5. The plasma membrane “fluid” isThe plasma membrane “fluid” is
a double layer of phospholipids.a double layer of phospholipids.
6. The plasma membrane “mosaic” isThe plasma membrane “mosaic” is
a collection of proteins.a collection of proteins.
7. Functions of MembraneFunctions of Membrane
MacromoleculesMacromolecules
• LipidsLipids
– Phospholipid bilayerPhospholipid bilayer
• Forms boundary to isolate cell contents fromForms boundary to isolate cell contents from
environmentenvironment
• Restricts passage of hydrophilic substancesRestricts passage of hydrophilic substances
across the membraneacross the membrane
– CholesterolCholesterol
• Increases bilayer strength, flexibilityIncreases bilayer strength, flexibility
• Reduces membrane fluidityReduces membrane fluidity
• Reduces permeability to water-solubleReduces permeability to water-soluble
substancessubstances
8. Functions of MembraneFunctions of Membrane
MacromoleculesMacromolecules
• ProteinsProteins
– Transport proteinsTransport proteins
• Regulate movement of water soluble substancesRegulate movement of water soluble substances
– Channel proteins have pores that allowChannel proteins have pores that allow
passage of ions and small water-solublepassage of ions and small water-soluble
moleculesmolecules
– Carrier proteins bind to molecules andCarrier proteins bind to molecules and
change shape for delivery across membranechange shape for delivery across membrane
9. Functions of MembraneFunctions of Membrane
MacromoleculesMacromolecules
• ProteinsProteins
– Receptor proteinsReceptor proteins
• Docking site for molecules outside the cellDocking site for molecules outside the cell
• Trigger internal cellular responseTrigger internal cellular response
– Recognition proteinsRecognition proteins
• Identification tagsIdentification tags
– Oligosaccharides aid inOligosaccharides aid in
cell-cell recognitioncell-cell recognition
• Cell-surface attachmentCell-surface attachment
sitessites
10. Applying Your KnowledgeApplying Your Knowledge
A.A. Which component would have anWhich component would have an
oligosaccharide attached?oligosaccharide attached?
B.B. Which component regulates membraneWhich component regulates membrane
fluidity?fluidity?
C.C. Which component assists in movement ofWhich component assists in movement of
water-soluble substances across thewater-soluble substances across the
membrane?membrane?
1.1. CholesterolCholesterol
2.2. Receptor ProteinReceptor Protein
3.3. Phospholipid BilayerPhospholipid Bilayer
4.4. Transport ProteinTransport Protein
5.5. Recognition ProteinRecognition Protein
11. Transport ProcessesTransport Processes
• Movement of substances across theMovement of substances across the
cell membranecell membrane
• Passive transportPassive transport
– Substances move from [high]Substances move from [high][low][low]
– No energy input requiredNo energy input required
– Simple Diffusion, Facilitated Diffusion,Simple Diffusion, Facilitated Diffusion,
OsmosisOsmosis
• Active transportActive transport
– Substances move from [low]Substances move from [low][high][high]
– Requires energy inputRequires energy input
– Protein carriers, Endocytosis, ExocytosisProtein carriers, Endocytosis, Exocytosis
12. Diffusion of Dye in WaterDiffusion of Dye in Water
Time 0Time 0
SteepSteep
ConcentrationConcentration
GradientGradient
Time 1Time 1
ReducedReduced
ConcentrationConcentration
GradientGradient
DispersingDispersing
Time 2Time 2
NoNo
ConcentrationConcentration
GradientGradient
RandomRandom
DispersalDispersal
13. Simple DiffusionSimple Diffusion
(extracellular fluid)
(cytoplasm)
Some moleculesSome molecules
diffuse freelydiffuse freely
acrossacross
(lipid soluble molecules, CO(lipid soluble molecules, CO22, O, O22, H, H22O )O )
Now known
to require
transport
proteins
called
aquaporins
14. Facilitated Diffusion:Facilitated Diffusion:
Use of Carriers to Assist with DiffusionUse of Carriers to Assist with Diffusion
Carrier protein
has binding site
for molecule
Molecule enters
binding site
Carrier protein changes
shape, transporting
molecule across membrane
Carrier protein resumes
original shape
(Inside Cell)(Inside Cell)(Inside Cell)(Inside Cell)
(Outside(Outside
Cell)Cell)
(Outside(Outside
Cell)Cell)
DiffusionDiffusion
ChannelChannel
ProteinProtein
DiffusionDiffusion
GradientGradient
