AP BIOLOGY:
MEMBRANES & TRANSPORT
Enduring Understandings
II.
B. Growth, reproduction and dynamic
homeostasis require that cells create and maintain
internal environments that are different from their
external environments.
1.
Cell membranes are selectively permeable due to their
structure.
2. Growth and dynamic homeostasis are maintained by the
constant movement of molecules across membranes.
3. Eukaryotic cells maintain internal membranes that partition
the cell into specialized regions.
Cells

The cell is the simplest collection of matter that
can live

Cells are open systems and continuously
exchange materials and energy with their
surroundings.
A Panoramic View of the Cell

All cells have basic features in common




Bounded by a plasma (cell) membrane
Filled with a semifluid substance, cytosol in which
organelles are found
Contain genetic material in the form of DNA
Have ribosomes, tiny organelles that make proteins
Internal Membranes

Eukaryotic cells have extensive and elaborate
internal membranes which partition the cell into
compartments or organelles

Archaea and Bacteria generally lack internal
membranes
Internal Membranes

Internal membranes in eukaryotes function to
facilitate cellular processes by:



Compartmentalizing reactants, substrates,
products, cofactors, inhibitors, etc.
Minimizing competing interactions
Increasing surface area where reactions can
occur
Discussion


How do these membrane functions help
maximize energy and materials for survival &
reproduction?
Can you give an example (even a very
general one)?
The Cell Membrane
AP Biology
Diffusion across cell membrane
 Cell membrane is the boundary between
inside & outside…

separates cell from its environment
Can it be an impenetrable boundary?
NO!
OUT
IN
food
carbohydrates
sugars, proteins
amino acids
lipids
salts, O2, H2O
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OUT
IN
waste
ammonia
salts
CO2
H2O
products
cell needs materials in & products or waste out
Cell membrane defines cell
 Cell membrane separates living cell from
aqueous environment

thin barrier = 8nm thick
 Controls traffic in & out of the cell

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allows some substances to cross more
easily than others
Phospholipids
 Phosphate head

hydrophobic
 Arranged as a bilayer
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“attracted to water”
hydrophilic
 Fatty acid tails

Phosphate
Fatty acid
“repelled by water”
Arranged as a Phospholipid bilayer
 Serves as a cellular barrier / border
sugar
H 2O
salt
polar
hydrophilic
heads
nonpolar
hydrophobic
tails
impermeable to charged particles
polar
hydrophilic
heads
waste
AP Biology
lipids
Membrane is a collage of proteins, lipids, &
carbohydrates embedded in the fluid matrix of the
lipid bilayer
https://www.youtube.com/watch?v=svAAiKsJaY&list=PLXwnjgs_UWpIQhMZ1YObcd338AHkQzPHW&feature=player_detailpage#t=98
Glycoprotein
Extracellular fluid
Glycolipid
Phospholipids
Cholesterol
Peripheral
protein
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Transmembrane
proteins
Cytoplasm
Filaments of
cytoskeleton
1972, S.J. Singer & G. Nicolson proposed Fluid Mosaic Model
Membrane carbohydrates
 Play a key role in cell-cell recognition

ability of a cell to distinguish one cell
from another
 antigens
important in organ &
tissue development
 basis for rejection of
foreign cells by
immune system

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Membrane fat composition varies
 Fat composition affects flexibility

membrane must be fluid & flexible
 about as fluid as thick salad oil

% unsaturated fatty acids in phospholipids
 keep membrane less viscous
 cold-adapted organisms, like winter wheat
 increase % in autumn

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cholesterol in membrane
Discussion
 Which kinds of molecules - polar or
non-polar - do you think are more likely
to be able to traverse the phospholipid
bilayer barrier?
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Selective Permeability
 Membrane becomes semi-permeable via
protein channels

specific channels allow specific material
across cell membrane
inside cell
NH
AP
Biology
3
salt
H 2O
aa
sugar
outside cell
Cell membrane is more than lipids…
 Transmembrane proteins embedded in
phospholipid bilayer

create semi-permeable channels
lipid bilayer
membrane
AP Biology
protein channels
in lipid bilyer membrane
Think back to amino acids…
How can you get a protein
to “stay put” in the membrane?
AP Biology
2007-2008
Classes of amino acids
What do these amino acids have in common?
nonpolar & hydrophobic
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Classes of amino acids
What do these amino acids have in common?
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polar & hydrophilic
Structure - Function!
 Within membrane

