Chapter 3 - Biological molecules - Water Flashcards
Describe the structure of water
A molecule of water (H2O) is made up of one oxygen atom (O) joined to two hydrogen atoms (H). These atoms are held together by two covalent bonds.
How is water a polar molecule ?
The covalent pair of electron is pulled more to the / oxygen nucleus than the hydrogen nucleus. Because electrons are negatively charged, this gives the oxygen atom an overall negative charge.
For the hydrogens, the electrons are being pulled away so they become slightly more positive.
This means that water has regions of negativity and positivity.
How do 2 molecules of wate join together ?
Via hydrogen bonds. These hydrogen bonds form between many water molecules, causing them to stick together and giving water some of its useful properties.
How do water molecules form hydrogen bonds?
The partially positive hydrogen end of one water molecule attracts towards the partially negative oxygen end of another molecule. This force of attraction is known as a hydrogen bond.
Name 7 roles of water.
1.Solvent/transport
2. Metabolite
3. Habitat
4. Density
5. High latent heat of evaporation
6. Cohesion
7. Temperature control
Describe how water acts as a solvent
Ions and polar molecules dissolve in water. (Non polar molecules won’t)
Substances can be transported in water if they can dissolve.
Eg:Glucose moves around while dissolved
Describes how water acts as a metabolite
Many chemical reactions involve water. For example it is a metabolite in metabolic reactions such as condensation and hydrolysis reactions which are used in forming and breaking chemical bonds
Describe how water acts as a habitat
Water is a liquid at room temp. This means it provides a constant environment for living things in rivers, lakes and seas.
Describe the density of water
Water molecules are held further apart in ice, making it less dense than water.
This causes ice to float, forming an insulating layer at the surface of ponds and lakes. This means the water below this layer does not freeze, allowing organisms within the water to move and survive.
Describe how water has high latent heat of vaporation
It takes a lot of energy to break the hydrogen bonds between water molecules. Therefore when water evaporates, it absorbs a large amount of heat before it turns into a water vapour.
So mammals are cooled dow when sweat evaporates and plants are cooled when water evaporates from mesophyll cells
Describe how water acts as a temperature control
Water has a high specific heat capacity, which means a lot of energy is needed to raise the temperature of 1 gram of water by 1°C.
The many hydrogen bonds between water molecules can absorb a lot of energy before being broken, so it takes a lot of energy to break the hydrogen bonds and heat the water.
The high specific heat capacity of water means that it is resistant to rapid changes in temperature. As many organisms are made up of water, this allows the body to remain at a fairly stable temperature.
Describe how water is cohesive
The hydrogen bonds between molecules pull them towards each other, so the stick together.
Columns of water in plant vascular tissues are pulled up the xylem from the roots.
Insects like pond-skaters can walk on water
