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By the end of this section, you will be able to:
  • Describe the molecular components that make up the cell membrane
  • Explain the major features and properties of the cell membrane
  • Differentiate between materials that can and cannot diffuse through the lipid bilayer
  • Compare and contrast different types of passive transport with active transport, providing examples of each

Despite differences in structure and function, all living cells in multicellular organisms have a surrounding cell membrane. As the outer layer of your skin separates your body from its environment, the cell membrane (also known as the plasma membrane) separates the inner contents of a cell from its exterior environment. This cell membrane provides a protective barrier around the cell and regulates which materials can pass in or out.

Structure and composition of the cell membrane

The cell membrane    is an extremely pliable structure composed primarily of back-to-back phospholipids (a “bilayer”). Cholesterol is also present, which contributes to the fluidity of the membrane, and there are various proteins embedded within the membrane that have a variety of functions.

A single phospholipid molecule has a phosphate group on one end, called the “head,” and two side-by-side chains of fatty acids that make up the lipid tails ( [link] ). The phosphate group is negatively charged, making the head polar and hydrophilic—or “water loving.” A hydrophilic    molecule (or region of a molecule) is one that is attracted to water. The phosphate heads are thus attracted to the water molecules of both the extracellular and intracellular environments. The lipid tails, on the other hand, are uncharged, or nonpolar, and are hydrophobic—or “water fearing.” A hydrophobic    molecule (or region of a molecule) repels and is repelled by water. Some lipid tails consist of saturated fatty acids and some contain unsaturated fatty acids. This combination adds to the fluidity of the tails that are constantly in motion. Phospholipids are thus amphipathic molecules. An amphipathic    molecule is one that contains both a hydrophilic and a hydrophobic region. In fact, soap works to remove oil and grease stains because it has amphipathic properties. The hydrophilic portion can dissolve in water while the hydrophobic portion can trap grease in micelles that then can be washed away.

Phospholipid structure

This diagram shows the structure of a phospholipid. The hydrophilic head group is shown as a pink sphere and the two tails are shown as yellow rectangles.
A phospholipid molecule consists of a polar phosphate “head,” which is hydrophilic and a non-polar lipid “tail,” which is hydrophobic. Unsaturated fatty acids result in kinks in the hydrophobic tails.

The cell membrane consists of two adjacent layers of phospholipids. The lipid tails of one layer face the lipid tails of the other layer, meeting at the interface of the two layers. The phospholipid heads face outward, one layer exposed to the interior of the cell and one layer exposed to the exterior ( [link] ). Because the phosphate groups are polar and hydrophilic, they are attracted to water in the intracellular fluid. Intracellular fluid (ICF) is the fluid interior of the cell. The phosphate groups are also attracted to the extracellular fluid. Extracellular fluid (ECF) is the fluid environment outside the enclosure of the cell membrane. Interstitial fluid (IF) is the term given to extracellular fluid not contained within blood vessels. Because the lipid tails are hydrophobic, they meet in the inner region of the membrane, excluding watery intracellular and extracellular fluid from this space. The cell membrane has many proteins, as well as other lipids (such as cholesterol), that are associated with the phospholipid bilayer. An important feature of the membrane is that it remains fluid; the lipids and proteins in the cell membrane are not rigidly locked in place.

Questions & Answers

all of the following substance move in in and out of cell except
Mohd Reply
all of the following substance move in and out of cells except
Which signaling molecule is most likely responsible for an increase in digestive activity?
Tonya Reply
Hi I think it is DNA
Homeostatasis return to the body to a healthy state after a stressful stimuli by producing
Ofosu Reply
There are some people suffering serious injured what will we do in doing X-ray?
Jefford Reply
observe bone arrangements n associated structures like soft tissues muscles in general the radiographical changes
epithelial tissue lines blood vessels
Laura Reply
what is difference between anabolism and catabolism in simple language
Chinaza Reply
anabolism simply means building up while catabolism breaking down
building up and breaking down
explain respiratory centers
Tharshana Reply
which enzyme help in excretion of bile
Sakshi Reply
how conversion of Beta-carotiene into vitamin-A takes place
best reference books for anatomy pls suggest that
what is gross anatomy
Saroj Reply
The study of large structure of the body
macroscopic anatomy (Gross anatomy)
manipulation of structures without aid of microscope due to their size
what is scrotum
Dakshit Reply
a bag of skin near the penis which contains the testicles
its me Sardar from kpk pakistan
Why are you on my digits
it is part if male organ that hold testis in position it also regulates temperature
how does it work?
Uriah Reply
I want to know the fertilization process in human
Nana Reply
Which plane divides the body into right and left parts
sagittal plane
Thank you
Homestatic regulations usually involves a (n) _ that detects a particular stimulus, and a(n) _that respond to the stimulus by communicating with a (n) _whose activity has an effect on the same stimulus.
A cell is producing proteins to be transported out of that cell. They will be processed on ribosomes that are
what are diseases in the bood
Azoyenime Reply
right hypochondriac rision how meain word
Ganesh Reply
Diabetes insipidus or diabetes mellitus would most likely be indicated by ________.
oral cavity how miean
Ganesh Reply
oral cavity how mean what
seems to me every one is here a doctor and worse part is that even they donot know what to ask.
they know what to ask the problem is how to ask it.too much broken english.
dear George ongeche I think if they ask these kind questions and telling themselves as HCPs or medical student's .these are the basic ones go ask something more interesting .

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Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
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