Transcript The Cell Membrane

CHAPTER 7
MEMBRANE STUCTURE
AND FUNCTION
AP Biology
Study guide #20-27
AP Biology
Diffusion
 2nd Law of Thermodynamics
Governs biological systems!
Universe tends to disorder!
Diffusion –
diffusion
movement from high>low concentration of THAT SUBSTANCE!
AP Biology
Diffusion of 2 Solutes
Each substance diffuses down its own
concentration gradient, independent of
concentration gradient of any other
substance.
AP Biology
Diffusion
•PASSIVE TRANSPORT
•No energy required
•So – How does that happen?
Brownian
Motion
AP Biology
Osmosis is the diffusion of water
across a membrane
 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
Osmosis
 250 times the volume of cell/second!
 Osmosis = NET movement of water across a




selectively permeable membrane driven by a
difference in solute concentration on either side
of the memrane.
“Free” water moves
Slide 2
Less solute = more free water
Water flows from low solute [ ] to high solute [ ]
Until equilibrium
AP Biology
osmosis
Osmosis
AP Biology
What determines when and what direction
water will move?
3 molecules of albumin
15 molecules
of
glucose
66,000 mw
180 mw
AP Biology
Isotonic? Hypertonic? Hypotonic?
A & B are isotonic
A & B are hypertonic to C
CAPisBiology
hypotonic to A and B
Osmotic Pressure
 Pressure generated by diffusion of water across




a membrane
When pressure is equal water flow will stop
Called “hydrostatic pressure” – water-stopping
pressure
Osmolarity- [ ] in terms of # of particles in a
volume of liquid
1 osmolar soln = 1 M of osmoltically active
particles per liter.
AP Biology
Red Blood Cells in NaCl solutions
100mOs
hypotonic
AP Biology
500 mOs
isotonic
hypertonic
Osmosis problems
Hydrostatic generator
Osmosis problems
AP Biology
End
Diffusion
Osmosis
AP Biology
Cell membrane
The Cell Membrane
AP Biology
2007-2008
Overview
 Cell membrane separates living cell from
nonliving surroundings

thin barrier = 8nm thick
 Controls traffic in & out of the cell


selectively permeable
allows some substances to cross more easily
than others
 hydrophobic vs hydrophilic
 Made of phospholipids, proteins & other
macromolecules
AP Biology
Phosphate
Phospholipids
 Fatty acid tails

hydrophobic
 Phosphate group head

hydrophilic
Fatty acid
 Arranged as a bilayer
Aaaah,
one of those
structure–function
examples
AP Biology
Phospholipid bilayer
polar
hydrophilic
heads
nonpolar
hydrophobic
tails
polar
hydrophilic
heads
AP Biology
AP Biology
Two generations of membrane models
How do we know?
phospholipid bilayer -1920’s;
1972; Singer, Nicholson
1930-60’s; models from EM views;
Dispersion model
??-Not as hydrophyllic as pure PL’s?
Hydrophyllic regions in aqueous
D & D - Hydrophyllic Protein sandwich
Hydrophobic in hydrophobic PL
??- all membranes identical???
FLUID MOSAIC MODEL!
??-amphipathic proteins? Solubile in H2O
Freeze Fracture evidence
AP Biology
Permanent model????
hydrophobic region in aqueous?
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
Evidence for the drifting of membrane proteins
Other Evidence:
microsurgery on cells
AP Biology
Membranes are fluid- like salad oil!
•held in place weak hydrophobic interactions
•PL’S drift laterally
•rarely flip-flop between layers
•larger proteins move slower
•some proteins guided by cytoskeleton
Fluid Membrane
“motors”
•some proteins anchored by cytoskeleton
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
Transmembrane
proteins
Peripheral
protein
Cytoplasm
AP Biology
Filaments of
cytoskeleton
The fluidity of membranes
Maintains and
increases/decreases fluidity
AP Biology
Fluidity influenced by temperature
cool - more solid- PL’s closely packed
if rich in unsaturated fatty acids - more fluid
than those rich in saturated fatty acids kinks prevent tight packing
cholesterol steroid- wedged between PL’s of
animal cells
•warm -limits mvmnt. of PL’s, reduces
fluidity
•cool - maintains fluidity, prevents tight
packing-EX: salmon
AP Biology
•Must be fluid to work w/ enzymes & be permeable
•Cells alter lipid makeup to adjust for temp. changes
EX: cold organisms ( winter wheat, salmon,
bears) increase % of unsat PL’s in autumn
- prevents solidifying membranes
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
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
Membrane Proteins
 Proteins determine membrane’s specific functions

