excrete ammonia which is highly toxic. (2 marks)
	b)	Describe how urea is formed from surplus amino acids in the mammalian liver. (3 marks) (5 lines were allowed for the answer.)

Northern Examinations and Assessment Board June‘96. Question 4.

3.	Below is an extract from an information sheet about the drug Frusemide.

	a)	Name the process described in the box labelled Y on the diagram. (1 mark)
	b)	Explain what causes water to pass out of the descending limb. (2 marks) (4 lines were allowed for the answer)
	c)	Sodium and chloride ions normally pass out of the ascending limb by active transport. Suggest two ways in which Frusemide might prevent this process occurring. (2 marks)
	d)	Explain how the rate of reabsorption of mineral salts from the collecting duct is controlled in a healthy person. (2 marks) (4 lines were allowed for the answer.)

Northern Examinations and Assessment Board March ‘98. Question 6.

4>	Explain the part played by the Loop of Henle and the collecting duct in the reabsorption of water in the kidney. (6 marks) (15 lines were allowed for the answer.)

Northern Examinations and Assessment Board June ‘96. Question 9.Part.

2.	a)	i)	Uric acid;
		ii)	(Since it is crystalline) it reduces water loss;
		iii)	Fish are surrounded by water; and so ammonia can be easily/quickly be diluted away;
	b)	Surplus amino acids are deaminated; and the ammonia reacts with ornithine/enters the ornithine cycle; which produces urea and regenerates ornithine;

3.	a)	Ultrafiltration/filtration;
		There is a higher concentration of sodium chloride in the (intercellular) fluid outside the descending limb; which means that the water potential is more negative outside; so, water leaves by osmosis; ( any 2 points)
	c)	The Frusemide might inhibit respiration (that supplies ATP for active transport); or it might bind to/block the transmembrane proteins/ion channels;
	d)	The hormone aldosterone; changes/increases the permeability of the duct to ions/sodium and potassium ions;