Majestic
Membranes
Chapter 7 Notes 
Plasma Membranes
Control Traffic in and out of cell
 Gatekeeper
 Selectively permeable –
allows some substances
to cross more easily than
others

Plasma Membranes


Composed of Lipids (mainly phospholipids),
proteins and carbohydrates
Phospholipids and proteins are
amphipathic – have a hydrophilic (water
loving) and a hydrophobic (water fearing)
region
History (Science as a Process)

Charles Overton (1895) – found that
substances that dissolve in lipids enter a
cell more rapidly than substances that are
insoluble in lipids

Hypothesis – membranes are made of lipids
History

Irving Langmuir (1917) – made artificial
membranes by adding phospholipids to
benzene
History

Gorter/Grendel (1925) – measured the
phospholipid content of membranes in red
blood cells and found just enough to cover
two layers of the cell

Hypothesis – Cell membranes must be
phospholipid bilayers acting as a stable boundary
between the fluid in the cell and the fluid outside
History

Davson/Danielli – Found that actual
membranes adhere to water more strongly
than artificial membranes

Hypothesis – Sandwich model – phospholipid
bilayers are sandwiched between two layers of
globular protein
History

Problems with the Sandwich model


Not all membranes look alike – some have more
protein content than others, some are thicker
than others
Membrane proteins have hydrophilic and
hydrophobic regions just like phospholipids
History

Singer/Nicolson (1972) – tried to revise the
sandwich model to account for problems

Hypothesis – membrane proteins are dispersed
and inserted into the phospholipids with their
hydrophilic heads on the outside and the
hydrophobic regions away from water
Fluid Mosaic Model

Mosaic of proteins floating in fluid layer of
phospholipids
Fluid Structure of Membranes

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Membranes are held together by
hydrophobic interactions
Lipids can drift laterally but don’t “flip-flop”
because their hydrophilic regions would
have to cross the hydrophobic core
Proteins can move, but more slowly
Some proteins are held in one place by
cytoskeleton
Fluid Structure of Membranes



Lower temperatures make the membrane
LESS fluid
More Unsaturated tails make the
membrane MORE fluid
Cholesterol


At warm temps – slows movement of lipids
At cool temps – prevents lipids from packing
tightly
Mosaic Structure of Membranes



Proteins are embedded in fluid
phospholipids
Proteins determine function of membrane
Different cell types have different proteins
embedded in membrane
Mosaic Structure of Membrane


Integral Proteins – penetrate into
hydrophobic core
Peripheral Proteins – loosely bound to
surface of membrane
Membrane Carbohydrates

Used in Cell-Cell recognition (distinguishing
one cell from another)


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Glycolipids – carbohydrates covalently bonded to
lipids
Glycoproteins – carbohydrates covalently
bonded to proteins
External – Oligosaccharides vary within a
species and from one cell type to the next (i.e.
different blood types are caused by different
carbohydrates on red blood cells)
Functions of Membrane Proteins


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Transport – can be hydrophilic channel or
pump that uses ATP
Enzymes – Active site exposed to
substances in nearby solution or work as a
chain in metabolic pathway
Signal Transduction – Binding sites for
chemical messengers that relay messages
to cell
Functions of Membrane Proteins



Intercellular joining – hook cells together
via junctions
Cell-Cell recognition – ID tags so that one
cell can recognize another
Attachment to ECM (extra-cellular matrix) –
helps maintain cell shape and coordinate
exchange between extra/intra cellular
Movement Across Membranes

Cell membranes are selectively permeable
– have some control over what enters and
exits the cell
Membrane Permeability


Depends on Lipid bilayer and proteins
embedded in it
Hydrophobic molecules


If small – can enter directly through
phospholipids (i.e. Oxygen and Carbon Dioxide)
Hydrophilic molecules and Large molecules

Use transport or channel proteins that span the
membrane
Passive Transport



Transport that does NOT require energy
Molecules move with the concentration
gradient from a high concentration to a low
concentration
Spontaneous process (decreases free
energy)
Passive Transport - Diffusion



