Transcript Chapter 8. Movement across the Membrane

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
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OUT
IN
waste
ammonia
salts
CO2
H2O
products
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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:


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peripheral proteins = loosely
bound to surface of membrane
integral proteins = penetrate into
lipid bilayer, often completely
spanning the membrane =
transmembrane proteins
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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
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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 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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Cholesterol
 Provides stability in animal cells
 Replaced with sterols in plant cells
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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
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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

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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
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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
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Transport summary
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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

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exocytosis
exocytosis
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Endocytosis
phagocytosis
pinocytosis
receptor-mediated
endocytosis
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fuse with
lysosome for
digestion
non-specific
process
triggered by
ligand signal
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The Special Case of Water
Movement of water across
the cell membrane
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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
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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
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Managing water balance
 Cell survival depends on balancing
water uptake & loss
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freshwater
balanced
saltwater
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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
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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
 they have to take up water &
pump out salt

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
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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:

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.2M or .8M?
20% or 80% water?