Cell Membrane and the
Movement across it!
AP Biology
2005-2006
Cell Membrane…more than just a barrier!
AP Biology
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
2005-2006
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
Phospholipid bilayer
 What molecules can get through directly?
inside cell
NH3
outside cell
AP Biology
lipid
salt
sugar aa
H 2O
fats & other lipids
can slip directly
through the
phospholipid cell
membrane, but…
what about other
stuff?
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
Membrane Protein Types
 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
AP Biology
A membrane is a collage of different proteins
embedded in the fluid matrix of the lipid bilayer
AP Biology
2005-2006
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

AP Biology
2005-2006
Cholesterol
 Provides stability in animal cells
 Replaced with sterols in plant cells
AP Biology
Getting through cell membrane
 Passive transport


No energy needed
Movement down concentration gradient
 Active transport

Movement against concentration gradient
 low  high

AP Biology
requires ATP
Diffusion
 2nd Law of Thermodynamics
- Universe tends towards disorder
 Diffusion

AP Biology
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
AP Biology
lipid
lipid
lipid
lipid
2005-2006
Diffusion of 2 solutes
 Each substance diffuses down its own
concentration gradient, independent of
concentration gradients of other
substances
AP Biology
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
 Some channel proteins open only in
presence of stimulus (signal)

stimulus usually different from
transported molecule
 ex: ion-gated channels
when neurotransmitters bind to a specific
gated channels on a neuron, these channels
open = allows Na+ ions to enter nerve cell
 ex: voltage-gated channels
change in electrical charge across nerve cell
membrane opens Na+ & K+ channels
AP Biology
2005-2006
Active transport
 Cells may need molecules to move
against concentration situation
need to pump against concentration
 protein pump
 requires energy
 ATP

Na+/K+ pump
in nerve cell
membranes
AP Biology
Transport summary
AP Biology
2005-2006
How about large molecules?
 Moving large molecules into & out of cell
requires ATP!
through vesicles & vacuoles
 endocytosis

 phagocytosis = “cellular eating”
 pinocytosis = “cellular drinking”
 receptor-mediated
endocytosis

AP Biology
exocytosis
exocytosis
2005-2006
Endocytosis
phagocytosis
pinocytosis
receptor-mediated
endocytosis
AP Biology
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
AP Biology
2005-2006
Osmosis is diffusion of water
 Diffusion of water from
high concentration of water to
low concentration of water

AP Biology
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
2005-2006
Managing water balance
 Cell survival depends on balancing
water uptake & loss
AP Biology
freshwater
balanced
saltwater
2005-2006
Hypotonicity
 animal cell in hypotonic
solution will gain water, swell
& possibly burst (cytolysis)
 Paramecium vs. pond water
 Paramecium is hypertonic
 H2O continually enters cell
 to solve problem, specialized
organelle, contractile vacuole
 pumps H2O out of cell = ATP

plant cell
 Turgid (turgor pressure)
 Cell wall
AP Biology
Hypertonicity
 animal cell in hypertonic
solution will loose water, shrivel
& probably die
 salt water organisms are
hypotonic compared to their
environment
 they have to take up water &
pump out salt

plant cells
 plasmolysis = wilt
AP Biology
2005-2006
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?