1. Membrane Structure & Function

2. Plasma Membrane Structure
Boundary that separates the living cell from its non-living environment
Selective permeability
Amphipathic~ hydrophobic & hydrophilic regions
Singer-Nicolson: fluid mosaic model

3. Plasma Membrane Structure Phospholipid Yellow- hydrophilic
Blue- hydrophobic
The two long chains coming off of the bottom of this molecule are made up of carbon and hydrogen. Because both of these elements share their electrons evenly these chains have no charge Molecules with no charge are not attracted to water; as a result water molecules tend to push them out of the way as they are attracted to each other. This causes molecules with no charge not to dissolve in water (this is why gasoline and water do not mix). At the other end of the phospholipid is a phosphate group and several double bonded oxygens. The atoms at this end of the molecule are not shared equally. This end of the molecule has a charge and is attracted to water.
If you mix phospholipids in water they will form these double layered structures. The hydrophillic ends will be in contact with water. The hydrophibic ends will face inwards touching each other.

4. Hydrophobic region of protein
Singer and Nicolson
In 1972, Singer and Nicolson, Proposed that membrane proteins are dispersed and individually inserted into the phospholipid bilayer of the plasma membrane
Phospholipid
bilayer
Hydrophilic region
of protein
Hydrophobic region of protein

5. Fluid Mosaic Model
A membrane is a fluid structure with a “mosaic” of various proteins embedded in it when viewed from the top
Phospholipids can move laterally a small amount and can “flex” their tails
Membrane proteins also move side to side or laterally making the membrane fluid

6. Freeze-fracture studies of the plasma membrane support the fluid mosaic model of membrane structure
A cell is frozen and fractured with a knife. The fracture plane often follows the hydrophobic interior of a membrane, splitting the phospholipid bilayer into two separated layers. The membrane proteins go wholly with one of the layers.

7. The Fluidity of Membranes
Phospholipids in the plasma membrane can move within the bilayer two ways
Lateral movement
(~107 times per second)
Flip-flop
(~ once per month)

8. The Fluidity of Membranes
The type of hydrocarbon tails in phospholipids affects the fluidity of the plasma membrane
Fluid
Viscous
Unsaturated hydrocarbon
tails with kinks
Saturated hydro-
Carbon tails

9. The Fluidity of Membranes
The steroid cholesterol Has different effects on membrane fluidity at different temperatures
Cholesterol

10. Membrane Proteins and Their Functions
A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer
Fibers of
extracellular
matrix (ECM)

11. Types of Membrane Proteins
Integral proteins
Penetrate the hydrophobic core of the lipid bilayer
Are often transmembrane proteins, completely spanning the membrane
EXTRACELLULAR
SIDE

12. Types of Membrane Proteins
Peripheral proteins
Are appendages loosely bound to the surface of the membrane

13. Membrane Structure Summary
Phospholipids~ membrane fluidity
Cholesterol~ membrane stabilization
“Mosaic” Structure~
Integral proteins~ transmembrane proteins
Peripheral proteins~ surface of membrane
Membrane carbohydrates ~ cell to cell recognition; oligosaccharides (cell markers); glycolipids; glycoproteins

14. Six Major Functions of Membrane Proteins
Figure 7.9
Transport. (left) A protein that spans the membrane
may provide a hydrophilic channel across the
membrane that is selective for a particular solute.
(right) Other transport proteins shuttle a substance
from one side to the other by changing shape. Some
of these proteins hydrolyze ATP as an energy source
to actively pump substances across the membrane.
Enzymatic activity. A protein built into the membrane
may be an enzyme with its active site exposed to
substances in the adjacent solution. In some cases,
several enzymes in a membrane are organized as
a team that carries out sequential steps of a
metabolic pathway.
Signal transduction. A membrane protein may have
a binding site with a specific shape that fits the shape
of a chemical messenger, such as a hormone. The
external messenger (signal) may cause a
conformational change in the protein (receptor) that
relays the message to the inside of the cell.
(a)
(b)
(c)
ATP
Enzymes
Signal
Receptor

15. Six Major Functions of Membrane Proteins
Cell-cell recognition. Some glyco-proteins serve as
identification tags that are specifically recognized
by other cells.
Intercellular joining. Membrane proteins of adjacent cells
may hook together in various kinds of junctions, such as
gap junctions or tight junctions
Attachment to the cytoskeleton and extracellular matrix
(ECM). Microfilaments or other elements of the
cytoskeleton may be bonded to membrane proteins,
a function that helps maintain cell shape and stabilizes
the location of certain membrane proteins. Proteins that
adhere to the ECM can coordinate extracellular and
intracellular changes
(d)
(e)
(f)
Glyco-
protein

16. The Role of Membrane Carbohydrates in Cell-Cell Recognition
Is a cell’s ability to distinguish one type of neighboring cell from another
Membrane carbohydrates
Interact with the surface molecules of other cells, facilitating cell-cell recognition
Immunity
Recognition is a key to the organism’s immune system’s ability to distinguish between self and invasion of foreign bodies

17. Synthesis and Sidedness of Membranes
Membranes have distinct inside and outside faces
This affects the movement of proteins synthesized in the endomembrane system (Golgi and ER)

18. Passive Transport SIMPLE DIFFUSION FACILITATED DIFFUSION Weeee!!!
cell uses no energy
molecules move randomly
concentration is from high to low
three types
SIMPLE DIFFUSION
FACILITATED DIFFUSION
OSMOSIS
high
low
Weeee!!!

