A1.1 Water Flashcards
A1.1.1—Water as the medium for life
Students should appreciate that the first cells originated in water and that water remains the medium in
which most processes of life occur.
Water with substances dissolved in it is an aqueous solution. The first living cells originated in aqueous solutions, sometimes called “primeval soup”
Reproduction: sperm swim to the egg through water; mammalian foetuses are supported by water in the uterus.
A medium is something in which processes can occur.
The eight processes of life happen in water, so after more than three billion years of evolution, water is still the
Movement: aquatic organisms swim through water or drift in currents; pumping of blood and sap transports substances dissolved in water.
medium for life.
Metabolism: because aqueous solutions are liquid, both the water and solutes dissolved in it are free to move
Response to stimuli: nerve impulses are movements of dissolved Nat and K* ions; hormone transport is in blood.
Excretion: urine is an aqueous solution of waste products; excretion of waste gases (for example, CO,) requires a moist surface.
and, in some cases, react chemically. Reactants and products of most chemical reactions in living organisms (= metabolism) are dissolved in water.
Nutrition: the reactions of both photosynthesis and digestion take place in aqueous solution.
Growth: cytoplasm is an aqueous solution, so cells must absorb water by osmosis to increase in size.
Homeostasis: blood plasma and tissue fluid are aqueous solutions that are regulated to form a stable and ideal internal environment for cells.
A1.1.2—Hydrogen bonds as a consequence of the polar covalent bonds within water molecules
Students should understand that polarity of covalent bonding within water molecules is due to unequal
sharing of electrons and that hydrogen bonding due to this polarity occurs between water molecules.
Students should be able to represent two or more water molecules and hydrogen bonds between them
with the notation shown below to indicate polarity.
A molecule is two or more atoms joined together by one or more covalent bonds. A covalent bond is formed when two atoms share a pair of electrons. In some cases, the nucleus of one of the atoms is more attractive to the electrons than the other, so the electrons are not shared equally. The consequence of this is that the covalent bond is polar, with one of the atoms having a slight positive charge and the other a slight negative charge. Molecules with polar covalent bonds have polarity. Water molecules are polar. A hydrogen nucleus is less attractive to electrons than an oxygen nucleus, so in a water molecule the two hydrogen atoms have a slight positive charge and the oxygen atom has a slight negative charge.
The two bonds between oxygen and hydrogen atoms in a water molecular are intramolecular. An intermolecular
bond can form between the positive pole of one water molecule and the negative pole of another water
molecule. This is called a hydrogen bond hydrogen bond Individual hydrogen bonds are weak, but water
molecules are small so relatively large numbers of the bonds form in a volume of water. Collectively, these
bonds influence the properties of water markedly
A1.1.3—Cohesion of water molecules due to hydrogen bonding and consequences for organisms
Include transport of water under tension in xylem and the use of water surfaces as habitats due to the
effect known as surface tension.
Water molecules stick to each other (cohere because of the hydrogen bonds that form between them.
* Strong pulling forces (tensions) are exerted to suck water up to the tops of the tallest trees in tubular xylem vessels. Because of the cohesion resulting from hydrogen bonds, the columns of water molecules in these vessels rarely break despite powerful suction forces.
* The surface of water on ponds is used as a habitat by s o m e animals, even though they are denser than water (e.g. the insect Gerris lacustris pictured right).
To break through the water surface, hydrogen bonds would have to be broken, which requires more energy than is available. This effect of water surfaces forming a cohesive structure that resists breakage is called surface tension.
Where do hydrogen bonds form?
A. Between the slight negative charge of hydrogen and slight positive charge of oxygen within a water molecule
B. Between the slight positive charge of hydrogen and slight negative charge of oxygen within a water molecule
C. Between the slight positive charge of hydrogen and slight negative charge of oxygen in different water molecules
D. Between the slight negative charge of hydrogen and slight positive charge of oxygen in different water molecules
[1]
Markscheme
C
Outline how hydrogen bonds form in water.
[3]
Markscheme
a. water (molecules) are polar/dipolar/have partially positive and negative poles/have δ+ and δ-;
b. attraction/bonding between positive and negative (poles);
c. hydrogen bond formed between hydrogen and oxygen; Reject if H and O in same molecule.
d. bond/attraction between different water molecules/intermolecular;
What are linked by hydrogen bonds?
A. Hydrogen and oxygen within a water molecule
B. Phosphate and sugar within a DNA molecule
C. Base and sugar between DNA nucleotides
D. Hydrogen and oxygen in different water molecules
[1]
Markscheme
D
Outline the reasons for secretion of sweat in humans.
[2]
Markscheme
cooling/removal of heat/lowering body temperature;
to prevent overheating
OR
to help maintain body temperature/for temperature homeostasis/for thermoregulation
OR
to keep temperature at 37 °C;
A1.1.4—Adhesion of water to materials that are polar or charged and impacts for organisms
Include capillary action in soil and in plant cell walls.
