Download The Plasma Membrane and Transport

The Plasma
Membrane
and Transport across it
Cell Membrane

Controls what enters and leaves the cell
 Like

water, nutrients and waste
The membrane is Selectively Permeable
Membrane selects which molecules can permeate
(pass through) into the cell.
 Like a window screen – allows what you want in
and keeps others out.

Structure of Plasma Membrane

Lipid Bilayer (2 layers of phospholipids)
Lipids with phosphate “heads” attached
 Head of phospholipid is polar



Hydrophilic (points outward, likes watery environment)
Fatty acids (tails) are nonpolar
 Hydrophobic (points inward, avoids water)
Other Membrane Molecules

Membrane Proteins - (transport)
Allow substances/waste to move in and out of the
cell.
 Creates a tunnel that can be closed or open.


Carbohydrates/ Other Proteins
Stick out of cell surface (id card for other cells to
see)
Cell Membrane Structures
Carb/Protein
(ID Cards)
Membrane Proteins
(tunnels)
Polar Heads
Fatty Acid Tails
How stuff gets into the cell

2 ways cell gets nutrients it needs and gets rid
of waste
 Passive

Requires no energy
 Active

Transport
Transport
Requires an input of energy
Passive
Transport

Occurs due to
concentration gradient
 Molecules
naturally move from areas
of higher concentration to lower
concentration (no energy required)
Diffusion – natural movement of
molecules
 Facilitated Diffusion – through protein
channels
 Osmosis – diffusion of water molecules

Diffusion


These odor molecules are diffusing from
the bottle (area of high concentration) to
the surrounding air (lower
concentration)
In nature, molecules
ALWAYS move from
areas of higher
concentration to areas of
lower concentration, as is
shown in this picture.
They move to reach
equilibrium

This also happens across
the cell membrane!
Facilitated Diffusion

Diffusion of molecules
through protein
channels in the cell
membrane
 Used
for substances
that the cell needs but
cannot pass through
the membrane
Large molecules
 Charged (ions)

Osmosis

The diffusion of water molecules is called
osmosis
 Just
like diffusion
 Water molecules move from areas of higher
conc. to areas of lower water conc.
 They will move until there are equal
concentrations of water (and solute)
molecules everywhere

Called equilibrium

Molecules still diffuse, but at equal rates
Osmosis



Blue – water molecules, easily pass through
membrane
Red – large (or charged) molecules, can’t
pass through membrane
The movement of water
molecules across a
membrane
Water moves from areas
of high water conc. to
areas of lower water conc.
This is how cells maintain
a stable internal
environment
 Homeostasis
environment
– reacting to
Passive Transport Video
Isotonic Solutions 
“Iso”
equal
Cells that are in equilibrium with the
surrounding solution do not experience
osmosis.
•These cells have the
same concentration
of solute (and water)
as the surrounding
solution
Hypotonic Solutions – “hypo”

lower
[solute] (solute concentration) is lower
outside the cell •More water outside, so water moves into
the cell, to try to reach equilibrium
•The cell swells with the extra water.
•Animal cells may swell so much,
they may burst (especially in pure
water)
•Plant cells – cell swells and membrane
pushes against the cell wall
•Like celery in water – crisp/ rigid
from pressure on cell walls
Hypertonic Solutions - “hyper”

higher
[solute] is higher outside of the cell.
•Less water outside the cell, more
inside. Water moves out of the cell.
•Animal cells shrivel because of
water loss.
•Plant cell membranes move
away from cell wall as water
moves out.
•Why plants wilt.
Other Forms of Transport

Active Transport – (needs energy)
 Large

Endocytosis – cell surrounds molecule(s), which
are then engulfed by cell


particle transport
doesn’t pass through membrane
Exocytosis - expulsion/secretion of materials.
Opposite of endocytosis
Active Transport (con’t)

Movement of molecules against (up) the
concentration gradient
 From
areas of lower concentration to areas of
higher concentration

For instance, if a cell needs a high concentration of
something (that would normally diffuse OUT of cell)
Summary!