<< Chapter < Page Chapter >> Page >
By the end of this section, you will be able to:
  • Understand the fluid mosaic model of cell membranes
  • Describe the functions of phospholipids, proteins, and carbohydrates in membranes
  • Discuss membrane fluidity

A cell’s plasma membrane defines the cell, outlines its borders, and determines the nature of its interaction with its environment (see [link] for a summary). Cells exclude some substances, take in others, and excrete still others, all in controlled quantities. The plasma membrane must be very flexible to allow certain cells, such as red blood cells and white blood cells, to change shape as they pass through narrow capillaries. These are the more obvious functions of a plasma membrane. In addition, the surface of the plasma membrane carries markers that allow cells to recognize one another, which is vital for tissue and organ formation during early development, and which later plays a role in the “self” versus “non-self” distinction of the immune response.

Among the most sophisticated functions of the plasma membrane is the ability to transmit signals by means of complex, integral proteins known as receptors. These proteins act both as receivers of extracellular inputs and as activators of intracellular processes. These membrane receptors provide extracellular attachment sites for effectors like hormones and growth factors, and they activate intracellular response cascades when their effectors are bound. Occasionally, receptors are hijacked by viruses (HIV, human immunodeficiency virus, is one example) that use them to gain entry into cells, and at times, the genes encoding receptors become mutated, causing the process of signal transduction to malfunction with disastrous consequences.

Fluid mosaic model

The existence of the plasma membrane was identified in the 1890s, and its chemical components were identified in 1915. The principal components identified at that time were lipids and proteins. The first widely accepted model of the plasma membrane’s structure was proposed in 1935 by Hugh Davson and James Danielli; it was based on the “railroad track” appearance of the plasma membrane in early electron micrographs. They theorized that the structure of the plasma membrane resembles a sandwich, with protein being analogous to the bread, and lipids being analogous to the filling. In the 1950s, advances in microscopy, notably transmission electron microscopy (TEM), allowed researchers to see that the core of the plasma membrane consisted of a double, rather than a single, layer. A new model that better explains both the microscopic observations and the function of that plasma membrane was proposed by S.J. Singer and Garth L. Nicolson in 1972.

The explanation proposed by Singer and Nicolson is called the fluid mosaic model    . The model has evolved somewhat over time, but it still best accounts for the structure and functions of the plasma membrane as we now understand them. The fluid mosaic model describes the structure of the plasma membrane as a mosaic of components—including phospholipids, cholesterol, proteins, and carbohydrates—that gives the membrane a fluid character. Plasma membranes range from 5 to 10 nm in thickness. For comparison, human red blood cells, visible via light microscopy, are approximately 8 µm wide, or approximately 1,000 times wider than a plasma membrane. The membrane does look a bit like a sandwich ( [link] ).

Questions & Answers

what are the characteristic of livingthing
Joy Reply
Movement Respiration Nutrition Irritability or sensitivity Growth Excretion Reproduction Adaptation Competition Death or Life Span
Emmanuel
what is the function of medulla oblongata
mbalenhle Reply
what is the strongest bone in the human body
Chionye
what is biology
Dauda Reply
is a branch of science which deals with the study of living thing
sheka
thanks
Dauda
Biology is got from two main words: bios: means life and Logos: means knowledge therefore, biology is a branch of science that deals with knowledge, life and functioning of living things.
Musoke
what are the characteristics of organism
Seldam
Responds to stimuli
Musoke
how do we draw a genetic diagram?
Clive Reply
Branches of variation
Mariama Reply
What is variation
Mariama Reply
this is the difference among different species
joe
natural selection was done by who
Faith Reply
by Charles Darwin
Godson
Charles Darwin
Charity
who proposed Cell doctrine
ANJOUS-YINKA Reply
biology introduction
Khab Reply
describe how the gurd cells regulate the movement of gases in and out of a leaf.
Tony
state the main functions of the leaf
kalisto Reply
what is the similarity between meiosis and mitosis
kalisto
meiosis-involves the division of sex cells mitosis-involves the division of body cells
ANGULA
ok
Osikhena
Regulation of blood sugar level
Kyara Reply
Describes larmack theory
Mayiik Reply
Mention five modes of nutrition in ecology
Mayiik
Saprophytic
Evans
commercialism mutualism saprophytic parasitic simboitism
Osikhena
what is biology
Imisi Reply
Biology is the study of life
Aisha
Yea
Mayiik
Biology is the study of biography
Musoke
yes
Osikhena
biology is the branch of science that deal with the study of living organisms
Dauda
Biology is the branch of science that due with living things and nor living things
Joy
Bio means life and logy means to study
Joy
what is insulin and where is it found in the body
Cecilia Reply
An illustration of the negative feedback mechanism in homeostasis
Cecilia
How we know
Nhial
That is not the meaning
Mayiik
what are chromosomes
Ibrahim
factor that affect the diffusion of substance
Nwafor Reply
nature of substances size increase in temperature
Quadri

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




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