Which of the Following is Not a Function of Lipids

Which of the Following is Not a Function of Lipids

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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© Jlcalvo/Dreamstime.com

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.

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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.


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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.


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Which of the following is a function of lipids?

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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.

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Which of the following is a function of lipids?

Lipids are molecules that contain hydrocarbons and make up the building blocks of the structure and function of living cells. Examples of lipids include fats, oils, waxes, certain vitamins (such as A, D, E and K), hormones and most of the cell membrane that is not made up of protein.

Lipids are not soluble in water as they are non-polar, but are thus soluble in non-polar solvents such as chloroform.

Which of the following is a function of lipids?

Human lipid bilayer – 3D Rendering. Image Credit: Crevis / Shutterstock

What do Lipids Consist of?

Lipids are mainly composed of hydrocarbons in their most reduced form, making them an excellent form of energy storage, as when metabolized the hydrocarbons oxidize to release large amounts of energy. The type of lipid found in fat cells for this purpose is a triglyceride, an ester created from glycerol and three fatty acids.

Where do Lipids Come From?

Excess carbohydrates in the diet are converted into triglycerides, which involves the synthesis of fatty acids from acetyl-CoA in a process known as lipogenesis, and takes place in the endoplasmic reticulum. In animals and fungi, a single multi-functional protein handles most of these processes, while bacteria utilize multiple separate enzymes. Some types of unsaturated fatty acids cannot be synthesized in mammalian cells, and so must be consumed as part of the diet, such as omega-3.

Acetyl-CoA is also involved in the mevalonate pathway, responsible for producing a wide range of isoprenoids, which include important lipids such as cholesterol and steroid hormones.

Hydrolysable and Non-hydrolysable Lipids

Lipids that contain an ester functional group are hydrolysable in water. These include neutral fats, waxes, phospholipids, and glycolipids. Fats and oils are composed of triglycerides, made up of glycerol (1,2,3-trihydroxypropane) and 3 fatty acids to form a triester. Triglycerides are found in the blood, and stored in fat cells. Complete hydrolysis of triacylglycerols yields three fatty acids and a glycerol molecule.

Non-hydrolyzable lipids lack such functional groups and include steroids and fat-soluble vitamins (A, D, E, and K).

Fatty Acids

Fatty acids are long chain carboxylic acids (typically 16 or more carbon atoms) which may or may not contain carbon-carbon double bonds.  The number of carbon atoms are almost always an even number and are usually unbranched.  Oleic acid is the most abundant fatty acid in nature.

The membrane that surrounds a cell is made up of proteins and lipids. Depending on the membrane’s location and role in the body, lipids can make up anywhere from 20 to 80 percent of the membrane, with the remainder being proteins. Cholesterol, which is not found in plant cells, is a type of lipid that helps stiffen the membrane. Image Credit: National Institute of General Medical Sciences

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Waxes/Fats and Oils

These are esters with long-chain carboxylic acids and long-alcohols. Fat is the name given to a class of triglycerides that appear as solid or semisolid at room temperature, fats are mainly present in animals. Oils are triglycerides that appear as a liquid at room temperature, oils are mainly present in plants and sometimes in fish.

Mono/Poly Unsaturated and Saturated

Those fatty acids with no carbon-carbon double bonds are called saturated. Those that have two or more double bonds are called polyunsaturated.  Oleic acid is monounsaturated, as it possesses a single double bond.

Saturated fats are typically solids and are derived from animals, while unsaturated fats are liquids and usually extracted from plants.

Unsaturated fats assume a particular geometry that prevents the molecules from packing as efficiently as they do in saturated molecules, leading to their propensity to exist as a liquid rather than a solid. Thus, the boiling point of unsaturated fats is lower than that of saturated fats.

Synthesis and Function of Lipids in the Body

Lipids are utilized directly, or otherwise synthesized, from fats present in the diet. There are numerous biosynthetic pathways to both break down and synthesize lipids in the body.

The main biological functions of lipids include storing energy, as lipids may be broken down to yield large amounts of energy. Lipids also form the structural components of cell membranes, and form various messengers and signaling molecules within the body.

Which of the following is a function of lipids?

Blood chemistry report showing normal liver function tests, and a lipid profile with high triglyceride levels. Image Credit: Stephen Barnes / Shutterstock

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Last updated Nov 4, 2018

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Which of the Following is Not a Function of Lipids

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