Cell Membrane and the
Movement across it!
2005-2006
Cell (plasma) membrane
 Cells need an inside & an outside…

separate cell from its environment

cell membrane is the boundary
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
Lipids of cell membrane
 Membrane is made of phospholipids

phospholipid bilayer
inside cell
phosphate
hydrophilic
lipid
hydrophobic
outside cell
AP Biology
Phospholipids
 Phospholipids are amphipathic
molecules, containing
hydrophobic and hydrophilic regions
AP Biology
Think about this……
 What does it mean when we say the cell
membrane is selectively permeable?
 What type(s) of molecules can pass
through the membrane most rapidly?
 Why is it difficult for water to cross the
cell membrane unassisted?
 How does facilitated diffusion differ from
diffusion?
Membrane Proteins
 Proteins determine most of membrane’s
specific functions

cell membrane & organelle membranes each
have unique collections of proteins
 Membrane proteins:


AP Biology
peripheral proteins = loosely
bound to surface of membrane
integral proteins = penetrate into
lipid bilayer, often completely
spanning the membrane =
transmembrane proteins
2005-2006
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
The many functions of proteins….
 Channel proteins – wide open passage
 Ion channels – gated
 Aquaporins – water only, kidney and





plant root only
Carrier proteins – change shape
Transport proteins – require ATP
Recognition proteins - glycoproteins
Adhesion proteins – anchors
Receptor proteins - hormones
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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

Cholesterol
 Provides stability in animal cells
 “temperature buffer” quality for membrane
 Replaced with sterols in plant cells
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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
Getting through cell membrane
 Passive transport


No energy needed
Movement down concentration gradient
 Active transport

Movement against concentration gradient
 low  high

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requires ATP
Diffusion
 2nd Law of Thermodynamics
- Universe tends towards disorder
 Diffusion

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movement from high  low concentration
Simple diffusion across membrane
Which way will
lipid move?
lipid
inside cell
low
lipid
lipid
lipid
lipid
lipid

high
outside cell
lipid
lipid
lipid
lipid
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lipid
lipid
lipid
lipid
Diffusion of 2 solutes
 Each substance diffuses down its own
concentration gradient, independent of
concentration gradients of other
substances
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Facilitated diffusion
 Move from HIGH to LOW concentration
through a protein channel
passive transport
 no energy needed
 facilitated = with help

AP Biology
2005-2006
Gated channels
 Proteins that open only in presence of stimulus
(signal)
 stimulus usually different from transported
molecule
 ex: ion-gated channels
 ex: voltage-gated channels
AP Biology
2005-2006
Active transport
 Cells may need molecules to move
against concentration gradient
need to pump against gradient
 protein pump
 requires energy
 ATP

Na+/K+ pump
in nerve cell
membranes
AP Biology
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Transport summary
AP Biology
2005-2006
How about large molecules?
 Moving large molecules into & out of cell
requires ATP(energy)!
through vesicles & vacuoles
 endocytosis

 phagocytosis = “cellular eating”
 pinocytosis = “cellular drinking”
 receptor-mediated
endocytosis

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exocytosis
exocytosis
2005-2006
Endocytosis
phagocytosis
pinocytosis
receptor-mediated
endocytosis
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fuse with
lysosome for
digestion
non-specific
process
triggered by
ligand signal
2005-2006
The Special Case of Water
Movement of water across
the cell membrane
2005-2006
Osmosis is diffusion of water
 Diffusion of water from
high concentration of water to
low concentration of water

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across a
semi-permeable
membrane
2005-2006
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
hypotonic
hypertonic
net movement of water
AP Biology
Managing water balance
 Cell survival depends on balancing
water uptake & loss
AP Biology
freshwater
balanced
saltwater
Hypotonicity
 animal cell in hypotonic
solution will gain water, swell
& possibly burst (cytolysis)
 Paramecium vs. pond water
 Paramecium is hypertonic
 H2O continually enters cell
 contractile vacuole - pumps H2O
out of cell = ATP

plant cell
 Turgid (turgor pressure)
 Cell wall
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Hypertonicity
 animal cell in hypertonic
solution will loose water, shrivel
& probably die
 salt water organisms are
hypotonic compared to their
environment

plant cells
 plasmolysis = wilt
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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 out2005-2006
of cell
AP Biology
Water Potential
 Water moves from a place of greater
water potential to a place of lesser
water potential (net).
 As the concentration of a solute
increases in a solution, the water
potential will decrease accordingly.

Which has the greater water potential:


Which has the greater water potential:

AP Biology
.2M or .8M?
20% or 80% water?