19. Simple Diffusion Diffusion
Is the tendency for molecules of any substance to spread out evenly into the available space
Move from high to low concentration
Down the concentration gradient

20. Effects of Osmosis on Water Balance
Is the movement of water across a semi-permeable membrane
It is affected by the concentration gradient of dissolved substances called the solution’s tonicity

21. Water Balance of Cells Without Walls
Tonicity – Three States
Is the ability of a solution to cause a cell to gain or lose water
Has a great impact on cells without walls

22. Isotonic Solutions If a solution is isotonic
The concentration of solutes is the same as it is inside the cell
There will be NO NET movement of WATER

23. Hypertonic Solution If a solution is hypertonic
The concentration of solutes is greater than it is inside the cell
The cell will lose water (PLASMOLYSIS)

24. Hypotonic Solutions If a solution is hypotonic
The concentration of solutes is less than it is inside the cell
The cell will gain water

25. Water Balance in Cells Without Walls
Animal cell. An animal cell fares best in an isotonic environment unless it has special adaptations to offset the osmotic uptake or loss of water.

26. Water Balance of Cells with Walls
Cell Walls
Help maintain water balance
Turgor pressure
Is the pressure of water inside a plant cell pushing outward against the cell membrane
If a plant cell is turgid
It is in a hypotonic environment
It is very firm, a healthy state in most plants
If a plant cell is flaccid
It is in an isotonic or hypertonic environment

27. Water Balance in Cells with Walls
Plant cell. Plant cells are turgid (firm) and generally healthiest in a hypotonic environment, where the uptake of water is eventually balanced by the elastic wall pushing back on the cell.

28. How Organisms Deal with Osmotic Pressure
Hypertonic
Bacteria and plants have cell walls that prevent them from over-expanding. In plants the pressure exerted on the cell wall is called tugor pressure.
Protists like paramecium has contractile vacuoles that collect water flowing in and pump it out to prevent them from over-expanding.
Salt water fish pump salt out of their specialized gills so they do not dehydrate.
Animal cells are bathed in blood. Kidneys keep the blood isotonic by remove excess salt and water. Paramecium: protist removing excess water video

29. How Will Water Move Across Semi-Permeable Membrane?
Solution A has 100 molecules of glucose per ml
Solution B has 100 molecules of fructose per ml
How will the water molecules move?
There will be no net movement of water since the concentration of solute in each solution is equal

30. How Will Water Move Across Semi-Permeable Membrane?
Solution A has 100 molecules of glucose per ml
Solution B has 75 molecules of fructose per ml
How will the water molecules move?
There will be a net movement of water from Solution B to Solution A until both solutions have equal concentrations of solute

31. How Will Water Move Across Semi-Permeable Membrane?
Solution A has 100 molecules of glucose per ml
Solution B has 100 molecules of NaCl per ml
How will the water molecules move?
Each molecule of NaCl will dissociate to form a Na+ ion and a Cl- ion, making the final concentration of solutes 200 molecules per mil. Therefore, there will be a net movement of water from Solution A to Solution B until both solutions have equal concentrations of solute

32. Facilitated Diffusion & Proteins
Facilitated diffusion is a type of Passive Transport Aided by Proteins that allow a specific substance to cross the membrane
Channel Proteins
In some cases they act in passive transport.
Molecules diffuse randomly through the opening, requiring no energy, moving from an area of high concentration to an area of low concentration.
Examples
ion channel
Glucose
Amino acids

33. Facilitated Diffusion & Proteins
Carrier proteins
Undergo a subtle change in shape that translocates the solute-binding site across the membrane
A carrier protein alternates between two conformations, moving a solute across the membrane as the shape of the protein changes. The protein can transport the solute in either direction, with the net movement being down the concentration gradient of the solute.

34. Filtration smaller molecules are forced through porous membranes
hydrostatic pressure important in the body
molecules leaving blood capillaries

35. This is gonna be hard work!!
Active Transport
Cell uses energy
Actively move molecules to where they are needed
Concentration is from low to high
Three types
PROTEIN PUMPS
EXOCYTOSIS
ENDOCYTOSIS
high
low
This is gonna be hard work!!

