Membranes

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Membranes

• fluid mosaic of molecules,
• dynamic
• flexible
• semi/selectively permeable barrier

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Artificial membranes behave like real
cellmembranes
phospholipids
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Phospholipids are a major component of cell
membranes
Phospholipids are made from 2 fatty acids, 1
glycerol and a phosphate group
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The structure of a phospholipid
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hydrophilic
hydrophobic
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The fluidity of membranes
Increases rigidity, decreases fluidity Cells can
change their membrane compositions
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Fluid mosaic model
What does each word mean in fluid mosaic
model?
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All membranes in all cells are a lipid bilayer
with scattered proteins and protein
complexes, However, the amount of cholesterol,
and types and amounts the glycolipids,
glycoproteins differ. Therefore some membrane
properties are shared and some are different.
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What evidence is there that the proteins in the
membrane are able to move around?
that the membrane is a fluid mosaic of
molecules?
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Evidence for the drifting of membrane
proteinsMouse and human cells fused to form
hybrid cells will have a mosaic of membrane
proteins
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The detailed structure of an animal cells plasma
membrane, in cross section
Always on outside
Transmembrane protein
determines flexibility
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The structure of a transmembrane protein
channel
What levels of structure are shown here?
Alpha helix
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1. What are transmembrane
proteins? 2. What do transmembrane
proteins do? There are 6 main catagories
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Form channels through which Molecules enter/
leave cells
Passive or active )
Are membrane enzymes that catalyze reactions
Receive signals (ligand molecules )and then
transfer a reaction through a cascade of other
Molecules within the cell
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Receptor proteins bind with membrane anitgens on
other cells and recognize them as self or foreign
Integrins stabilize structure
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Sidedness of the plasma membrane
The plasma membrane is added to and taken from
during exocytosis and endocytosis The membrane
has two different sides caused by different
molecules in the membrane
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• Cell to cell recognition a membrane
• function
• identifies cell neighbors
• identifies invaders ( foreign cells )
• is achieved by identifying specific
• glycolipids and glycoproteins on the
• other cell as self or foreign
• cell markers are called antigens
• eg. Blood typing antigens for A,B AB O
• blood are glycoproteins

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Your antibodies
Host makes antibodies against the antigens
Invader is killed
Foreign cell enters with its antigens on its
membrane
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Methods of transport occurs acrossthe membrane

• Dialysis and osmosis
• Facilitated transport
• Active Transport
• Endocytosis
• Exocytosis
• Co transport
• Specialized channels for transport of
• specific substances

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Figure 8.10 The diffusion of solutes across
membranes
dialysis
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Figure 8.11 Osmosis diffusion of water across a
membrane from an area of high tolow water
potential
Water potential the tendency of water to leave
one area and move to another
Non diffusible solute
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Figure 8.12 The water balance of living cells-
water moves from high to low water potential
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Turgid cells, large central vacuoles are expanded
with water. High pressure potential inside
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Hypertonic solution outside causes plasmolysis
Cell membrane shrinks, compressing cell contents
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The contractile vacuole of Paramecium an
evolutionary adaptation for osmoregulation- uses
ATP to actively pump out water
H20
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• Facilitated diffusion aided diffusion
• Molecules move with their gradient but pass
• through specific protein channels
• ( transport proteins help the diffusion )
• No energy is expended

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Figure 8.14 Two models for facilitated diffusion
Aquaporins are special protein channels that
let water into/out of cells
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• Active transport involves the use of energy /
  ATP work
• molecules are pumped into or out of the cell
  against a gradient ( chemical or
• electrochemical gradient )
• the cell membrane encloses extracellular
• material endocytosis
• a) - phagocytosis - particles
• b) pinocytosis - liquid

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The sodium-potassium pump a specific case of
active transport that adds to the membrane
potential 3 Na out/ 2Kin. A
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Animal cell membranes have a voltage across them
called a membrane potential The cell inside is
negative compared to the outside. ( - 50 -
200 millivolts ) This favors cation ()
movement into the cell and anion (-)
movement out of the cell An electrochemical
gradient exists across the cell membrane when
animal cells pump Na ions out. The
sodium-potassium pump is an electrogenic pump.
A pump that generates a voltage difference across
the membrane
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A high concentration of a substance is an area
of high in potential energy Cells build up ions
on one side of the membrane and then allow them
to flow back along their gradient to
release energy for work Eg sodium-potassium pump
in animal cells H pump in plants, fungi,
bacteria
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An electrogenic pump used in plants, fungi
andbacteria pushes H out
Outside net charge
Area of potential energy
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Cotransport- one transported substance drives
another one
High H conc outside has high potential energy.
As H ions move back into cell they fall down
their gradient, losing energy. That energy
brings sucrose across the membrane
in plants
High sucrose
Active transport)
Low sucrose
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Review passive and active transport compared
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The three types of endocytosis in animal cells
particles
liquids