IB Biology - Biomolecules: Water

Structure of water

-2 hydrogen atoms covalently bonded to an oxygen atom

What causes water's polarity?

-although covalent bonding involves electron sharing, they are not shared equally between atoms

Describe polarity

the slight difference in charge that occurs at the different poles of the water molecule

Charge of oxygen

due to having high electronegativity attracts electrons more strongly, forming a slightly negative charge

Charge of hydrogen

the hydrogen atoms have weaker attraction towards electrons, resulting in a slightly positive charge

Water's asociations with other polar molecules or charged ions

-due to it's polarity it can form weak associations

-the slightly negative poles will attract the slightly positive poles of other molecules and vice versa

Hydrogen bonds

intermolecular associations formed as a consequence of the polar covalent bonds within water molecules

-they from when a slightly positive hydrogen atom is attracted to a slightly negative fluorine, oxygen or nitrogen atom of another molecule

-hydrogen bonds are stronger than the standard polar associations due to the high electronegativity of F, O & N

-hydrogen bonding is responsible for several unique properties of water: including thermal, cohesive and solvent properties

Cohesive properties of water

a water molecule is able to stick to other water molecules via the formation of hydrogen bonds

How does water's cohesiveness work?

-the hydrogen bonding between water molecules allows the liquid to resist low levels of external force (creating surface tension)

-the higher surface tension of water makes it sufficiently dense for certain smaller organisms to move along its surface

Real life example of water cohesion

water striders are small insects that can move along the surfface of water due to its high surface tension

How do water striders work?

-they distribute their weight via long legs that contain thousands of microscopic hairs to trap air and increase buoyancy

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

Adhesive properties of water

a water molecule is able to stick to other polar or charged molecules via the formation of polar associations

How does water's adhesion work?

-attraction to charged or polar surfaces (such as cellulose cell walls) allows water to flow in opposite to gravitational forces (capillary action)

-the strength of the capillary action will be dependent on the diameter of the pore through which the water moves (smaller diameter = more action)

Capillary action & movement of water through soil

-capillary action is also responsible for the movement of water through the soil from the deeper water table

-the type of soil will influence the strength of the capillary action, affecting the efficacy of agricultural practices

Capillary action's role in plant stems

-capillary action is necessary to transport water up plant stems via a transpiration stream

-the loss of water vapour from the leaves (via evaporation) and the absorption of water into the roots (via osmosis) creates a pressure gradient

-water will move along the gradient by using capillary action and cohesion to be transported up the stem of the plant via narrow xylem vessels

Solvation

the process by which solvent molecules surround and interact with solute molecules

Solute

-any substance (usually a solid) that is disolved by a solvent molecule to form a solution

-solutes that are able to be dissolved by a solvent are considered to be soluble (insoluble substances will not form a solution)

The universal solvent:

water, due to it's capacity to dissolve a large number of substances

What can water dissolve?

-any substance that contains charged particles (ions) or electronegative atoms (polarity)

-the polar regions of the water molecule associate with molecular surfaces that have an opposing charge, forming dispersive hydration shells

Metabolic medium

the capacity of water to dissolve a large variety of substances makes it an important medium for metabolic reactions

-substances dissolved into aqueous solutions are more likely to collide with enzymes and undergo necessary chemical reactions

-water can also promote enzyme activity by absorbing heat from exothermic reactions and maintain acid-base neutrality

Hydrophilic

-substances that freely associate and readily dissolve in water

-includes all polar molecules and ions

Hydrophobic

-substances that do not freely associate and readily dissolve in water

-includes all large non-polar molecules (lipids)

Transport medium

the movement of water-soluble substances in plants and animals involves different transport systems

-in animals, the blood plasma transports dissolved solutes: including amino acids, simple sugars, water (urea) and a small amount of gases (O2 & CO2)

-in vascular plants, mineral ions are transported xylem vessels while dissolved nutrients are transported via the phloem

Not water soluble and not freely transported substances in an aqueous environment

-in animals, lipids are packaged with proteins to form water soluble lipoproteins that can be transported via the bloos

-the mechanism of water transport in plants is not well understood, but may involve conjugation to amino acids and transport via the phloem

Physical properties of water

-contibutes to water being the medium of life as its properties sustain living processes

-properties include buoyancy, viscosity, thermal conductivity and specific heat capacity

Buoyancy

-the upward force applied to an object in a medium and is determined by the medium's density

-as water is more dense than air, it applies a greater upward force which allows objects to float in water

-the capacity of an object to float in water will be determined by its relative weight (heavier objects will sink)

Viscosity

-a measure of a fluid's tendency to flow (more viscous fluids are more resistant to flow)

-water is more viscous than air as it can form hydrogen bonds which increase the friction of flowing molecules

-water can dissolve many solutes and these solutes can increase the solute's viscosity

Thermal conductivity

-a measure of a medium's ability to move heat across a temperature gradient

-water absorbs and transfers hear more readily than air because water particles are packed more tightly together

Specific heat capacity

-the amount of energy required to raise the temperature of 1g of substance by 1 degree C

-water has a higher specific hear capacity than air (highest of any liquid) as its hydrogen bonds require additional energy to break

-the high specific heat capacity of water makes it an effective coolant in sweat (sweat absorbs a lot of thermal energy resulting in high latent of vaporisation

Biological consequences of water's physical properties

-the different physical properties of water and air influence the various adaptations and behaviours of terrestrial and aquatic organisms

-the black throated loon and ringed seal both live in the Artic and spend time on land and in water

Black-Throated Loon adaptations

-the loon has lighter (less dense) bones, allowing it to float on water (due to buoyancy) - however, the bones are not hollow like in many other birds of flight (allowing it to dive under water)

-loons have difficulty walking on land because their legs are located at the rear to better propel them through water (higher viscosity)

-the loon's feathers form an interlocking structure that functions as a barrier to water, preventing heat loss (water has higher thermal conductivity

Ringed seal

-the seal has denser bones than the loon, allowing it to stay submerged upon diving (less buoyant)

-the seal possesses a streamlined body to better propel them through water (higher viscosity than air)

-ringed seals have an outer coat of fur that traps air for waterprooing and also has a thick layer of blubber to prevent heat loss while in water

-ringed seals do not have many effective cooling mechanisms because water temperatures are generally stable (due to specific heat capacity). This makes the seal particularly vulnerable to climate change

Origin of Water

earth's water must have an extra planetary origin as Earth formed (4.5 BYA) in an environment too hot for water to condense into liquid