36. Protein changes shape to move molecules: this requires energy!
Sodium Potassium Pumps
Protein Pumps
Some transport proteins actively use energy from ATP in the cell to drag molecules from area of low concentration to areas of high concentration (working directly against diffusion)
An example of this is the sodium/potassium pump. Here the energy of a phosphate (shown in red) is used to exchange sodium atoms for potassium atoms.
Protein changes shape to move molecules: this requires energy!

37. Active Transport
The sodium-potassium pump is one type of active transport system
Na+ binding stimulates
phosphorylation by ATP.
Na+
Cytoplasmic Na+ binds to
the sodium-potassium pump. 1
K+ is released and Na+
sites are receptive again;
the cycle repeats.
Phosphorylation causes the
protein to change its conformation, expelling Na+ to the outside.
Extracellular K+ binds to the
protein, triggering release of the
Phosphate group. 6
Loss of the phosphate
restores the protein’s
original conformation.
CYTOPLASM
[Na+] low
[K+] high P
ATP
ADP K+
[Na+] high
[K+] low
EXTRACELLULAR
FLUID P
P i

38. Comparison of Passive & Active Transport
Passive transport. Substances diffuse spontaneously
down their concentration gradients, crossing a
membrane with no expenditure of energy by the cell.
The rate of diffusion can be greatly increased by transport
proteins in the membrane.
Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP.
Diffusion. Hydrophobic
molecules and (at a slow
rate) very small uncharged
polar molecules can diffuse through the lipid bilayer.
Facilitated diffusion. Many hydrophilic substances diffuse through membranes with the assistance of transport proteins,
either channel or carrier proteins.
ATP

39. Maintenance of Membrane Potential by Ion Pumps
Is the voltage difference across a membrane
An electrochemical gradient
Is caused by the concentration electrical gradient of ions across a membrane
An electrogenic pump
Is a transport protein that generates the voltage across a membrane

40. Permeability of the Lipid Bilayer
Hydrophobic molecules
Are lipid soluble and can pass through the membrane rapidly
Polar molecules
Do NOT cross the membrane rapidly
Transport proteins
Allow passage of hydrophilic substances across the membrane

41. Co-transport Co-transport
Occurs when active transport of a specific solute indirectly drives the active transport of another solute
Involves transport by a membrane protein
Driven by a concentration gradient

42. Example of Co-transport
Co-transport: active transport driven by a concentration gradient

43. Bulk Transport In Exocytosis
Transport vesicles migrate to the plasma membrane, fuse with it, and release their contents
In Endocytosis
The cell takes in macromolecules by forming new vesicles from the plasma membrane

44. Endocytosis Taking large molecules into a cell Uses energy
Cell membrane in-folds
pinocytosis – substance is mostly water
phagocytosis – substance is a solid
receptor-mediated endocytosis – requires the substance to bind to a membrane-bound receptor

45. Endocytosis In phagocytosis, a cell engulfs a particle by
Wrapping pseudopodia
around it and packaging
it within a membrane-
enclosed sac large
enough to be classified
as a vacuole. The
particle is digested after
the vacuole fuses with a
lysosome containing
hydrolytic enzymes.
PHAGOCYTOSIS
In pinocytosis, the cell
“gulps” droplets of
extracellular fluid into tiny
vesicles. It is not the fluid
itself that is needed by the
cell, but the molecules
dissolved in the droplet.
Because any and all
included solutes are taken
into the cell, pinocytosis is nonspecific in the substances it transports.

46. RECEPTOR-MEDIATED ENDOCYTOSIS
Ligand
Coat protein
Coated
pit
vesicle
A coated pit
and a coated
vesicle formed
during
receptor-
mediated
endocytosis
(TEMs).
Plasma
membrane
Coat
protein
Receptor-mediated endocytosis enables the
cell to acquire bulk quantities of specific
substances, even though those substances
may not be very concentrated in the
extracellular fluid. Embedded in the
membrane are proteins with
specific receptor sites exposed to
the extracellular fluid. The receptor
proteins are usually already clustered
in regions of the membrane called coated
pits, which are lined on their cytoplasmic
side by a fuzzy layer of coat proteins.
Extracellular substances (ligands) bind
to these receptors. When binding occurs,
the coated pit forms a vesicle containing the
ligand molecules. Notice that there are
relatively more bound molecules (purple)
inside the vesicle, other molecules
(green) are also present. After this ingested
material is liberated from the vesicle, the
receptors are recycled to the plasma
membrane by the same vesicle.
Endocytosis

47. Exocytosis Forces large molecules out of cell
Membrane surrounding the material fuses with cell membrane
Cell changes shape – requires energy
Hormones or wastes released from cell

48. Transcytosis endocytosis followed by exocytosis
transports a substance rapidly through a cell
HIV crossing a cell layer