CELL MEMBRANES
Professor Andrea Garrison
Biology 11
Illustrations ©2010 Pearson Education, Inc.
unless otherwise noted
Plasma Membranes
• Plasma membrane = external cell membrane
– Surrounds cell
– Separates cell’s internal environment
from external environment
– Regulates passage of molecules
in/out of cell
– Allows homeostasis
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Plasma Membranes
• Cell cytoplasm about 80% water
• Extracellular fluid (outside cell)
about 80% water
• Water has a drastic effect on cell
structure
– Water very unique molecule
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Water (H2O)
2 hydrogen atoms and 1 oxygen atom
Bond—represents
sharing of 2 electrons
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Water (H2O)
• H2O is polar
molecule
– Electrons
spend more
time around
oxygen
– Molecule
slightly
charged on
each end
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Water (H2O)
• Water forms
hydrogen bonds
– Opposite charges
on different ends
of molecules
attract each
other
– H-bonds are very
weak
• Break easily
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Properties of water related to H-bonds
High surface tension
• H-bonds hold
water molecules
together
• Takes significant
weight to break
the bonds
Raft Spider, Europe, 3” legspan
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Properties of water related to H-bonds
High surface tension
• H-bonds hold water
molecules together
• Evaporation from
leaves acts like
sucking on straw to
pull water up
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Properties of water related to H-bonds
High heat capacity
• Heat makes molecules
vibrate
• Temperature is a
measure of the
amount of vibration
• Some energy has to
break H-bonds before
they vibrate
• Water absorbs a lot of
energy before
temperature increases
Cell Membranes ©2012 Amy Garrison
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Properties of water related to H-bonds
High heat capacity
• Water moderates
climate
Cell Membranes ©2012 Amy Garrison
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Properties of water related to H-bonds
High heat capacity
• Water moderates
climate
• Water moderates
body temperature
• Water cools by
evaporative cooling
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Properties of water related to H-bonds
Ice floats
• Liquid water is
dense
– H-bonds
constantly break
and reform
• Solid water is less
dense
– H-bonds last
longer
– Arrangement
more spacious
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Properties of water related charge
Universal solvent
• Charges on water
molecule allow it
to dissolve many
substances
• Charges attract
opposite charges
on other
molecules, such
as Na+Cl-
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Properties of water related to charge
Does not react with nonpolar (non-charged)
molecules
Oil has no charge
Vinegar has water in
it and is charged
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Properties of water related charge
• Based on what you know about how water
reacts with charged and non-charged
molecules, what type of molecules would you
expect your cell membranes to be made of?
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Fluid Mosaic Model of Plasma
Membrane
• Plasma membrane a
mosaic
– Phospholipids
– Proteins
• Fluid structure
– Molecules move around
freely in the membrane
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Fluid Mosaic Model of Plasma
Membrane
• Phospholipids
Has H2O
– Form bilayer
– Foundation of
membrane
Has H2O
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Fluid Mosaic Model of Plasma
Membrane
• Phospholipids
+
Hydrophilic head
•charged
• water-loving
• attracted to water
Hydrophobic tails
•No charge
• water-fearing
• repel water
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Fluid Mosaic Model of Plasma
Membrane
• Phospholipids
Has H2O
– Hydrophilic heads
• Contact water inside
and outside cell
– Hydrophobic tails
• No contact with water
Has H2O
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Fluid Mosaic Model of Plasma
Membrane
• Phospholipids
Has H2O
– Prevent most
substances from
crossing membrane
• Only very small
molecules with little or
no charge can get
through
Has H2O
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Fluid Mosaic Model of Plasma
Membrane
• Proteins
Has H2O
– Allow specific
substances to get
through (based on
shape)
– Hydrophilic portions
• Contact water
– Hydrophobic portions
• No contact with water
Has H2O
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Fluid Mosaic Model of Plasma
Membrane
Has H2O
Has H2O
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Functions of Membrane Proteins
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Functions of Membrane Proteins
=receptor protein;
Binds to molecule
and causes rxn
inside cell
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Functions of Membrane Proteins
Transports molecules across membrane
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Functions of Membrane Proteins
Recognition
Proteins--ID body cells and types of cells
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Functions of Membrane Proteins
Adhesion
proteins—holds cells together
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Membrane Permeability
• Let’s look at the plasma membrane’s
regulation of transport in/out of cell?
