water Flashcards
functions
1) Water is a reactant in loads of important chemical reactions, including hydrolysis reactions (see page 22).
2)Water is a solvent, which means some substances dissolve in it. Most biological reactions take place
in solution (e.g. in the cytoplasm of eukaryotic and prokaryotic cells) so water’s pretty essential.
3)Water transports substances. The fact that it’s a liquid and a solvent means it can easily transport all sorts of materials, like glucose and oxygen, around plants and animals.
4) Water helps with temperature control because it has a high specific heat capacity and a high latent heat of evaporation (see below).
5) Water is a habitat. The fact that it helps with temperature control, is a solvent and becomes less dense when it freezes (see next page) means many organisms can survive and reproduce in it.
structure
1) A molecule of water (H,O) is one atom of oxygen (O)
2)
joined to two atoms of hydrogen (H,) by shared electrons.
Because the shared negative hydrogen electrons are pulled towards the oxygen atom, the other side of each hydrogen atom is left with a slight positive charge.
3) The unshared negative electrons on the
4)
oxygen atom give it a slight negative charge.
This makes water a polar molecule — it has a partial negative (&-) charge on one side and a partial positive (&+) charge on the other.
5) The slightly negatively-charged oxygen atoms attract the slightly positively-charged hydrogen atoms of other water molecules.
6)
This attraction is called hydrogen bonding and it gives water some of its useful properties.
properties
Hydrogen Bonds Give Water a High Specific Heat Capacity
2)
Specific heat capacity is the energy needed to raise the temperature of 1 gram of a substance by 1 °C.
The hydrogen bonds between water molecules can absorb a lot of energy.
So water has a high specific heat capacity - it takes a lot of energy to heat it up.
3)This means water doesn’t experience rapid temperature changes, which is one of the properties that makes it a good habitat — the temperature under water is likely to be more stable than it is on land.
Hydrogen Bonds Also Give Water a High Latent Heat of Evaporation
1) It takes a lot of energy (heat) to break the hydrogen bonds between water molecules.
2) So water has a high latent heat of evaporation — a lot of energy is used up when water evaporates.
3) This is useful for living organisms because it means water’s great for cooling things. This is why some mammals, like us, sweat when they’re too hot. When sweat evaporates, it cools the surface of the skin.
Water’s Polarity Makes it Very Cohesive
1) Cohesion is the attraction between molecules of the same type (e.g. two water molecules).
Water molecules are very cohesive (they tend to stick together) because they’re polar.
2) This helps water to flow, making it great for transporting substances. It also helps water to be transported up plant stems in the transpiration stream (see page 91).
Water’s Polarity Also Makes it a Good Solvent
1) A lot of important substances in biological reactions are ionic (like salt, for example). This means they’re made from one positively-charged atom or molecule and one negatively-charged atom or molecule (e.g. salt is made from a positive sodium ion and a negative chloride ion).
2) Because water is polar, the slightly positive end of a water molecule will be attracted to the negative ion, and the slightly negative end of a water molecule will be attracted to the positive ion.
3) This means the ions will get totally surrounded by water molecules — in other words, they’ll dissolve.
4) Water’s polarity makes it useful as a solvent in living organisms. E.g. in humans, important ions (see p. 29) can dissolve in the water in blood and then be transported around the body.
Water’s Less Dense When it’s Solid
1) At low temperatures water freezes - it turns from a liquid to a solid.
2) Water molecules are held further apart in ice than they are in liquid water because each water molecule forms four hydrogen bonds to other water molecules, making a lattice shape. This makes ice less dense than liquid water - which is why ice floats.
3)
This is useful for living organisms because, in cold temperatures, ice forms an insulating layer on top of water — the water below doesn’t freeze. So organisms that live in water, like fish, don’t freeze and can still move around.
