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7/30/2019 Ch 06 Cell Membrane Diffusion
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AP Biology
The Cell Membrane
Presented by:
Dr Haseeb1st year MDS student
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AP Biology
contents
Overview
Structure
functions
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AP Biology
Overview
Cell membrane separates living cell fromnonliving surroundings
thin barrier = 8nm thick
Controls traffic in & out of the cell
selectively permeable
allows some substances to cross more easilythan others
hydrophobic vs hydrophilic
Made of phospholipids, proteins & othermacromolecules
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AP Biology
Phospholipids
Fatty acid
Phosphate
Fatty acid tails hydrophobic
Phosphate group head
hydrophilic
Arranged as a bilayer
Aaaah,one of those
structurefunctionexamples
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AP Biology
Phospholipid bilayer
polarhydrophilic
heads
nonpolarhydrophobic
tails
polarhydrophilic
heads
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AP Biology
More than lipids
In 1972, S.J. Singer & G. Nicolsonproposed that membrane proteins areinserted into the phospholipid bilayer
Its like a fluidIts like a mosaic
Its the
Fluid Mosaic Model!
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AP Biology
Membrane is a collage of proteins & other moleculesembedded in the fluid matrix of the lipid bilayer
Extracellular fluid
Cholesterol
Cytoplasm
Glycolipid
Transmembraneproteins
Filaments of
cytoskeleton
Peripheralprotein
Glycoprotein
Phospholipids
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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
cholesterol in membrane
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AP Biology
Membrane Proteins
Proteins determine membranes 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)
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AP Biology 2007-2008
Why areproteins the perfectmolecule to build structures
in the cell membrane?
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AP Biology
Classes of amino acids
What do these amino acids have in common?
nonpolar & hydrophobic
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AP Biology
Classes of amino acids
What do these amino acids have in common?
polar & hydrophilic
I like thepolar onesthe best!
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AP Biology
Proteins domains anchor molecule
Within membrane nonpolar amino acids
hydrophobic
anchors protein
into membrane
On outer surfaces ofmembrane
polar amino acids hydrophilic
extend intoextracellular fluid &
into cytosol
Polar areasof protein
Nonpolar areas of protein
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AP Biology
NH2
H+
COOH
Cytoplasm
Retinalchromophore
Nonpolar(hydrophobic)
-helices in thecell membrane H+
Porin monomer
-pleated sheets
Bacterialoutermembrane
proton pump channelin photosynthetic bacteria
water channelin bacteria
function throughconformational change =
shape change
Examples
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AP Biology
Many Functions of Membrane Proteins
Outside
Plasmamembrane
InsideTransporter Cell surface
receptorEnzymeactivity
Cell surfaceidentity marker Attachment to thecytoskeletonCell adhesion
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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 byimmune system
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AP Biology
Thank you
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AP Biology
Any Questions??
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AP Biology
Thank you
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AP Biology
Movement across the
Cell Membrane
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AP Biology
Diffusion
2nd Law of Thermodynamicsgoverns biological systems
universe tends towards disorder (entropy)
Diffusion
movement from high low concentration
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AP Biology
Diffusion
Move from HIGH to LOW concentration passive transport
no energy needed
diffusion osmosis
movement of water
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AP Biology
Diffusion across cell membrane
Cell membrane is the boundary betweeninside & outside
separates cell from its environment
INfood
carbohydratessugars, proteinsamino acidslipidssalts, O2,H2O
OUTwaste
ammoniasaltsCO2H2Oproducts
cell needs materials in & products or waste out
IN
OUT
Can it be an impenetrable boundary? NO!
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AP Biology
Diffusion through phospholipid bilayer
What molecules can get through directly? fats & other lipids
inside cell
outside cell
lipid
salt
aa H2Osugar
NH3
What molecules can
NOT get throughdirectly?
polar molecules
H2O
ions salts, ammonia
large molecules
starches, proteins
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AP Biology
Channels through cell membrane
Membrane becomes semi-permeablewith protein channels
specific channels allow specific material
across cell membrane
inside cell
outside cell
sugaraaH2O
saltNH3
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AP Biology
Facilitated Diffusion
Diffusion through protein channels channels move specific molecules across
cell membrane
no energy needed
The Bouncer
open channel = fast transport
facilitated = with help
high
low
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AP Biology
Active Transport
The Doorman
conformationalchange
Cells may need to move molecules againstconcentration gradient
shape change transports solute fromone side of membrane to other
protein pump costs energy = ATP
ATP
low
high
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AP Biology symportantiport
Active transport
Many models & mechanisms
ATP ATP
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AP Biology
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 againstconcentration gradient
low high
uses a protein pump
requires ATPATP
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AP Biology
Transport summary
simplediffusion
facilitateddiffusion
activetransport
ATP
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AP Biology
How about large molecules?
Moving large molecules into & out of cell through vesicles & vacuoles
endocytosis
phagocytosis= cellular eating pinocytosis= cellular drinking
exocytosis
exocytosis
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AP Biology
Endocytosis
phagocytosis
pinocytosis
receptor-mediatedendocytosis
fuse withlysosome fordigestion
non-specificprocess
triggered bymolecularsignal
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AP Biology 2007-2008
The Special Case of Water
Movement of water acrossthe cell membrane
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AP Biology
Osmosis is diffusion of water
Water is very important to life,so we talk about water separately
Diffusion of water from
high concentrationof water tolow concentrationof water
across asemi-permeable
membrane
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AP Biology
Concentration of water
Direction of osmosis is determined bycomparing total solute concentrations
Hypertonic - more solute, less water
Hypotonic - less solute, more water
Isotonic - equal solute, equal water
hypotonic hypertonic
water
net movement of water
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AP Biology freshwater balanced saltwater
Managing water balance
Cell survival depends on balancingwater uptake & loss
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AP Biology
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
balanced
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AP Biology
Managing water balance
Hypotonic a cell in fresh water
example: Paramecium
problem: gains water,
swells & can burst
water continually enters
Parameciumcell
solution: contractile vacuole
pumps water out of cell
ATP
plant cells
turgidfreshwater
ATP
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AP Biology
Water regulation
Contractile vacuole in Paramecium
ATP
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Managing water balance
Hypertonic a cell in salt water
example: shellfish
problem: lose water & die
solution: take up water orpump out salt
plant cells plasmolysis= wilt
saltwater
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AP Biology
Aquaporins
Water moves rapidly into & out of cells evidence that there were water channels
1991 | 2003
Peter Agre
John Hopkins
Roderick MacKinnon
Rockefeller
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AP Biology
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
.05 M .03 M
Osmosis
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AP Biology
Any Questions??
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Thank you