A1.1 - water (2a)

Question 1

why is water known as the medium of life?

Answer 1

the first cells originated in water, with:
- oceans provided the solvent for biochemical reactions
- membranes evolved to separate the water in the cytoplasm from “ocean water”

Question 2

what processes does water remains the medium in which they occur?

Answer 2

• making up cytoplasm in cells
• making up fluids inside organelles
• making up tissue fluid or fluids between cells
• transporting molecules around organisms and inside or outside of cells
• range of habitats, including lakes, rivers and oceans

Question 3

why are the bonds in water polar?

Answer 3

unlike other covalent bonds, such as C-H, the electrons are not equally shared between the oxygen and hydrogen atoms

Question 4

what is the dipolarity of water?

Answer 4

water molecules are polar as:
- hydrogen is a partially positive atom
- oxygen is a partially negative atom

Question 5

what is the impact of the polarity of water on their intermolecular interactions?

Answer 5

water’s molecules polarity allows them to interact by forming hydrogen bonds with each other
- these bonds are constantly forming and breaking

Question 6

how do hydrogen bonds form in water?

Answer 6

• water molecules have a partially positive and negative pole
• there is an attraction between the opposite poles
• hydrogen bonds form between hydrogen and oxygen of different water molecules

Question 7

what is cohesion?

Answer 7

attraction of water molecules to other water molecules

Question 8

what causes cohesion?

Answer 8

cohesion occurs due to hydrogen bonding, as a result of the dipolarity of water

Question 9

what are the 2 benefits of cohesion to organisms?

Answer 9

cohesion can benefit living organisms by:
- allowing transport of water under tension in the xylem against gravity as a column of water molecules
- acting as water surface habitats due to surface tension effect

Question 10

what is adhesion?

Answer 10

water’s attraction to other molecules due to hydrogen bonding
- can be polar or charged materials

Question 11

what causes adhesion?

Answer 11

adhesion occurs due to hydrogen bonding, as a result of the dipolarity of water molecules

Question 12

what is capillary action?

Answer 12

capillary action is where movement due to cohesion is supported by adhesion to surrounding polar molecules, preventing the water moving back to its original location
- adhesion leads to capillary action

Question 13

what is the role of adhesion and capillary action in the xylem?

Answer 13

in plant cells, adhesion to cellulose generates tension and aids the upwards movement of water and stopping it from dropping in the xylem

Question 14

what is the role of adhesion and capillary action in the soil?

Answer 14

in the soil, water moves through microscopic channels

Question 15

what are the solvent properties of water?

Answer 15

water can dissolve many hydrophilic substances:

polar molecules dissolve in water due to attraction between partial charges on polar groups of the solute
- such as glucose where the partially negative charges on the solute are attracted to the partially positive charges on the water molecules

ionic substances dissolve in water because they dissociate into ions and water molecules cluster around these
- partially positive hydrogen is attracted to negative ion and partially negative oxygen is attracted to the positive ion

Question 16

how do the solvent properties of water relate to its roles of:
1. transport
2. metabolism

Answer 16

1. water is a liquid at most biological temperatures, meaning the wide variety of dissolved molecules can flow throughout the organism
2. metabolism is controlled by enzymes, and these proteins dissolve in the water of the cytoplasm (and other fluids), and allow the reactions to occur at a fast enough rate to support life

Question 17

what are hydrophobic molecules?

Answer 17

hydrophobic molecules are insoluble in water, but can pass through cell membranes

Question 18

how do the functions of some molecules in cells depend on them being hydrophobic and insoluble, including hormones, proteins and epidermal cells?

Answer 18

1. steroid hormones, such as testosterone, can pass directly through the plasma membrane and nuclear membrane to influence gene expression
2. membrane-bound proteins have hydrophobic regions that embed into a cell membrane, and hydrophilic regions that extend into the cytoplasm and tissue fluid
   - the embedded hydrophobic region keeps the protein attached to the membrane
3. epidermal cells of leaves secrete a hydrophobic wax that forms the cuticle
   - this acts as a barrier to reduce water loss through evaporation

Question 19

what are the 4 physical properties of water and air?

Answer 19

• buoyancy
• viscosity
• specific heat capacity
• thermal conductivity

Question 20

what is buoyancy?

Answer 20

bouyancy is the upward force exerted on an object, equal to weight of the displaced medium

Question 21

how does buoyancy compare between air and water?

Answer 21

water - big upward force due to increased pressure from water below than air above (allowing organisms to float)

air - small upward force as weight of air displaced is less than the same volume of water

Question 22

what is viscosity?

Answer 22

viscosity is the resistance to an object moving through a medium

Question 23

how does viscosity compare between air and water?

Answer 23

water is more dense, so has a higher viscosity

air is less dense, so has a lower viscosity

Question 24

what is thermal conductivity?

Answer 24

thermal conductivity is the ability of a substance to transfer heat

Question 25

how does thermal conductivity compare between air and water?

Answer 25

water has a high thermal conductivity

air has a low thermal conductivity

Question 26

what is specific heat capacity?

Answer 26

energy required to raise 1kg of a substance by 1C

Question 27

how does specific heat capacity compare between air and water?

Answer 27

water - high specific heat capacity as a lot of energy is needed to break the hydrogen bonds

air - low specific heat capacity as less energy is needed to change the temperature of the gas

Question 28

what are characteristics of the black throated loon in response to water’s physical properties?

Answer 28

black-throated loon - bird that lives in cold regions of the northern hemisphere

• high buoyancy of water allows the bird to stay in water for long period of times
• webbed feet and streamlined body helps overcome high viscosity so less energy is needed
• glands produce oil to waterproof the feathers
• high thermal conductivity means that birds lose more body heat on water
• water is warmer in cold weather due to high specific heat capacity

Question 29

what are characteristics of the ringed seal in response to water’s physical properties?

Answer 29

ringed seal - found in cold regions of the northern hemisphere

• high buoyancy of water allows the ringed seal to keep snout above water to access air
• padded feet and streamlined body helps overcome high viscosity so less energy is needed
• thick blubber minimises heat loss and provides insulation and protection to internal organs
• water is warmer in cold weather due to high specific heat capacity