Polar areas
of protein
nonpolar amino acids
 hydrophobic
 anchors protein
into membrane
 On outer surfaces of
membrane in fluid

polar amino acids
 hydrophilic
 extend into
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extracellular fluid &
into cytosol
Nonpolar areas of protein
+
H
H+
Examples
Retinal
chromophore
NH2
aquaporin =
water channel in bacteria
Porin monomer
H 2O
b-pleated sheets
Bacterial
outer
membrane
Nonpolar
(hydrophobic)
a-helices in the
cell membrane
COOH
H++
H
Cytoplasm
proton pump channel
in photosynthetic bacteria
H O
AP Biology 2
Membrane Proteins
 Proteins determine membrane’s specific functions

cell membrane & organelle membranes each have
unique collections of proteins
 Classes of membrane proteins:

peripheral proteins
 loosely bound to surface of membrane
 ex: cell surface identity marker (antigens)

integral proteins
 penetrate lipid bilayer, usually across whole membrane
 transmembrane protein
 ex: transport proteins
 channels, permeases (pumps)
AP Biology
Many Functions of Membrane Proteins
“Channel”
Outside
Plasma
membrane
Inside
Transporter
Enzyme
activity
Cell surface
receptor
Cell adhesion
Attachment to the
cytoskeleton
“Antigen”
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Cell surface
identity marker
Movement across the
Cell Membrane
AP Biology
2007-2008
Diffusion
 2nd Law of Thermodynamics
governs biological systems

universe tends towards disorder (entropy)
 Diffusion
movement from HIGH  LOW concentration
APBiology
Simple Diffusion
 Move from HIGH to LOW concentration
“passive transport”
 no energy needed

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diffusion
movement of water
osmosis
Discussion
 1) Thinking about molecular activity…
WHY do you think molecules show a
net movement from where they are
highly concentrated to where they are
less highly concentrated?
 2) What kinds of molecules (or
characteristics of molecules) do you
think could conduct simple diffusion
across a cell membrane?
AP Biology
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.html
Facilitated Diffusion
 Diffusion through protein channels


channels move specific molecules (size, charge
can vary) across cell membrane and along/down
concentration gradient
facilitated = with help
no energy needed
open channel = fast transport
HIGH
LOW
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How about really large molecules?
 Moving large molecules into & out of cell
through vesicles & vacuoles
 endocytosis

 phagocytosis = “cellular eating”
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__phagocytosis.html
 pinocytosis = “cellular drinking”

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exocytosis
exocytosis
Endocytosis
phagocytosis
fuse with
lysosome for
digestion
pinocytosis
non-specific
process
receptor-mediated
endocytosis
triggered by
molecular
signal
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The Special Case of Water
Movement of water across
the cell membrane
AP Biology
2007-2008
Osmosis is just diffusion of water
 Diffusion of water from
HIGH concentration of water to
LOW concentration of water

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across a
semi-permeable
membrane
Aquaporins
 Water moves rapidly into & out of cells

Aquaporins = water channels
 protein channels allowing flow of water
across cell membrane
AP Biology
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html
Concentration of water
 Direction of osmosis is phrased in terms of
total solute concentrations compared to the
other side

Hypertonic - MORE solute, less water

Hypotonic - LESS solute, more water

Isotonic - EQUAL solute, equal water
water
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hypotonic
hypertonic
net movement of water
Discussion
 Suppose the following concentrations
are in effect:
Outside membrane = 50% solvent, 50%
solute
 Inside membrane = 30% solvent, 70%
solute

 What is the tonicity of each side? In
which direction will water osmose?
AP Biology
Discussion
.05 M
.03 M
Cell (compared to beaker)  hypertonic or hypotonic
Beaker (compared to cell)  hypertonic or hypotonic
Which way does the water flow?  in or out of cell
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Discussion
 What tonicity of solution was each cell
suspension placed in?
AP Biology
Managing water balance
 Cell survival depends on balancing
water uptake & loss
AP Biology
freshwater
balanced
saltwater
1
Managing water balance
 Hypotonic

a cell in fresh water

high concentration of water around cell
 problem: cell gains water,
swells & can burst
KABOOM!
 example: Paramecium
 ex: water continually enters
Paramecium cell
 solution: contractile vacuole
 pumps water out of cell
ATP
 ATP

plant cells
No problem,
here
 turgid = full
 cell wall protects from bursting
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freshwater
Pumping water out
 Contractile vacuole in Paramecium
ATP
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2
Managing water balance
 Hypertonic
I’m shrinking,
a cell in salt water I’m shrinking!
 low concentration of water
around cell

 problem: cell loses water &
can die
 example: shellfish
 solution: take up water or
pump out salt

plant cells
I will
survive!
 plasmolysis = wilt
 can recover
AP Biology
saltwater
3
Managing water balance
 Isotonic
That’s
perfect!

