Cell Membranes and
Movement Across
Membranes (Transport)
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
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Phospholipid bilayer
 What molecules can get through directly?
inside cell
NH3
outside cell
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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, only active under



specific conditions
Aquaporins – move water only, found
primarily in animal kidneys and plant roots
Carrier proteins – conformational change
(change in shape occurs)
Transport proteins – require ATP
AP Biology
Membrane Protein Types
 Recognition proteins – glycoproteins,
carbohydrates are attached (too
hydrophilic to be directly on membrane)
 Adhesion proteins – anchors to
surrounding cells/tissues
 Receptor proteins – hormones; receptors
that start signal transduction pathways
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

high  low
 Active transport

Movement against concentration gradient
 low  high

requires ATP
Think: does it take more energy to move water
down a waterfall (from high to low) or up a
waterfall (from low to high)?
AP Biology
Diffusion
 Diffusion

movement from high  low concentration
 2nd Law of Thermodynamics
- Universe tends towards disorder
AP Biology
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
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Facilitated diffusion
 Movement 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 the
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 (taking molecules in)

 phagocytosis = “cellular eating”
 pinocytosis = “cellular drinking”
 receptor-mediated
endocytosis

Exocytosis
 Molecules exiting
AP Biology
Exocytosis 
2005-2006
Endocytosis
phagocytosis
pinocytosis
receptor-mediated
endocytosis
AP Biology
fuse with
lysosome for
digestion
non-specific
process
triggered by
ligand signal
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 a hypotonic
solution will gain water,
swell & possibly burst (cytolysis)
 Paramecium vs. pond water
 Paramecium is hypertonic
 H2O continually enters cell
 specialized organelle, contractile
vacuole, pumps excess H2O out
of cell; requires ATP
 plant cell in a hypotonic
solution is turgid
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 Turgor pressure
 Cell wall exerts force
Hypertonicity
 animal cell in hypertonic
solution will lose water,
shrivel & probably die
 salt water organisms are
hypotonic compared to their
environment
 they have to take up water &
pump out salt
 plant cell in a hypertonic
solution experiences plasmolysis
 Vacuole shrinks; cell membrane pulls
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away from cell wall
 Wilting
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?