Oil. Does it mix with water? No. Biologically, why is this important?
Oil is a lipid. The property of chemically not being able to mix with water gives lipids some very important biological functions. A particular type of lipid - the phospholipid - is the main component of the outer membrane of all cells. Why?
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Lipids are organic compounds that contain mainly carbon, hydrogen, and oxygen. They include substances such as fats and oils, as well as waxes, sterols, some vitamins (A, D, E and K) and phospholipids. Lipid molecules consist of fatty acids, with or without additional molecules. Fatty acids are organic compounds that have the general formula CH3(CH2)nCOOH, where n usually ranges from 2 to 28 and is always an even number.
A distinguishing feature of lipids is that they are insoluble in water. The main biological functions of lipids include energy storage, as the major structural component of cell membranes, and as important signaling molecules.
Saturated and Unsaturated Fatty Acids
Fatty acids can be saturated or unsaturated. The term saturated refers to the placement of hydrogen atoms around the carbon atoms. In a saturated fatty acid, all the carbon atoms (other than the carbon in the -COOH group) are bonded to as many hydrogen atoms as possible (usually two hydrogens). Saturated fatty acids do not contain any other groups except the -COOH. This is why they form straight chains, as shown in Figure below. Because of this structure, saturated fatty acids can be packed together very tightly. This allows organisms to store chemical energy very densely. The fatty tissues of animals contain mainly saturated fatty acids.
Saturated and Unsaturated Fatty Acids. Saturated fatty acids include arachidic, stearic, and palmitic fatty acids. Unsaturated fatty acids include all the other fatty acids in the figure. Notice how all the unsaturated fatty acids have bent chains, whereas the saturated fatty acids have straight chains.
In an unsaturated fatty acid, some carbon atoms are not bonded to the maximum number of hydrogen atoms. This is because they are bonded to one or more additional groups, including double and triple bonds between carbons. Wherever these other groups bind with carbon, they cause the chain to bend - they do not form straight chains (Figure above). This gives unsaturated fatty acids different properties than saturated fatty acids. For example, unsaturated fatty acids are liquids at room temperature whereas saturated fatty acids are solids. Unsaturated fatty acids are found mainly in plants, especially in fatty tissues such as nuts and seeds.
Unsaturated fatty acids occur naturally in the bent shapes shown in Figure above. However, unsaturated fatty acids can be artificially manufactured to have straight chains like saturated fatty acids. Called trans fatty acids, these synthetic lipids were commonly added to foods, until it was found that they increased the risk for certain health problems. Many food manufacturers no longer use trans fatty acids for this reason.
These plant products all contain unsaturated fatty acids.
Types of Lipids
Lipids may consist of fatty acids alone or in combination with other compounds. Several types of lipids consist of fatty acids combined with a molecule of alcohol:
- Triglycerides are the main form of stored energy in animals. This type of lipid is commonly called fat. Triglycerides are composed of a glycerol and three fatty acid chains. An example is shown in Figure below. In humans, triglycerides are a mechanism for storing unused calories, and their high concentration in blood correlates with the consumption of excess starches and other carbohydrate-rich foods.
- Phospholipids are a major component of the membranes surrounding the cells of all organisms, as they have the ability to form bilayers. The structure of the phospholipid molecule consists of two hydrophobic tails (a diglyceride made of two fatty acid chains) and a hydrophilic head (a phosphate group, PO43-).
- Steroids (or sterols) have several functions. Sterols are a subgroup of steroids. The sterol cholesterol is an important part of cell membranes and plays other vital roles in the body. Cholesterol is a precursor to fat-soluble vitamins and steroid hormones. Steroid hormones include the male and female sex hormones. Sterols also have roles as second messengers in signalling pathways.
Triglyceride Molecule. The left part of this triglyceride molecule represents glycerol. Each of the three long chains on the right represents a different fatty acid. From top to bottom, the fatty acids are palmitic acid, oleic acid, and alpha-linolenic acid. The chemical formula for this triglyceride is C55H98O6. KEY: H=hydrogen, C=carbon, O=oxygen
Lipids and Diet
Humans need lipids for many vital functions, such as storing energy and forming cell membranes. Lipids can also supply cells with energy. In fact, a gram of lipids supplies more than twice as much energy as a gram of carbohydrates or proteins. Lipids are necessary in the diet for most of these functions. Although the human body can manufacture most of the lipids it needs, there are others, called essential fatty acids, that must be consumed in food. Essential fatty acids include omega-3 and omega-6 fatty acids. Both of these fatty acids are needed for important biological processes, not just for energy.
Although some lipids in the diet are essential, excess dietary lipids can be harmful. Because lipids are very high in energy, eating too many may lead to unhealthy weight gain. A high-fat diet may also increase lipid levels in the blood. This, in turn, can increase the risk for health problems such as cardiovascular disease. The dietary lipids of most concern are saturated fatty acids, trans fats, and cholesterol. For example, cholesterol is the lipid mainly responsible for narrowing arteries and causing the disease atherosclerosis.
- cholesterol: A steroid alcohol that is present in animal cells and body fluids, regulates membrane fluidity, and functions as a precursor molecule in various metabolic pathways.
- essential fatty acids: Fatty acids that humans and other animals must ingest.
- fatty acids: A component of triglycerides and phospholipids; a carboxylic acid (-COOH) with a long hydrocarbon tail, which is either saturated or unsaturated.
- lipid: Organic compound such as fat or oil.
- phospholipid: A major component of the cell membrane; consists of two hydrophobic tails and a hydrophilic phosphate head group.
- saturated fatty acid: Molecule in lipid with carbon atoms bonded to the maximum number of hydrogen atoms; contains only single bonds between carbon atoms.
- second messenger: A molecule that relays a signal from a receptor on the cell surface to target molecules inside the cell.
- steroid: A type of lipid; examples include cholesterol and the sex hormones.
- trans fatty acid: Unsaturated fatty acid artificially manufactured to have straight fatty acid chains, like saturated fatty acids.
- triglycerides: The main form of stored energy in animals; commonly called fat; composed of a glycerol and three fatty acid chains.
- unsaturated fatty acid: Molecule in lipid with double or triple bonds between carbon atoms; does not contain the maximum number of hydrogen atoms.
- Lipids are organic compounds that consist of carbon, hydrogen, and oxygen. They are made up of fatty acids and other compounds. They provide cells with energy, store energy, and help form cell membranes.
Use this resource to answer the questions that follow.
→Biology for AP* →Search: Structure and Function of Fats
- Are fats considered a macromolecule? Why or why not?
- Describe the structure of triglyceride molecules. How do triglycerides form?
- What are the roles of triglycerides and phospholipids?
- Which are non-polar molecules, triglycerides or phospholipids?
- What determines a fat's function?
- What are lipids? Give examples of lipids?
- Why do molecules of saturated and unsaturated fatty acids have different shapes?
- Describe the structure and role of phospholipids.
- What type of organic compound is represented by the formula CH3(CH2)4COOH? How do you know?
- What are essential fatty acids?