Transcript Cell membrane and transport notes

Cell Membrane and the
Movement across it!
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
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
polar
hydrophilic
heads
nonpolar
hydrophobic
tails
polar
hydrophilic
heads
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?
A membrane is a collage of different proteins
embedded in the fluid matrix of the lipid bilayer
AP Biology
2005-2006
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
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
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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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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
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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
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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
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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
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2005-2006
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
 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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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
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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?
Animal systems evolved to
support multicellular life
single cell
aa
O2
CH
CHO
CO2
aa
NH3
CHO
O2
O2
CH
aa
CO2
CO2
aa
NH3
CO2
NH3
CO2
O2
NH3
CO2
CO2
aa
NH3
CH
NH3
NH3
CO2
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CO2
NH3
CO2
intracellular
waste
O2
NH3
but what
if the
cells are
clustered?
CHO
CO2
aa
Diffusion too slow!
extracellular
waste
for nutrients in & waste out
Overcoming limitations of diffusion
 Evolution of exchange systems for
distributing nutrients
 circulatory system
 removing wastes
aa
 excretory system
 gas exchange
O2
 respiratory system

systems to support
multicellular organisms
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aa
CO2
CO2
NH3
NH3
CO2
NH3
CO2
CO2
NH3
O2
NH3
NH3
CO2
NH3
NH3
CO2
CH
CO2
CO2
CHO
CO2
aa
When is Diffusion Needed?
 Respiratory and Circulatory systems

Oxygen/CO2 transport into and out of
bloodstream
 Skins, gills, alveoli, capillaries
 Excretory systems

Movement of wastes into or out of blood
 Skin, nephridia, nephrons, gills
Q: What do these systems have in common
for diffusion to occur? Large surface area
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Gas exchange in many forms…
one-celled
amphibians
echinoderms
insects
fish
mammals
cilia
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•
size
water vs. land
•
endotherm vs. ectotherm
Counter current exchange system
 Water carrying gas flows in one direction,
blood flows in opposite direction
AP Biology
just keep
swimming….
How counter current exchange works
70%
front
40%
100%
15%
water
60%
30%
counter90%
5%
current
blood
50% 70%
100%
50% 30%
concurrent
water
5%
blood
 Blood & water flow in opposite directions

AP Biology
maintains diffusion gradient over whole length
of gill capillary
maximizing O2 transfer from water to blood
back
Gas Exchange on Land
 Advantages of terrestrial life

air has many advantages over water
 higher concentration of O2
 O2 & CO2 diffuse much faster through air
 respiratory surfaces exposed to air do not have to
be ventilated as thoroughly as gills
 air is much lighter than water & therefore
much easier to pump
 expend less energy moving air in & out
 Disadvantages

keeping large respiratory surface moist
causes high water loss
 reduce water loss by keeping lungs internal
AP Biology
Intracellular Waste
 What waste products
Animals
poison themselves
from the inside
by digesting
proteins!
are made inside of cells?

what do we digest our food into…
 carbohydrates = CHO  CO2 + H2O
 lipids = CHO  CO2 + H2O
 proteins = CHON  CO2 + H2O + N
 nucleic acids = CHOPN  CO2 + H2O + P + N
cellular digestion…
cellular waste
NH2 =
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ammonia
H| O
||
H
N –C– C–OH
|
H
R
CO2 + H2O
Nitrogen waste
 Aquatic organisms


can afford to lose water
ammonia
 most toxic
 Terrestrial


need to conserve
water
urea
 less toxic
 Terrestrial egg
layers



need to conserve water
need to protect
embryo in egg
uric acid
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 least toxic
H
Land animals
 Nitrogen waste disposal on land
H
H
H
need to conserve water
 must process ammonia so less toxic

N
C
O
N
 urea = larger molecule = less soluble = less toxic
 2NH2 + CO2 = urea
Urea
 produced in liver
costs energy

kidney
to synthesize,
but it’s worth it!
 filter solutes out of blood
 reabsorb H2O (+ any useful solutes)
 excrete waste
 urine = urea, salts, excess sugar & H2O

AP Biology

urine is very concentrated
concentrated NH3 would be too toxic
mammals
Animal Osmoregulation (blood solute levels)
 Kidneys – generate urine by filtering
wastes from blood

Nephrons – basic unit of kidney
 Water will move towards urine or blood as it
becomes hypertonic via AQUAPORINS
 Anti-Diuretic Hormone increases water
movement back to blood….why?
 Blood pressure is highly regulated by the
kidney because without pressure, there is
no filtration = toxic blood = death!!!
AP Biology
Osmotic control in nephron
 How is all this re-absorption achieved?
tight osmotic
control to reduce
the energy cost
of excretion
 use diffusion
instead of
active transport
wherever possible

the value of a
counter current
exchange system
AP Biology
Osmoregulation
hypotonic
 Water balance vs. Habitat

freshwater
 hypotonic to body fluids
 water flow into cells & salt loss

saltwater
 hypertonic to body fluids
 water loss from cells

hypertonic
land
 dry environment
 need to conserve water
 may also need to conserve salt
Why do all land animals have to conserve water?
 always lose water (breathing & waste)
AP
may
lose life while searching for water
Biology