B4. Water Flashcards
The importance of water
Water is vital to living organisms. It makes up about 80% of a cell’s contents and has loads of important functions, inside and outside cells: (4 things)
- Water is a metabolite in loads of important metabolic reactions, including condensation and hydrolysis reactions
- Water is a solvent, which means some substances dissolve in it. Most metabolic reactions take place in solution (e.g. in the cytoplasm of eukaryotic and prokaryotic cells) so water’s pretty essential.
- Water helps with temperature control because it has a high latent heat of vaporisation and a high specific heat capacity
- Water molecules are very cohesive (they stick together), which helps water transport in plants as well as transport in other organisms.
Structure of water - Polarity of water
A molecule of water (H,O) is one atom of oxygen (O) joined to two atoms of hydrogen (H) by shared __________
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. The unshared negative electrons on the oxygen atom give it a slight negative charge. This makes water a ______molecule - it has a slight (partial) negative charge on one side and a slight (partial) positive charge on the other.
Polarity of water
A molecule of water (H,O) is one atom of oxygen (O) 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. The unshared negative electrons on the oxygen atom give it a slight negative charge. This makes water a polar molecule - it has a slight (partial) negative charge on one side and a slight (partial) positive charge on the other.
Structure of water - Hydrogen bonding
What are hydrogen bonds?
Where do they form?
Hydrogen bonding
Hydrogen bonds are weak bonds between a slightly positively charged hydrogen atom in one molecule and a slightly negatively charged atom in another molecule. Hydrogen bonds form between water molecules because the slightly negatively charged oxygen atoms of water attract the slightly positively charged hydrogen atoms of other water molecules. This hydrogen bonding gives water some of its useful properties.
Properties of water - Important metabolite
What type of reactions?
What exactly happens in these reactions?
Use amino acids as a example?
Many metabolic reactions involve a condensation or hydrolysis reaction. A hydrolysis reaction requires a molecule of water to break a bond. A condensation reaction releases a molecule of water as a new bond is formed. For example, amino acids are joined together to make polypeptides (proteins) by condensation reactions. Energy from ATP is released through a hydrolysis reaction
Properties of water - Good solvent
A lot of important substances in biological reactions are _____(like ___, for example). This means they’re made from one __________ __________ ____or molecule and one _____________ __________ _____or molecule (e.g. ____is made from a positive sodium ion and a negative chloride ion).
Because water is _____, the __________positively charged end of a water molecule will be attracted to the negative ion, and the _________negatively charged end of a water molecule will be attracted to the positive ion. This means the ions will get totally surrounded by water molecules in other words, they’ll __________.
So water’s polarity makes it useful as a solvent (a substance capable of dissolving another substance). This means living organisms can take up useful substances (e.g, mineral ions) _________in water and these dissolved substances can be _____________around the organism’s body.
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).
Because water is polar, the slightly positively charged end of a water molecule will be attracted to the negative ion, and the slightly negatively charged end of a water molecule will be attracted to the positive ion. This means the ions will get totally surrounded by water molecules in other words, they’ll dissolve.
So water’s polarity makes it useful as a solvent (a substance capable of dissolving another substance). This means living organisms can take up useful substances (e.g, mineral ions) dissolved in water and these dissolved substances can be transported around the organism’s body.
Properties of water - High latent heat of vaporisation
What does this mean?
Energy?
Why is it useful?
How is it useful?
Water evaporates (vaporises) when the hydrogen bonds holding water molecules together are broken. This allows the water molecules on the surface of the water to escape into the air as a gas. It takes a lot of energy (heat) to break the hydrogen bonds between water molecules, so a lot of energy is used up when water evaporates. This means water has a high latent heat of vaporisation lots of heat is used to change it from a liquid to a gas.
This is useful for living organisms because it means they can use water loss through evaporation to cool down without losing too much water. When water evaporates it carries away heat energy from a surface, which cools the surface and helps to lower the temperature (e.g. when humans sweat to cool down).
Properties of water - Can buffer (resist) changes in temperature
How?
2 reasons for why this is useful?
Hydrogen bonds give water a high specific heat capacity—this is the energy needed to raise the temperature of 1 gram of a substance by 1 °C. When water is heated, a lot of the heat energy is used to break the hydrogen bonds between the water molecules. This means there is less heat energy available to actually increase the temperature of the water. So water has a high specific heat capacity-it takes a lot of energy to heat it up.
This is useful for living organisms because it means that water doesn’t experience rapid temperature changes. This makes water a good habitat because the temperature under water is likely to be more stable than on land.
The water inside organisms also remains at a fairly stable temperature- helping them to maintain a constant internal body temperature.
Properties of water - Very cohesive
What does this mean?
Why is this the case?
Where is it useful?
What does it allow?
Very cohesive
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. Strong cohesion helps water to flow, making it great for transporting substances. For example, it’s how water travels in columns up the xylem (tube-like transport cells) in plants. Strong cohesion also means that water has a high surface tension when it comes into contact with air. This is the reason why sweat forms droplets, which evaporate from the skin to cool an organism down. It’s also the reason that pond skaters, and some other insects, can ‘walk’ on the surface of a pond.