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Membrane Permeability
• Plasma membrane freely permeable (cannot
regulate):
– Water
– Oxygen
– Carbon dioxide
• These molecules move across membrane via
diffusion
– Diffusion = movement of molecules from region of
higher concentration to region of lower concentration
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Diffusion (Passive Transport)
• Requires no energy on part of cell; tends toward
equilibrium
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Membrane Permeability
• Diffusion of oxygen and carbon dioxide are not
a problem in normal situations
– Inhaled O2 diffuses into the blood in the lungs and
out of the blood into the cells of the body
– Waste CO2 diffuses out of the body cells into the
blood and out of the blood into the lungs to be
exhaled
• Diffusion of water can be a problem
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Osmosis
• Special case of diffusion
• Diffusion of water (high concentration
water low concentration water) across
selectively permeable membrane
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Osmosis
To understand osmosis, must understand
solutions
SOLUTION = SOLUTE + SOLVENT
Dissolved +
particles
water
Concentration of solution = % solute
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Osmosis
• Concentration of solution considers %solute
• Osmosis considers %water (solvent)
• 90% salt solution = 90% salt and 10% water
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Osmosis
Which has greater salt?
90% salt
or
50% salt?
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Osmosis
Which has greater salt?
90% salt
or
50% salt?
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Osmosis
Which has greater salt?
90% salt
or
50% salt?
Which has greater water?
90% salt
or
50% salt?
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Osmosis
Which has greater salt?
90% salt
or
50% salt?
Which has greater water?
90% salt
or
50% salt?
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Osmosis
Which has greater salt?
90% salt
or
50% salt?
Which has greater water?
90% salt
or
50% salt?
So more salt means less water,
And vice-versa
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Osmosis
• Which side has more water?
• What direction will water move?
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Osmosis
• Water moves from side where water is greater to
side where water is less (low salt to high salt)
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Osmosis
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Osmosis
• Comparing a cell to the surrounding solution
– Isotonic solution has equal solute to cell
• 0.9% salt solution surrounds 0.9% cell
– Hypotonic solution has less solute than cell
• 0.1% salt solution surrounds 0.9% cell
– Hypertonic solution has more solute than cell
• 10% salt solution surrounds 0.9% cell
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Osmosis
• Isotonic solution
surrounds cell
• No net movement of
water
• Cell remains healthy
0.9%
salt
0.9%
salt
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Osmosis
• Hypotonic solution
surrounds cell
• Water enters cell
• Cell swells/bursts
0.1%
salt
0.%
0.9%
salt
salt
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Osmosis
• Hypertonic solution
surrounds cell
• Water leaves cell
• Cell shrivels
10%
salt
0.9%
salt
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Osmosis
• Plant and
animal cells
affected by
osmosis
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Osmosis
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Plant Turgor—Effects of Osmosis
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Membrane Permeability
•Regulated substances
-- Move across membrane via transport proteins
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Membrane Permeability
•Regulated substances
-- Move across membrane via transport proteins
• Facilitated diffusion
• Active transport
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Membrane Permeability
• Regulated substances
– Facilitated diffusion
Higher solute
concentration
• Diffusion
• Requires help of
transport protein
• Requires no energy
(passive)
Lower solute
concentration
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Membrane Permeability
• Regulated substances
– Active transport
• Molecules move from
lower to higher
concentration
• Requires help of transport
protein
• Requires energy (active)
• ̴1/3 cell’s energy
expended in active
transport
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Summary of Transport Mechanisms
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Membrane Transport
• Not all substances transported one molecule
at a time
– Bulk transport
• Moves molecules in bulk
• Moved via transport vesicle
• Requires energy
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Membrane Transport
• Bulk transport
– Endocytosis
– Exocytosis
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Membrane Transport
• Bulk transport
– Endocytosis
• Substance moved into cell via pinching of membrane into vacuole
Outside of cell
Plasma
membrane
Cytoplasm
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Membrane Transport
• Bulk transport
– Endocytosis
• Substance moved into cell via pinching of membrane into vacuole
• Phagocytosis vs. pinocytosis
Outside of cell
Plasma
membrane
Cytoplasm
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Membrane Transport
• Bulk transport
– Exocytosis
• Substance moved out of cell via fusion of vesicle with
membrane
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