Amino Acid Metabolism | Carbohydrate Metabolism
F>ats are known as lipids. The word lipid comes from the Greek word "lipos" which means fat and is used to describe fats, oils, waxes and other related compounds. Fats are solid at room temperature while oils are liquid.
Lipids, like carbohydrates, are composed of carbon, hydrogen and oxygen, but differ in the ratio of each molecule. For example, the molecular structure of the lipid stearin is C57H110O6 . The ratio of H:O is 18.3:1 in this fat, while in carbohydrates it is a constant 2:1.
Types Of Fats
Lipids are classified into one of three groups: simple lipids, compound lipids, or derived lipids.
- Simple lipids, also called "neutral fats" are made of triglycerides. A triglyceride is a combination of three fatty acids bonded to a glycerol molecule. The bulk of dietary fats (98%) are triglycerides.
Fat is also stored in the body in the form of triglycerides. Simple lipids are further classified as saturated or unsaturated fatty acids.
Saturated Fatty Acid
- Saturated fatty acids consist of only single bonds between carbon atoms with all remaining bonds attached to hydrogen atoms. The molecule is then bonded to as many hydrogen atoms as possible, hence the name saturated.
This lack of double bonds causes the three fatty acids to pack tightly together, making it harder to breakdown. These saturated fats are found in animal products (beef) and dairy products (milk, butter).
Unsaturated Fatty Acid
- Unlike saturated fatty acids, unsaturated fatty acids contain one or more double bonds along its main carbon chain. Each double bond reduces the number of hydrogen atoms that can bond to the fatty acid.
The double bonds also put a "kink" in the fatty acids, which prevents them from packing tightly together. Unsaturated fatty acids are found in plant sources.
The term monounsaturated fatty acid is used to describe a fatty acid with one double bond (ex: olive oil) and the term polyunsaturated fatty acid describes a fatty acid with two or more double bonds (flax seed oil).
The process of hydrogenation involves changing unsaturated fatty acids into saturated fatty acids by bubbling liquid hydrogen into the oils. This reduces double bonds to single bonds and creates a harder fat and increases the fats melting temperature. This process is used to make chocolate candies that "melt in your mouth and not in your hand."
This process can also result in the formation of trans-fatty acids. A trans-fatty acid is formed when a hydrogen atom moves from its natural position (cis position) to the opposite side of a double bond (trans position). Trans-fatty acids have been linked with heart disease and have recently been required to be listed on food labels.
Compound & Derived Lipids
A compound lipid is a triglyceride combined with other chemicals.
- Phospholipids - One or more fatty acids combined with a phosphorus group and a nitrogen base.
- Glycolipids - Fatty acids combined with glucose and nitrogen.
- Lipoproteins - Lipids combined with proteins. Lipoproteins serve as the body's lipid transport system.
- Derived lipids contain hydrocarbon rings instead of chains.
- Cholesterol - A waxy, fat-like substance present in every cell in the body and in many foods. Some cholesterol in the blood is necessary - but a high level can lead to heart disease.
Ingestion & Digestion
Because dietary fat is water insoluble, it condenses into large lipid droplets in the upper stomach. Digestion of fats takes place almost completely in the small intestine. In the small intestine, the pancreatic enzyme lipase catalyzes (speeds up) the splitting of the bonds between the fatty acids and glycerol, producing a monoglyceride (glycerol + 1 fatty acid) and two fatty acids.
This reaction can only take place on the surface of the lipid droplet though because lipase is a water-soluble enzyme. In order to speed up the digestive process, the large lipid droplets are emulsified into smaller droplets, which create more surface area for the lipase to act on.
In order to further increase the speed of digestion, micelles are formed by the action of bile salts. Micelles are similar to the emulsion droplets but are even smaller. Micelles continuously breakdown and reform, when they breakdown the fat digestion products are able to diffuse across the intestinal lining.
During their passage through the epithelial cells, the monoglyceride and fatty acids resynthesize into triglycerides. This resynthesis, which occurs in the smooth endoplasmic reticulum, lowers the concentration of free fatty acids and monoglycerides, which maintains the concentration gradient and allows other fatty acids and monoglycerides to diffuse.
The now resynthesized fat aggregates into small droplets again. The droplets exit the cell by pinching off of the endoplasmic reticulum into the interstitial fluid. At this point, the droplets are known as chylomicrons, which contain triglycerides and other lipids (cholesterol, fat-soluble vitamins, etc.).
