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Cell Transport Mechanisms and Homeostasis
Cell Cytoplasm is full of many different Substances.
Mostly WATER as the Solvent making movement and reactivity of the various molecules possible.
≈ 80% Water!
Lots of dissolved Ions – but different concentrations than in the Blood.
Sugars and Proteins and other molecules.
An IV represents Balance with the Cells of your Blood. Not the same Stuff, but equal amounts!
5% Sugar (Dextrose or Ringer’s Lactate) or 0.9% Salt = Cell.
If the Balance is
wrong, your blood cells will
not survive or function.
Pre-Made!
Precisely Balanced!
Cells!
5%
Cell Transport Mechanisms and Homeostasis
OUTSIDE = Extracellular Fluid.
Contains all sorts of things that cells need and that the Organism Provides. Your Digestive Tract is Choosy, but not perfectly Picky!
INSIDE = Cytoplasm.
THE RIGHT STUFF!
Contains a Precisely Balanced Solution of all of the RIGHT STUFF! This is Homeostasis! Must be able to Transport the RIGHT STUFF IN!
The Cell NEEDS Specific Things:
The right Ions, as many Amino Acids as Possible, Glucose, “Its” Proteins, etc…
Other things are Kicked OUT.
Selective Permeability allows Cells to Choose and Move the RIGHT STUFF –
CONTROLLED!!
RR
RR
r
Cell Transport Mechanisms and Homeostasis
Transport Proteins can do their job because they are specific in SHAPE! Only Proteins can
do this because they are so Variable!
? ??
But then the Chain?
But Remember!!
VARIABLE in SHAPE!!
Opening COULD BE: OR OTHERS!
Hydrophobic Core
Inside Membrane!
Hydrophilic Edges
Outside Membrane!
H20
Water is the Solvent!
Dry Solutes cannot move – they are
bound together as a Solid.
Water diffuses into the Solute (High to low conc.)
and dissolves it!
H20
H20
Water IS the SOLVENT and breaks-up the SOLUTES from each other.
Once the SOLUTES are separate, they WILL SCATTER until
completely distributed as a Solution.
If there is no Restriction to the Movement of the Solutes (Remember – Water will move by itself!),
they will form a SOLUTION!
Solutes Stick Together!!
BUT! Solvents Stick to Solutes!
DIFFUSION leads to Equal Concentrations Everywhere!
NORMAL CONDITION!
Cells will HAVE TO Choose and Separate what is in the Cytoplasm. This Membrane is NOT doing
that here, but Cell’s Membrane will be SELECTIVELY PERMEABLE!
INSIDE ≠ OUTSIDE
Cells DO NOT Really WANT This! Must have the RIGHT STUFF in the
RIGHT Place!
Cell Transport Mechanisms and Homeostasis
DIFFUSION is the most basic form of molecular movement. In Diffusion, substances scatter as much as possible. As they
collide and scatter, they will spread out.
Begin with packed Solutes = High Concentration
Once D
iffusi
on has
pro
ceed
ed, t
he so
lute
s hav
e bee
n
disru
pted a
nd sca
ttere
d as
much
as
possib
le w
ithin
thei
r
spac
e. D
iffusi
on mak
es s
ubstan
ces
spre
ad o
ut com
plete
ly
and is
GOIN
G TO H
APPEN UNLESS IT
IS S
TOPPED!!
End with randomly separated Solutes =
Lowest Concentration =
SOLUTION!!
Always Moving!
Diffusion WILL MAKE
ALL SOLUTIONS THE SAME!!
Cell Transport Mechanisms and HomeostasisBUT REMEMBER! The Cell is surrounded by a Plasma
Membrane that IS NOT just Open to Anything/Everything. The Plasma Membrane Controls Access to the Cell!
Polar “Heads” line-up with Aqueous Surface Inside and
Outside. Hydrophilic = Loves Water!
Fatty Acid “Tails” are Non-Polar and avoid
Water Inside the Membrane = Fat
Sandwich!
Hydrophobic = Fears Water.
Most Substances CANNOT pass through the membrane – would need to be Polar AND
NonPolar -so the Bilayer is a real boundary!
But they CAN Pass through “their”
Transport Protein IF IT IS OPEN!
Cell Transport Mechanisms and Homeostasis
SIMPLE DIFFUSION
The simplest process of molecular Transport.
Always High conc. To low conc. Always moves to make balance on both sides. Diffusion is trying to force everything to be a Solution. That is NOT always what the Cell wants!