Whereas cohesion is water molecules sticking to each other, adhesion is water sticking to another substance. Adhesion happens if the other substance is hydrophilic. Hydrophilic substances are attractive to water because they can make intermolecular bonds with the water molecules. Polar and charged materials are hydrophilic.
* Polar substances such as cellulose in plant cell walls form hydrogen bonds with water. Cell walls therefore tend to remain saturated with water and draw more water from the nearest supply if they have become unsaturated due to evaporation, for example when a leaf transpires.
* Capillary action is water being drawn through narrow spaces because it adheres to the surfaces of the spaces. Water moves through pores in dry
soils by this process because the solids in the soil-humus (dead organic matter) and particles of sand, silt and clay-are hydrophilic. The water can move upwards.
A1.1.5—Solvent properties of water linked to its role as a medium for metabolism and for transport in
plants and animals
Emphasize that a wide variety of hydrophilic molecules dissolve in water and that most enzymes catalyse
reactions in aqueous solution. Students should also understand that the functions of some molecules in
cells depend on them being hydrophobic and insoluble
A wide range of hydrophilic molecules and ions dissolve in water. Polar molecules dissolve by forming hydrogen bonds with water. lons with positive charges form electrostatic interactions with the & poles of water
molecules and negative ions with the &t poles. For example, sodium chloride dissolves in water like so: Urea molecules are polar and dissolve with hydrogen bonds between positive and negative poles of the urea and water molecules.
Most chemical reactions in cells are catalysed by enzymes and take place in aqueous solution, which allows both enzymes and substrates to move, so substrate-active site collisions can occur. Transport systems in plants and animals rely on the solvent properties. Blood plasma, xylem sap and phloem sap are all aqueous solutions that are pumped to transfer the solutes from one part of the plant or animal to another. If a substance is not hydrophilic it is said to be hydrophobic. This does not mean that it is repelled by water, but that water molecules are more strongly attracted to each other than to the non-polar molecules of hydrophobic substances. Hydrophobic substances are therefore insoluble in water. They can also form a barrier to water and hydrophilic substances. The wax on leaf surfaces and oils on human skin are hydrophobic and prevent dehydration. Hydrocarbon tails in the centre of cell membranes form a barrier that allows cells to regulate the movement of hydrophilic substances across the membrane.
Outline how the properties of water make it an ideal transport medium in plants.
[4]
Markscheme
a. polarity of water;
b. hydrogen bonds between water molecules;
c. cohesion between water molecules/water molecules stick together;
d. cohesion allows tensions/low pressures/transpiration pull/movement upward/against gravity;
e. adhesion to cellulose/cell walls generates tensions/pull (in xylem)
OR
adhesion to xylem walls/vessel walls causes capillary rise/upward movement;
f. solvent for many substances / many substances dissolve;
g. liquid at most temperatures experienced by plants / liquid so can flow;
A1.1.6—Physical properties of water and the consequences for animals in aquatic habitats
Include buoyancy, viscosity, thermal conductivity and specific heat capacity. Contrast the physical
properties of water with those of air and illustrate the consequences using examples of animals that live in
water and in air or on land, such as the black-throated loon (Gavia arctica) and the ringed seal (Pusa
hispida).
Air and water have markedly different physical properties, with consequences for animals that swim in water or fly in the air. This can be illustrated with black-throated loons (birds that dive in water to feed and also fly) and the ringed seal (a mammal that only comes out of the water to breed). Buoyancy-solids float in fluids if their density is lower. Consider the densities in the table. The seal’s density is close to that of seawater so it can float easily. The loon’s density is far higher than that of air so it beats its wings to remain airborne.
Pure water1,000
Seawater,1025
Air1.225
Loon 700-850
Seal 1,025-1,045
Density (mg per cm?)
Viscosity-resistance to flow of a fluid due to cohesion between the molecules. Water has high viscosity due
to hydrogen bonding. Air’s viscosity is more than 50 times less. The seal has to overcome much more resistance when it swims through water than the loon when it flies in air. Thermal conductivity-the ability of a material to
transfer heat. Water’s thermal conductivity is more than 2 0 times that of air. The seal is therefore more
vulnerable to hypothermia in the cold northerly habitats of these species.
Specific heat capacity-the quantity of heat needed to raise the temperature of a gram of a material by one degree. It takes 3,500 times as much heat to
raise the temperature of a given volume of water by a d e g r e e than air b e c a u s e water is much denser. Water therefore heats up and cools down much more slowly than air; marine and freshwater habitats are much more thermally stable than terrestrial habitats.
Distinguish between the thermal properties of air and water as they relate to the habitat of animals.