cell membrane & organelle membranes each have
unique collections of proteins
 Membrane proteins:

peripheral proteins
 loosely bound to surface of membrane
 cell surface identity marker (antigens)

integral proteins
 penetrate lipid bilayer, usually across whole membrane
 transmembrane protein
 transport proteins
 channels, permeases (pumps)
AP Biology
2 major types of membrane proteins.
Peripheral proteins - not embedded; bound
to inner/outer surface; may be
connected to integral proteins
Integral proteins - penetrate bilayer;
- Transmembrane protein
•hydrophobic regions of nonpolar aa’s
in contact with bilayer core;
often alpha helices
•hydrophilic regions of aa’s in contact
w/ environment – beta pleated
AP Biology
- Unilateral – partially through; coupled
Protein’s domain anchor molecule
 Within membrane

Polar areas
of protein
nonpolar amino acids
 hydrophobic
 anchors protein
into membrane
 On outer surfaces of
membrane

polar amino acids
 hydrophilic
 extend into
AP Biology
extracellular fluid &
into cytosol
Nonpolar areas of protein
H+
Examples
Retinal
chromophore
NH2
water channel
in bacteria
Porin monomer
b-pleated sheets
Bacterial
outer
membrane
Nonpolar
(hydrophobic)
a-helices in the
cell membrane
COOH
H+
Cytoplasm
proton pump channel
in photosynthetic bacteria
AP Biology
function through
conformational change =
shape change
Many Functions of Membrane Proteins
Outside
Plasma
membrane
Inside
Transporter
Enzyme
activity
Cell surface
receptor
Cell surface
identity marker
Cell adhesion
Attachment to the
cytoskeleton
Uniport
Symport
Antiport
AP Biology
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
Some proteins reinforce shape of cell
•cytoplasmic side,
•some connect to cytoskeleton
• exterior side,
•some attach to fibers of ECM
•ECM = extracellular matrix
AP Biology
Sidedness of the plasma membrane
What makes
the
endomembrane
system
work?
AP Biology
Membranes are bifacial and sided
•may differ in lipid composition
•proteins have a direction orientation
•outer surface has carbo’s
•asymmetry begins w/ synthesis in ER
•proteins in plasma membrane provide a
variety of major cell functions
AP Biology
The detailed structure of an animal cell’s plasma membrane, in cross section
AP Biology
Cell-cell recognition = distinguish one type
of neighboring cell from another
important in cell sorting
•organization into tissues and organs
during development
basis for rejection of foreign cells by
immune system
•key in on surface molecules, often carbs
AP Biology
Carbo’s - usually branched
oligosaccharides = < 15 monomers
•may be covalently bonded to
•lipids= glycolipids
•proteins = glycoproteins
•external OS’s vary from species to species,
individual to individual, and even from cell type
to cell type w/in same individual - identification
marks each cell type as distinct
•blood types (A, B, AB, O)-RBCs
•Glycocalyx of animal cells – fuzzy!
AP Biology
End membranes
AP Biology
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
Diffusion
 Move from HIGH to LOW concentration
“passive transport”
 no energy needed

AP Biology
diffusion
movement of water
osmosis
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
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
 salts, ammonia

large molecules
 starches, proteins
AP Biology
Channels through cell membrane
 Membrane becomes semi-permeable
with protein channels

specific channels allow specific material
across cell membrane
inside cell
NH
AP
Biology
3
salt
H 2O
aa
sugar
outside cell
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



shape change transports solute from
one side of membrane to other
protein “pump”
conformational change
“costs” energy = ATP
low
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 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
Managing water balance
 Isotonic

animal cell immersed in
mild salt solution
 example:
blood cells in blood plasma
 problem: none
 no net movement of water

flows across membrane equally,
in both directions
 volume of cell is stable
AP Biology
balanced
Managing water balance
 Hypotonic

a cell in fresh water
 example: Paramecium
 problem: gains water,
swells & can burst
 water continually enters
Paramecium cell
 solution: contractile vacuole
 pumps water out of cell
ATP
 ATP

plant cells
 turgid
AP Biology
freshwater
Water regulation
 Contractile vacuole in Paramecium
ATP
AP Biology
Managing water balance
 Hypertonic

a cell in salt water
 example: shellfish
 problem: lose water & die
 solution: take up water or
pump out salt

plant cells
 plasmolysis = wilt
AP Biology
saltwater
1991 | 2003
Aquaporins
 Water moves rapidly into & out of cells

AP Biology
evidence that there were water channels
Peter Agre
Roderick MacKinnon
John Hopkins
Rockefeller
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