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Cell Membranes Chapt 5

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Every cell is encircled by a membrane and most cells contain ... http://www.the-aps.org/education/lot/cell/HotT.htm. Transport Proteins. Facilitated Diffusion ... – PowerPoint PPT presentation

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Title: Cell Membranes Chapt 5


1
Cell Membranes Chapt 5
www.cellsalive.com/
2
The Cell Membrane
3
Cell Membrane
At Very High Magnification in color
4
Membrane Structure
http//users.rcn.com/jkimball.ma.ultranet/BiologyP
ages/C/CellMembranes.html
5
Cell Membrane
  • Every cell is encircled by a membrane and most
    cells contain an extensive intracellular membrane
    system. Membranes fence off the cell's interior
    from its surroundings. Membranes let in water,
    certain ions and substrates and they excrete
    waste substances. They act to protect the cell.
  • Without a membrane the cell contents would
    diffuse into the surroundings, information
    containing molecules would be lost and many
    metabolic pathways would cease to work
  • The cell would die!

www.biologie.uni-hamburg.de/b-online/e22/22.htm
6
Cell Membranes
  • Surround all cells
  • Fluid-like composition…like soap bubbles
  • Composed of
  • Lipids in a bilayer
  • Proteins embedded in lipid layer (called
    transmembrane proteins)
  • And, Proteins floating within the lipid sea
    (called integral proteins)
  • And Proteins associated outside the lipid bilayer
    (peripheral).

7
Membrane Lipids
  • Composed largely of phospholipids
  • Phospholipids composed of….glycerol and two fatty
    acids PO4 group
  • P-Lipids are polar molecules…

P-Lipids are represented like this
Text pg. 81
8
Membrane Lipids form a Bilayer
Outside layer
Inside Layer
9
Quiz
  • If Phospholipids are polar, which end seeks out
    water and which avoids water?

10
Phospholipid Molecule Model

phosphate (hydrophilic)
glycerol fatty acids (hydrophobic)
11
Membrane Proteins
  • Integral embedded within bilayer
  • Peripheral reside outside hydrophobic region of
    lipids

Text pg. 80
12
Membrane Proteins
Text pg 80
13
Integral membrane proteins
14
Peripheral membrane proteins
Integral
15
Membrane Models
  • Fluid Mosaic Model - lipids arranged in bilayer
    with proteins embedded or associated with the
    lipids.

16
Fluid Mosaic Membrane
Text pg 80
17
Evidence for the Fluid Mosaic Model (Cell Fusion)
18
More Evidence for the Fluid Mosaic Model
19
Membrane Functions
  • allows for different conditions between inside
    and outside of cell
  • subdivides cell into compartments with different
    internal conditions
  • allows release of substances from cell via
    vesicle fusion with outer membrane

http//www.emc.maricopa.edu/faculty/farabee/BIOBK/
exocyt.gif
20
Membrane Permeability
  • Biological membranes are physical barriers..but
    which allow small uncharged molecules to pass…
  • And, lipid soluble molecules pass through
  • Big molecules and charged ones do NOT pass
    through

21
How to get other molecules across membranes??
  • There are two ways that the molecules typically
    move through the membrane
  • passive transport and active transport
  • Active transport requires that the cell use
    energy that it has obtained from food to move the
    molecules (or larger particles) through the cell
    membrane.
  • Passive transport does not require such an energy
    expenditure, and occurs spontaneously.

22
Membrane Transport Mechanisms I. Passive Transport
  • Diffusion- simple movement from regions of high
    concentration to low concentration
  • Osmosis- diffusion of water across a
    semi-permeable membrane
  • Facilitated diffusion- protein transporters which
    assist in diffusion

Text pg 89
23
Membrane Transport Mechanisms II. Active Transport
  • Active transport- proteins which transport
    against concentration gradient.
  • Requires energy input

Text pg 89
24
Diffusion
  • Movement generated by random motion of particles.
    Caused by internal thermal energy.
  • Movement always from region of high free
    energy(high concentration) to regions of low free
    energy (low conc.)

Text pg 86
25
Osmosis
  • Movement of water across a semi-permeable
    barrier.
  • Example Salt in water, cell membrane is
    barrier. Salt will NOT move across membrane,
    water will.