animal cell immersed in
mild salt solution

no difference in concentration of
water between cell & environment
 problem: none
 no net movement of water
flows across membrane equally, in
both directions
I could
 cell in equilibrium
be better…

 volume of cell is stable
 example:
blood cells in blood plasma
 slightly salty IV solution in hospital
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balanced
Discussion
 Grocers regularly spray produce with
water. Not just to make them shiny!
Use what you’ve learned about
transport to explain, what are the other
reasons why they do this?
AP Biology
Discussion
 You’ve seen warning signs driving that
areas are “salt-free.” These roads are
not salted, because the salt dissolves
in melted snow, running off to the side
of the road, and… what effect does this
have on the plants there?
AP Biology
Discussion
 If you sprinkle fruit like a strawberry
with sugar, it will soon have glistening
beads all over it. Why? What
happened?
AP Biology
P.S.: Osmolarity

Osmolarity of solution = like molarity, but
measures osmoles of solute per volume
rather than moles of solute per volume

1 osmole = 1 mole (6.022 x 1023) of solute
particles



What is the molarity of 1 mole of NaCl in 1 L of water?
What is the osmolarity of 1 mole of NaCl in 1 L of
water?
Not used in AP Bio calculations, and if it’s in a problem,
can generally be treated as interchangeable with
molarity
Water Potential

Water or osmotic potential: the tendency of water to
experience net movement from one place to another


It amounts to being the free energy of water
 = p + s
  = water potential (net movement of water into the
environment with lower )
 p = pressure potential (Physical pressure on the solution,
such as that exerted by cell walls)
 s = solute potential (Effect of solute concentration. No
solute, s = 0. More solute -> more negative s)
Water Potential


p requires no particular calculation
s = -iCRT




i = ionization constant, the degree to which the
molecule ionizes in water. (typically equals
number of ions. For NaCL, i = 2.0. For sucrose,
which doesn’t ionize in water, i = 1.0)
C = solution molarity
R = the pressure constant (0.0831 liter bars/moleK)
T = solution temperature in degrees Kelvin (ºC +
273º)
Discussion

Suppose I have an 0.18 M solution of sodium
chloride at atmospheric pressure (i.e., no
pressure potential). It’s at 25º C. What is its
water potential?
Discussion - Answer
Discussion



 = -8.91 bars, the water potential of our salt
solution
Suppose I put a plant cell in that solution in
which the pressure potential = 3 and the
solute potential = -6.
Will osmotic pressure favor the net
movement of water into the cell, out of the
cell, or both equally?
Discussion - Answer
Limits on Cell Size


Metabolic requirements impose upper limits
on size that is practical for a cell
Volume grow proportionately more than its
surface area


The plasma membrane allows passage of nutrients,
oxygen, and wastes to service the
entire volume of the cell
DNA must be able to
give enough instructions
for the entire cell’s
proteins
Surface Area to Volume Ratio




What is the surface area of a 1x1x1 cm
cube? A 2x2x2? 3x3x3?
What are the volumes of these cubes?
What is the surface area to volume ratio of
each cube?
Which cube will be best at transporting
materials in and out of it, as well as efficiently
transporting materials within it?
Active Transport
AP Biology
2007-2008
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html
Active Transport
 Cells may need to move molecules against
concentration gradient



conformational shape change transports solute
from one side of membrane to other
protein “pump”
“costs” energy = ATP LOW conformational change
ATP
HIGH
AP Biology
“The Doorman”
Passive transport
Transport
Type
Type of
molecule
transported?
Concentration
gradient
Through
protein?
Uses energy
(typically
ATP)?
Is diffusion?
AP Biology
Simple
diffusion
Facilitated
diffusion
Osmosis
Active
transport
Summary
 Passive Transport

Simple diffusion
 diffusion of nonpolar, hydrophobic molecules
 lipids
 HIGH  LOW concentration gradient

Facilitated transport
 diffusion of polar, hydrophilic molecules
 through a protein channel
 HIGH  LOW concentration gradient

Osmosis
 diffusion of water
 Active transport

diffusion against concentration gradient
 LOW  HIGH
uses a protein pump
AP Biology
 requires ATP

ATP
Transport summary
simple
diffusion
facilitated
diffusion
active
transport
AP Biology
ATP
Diffusion through phospholipid bilayer
 What molecules can get through directly?

fats & other lipids
inside cell
NH3
 What molecules can
lipid
salt
NOT get through
directly?

polar molecules
 H 2O

outside cell
sugar aa
H 2O
ions (charged)
 salts, ammonia

large molecules
 starches, proteins
AP Biology