2. Membrane StructureMembrane Structure
& Function& Function
The selectively
permeable plasma
membrane acts as traffic
control for the cell,
allowing only certain
things in at any given
time
3. Cell Membranes are FluidCell Membranes are Fluid
Mosaics of Lipids & ProteinsMosaics of Lipids & Proteins
*Phospholipids are
amphipathic-
both
hydrophobic &
hydrophilic
The structure
&
arrangement
of the
phospholipid
bilayer is a
result of the
molecular
properties of
phospholipids
4. Membrane Structure ResultsMembrane Structure Results
in Selective Permeabilityin Selective Permeability
The fluid mosaic model explains how membranes regulate
cellular traffic
--it explains how form fits function
Fibers of
extracellular
matrix
Cytoskeleton Cytoplasm
Attachment to
cytoskeleton and
extracellular
matrix
Cell signaling
Enzymatic activity
Transport
Intercellular
joining Cell-cell
recognition
Cytoplasm
5. The Permeability of the Lipid
Bilayer
Small nonpolar (hydrophobic) molecules can pass
freely across the membrane
Polar molecules pass through slowly or not at all;
they require the aid of transport proteins
6. Transport Proteins
Transport proteins are just as selective as the lipid
bilayer.
Two types of transport proteins:
1. Channel Proteins- a hydrophilic
tunnel through which hydrophilic
molecules & ions can pass
through
2. Carrier Proteins- Hold onto
molecules and change shape
in a way that shuttles them
across the membrane
What kind of molecules will the transport
proteins help moved across the
membrane? Why?
7. Passive Transport is Diffusion of
a Substance Across a Membrane
With No Energy Requirement
Molecules (above 0 K)
are always in motion
Diffusion is the tendency
for
molecules to move
down their
concentration
gradient until a dynamic
equilibrium is reached.
8. Passive Transport is Diffusion of
a Substance Across a Membrane
With No Energy Requirement
O2 can
diffuse
into a cell
from
blood &
CO2 can
diffuse out
and get
carried
back to
lungs to
be
exhaled
9. Effects of Osmosis on Water
Balance
Osmosis is the
diffusion of
water
molecules from
areas of high
(free) water
concentration
to areas of
lower
concentration
10. Quick Think
Oxygen & carbon dioxide can cross
the lipid bilayer without help from
membrane proteins. What properties
allow this to occur?
Water sometimes needs the help of a
transport protein (aquaporins) to
cross the membrane. Why?
11. = solute
Water Balance of CellsWater Balance of Cells WithoutWithout
Walls e.g.Walls e.g.
There are 3 states of tonicity when comparing two solutions:
1. Isotonic: No net movement of
water across the cell membrane
Iso= equal
Animal CellsAnimal Cells
12. =solute
Water Balance of CellsWater Balance of Cells WithoutWithout
Walls e.g. Animal CellsWalls e.g. Animal Cells
There are 3 states of tonicity when comparing two solutions:
2. Hypertonic: A solution that
is relatively hypertonic
contains more
non-penetrating solutes than
the solution that it is being
compared to
Hyper = More
13. Water Balance of CellsWater Balance of Cells WithoutWithout
Walls e.g. Animal CellsWalls e.g. Animal Cells
There are 3 states of tonicity when comparing two solutions:
3. Hypotonic: A solution that is relatively
hypotonic contains fewer
non-penetrating solutes than the solution
that it is being compared to
Hypo = less
=solute
14. Quick Think
Where will water move and why?
1. A cell placed in a 10% salt solution (that’s very
salty).
2. A cell placed in distilled water.
3. A cell placed in an isotonic solution.
15. Water Balance of CellsWater Balance of Cells WithoutWithout
Walls e.g. Animal CellsWalls e.g. Animal Cells
Osmoregulation- The control of water balance within a cell.
Various methods of osmoregulation are present in organisms
that are specially adapted to living in hypertonic or hypotonic
environments:
Contractile Vacuole
16. Quick Think
If a Paramecium were to swim from a hypotonic
environment to an isotonic one, would the
activity of its contractile vacuole increase or
decrease? Why?
