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A lipid is any of various organic compounds that are insoluble in water. They include fats, waxes, oils, hormones, and certain components of membranes and function as energy-storage molecules and chemical messengers. Together with proteins and carbohydrates, lipids are one of the principal structural components of living cells. Lipids are a diverse group of compounds and serve many different functions. At a cellular level, phospholipids and cholesterol are some of the primary components of the membranes that separate a cell from its environment. Lipid-derived hormones, known as steroid hormones, are important chemical messengers and include testosterone and estrogens. At an organismal level triglycerides stored in adipose cells serve as energy-storage depots and also provide thermal insulation. Lipid rafts are possible areas of the cell membrane that contain high concentrations of cholesterol and glycosphingolipids. The existence of lipid rafts has not been conclusively established, though many researchers suspect such rafts do indeed exist and may play a role in membrane fluidity, cell-to-cell communication, and infection by viruses. lipid, any of a diverse group of organic compounds including fats, oils, hormones, and certain components of membranes that are grouped together because they do not interact appreciably with water. One type of lipid, the triglycerides, is sequestered as fat in adipose cells, which serve as the energy-storage depot for organisms and also provide thermal insulation. Some lipids such as steroid hormones serve as chemical messengers between cells, tissues, and organs, and others communicate signals between biochemical systems within a single cell. The membranes of cells and organelles (structures within cells) are microscopically thin structures formed from two layers of phospholipid molecules. Membranes function to separate individual cells from their environments and to compartmentalize the cell interior into structures that carry out special functions. So important is this compartmentalizing function that membranes, and the lipids that form them, must have been essential to the origin of life itself. lipid; oogonium Water is the biological milieu—the substance that makes life possible—and almost all the molecular components of living cells, whether they be found in animals, plants, or microorganisms, are soluble in water. Molecules such as proteins, nucleic acids, and carbohydrates have an affinity for water and are called hydrophilic (“water-loving”). Lipids, however, are hydrophobic (“water-fearing”). Some lipids are amphipathic—part of their structure is hydrophilic and another part, usually a larger section, is hydrophobic. Amphipathic lipids exhibit a unique behaviour in water: they spontaneously form ordered molecular aggregates, with their hydrophilic ends on the outside, in contact with the water, and their hydrophobic parts on the inside, shielded from the water. This property is key to their role as the fundamental components of cellular and organelle membranes. Although biological lipids are not large macromolecular polymers (e.g., proteins, nucleic acids, and polysaccharides), many are formed by the chemical linking of several small constituent molecules. Many of these molecular building blocks are similar, or homologous, in structure. The homologies allow lipids to be classified into a few major groups: fatty acids, fatty acid derivatives, cholesterol and its derivatives, and lipoproteins. This article covers the major groups and explains how these molecules function as energy-storage molecules, chemical messengers, and structural components of cells. Fatty acids rarely occur as free molecules in nature but are usually found as components of many complex lipid molecules such as fats (energy-storage compounds) and phospholipids (the primary lipid components of cellular membranes). This section describes the structure and physical and chemical properties of fatty acids. It also explains how living organisms obtain fatty acids, both from their diets and through metabolic breakdown of stored fats. Biological fatty acids, members of the class of compounds known as carboxylic acids, are composed of a hydrocarbon chain with one terminal carboxyl group (COOH). The fragment of a carboxylic acid not including the hydroxyl (OH) group is called an acyl group. Under physiological conditions in water, this acidic group usually has lost a hydrogen ion (H+) to form a negatively charged carboxylate group (COO−). Most biological fatty acids contain an even number of carbon atoms because the biosynthetic pathway common to all organisms involves chemically linking two-carbon units together (although relatively small amounts of odd-number fatty acids do occur in some organisms). Although the molecule as a whole is water-insoluble by virtue of its hydrophobic hydrocarbon chain, the negatively charged carboxylate is hydrophilic. This common form for biological lipids—one that contains well-separated hydrophobic and hydrophilic parts—is called amphipathic. In addition to straight-chain hydrocarbons, fatty acids may also contain pairs of carbons linked by one or more double bonds, methyl branches, or a three-carbon cyclopropane ring near the centre of the carbon chain. Get a Britannica Premium subscription and gain access to exclusive content. Subscribe Now
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Lipids are fatty, waxy, or oily compounds that are essential to many body functions and serve as the building blocks for all living cells. Lipids help regulate hormones, transmit nerve impulses, cushion organs, and store energy in the form of body fat. The three main types of lipids are phospholipids, sterols (including the different types of cholesterol), and triglycerides (which account for over 95% of lipids in food). Lipids are found in higher quantities in fried foods, animal fats, and dairy products like cream, butter, and cheese. Though lipids are important to your health, consuming excessive amounts in food can lead to diseases like atherosclerosis ("hardening of the arteries"), hypertension (high blood pressure), and coronary artery disease. This article describes the three main types of lipids and what each of them do. It also explains what happens when blood lipids are high and how healthcare providers can measure this with a lipid panel test.
The three main types of lipids are phospholipids, sterols, and triacylglycerols (also known as triglycerides). Phospholipids make up the outer layer of cells in the bodies of animals and humans. They create a protective layer around the cells. Most people do not have to worry about phospholipids. However, they can cause problems for people with a rare autoimmune disorder called antiphospholipid syndrome (APS). APS often happens in people with lupus—especially women.
