Cell membrane
• Also known as plasma membrane.
• Function: Maintains homeostasis within the cell
by being selectively permeable (meaning that it
will some things into the cell and keep others
out)
Cell membrane continued PHOSPHOLIPID
• Cell membranes are made primarily of a phospholipid
bilayer
• Phospholipids have 2 main regions:
• Head  negative charge, hydrophilic. Heads point toward
the inside and outside of the cell (toward water)
• 2 fatty acid Tails  nonpolar, hydrophobic. Tails point to the
center of the membrane (away from water)
HYDROPHILIC 
HYDROPHOBIC 
2
Membranes form spontaneously
– This can be demonstrated when a mixture of
phospholipids and water are shaken, the
phospholipids organize into bilayers
surrounding water-filled bubbles
Water-filled
bubble made of
phospholipids
• This formation of membrane enclosed
collections of molecules was a critical step
in the evolution of the first cell.
Fluid Mosaic model
• “mosaic” – surface made of small pieces
• Has diverse protein molecules embedded in a framework
of phospholipids.
• “fluid” – most molecules can drift about in the membrane.
• The double bonds in the unsaturated fatty acid tails of
many phospholipids produce kinks that prevent them
from packing tightly together. (fluid as salad dressing)
• The steroid cholesterol wedged in the bilayer in animal
cells helps stabilize the membrane in warm temps., and
keeps the membrane fluid at lower temps.
Types of proteins within the cell
membrane
• 1) Glycoproteins – involved in cell to cell
recognition.
• Carbohydrates outside the surface of the
cell membrane function as “id tags”.
– Cells in an embryo can sort themselves into
tissue & organs
– Immune system to recognize and reject
foreign cells (such as bacteria)
– Form junctions between cells.
Types of proteins within the cell
membrane
• 2) Enzymes - Many membrane proteins are
enzymes that carry out reactions. Remember that
enzymes speed up reactions by lowering activation
energy.
Enzymes
Types of proteins within the cell
membrane
• 3) Receptors – Proteins that receive chemical signals
(called ligands) from other cells and cause a reaction to
occur in the cell
• Has a shape that fits a specific messenger, such as a hormone. It
can either turn on a process directly or initiate a signal transduction
pathway (a series of steps to turn on a process).
Messenger molecule
(ligand)
Receptor
Activated
molecule
Types of proteins within the cell
membrane
• 4) Transport proteins • Molecules that are large, polar/ionic (hydrophilic) molecules must use a
transport protein to move across a cell because they cannot mix with
the hydrophobic tails of the phospholipid bilayer
– Small, nonpolar (hydrophobic) molecules can
move directly across the phospholipid bilayer
because they can interact with the nonpolar
(hydrophobic) tails
– These differences in solute movement is what
allows for selective permeability.
• 3 different types of Transport Proteins (all are
specific to what they will move)
1. Channel – move polar/ionic molecules down their
concentration gradient
2. Pump – move polar/ionic molecules against their
concentration gradient
3. Carrier – bind to one (or a few) specific molecules and
move then individually across the membrane; much
slower than channel protein
Concentration and
concentration gradient
• Concentration = amount of solute / amount of solvent
• Concentration gradient = difference in concentration
across the cell membrane (intracellular vs.
extracellular)
Extracellular Matrix (ECM) – Present only in animal cells, Helps hold
together tissues, protects and supports the plasma membrane
Glycoprotein
complex with long
polysaccharide
Collagen fiber
EXTRACELLULAR FLUID
Glycogen of
glycoprotein
Glycoprotein
Plasma
membrane
Microfilaments
CYTOPLASM
11
Passive Transport
• Passive transport – cell performs no work to move
molecules into or out of the cell because they are moving
DOWN their CONCENTRATION GRADIENT.
– Small, nonpolar molecules move directly across the
plasma membrane
• Remember that the tails of phospholipids are
nonpolar (hydrophobic) so other nonpolar
(hydrophobic) things can move through here
• Example: In our lungs, oxygen enters red blood
cells, and carbon dioxide passes out by passive
transport.
