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By the end of this section, you will be able to:
  • Describe the four major types of lipids
  • Explain the role of fats in storing energy
  • Differentiate between saturated and unsaturated fatty acids
  • Describe phospholipids and their role in cells
  • Define the basic structure of a steroid and some functions of steroids
  • Explain the how cholesterol helps to maintain the fluid nature of the plasma membrane

Lipids include a diverse group of compounds that are largely nonpolar in nature. This is because they are hydrocarbons that include mostly nonpolar carbon–carbon or carbon–hydrogen bonds. Non-polar molecules are hydrophobic (“water fearing”), or insoluble in water. Lipids perform many different functions in a cell. Cells store energy for long-term use in the form of fats. Lipids also provide insulation from the environment for plants and animals ( [link] ). For example, they help keep aquatic birds and mammals dry when forming a protective layer over fur or feathers because of their water-repellant hydrophobic nature. Lipids are also the building blocks of many hormones and are an important constituent of all cellular membranes. Lipids include fats, oils, waxes, phospholipids, and steroids.

Photo shows a river otter swimming.
Hydrophobic lipids in the fur of aquatic mammals, such as this river otter, protect them from the elements. (credit: Ken Bosma)

Fats and oils

A fat molecule consists of two main components—glycerol and fatty acids. Glycerol is an organic compound (alcohol) with three carbons, five hydrogens, and three hydroxyl (OH) groups. Fatty acids have a long chain of hydrocarbons to which a carboxyl group is attached, hence the name “fatty acid.” The number of carbons in the fatty acid may range from 4 to 36; most common are those containing 12–18 carbons. In a fat molecule, the fatty acids are attached to each of the three carbons of the glycerol molecule with an ester bond through an oxygen atom ( [link] ).

The structures of glycerol, a fatty acid, and a triacylglycerol are shown. Glycerol is a chain of three carbons, with a hydroxyl (OH) group attached to each carbon. A fatty acid has an acetyl (COOH) group attached to a long carbon chain. In triacylglycerol, a fatty acid is attached to each of glycerol’s three hydroxyl groups via the carboxyl group. A water molecule is lost in the reaction so the structure of the linkage is C-O-C, with an oxygen double bonded to the second carbon.
Triacylglycerol is formed by the joining of three fatty acids to a glycerol backbone in a dehydration reaction. Three molecules of water are released in the process.

During this ester bond formation, three water molecules are released. The three fatty acids in the triacylglycerol may be similar or dissimilar. Fats are also called triacylglycerols or triglycerides because of their chemical structure. Some fatty acids have common names that specify their origin. For example, palmitic acid, a saturated fatty acid    , is derived from the palm tree. Arachidic acid is derived from Arachis hypogea, the scientific name for groundnuts or peanuts.

Fatty acids may be saturated or unsaturated. In a fatty acid chain, if there are only single bonds between neighboring carbons in the hydrocarbon chain, the fatty acid is said to be saturated. Saturated fatty acids are saturated with hydrogen; in other words, the number of hydrogen atoms attached to the carbon skeleton is maximized. Stearic acid is an example of a saturated fatty acid ( [link] )

The structure of stearic acid is shown. This fatty acid has a hydrocarbon chain seventeen residues long attached to an acetyl group. All bonds between the carbons are single bonds.
Stearic acid is a common saturated fatty acid.

When the hydrocarbon chain contains a double bond, the fatty acid is said to be unsaturated . Oleic acid is an example of an unsaturated fatty acid ( [link] ).

Questions & Answers

why do a dog eat it's first come
Jonathan Reply
what is meoisis
Mary
Meiosis is a biological processes that involves the division of sex cells
Musoke
what is C4 in photosynthesis
Kirubel Reply
carbon the forth
Elsabet
Carbon fourth
Malekhotla
l don't know,it may be
Elsabet
How the urine is produced?
Malekhotla
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Chris Reply
what is the difference between mitosis and meiosis
Agera Reply
how can a human being die by drinking too much of water
Olaleke Reply
Scientific name of a dog in canis familiarise?
Jaymez Reply
what's biology?
Jaymez
Biology is the study of living organism and their interaction with one another and their environment
Sharon
an other questions please
Elsabet
what is biosphere.
Chinyi
where are pollen grains produced pls.
Chinyi
And what are emphatic factors.
Chinyi
@Chinyi Bahiyah -pollen grains are produced in the anther of plants. the anther is one part of the male part of the plant the other being filament and they together form the stamen. hoped that helped :) !!
xjuicy_editzz
biosphere is the regions surface and atmosphere of the earth (or other planets) where living organism's exist.
Esnart
what is science?
Benedict Reply
Science is the method of acquiring skills and knowledge through experimentation and observation in the World Series
Richmond
what is ecology
Bitwell
is the study that deals with the interaction between living things and their environment
Alexander
what is the main fuction of root hair cells?
Liteboho
the main function is to uptake of water and nutrients from the rhizosphere.
Benedict
State the genius to where the dog belong.
Jaymez
Science is the study of living things and non living thing
Chris
where does meiosis occur in male plant reproductive organ
Gimai Reply
plant cell is plasmolysed as a result of the
Favour Reply
of the plant losing water
Esnart
"Head of a pin" means what?
Theresah Reply
types of genotype and types of blood group
Adebusuyi Reply
what is biology
Musa Reply
is the study of cell and structure of an organism
Atem
is the study of characteristics,life processes and phenomana of living organism
Atem
the study of living things
Jonathan
What is Monomer of protein?
Dimpho Reply
What's nutrition and types of nutrition
Jessica Reply
Why is cheetah d fastest animal
Jessica Reply
because they are capable of reaching speeds of up to 70mph
Abu
Thanks
Jessica
what is diffusion?
Chris
Diffusion is the movement of molecules from high concentration to low concentration
Malekhotla
why do cats eat there kitten
Elsabet

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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