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Power point notes- Cell membrane
Slide2: Artificial membranes. Phospholipids are amphipathic molecules- they have a hydrophobic and
hydrophilic section. When they come in contact with water (since water is polar) the hydrophilic section
sticks into water and the hydrophobic section is held away. Please note what we talked about in classwater is polar because of oxygen’s electronegativeity. Oxygen pulls electrons toward itself making it
slightly negative-the hydrogen atoms are left slightly positive. It is important to note that proteins that
span the cell membrane are also amphipathic-the portion inside the lipid bilayer is non-polar and therefore
hydrophobic.
Slide 3: The fluidity of membranes. The phospholipids move laterally (2micrometers.sec) and they
tumble. Proteins too can move in the matrix. Some move in a directed manner, others are anchored
(cytoskeleton). The fluidity of the membrane is related to the fatty acid tails of the of the phospholipid
molecule. When the tails are made from saturated fatty acids the phospholipids pack closely together-this
leads to less fluidity. If the tails are unsaturated they have double bonds and these bonds make the fatty
acids ‘kinky”!!! the kinks prevent the phospholipids from packing closely-this leads to greater fluidity.
Cholesterol is needed in the membrane to reduce the fluidity at normal temperatures. In cooler
temperatures the cholesterol prevents packing of the phospholipids so fluidity is maintained-this is
essential for proper membrane functioning-ex. Membrane transport
Slide 4: The membrane is a mosaic of different components. It is essentially a lipid bilayer “studded”
with proteins. The proteins can be “integral” i.e. they span the bilayer. These proteins must be
amphipathic-the hydrophobic region is embedded within the lipid bilayer. Other proteins are peripheralthey are loosely bound to the surface. Please note that the carbohydrates that are part of the glycolipids
and glycoproteins are on the outside of the cell-they function in cellular recognition. The selective
permeability of the membranes comes from the hydrophobic bilayer and the role of specific transport
proteins.
Slide 5: It should be noted that proteins while thought of as blobs or 3D structures embedded in the
membrane are actually long chains of amino acids that take on coiled and folded shapes much like a
ribbon. The alpha helix identified in this slide is common to proteins that stretch and flex.
To show that membrane proteins are able to drift researchers labelled the plasma membrane proteins of a
mouse cell and a human cell with two different markers and fused the cells. Using a microscope, they
observed the markers on the hybrid cell. The mixing of the mouse and human membrane proteins
indicates that at least some membrane proteins move sideways within the plan of the plasma membrane.
Slide6: Membranes have an inside and an outside surface. The cell membrane is built by the golgi
complex and the endoplasmic reticulum. Proteins made by ribosomes associated with the rough ER are
packed in vesicles that drift to the golgi where they are modified and then released to fuse with the
membrane. Note how the internal surface of the vesicle becomes the external membrane surface.
Slide 7: Membrane protein function. 1. Transport-active (requiring ATP) and facilitated (hydrophilic
pathway through the protein) 2. Enzyme activity- proteins in the membrane may function to break
molecules apart or to synthesize new molecules 3. Signal Transduction-protein hormones, for example,
act on a surface protein to send a signal to the inside of the cell. Transduction is a term that relates to a
cascade or domino effect of many more molecules being activated as the reaction proceeds 4. Cell
junctions a) plasmodesmata-channels between plant cells b) desmosomes-“rivets’ holding cells together
c) gap junctions-cytoplasmic channels that allow salt ion exchange, amino acids and sugars to move
between cells d) tight juctions- neighbouring cells are fused togethher ( ex. Intestinal cells) forming a
tissue 5. Cell to cell recognition-glycoproteins 6. Attachment points for the ECM and cytoskeleton .
Slide 8: Diffusion. Molecules move randomly due to their natural vibration derived from heat. This
natural movement is called “Brownian Motion”. Molecules move from an area of high concentration to
and area of low concentration (concentration gradient) when the molecules are equally balanced in an
area, their movement does not stop and the molecules are in what is called dynamic equilibrium. This
process of diffusion is passive- it requires no energy input by the cell.
Slide 9: Osmosis. The diffusion of water across a semi-permeable membrane. In this slide note the labelsthe hypotonic solution has less solute that the hypertonic solution. Therefore it has proportionately more
water and the water will try to move from high to low- or if you like from the hypo to the hypertonic side.
Note that the solute molecules are too large to move through the membrane.
Slide 10: this is an important slide. Please note the direction of water movement in each case and the
effect of this water movement on the cell. Also note the vocabulary- Turgid (turgor pressure); flaccid and
plasmolysis.
Slide 11 and 12: These images of a paramecium introduce the idea of osmoregulation in animals.
Paramecia are freshwater critters- we will be looking at these a little later on in the year. They are
hypertonic to their surroundings and therefore will gain water. To prevent lysis excess water is
accumulated into a contractile vacuole. This vacuole periodically empties allowing the cell to maintain
homeostasis.
Slide 13: Facilitated diffusion. The actual process of protein function is unknown but this slide suggests
two possibilities. Specific shapes of proteins fit the specific molecules to be transported. Please note that
the protein can change shape in order to facilitate transport. Charges on the molecule are likely matched to
charges on the membrane protein-this acts to specify shape (position of the charge) and draw the molecule
into the protein. Water which is a polar molecule moves through tiny channels called aquaporins. Please
see the Living Systems text book for a good diagram and description of gated channels.
Slide 14: the sodium potassium pump. This is a specific example of active transport. The addition of ATP
provides energy for transport. When ATP is used a phosphate molecule is dropped off phosphorylation)
to the transport protein. This energizes the protein and it changes shape. The shape change in this case
pushes sodium ions out of the cell. Potassium ions then enter the proteins and this triggers the shape of the
protein to return to normal, the potassium is moved into the cell and phosphate id released. The free
phosphate is eventually reattached to ADP to make another ATP molecule. Note that active transport leads
to an accumulation of a molecule-against the concentration gradient.
Slide 15: Summary of membrane transport. Note the differences
Slide 16 and 17: These two slides introduce the idea of active transport creating a proton gradient (
hydrogen ions) that can be used to the cells advantage. We will discuss later that the mitochondrion
chloroplast use a proton gradient to generate ATP.
Slide 18: These are examples of what we call “mass transport” into the cell ENDOcytosis and out of the
cell EXOcytosis.
Notes: modified from Mr. I Morrison Citadel High.