– Polar molecules can also move by passive transport if
they are moving down their concentration gradient,
but they must have transport (channel or carrier)
proteins to provide a pathway.
Passive Transport
• Passive transport will continue until equilibrium is
reached. At this point, the amount of solute moving into
and out of the cell will be equal.
• Once equilibrium is reached, there is still movement of
particles, but no net change in concentration.
3 Types of Passive Transport
• 1. Diffusion – the movement of small, nonpolar
molecules directly across the phospholipid bilayer down
their concentration gradient. Diffusion also describes the
tendency for particles of any kind to spread out evenly in
an available space, moving from highly concentrated
areas, to low concentrated areas.
– Ex. Ink spreading out in water, Perfume (and other smells)
diffusing across a room
3 Types of Passive Transport Diffusion continued
• Requires NO work
• It is caused by the RANDOM THERMAL MOVEMENT of
molecules.
• Molecules are constantly in motion (fast when they
are hot and slow when they are cold). When they are
in areas of high concentration, they collide with other
molecules and move in the opposite direction.
• Although movement is random, there is a net
movement of particles from high to low concentration
because in an area of low concentration molecules
ARE NOT colliding and bouncing in the other
direction as frequently. Once in equilibrium, they
collide and move in opposite directions equally.
3 Types of Passive Transport
• 2. Facilitated diffusion – movement of a large,
polar, or ionic molecule down its concentration gradient
using a transport protein (because it can’t move across
the phospholipid bilayer)
– Facilitate means to help so facilitated diffusion is just like
diffusion except with the help of a transport protein
– Does NOT require energy because it is moving down its
concentration gradient
• Substances that use facilitated diffusion:
–
–
–
–
Sugars
amino acids
Ions
water
Transport
protein
Solute
molecule
3 Types of Passive Transport
• 3. Osmosis – diffusion of WATER across the
membrane using aquaporins (channel proteins for water
because water is polar and thus can’t move across
phospholipid bilayer).
• The net movement of water down its own concentration
gradient which will always be in the opposite direction of
diffusion.
Water
molecule
Solute molecule with
cluster of water molecules
Net flow of water
3 Types of Passive Transport –
Osmosis Continued
Osmosis operates in the opposite direction of diffusion
because a high concentration of solute means a low
concentration of water. This is because water adhesively
bonds to the solute and is thus no longer free to move by
itself.
Osmosis – Tonicity
• Solutions of various tonicities (ability of a
solution to make a cell gain or lose water) can
have three different effects on plant & animal
cells.
• Isotonic solutions:
– (iso – the same) (tonos – tension)
• The solute concentration in the external
environment is equal to that of the cell.
– The cell’s volume remains constant. It gains
water at the same rate that it loses water.
• Plasma that transports red blood cells.
• Intravenous fluid administered in hospitals.
• Marine animals are isotonic to seawater.
Osmosis – Tonicity continued
• Hypotonic solution:
– (hypo – below)
• The solute concentration in the
external environment is below
that of the cell.
• The cell gains water, swells,
and may burst (lyse).
– The cell’s volume increases. It gains water faster than
it loses water.
Osmosis – Tonicity continued
• Hypertonic solution:
– (hyper – above)
• The solute concentration in
the external environment is
above that of the cell.
• The cell loses water, shrivels,
and can die from water loss.
This is referred to as plasmolysis. In plants the
cell membrane pulls away from the cell wall.
– The cell’s volume decreases. It loses water faster that
it gains water.
Osmosis – Tonicity Continued
• Water balance differs slightly for plant cells vs. animal cells.
• Animal cells prefer isotonic environments.
• Plant cells prefer hypotonic environments.
– The cell wall of plants exerts pressure on the cell,
preventing it from taking in too much water and bursting.
Isotonic solution
Hypotonic solution
Hypertonic solution
(A) Normal
(B) Lysed
(C) Shriveled
Animal
cell
Plasma
membrane
Plant
cell
(D) Flaccid
(E) Turgid
(F) Shriveled
(plasmolyzed)
Active Transport
• A cell expends energy to move a solute against
its concentration gradient – that is toward the
side where there is more solute.
