ap biology 2005-2006 cell membranes and movement across membranes (transport)
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AP Biology2005-2006
Cell Membranes and Movement Across Membranes (Transport)
AP Biology2005-2006
Cell Membrane…more than just a barrier!
AP Biology2005-2006
Cell (plasma) membrane Cells need an inside & an outside…
separate cell from its environment cell membrane is the boundary
INfoodcarbohydratessugars, proteinsamino acidslipidssalts, O2, H2O
OUTwasteammoniasaltsCO2
H2O products
cell needs materials in & products or waste out
IN
OUT
Can it be an impenetrable boundary? NO!
AP Biology
Lipids of cell membrane Membrane is made of phospholipids
phospholipid bilayer
phosphate
lipid
hydrophilic
hydrophobic
inside cell
outside cell
AP Biology
Phospholipid bilayer What molecules can get through directly?
inside cell
outside cell
fats & other lipidscan slip directly through the phospholipid cell membrane, but…
what about other stuff?
lipid
salt
aa H2Osugar
NH3
AP Biology2005-2006
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 proteins
AP Biology
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
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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 Biology2005-2006
A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer
AP Biology2005-2006
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
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
2nd Law of Thermodynamics - Universe tends towards disorder
Diffusion movement from high low concentration
AP Biology2005-2006
Simple diffusion across membrane
inside cell
outside cell
Which way will lipid move?
low
high
lipid
lipid
lipid
lipid
lipid
lipid lipid
lipidlipid
lipid
lipid
lipid
lipid
lipid
AP Biology
Diffusion of 2 solutes Each substance diffuses down its own
concentration gradient, independent of concentration gradients of other substances
AP Biology2005-2006
Facilitated diffusion Movement from HIGH to LOW
concentration through a protein channel passive transport; no energy needed facilitated = with help
AP Biology2005-2006
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 channelschange in electrical charge across nerve cell membrane opens Na+ & K+ channels
AP Biology
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 Biology2005-2006
Transport summary
AP Biology2005-2006
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
Exocytosis
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Endocytosis
phagocytosis
pinocytosis
receptor-mediated endocytosis
fuse with lysosome for digestion
non-specificprocess
triggered byligand signal
AP Biology2005-2006
The Special Case of Water
Movement of water across the cell membrane
AP Biology2005-2006
Osmosis is diffusion of water Diffusion of water from
high concentration of water to low concentration of water across a
semi-permeable membrane
AP Biology2005-2006
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
hypotonic hypertonic
water
net movement of water
AP Biology2005-2006
Managing water balance Cell survival depends on balancing
water uptake & loss
freshwater balanced saltwater
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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
Turgor pressure Cell wall exerts force
AP Biology2005-2006
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 away from cell wall
Wilting
AP Biology2005-2006
Cell (compared to beaker) hypertonic or hypotonic
Beaker (compared to cell) hypertonic or hypotonic
Which way does the water flow? in or out of cell
.05 M .03 M
Osmosis…
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:
.2M or .8M? Which has the greater water potential:
20% or 80% water?