Transcript Cell Transport

Cell Transport
Overview of Passive Transport
• Cell membrane is selectively permeable
as CM can regulate what comes in and out
• Passive transport: movement of particles
across a semi-permeable membrane
without an input of energy from the cell
• 3 types:
– Simple Diffusion
– Facilitated Diffusion
– Osmosis
Simple Diffusion
• Random movement of particles from an area of
high concentration to an area of low
concentration
• Movement of particles from [high] to [low] is
called movement down a concentration
gradient
Factors that affect Rate of Diffusion
1. Size of molecule
–
Rate of diffusion ↓ with ↑ molecule size
2. Polarity of molecule
–
Some substances can diffuse easily than others (eg.
Small non-polar molecules like lipids & steroid
hormones can diffuse across CM compared to large
polar molecules like glucose which cannot get
across).
3. Charge of ions & molecules
–
Charged ions/molecules cannot get across.
Factors that affect Rate of
Diffusion
4. Temperature
– ↑ T = ↑ rate of diffusion (particles have more
energy and move faster)
5. Pressure
– ↑ P = ↑ rate of diffusion (more force on
particles, pushing them across membrane)
Diffusion through Phospholipid Bilayer
What molecules can get through directly?
– fats & other lipids
– water & small polar molecules
inside cell
lipid
H2O
salt
 What molecules can
NOT get through
directly?

Large polar molecules
 glucose, amino acids

Ions
 salts
NH3
outside cell
sugar aa

Large molecules
 starches, proteins
Channels through Cell Membrane
• Membrane becomes semi-permeable
with protein channels
– specific channels allow specific material
across cell membrane
inside cell
H+
H2O
salt
aa
sugar
outside cell
Facilitated Diffusion
• Diffusion of ions or molecules across CM
from [high] to [low] by using a membrane
protein (channel proteins & carrier
proteins).
• Proteins must be specialized to aid the
diffusion of these molecules as it has a
specific fit.
Channel Proteins
• Highly specific as structure of
channel protein determines what
molecules can move through it
• Some channel proteins are open
24/7, others have gates that
open/close in response to
signals
• Allow passage of charged ions
(eg. Na+) and polar molecules
(that cannot simply diffuse
across CM)
Carrier Proteins
• In comparison to channel proteins, can
transport larger molecules (eg. glucose,
AAs)
• Because they can only bind to a few at a
time, CPs have lower rate of diffusion
compared to channel proteins
CARRIER
PROTEINS
high concentration outside cell
lower concentration inside cell
a) molecule bounces b) carrier protein
into a specific
binds molecule
carrier protein
d) carrier protein resumes
its shape, as seen in (a)
c) carrier changes
shape & flips
over, bringing
molecule into
cell
Osmosis
• Diffusion of water molecules across a
selectively permeable membrane
• Cells are ideally bathed in isotonic
solutions (where conc. inside cell equals
conc. outside cell)
• H2O molecules move from a side with
[high] to a side with [low] across a
selectively permeable membrane to
obtain equilibrium
– SP membrane allows water to go through, but
not other molecules
Osmosis happens when
there is a difference in
solute conc.
SP membrane keeps
solutes molecules
from moving to other
side! (impermeable to
them)
Result:
Equal conc. of
solution on BOTH
sides
1. Isotonic – solution has
the same solute conc.
as another solution.
Nothing happens to cell
size.
2. Hypertonic – solution
has higher solute conc.
than another solution
Water moves OUT to
adjust conc. = cell
SHRINKS
3. Hypotonic – solution
has lower solute conc.
than another solution
Water moves IN to
adjust conc. = cell
SWELLS & may burst
isotonic
1.
hyper
2.
hypo
hypo
hyper
3.
Hyper = more solute, less water
Hypo = less solute, more water
Note: the
solution with the
higher
concentration is
designated
“hypertonic”
.05 M
Note: the
solution with the
lower
concentration is
designated
“hypotonic”
Osmosis
.03 M
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
Active Transport
• As living beings, we NEED energy to sustain
life processes.
• Active transport is the transport of a molecule
across a membrane AGAINST its
concentration gradient and requires energy.
• We eat food containing nutrients for energy,
glucose being one of them.
– Recall: glucose cannot be stored inside body as it is
water-soluble, so it must be converted into
glycogen. This conversion MAKES energy in the
form of adenosine triphosphate (ATP) = main
source of energy for cells
– ATP reacts with certain compounds to release
energy to drive life processes
ATP Molecule
• Derived from an adenosine nucleotide
• Has 3 phosphate groups
• Breaking of end phosphate group from ATP
molecule releases energy, which can be used
by the cell
1. CM Carrier Protein Pump
• Bit similar to facilitated diffusion, but…
• Cell energy (ATP) is used to move
substance across CM against
concentration gradient (low  high)
1. Substance (eg.
protein, vitamin,
molecule) binds to
carrier protein.
2. Carrier protein uses
energy (ATP) to flip
over & “pumps”
(releases) substance to
other side of the
membrane.
3. Carrier protein flips
back to get another
substance.
HIGH CONCENTRATION
DIFFUSION
LOW CONCENTRATION
• Ion pumps are CM carrier proteins
that use ATP to pump ions from [low]
to [high].
• An example of this is the Na+-K+ pump
(plays role in nervous system)
1. [Na+] low & [K+] high inside
cell. Na+ ions bind to pump.
2. ATP molecule binds to
pump. ATP hydrolyzed,
releasing energy. One
phosphate group stays
attached to pump.
3. Energy allows pump to
change shape. Na+ ions exit
cell.
4. K+ ions bind to pump. This
causes the phosphate group
to detach from pump.
5. & 6. Pump changes shape
to release K+ ions into cell.