17. Water Balance of CellsWater Balance of Cells WithWith
Walls e.g.Walls e.g. Plant CellsPlant Cells
…Prokaryotes & Fungi Too!
Hypotonic solutions, like rain, cause turgor (firmness) in
plants because the cell wall pushes back against the intake
of additional water after a certain point:
Turgor= nice, healthy plant
When plant cells are isotonic with their
environment, they become flaccid
18. Water Balance of CellsWater Balance of Cells WithWith
Walls e.g. PlantsWalls e.g. Plants
In hypertonic solutions, cells with
walls experience
plasmolysis- they lose water to
their environment until the
cell membrane shrivels & pulls
away from the cell wall. The
result is a wilted & possibly dead
plant (or other organism)
= solute
19. Facilitated Diffusion: PassiveFacilitated Diffusion: Passive
Transport Aided by ProteinsTransport Aided by Proteins
Facilitated diffusion is the use of transport proteins, like
channel & carrier proteins, to speed up the diffusion of
molecules across a membrane.
A specific type of channel proteins are ion channels which are often
gated channels-the presence of a stimulus causes them to open/close
Example- the
presence of a
certain neuro-
transmitter, like
acetylcholine,
would cause the
gated channels of
a nerve cell to
open and let Na+
into the cell
21. Active Transport Uses Energy to MoveActive Transport Uses Energy to Move
SolutesSolutes AgainstAgainst Their GradientsTheir Gradients
PassivePassive
TransportTransport
Includes osmosis,
diffusion, & facilitated
diffusion
ActiveActive
TransportTransport
Molecules move down
concentration gradient
Molecules are moved
against concentration
gradient
Doesn’t require
expenditure of energy
Requires energy
No assistance,
channel proteins &/or
carrier proteins
Carrier proteins
23. Maintenance of MembraneMaintenance of Membrane
Potential by Ion PumpsPotential by Ion Pumps
All cell membranes have voltage across them due to a separation
of opposite charges.
TheThe membrane potentialmembrane potential is a result of a higheris a result of a higher negativenegative ionion
concentration inside the cell relative to the outside of the cellconcentration inside the cell relative to the outside of the cell
TheThe electrochemical gradientelectrochemical gradient can cause positive ions tocan cause positive ions to
diffuse into the celldiffuse into the cell
24. Creating Membrane Potential-Creating Membrane Potential-
Two ExamplesTwo Examples
Ion pump=Sodium-Potassium Pump
Electrogenic Pump = Proton Pump
25. Cotransport: Coupled TransportCotransport: Coupled Transport
by a Membrane Proteinby a Membrane Protein
Cotransport is
when the action
of one proton
pump creates the
electrochemical
gradient to power
another transport
protein
The natural flow of H ions back
The natural flow of H ions back
in sorta “sucks” in the other
in sorta “sucks” in the other
desired molecule
desired molecule
26. Bulk Transport Across the PlasmaBulk Transport Across the Plasma
Membrane Occurs by Exocytosis &Membrane Occurs by Exocytosis &
EndocytosisEndocytosis
Exocytosis: Transport
vesicles from golgi
apparatus move
to cell membrane. The
vesicle membrane fuses
with the cell
Membrane, and the
contents is expelled out of
the cell
27. Endocytosis: A section of the plasma membrane sinks
inward & pinches off to form a vesicle that transports
materials into the cell; Three kinds:
Phagocytosis: “Cell eating”
Pseudopodia of the cell membrane “reach
out” & engulf particles. These particles are
then digested when a vacuole fuses with a
lysosome
Pinocytosis: “Cell drinking”
Part of the cell membrane caves in and
gulps extracellular fluid & the molecules
dissolved in it
Receptor-Mediated Endocytosis:
Allows for the bulk transport of specific
extracellular particles. The vesicle forms
only when certain ligands bind to receptor
proteins on the cell membrane.
28. Quick Think
The carbohydrates attached to some of
the proteins and lipids of the cell
membrane are added as the membrane is
made and refined in the ER and Golgi.
The new membrane then forms transport
vesicles that travel to the cell surface. On
which side of the vesicle membrane are
the carbohydrates?
