1. Membrane Structure and Function

2. What You Must Know: Why membranes are selectively permeable.
The role of phospholipids, proteins, and carbohydrates in membranes.
How water will move if a cell is placed in an isotonic, hypertonic, or hypotonic solution.
How electrochemical gradients are formed.

3. Cell Membrane Plasma membrane is selectively permeable
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Cell Membrane
Plasma membrane is selectively permeable
Allows some substances to cross more easily than others
Containment and separation
Material Exchange – selectively permeable
Information Detection – receptors – selectively bind molecules, hormones or some chemicals from outside the body
Identification – glycolipids and glycoproteins
Attachment and reinforcement
Movement – pushing or pulling against the cytoskeleton
Metabolism- some metabolic enzymes attached to or embedded in the membrane
Fluid Mosaic Model
Fluid: membrane held together by weak interactions
Mosaic: phospholipids, proteins, carbs
Structure:
Lipid to protein ratio 1:1 by mass
7.5 nm to 10 nm thick
Lipid bilayer – can form spontaneously
Most abundant – phospholipid
Cholesterol (not in bacteria)
Glycolipids
All are amphipathic – one polar end and one nonpolar end

4. Early membrane model (1935) Davson/Danielli – Sandwich model
phospholipid bilayer between 2 protein layers
Problems: varying chemical composition of membrane, hydrophobic protein parts

5. The freeze-fracture method: revealed the structure of membrane’s interior

6. Fluid Mosaic Model

8. Phospholipids Bilayer Amphipathic = hydrophilic head, hydrophobic tail
Hydrophobic barrier: keeps hydrophilic molecules out

9. Membrane fluidity - must be fluid to work properly (salad oil)
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Low temps: phospholipids w/unsaturated tails (kinks prevent close packing). Rich in unsaturated fatty acids are more fluid than those rich with saturated fatty acids.
Cholesterol resists changes by:
limit fluidity at high temps
hinder close packing at low temps
Adaptations: bacteria in hot springs (unusual lipids); winter wheat ( unsaturated phospholipids)
Membranes must be fluid to work properly; they are usually about as fluid as salad oil
As temperatures cool, membranes switch from a fluid state to a solid state
The temperature at which a membrane solidifies depends on the types of lipids
Fluidity depends on which lipids are present
Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids
Self-sealing
Can fuse with other membranes

10. The steroid cholesterol has different effects on membrane fluidity at different temperatures
At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids
At cool temperatures, it maintains fluidity by preventing tight packing
Cells in culture which cannot synthesize cholesterol break and die

11. Plasma membrane of mammalian red blood cell

12. Membrane Proteins Integral Proteins Peripheral Proteins
Embedded in membrane
Determined by freeze fracture
Transmembrane with hydrophilic heads/tails and hydrophobic middles
Extracellular or cytoplasmic sides of membrane
NOT embedded
Held in place by the cytoskeleton or ECM
Provides stronger framework

13. Integral & Peripheral proteins
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Other are immobile because they are attached to structures
Fluid in that molecules continuously removed and replaced with newly made molecules
Integral & Peripheral proteins

14. Transmembrane protein structure
Hydrophobic interior
Hydrophilic ends

15. Some functions of membrane proteins

16. Functions of membrane proteins

17. The Role of Membrane Carbohydrates in Cell-Cell Recognition
Cells recognize each other by binding to surface molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane
Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins)
Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual

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CD4 is a glycoprotein
CCR5 is a protein. Most common form of the mutation results in an integral protein that does not jut out of the cell membrane. 20% of Caucasians are heterozygotes and have a reduced chance of infection and decreased severity of infection. 1% of Caucasians are virtually immune to HIV by being homozygous for this mutation.

19. Carbohydrates Function: cell-cell recognition; developing organisms
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Carbohydrates
Function: cell-cell recognition; developing organisms
Glycolipids, glycoproteins
Eg. blood transfusions are type-specific
Blood type antigens are glycoproteins.