[4]
Markscheme
water has a (much) higher specific heat capacity (than air);
water requires more energy gain/loss to change temperature;
(water) provides more stable thermal habitats than air / temperature of water bodies remains more constant than air temperature;
water has a higher thermal conductivity than air;
more heat is transferred from organisms to water than to air;
water is a good evaporative coolant and air is not;
aquatic mammals have (proportionally) more body fat as insulation;
What is the benefit to living organisms that water has a high specific heat capacity?
A. Heat can be lost from the skin when sweat evaporates.
B. Aquatic environments do not have a great fluctuation in their temperature.
C. The amount of heat stored by water is highly predictable.
D. It allows water to be a solvent for chemical reactions at body temperature.
[1]
Markscheme
B
Outline how the properties of water make it an effective coolant for the body.
[3]
Markscheme
hydrogen bonds hold water molecules together/make water molecules cohere;
evaporation requires breaking of hydrogen bonds / heat needed to break hydrogen bonds
water has a high heat of vaporization/high latent heat;
evaporation of water/sweat removes heat from/cools the skin/body;
A1.1.7—Extraplanetary origin of water on Earth and reasons for its retention
The abundance of water over billions of years of Earth’s history has allowed life to evolve. Limit hypotheses
for the origin of water on Earth to asteroids and reasons for retention to gravity and temperatures low
enough to condense water.
The Earth was formed from gas and dust about 4.5 billion years ago. Initially any water would have boiled and been lost, so the 1.4 billion cubic kilometres of water now on Earth must have arrived later, when the Earth had cooled down, so the water could remain liquid and be retained.
The leading hypothesis is that the water was brought when asteroids containing ice collided with the early Earth. The ice would have melted, adding to the volume of liquid water in the growing oceans.
Two factors promote the retention of water in a liquid state on Earth:
* strong gravitational pull due to Earth’s size intensity of sunlight due to distance from the Sun keeps the Earth below 100°C and mostly above 0°C
A1.1.8—Relationship between the search for extraterrestrial life and the presence of water
Include the idea of the “Goldilocks zone”.
Water is a unifying feature in biology-it is the medium of life for all species on Earth. We may reasonably expect life only to be found on planets in the universe that have liquid water. When we are searching for extraterrestrial life, we need not look on dry planets!
To retain liquid water, a planet must be large enough for gravity to be strong and temperatures must be warm enough for ice to melt, but not so hot that water boils. The planet must be in the “Goldilocks zone” which is the range of distances from a star that keep temperatures between 0 and 100°C.
Planet
Distance from
Sun (km)
Average
temperature (°C)
Venus
108 × 106
+462
Earth
149 × 106
+14
Mars
216 × 106
-60
Meanings of terms
Acid—a substance that can donate one or more protons and so become negatively charged, for example deoxyribonucleic acid (DNA).
Atom-a unit of matter with positively charged protons grouped in the nucleus and negatively charged electrons in orbitals around the nucleus; atoms have no net charge because the number of protons and electrons is equal.
Base-a substance that can accept one or more protons
(H*) and so become positively charged, for example adenine and other bases in DNA.
Catalyst-a substance that increases the rate of a reaction but is itself unchanged at the end of the reaction.
Concentration-the amount of substance per unit volume.
Condensation-a reaction in which two molecules are combined into one molecule and water is eliminated.
Covalent bond—a region of high electron density between two atoms due to the sharing of a pair of electrons, which attracts the nuclei of both atoms, holding them together.
Hydrogen bond-an attraction between an electronegative atom (such as oxygen) and a hydrogen atom bonded to another electronegative atom.
Hydrolysis-the separation of one molecule into two using hydrogen (H) and hydroxyl groups (OH) released by splitting a water molecule.
Hydrophilic-having an affinity (attraction) for water.
Hydrophobic-having a low affinity for water and more affinity for non-polar molecules.
Intermolecular force an attraction between molecules.
Intramolecular force —a bond between atoms within a molecule.
lon-an atom or molecule that has become positively charged by losing one or more electrons or negatively charged by gaining one or more electrons.
lonic bond—a bond formed by attraction between positively- charged and negatively charged ions or between positively- charged and negatively charged groups on molecules.
Isotope one of the two or more alternative forms of an element, with the same number of protons and electrons per atom as other forms, but a different number of neutrons.
Macromolecule a molecule with a molecular mass of more than 10,000
Molecule two or more atoms joined together by one or more covalent bonds.
Oxidation-a reaction in which hydrogen is removed, electrons are removed, or oxygen is added.
Pigment-a substance that absorbs wavelengths of visible light and so appears coloured.
Polymer-a molecule consisting of a series of many subunits linked by covalent bonds; an oligomer has fewer than 10 subunits.
Radioactive a type of isotope with an unstable nucleus which can emit radiation (a, B or y rays).
Reduction- a reaction in which hydrogen is added, electrons are added, or oxygen is removed.
Solute- a dissolved substance in a solution.
Solvent-a liquid that can dissolve other substances to make a solution.
Synthesis-the production of more complex molecules from simpler substances by one or more chemical reactions.
