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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 is it so hard to get to the questions
Eric Reply
why is it so hard to get to questions?
Eric
what types of proteins which are found in hair,hooves and tendons?
Kapalu Reply
what is blood cells
Isaac Reply
It's cells of the blood
Kaique
can biology be also be define has a scientific study that deal with life?
Stanley Reply
do you care asking you a question
Afolayan Reply
name the gas that diffuses into the plants leaves on a bright, sunny day
Tuhemwe Reply
explain why this gas diffuses into the plant's leaves
Tuhemwe
name two gases that diffuses out of the plant's leaves on a bright sunny day
Tuhemwe
Biology olympaid
Khan
what is ecosystem
Rondy Reply
why does a car move yet it is not a living thing?
Sserujja
this is because of the engine and fuel placed on
Gift
I also think it because force has been apply to it.
Stanley
It is aplace where organísms live and associate with each other.
Nasib
what is the reason of swelling due to fracture
Ejaz Reply
Sngy alaka
Khan
how do animals reproduce
Kelvin Reply
be specific because there are so many types of animals and how they reproduce are different
Sheillah
yeah
Chinwendu
just be specific with a particular animal
Stanley
what happens when you sneeze
Asuquo Reply
The sneeze center sends out a signal to tightly close your throat, eyes and mouth. Your chest muscles contract and compress your lungs while your throat muscles relax. All of that means air, saliva and mucus is forced out of your nose and mouth. AAAAAHHHH-CHOOOO.
Hassan
how are you
Ayouba
hy
Ejaz
hii
Ishitha
hello
mahesha
Hello
Madu
Am fine
Abavon
how far
Abavon
hello
Sheillah
with why
Sheillah
with what
Sheillah
I have a question plzz
Sheillah
what is globsl warming
Rondy
Global warming is the overall rise in temperature of the Earth itself which is caused by multiple factors. Such factors include: greenhouse gases, the burning of fossil fuels and other forms of human activity.
Maryam
It's overall effects are known as climate change.
Maryam
thanks
Rondy
good
Haider
No problem.
Maryam
Thank you.
Maryam
what is creatinine?
Haider
Creatinine is a waste product produced in the body during muscle metabolism. This waste product is expelled from the body through urine. Here is it's formula:  C4H7N3O
Maryam
Formula: C4H7N3O
Maryam
ok
Haider
ok
Stephen
Outline the process of cell in the body interms of it's function.
Aliruku Reply
what communication method used mainly by plants
Jerda Reply
what is biology
james Reply
Biology is the study of life
Omoloju
yah
John
Biology is natural science that studies life and living orgasms.
Hilda
any questions from colleague
Sheillah
define nerve impulse
Karlin Reply
A nerve impulse is the way nerve cells (neurons) communicate with one another. Nerve impulsesare mostly electrical signals along the dendrites to produce a nerve impulse or action potential. The action potential is the result of ions moving in and out of the cell.
Sam
What is the Molecular Biology?
Service Reply
The branch of biology that deals with the structure and function of the macromolecules (e.g. proteins and nucleic acids) essential to life.
Sam
what is botany
Sillah
Description Description Botany, also called plant science, plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field.
Sam

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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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