Fat is where it's at

This week's topic is lipids, AKA fat. Lipids are water insoluble, or immiscible. They are, however, soluble in some organic solvents such as acetone and benzene. Lipids are a very concentrated source of energy, and therefore may contribute to obesity, which contributes to the mortality rate in the US. Types of lipids include fatty acids, triglycerides, phospholipids, cholesterol, eicosanoids, lipoproteins, and the fat soluble vitamins A, D, E, and K.
The fat cells in our body (also known as adipocytes or adipose tissue) serve to store fat. The form that fat is stored in is triacyglycerol, or triglyceride.
Fatty acids are not found alone in nature or in the body. They are part of the structure of triacylglycerols. The structure of fatty acids is a chain of CH with carboxyl group (COOH) at one end of the chain. A saturated fatty acid has no double bonds. Unsaturated fatty acids have at least one double bond. Monounsaturated has one double bond, and polyunsaturated has two or more double bonds.
The number of carbons in fatty acids is usually an even number in nature. The most abundant have 16, 18, or 20 carbons.
To name the fatty acid, you start at the carboxyl group end and number the carbons. Then you count the number of double bonds and note it like this - example 18:1 is a fatty acid with an 18 carbon chain and one double bond. In this case, the double bond occurs between carbon # 9 and # 10, so you note this by adding a delta 9 after the name. This one is 18:1 (delta)9, or oleic acid. A fatty acid named 18:2 (delta) 9,12 has 18 carbons and two double bonds, occurring at carbon #9 and #12. There is always a difference of 3 carbons between the double bonds.
Monounsaturated and polyunsaturated fats come from plant sources and include canola oil and olive oil (mono) and vegetable, sunflower, and safflower oil (poly).
Tropical oils include palm and coconut. Although they come from plant sources, they are highly saturated and should be limited. This is confusing to some people because typically saturated fats are more solid at room temperature. Tropical oils appear more liquid at room temperature due to their content of shorter chains of fatty acids.
Most saturated fats are from animal sources: butter, beef fat, and lard.
The essential fatty acids are omega 6 (linoleic acid) and omega 3 (linolenic acid). They are essential because they cannot be synthesized in the body and must be obtained from the diet.
The omega fatty acids are named differently than others. Instead of starting the numbering at the carboxyl end, you start at the methyl end. So the first double bond on an omega 3 fatty acid is at the 3rd carbon from the methyl end.
Omega 3 fatty acids have been shown to decrease the incidence of cardiovascular disease (CVD) by decreasing blood cholesterol levels and lowering blood clotting. They are also thought to be good for brain development and functioning, vision, the immune system, and memory. Linolenic acid is just one type of omega 3 fatty acid, and comes from plant sources. Other types, such as eicosapentaenoic acid (EPA) and docohexaenoic acid (DHA) come from fish oils. EPA and DHA are both highly unsaturated. EPA contains 5 double bonds, and DHA contains 6. Omega 3 fatty acids produce the important biomolecules prostoglandins, leukotrienes, and thromboxanes.
Hydrogenation is a process that turns polyunsaturated fats into saturated fats by adding hydrogen at the double bonds. This process generates trans fatty acids when vegetable oils are partially hydrogenated - i.e. not all of the double bonds are filled with hydrogen. When this happens the double bond is twisted from the cis (hydrogens are on the same side of the double bond) to trans (hydrogens are on opposite sides of the double bond). If the fat gets fully hydrogenated, this makes a saturated fatty acid, not a trans fat.
Trans fats are a huge health concern because they increase the risk of heart disease. They increase bad cholesterol and prevent good fats from functioning, promoting heart disease and circulatory disorders. They also depress the immune system, interfere with pregnancy (can cause low birth weights and poorer quality of breast milk), increase insulin resistance which can worsen diabetes and hypertension, and disturb liver function.
Other reactions that fat can undergo include emulsification, oxidation, and anti-oxidation.
Emulsification is the breaking of large fat molecules into smaller particles, such as the homogenization of milk. Bile and lecithin are able to undergo this process.
Oxidation results in the spoilage/rancidity of fats. This occurs when the double bond forms a peroxide and results in a bad taste and smell.
The fat soluble vitamin E can act as an antioxidant, and prevent spoilage. That is why it is added to products that contain fat. Vegetable oil naturally has some vitamin E in it, but more is often added to prolong shelf life.
