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AP Biology
The Cell membrane
M. Saifur Rohman, dr. SpJP. PhD. FICA Department of cardiology and Vascular Medicine, Lab of Biochemistry, Faculty of Medicine, Brawijaya
University
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AP Biology 2007-2008
The Cell Membrane
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AP Biology
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
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AP Biology
Plasma Membrane Functions
Maintain a high concentration of materials in the cell.
Keep harmful materials out.
Control the movement of materials into and out of the cell.
Let the cell sense its environment.
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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
structure–functionexamples
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AP Biology
Phospholipid bilayer
polarhydrophilic
heads
nonpolarhydrophobic
tails
polarhydrophilic
heads
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AP Biology
The Plasma Membrane is Semipermeable
Small molecules and larger hydrophobic molecules move through.
Ions, hydrophilic molecules larger than water, and large molecules such as proteins do not move through the membrane on their own.
The physical properties of phospholipids account for membrane assembly and many of its properties.
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AP Biology
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!
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AP Biology
The Plasma Membrane – Gateway to the Cell
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AP Biology
Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer
Extracellular fluid
Cholesterol
Cytoplasm
Glycolipid
Transmembraneproteins
Filaments ofcytoskeleton
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 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)
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AP Biology 2007-2008
Why areproteins the perfect
molecule 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?What do these amino acids have in common?
nonpolar & hydrophobicnonpolar & hydrophobic
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AP Biology
Classes of amino acids
What do these amino acids have in common?What do these amino acids have in common?
polar & hydrophilicpolar & 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 of membrane polar amino acids
hydrophilic extend into
extracellular 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 channel in photosynthetic bacteria
water channel in bacteria
function through conformational change = shape change
function through conformational change = shape change
Examples
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AP Biology
Many Functions of Membrane Proteins
Outside
Plasmamembrane
InsideTransporter Cell surface
receptorEnzymeactivity
Cell surface identity marker
Attachment to thecytoskeleton
Cell adhesion
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AP Biology
Proteins Are Critical to Membrane Function
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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 by immune system
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AP Biology
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AP Biology
Any Questions??
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AP Biology 2007-2008
Movement across the Cell Membrane
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AP Biology
Diffusion 2nd Law of Thermodynamics
governs biological systems universe tends towards disorder (entropy)
Diffusion movement from high low concentration
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 between
inside & outside… separates cell from its environment
INfoodcarbohydratessugars, proteinsamino acidslipidssalts, O2, H2O
OUTwasteammoniasaltsCO2
H2O products
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 through directly?
polar molecules H2O
ions salts, ammonia
large molecules starches, proteins
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AP Biology
Channels through cell membrane Membrane becomes semi-permeable
with 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”“The Bouncer”
open channel = fast transport
facilitated = with help
high
low
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AP Biology
Active Transport
“The Doorman”“The Doorman”
conformational change
Cells may need to move molecules against concentration gradient shape change transports solute from
one side of membrane to other protein “pump” “costs” energy = ATP
ATP
low
high
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AP Biologysymportantiport
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 against concentration gradient
low high uses a protein pump requires ATP
ATP
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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
Three Forms of Transport Across the Membrane
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AP Biology
Endocytosis
phagocytosis
pinocytosis
receptor-mediated endocytosis
fuse with lysosome for digestion
non-specificprocess
triggered bymolecular signal
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AP Biology 2007-2008
The Special Case of Water
Movement of water across the 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 concentration of water to low concentration of water across a
semi-permeable membrane
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AP Biology
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
hypotonic hypertonic
water
net movement of water
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AP Biology freshwater balanced saltwater
Managing water balance Cell survival depends on balancing
water 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
Paramecium cell
solution: contractile vacuole pumps water out of cell ATP
plant cells turgid
freshwater
ATP
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AP Biology
Water regulation Contractile vacuole in Paramecium
ATP
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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
saltwater
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AP Biology
Aquaporins Water moves rapidly into & out of cells
evidence that there were water channels
1991 | 2003
Peter AgreJohn Hopkins
Roderick MacKinnonRockefeller
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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
Review
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AP Biology
Any Questions??
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AP Biology
Thank you