Biological Molecules - Water Flashcards
What is the structure of water?
2 atoms of hydrogen covalently bonded to a molecule of oxygen. There are intermolecular forces between water molecules.
What is a polar molecule?
When electrons are not evenly distributed across the molecule. The hydrogen is slightly positive and the oxygen is slightly negative, but there is no overall charge. Water is therefore described as dipolar. The positive pole of one water molecule is attracted to the negative pole of another. The attraction between the charges forms the hydrogen bond between water molecules. Each bond is weak but together allow water to stick together. The number of bonds gives considerable strength.
What is electronegative?
Oxygen is electronegative. Electrons are more attracted to the oxygen atoms within the covalent bond because oxygen has more protons.
What is cohesion?
Cohesion is the attraction between two water molecules due to the polar nature of the molecule. Water molecules form hydrogen bonds (slightly positive hydrogen attracts to slightly negative oxygen of another molecule) and stick together. The large cohesive forces allow water to be pulled up a tube in a continuous column (e.g. xylem).
What does cohesion result in?
More energy being needed to separate water molecules.
- higher boiling point than expected
- high specific heat capacity
- high latent heat of vaporisation
What is surface tension?
At an air-water surface, the cohesion between water molecules produces surface tension. Within a column of water, cohesion also explains why the column does not break when water molecules are pulled up a narrow tube. When water molecules meet air, they are pulled back into the body of water. Water acts like a skin as it is strong enough to support small organisms such as pond skaters.
What is the transpiration stream?
Cohesion allows a continuous stream of water in the transpiration stream, moving water up the xylem in plants without any breaks.
If there were breaks, excessive water would be stored in the cells before the break, and the cell would burst under osmotic pressure.
What is adhesion?
Adhesion refers to the tendency of water molecules to be attracted, or “stick” to other substances (this causes capillary action). This is a result of the covalent bond between the two hydrogen atoms and the one oxygen atom in the water molecule. Because it is polar, it is attracted to substances that have charges.
Why is oxygen highly electronegative?
It has relatively a lot of positive charge in its nucleus, with only two electron shells to shield it. The positive nucleus isn’t shielded very well, attracting electrons. The oxygen atom attracts an “unequal share” of electrons.
What are the properties of water?
- It is a metabolite in many metabolic reactions, including condensation and hydrolysis reactions which are used in the forming and breaking of chemical bonds. A cell’s metabolic reactions occur in aqueous solution, and water is a major raw material in photosynthesis.
- It is an important solvent in which metabolic reactions occur. Water readily dissolves other substances (e.g. gases such as oxygen and carbon dioxide; wastes such as ammonia and urea; inorganic ions and small hydrophilic molecules such as amino acids, monosaccharides and ATP; and enzymes, whose reactions take place in solution.)
- It has a relatively high heat capacity, buffering changes in temperature.
- It has a relatively large latent heat of vaporisation, providing a cooling effect with little loss of water through evaporation.
- It has strong cohesion between water molecules; this supports columns of water in the tube-like transport cells of plants (xylem vessels) and produces surface tension where water meets air (which allows small organisms such as Pond Skaters to walk along it).
Why does water have a high specific heat capacity?
Water’s high heat capacity is a property caused by hydrogen bonding among water molecules. When heat is absorbed, hydrogen bonds are broken and water molecules can move freely. When the temperature of water decreases, the hydrogen bonds are formed and release a considerable amount of energy. Water has the highest specific heat capacity of any liquid. Specific heat is defined as the amount of heat one gram of a substance must absorb or lose to change its temperature by one degree Celsius.
Even though each hydrogen bond by itself is weak compared to the bonds in the molecules themselves, there are many neighbours around each water molecule, which means that water can store a significant amount of energy in these bonds. As you add heat to water, instead of speeding up the molecules (increasing their kinetic energy and thus temperature) the hydrogen bonds get broken up, soaking in the additional heat without much change in temperature.
Why does water have a high latent heat of vaporisation?
Hydrogen bonding means that it requires a lot of energy to evaporate 1 gram of water (turn it into water vapour). Therefore water has a relatively large latent heat of vaporisation, meaning evaporation of water provides a cooling effect with little water loss e.g. sweating.
Why is having a high specific heat capacity important?
Water acts as a temperature buffer. The temperature of the water in ponds, lakes and seas does not fluctuate very much compared to the air above them. This is important for enzyme controlled reactions and animals who cannot regulate their own temperature (cold-blooded animals). It is also important in making oceans a stable environment for aquatic organisms to live.
Why is having a high latent heat of vaporisation important?
The hydrogen bonding means that a large amount of heat is required to evaporate water. Evaporation of water such as sweat in mammals is therefore a very effective means of cooling because body heat is used to evaporate the water.
Why is water a solvent?
Water is a solvent allowing gases to readily diffuse as well as enzymes and waste products e.g. ammonia and urea. Because it has polar molecules, water is attracted to any substance that is also polar.
Only triglycerides and large polymers do not dissolve in water.
Ions and polar molecules will dissolve in water but non-polar molecules will not.
