Transcript Cell Membrane - holyoke

Cell Membrane
“Keeping it all Together”
Cell Membrane
► When
you think about a membrane, imagine it is
like a big plastic bag with some tiny holes.
► That bag holds all of the cell pieces and fluids
inside the cell and keeps any nasty things outside
the cell. The holes are there to let some things
move in and out of the cell.
Cell Membrane cont…
► The
cell membrane is what defines the cell
and keeps its components separate from
outside cells or organisms.
Cell membrane
►
The cell membrane is composed of a double
layer of fat cells called a lipid bilayer in
which membrane proteins are embedded.
► The structure of the lipid bilayer prevents
the free passage of most molecules into and
out of the cell.
Cell Membrane
► Fluid
Mosaic Model:
Cell Membrane
► The
composition of nearly all cell
membranes is a double-layered sheet called
a lipid bilayer.
Lipid bilayer
The Lipid Bilayer
► The
lipid bilayer is a universal component of
all cell membranes
► Its role is critical because its structural
components provide the barrier that marks
the boundaries of a cell.
► The structure is called a "lipid bilayer"
because it is composed of two layers of fat
cells organized in two sheets.
Lipid Bilayer Structure
► The
structure of the lipid bilayer explains its
function as a barrier
► Lipids are fats, like oil, that are insoluble in
water. There are two important regions of a
lipid that provide the structure of the lipid
bilayer.
Lipid Bilayer Structure cont…
lipid molecule contains a hydrophilic
region or polar head region and a
► Each
hydrophobic region or non polar tail
region.
The hydrophilic region is attracted to
aqueous water conditions while the
hydrophobic region is repelled from
such conditions
Phospholipids
► The
most abundant class of lipid molecule
found in cell membranes is the
phospholipid.
Properties of Lipid Bilayer
► Most
important property is that it is highly
impermeable.
► Impermeable means it does not allow
molecules to pass through it easily
► Some molecules require the assistance of
other structures.
Another Bilayer Property:
► Another
property of the bilayer is it’s
fluidity.
► The bilayer contains lipids and proteins
► The bilayer's fluidity allows these structures
mobility within the lipid bilayer
Quiz Checkpoint:
► What
is the main function of the lipid
bilayer?
Answer to Question #1
► The
lipid bilayer acts as a barrier between
the inside and outside of the cell. It is highly
impermeable and does not allow most
molecules to freely pass through it into or
out of the cell.
Quiz Question #2
► Why
is the structure called a lipid bilayer?
Answer to Question #2
is called a lipid bilayer because it is
composed of two layers of fat, or lipid,
molecules.
► It
Quiz Question #3 – Fill in the Blank
►A
phospholipid molecule contains two
distinct regions. The __________ region is
attracted to water and the ___________
region is repelled from water.
Answers to Question #3
► Hydrophilic;
Hydrophobic
Membrane Proteins
the lipid bilayer provides the
structure for the cell membrane,
membrane proteins allow for many of the
interactions that occur between cells.
► While
Membrane Proteins cont…
► Membrane
proteins perform various
functions, and this diversity is reflected in
the significantly different types of proteins
associated with the lipid bilayer.
Classifying Membrane Proteins
► Proteins
are generally broken down into the
smaller classifications of integral proteins,
peripheral proteins, and lipid-bound
proteins.
Integral Proteins
► Integral
proteins are embedded within the
lipid bilayer.
► They cannot easily be removed from the cell
membrane without the use of harsh
detergents that destroy the lipid bilayer.
► Integral proteins float rather freely within
the bilayer, much like oceans in the sea.
Peripheral Proteins
► Peripheral
proteins are attached to the
exterior of the lipid bilayer.
► They are easily separable from the lipid
bilayer, able to be removed without harming
the bilayer in any way.
► Peripheral proteins are less mobile within
the lipid bilayer
Transmembrane Proteins
► Lipid-bound
proteins are located entirely
within the boundaries of the lipid bilayer
► Often act as “channels” for molecules to
pass through the membrane!
The Cell Membrane
Membrane Transport
► We
have discussed how the lipid bilayer acts as an
efficient barrier by only allowing a very small
number of non-polar molecules to freely enter or
exit a cell. While for the most part this selectivity
is a valuable function and allows the cell to
maintain its integrity, cells do need to move
certain large, polar molecules such as amino
acids, sugars, and nucleotides across their
membranes. As a result, cell membranes
require specific structures that allow for the
transport of certain molecules.
Membrane Transport
► There
are a number of different ways that
molecules can pass from one side of a cell
membrane to the other
► Some such means, like diffusion and
osmosis, are natural processes that require
no expenditure of energy from the cell
and are called passive transport.
Membrane Transport
Other methods of transport do require
cellular energy and are called active
transport
► In addition to these two forms of transport,
there exist other forms of transport such as
endocytosis and exocytosis, which will
be discuss later.
►
Transport across the membrane:
► http://biologycorner.com/bio1/diffusion.html
Checkpoint Quiz
► QUESTION
1:
The cell membrane is made up of a _____
bilayer.
Carbohydrate
Lipid
Protein
None of the Above
Answer to Question 1
► Cell
membranes are also described as lipid
bilayers. There are two layers of
phospholipids with proteins embedded in
the layers. The membrane is able to move
as needed by the cell.
Quiz
► QUESTION
2:
All eukaryotic cells are surrounded by a cell
membrane.
True
False
Answer to Question 2
► Eukaryotic
cells have an organized nucleus
and cell organelles. All of these cells have a
membrane. Plant cells have a cell
membrane and cell walls.
Quiz
► QUESTION
3:
The lipids in the bilayer have tails that are...
Hydrophilic
Hydrophobic
Answer to Question 3
► The
tail end of a phospholipid molecule is
hydrophobic. The tails do not like to be near
water and arrange themselves so that tails
meet tails and the heads face the cellular
fluids and exterior of the cell.
Quiz
► Question
4: The heads of the lipids in the
cell membrane are...
Hydrophilic
Hydrophobic
Answer to Question 4:
► The
head end of a phospholipid molecule is
hydrophilic. The heads like to be near water
and arrange themselves so that tails meet
tails and the heads face the cellular fluids
and exterior of the cell.
Quiz
► QUESTION
5:
Some molecules can pass through a cell
membrane without help.
True
False
Answer to Question 5
► Some
molecules can pass through the
cellular membrane by a process called
diffusion. Other molecules are too large to
pass through the membrane and must be
helped across the lipid bilayer.
Quiz
► QUESTION
6:
You will also find proteins in the lipid bilayer.
True
False
Answer to Question 6
► There
are proteins embedded in the cell
membrane. Some of the proteins pass
through the membrane and others are only
attached to the surface. The proteins serve
several functions.