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cell membranes and cytoplasm
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Lecture notes of 7 pages for the course foundations of life at UWS (.)
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Cell membranes and cytoplasmLearning Outcomes
1. Summarise the main functions of cellular membranes.
2. List the main components of a cell membrane.
3. Draw a labelled diagram of a cell membrane.
4. Discuss the 3 main types of membrane transport.
Membrane Function
Protecting the cell from the outside environment and maintaining homeostasis
Protects cell by maintaining the cells structure-binds to the cytoskeleton and the
extracellular matrix.
Controlling the cells cytoplasmic contents
Sensing the environment
It detects key molecules and other cells in its environment.
E.g., Marker molecules on cell membrane identify it to other cells.
Proteins embedded in membrane can sense and respond to signals.
Plasma membrane is selectively permeable.
All biological membranes have similar structure.
Functions of each type of membrane depend on the composition of specific proteins and
carbs embedded in the membrane.
E.g., Nuclear membrane separates nucleus from the rest of cytoplasm and regulates flow of
materials from cytoplasm to nucleus.
Membrane Structure
Phospholipid bilayer
Globular proteins inserted in the bilayer.
Fluid mosaic model
Mosaic of proteins floats in or on the fluid lipid bilayer
Water surrounds and fills cells.
Plasma membrane must not be water soluble to maintain barrier between cell and exterior.
Best kind of molecule suited for this is an oily, hydrophobic molecule such as a lipid- will
pack together and serve as a barrier to water.
Primary molecules that make up biological membranes are phospholipids.
Carbohydrate components of the membrane (glycolipids and glycoproteins) are used mainly
in cellular identification.
Sterols e.g., Cholesterol, help the membrane maintain the proper fluidity.
Fluid Mosaic
Like a tile mosaic, plasma membrane created from many individual parts.
The plasma membranes components are in constant motion.
Consistency of the plasma membrane Is like a sheet of oil.
Proteins and other molecules float in the sheet
The motion and varied units are the reason the plasma membrane is called the fluid mosaic.
Frye and Edidins Membrane Fusion Experiment
, Bilayers are fluid.
Hydrogen bonding of water holds the 2 layers together.
Individual phospholipids and unanchored proteins can move through the membrane.
An elegant experiment conducted by Larry Frye and Michael Ediden demonstrated the
results of this motion.
Used fluorescent antibodies to label plasma membrane proteins from a mouse cell.
Then used a different colour of fluorescent antibodies to label the plasma membrane
proteins of a human cell.
Then they fused these 2 cells together
Resulting cell shows an even distribution of both colours throughout the entire membrane.
Thus, membrane components are moving withing the membrane.
The research has deepened scientists understanding of membrane fluidity and its role in cell
function.
Further work such as that done by Ediden in 1997 has built on this to show that fluidity is not
uniform for all regions of the membrane.
Microscopes
Electron microscopy has provided much structural evidence to support the fluid mosaic
model.
Both transmission electron microscope (TEM) and scanning (SEM) used to study membranes
Samples need to be prepared.
One method is to embed specimen in resin.
Cut with microtome (<1um thick)
Shavings placed on grid and beam of electrons passed through.
At high magnification and with good resolution from the EM we can see the double layers of
the membrane
Structure of Phospholipids
Primary molecule that makes up bilayer
A phospholipid is an amphipathic molecule-contains hydrophobic and hydrophilic regions.
Have a phosphate-containing hydrophilic head and a hydrophobic tail.
Structure consists of glycerol-a 3 carbon polyalcohol-and 2 fatty acids attached to the
glycerol.
Non-polar/hydrophobic-phosphate group attached to the glycerol.
Polar/hydrophilic
spontaneously forms a bilayer.
fatty acids on the outside
phosphate groups are on both surfaces.
causes them to spontaneously orient hydrophilic heads towards H2O and hydrophobic tails
away from H2O.
inner and outer surfaces of the plasma membrane must remain stable when in contract with
water, so these surfaces must be hydrophilic.
a bilayer results when numerous phospholipids are surrounded by water.
Membrane Fluidity and Sterols
a membranes hydrophobic inner region forms a barrier that cannot easily be crossed by
hydrophilic molecules.
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