Molecule inMolecule in
TransitTransit
15. OsmosisOsmosis
• Diffusion of water across aDiffusion of water across a
differentially permeable membranedifferentially permeable membrane
• Water moves from [high]Water moves from [high] [low][low]
17. Normal RBCs
Isotonic SolutionIsotonic Solution
The Effects of OsmosisThe Effects of Osmosis
Equal movement of water
into and out of cells
Net movement of
water out of cells Net movement of
water into cells
Shriveled RBCs
Swollen RBCs
Hypertonic SolutionHypertonic Solution Hypotonic SolutionHypotonic Solution
18. Active Transport with Protein CarrierActive Transport with Protein Carrier
Inside CellInside CellInside CellInside Cell
Outside CellOutside CellOutside CellOutside Cell
EnergyEnergy
ProviderProvider
Spent EnergySpent Energy
ProviderProvider
TransportedTransported
MoleculeMolecule
1 2 3 4
20. EndocytosisEndocytosis
• Active process for movement ofActive process for movement of
large molecules and organismslarge molecules and organisms
– Substance is taken in by vesicleSubstance is taken in by vesicle
formed from cell membraneformed from cell membrane
– Phagocytosis: solid substance inPhagocytosis: solid substance in
vesiclevesicle
– Pinocytosis: liquid droplets in vesiclePinocytosis: liquid droplets in vesicle
– Receptor-Mediated Endocytosis:Receptor-Mediated Endocytosis:
incoming substance binds to receptorincoming substance binds to receptor
21. Receptor-mediated Endocytosis IReceptor-mediated Endocytosis I
Begins with a shallowBegins with a shallow
pit in plasma membranepit in plasma membrane
Pit is coated withPit is coated with
proteinprotein
(cytoplasm)
(extracellular fluid)
coated
pit
protein
coating
extracellular particles
bound to receptors
plasma membrane
Pit deepensPit deepens
aa bb
0.1 Micrometer
22. Receptor-mediated Endocytosis IIReceptor-mediated Endocytosis II
Pit deepens further andPit deepens further and
begins to pinch offbegins to pinch off
Eventually becomes aEventually becomes a
coated vesiclecoated vesicle
ddcc
0.1 Micrometer
coated
vesicle
23. ExocytosisExocytosis
• Substance is expelled after beingSubstance is expelled after being
enclosed in a vesicle within the cellenclosed in a vesicle within the cell
• Used to move large molecules outUsed to move large molecules out
of the cellof the cell
25. Applying Your KnowledgeApplying Your Knowledge
A.A. Which methods allow movement intoWhich methods allow movement into
the cell?the cell?
B.B. Which methods involve movement ofWhich methods involve movement of
liquid?liquid?
C.C. Which methods involve movementWhich methods involve movement
from [low]from [low] [high]?[high]?
1.1. Facilitated DiffusionFacilitated Diffusion
2.2. EndocytosisEndocytosis
3.3. OsmosisOsmosis
4.4. ExocytosisExocytosis
5.5. Active TransportActive Transport
6. Simple Diffusion6. Simple Diffusion
7. Pinocytosis7. Pinocytosis
8. Receptor-mediated8. Receptor-mediated
endocytosisendocytosis
9. Phagocytosis9. Phagocytosis
1, 2, 3, 5, 6, 7, 8, 91, 2, 3, 5, 6, 7, 8, 9
2, 3, 6, 72, 3, 6, 7
2, 4, 5, 7, 8, 92, 4, 5, 7, 8, 9
26. Cell Attachment StructuresCell Attachment Structures
DesmosomeDesmosome
desmosome
Protein strands
holding cells
together
Protein filaments
in cytoplasm
Small intestine Plasma membrane
(edge view)
Cells lining
small intestine
Tight JunctionTight Junction
Tight junctions
formed by strands
of protein
Plasma membrane
(edge view)
Cells lining
bladder
Tight junctions
seal membranes
of adjacent cells
together
27. Cell Communication StructuresCell Communication Structures
Gap JunctionsGap Junctions
desmosome
Gap Junctions:
pairs of channels
connect insides of
adjacent cells
Liver Plasma membrane
(edge view)
Liver cells
PlasmodesmataPlasmodesmata
Plasmodesmata
connect insides
of adjacent cells
Plasma membrane
(edge view)
Root cells
Cell wall
(edge view)
Middle lamella
(edge view)
Root
AnimalsAnimals PlantsPlants
Editor's Notes
Many of these protein molecules (blue in this illustration) float around in the membrane like icebergs on the polar sea. The proteins serve several functions:
Transport proteins: Regulate the movement of water-soluble molecules through the plasma membrane.
Channel Proteins: Form pores that allow small molecules to pass.
Carrier Proteins: Have binding sites that bind specifically to specific proteins and move the molecules through the membrane.
