Colonie High AP Biology
DeMarco/Goldberg
Chapter 5
Diffusion
 2nd Law of Thermodynamics
The Plasma Membrane
governs biological systems

Universe tends towards disorder—ENTROPY!
 Diffusion

Diffusion of 2 solutes
 Each substance diffuses down its own
concentration gradient, independent of
concentration gradients of other
substances
movement from high  low concentration
Diffusion
 Move from HIGH to LOW concentration


“passive transport”
no energy needed
diffusion
Cell (plasma) membrane
 Cells need an inside & an outside…

separate cell from its environment

cell membrane is the boundary
IN
food
carbohydrates
sugars, proteins
amino acids
lipids
salts, O2, H2O
IN
Building a membrane
 With what do you build a barrier that
keeps the watery contents of the cell
separate from the watery environment?
OUT
OUT
osmosis
waste
ammonia
salts
CO2
H2O
products
Your choices
 carbohydrates?
 proteins?
 nucleic acids?
 lipids?
Colonie High AP Biology
DeMarco/Goldberg
Lipids of cell membrane
 Membrane is made of phospholipids

Phospholipids
phospholipid bilayer
inside cell
hydrophilic
phosphate
hydrophobic
outside cell
fatty acid tails
Semi-permeable membrane
 Need to allow passage through the

membrane for a lot of stuff!
But it needs to control what gets in or
out

Simple diffusion across membrane
lipid
inside cell
lipid
lipid
lipid
lipid
lipid
membrane needs to be semi-permeable
sugar
aa
lipid
H2O
salt
NH3
outside cell
lipid
lipid
lipid
lipid
Phospholipid bilayer
 What molecules can get through directly?
inside cell
NH3
outside cell
sugar aa
salt
H2O
Fats and other nonpolar (hydrophoblic)
molecules can slip
directly through the
phospholipid bilayer
membrane, but…
…what about all the
other stuff?
lipid
lipid
lipid
Permeable cell membrane
 Need to allow other material through

lipid
lipid
membrane needs to be permeable to…
 all materials a cell needs to bring in
 all wastes a cell needs excrete out
 all products a cell needs to export out
inside cell
Haa
sugar
2O
“holes” or channels in
cell membrane allow
polar (hydrophilic)
materials in & out
outside cell
NH
salt3
lipid
Colonie High AP Biology
DeMarco/Goldberg
Diffusion through a channel
 Movement from high to low
sugar
inside cell sugar
sugar sugar
Semi-permeable cell membrane
 But the cell still needs control

membrane needs to be semi-permeable
 specific channels allow specific material in & out
sugar
inside cell
H2O
aa
sugar
outside cell
sugar
sugar sugar
sugar
NH3
sugar
sugar
salt
outside cell
sugar
So… how do you build a selectively
permeable cell membrane?
 What molecule will sit “comfortably” in a
Why proteins?
 Proteins are mixed molecules

bi-lipid
membrane
_________ channels
in bi-lipid membrane
some hydrophobic amino acids
 stick in the lipid membrane
 anchors the protein in membrane
phospholipid bilayer forming channels?

some hydrophilic amino acids
 stick out in the watery
fluid in & around cell
 specialized “receptor”
for specific molecules
Membrane Proteins
 Proteins determine most of membrane’s
specific functions

cell membrane & organelle membranes each
have unique collections of proteins
 Membrane proteins:


peripheral proteins = loosely
bound to surface of membrane
integral proteins = penetrate into
lipid bilayer, often completely
spanning the membrane
 a.k.a. transmembrane proteins
Facilitated Diffusion
 Movement from HIGH to LOW
concentration through a protein channel
passive transport
no energy needed
 facilitated = with help


Colonie High AP Biology
DeMarco/Goldberg
Facilitated Diffusion
 Globular proteins act as doors in membrane

channels to move specific molecules
through cell membrane
open channel = fast transport
[high]
Osmosis is diffusion of water
 Water is very important, so we talk
about water separately—but same idea!
 Diffusion of water from
high concentration of water to
low concentration of water

[low]
across a
semi-permeable
membrane
“The Bouncer”
Concentration of water
 Direction of osmosis is determined by
comparing total solute concentrations!
Managing water balance
 Cell survival depends on balancing
water uptake & loss
hypertonic - more solute, less water
 hypotonic - less solute, more water
 isotonic - equal solute, equal water

molecule of solute
hypotonic
hypertonic
net movement of water
freshwater
Managing water balance
 Isotonic

animal cell immersed in
isotonic solution
balanced
saltwater
Managing water balance
 Hypotonic

 blood cells in blood
 no net movement of water
animal cell in hypotonic
solution will gain water,
swell & burst
 Paramecium vs. pond water
 Paramecium is hypertonic
 H2O continually enters cell
 to solve problem,
across plasma membrane
 water flows across
membrane, at same rate
in both directions
 volume of cell is stable
specialized organelle,
contractile vacuole
 pumps H2O out of cell
using ATP

plant cell
 turgid
Colonie High AP Biology
DeMarco/Goldberg
1991 | 2003
Managing water balance
 Hypertonic

