The Cell Membrane
AP Biology
2007-2008
Phospholipids
 Phosphate head

“attracted to water”
hydrophilic
 Fatty acid tails

Phosphate
hydrophobic
 Arranged as a bilayer
Fatty acid
“repelled by water”
Aaaah,
one of those
structure–function
examples
AP Biology
Arranged as a Phospholipid bilayer
 Serves as a cellular barrier / border
sugar
H 2O
salt
polar
hydrophilic
heads
nonpolar
hydrophobic
tails
impermeable to polar molecules
polar
hydrophilic
heads
waste
AP Biology
lipids
Cell membrane defines cell
 Cell membrane separates living cell from
aqueous environment

thin barrier = 8nm thick
 Controls traffic in & out of the cell

allows some substances to cross more
easily than others
 hydrophobic (nonpolar) vs. hydrophilic (polar)
AP Biology
Permeability to polar molecules?
 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-permeabe channels
lipid bilayer
membrane
AP Biology
protein channels
in lipid bilyer membrane
Why are
proteins the perfect
molecule to build structures
in the cell membrane?
AP Biology
2007-2008
Classes of amino acids
What do these amino acids have in common?
nonpolar & hydrophobic
AP Biology
Classes of amino acids
What do these amino acids have in common?
I like the
polar ones
the best!
AP Biology
polar & hydrophilic
Proteins domains anchor molecule
 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
function through
conformational change =
protein changes shape
Many Functions of Membrane Proteins
“Channel”
Outside
Plasma
membrane
Inside
Transporter
Enzyme
activity
Cell surface
receptor
Cell adhesion
Attachment to the
cytoskeleton
“Antigen”
AP Biology
Cell surface
identity marker
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
Cell membrane must be more than lipids…
 In 1972, S.J. Singer & G. Nicolson
proposed that membrane proteins are
inserted into the phospholipid bilayer
It’s like a fluid…
It’s like a mosaic…
It’s the
Fluid Mosaic Model!
AP Biology
Membrane is a collage of proteins & other molecules
embedded in the fluid matrix of the lipid bilayer
Glycoprotein
Extracellular fluid
Glycolipid
Phospholipids
Cholesterol
Peripheral
protein
AP Biology
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

AP Biology
Any Questions??
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Movement across the
Cell Membrane
AP Biology
2007-2008
Diffusion
 2nd Law of Thermodynamics
governs biological systems

universe tends towards disorder (entropy)
 Diffusion

AP Biology
movement from HIGH  LOW concentration
Simple Diffusion
 Move from HIGH to LOW concentration
“passive transport”
 no energy needed

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diffusion
movement of water
osmosis
Facilitated Diffusion
 Diffusion through protein channels


channels move specific molecules across
cell membrane
facilitated = with help
no energy needed
open channel = fast transport
HIGH
LOW
AP Biology
“The Bouncer”
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”
Active transport
 Many models & mechanisms
ATP
AP Biology
ATP
antiport
symport
Getting through cell membrane
 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
 Active transport

diffusion against concentration gradient
 LOW  HIGH


AP Biology
uses a protein pump
requires ATP
ATP
Transport summary
simple
diffusion
facilitated
diffusion
active
transport
AP Biology
ATP
How about large molecules?
 Moving large molecules into & out of cell
through vesicles & vacuoles
 endocytosis

 phagocytosis = “cellular eating”
 pinocytosis = “cellular drinking”

AP Biology
exocytosis
exocytosis
Endocytosis
phagocytosis
fuse with
lysosome for
digestion
pinocytosis
non-specific
process
receptor-mediated
endocytosis
triggered by
molecular
signal
AP Biology
The Special Case of Water
Movement of water across
the cell membrane
AP Biology
2007-2008
Osmosis is just diffusion of water
 Water is very important to life,
so we talk about water separately
 Diffusion of water from
HIGH concentration of water to
LOW concentration of water

AP Biology
across a
semi-permeable
membrane
Concentration of water
 Direction of osmosis is determined by
comparing total solute concentrations

