1. Maintaining a balance (booklet 3)
Year 12
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2. Excretion—wastes, water and salt balance
Plants and animals regulate the concentration of gases, water and waste products of metabolism in cells and in interstitial fluid.
The maintenance of relatively constant concentrations of gases, water and waste products in the internal environment of living organisms is an important aspect of homeostasis. The concentration of these substances directly affects metabolism in cells.
Excretion is the process by which waste products, which have been produced as a result of metabolism, are removed from the body.
Note: Excretion in animals should not be confused with elimination,
which is the removal of unabsorbed food from the body, since this
undigested food was never a part of the metabolic functioning of the body.

3. The importance of excretion (and water and salt balance) in humans and other animals
Enzymes are substrate specific, and do not function effectively in an environment where there is an accumulation of waste products or the optimal concentrations of water and dissolved substances are not maintained.
The excretory system regulates the concentrations of essential substances and wastes in the internal environment by:
removing metabolic wastes (e.g. nitrogenous wastes, carbon dioxide)
maintaining optimal water and salt levels (i.e. osmoregulation).

4. The importance of excretion (and water and salt balance) in humans and other animals
explain why the concentration of water in cells should be maintained within a narrow range for optimal function
Water makes up at least two-thirds of the body composition of most living organisms, and the internal concentration of water and dissolved substances in cells is similar to that of sea water. Water is the solvent that forms the basic aquatic medium of cytoplasm in cells and also of body fluids.

5. The importance of excretion (and water and salt balance) in humans and other animals
Solutes that dissolve in water in living organisms include inorganic dissolved ions (e.g. sodium, potassium, chloride and hydrogen ions) and organic solutes (end products of digestion, e.g. glucose and amino acids), as well as urea and ammonia (nitrogenous wastes).

6. The importance of excretion (and water and salt balance) in humans and other animals
Changes in water concentration lead to corresponding changes in solute concentration in cells. The relative concentration of solutes to the water in cells determines the osmotic pressure of cells.
The net direction of water movement is dependent on the osmotic gradient - water moves from a high to a low water concentration through the selectively permeable cell membrane. The movement of water into and out of cells therefore depends directly on the concentration of solutions both inside and outside the cells.

7. The importance of excretion (and water and salt balance) in humans and other animals
Water provides the necessary medium in which all chemical reactions of metabolism can occur.
Chemical reactions in cells can proceed only if the reactants are dissolved in water.
Water and solute concentration in cells and fluids in living organs must be maintained at a relatively constant level so that these cellular reactions can take place.

8. The importance of excretion (and water and salt balance) in humans and other animals
If the balance of water and solutes in cells is not maintained:
Too much water may move into cells, causing them to burst (if animal cells). Too much water may also dilute reactants and slow down metabolism.
Too much water may move out, causing the cell contents to shrink, and the cytoplasm to become too concentrated for cell functioning.

9. The importance of excretion (and water and salt balance) in humans and other animals
Osmotic pressure:
Can affect the pH in cells. E.g. too little water leads to an increase in the concentration of solutes such as carbon dioxide and this lowers pH.
Is responsible for structural support in many living organisms. E.g. in plants the outward osmotic pressure of the vacuole is counteracted by the inward pressure of the cell wall, making plant cells turgid.

10. The importance of excretion (and water and salt balance) in humans and other animals
explain why the removal of wastes is essential for continued metabolic activity
Any accumulation of wastes may be toxic to cells and so metabolic wastes must be removed from the body to maintain homeostasis. If wastes are not continuously removed, their levels in the body will increase and alter conditions in the internal environment. Example:
The build-up of nitrogenous wastes (e.g. ammonia) increases pH in cells, which results in them becoming more alkaline
The build-up of carbon dioxide lowers pH, which results in the internal environment becoming more acidic.
These pH changes can slow down or inhibit enzyme functioning in metabolism.

11. The role of the kidney in excretion and osmoregulation
The Excretory System
The excretory system is a group of organs that function together to remove metabolic wastes from the tissues of an organism and expel them to the outside.
The kidneys are the main excretory organs responsible for removing nitrogenous wastes from the bodies of vertebrate animals.

12. The role of the kidney in excretion and osmoregulation
Urine flows from each kidney, down its ureter to the bladder for storage, and then to the outside via the urethra
The function of the kidney is to filter the blood that enters it, removing wastes (in solution) from the bloodstream so that they can be excreted from the body. This filtration is carried out in millions of tiny excretory units called nephrons.
Urine is the excretory solution finally produced by the nephrons, and drains out of the kidney to be removed from the body.

13. Active and passive transport (diffusion and osmosis) in kidney functioning
distinguish between active and passive transport and relate these to processes occurring in the mammalian kidney
Movement of materials into and out of cells takes place either passively or actively.
Passive movement includes the processes of diffusion and osmosis. These types of movement require no energy input, since molecules move along a concentration gradient.
Active transport requires an input of cellular energy to actively move molecules against a concentration gradient.
Diffusion is the movement of any molecule from a region of high concentration to a region of low concentration of that substance, until equilibrium is reached. This does not require an energy input.
Osmosis is the movement of water molecules from a region of high water concentration to a region of low water concentration through a selectively permeable membrane. This also does not require energy.

14. Active and passive transport (diffusion and osmosis) in kidney functioning
Within the kidney, the movement of substances between the bloodstream and excretory fluid in the nephrons involves both active and passive transport.

15. Active and passive transport (diffusion and osmosis) in kidney functioning
Passive transport in the kidney
A balance in the optimal concentrations of blood chemicals is maintained by the selective excretion of wastes, as well as any excess water and salts in urine. Therefore the ability of the kidney to alter the urine concentration plays a vital role in homeostasis.

