Lipids Flashcards
What are lipids
Lipids, commonly known as fats and oils, are molecules containing the elements carbon, hydrogen and oxygen. Generally, fats and lipids that are solid at room temperature and oils are lipids that are liquid at room temperature.
Lipids are non-polar molecules as the electrons in the outer orbitals that form the bonds are more evenly distributed than in polar molecules. This means that there are no positive or negative areas within the molecules and for this reason lipids aren’t soluble in water. Oil and water don’t mix.
Lipids are large complex molecules known as macromolecules, which are built from repeating units, or monomers, like polysaccharides.
How is a triglyceride made
Made by combining one glycerol molecule with three fatty acids.
Glycerol is a member of a group of molecules called alcohols
Fatty acids belong to a group of molecules called carboxylic acids-they consist of a carboxyl group (-COOH) with a hydrocarbon chain attached.
What do both glycerol and fatty acid molecules contain
Hydroxyl (OH) groups. The hydroxyl groups interact, leading to the formation of three water molecules and bonds between fatty acids and the glycerol molecules. These are called ester bonds and this reaction is called esterification. Esterification is another example of a condensation reaction.
When triglycerides are broken down, three water molecules need to be supplied to reverse the reaction that formed the triglyceride. Another hydrolysis reaction.
Are fatty acids saturated or unsaturated
Fatty acid chains that have no double bonds present between carbon atoms are called saturated, because all the carbon atoms form the maximum number of bonds with hydrogen atoms.
A fatty acid with double bonds between some carbon atoms is called unsaturated. If there is just one double bond it is called monounsaturated. If there are two or more double bonds it is called polyunsaturated. The presence of double bonds causes the molecule to kink or bend and they therefore cannot pack so closely together. This makes them liquid at room temperature rather than solid, and they are therefore described as oils rather than fats.
Plants contain unsaturated triglycerides, which normally occur as oils, and tend to be more healthy in the human diet than saturated triglycerides, or fats. There has been some research evidence that in excess, saturated fats can lead to coronary heart disease, however the evidence remains inconclusive. An excess of any type of fat can lead to obesity , which also puts a strain on the heart.
Phospholipids
Are modified triglycerides and contain the element phosphorus along with carbon, hydrogen and oxygen. Inorganic phosphate ions are found in the cytoplasm of every cell. The phosphate ions have extra electrons and so are negatively charged, making them soluble in water.
One of the fatty acid chains in a triglyceride molecule is replaced with a phosphate group to make a phospholipid.
Phospholipids are unusual because, due to their length, they have a non-polar end or tail (fatty acid chains) and a charged end or head (phosphate group). The non-polar tails are repelled by water (but mix readily with fat). They are hydrophobic. The charged heads (often incorrectly called polar ends) will interact with, and are attracted to water, they hydrophilic.
How do phospholipids interact with water
As a result of their dual hydrophobic/hydrophilic structure, phospholipids behave in an interesting way when they interact with water. They will form a layer on the surface of the water with the phosphate heads in the water and the fatty acid tails sticking out of the water. Because of this they are called surface active agents or surfactants for short.
They can also form structures based on a two-layered sheet formation (bilayer) with all of their hydrophobic tails pointing toward toward the centre of the sheet, protected from the water by the hydrophilic heads. It is as a result of this bilayer arrangement that phospholipids play a key role in forming cell membranes. They are able to separate an aqueous environment in which cells usually exist from the aqueous cytosol within cells. It is thought that this is how the first cells were formed and, later on, membrane-bound organelles within cells.
What are sterols
Also known as steroid alcohols, are another type of lipid found in cells. They are not fats or oils and have little in common with them structurally. They are complex alcohol molecules, based on a four carbon ring structure with a hydroxyl (OH) group at one end.
How are sterols similar to phospholipids
They have dual hydrophilic/hydrophobic characteristics. The hydroxyl group is polar and therefore hydrophilic and the rest of the molecule is hydrophobic.
What is cholesterol
A sterol. The body manufactures cholesterol primarily in the liver and intestines. It has an important role in the formation of of cell membranes, becoming positioned between the phospholipids with the hydroxyl group at the edge of the membrane. This adds stability to cell membranes and regulates their fluidity by keeping membranes fluid at low temperatures and stopping them becoming too fluid at high temperatures.
Vitamin D, steroid hormones and bile are all manufactured using cholesterol.
What are roles of lipids due to their non-polar nature
-membrane formation and the creation of hydrophobic barriers
-hormone production
-Electrical insulation necessary for impulse transmission.
-waterproofing, for example in birds feathers and on plant leaves.
Roles of lipids (triglycerides) when stored under skin and around vital organs
-thermal insulation to reduce heat loss for example in penguins
-cushioning to protect vital organs such as the heart and kidneys.
-buoyancy for aquatic animals like whales
Identification of lipids
Lipids can be identified in the laboratory by a simple test known as emulsion test. Sample mixed with ethanol. Resulting solution is mixed with water and shaken. If a white emulsion forms as a layer on top of the solution this indicates the presence of a lipid. If the solution remains clear the test is negative.