20. Selective Permeability
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Selective Permeability
Small molecules (polar or nonpolar) cross easily (hydrocarbons, hydrophobic molecules, CO2, O2)
Hydrophobic core prevents passage of ions, large polar molecules .
Nonpolar molecules like hydrocarbons can dissolve and move through.
Transport proteins allow hydrophilic substances to move.
Channel proteins (aquaporins) have a hydrophilic channel to allow certain molecules or ions to use a tunnel to cross
Carriers bind to molecules and change shape to shuttle molecules across the membrane. Specific for the substances they move.
Physical processes
Osmosis & Diffusion
Carrier mediated
Channel proteins & Carrier proteins

21. Passive Transport NO ENERGY (ATP) needed!
Diffusion down concentration gradient (high  low concentration)
Eg. hydrocarbons, CO2, O2, H2O

22. Diffusion

23. Osmosis: diffusion of H2O

24. Osmotic pressure Turgor pressure
Concentration of dissolved substances in a solution
Isotonic: equal solute concentration
Hypertonic: loses water in plasmolysis
Hypotonic: gains water and swells
Turgor pressure
Internal hydrostatic pressure in walled cells

25. External environments can be hypotonic, isotonic or hypertonic to internal environments of cell

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28. Facilitated Diffusion
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Facilitated Diffusion
Transport proteins (channel or carrier proteins) help hydrophilic substances cross
Two ways:
Provide hydrophilic channel
Loosely bind/carry molecule across
Eg. ions, polar molecules (H2O, glucose)

29. Glucose Transport Protein (carrier protein)

30. Aquaporin: channel protein that allows passage of H2O
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Aquaporin: channel protein that allows passage of H2O
Found in animals, plants and some bacteria

32. Active Transport Requires ENERGY (ATP)
Proteins transport substances such as ions or large molecules against concentration gradient (low  high conc.)
Eg. Na+/K+ pump, proton pump

33. Electrogenic Pumps: generate voltage across membrane
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Electrogenic Pumps: generate voltage across membrane
Na+/K+ Pump
Proton Pump
Pump Na+ out, K+ into cell
Nerve transmission
Push protons (H+) across membrane
Eg. mitochondria (ATP production)
Na/K pump utilizes about 20% of ATPs produced by humans daily.

34. Eg. sucrose-H+ cotransporter (sugar-loading in plants)
Cotransport: membrane protein enables “downhill” diffusion of one solute to drive “uphill” transport of other
Eg. sucrose-H+ cotransporter (sugar-loading in plants)
Go to 1 min

35. Osmoregulation Control solute & water balance
Contractile vacuole: “bilge pump” forces out fresh water as it enters by osmosis
Eg. paramecium caudatum – freshwater protest

36. Bulk Transport Transport of proteins, polysaccharides, large molecules
Endocytosis: take in macromolecules, form new vesicles
Exocytosis: vesicles fuse with cell membrane, expel contents

37. Types of Endocytosis Phagocytosis: “cellular eating” - solids
Pinocytosis:
“cellular drinking” - fluids
Receptor-Mediated Endocytosis:
Ligands bind to specific receptors on cell surface

41. Membrane Transport in Cells Symport, Antiport, Co transport 3D Animation-Eplanation

42. Passive vs. Active Transport
Little or no Energy
High  low concentrations
DOWN the concentration gradient
eg. diffusion, osmosis, facilitated diffusion (w/transport protein)
Requires Energy (ATP)
Low  high concentrations
AGAINST the concentration gradient
eg. pumps, exo/endocytosis

44. Understanding Water Potential

45. Water potential equation:
Water potential (ψ): H2O moves from high ψ  low ψ potential
Water potential equation:
ψ = ψS + ψP
Water potential (ψ) = free energy of water
Solute potential (ψS) = solute concentration (osmotic potential)
Pressure potential (ψP) = physical pressure on solution; turgor pressure (plants)
Pure water: ψP = 0 MPa
Plant cells: ψP = 1 MPa

46. Calculating Solute Potential (ψS)
ψS = -iCRT
i = ionization constant (# particles made in water)
C = molar concentration
R = pressure constant ( liter bars/mole-K)
T = temperature in K ( C)
The addition of solute to water lowers the solute potential (more negative) and therefore decreases the water potential.

47. Where will WATER move? From an area of:
higher ψ  lower ψ (more negative ψ)
low solute concentration  high solute concentration
high pressure  low pressure

48. Which chamber has a lower water potential?
Which chamber has a lower solute potential?
In which direction will osmosis occur?
If one chamber has a Ψ of kPa, and the other kPa, which is the chamber that has the higher Ψ?