16. Active and passive transport (diffusion and osmosis) in kidney functioning
Within the kidney tubules, there is a two-way movement of substances:
Waste substances pass from the bloodstream into the kidney tubules, to be excreted in urine (filtration and secretion)
Substances required by the body are removed from the urine in the kidney tubules (before it is excreted) and returned to the bloodstream (reabsorption)

17. Active and passive transport (diffusion and osmosis) in kidney functioning
Passive transport moves water (by osmosis), and some nitrogenous wastes such as urea and ammonia (by diffusion) in the kidneys of mammals.
Only excess water and salts are excreted.
Homeostasis requires that sometimes water and salts should be conserved and at other times they should be excreted. Salt movement is via active transport and this in turn draws water by osmosis (passive transport).

18. Active and passive transport (diffusion and osmosis) in kidney functioning
Active transport in the kidney
Active transport involves a carrier protein that spans the membrane and this carrier molecule can actively move chemicals from a low to a high concentration, utilising cellular energy.
Active transport moves mainly sodium ions, glucose, amino acids and hydrogen ions across the wall of the nephron.
Active transport is the movement of molecules from an area of low
concentration to a region of high concentration, requiring an input of
energy.

19. Active and passive transport (diffusion and osmosis) in kidney functioning
All glucose and amino acids are reabsorbed by kidney cells so that they are not lost in urine
Additional nitrogenous wastes (uric acid) and hydrogen ions (H⁺) are removed from blood capillaries and are added to urine in the kidney tubules
A ‘sodium pump’ mechanism in the kidney tubules actively transports ions (salts) from the urine back into the kidney cells. This process also brings about the conservation of water within the body by osmosis. Water is drawn by the osmotic pull of the salts in solution.
Active transport is the movement of molecules from an area of low
concentration to a region of high concentration, requiring an input of
energy.

20. Why osmosis and diffusion may be inadequate for WASte removal
Explain why the processes of diffusion and osmosis are inadequate in removing dissolved nitrogenous wastes in some organisms
Problems with relying on diffusion
The rate of movement is to slow to just rely on diffusion.
Nitrogenous wastes and toxins (e.g. drugs) must be dissolved in water when they are removed. If their removal by the kidney was dependent on diffusion only, wastes would be able to move only if they were more concentrated inside the cells or the bloodstream than in the fluids outside. As their concentrations begin to equalise, their movement would slow down and eventually stop.

21. Why osmosis and diffusion may be inadequate for WASte removal
Not all wastes can be removed by diffusion.
Since nitrogenous wastes are toxic, it is essential they are all removed. If concentrations within the blood and urine equalised and no further wastes were removed, their accumulation would change the pH of cells and become toxic.
Active transport is therefore essential at this point to move wastes against the concentration gradient from blood into urine in the kidney.

22. Why osmosis and diffusion may be inadequate for WASte removal
Problems with relying on osmosis
Too much water may be lost in urine.
If urine contains a large number of nitrogenous wastes in solution, water will be drawn into the urine by osmosis to dilute the wastes and try to equalise the concentration of fluid inside the urine and in the surrounding kidney. This means the loss of large amounts of water from the body – a loss many terrestrial animals cannot afford.

23. Why osmosis and diffusion may be inadequate for WASte removal
Movement of water may make wastes too dilute for excretion by diffusion.
Organisms that live in freshwater environments have a different problem. Osmosis results in water moving into the body tissues from the surrounding environment. Although this dilutes the toxic wastes in the body, it lowers the concentration gradient and slows down their excretion by diffusion. Therefore a mechanism is essential to remove wastes against a concentration gradient.

24. Why osmosis and diffusion may be inadequate for WASte removal
Solution to the problems – combined active transport and osmosis
Active transport is quicker and more effective than diffusion as it removes most wastes, even against a concentration gradient.
It can also be used to pump salts from urine back into the kidney tissues and these in turn will draw water with them (by osmosis), ensuring in this way that the amount of water lost in urine does not affect the body’s water balance.
(Note: Water cannot be moved directly by active transport.)

25. Questions
Explain why the removal of wastes is essential for continued metabolic activity
Distinguish between active and passive transport and relate these to processes occurring in the mammalian kidney
Explain why the processes of diffusion and osmosis are inadequate in removing dissolved nitrogenous wastes in some organisms

26. Investigating the structure of a mammalian kidney
perform a first-hand investigation of the structure of a mammalian kidney by dissection, use of a model or visual resource and identify the regions involved in the excretion of waste products
Next term 

27. Microscopic structure and the functioning of the mammalian kidney
The basic functional unit within the kidney is a microscopic tubule called the nephron, the smallest structural part of a kidney that is capable of producing urine.
Each kidney contains millions of these tiny units, which coil and twist across both the cortex and medulla.
Function of the nephron video -

28. Microscopic structure and the functioning of the mammalian kidney
A nephron consists of four functional parts:
The Bowman’s capsule
A proximal (first) convoluted tubule
The loop of Henle
A distal (second) convoluted tubule which leads into a collecting duct

29. Microscopic structure and the functioning of the mammalian kidney
Bowman’s capsule is an enlarged part of the nephron tubule. It is a double walled sac, indented on one side to accommodate a spherical network of blood capillaries called the glomerulus.
The hollow part the Bowman’s capsule is filled with glomerular filtrate. This fluid continues its flow along the length of the nephron, where its chemical composition is adjusted, and by the time it reaches the collecting tubule at the far end, it is known as urine.

30. Holiday Homework check!
revision questions - booklet 2
read and highlight pages 79-82
complete nephron worksheet (check )
complete MAB question booklet