Hypertonic - more solute, less water

Hypotonic - less solute, more water

Isotonic - equal solute, equal water
water
AP Biology
hypotonic
hypertonic
net movement of water
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
AP Biology
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
I

plant cells
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
AP Biology
balanced
1991 | 2003
Aquaporins
 Water moves rapidly into & out of cells

evidence that there were water channels
 protein channels allowing flow of water
across cell membrane
AP Biology
Peter Agre
Roderick MacKinnon
John Hopkins
Rockefeller
Do you understand Osmosis…
.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
AP Biology
Any Questions??
AP Biology
Ghosts of Lectures Past
(storage)
AP Biology
2007-2008
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
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

AP Biology
cholesterol in membrane
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
AP Biology
OUT
IN
waste
ammonia
salts
CO2
H2O
products
cell needs materials in & products or waste out
Regulating the Internal
Environment
Water Balance &
Nitrogenous Waste
Removal
AP Biology
2006-2007
Animal systems evolved to
support multicellular life
aa
O2
CH
CHO
CO2
aa
NH3
CHO
O2
O2
CH
aa
CO2
aa
NH3
CO2
NH3
CH
CO2
CO2
NH3
NH3
CO2
AP Biology
NH3
NH3
CO2
CO2
aa
O2
NH3
NH3
CO2
O2
intracellular
waste
CO2
CHO
CO2
aa
Diffusion too slow!
extracellular
waste
Overcoming limitations of diffusion
 Evolution of exchange systems for
distributing nutrients
 circulatory system
 removing wastes
 excretory system

CO2
CO2
aa
CO2
CO2
O2
NH3
CO2
systems to support
multicellular organisms
AP Biology
NH3
CO2
CO2
NH3
NH3
CO2
CH
NH3
NH3
CO2
aa
O2
NH3
NH3
CHO
CO2
aa
Osmoregulation
hypotonic
 Water balance

freshwater
 hypotonic
 water flow into cells & salt loss

saltwater
 hypertonic
 water loss from cells

hypertonic
land
 dry environment
 need to conserve water
 may also need to conserve salt
Why do all land animals have to conserve water?
 always lose water (breathing & waste)
AP
may
lose life while searching for water
Biology
Intracellular Waste
 What waste products?

Animals
poison themselves
from the inside
by digesting
proteins!
what do we digest our food into…
 carbohydrates = CHO  CO2 + H2O
 lipids = CHO  CO2 + H2O
lots!
 proteins = CHON  CO2 + H2O + N
very
little
 nucleic acids = CHOPN  CO2 + H2O + P + N
cellular digestion…
cellular waste
NH2 =
AP Biology
ammonia
H| O
||
H
N –C– C–OH
|
H
R
CO2 + H2O
Nitrogenous waste disposal
 Ammonia (NH3)

very toxic
 carcinogenic

very soluble
 easily crosses membranes

must dilute it & get rid of it… fast!
 How you get rid of nitrogenous wastes depends on

who you are (evolutionary relationship)

where you live (habitat)
aquatic
AP Biology
terrestrial
terrestrial egg layer
Nitrogen waste
 Aquatic organisms


can afford to lose water
ammonia
 most toxic
 Terrestrial


need to conserve
water
urea
 less toxic
 Terrestrial egg
layers



need to conserve water
need to protect
embryo in egg
uric acid
AP Biology
 least toxic
Freshwater animals
 Water removal & nitrogen waste disposal

remove surplus water
 use surplus water to dilute ammonia & excrete it
 need to excrete a lot of water so dilute ammonia &
excrete it as very dilute urine
 also diffuse ammonia continuously through gills or
through any moist membrane

overcome loss of salts
 reabsorb in kidneys or active transport across gills
AP Biology
H
Land animals
 Nitrogen waste disposal on land
H
H
H
need to conserve water
 must process ammonia so less toxic

N
C
O
N
 urea = larger molecule = less soluble = less toxic
 2NH2 + CO2 = urea
Urea
 produced in liver
costs energy

kidney
to synthesize,
but it’s worth it!
 filter solutes out of blood
 reabsorb H2O (+ any useful solutes)
 excrete waste
 urine = urea, salts, excess sugar & H2O

