chapter 3 p3 Flashcards
Lipids
- commonly known as fats and oils, are molecules containing the elements carbon, hydrogen, and oxygen.
- Generally, fats are 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 there are no positive or negative areas within the molecules and for this reason lipids are not soluble in water.
- Oil and water do not mix.
- Lipids are large complex molecules known as macromolecules, which are built from repeating units, or monomers, like polysaccharides.
Triglycerides
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A triglyceride is 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.
both of these molecules contain hydroxyl (OH) groups.
Triglycerides
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The hydroxyl groups interact, leading to the formation of three water molecules and bonds between the fatty acids and the glycerol molecule.
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.
This is another example of a hydrolysis reaction
Saturated and unsaturated: p1
Fatty acid chains that have no double bonds present between the carbon atoms are called saturated, because all the carbon atoms form the maximum number of bonds with hydrogen atoms (i.e., they are saturated with hydrogen atoms).
A fatty acid with double bonds between some of the 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.
Saturated and unsaturated: p2
The presence of double bonds causes the molecule to kink or bend (Figure 2) 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 (solid) fats.
There has been some 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:
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Phospholipids are modified triglycerides and contain the clement phosphorus along with carbon, hydrogen, and oxygen.
Inorganic phosphate ions (PO,) 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:
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Phospholipids are unusual because, due to their length, they have a non-polar end or tail (the fatty acid chains) and a charged end or head (the 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 are hydrophilic.
structure of a phospholipid
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 (Figure 4).
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 (a bilayer) with all of their hydrophobic tails pointing toward the centre of the sheet, protected from the water by the hydrophilic heads (Figure 5).
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.
Sterols:
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.
Like phospholipids, however, they have dual hydrophilic/hydrophobic characteristics.
The hydroxyl group is polar and therefore hydrophilic and the rest of the molecule is hydrophobic.
Cholesterol
- is a sterol.
- The body manufactures cholesterol primarily in the liver and intestines.
- It has an important role in the formation of cell membranes, becoming positioned between the phospholipids with the hydroxyl group at the periphery 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.
Roles of lipids:
- Due to their non-polar nature, lipids have many biological roles.
These include: - 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.
Lipids, triglycerides in particular, also have an important role in long-term energy storage. They are stored under the skin and around vital organs, where they also provide:
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 the emulsion test. First, the sample is mixed with ethanol.
The 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.
Changing health advice:
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It can be confusing because health advice constantly changes.
The way that new advice is issued in the media from new findings is partly responsible.
The validity of the research has not usually been evaluated, the science is often not easy to explain, and as the majority of the general public do not have scientific background they are not aware of the fluid nature of scientific understanding.
Scientific knowledge is also constantly changing as technology develops and so our understanding of biological processes increases.
