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Transport Movement across
the Cell Membrane
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Lipids of cell membrane
Membrane consists primarily of phospholipids
phospholipid bilayer
phosphate
lipid
hydrophilic
hydrophobic
inside cell
outside cell
S1
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The Fluidity of Membranes
Phospholipids in the membrane are
fluid – like (can move easily)
Figure 7.5 A
Lateral movement
(~107 times per second) Flip-flop
(~ once per month)
(a) Movement of phospholipids
S2
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Semi-permeable membrane
Will allow passage through the membrane
But need to control what gets in or out
membrane is semi-permeable
So how does a semi-
permeable membrane work?
aa H2O sugar lipid salt NH3
S3
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Phospholipid bilayer
What molecules can get through directly?
inside cell
outside cell
Small lipids can slip directly through the phospholipid cell membrane.
lipid salt
aa H2O sugar
NH3
S4
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Getting through cell membrane Diffusion
Movement of molecules from a high concentration to a low concentration until it’s equal
Facilitated Diffusion
transport of larger molecules
through a protein channel Movement from high low
Active transport Movement from low high
uses a protein channel (pump)
requires ATP energy
S5
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Diffusion (passive transport)
movement from high low concentration
S6
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Diffusion of two solutes (passive)
Each substance diffuses down its own
concentration gradient, independent of
concentration gradients of other
substances
S7
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Osmosis, the diffusion of water
Water goes from HIGH to LOW concentration
“passive transport”
no energy needed (does not require ATP)
diffusion osmosis S8
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Simple diffusion across membrane
inside cell
outside cell
Which way will
lipid move?
low
high
lipid
lipid
lipid
lipid
lipid
lipid lipid
lipid lipid
lipid
lipid
lipid
lipid
lipid
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Facilitated Diffusion through a Channel
Movement from high to low
inside cell
outside cell
sugar
sugar
sugar
sugar
sugar sugar
sugar sugar
sugar sugar
sugar
Which way
will sugar
move?
low
high
sugar
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Semi-permeable cell membrane
But the cell still needs control
membrane needs to be semi-permeable
specific channels allow
specific material in & out
inside cell
outside cell
sugar aa H2O
salt NH3
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Active Transport (needs ATP energy)
“The Doorman”
shape change transports high
low
Membrane proteins act as a PUMP for specific molecules
Uses ATP –energy
Proteins act as pumps and channels
S9
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Active transport can move materials from
low to high concentration
Cells may need molecules to move
against concentration situation
protein pump
requires energy
ATP for NRG
Na+/K+ pump in nerve cell
membranes
S10
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Transport summary
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How about large molecules?
Moving large molecules into & out of cell
through vesicles & vacuoles
Endocytosis (moving into cell)
phagocytosis = “cellular eating”
pinocytosis = “cellular drinking”
receptor-mediated
endocytosis
Exocytosis (moving out)
exocytosis S11
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Endocytosis
phagocytosis
pinocytosis
receptor-mediated
endocytosis
fuse with
lysosome for
digestion
non-specific
process
triggered by
chemical signal
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More than just a barrier…
Expanding our view of cell membrane
beyond just a phospholipid bilayer barrier
phospholipids plus…
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A membrane is a collage of different proteins embedded
in the fluid matrix of the lipid bilayer
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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 =
transmembrane protein
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Membrane Carbohydrates
Play a key role in cell-cell recognition
ability of a cell to distinguish neighboring
cells from another
important in organ &
tissue development
basis for rejection of
foreign cells by
immune system
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Any Questions?
Fluid Mosaic Model