Tendency for molecules to spread out in
available space (due to kinetic energy of
molecules)
If a substance is more concentrated on one
side of the membrane, it will move across
the membrane until it reaches equilibrium
Not everything can pass through easily
Passive Transport – Facilitated
Diffusion



Diffusion of hydrophilic solutes across a
membrane
No energy needed
Osmosis – diffusion of water through a
channel protein (aquaporin) due to
hydrophobic portion of phospholipids
Osmosis Comparison Terms



Hypertonic – solution with a higher
concentration of solutes (less water)
Hypotonic – solution with a lower
concentration of solutes (more water)
Isotonic – solution with equal solute and
water concentrations
Osmosis


Water always moves from a HYPOTONIC
to a HYPERTONIC solution
If two solutions are isotonic, water will
move, but at the same rate in both
directions
Osmoregulation

Cells without walls – like isotonic
environments


In hypertonic solution these cells tend to lose
water
In hypotonic solution, these cells tend to gain
water
Osmoregulation


Cells with walls – Like hypotonic environment
Cell wall prevents cell from taking up too much
water



When cell is full = turgid – healthy state for plant cells
In isotonic environment – Water will not enter plant –
becomes flaccid (shrunken)
In hypertonic environment – Cells lose water (plasmolysis)
and usually die
Passive Transport – Transport Protein

Transport proteins have properties of enzymes

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Specific for solute it transports (like substrate)
Can be saturated (working as fast as possible)
Can be inhibited
Catalyze a PHYSICAL process (not chemical)
Can be a channel
Can be gated – open and close as needed
Active Transport



Cell expends energy to move molecules
AGAINST the concentration gradient (from
a low concentration to a high concentration)
Used to help cell to obtain molecules it
needs from surroundings even if there is
not a high concentration
ATP supplies the energy
Active Transport – Protein Pump



Uses proteins embedded in membrane
ATP transfers a phosphate to the protein
causing it to change shape
(phosphorylation).
When it changes shape the solute moves
across the membrane

i.e. sodium/potassium pump
Pump Animation

http://highered.mcgrawhill.com/sites/0072495855/student_view0/c
hapter2/animation__how_the_sodium_pota
ssium_pump_works.html
Active Transport – Electrogenic
Pumps



Used to maintain membrane voltage
Cytoplasm is more negatively charged than
extracellular fluid so positively charged
molecules diffuse in to cell (opposite
charges attract)
Pumps actively transport ions back out to
keep maintain the voltage
Active Transport - Cotransport


ATP powered pump indirectly drives the
active transport of other solutes
Proton pump actively transports protons
(H+) out of the cell using ATP. + charge
builds up and as the H+ moves back into
the cell, it carries another substance with it

i.e. sucrose/H+ cotransporter is used to put
sucrose into special leaves in cell
Cotransporter animation

http://programs.northlandcollege.edu/biolog
y/Biology1111/animations/active3.swf
Active Transport - Exocytosis



Used to SECRETE macromolecules
Transport vesicles from golgi carry large
molecules to plasma membrane
Membrane of vesicle fuses with plasma
membrane and “dumps” macromolecule
out of cell

i.e. pancreas secretes insulin into blood
Active Transport - Endocytosis


Used in bring IN macromolecules
Phagocytosis – “cell eating”


Cell engulfs particle by wrapping pseudopodia
around it and packaging it in a vesicle
A lysosome will then eat the membrane and
release the particle into the cell
Phagocytosis Animation

http://animations.3d4medical.com/Phagocyt
osis-animation_AN2435.html
Active Transport - Endocytosis

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Pinocytosis – “cell drinking” – droplets of
extracellular fluid are engulfed in vesicles
Non-specific – any solutes in the fluid are
brought into the cell
Active Transport - Endocytosis

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Receptor-Mediated Endocytosis – VERY specific
Proteins in membrane have specific receptor sites
“Ligands” bind to these sites
Plasma membrane forms vesicle around them
Cell can acquire large quantities of specific
substances (like shopping at COSTCO )
i.e. cholesterol brought in to cells by this method