Download Cell Membrane

Cell Membrane
“Keeping it all Together”
►
Cell Membrane
► Defines
the cell
► About 8 nm thick (8,000 to equal the
thickness of one page of paper)
► Helps to regulate the cell (maintain
equilibrium/homeostatic state)
► Selectively permeable
► Fluidity (lipids/proteins can shift laterally)
Gateway to the Cell
Cell Membrane Architecture
► Lipids
and Proteins – staple ingredients Most
abundant lipids – phospholipids
► Phospholipids are amphipathetic
(philic/phobic)
► Fluid Mosaic Model
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
Cell Membrane
Freeze Fracture
►A
cell membrane can be split (with a knife)
into it’s two layers revealing the
ultrastructure of the membrane’s interior.
Membrane Proteins
the lipid bilayer provides the
structure for the cell membrane,
membrane proteins allow for many of the
interactions that occur between cells.
► More than 50 kinds of proteins found (so
far) in the plasma membrane of RBC’s for
example.
► While
Classifying Membrane Proteins
► Proteins
are generally broken down into the
smaller classifications of integral proteins
and peripheral proteins
Integral Proteins
► Integral
proteins are embedded within the
lipid bilayer and partially penetrate the
hydrophobic core.
► Many are transmembrane proteins
► Example of an integral protein is “integrins”.
These attachments combine to give animal
cells a stronger framework that the plasma
membrane could alone.
Peripheral Proteins
► Peripheral
proteins are not embedded in the
lipid bilayer at all
► They are appendages loosely bound to the
surface of the membrane, often to exposed
parts of the integral proteins.
► Some are attached to the cytoskeleton of
the cell.
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.
►
Passive Transport
► Diffusion
is the natural phenomenon in
which nonpolar molecules naturally flow
from an area of higher concentration to an
area of lower concentration ► Diffusion
► Osmosis
DOES NOT require energy!
is a similar process, but refers
specifically to water molecules.
Diffusion of Liquids
http://www.tvdsb.on.ca/we
stmin/science/Sbi3a1/cells/
Osmosis.htm
Active Transport
Requires energy or
ATP
Moves materials
from LOW to HIGH
concentration
AGAINST
concentration
gradient
Transport Proteins
► Both
of passive and active transport are
mediated with the help of transmembrane
proteins that act as transporters
► Two main classes of transport proteins
are carrier proteins and channel
proteins.
Transport Proteins cont…
► For
the most part, carrier proteins mediate
active transport while channel proteins
mediate passive transport
Transport Proteins cont…
► Transport
proteins are critical to cell life and
cell interactions. They allow for the
proper distribution of ions and
molecules in multicellular organisms.
Additionally, they can help to maintain
proper intra- and extra-cellular pH
levels, facilitate communication
between cells, and are involved
numerous other essential functions
including protein sythesis.
Three Forms of Transport Across the Membrane
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.
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.
Quiz
► QUESTION
6:
You will also find proteins in the lipid bilayer.
True
False
Overview of Membrane Transport
For a visual of the processes discussed, check
out:
► http://phschool.com/webcodes10/index.cfm
?wcprefix=cbe&wcsuffix=3076&fuseaction=
home.gotoWebCode&x=0&y=0
Read section 7-3 and answer
questions 1-6 on page 189
► Copy Figure 7-16 into notes
► Assignment: