Water: the medium for life

•       Earth has existed for 4.5 billion years

•       Water has existed on Earth for 3.8 billion years

•       First cells on Earth originated 2000 mya in water.

•       Water blocked harmful UV rays from destroying these cells leading to evolution of life forms

•       65%-95% by mass of most multicellular organisms is water

•       80% of human cells is water

Why is water important to life?

•       Provides a medium in which enzymes dissolve to work

•       Provides a chemically stable medium for life processes to operate

Atoms, Molecules and Covalent bonds

•       There are 118 chemical elements

•        These elements are the fundamental building blocks of matter.

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•       The structure of atoms includes protons and neutrons in the central nucleus and electrons in outside orbitals.

•       The outermost electrons are known as valence electrons.

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•       Pairs of valence electrons can be shared between atoms forming covalent bonds to form a molecule.

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•       A water molecule is formed by covalent bonding between 1 Oxygen atom and 2 hydrogen atoms sharing their valence electrons.

•       Valence electrons can be lost by an atom, creating a cation, or gained by an atom, creating an anion.

Polarity in water molecules

•       Electrons are not equally shared between oxygen and hydrogen atoms in a water molecule

•       Water molecule has the shape of letter V.

•       The nucleus of oxygen atom draws electrons away from hydrogen atoms.

•       There is a net negative charge on the oxygen atom and net positive charge on hydrogen atoms

Water molecules show Dipolarity

•       A polar molecule is one having positively & negatively charged areas because of an uneven distribution of electrons.

•       Water molecule has a:

1. Negative pole – Area with more electrons around oxygen atom

2. Positive pole – Area with fewer electrons near the hydrogen atom nuclei.

Hydrogen bond between water molecules

•       The positively charged hydrogen atoms are attracted to negatively charged oxygen atoms of neighbouring water molecules.

•       These (weak) attractive forces are called hydrogen bonds

Hydrogen bonds:

•       Are weaker than covalent bonds

•       Are strong enough to hold water molecules together

•       Attract water molecules to a charged surface or charged particles

•       Are largely responsible for the unique properties of water

Properties of water molecules :

Cohesion

•       Force by which individual molecules of the same type attract and associate (stick together)

•       H-bonds make water molecules stick to each other more strongly.

•       Cohesion is important in transporting water through xylem in vascular plants

•       In the leaves of a tree, water evaporates from the surface of inner cell walls into air spaces in the leaf.

•       As water molecules evaporate, they are replaced by water from within the cells

•       More water molecules move from the xylem into the leaf cells, creating tension, a force to pull water against gravity.

•       This tension is transmitted down a continuous column (cohesive force) of water molecules all the way to the root.

Cohesion and surface tension

•       There are no neighbouring water molecules above the surface of water.

•       These water molecules exhibit stronger attractive forces with water molecules below them and with nearest neighbours on the surface.

•       The net inward force causes molecules on the surface to contract and resist being stretched or broken.

•       This is called surface tension.

Consequence of surface tension to organisms

•       Water striders (Gerris lacustris, also called pond skaters) live on the surface of calm and unpolluted bodies of water.

•       Mass of the insect is not great enough to break the surface tension.

•       Waxy cuticle prevents the insect from getting wet.

•       When the pond skater moves over the surface of water:

1. Surface tension supports it

2. The surface of water is depressed but hydrogen bonds hold it together

Adhesion of water and its impact on organisms

•       The attraction between water and other materials

•       Adhesion make surfaces “wet”

•       Hydrogen bonds are readily formed between molecules and a hydrophilic substance

•       Water adheres to most surfaces and can be drawn up long columns like xylem

n.b. cohesion is a far stronger force in xylem transport than adhesion – cohesion can resist tension

Viscosity

•       Water molecules slide past each other very easily, below the surface

•       Hence water flows through tiny capillaries, gaps and pores.

Capillary action in soils

•       Cohesion and adhesion causes water to be pulled through narrow tubes and spaces – capillary action

•       Occurs when adhesion is greater than cohesion

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•       Channels between soil particles (where plant roots are) behave like capillary tubes.

•       Adhesion between water molecules and these capillary tubes draws up water, above the water table

•       Hence soil does not dry out around plant roots.

Capillary action in plant cell walls

•       Cell wall in plant cells are made of a fibrous material called cellulose

•       As water enters a leaf cell from the xylem, water is pulled by capillary action into the spaces between the cellulose fibers

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•       Water evaporates from the surface of spongy mesophyll cells into the air spaces inside the leaf and then out through the stomata

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•       Capillary action draws out more water from the xylem into the walls of these cells, preventing these cells from drying out.

•       These events draw water up continuously through the plant.

Water is a liquid at room temperature

•       The potential energy of hydrogen bonds in water is greater than the KE of its molecules upto 100⁰C.

•       Hence hydrogen bonds pull water molecules very close to each other

•       This makes water liquid at normal temperature and pressure.

Solvent properties of water and polar substances

•       Ionic substances (extremely polar) like (NaCl) dissociate into anions (negative charge) and cations (positive charge) while dissolving in water

•       These ions become surrounded by a layer of orientated water molecules

•       Amino acids are hydrophilic as they have ionized groups - positively charged amino (‒NH₃⁺) groups and negatively charged carboxyl (‒COO⁻) groups.