Unlike amino acids and glucose, the chylomicron travels through the lymphatic system instead of the blood. The lymph eventually flows into the systemic circulation.
Once in the circulation, the fatty acids are released from the chylomicron. This occurs primarily in the adipose tissue capillaries. The fatty acids then enter adipocytes (fat cells) and combine with -glycerol phosphate, from glucose metabolism, to form triglycerides. The fatty acids can also be used for energy in other cells (except for the nervous system).
In order for a fatty acid to be used so a cell can obtain energy, it must be transported through the blood to that cell then cross into the matrix of a mitochondria. The fatty acid undergoes beta-oxidation, which creates hydrogen atoms (for oxidative phosphorylation) and acetyl CoA (for the Krebs cycle).
What Is The Lymphatic System?
The tissues and organs that produce, store, and carry white blood cells that fight infections and other diseases. This system includes the bone marrow, spleen, thymus, lymph nodes, and lymphatic vessels (a network of thin tubes that carry lymph and white blood cells). Lymphatic vessels branch, like blood vessels, into all the tissues of the body.
Stored Triglycerides (Fat)
While all cells contain some fat, it is mainly stored in muscle (intramuscular triglycerides) and in body fat. Body fat is known as adipose tissue and is the body's main fat storage site. Adipose tissue is divided into individual cells called adipocytes.
These adipocytes hold stored triglyceride (1 glycerol molecule bonded to 3 fatty acids) droplets, which serve as a source of energy for the body. These droplets make up 95% of adipocytes volume. In order for this storage of potential energy (60,000-100,000 kcal) to be able to be used, it must be mobilized through lipolysis.
Lipolysis involves hydrolyzing (splitting with water) the triglycerides into a glycerol molecule and 3 separate fatty acids (FFA). This reaction is catalyzed by the enzyme hormone-sensitive lipase (HSL):
Once the fatty acids diffuse (exit) from the adipocytes, they bind to plasma albumin (a protein in the blood) in order to be transported to active tissues.
Fatty Acid Catabolism
When the albumin-FFA complex reaches muscle tissue, the FFAs are released and are transported into a muscle cell. Once in the muscle cell, the FFAs can reesterfy (rebind) with glycerol to form triglycerides or bind with intramuscular proteins to be used for energy in the mitochondria.
In the mitochondria, the fatty acids undergo beta-oxidation. During beta-oxidation, fatty acids are coverted to acetyl-CoA. Once the entire fatty acid molecule is degraded into acetyl-CoA molecules, they are sent into the citric acid cycle. ATP phosphorylates this reaction, then water is added, and the H+ are passed to NAD+ and FAD to be sent to the respiratory chain to be oxidized.
Beta-Oxidation is an aerobic reaction because oxygen is needed to bind with the H+. Remember, without oxygen, the H+ stay bound to NAD+ and FAD and the respiratory chain cannot transfer electrons and NADH and FADH2 levels accumulate, which stops fatty acid catabolism.
The water-soluble glycerol molecule formed from lipolysis can diffuse from the adipocytes into the circulation. The liver can use the glycerol in the circulation to form glucose through gluconeogenesis. Glycerol is accepted as 3-phosphoglyceraldehyde, which degrades to pyruvate to be oxidized for ATP in the citric acid cycle.
What Is The Citric Acid Cycle?
More commonly known as the Krebs Cycle, The Citric Acid Cycle is one of 3 stages of cellular respiration, along with glycolysis and electron transport/oxidative phosphorylation.
The H+ are accepted by NAD+ and sent to the electron transport chain. Glycerol also has a gluconeogenic role as it provides carbon skeletons for glucose synthesis. Glycerol is an important fuel source.
Lipids serve as an energy source and reserve (adipose tissue), protect organs, provide insulation, and carry fat-soluble vitamins. Despite the low-fat diet craze, dietary fat is very important.
Like essential amino acids, there is an essential fatty acid: Linoleic acid. Linoleic acid is a polyunsaturated fatty acid that the body cannot synthesize. This acid is vital for maintaining cell membrane integrity, growth, reproduction, and other vital functions. To obtain this fatty acid, one must eat it.
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It is recommended that less than 30% of one's calories come from fat, with only 20-30% of that being from saturated fat and 70-80% from unsaturated fats. For example, if someone consumes 60 grams of fat, only 12-18 grams of that should be from saturated fats.
The other 42-48 grams should come from unsaturated fats found in foods like flax seeds, olive oil, nuts, and fish (which can be obtained from fish oil supplements).
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