<< Chapter < Page Chapter >> Page >
An illustration of a phospholipids bilayer is shown. The phospholipids bilayer consists of two layers of phospholipids. The hydrophobic tails of the phospholipids face one another while the hydrophilic head groups face outward.
The phospholipid bilayer is the major component of all cellular membranes. The hydrophilic head groups of the phospholipids face the aqueous solution. The hydrophobic tails are sequestered in the middle of the bilayer.

The head is the hydrophilic part, and the tail contains the hydrophobic fatty acids. In a membrane, a bilayer of phospholipids forms the matrix of the structure, the fatty acid tails of phospholipids face inside, away from water, whereas the phosphate group faces the outside, aqueous side ( [link] ).

Phospholipids are responsible for the dynamic nature of the plasma membrane. If a drop of phospholipids is placed in water, it spontaneously forms a structure known as a micelle, where the hydrophilic phosphate heads face the outside and the fatty acids face the interior of this structure.

Steroids

Unlike the phospholipids and fats discussed earlier, steroids have a fused ring structure. Although they do not resemble the other lipids, they are grouped with them because they are also hydrophobicand insoluble in water. All steroids have four linked carbon rings and several of them, like cholesterol, have a short tail ( [link] ). Many steroids also have the –OH functional group, which puts them in the alcohol classification (sterols).

 The structures of cholesterol and cortisol are shown. Each of these molecules is composed of three six-carbon rings fused to a five-carbon ring. Cholesterol has a branched hydrocarbon attached to the five-carbon ring, and a hydroxyl group attached to the terminal six-carbon ring. Cortisol has a two-carbon chain modified with a double-bonded oxygen, a hydroxyl group attached to the five-carbon ring, and an oxygen double-bonded to the terminal six-carbon ring.
Steroids such as cholesterol and cortisol are composed of four fused hydrocarbon rings.

Cholesterol is the most common steroid. Cholesterol is mainly synthesized in the liver and is the precursor to many steroid hormones such as testosterone and estradiol, which are secreted by the gonads and endocrine glands. It is also the precursor to Vitamin D. Cholesterol is also the precursor of bile salts, which help in the emulsification of fats and their subsequent absorption by cells. Although cholesterol is often spoken of in negative terms by lay people, it is necessary for proper functioning of the body. It is a component of the plasma membrane of animal cells and is found within the phospholipid bilayer. Being the outermost structure in animal cells, the plasma membrane is responsible for the transport of materials and cellular recognition and it is involved in cell-to-cell communication.

For an additional perspective on lipids, explore the interactive animation “Biomolecules: The Lipids”

.

Section summary

Lipids are a class of macromolecules that are nonpolar and hydrophobic in nature. Major types include fats and oils, waxes, phospholipids, and steroids. Fats are a stored form of energy and are also known as triacylglycerols or triglycerides. Fats are made up of fatty acids and either glycerol or sphingosine. Fatty acids may be unsaturated or saturated, depending on the presence or absence of double bonds in the hydrocarbon chain. If only single bonds are present, they are known as saturated fatty acids. Unsaturated fatty acids may have one or more double bonds in the hydrocarbon chain. Phospholipids make up the matrix of membranes. They have a glycerol or sphingosine backbone to which two fatty acid chains and a phosphate-containing group are attached. Steroids are another class of lipids. Their basic structure has four fused carbon rings. Cholesterol is a type of steroid and is an important constituent of the plasma membrane, where it helps to maintain the fluid nature of the membrane. It is also the precursor of steroid hormones such as testosterone.

Questions & Answers

what is nutrients
Patrick Reply
what are the types of skeleton
Tanyi Reply
endo skeleton and exo skeleton
Faith
or types of skeletons There is exo skeleton found in class insecta Exo skeleton found in phylum chordata and lastly hydrostatic skeleton found mainly in earth worms
Daniel
and hydrostatic skeleton
Godbless
what is life?
Jerson Reply
life is when any living thing breath or respirate.
Lucky
I want to join
Patrick
me too
Favour
me two
Monday
me too
Bengazy
me too
Maggie
I here
Yahaya
Does anybody knows another biology app that brings good understand
Lucky
give an example of a phylum where all members have open circulatory
Omoro Reply
anthropoda
Daniel
phylum Anthropoda
Daniel
how do u change the unit of temperature from degree celsius to degree Fahrenheit
Maggie
what are beneficial effects in industrial
Nnamani Reply
create employment
ibrahim
provide food
ibrahim
pollution
ibrahim
dirty the environment
ibrahim
pollution and lack of knowledge about how to manage waste
Alex
what is gametophyte
Ibrahim Reply
what is biology
Basiru Reply
Biology deals with the study of living organisms and their interactions with each other and the environment.
Quartey
biology simply means the study of living and non-livingthings.
Shamstar
what is the meaning of ubiquitous
Hamziyatu Reply
Under what topic is this question
fred
three branches of biology
Alexander Reply
what is the cell
Olajide Reply
what is life
Olajide
cell is the functional and structural unit of life
Shweta
life is a problem solving entity
Atanga
what is the characteristics of plant cell
Quaku
Describe the process that results in the formation of a tetrad.
Michael Reply
what is biology
Ndukwe Reply
biology is the study of life
Oghenemine
what is biology
Jimmy Reply
the study of life
Hamziyatu
what is the Biosphere?
Kabel Reply
I want to know what classifications of living organisms and their names
Quaku
are classified into plants and animals eukaryotic and prokaryotic cell.
Lokwe
what is biology?
Turabi Reply
what is the meaning of Biologists
Turabi
what is the meaning of Biologists
Turabi
what is the meaning of Biologists
Turabi
are those who are specialized in Biology
Nicode
Is the study of living things,
Kabel
what is the ecosystem?
Kabel
bio mean life logus means study so the study of living things or living organisms is called biology
Faraq
Ecosystem consists of the living organisms in a particular area together with the abiotic factors.
Quartey

Get the best Biology course in your pocket!





Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Biology' conversation and receive update notifications?

Ask