Receptor Proteins: Trigger specific cellular responses when certain molecules bind to them.
Recognition Proteins: Serve as ID tags and cell-surface attachment sites.
A triglyceride is a glycerol molecule with three fatty acids attached (p42).
Triglycerides are familiar fats and oils.
The fatty acids may be of the same type or different types.
Different fatty acids may have differing number of carbon atoms in their hydrocarbon chain.
Some fatty acids have double bonds and are called unsaturated.
Those with no double bonds are called saturated.
Phosphoplipids are like triglycerides with one fatty acid swapped out and a polar group containing phosphorous swapped in its place.
Phospholipids are abundant in biological membranes.
They self assemble into lipid bilayers characteristic of the plasma membrane and organnelle membranes (fig 5-2).
Eukaryotic Cell
Eukaryotic Cell
This refers to the types of processes used by cells to get materials in and out across the cell membrane.
The membrane is said to be “semi-permeable.”
This means that it will allow spontaneous passage of some materials but others must use special processes to get across.
A concentration gradient is a difference in the number of molecules or ions of a given substance in two adjoining regions.
Molecules constantly collide and tend to move according to existing concentration gradients.
The net movement of like molecules down a concentration gradient (high to low) is simple diffusion.
Gradients in temperature, electric charge, and pressure, can influence movements.
Passive transport: Follows a concentration gradient
Passive in the sense that the process does not require energy.
It just happens naturally.
Diffusion is passive transport.
Due to the random movement of molecules in liquids & gases (Brownian motion).
A concentrated material will tend to become more uniformly dispersed in the space made available to it.
Water can diffuse across the cell membrane unassisted.
Facilitated transport: Also follows a concentration gradient, but requires gateway to get through.
In this case, the particular materials would not spontaneously cross the pure phospholipid membrane, even if encouraged by a concentration gradient.
A facilitator molecule is produced in the membrane that serves as a gateway for the material.
The material can now cross through the gateway spontaneously whenever a concentration gradient exists.
Active transport: Flow up-hill against a concentration gradient; requires energy expenditure to keep going.
The required energy is almost always provided directly by ATP.
The ATP donates a phosphate to a specific gateway molecule which then “pumps” the desired molecule across the membrane, even if it goes against a concentration gradient.
The ATP energy is used to drive the pump.
Shows successive stages in the dispersal of red dye molecules after being dropped into water.
Note that the molecules to be transported (little balls) are in higher concentration outside the cell than inside the cell.
The molecules are of a type that cannot get to the inside of the cell by going directly through the membrane.
The protein channel provides a passage way for the molecules to follow the concentration gradient.
Osmosis is the passive movement of water across a differentially permeable membrane in response to solute concentration gradients, pressure gradients, or both.
Special case of diffusion.
Involves the diffusion of water across a membrane that has solutions of differing concentration on opposite sides of the membrane.
The water moves more abundantly toward the side with the highest concentration of dissolved materials.
The concentration of water is obviously lower on the side with the highest concentration of dissolved materials.
So the water is simply diffusing from a higher concentration of water to a lower concentration of water.
Example: Pure water on one side; sugar solution on other (ƒ5-5).
Sugar molecules can’t cross, but crowd membrane pores on syrupy side.
Water crosses faster toward sugar.
Net movement of water ====> sugar solution
There is a net increase in volume on the side that has the higher concentration of sugar.
The side of the membrane with the lower concentration of sugar experiences a loss of volume.
This persistent movement can result in a build up of pressure known as osmotic pressure.
Note that molecules of water are constantly moving in BOTH directions across the cell membrane.
Water will move faster FROM a low concentration of WATER than it moves AWAY from it.
Though water is moving in both directions, there is a net gain of water on the side that starts out with less water (more stuff dissolved in the water).
Note that the concentration of the molecules is higher outside the cell than inside, but that the molecules are moving out, against the gradient. This always requires energy, thus the term “active” transport.
Here are some special adaptations of endocytosis.
Pinocytosis is when a vesicle forms around and then surrounds and engulfs the liquid from outside the cell.
Phagocytosis is when the vesicle forms around a solid particle like in slide 29.
Receptor-mediated Endocytosis uses special receptor molecules that;
Bind to specific molecules outside the cell
Aggregate together as more and more of the receptors bind target molecules
Cause the membrane to warp inwards where they aggregate
Eventually pinch off to form a “coated” vesicle.
This shows the steps in exocytosis.
A vesicle on the inside of the cell contains a cell product destined for export.
The membrane of the vesicle merges with the plasma membrane, and the contents of the vesicle are now outside the cell.