Triglycerides are what we typically think of when we think of fat. It is the most abundant form both in our body and in nature (up to 95% of all fat). It is a neutral fat, meaning it is not polar and does not react with water.
The structure of a triglyceride is a glycerol with 3 fatty acids (acyl groups) attached, or estrified.
A diglyceride would have two attached fatty acids, and a monoglyceride would have 1.
Glycerol is the backbone of these structures. It contains 3 carbons with a hydroxyl group. The fatty acids get attached to the carbons in this backbone. In a monoglyceride, the fatty acid is attached to carbon #1. On a diglyceride, the fatty acid groups are attached to carbon #1 and #3.
Phospholipids are another important type of lipid. They are known as structural lipids because they are the main lipid found in the plasma membrane of cells. Human cells are all different, but the common feature is the plasma membrane. It keeps the structure of the cell intact which allows all of its contents to stay inside.
Human/animal cells also contain the glycocalyx, a carbohydrate rich area that includes glycoprotein and glycolipids.
Proteins in the cell may be integral (penetrating the cell membrane) or peripheral (stay on the surface of the membrane).
Cholesterol is also found in the plasma membrane. It has a 4 ring structure, and like phospholipid has a polar OH group which is hydrophyllic, and a tail group (including the ring structure) that is hydrophobic and hides inside the membrane. Cholesterol is synthesized in the body and is used to make cholic acid, which is part of bile. Because it is both hydrophobic and hydrophillic it can act as an emulsifier. Cholesterol is also used to make hormones like estradiol, a female sex hormone.
There are 4 types of transport mechanisms for moving molecules through the plasma membrane of cells: diffusion (simple or facilitated), active transport, and endocytosis.
Diffusion allows molecules to move from areas of high concentration to areas of low concentration, without requiring energy because it is a passive process. Simple diffusion simply moves the molecules/ions between lipids or other parts of the membrane. Sometimes pores are formed, which are known as gated channels for them to move through.
Facilitative diffusion requires a carrier protein which attaches to the molecule and undergoes a transformational change, allowing the molecule to move through.
Active transport also requires a carrier molecule, as well as energy. It allows movement from areas of low concentration to high concentration. An example is Sodium/potassium/ATPase in which energy is released when sodium binds to the carrier protein, releasing ADP and phosphate. Phosphate then attaches and undergoes a conformational change, dumping sodium into the extracellular fluid. Potassium then attaches at the binding site, releasing the phosphate group and undergoing another conformational change which in turn brings in the molecule and released potassium inside the cell.
Endocytosis occurs when the molecules approach the membrane, and it folds in to move them inside the cell.
Saturation phenomenon is the idea that the maximum velocity has been reached when all of the binding sites of transporter proteins are occupied.
Phospholipids are polar lipids, meaning that they contain molecules that react with water, such as nitrogen and oxygen. The basic structure is a glycerol backbone with 2 fatty acid chains plus a phosphate group. There will be another group attached to the phosphate group, known as the head group, which allows for classification of that specific phospholipid.
Phospholipids are distinct in that they contain a polar head and a hydrophobic tail in the same molecule. Because of this they are called ampipathic molecules. The head is hydrophillic (likes to react with water) and the tail is hydrophobic, or immiscible (does not like to react with water). This allows them to form a structural bilayer in the cell membrane. The hydrophyllic ends face outward while the hydrophobic ends remain inward. Lecithin is the major phospholipid found in the membrane. It can be synthesized in the body, but is also found in foods such as liver, egg yolk, and soybeans. Lecithin is an effective emulsifier.
Phospholipids are also found in lipoproteins, which are the transport vehicles for lipid in the blood. Lipids need help being transported because they are water immiscible. Therefore, they are hidden inside the lipoproteins. The hydrophyllic part of the phospholipid faces outward from the surface of the lipoprotein to allow it to freely move through the blood.
There are several types of lipoproteins: chylomicrons, which are made in the intestine; VLDL, which is made in the intestine and liver; LDL, which is made from VLDL in the blood; and HDL, which is made in the intestine and liver. The density depends upon the amount of protein and fat. A chylomicron has the highest amount of fat and the lowest protein, so it is the lowest density. HDL is the highest density due to its high protein content relative to fat.