2.1.2b Water Flashcards
How much of a cell is water
Water makes up about 80% of a cell’s contents
Functions of water
- Water is a REACTANT in many chemical reactions (eg. hydrolysis)
- Water is a SOLVENT, many substances dissolve in it (eg. ions in the water in the bloodstream). Most biological reactions take place in a solution
- Water TRANSPORTS substances sa. glucose, mineral ions, oxygen
- Water is involved in TEMPERATURE CONTROL bc it has a high specific heat capacity & high latent heat of evaporation
- Water is a HABITAT. Nutrients can be dissolved in water. It contains oxygen which is essential to life. Many organisms can survive/reproduce in it
What atoms make up water molecules
H2O contains:
- 2 Hydrogen (H) atoms
- 1 Oxygen (O) atom
The atoms are covalently bonded, meaning they bond by sharing outer electrons
See diagram of the atoms in H2O on slide 3-7
Structure of a water molecule (charge)
- Bc the shared negative hydrogen electrons are pulled towards the oxygen atom, the other side of each hydrogen atom is left w a slight positive charge
- The unshared electrons on the oxygen atom give it a slight negative charge
This makes water a POLAR MOLECULE - it has a partial negative (8-) charge on one side & a partial positive (8+) charge on the other
What is hydrogen bonding
The slightly negatively-charged oxygen atoms attract the slightly positively-charged hydrogen atoms of other water molecules.
This attraction is called hydrogen bonding & it gives water some of its useful properties
Structure of water related to its function
- Hydrogen bonds give water a high specific heat capacity
- Hydrogen bonds also give water a high latent heat of evaporation
- Water’s polarity makes it very cohesive
- Water’s polarity also makes it a good solvent
- Water is less dense when its solid
Structure of water related to its function: High specific heat capacity
Hydrogen bonds give water high specific heat capacity.
- SHC is the energy needed to raise the temp of 1g or a substance by 1dc.
- The hydrogen bonds between water molecules can absorb lots of energy. So water has a high SHC - it takes lots of energy to heat up
- This means water doesnt experience rapid temp changes, which is one of the properties that makes it a good habitat. The temperature underwater is likely to be more stable than it is on land
Structure of water related to its function: High latent heat of evaporation
Hydrogen bonds give water a high latent heat of evaporation
- It takes a lot of energy (heat) to break the hydrogen bonds between water molecules. So water has a high LHoE as a lot of energy is used up when water evaporates
- This is useful for organisms bc it means water’s great for cooling things. This is why some mammals, like us, sweat when theyre too hot. When sweat evaporates, it cools the surface of the skin
Structure of water related to its function: Very cohesive
Waters polarity makes it very cohesive
- Cohesion is the attraction between molecules of the same type (eg. 2 water molecules). Water molecules are very cohesive (tend to stick tg) bc theyre polar
- This helps water to flow, making it great for transporting substances. Also helps water to be transported up plant stems in the transpiration stream
Structure of water related to its function: Good solvent
Waters polarity makes it a good solvent
- A lot of important substances in biological reactions are ionic (like salt), meaning theyre made from 1 positive and 1 negatively-charged atom.
- Bc water is polar, the slight positive end of a water molecule will be attracted to the negative ions, & the slightly negative end will be attracted to the positive ion
- This means the ions will get totally surrounded by water molecules - in other words, they’ll dissolve
- Waters polarity makes it useful as a solvent in organisms. Eg. in humans, important ions can dissolve in the water in blood & then be transported around the body. It is also able to dilute toxic substances
See showbie slide 12 to see diagram of ions dissolved in water
Structure of water related to its function: Less dense when solid
Water is less dense when solid
- At low temps water freezes, turning from liquid to solid
- Water molecules are held further apart in ice than they are in liquid water. This is bc each water molecule forms 4 hydrogen bonds to other water molecules, making a lattice shape. This makes ice less dense than liquid water, which is why ice floats
- This is useful for organisms bc, in cold temps, ice forms an insulating layer on top of water - the water below doesn’t freeze, so organisms that live in water dont freeze & can still move around
