Lipids are fats. Remember, lipids are not made out of building blocks like carbohydrates, proteins, and nucleic acids. Instead, there are just different types of lipids. Here are the types of lipids that you have learned about:
The main thing that these types of lipids have in common is that they are all insoluble in water. Also, triglycerides and phospholipids both contain fatty acids and glycerol. You may want to go back and review what you learned about lipids back in the lipid page of Unit 2.
What lipids do we eat?
As far as quantity of a type of lipid, we mainly ingest triglycerides. We get them whenever we eat butter, oil, or animal fat. Of course, we also get phospholipids and steroids (mainly cholesterol) in our diets, just not as much as the triglycerides. A few phospholipids that we need are ones with fatty acids that we cannot make from scratch-- therefore, we have to ingest certain fatty acids. The fatty acids that we have to ingest are called essential fatty acids (and we get them in our normal diets).
Triglycerides are mainly used for energy or energy reserves. You see, we can break down triglycerides and use the products of this breakdown for cellular respiration, or we can store triglycerides as fat. This requires a bit more of an explanation. After we eat triglycerides, our digestive system breaks them down into fatty acids and glycerol for absorption. This absorption is shown a bit in Figure 17.44 (on page 690). The fatty acids are absorbed for the production of chylomicrons. Fatty acids enter the epithelial cells of the small intestine and are turned into chylomicrons with the help of the smooth endoplasmic reticulum. Once the chylomicrons are formed, they diffuse into the lacteal and eventually find their way back into the blood. Fatty acids can be drawn out of the chylomicrons into tissues that need them as the chylomicrons travel through the blood.
Fatty acids can be used in cellular respiration-- they do not have to be turned into glucose first. Instead, the fatty acids are broken down into 2-carbon molecules that enter the cellular respiration events to make ATP. You see, glucose, a 6-carbon molecule, gets broken down into 3-carbon and then 2-carbon molecules as it goes through cellular respiration. Therefore, the 2-carbon molecules from fatty acids just join into the process where the 2-carbon molecules are normally used. Because they join in a little late in the process, they don't yield as much energy as glucose does, but they still generate some ATP.
Fatty acids can also be used to build storage triglycerides (fat for later use) and phospholipids for our membranes.
Cholesterol is important for manufacturing certain hormones, for helping to make up the lipid bilayer of cells, and for making bile salts.
There really isn't such a thing. Fatty acids and steroids are about as small as they get.
Getting and storing lipids
In the carbohydrate page I gave you names for the chemical reactions that occur to make or breakdown carbohydrates. Here, there are no analogous names that you need to know. But, I would like you to know that the liver orchestrates the breakdown and synthesis of lipids.
How much lipid do we need?
We don't need much lipid-- only enough to get the essential fatty acids. Other than that, we can always make what we need. It is recommended that a person eat less than 30% of their daily calories from lipids.
© 2011 STCC Foundation Press