Question

Describe the principles of diffusion and osmosis across cell membranes and how organisms maintain an optimal internal fluid environment. Can you please talk about:

a. the mechanisms by which freshwater paramecia rid their bodies of excess water

b. how invertebrates maintain osmotic balance and waste removal using four invertebrate types as examples: flatworms, annelid

worms, insects, and hemichordates

c. the general mechanisms of salt balance and metabolic waste removal in the kidney of a terrestrial vertebrate

d. a description of the special functions of the "multiplier effect" of mammalian kidneys and of the hormone ADH in regulating

salt and water balance

Answer 1

Osmosis is the diffusion of water through a semipermeable membrane according to the concentration of gradient of water across the membrane.

Diffusion is the transport of material across membranes and within cells .Osmosis transports water only across amembrane.Osmosis is a special case of diffusion.

Principle of diffusion is -

• Molecules move around and will spread evenly throughout the medium
• In a hypotonic solution the extracellular fluid has a lower concentration of solutes than the fluid inside the cell and thus the water enter's the cell.Thus placing an cell in hypotonic solution will lead to enter of water into the cell and cause lysis of the cell
• In hypertonic solution,the extracellular fluid has a higher concentration of solutes than the cells cytoplasm , thus water leaves the cell .And causes cells to shrink up.
• In isotonic solution both the ECF and cytoplasm will have the same concentration thus there will be no net change and when cell is paced in an isotonic solution the cell will be of its normal appearance.

a Paramecia are single celled protists that are found in aquatic habitats.They have cilia all over the body for both locomotion as well as food intake and water movemnent.The paramecium has vacuoles which are membrane enclosed Pockets .There are mainly two types of vacuoles based on their function in paramecium they are 1 Food vacuole Contractile vacuoles.The food vacuoles are used to store food and these then fuse with lysosomes that digest the food .The contractile vacuoles help in Osmoregulation discharge of excess water.These contractile vacuoles are filled with water by canals and in case of osmotic gradient change these contractile vacuoles collapse and cause release of water from pellicle.

b Flatworms have an excretory system that is made up of 2 vesicles or tubules These tubules have cells referred to as flame cell which consists of numerous cilia that propel waste from the body through the excretory pores.These flame cells are also involved in drawing water from interstitial fluid and maintenance of osmotic balance.

Annelids or earthworms have a slightly complex excretory system reffered to as nephridia which is present on each segment they also have tubular structures with cilia on them which help in excreting out the metabolic wastes from the body to the outside surface through spores referred to as nephridiopore.These annelids have specialized tubular reabsorption centres that allow for osmoregulation and water reabsorption in case of change in the osmotic gradient.It also allows for excess water removal .

Arthropods or insects have a different mechanism in which they use structures referred to as Malpighian tubules that are convoluted as well as lined by microvilli which increases the Surface area of reabsorption and osmotic balance.There are potasium pumps lined in the wall of these tubules that causes K+ ions ans well as water to move out from the cell as well as helps to reabsorb water when the organisms face a low water condition thus helping in osmoregulation

Hemichordates also have the same function as mentioned above.

c The osmoregulation in terrestrial vertebrates are regulated and monitored by osmoregulators .These osmoregulators maintains the fluid balance and salt balace.In terrestrial vertebrates kidneys are the organs responsible for salt balance and osmoregulataion.For example, condition of decreased water potential will be detected by the osmoreceptor in hypothalamus which stimulates release of ADH from pituitary gland and increases the permeability of walls of collecting ducts of kidney leading to reabsorption of water from the kidneys to maintain the salt and water balance.Waste product generated in the vertebrates are mainly in the form of ammonia which is a toxic product and must be removed from the cell .This filtration of the waste products which are present in the blood is achieved by the Kidney wherein filtration structures Referred to as nephron filter out the ammonia and form the glomerular filtrate and is excreted out of the body as urine .

d Multiplier effect of mammalian kidneys- This counter current multiplier is demonstrated in the loop of Henle which uses energy to create a concentration gradient.In loop of Henle the permeability of the membrane changes because the descending limb is permeable to water but not permeable to solutes and the ascending limb of loop of Henle is permeable to solutes but not for water .Moreover the loop of Henle is present in the renal medulla region which naturally has a high salt Concentration and tends to absorb water from renal tubule and concentrates the filtrate the osmotic gradient increases as it moves deeper into the medulla since the two limbs of loop of Henle perform opposing function it produces a countercurrent multiplier system.

Anti diuretic hormone plays an important role in osmoregularity .It helps in lowering osmolality by reducing sodium concentration and by increasing water reabsorption in the kidneys helping to dilute body fluids.They also insert water channels that is Aquaporins into the kidney tubules these channels transport solute free water back to blood and causing a decrease in plasma osmolality .ADH is released by the pituitary gland in response to dehydration causing kidneys t become more permeable to water and inserting Aquaporins into the tubules to cause reabsorption of water.Thus ADH plays an important role in water balanocele and salt regulation