Importance of water in organisms

Question 1

Substances reabsorbed in the proximal convoluted tubule

Answer 1

1) Active transport of 3 sodium ions out epithelial cells, lining the PCT, into tissue fluid, into blood.
2) Lowers concentration of sodium ions inside cell.
3) Sodium ions diffuse down concentration gradient, through number of different co-transport proteins in cell-surface membrane of epithelial cells of PCT - facilitated diffusion.
4) Each type of co-transport protein transports sodium ions and either glucose or amino acids or chloride ions or vitamins together into epithelial cell.
5) They diffuse down their concentration gradient via facilitated diffusion (channel proteins) into the capillaries.
6) Diffusion of soluble substances into epithelial cell lowers water potential, so water moves into epithelial cell of PCT, into tissue fluid, before diffusing across endothelial cell and into capillary via osmosis.

Question 2

Low water potential in blood

Answer 2

1) Detected by osmoreceptors in hypothalamus.
2) Hypothalamus makes more ADH.
3) ADH to blood via pituitary gland.
4) To get cells in DCT and CD to respond.
5) Walls of collecting duct/CT allows more water from filtrate -> blood.
6) Concentration of urine increases so drink more.

Water needed to hydrolyse glycogen to glucose in glycogenolysis. If this does not occur, there will be less metabolic reactions like muscle contraction.

Question 3

Photolysis of water

Answer 3

1) Light energy splits water molecules into hydrogen, oxygen, and electrons.
2) Electrons produced are used to reduce the chlorophyll molecule - restoring the electrons it had lost.
3) Oxygen produced is a by-product which is used in respiration or leaves the cell and then the leaf through stomata.
4) Protons are taken up by final electron acceptor - NADP.
5) NADP gains electrons, becomes reduced, becoming reduced NADP, which (along with ATP) is used in the light-independent reaction.

Question 4

Properties and facts of water

Answer 4

Metabolite used to break down complex molecules via hydrolysis.
Chemical reactions take place in aqueous medium.
Raw metabolite in photosynthesis.
Cohesion: water molecules ‘stick together’ by forming hydrogen bonds with other water molecules.
Adhesion: tendency of water molecules to stick to other substances
Cohesion causes surface tension (allows some organisms to walk on water).

High specific heat capacity:
Acts as a buffer against sudden changes in temperature/
Provides thermostable aquatic environment but also within fluids of terrestial organisms.
Minimises temperature fluctuations in cells.
Provides optimum rate of reactions.
Prevents enzymes denaturing.

Question 5

Water transported up the xylem

Answer 5

1) Water evaporates from leaves.
2) Water molecules form hydrogen bonds between one another, stick together via cohesion.
3) Water forms continuous unbroken column.
4) As water evaporates in leaves, water molecules drawn up xylem as result of cohesion.
5) Column of water is therefore pulled up xylem by transpiration - transpiration stream.
6) Places xylem under negative pressure so there is tension within xylem - cohesion-tension theory.