Aquaporins
 Water moves rapidly into & out of cells
animal cell in hypertonic
solution will lose water,
shrivel & probably die

evidence that there were water channels
 salt water organisms are
hypotonic compared to
their environment
 they have to take up
water & pump out salt

plant cells
 plasmolysis = wilt
Peter Agre
Roderick MacKinnon
John Hopkins
Rockefeller
Osmosis in Action
Fluid & solutes flows
out of capillaries to
tissues due to blood
pressure
Lymphatic
capillary
 plasma proteins  osmotic
 “bulk flow”
pressure in capillary
BP > OP
.05 M
Interstitial fluid flows
back into capillaries
due to osmosis
BP < OP
Interstitial
fluid
.03 M
Blood
flow
85% fluid returns
to capillaries
Capillary
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
15% fluid returns
via lymph
Arteriole
Active Transport
 Cells may need molecules to move
against concentration situation
need to pump against concentration
 protein pump
 requires energy
 ATP

Venule
Active Transport
 Globular proteins act as ferry for specific
molecules
shape change transports solute from one side
of membrane to other  protein “pump”
 “costs” energy (ATP)

conformational change
Na+/K+ pump
in nerve cell
membranes
“The Doorman”
Colonie High AP Biology
DeMarco/Goldberg
Active Transport
 Many models & mechanisms


uniports, symports, antiports
primary vs. secondary active transport
using ATP
using ATP
Absorption of Nutrients
 Passive transport

fructose
 Active (protein pumps) transport

pump amino acids, vitamins & glucose
 against concentration gradients across
intestinal cell membranes
 allows intestine to absorb much higher
proportion of nutrients in the intestine than
would be possible with passive diffusion
 worth the cost of ATP!
Absorption by Small Intestines
 Absorption through villi & microvilli

finger-like projections
 increase surface area for absorption
Gated channels
 Some channel proteins open only in
presence of stimulus (signal)

stimulus usually different from
transported molecule
 ex: ion-gated channels
when neurotransmitters bind to a specific
gated channels on a neuron, these channels
open = allows Na+ ions to enter nerve cell
 ex: voltage-gated channels
change in electrical charge across nerve cell
membrane opens Na+ & K+ channels
Getting through cell membrane
 Passive transport

diffusion of hydrophobic (lipids) molecules
 high  low concentration gradient
 Facilitated transport


diffusion of hydrophilic molecules
through a protein channel
 high  low concentration gradient
 Active transport

diffusion against concentration gradient
 low  high
uses a protein pump
 requires

Transport summary
Colonie High AP Biology
How about large molecules?
 Moving large molecules into & out of cell
DeMarco/Goldberg
Endocytosis
phagocytosis
fuse with
lysosome for
digestion
pinocytosis
non-specific
process
receptor-mediated
endocytosis
triggered by
ligand signal
through vesicles & vacuoles
 endocytosis

 phagocytosis = “cellular eating”
 pinocytosis = “cellular drinking”
 receptor-mediated
endocytosis

exocytosis
Receptor-Mediated Endocytosis
Cell Junctions
Cell Recognition and Adhesion
 Cells  Tissues
Plant cell wall
 Structure

binding can be homotypic (between
molecules of the same protein) or heterotypic
(between different but complementary
proteins)
cellulose
primary cell wall
 secondary cell wall
 middle lamella = sticky polysaccharides


Colonie High AP Biology
DeMarco/Goldberg
Intercellular junctions
 Plant cells

plasmodesmata
Animal cell surface
 Extracellular matrix

 channels allowing
cytosol to pass
between cells
collagen fibers in
network of
glycoproteins
 support
 adhesion
 movement
plasmodesmata
 regulation
Intercellular junctions in animals
Intercellular junctions
 Animal cells

tight junctions
 membranes of adjacent cells fused
forming barrier between cells
 forces material through cell membrane

gap junctions
 communicating junctions
 allow cytoplasmic movement between
adjacent cells

desmosomes
 anchoring junctions
 fasten cells together in strong sheets
More than just a barrier…
 Expanding our view of cell membrane
beyond just a phospholipid bilayer
barrier

phospholipids plus…
…proteins. Plus…
 In 1972, S.J. Singer & G. Nicolson
proposed that membrane proteins are
inserted into the phospholipid bilayer
Colonie High AP Biology
Membrane Carbohydrates
 Play a key role in cell-cell recognition
 called glycoproteins
ability of a cell to
distinguish
neighboring cells
from another
 important in organ &
tissue development
 basis for rejection of
foreign cells by
immune system
(ABO blood system)

Fluid Mosaic Model
A membrane is a collage of different proteins
embedded in the fluid matrix of the lipid bilayer.
DeMarco/Goldberg
Membranes provide a variety of cell functions