50. Sample Problem
Calculate the solute potential of a 0.1M NaCl solution at 25°C.
If the concentration of NaCl inside the plant cell is 0.15M, which way will the water diffuse if the cell is placed in the 0.1M NaCl solution?

51. Cell Communication

52. Cells communicate by cell signaling Signaling molecules include
Neurotransmitters
Hormones
Regulatory molecules

53. Cell signaling involves
Synthesis and release of signaling molecule
Transport to target cells
Reception by target cells
Signal transduction
Response by the cell
Termination of signal

54. Signal transduction

55. Attachments Between Cells
Multicellular cells must be held together – interactions between the cell and environment
Cell walls made inside the cell and released to the outside by exocytosis
Cellulose fibers made outside the cell by enzymes that are part of the plasma membrane
Animal body makes protein fibers and polysaccharide chains and moved out by exocytosis.
Cells attach to ECM by some of the membranes proteins

56. Cells in close contact often develop intercellular junctions
Anchoring junctions
Desmosomes
Adhering junctions
Tight junctions
Gap junctions
Plasmodesmata

57. Tight junction or "occluding junction" (zonula occludens)
Tight junction or "occluding junction" (zonula occludens). This is shown as the top junction in the drawing. At this site, membrane glycoproteins and associated "glue" bind the cells together like double-sided "strapping tape"

58. Tight Junctions Animal Cells
Seal the outer edges of the cell’s membranes together.
Forms such a tight barrier that even the smallest molecules cannot pass through.
Whatever enters the body must pass across the cell membrane.
Looks like 2 pieces of fabric quilted by protein stitches
Divides the pm into 2 zones

59. Tight
junctions

61. Beyond tight junctions- Adherens Junctions and further out desmosomes.
Hold epithelial cells together in the presence of mechanical action.
Membranes of neighboring cells are held together proteins that extend through the membrane and link up in the intercellular space.
Cytoplasmic side is a dense plate of proteins which link to structural protein filaments in the cytoplasm.
Adherens junctions attach to bundles of actin filaments
Desmosomes to intermediate filaments

62. Desmosomes

63. Communication Between Cells
Chemical messengers which attach to receptors.
Gap Junctions – direct cytoplasm to cytoplasm connections.
Permits ions and small molecules to pass directly through.
Pipe-like channel proteins
Each pipe contains a ring of 6 cylindrical proteins that stick through the pm and butts up against another pipe in an adjacent cell.

64. Gap Junctions Electrical coupling in heart coordinates the heart beat
Muscle contractions
Embryonic development

65. Gap junctions allows communication between cells
Gap junctions allows communication between cells. Small molecules or ions can pass through. The freeze-fracture /freeze etch image shows the internal view of the gap junction on the left. The proteins look like little donuts which reflects the fact that they are actually a channel. These proteins are "connexon" molecules. The side facing the cytoplasm (called the P face) is shown in the center panel. The region looks like aggregated lumps. Finally, the typical electron microscopic view is seen in the third panel. This shows a thin line between the two plasma membranes indicating a "gap junction".

66. There are several ways to prove the cells are communicating by gap junctions. First, one can identify the connexon molecules by immunocytochemical labeling. Second, one can identify the actual junctional complex with freeze-fracture/freeze etch. To see if they are functional, however, one needs to inject one cell with a dye and watch to see if it is transferred to another cell. This cartoon diagrams a view of a gap junction showing molecules that can freely pass. Ions pass and in this way the cells can be electrically coupled together. Other small molecules that pass through include cyclic AMP (a second messenger) and the dye marker fluorescein. This last compound enables the scientist to study transport throught the gap junction.

67. Gap junctions

68. Plasmodesmata
Plants
Cytoplasmic bridges which pass through openings in the cell wall
In effect makes plant cells continuous with one another

69. Plasmodesmata

70. Microvilli
20 fold expansion of surface area.
Finger-like projections

71. Microvillus It is covered by a plasma membrane and encloses cytoplasm and microfilaments. Typically microvilli are found in absorptive cells, whenever there is a need for an increase in surface area.

72. This figure shows a scanning electron micrograph of the luminal surface of the oviduct. It illustrates one difference between cilia and microvilli. The longer projections are cilia and the shorter projections are microvilli.