Search for notes by fellow students, in your own course and all over the country.
Browse our notes for titles which look like what you need, you can preview any of the notes via a sample of the contents. After you're happy these are the notes you're after simply pop them into your shopping cart.
Document Preview
Extracts from the notes are below, to see the PDF you'll receive please use the links above
• KHANacademy
Subject: Biology
Search for subjects, skills, and videos Home
Noman
Structure of the plasma membrane
Introduction
Each cell of your body is encased in a tiny bubble of membrane, one which has approximately the
consistency of salad oil
...
It not only
defines the borders of the cell, but also allows it to interact with its environment in a controlled way
...
It also needs proteins, which are involved in cross-membrane
transport and cell communication, and carbohydrates, which decorate both the proteins and lipids
and help cells recognize each other
...
Fluid mosaic model
The currently accepted model for the structure of the plasma membrane, called the fluid mosaic
model, was first proposed in 1972
...
According to the fluid mosaic
model, the plasma membrane is a mosaic of components—primarily, phospholipids, cholesterol, and
proteins—that can move about freely and fluidly in the plane of the membrane
...
Interestingly enough, this fluidity means that if you insert a very fine needle
into a cell, the membrane will simply part to flow around the needle; once the needle is removed, the
membrane will flow back together seamlessly
...
The principal components of the plasma membrane are lipids (phospholipids and cholesterol),
proteins, and carbohydrate groups that are attached to some of the lipids and proteins
...
Biological membranes usually involve two layers of phospholipids with their tails pointing
inward, an arrangement called a phospholipid bilayer
...
Membrane proteins may extend partway into the plasma membrane, cross the membrane
entirely, or be loosely attached to its inside or outside face
...
The proportions of proteins, lipids, and carbohydrates in the plasma membrane vary between
different types of cells
...
Phospholipids
Phospholipids, arranged in a bilayer, make up
the basic fabric of the plasma membrane
...
The hydrophilic, or “water-loving,” portion of a
phosopholipid is its head, which contains a
negatively charged phosphate group as well as
an additional small group (of varying identity, “R”
in the diagram below), which may also or be
charged or polar
...
The hydrophobic, or “water-fearing,” part of a
phospholipid consists of its long, nonpolar fatty
acid tails
...
poorly with water
...
The phospholipid bilayer formed by these interactions makes a good barrier
between the interior and exterior of the cell, because water and other polar or charged substances
cannot easily cross the hydrophobic core of the membrane
...
)
Thanks to their amphipathic nature,
phospholipids aren’t just well-suited to form a
membrane bilayer
...
If the
phospholipids have small tails, they may form a
micelle (a small, single-layered sphere), while if
they have bulkier tails, they may form a
liposome (a hollow droplet of bilayer
membrane)
...
Proteins are the second major component of
plasma membranes
...
Integral membrane proteins are, as their name suggests, integrated into the membrane: they have at
least one hydrophobic region that anchors them to the hydrophobic core of the phospholipid bilayer
...
The portions of an integral membrane protein found inside the membrane are hydrophobic, while
those that are exposed to the cytoplasm or extracellular fluid tend to be hydrophilic
...
A typical membrane-spanning segment consists of 20-25 hydrophobic amino
acids arranged in an alpha helix, although not all transmembrane proteins fit this model
...
Some act as
ion channels or transporters, selectively allowing certain molecules to pass through the plasma
membrane
...
Still others function in cell-cell
recognition, help the cell stick to its neighbors or
the surrounding matrix, or play structural roles
...
Peripheral membrane proteins are found on the
outside and inside surfaces of membranes,
attached either to integral proteins or to
phospholipids
...
Peripheral membrane proteins play a
number of important roles, like providing
attachment sites for cytoskeletal fibers and
relaying signals from receptor proteins
...
In general, they are found on
the outside surface of cells and are bound either to proteins (forming glycoproteins) or to lipids
(forming glycolipids)
...
Along with
membrane proteins, these carbohydrates form distinctive cellular markers, sort of like molecular ID
badges, that allow cells to recognize each other
...
Membrane Fluidity
The structure of the fatty acid tails of the phospholipids is important in determining the properties of
the membrane, and in particular, how fluid it is
...
Unsaturated fatty acids, on the other
hand, contain one or more double bonds, often resulting in a bend or kink
...
Phospholipids with unsaturated fatty acid tails cannot pack together as tightly because of the
bent structure of the tails
...
Organisms that live in cold environments, like cold-water fish, tend to have more unsaturated fatty
acids in their cell membranes, keeping the membranes fluid at low temperatures
...
Cholesterol, which is embedded among the phospholipids in the membrane, helps to
minimize the effects of temperature on fluidity
...
In this way, cholesterol
expands the range of temperatures at which a membrane maintains a functional, healthy fluidity
...
[Attribution and references]
×
How helpful was this article to you?
The article was great and really helpful
...
The article was okay but wasn't very interesting to me
...
Send feedback
Be specific, and
indicate a location in
the text:
In the second paragraph,
how is the moon large
enough to block the sun?
Isn't the sun way larger?
Have something that's
not a question about
this content?
Post a tip or thanks
Join our help
discussions
Report a technical
problem with the site
Request a feature
Report a mistake in
the video
This discussion area is not meant for answering
homework questions
...
It's been a while since I did A levels (I'm
refreshing my memory as I'm going to study medicine after my
maths degree) but I do think this is just a more compact source of
the same information
...
Answer this question
...