A common name for APS is "sticky blood." That's a good description of what happens inside the body when these key lipids cannot do their jobs.
In people with APS, the proteins on phospholipids get attacked, and the protective layers are damaged. As APS attacks blood cells and vessels, it increases a person's risk for blood clots that can lead to heart attacks and strokes. APS also may lead to pregnancy complications. Sterols are a kind of steroid. Steroids are a group of hormones the body makes using lipids. You might have heard about plant sterols or phytosterols. These substances are found in foods and offer many health benefits. Phytosterols are similar to the main sterol in humans, called cholesterol. Most of your body's cholesterol is produced in the liver and the intestines, and about 20% comes from our diet. Cholesterol is found in every cell in the body and takes part in many important body functions, like making hormones and vitamin D. Cholesterol is also key for making bile salts, the substances that help the body break down fat and absorb vitamins. Since cholesterol is a fat, and is not water-soluble, it requires protein to carry it through the bloodstream. Cholesterol (and triglycerides) are transported through the bloodstream by particles consisting of lipids and proteins, called lipoproteins. There are several types of lipoproteins, but the two major ones are high-density lipoprotein (HDL) and low-density lipoprotein (LDL). HDL is called “good” cholesterol because it absorbs cholesterol and brings it back to the liver. LDL is called “bad” cholesterol because it can build up in the blood vessels. Triglycerides are the most common type of lipid in our body and come from fats and oils in our diet. Triglycerides are important because they give us energy. The lipids we consume in our diet can be saturated or unsaturated fats. Saturated fats are solid at room temperature and are typically derived from animal products such as meat, butter, and cheese. Unsaturated fats are usually liquid at room temperature and are often, but not always, derived from plants.
While "lipids" and "fat" are sometimes used interchangeably, fat (a.k.a. fatty acids) is only one type of lipid. Unsaturated fats have essential nutrients commonly knowns as omega-3 fatty acids and are found in foods like tuna, salmon, nuts, seeds, avocados, and leafy vegetables. These unsaturated fatty acids can help reduce inflammation, blood pressure, and triglycerides in the body. They also lower the risk of sudden death from a heart attack and prevent blood clots from forming. Trans fats, or "partially hydrogenated oils," are artificially made to have a certain texture desired for processed foods. They also increase the shelf life of baked goods. Eating foods that contain trans fats can lead to high levels of LDL (bad) cholesterol and can lower your HDL (good) cholesterol. So, what foods raise triglyceride levels? Actually, it is all of those extra calories we consume from food and drinks, especially from alcohol and carbohydrates, that end up stored in the form of triglycerides in our fat (adipose) tissue. Over time, if you aren't burning those extra calories, you can develop high triglycerides and excess fat storage that leads to various metabolic conditions such as obesity, insulin resistance, diabetes, and even acute pancreatitis. Lipids have a range of functions, including:
Lipids are a very important part of your nervous system. One place you'll find lipids is in the fatty tissue sleeves that protect your nerve cells and increase the conduction of their impulses (myelin sheaths). Lipids make it possible for the body to use vitamins. Lipids also help the body absorb fat-soluble vitamins A, D, E, and K. Here are just a few reasons why your body needs these vitamins:
Lipids are needed for the production of certain hormones, including estrogen, testosterone, and cortisol. Here are a few important jobs that hormones have:
Lipids are essential for your health. However, having too many of them can put you at a higher risk of medical conditions like liver disease and heart disease. Cardiovascular disease is the leading cause of death in the United States. The World Health Organization (WHO) estimates that 17.9 million people worldwide die from cardiovascular disease each year. A buildup of LDL cholesterol can clog the arteries (atherosclerosis). These 'clogs' can narrow the opening of the artery and lead to a heart attack or stroke. High LDL levels are linked to diets rich in saturated fats, such as fatty or processed meats, cream-based sauces, cheese, deep-fried foods, and processed foods. In addition to following a good diet, you can help to control your level of LDL cholesterol by keeping an ideal weight, getting plenty of exercise, and not smoking. These same lifestyle choices can also help boost your HDL cholesterol, as can including plenty of fish, olive oil, vegetables, legumes, nuts and high-fiber fruits in your diet. Your healthcare provider may want you to have a lipid panel test as part of your routine annual physical. They might also want you to have the test if you are at risk for cardiovascular disease. A lipid panel is also called a cholesterol test. It uses a blood sample to determine your total cholesterol levels (overall), LDL, HDL, and triglycerides. From these values, the lab will calculate your cholesterol ratio. The results of your cholesterol test can help your provider determine your risk for cardiovascular disease, including heart attack or stroke. The three types of lipids—phospholipids, sterols, and triglycerides—are needed for many essential functions in the body. They also offer many health benefits. Triglycerides and cholesterol may pose health risks if your levels are too high. The foods that you eat can increase your "bad" LDL cholesterol, lower your "good" HDL cholesterol, and raise your triglycerides, which can lead to health consequences. If you're concerned about your lipid levels, talk to your healthcare provider. A lipid panel test will give you the information that you need to begin making lifestyle changes, like getting more exercise and changing your diet, to help lower your levels. When you're looking at the results of your cholesterol test, remember that not all lipids are created equal. High levels of LDL pose a serious risk to your health, but higher levels of HDL can help get your cholesterol under control. Frequently Asked Questions
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