Question

Carbohydrates and lipids are important molecules found in living things.
Part A: Describe how the general structure of a carbohydrate differs from the general structure of a lipid.
Part B: Describe one function that is common to both carbohydrates and lipids.
Part C: A phospholipid is a common type of lipid molecule that is hydrophilic at one end and hydrophobic at the other. Explain how this structural characteristic of a phospholipid contributes to the formation of plasma membranes.
Type your responses to parts A, B, and C in the box below.

Answer 1

Part A: There are many different kinds of bonds that hold molecules together. Some molecules are held together by covalent bonds between the atoms. These tend to require a lot of energy to form - hence in biologic systems they usually require enzymes to be made, and they are very stable so they don't break easily. Some molecular assemblies are held together by non-bonded interactions which are far more transient. That is a glaring hint for you.

Answer 2

Part A-Carbohydrates are soluble in water due to the fact that contain an OH group that can hydrogen bond with water. Lipids, contain long chains of hydrocarbons (made of hydrogens and carbon) and are hydrophobic. Lipids are important to the lipid bilayer found in cells, which contribute to fluidity and stability.
Part B- Both carbohydrates and lipids are sources of energy for the human body. However, carbohydrates are the main source of energy.
Part C- A phospholipid contains a hydrophilic head and a hydrophobic tail. The head contains a phosphate and alcohol (glycerol or sphingosine) joined to hydrophobic fatty acid tails by phosphodiesterase linkages. The fact that the head contains an OH group (alcohol) gives the phospholipid the ability to hydrogen bond outside of the lipid bilayer while the hydrophobic ends are inside the bilayer. An example of this mechanism are micelles.

Answer 3

Part A: How is the general structure of a carbohyrdrate different from the structure of a lipid?

Well, carbohydrates are simply 'hydrated carbon'. It has a general formula of (CH2O)n which, if you look, means one carbon for every water molecule. Anyways, carbohydrates typically contain hydroxyl groups. Notice that carbohydrates engage In condensation reactions, for instance two monomers react at the hydroxyl groups to produce water and link via a glycosidic linkage. If you look at say, for instance, at a molecule of glucose, you will see the formula is C6H12O6, and if you look at the structure you will see that their are 6 carbon atoms AND 6 hydroxyl groups (-OH)... Based on this information we can realise that carbohydrates have polar bonds (O-H) which can form hydrogen bonds with water molecules. Therefore, we know that carbohydrates are soluble in water. However, polysaccharides such as starch are not soluble in water - it appears as the chain increases, solubility decreases. Overall, carbohydrates are broadly hydrophilic (Water Loving).

What about lipids? Lipids are another category of compounds. Lipids tend to be insoluble in water, therefore they are hydrophobic. Lipids are derived from a chemical reaction of glycerol and fatty acids. Since glycerol is trihedric (three -OH groups) they react with three fatty acids to produce ester linkages. Remember esters are formed by the reaction of an organic weak acid (R-COOH) and alcohol (in this case glycerol). Due to the overall polarity of lipids, they are generally non-polar due to the carbon chain attached to the (-COOH group) R-COOH. Due to the non-polar nature of lipids, they are insoluble in water but soluble in non-polar substances such as ethanol. Overall, lipids are broadly hydrophobic.

To summarise the differences

Carbohydrates contain glycosidic bonds whereas lipids contain ester linkages

Carbohydrates are generally hydrophilic, however as their chain increases in size it becomes less soluble in water; whereas lipids are generally hydrophobic, even when their chain size is relatively small.

Part B: What function is common to both carbohydrates and lipids? Well we know that carbohydrates and fats (lipids) release energy upon oxidation, so they can be used as energy. Indeed they are, glucose is used as energy and fats are used as energy during times of starvation. We also know that proteins can be used as energy but they produce nitrogen based waste whereas lipids and carbohydrates do not.

So, they can both USED as energy and they can be STORED as an energy source (glycogen, fat).

Part C: How does the hydrophobic end and hydrophilic end of a phospholipid contribute to the formation of plasma membrane(s). A phospholipid is a lipid which has had one of the fatty acid chains substituted with a phosphate group. The result is a compound that is not a "true lipid" but is instead a compound of (generally) a phosphate group, glycerol and fatty acid tails. The phosphate group contributes to the water soluble head, whereas as the fatty acid tails contribute to the water insoluble tails. This means that the orientation of phospholipids is to minimise repulsion, so they form a bilayer, which makes the tails face away from the water and the heads face towards them. The tails face inwards, head outwards.