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•       Sugars have slightly charged hydroxyl (–OH) groups, which forms hydrogen bonds with water to dissolve in it.

•       These hydrophilic molecules become chemically reactive and are free to move around inside the cell in soluble form.

Solvent properties of water and non-polar substances

•       Non-polar substances are hydrophobic and are repelled by water.

•       Cell membrane is made from phospholipids.

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•       Each phospholipid molecule has a hydrophilic part which forms the head and two hydrophobic tails.

•       While forming cell membrane, the phospholipids orient themselves to form a bilayer.

•       Hydrophilic heads of the bilayer are in contact with aqueous environment inside and outside the cell.

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•       Hydrophobic tails are oriented inwards away from the watery environment.

•       CO₂ is more soluble in water than oxygen and nitrogen.

•       Some CO₂ dissolves in water to form carbonic acid (H₂CO₃) which dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻)

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•       Oxygen gas is a non-polar substance.

•       Poor solubility of oxygen has resulted in evolution of respiratory pigments like haemoglobin which has efficient oxygen-carrying capacity than water.

Solvent properties of water and metabolic reactions

•       Most enzymes become active forms only in water and hence many enzyme-catalysed reactions occur in water

•       Water is a requirement for some enzymes to maintain its shape, stability and functionality.

•       Hydrogen bonds often act as bridges between the active site of enzymes and its substrates.

Solvent properties of water and transport

•       Polar substances like amino acids and sugars are transported from cell to cell in a watery medium in multicellular plants and animals.

•       Blood in animals have plasma which uses water as a solvent to transport small, hydrophilic molecules.

•       Plants transport sucrose and amino acids which are dissolved in the phloem sap

Movement of water through cell membrane

•       Water moves through special transmembrane proteins called aquaporins.

•       These proteins have a hydrophilic interior which allows water to move inside or outside a cell

•       These proteins are anchored in place in the membrane by a hydrophobic exterior

Ø  Buoyancy

•       The ability of any fluid to provide a vertical upward force on an object placed in/on it.

•       Liquid water has high density allowing materials that are less dense to float on it.

•       Air is less dense than water, so less buoyant as well

Ø  Viscosity

•       Viscosity is resistance to flow

•       Related to how much energy is needed to change the shape of a liquid.

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•       Water has low viscosity compared with other liquids, but greater viscosity than air.

•       Water molecules slide past each other very easily, below the surface

•       Hence water flows through tiny capillaries, gaps and pores.

Ø  Specific heat capacity

•       The amount of energy required to raise the temperature of 1 kg of a substance by 1⁰C.

•       Water has a higher specific heat capacity than air

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•       More energy is needed to overcome the intermolecular forces (hydrogen bonds) in water to increase its temperature

•       Aquatic environments therefore, have stable temperature.

•       Water has the ability to absorb and lose heat without undergoing large temperature change.

•       This protects cells and organisms from large changes in temperature fluctuations.

Ø  Thermal conductivity

•       The ability of a substance to transfer heat when there is a temperature difference.

•       Water has a high thermal conductivity compared with other liquids (except liquid metals).

•       Air has a much lower thermal conductivity than water.

Physical properties of water and its consequences to Ringed Seal and Black-throated loon

Refer to WS

Extraplanetary origin of water on Earth

•       Extremely hot conditions on Earth 4.5 b.y.a

•       Impossible for water vapour to condense into liquid water

•       Asteroids and meteorites are most likely Earth’s source of water

•       Asteroids still contain ice and amino acids – critical for evolution of life

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•       Meteorites (break off from asteroids) like carbonaceous chondrites contain water trapped in mineral crystals

•       These contain isotopes of hydrogen (deuterium and protium), which closely matches with that of ocean water.

•       Two 4.5-billion-year-old meteorites support this hypothesis

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•       The meteorite eucrite achondrite (origin from asteroid Vesta in the asteroid belt between Jupiter and Mars) has deuterium protium ratio that matches with current Earth.

•       Meteorites heated up on impact with Earth releasing water vapour which were trapped by Earth’s gravity

•       When surface temperature of Earth became cooler, water vapour condensed into liquid water.

•       Earth’s gravity retained this water.

•       The presence of liquid water as a solvent and medium for metabolism is considered necessary for life.

Goldilock Zone

•       Astrobiologists believe extraterrestrial life will be found only in the presence of liquid water on other planets or natural satellites.

•       The habitable zone, or Goldilocks zone, refers to the orbital distance from a star that will result in liquid water.

•       Earth is in the Goldilocks zone because its distance from its star (the Sun) is neither too hot nor too cold to prevent liquid water – it is just the right distance.

Search for extraterrestrial life

•       Wavelengths of light are absorbed or reflected when a planet passes in front of its nearest star.

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•       By analysing the light (transit spectroscopy) it is possible to find whether the atmosphere of the planet contains water.

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•       This technique is used to establish that exoplanets in the Goldilocks zone (right distance from its star and right size) like Kepler-186f may have a “water signature” and therefore extraterrestrial life.