High Concentration
Outside
Low Concentration
Inside.
Concentration Gradient
High to low.
IF THIS Happened ALL of the TIME:
Cytoplasm would be the SAME as the
Outside of the Cell.
It is NOT, so there have to be ways to restrict movement of various
molecules.
AND THERE ARE!!
Cell Transport Mechanisms and Homeostasis
Facilitated Diffusion: Passive Transport through a Selectively Permeable Membrane.
Only a few substances can move through a Plasma Membrane through Simple Diffusion. Everything else will require a Transport Protein that forms an openable and closeable Hole through the Lipids.
Naturally moving H L, No Energy required!
Naturally moving L H, No Energy required!
The Transport Protein must
have an opening that “suits” the
substance. Shape, Charge, Size, etc… all
must be Right!
Since each substance requires a different
Transport Protein – There are LOTS in the
membrane of every Cell!!
Can Move Anything IF IT
FITS!!
Cell Transport Mechanisms and Homeostasis
Active Transport
Cells have very Specific Needs and sometimes need to “Pack” molecules in and out of the membrane.
That would be moving AGAINST the Concentration Gradient and is impossible in Diffusion. To move things from low to High concentration requires Energy = ATP!
ATP
ADP
Wants Out!
Why?! But?!
Wants In?! Why Not?!
But, The Cell NEEDS these molecules and must “Pump” them IN! That is “Work”!
Work requires?
The substance will bind with “its” Transport
Protein, but waits for?
Once “Energized”, the substance is “pumped”
against the Concentration Gradient to make one side MORE
Concentrated! Anti-Diffusion!
This is h
ow the Cell “
Get’s” what it
NEEDS!!
Diffusio
n makes everything the sa
me, but
Active Transport can Force High
Concentrations to Happen!!
Cell Transport Mechanisms and Homeostasis
OSMOSIS: The reality of molecular Transport. Everything is Aqueous!!
Most Molecules CANNOT pass through the Cell Membrane directly. However, Water is a small molecule and can “leak” through the various Transport Proteins and CANNOT be completely Stopped! Water is Always Moving!!
This can be a Problem
because?
High Conc. of the RIGHT STUFF Inside! Wants OUT (!), But cannot exit. It was moved inside by the cell.
Low Conc. Outside, but it
cannot penetrate the Membrane.
Not Equal.
But Water is everywhere as a solvent. It is always sticking to the various Solutes of the Environment. They have to Polar
if they are Dissolved!!
In Osmosis, water moves to make both
sides Equal!
The Solutes (Hearts) cannot
move (NOW!), but Water can always
penetrate the Membrane!
Cell Transport Mechanisms and Homeostasis
Endocytosis
The previous forms of Membrane Transport moved one molecule at a time. Sometimes Cells want to engulf Large Amounts of a substance – often Food Particles.
Endo = Inside, Cyto = Cell. This is often referred to as Bulk Transport. Phopholipid Bilayers will merge and separate if the Cytoskeleton attaches and pulls on them.
In Animals, this is a captured Food
Vacuole.FOOD
Membrane and Food pinched into Cell.
Transported on to be
Digested/Used!
Cell Transport Mechanisms and Homeostasis
ExoCytosis
What goes in – Must come Out! After it has been used and Processed by the Cell! Or Made for Export!
Exo = Outside, Cyto = Cell. This form of Bulk Transport removes materials from the Cell. After the Food Vacuole has been “used”, the waste that is still in the “container” can be expelled.
Cell Transport Mechanisms and Homeostasis
Cells HAVE to be in “Balance” because of Osmosis. The Water molecules cannot be completely stopped, so the Cytoplasm WILL BE in equal concentration with the Extracellular Fluid in Animal Cells.
However, the inside and Outside will be VERY DIFFERENT because of Membrane Transport.
Extracellular FluidExtracellular Fluid
CytoplasmCytoplasm
Some substances are
Blocked = Kept Out!
Some Substances are let/carried In = Selective!
Many Substances are Kept/Trapped Inside!
Waste/Used Substances
are expelled = Exocytosis!
Selective
Permeability
Cell Transport Mechanisms and Homeostasis
Animal Cells are Delicate and require very precise Transport Processes to stay in Homeostasis. No Extra Support.
Other Cells Possess a external Cell Wall that helps allow them to survive changing external conditions.
WATER!
But this is OK because the Vacuole holds the water
and keeps it separate from the Cytoplasm!
Why is it Important to keep the Water OUT of
the Cytoplasm?!