– Transport proteins are used to pump solutes
against their concentration gradient (from low to
high). ATP provides the energy to do this.
– This is done to build up the concentration gradient
(and potential energy) to be used later. In other
words, you put in a little bit of energy now to get
back a lot of energy later.
Steps for Active Transport
1. Solute on the inside of the cell binds to an
active site on a transport protein.
2. ATP then transfers one of its phosphate
groups to the transport protein (this is called
phosphorylating the pump).
Steps for active transport
continued
3. Causing the protein to change shape, so that
the solute is released on the other side of the
membrane.
4. Then the phosphate group detaches, and the
transport protein returns to its original shape.
Example of Active Transport
• Sodium-Potassium pump: a transport protein
that helps generate nerve signals.
• Creates a higher concentration of K+ and a lower
concentration of Na+ inside the cell.
• The transport protein constantly shuttles the K+
into the cell, and the Na+ out of the cell.
– Because you maintain this large concentration
gradient across your nerve cells, there is lots
of potential energy because they want to
return to equilibrium. When you send a nerve
signal, you open up ion channels that allow
NA+ and K+ to rush back through them. This
is why you can send nerve signals so quickly.
Na+/K+ pumps
EXTRACELLULAR [Na+] high
FLUID
[K+] low
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
CYTOPLASM
[Na+] low
[K+] high
Na+
Cytoplasmic Na+ bonds to
the sodium-potassium pump
P
ATP
P
ADP
Na+ binding stimulates
phosphorylation by ATP.
Phosphorylation causes
the protein to change its
conformation, expelling Na+
to the outside.
Loss of the phosphate
restores the protein’s
original conformation.
K+ is released and Na+
sites are receptive again;
the cycle repeats.
P
P
Extracellular K+ binds
to the protein, triggering
release of the phosphate
group.
28
Types of Cellular Transport
• PASSIVE
• ACTIVE
• Does not require
energy.
• Requires energy from
ATP.
• Goes with the
concentration gradient
(high to low).
• Diffusion, Facilitated
Diffusion, Osmosis
• Goes against the
concentration gradient
(low to high).
• Active Transport
Diffusion
Requires no energy
Requires energy
Passive transport
Active transport
Facilitated
diffusion
Higher solute concentration
Osmosis
Higher water
concentration
Higher solute
concentration
Solute
Water
Lower solute
concentration
Lower water
concentration
Lower solute
concentration
Vesicular transport (**Add to notes:
Also called bulk transport) - Movement
of MACROMOLECULES
Some molecules are too large to move across even with a
transport protein. Movement of these macromolecules can
be achieved by using vesicles in the following 2 methods:
1. Exocytosis – removal (or exit) of macromolecules from
cell
2. Endocytosis – entry of macromolecules into cell
a) Phagocytosis
b) Pinocytosis
c) Receptor-mediated endocytosis
1. Exocytosis
•
Exocytosis – (exo – outside) export bulky materials
such as proteins or polysaccharides.
A transport vesicle filled with macromolecules buds from the
Golgi body.
1. Moves to the cell membrane.
2. Vesicle fuses with the cell membrane.
3. Vesicle contents spill out of the cell.
4. Vesicle membrane becomes part of the cell membrane.
2. Endocytosis
•
Endocytosis – (endo – inside) a cell
takes in substances.
1. A depression forms in the cell membrane.
2. Material outside the cell sits within this
depression.
3. The depression pinches in and forms a
vesicle (containing materials).
3 types of endocytosis
• Phagocytosis – “cellular eating” A cell
engulfs a particle by wrapping extensions
called pseudopodia around it and
packaging it within a vacuole.
•Vacuole then fuses
with a lysosome,
which digests the
contents.
3 types of endocytosis
• Pinocytosis – “cellular drinking” A cell
gulps droplets of fluid into tiny vesicles.
3 types of endocytosis
•
Receptor – mediated endocytosis –
1. Receptor proteins for specific molecules are
embedded in cell membrane.
2. These receptors have picked up particular
molecules.
3. Then the cell membrane pinches off to form vesicle
containing receptors and their attached molecules.
• Used to take in cholesterol
(LDL) from the blood.