IB biology: topic 2: water

why is water polar? what are the charges?

• oxygen has higher electronegativity and attracts electrons more strongly - polarity.

• oxygen is delta negative

• hydrogens are delta positive

what type of interactions does water have? what are examples of this?

• water can form hydrogen bonds.

• thermal properties.

• cohesive / adhesive properties.

• solvent properties.

outline water’s thermal properties

• water can absorb lots of energy before changing state as extensive hydrogen bonds require lots of energy to overcome.

• relatively slow to change temperature and thus supports the maintenance of constant conditions.

outline water as a cooling system within organisms

• evaporation requires an input of energy.

• this energy comes from the surface of the skin when it is hot, therefore when the sweat evaporates the skin is cooled.

• water has high SHC, absorbs a lot of thermal energy before it evaporates - highly effective coolant.

outline water’s cohesive/adhesive properties

• cohesion - water “sticks“ to other water molecules.

• adhesion - water “sticks” to other molecules that are polar

compare water and methane

• similar size and weight.

• water polar methane non-polar.

• water hydrogen bonding methane weak dispersion forces.

• water higher melting point/boiling point.

• water higher SHC and SLH.

why does water display surface tension

• hydrogen bonding between water molecules (COHESION) allows the liquid to resist low levels of external force (surface tension).

• high surface tension of water makes it sufficiently dense for certain smaller organisms to move along its surface.

why does water display capillary action

• attraction to charged or polar surfaces (e.g. glass - ADEHSION) allows water to flow in opposition of gravitational forces (capillary action).

• capillary action is necessary to allow water to be transported up plant stems via a transpiration stream.

outline the solvent properties of water

• water can dissolve any substance that contains ions or polar molecules.

• occurs because the polar attraction of water molecules can sufficiently weaken intramolecular forces and result in the dissociation of the atoms.

• dispersive hydration shells form between charged regions of water and atoms of opposite charge.

how are insoluble molecules transported around the body.

• lipids (fats and cholesterol) are non-polar and hydrophobic and hence will not dissolve in water

• they form complexes with proteins (lipoproteins) in order to move through the bloodstream.

• hydrophilic portions of proteins, cholesterol and phospholipids will face outwards and shield internal hydrophobic components.

how are amino acids transported around the body?

amino acids will be transported in the blood in an ionised state

how is oxygen transported around the body?

oxygen is soluble in water but in low amounts – most oxygen is transported by haemoglobin within red blood cells

how is glucose transported around the body?

glucose contains many hydroxyl groups (–OH) which may associate with water and thus can freely travel within the blood

how do water soluble molecules move about the body?

water soluble substances will usually be able to travel freely in the blood plasma, whereas water insoluble substances cannot

what are hydrophillic substances?

substances that freely associate and readily dissolve in water - polar molecules and ions

what are hydrophobic substances?

substances that do not freely associate or dissolve in water - large non-polar molecules