AP Biology

urine is very concentrated
concentrated NH3 would be too toxic
mammals
Egg-laying land animals
 Nitrogen waste disposal in egg
no place to get rid of waste in egg
 need even less soluble molecule

 uric acid = BIGGER = less soluble = less toxic

birds, reptiles, insects
itty bitty
living space!
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
AP Biology
Uric acid
 Polymerized urea
And that folks,
is why most
male birds don’t
have a penis!
large molecule
 precipitates out of solution

 doesn’t harm embryo in egg
 white dust in egg
 adults still excrete N waste as white paste
 no liquid waste
 uric acid = white bird “poop”!
O
H
H
N
N
O
O
N
N
AP Biology
H
H
Mammalian System
 Filter solutes out of blood &
blood
filtrate
reabsorb H2O + desirable solutes
 Key functions

filtration
 fluids (water & solutes) filtered out
of blood

reabsorption
 selectively reabsorb (diffusion)
needed water + solutes back to blood

secretion
 pump out any other unwanted
solutes to urine

excretion
 expel concentrated urine (N waste +
AP Biology
solutes + toxins) from body
concentrated
urine
Mammalian Kidney
inferior
vena cava
aorta
adrenal gland
kidney
ureter
bladder
urethra
AP Biology
nephro
n
renal vein
& artery
epithelial
cells
Nephron
 Functional units of kidney

1 million nephrons
per kidney
 Function


filter out urea & other
solutes (salt, sugar…)
blood plasma filtered
into nephron
 high pressure flow

AP Biology
selective reabsorption of
valuable solutes & H2O
back into bloodstream
 greater flexibility & control
why
selective reabsorption
& not selective
filtration?
“counter current
exchange system”
How can
different sections
allow the diffusion
of different
molecules?
Mammalian kidney
 Interaction of circulatory
& excretory systems
 Circulatory system

glomerulus =
ball of capillaries
Bowman’s
capsule
Proximal
tubule
Distal
tubule
Glomerulus
 Excretory system



nephron
Bowman’s capsule
loop of Henle





AP Biology
proximal tubule
descending limb
ascending limb
distal tubule
collecting duct
Glucose
Amino
acids
H2O
Mg++ Ca++
H2O
Na+ ClH2O
H2O
Na+ Cl-
H2O
H2O
Loop of Henle
Collecting
duct
Nephron: Filtration
 At glomerulus

filtered out of blood
 H2O
 glucose
 salts / ions
 urea

not filtered out
 cells
 proteins
AP Biology
high blood pressure in kidneys
force to push (filter) H2O & solutes
out of blood vessel
BIG problems when you start out
with high blood pressure in system
hypertension = kidney damage
Nephron: Re-absorption
 Proximal tubule

reabsorbed back into blood
 NaCl
 active transport
of Na+
 Cl– follows
by diffusion
 H2O
 glucose
 HCO3 bicarbonate
 buffer for
AP Biology
blood pH
Descending
limb
Ascending
limb
Nephron: Re-absorption
structure fits
 Loop of Henle
function!

descending limb
 high permeability to
H2O
 many aquaporins in
cell membranes
 low permeability to
salt
 few Na+ or Cl–
channels

reabsorbed
 H2O
AP Biology
Descending
limb
Ascending
limb
Nephron: Re-absorption
structure fits
 Loop of Henle
function!

ascending limb
 low permeability
to H2O
 Cl- pump
 Na+ follows by
diffusion
 different membrane
proteins

reabsorbed
 salts
 maintains osmotic
AP Biology
gradient
Descending
limb
Ascending
limb
Nephron: Re-absorption
 Distal tubule

reabsorbed
 salts
 H2O
 HCO3 bicarbonate
AP Biology
Nephron: Reabsorption & Excretion
 Collecting duct

reabsorbed
 H2O

excretion
 concentrated
urine passed
to bladder
 impermeable
lining
AP Biology
Descending
limb
Ascending
limb
Osmotic control in nephron
 How is all this re-absorption achieved?
tight osmotic
control to reduce
the energy cost
of excretion
 use diffusion
instead of
active transport
wherever possible

the value of a
counter current
exchange system
AP Biology
why
selective reabsorption
& not selective
filtration?
Summary
 Not filtered out


cells
 proteins
remain in blood (too big)
 Reabsorbed: active transport


Na+
Cl–
amino acids
 glucose

 Reabsorbed: diffusion


Na+
H2O

Cl–
 Excreted


AP Biology
urea
excess H2O
 excess solutes (glucose, salts)
toxins, drugs, “unknowns”
Any Questions?
AP Biology
2006-2007
Don’t get batty…
Ask Questions!!
AP Biology
2006-2007