Text pg 87
26
Osmosis in Hypertonic medium



27
  • Hypertonic solutions- shrink cells

28
Osmosis in Hypotonic medium


29
  • Hypotonic solutions- swell cells

30
Endocytosis
  • Transports macromolecules and large particles
    into the cell.
  • Part of the membrane engulfs the particle and
    folds inward to bud off.
  • Fig. 5.16

31
Endocytosis
32
Putting Out the Garbage
  • Vesicles (lysosomes, other secretory vesicles)
    can fuse with the membrane and open up the the
    outside…

33
Exocytosis (Cellular Secretion)
34
Movies!
35
Membrane Permeability
  • 1) lipid soluble solutes go through faster
  • smaller molecules go faster
  • 1) uncharged weakly charged go faster
  • 2) Channels or pores may also exist in membrane
    to allow transport

1
2
36
Cellular Membranes
  • REVIEW
  • Importance of Membranes
  • Membrane Structure
  • Proteins
  • Fluid Mosaic model
  • Permeability
  • Types of Transport
  • Passive and Active

37
Types of Protein Transporters Ion Channels
  • work by facilitated diffusion No E!
  • deal with small molecules... ions
  • open pores are gated- Can change shape.
  • How?
  • How much gets in?
  • important in cell communication

38
Ion Channels
  • Work fast No conform. changes needed
  • Not simple pores in membrane
  • specific to different ions (Na, K, Ca...)
  • gates control opening
  • Toxins, drugs may affect channels
  • saxitoxin, tetrodotoxin
  • cystic fibrosis

39
Toxins…how they work
40
Cystic Fibrosis
  • Fatal genetic disorder
  • Mucus build-up results in lung and liver failure
  • Patients die between 4 and 30 yrs.
  • Single gene defect
  • 1 in 25 Caucasians carry 1 bad gene copy
  • 1 in 2500 kids has it in Canada
  • Testing

41
CF Cont…
  • Proteins for diffusion of salt into the airways
    don't work.     
  • Less salt in the airways means less water in
    the airways. 
  • Less water in the airways means mucus layer is
    very             sticky (viscous).
  • Sticky mucus cannot be easily moved to clear
    particles from the lungs.    
  • Sticky mucus traps bacteria and causes more
    lung infections.

http//www.the-aps.org/education/lot/cell/HotT.htm
42
Transport Proteins Facilitated Diffusion
Active Transport
  • move solutes faster across membrane
  • highly specific to specific solutes
  • can be inhibited by drugs

43
Types of Protein Transporters
  • A. Facilitated Diffusion
  • Assist in diffusion process.
  • Solutes go from High conc to Low conc.
  • Examples Glucose transporters

http//bio.winona.msus.edu/berg/ANIMTNS/FacDiff.ht
m
Text pg 88
44
Facilitated Diffusion The Glucose Transporters
  • Transport of glucose into cells mediated by
    proteins in the GLUT (GLUcose Transport) family
    of transporters. There are 7 different, but
    related, proteins. But, only four (GLUT1-4) are
    known to be involved in glucose transport.
  • All GLUT proteins share a set of similar
    structural features and are all about 500 amino
    acids in length (giving them a predicted
    molecular weight of about 55,000 Daltons)
  • Glucose uptake shows saturation and glucose
    uptake can be inhibited by drugs
  • A classic Membrane Transport protein

45
Glucose Transporter Characteristics
  • integral protein spans the membrane
  • 12 alpha helices woven into membrane
  • 55,000 mol. wt.
  • Text pg. 88

46
Glucose Transporter How it works..
  • glucose binds to outside of transporter
    (exterior side with higher glucose conc.)
  • glucose binding causes a conform. change in
    protein
  • glucose drops off inside cell
  • protein reassumes 1st configuration

47
Types of Protein Transporters Active Transport
  • carrier proteins
  • go against the concentration gradients Low to
    High
  • require Energy to function (ATP, PEP, light
    energy, electron transport)

48
Membrane Transport Active transport
  • Movement from region of low free energy(low
    concentration) to regions of high free energy
    (high conc.)
  • Requires energy input

49
Active Transport Sodium-Potassium Pump
Na low
Na high
K low
K high
Balance of the two ions goes hand-in-hand
ATP required for maintenance of the pump
50
The sodium/potassium pump
  • All nerve and muscle cells have a high internal
    potassium ion concentration and a low internal
    sodium ion concentration. Ki166 mM Ko5 mM
    Nai18 mM Nao135 mM.
  • Early on, it was thought that the nerve and
    muscle membranes were relatively impermeable to
    these ions and that the difference in ionic
    concentration was set up in early development of
    the cells. The membrane then became impermeable.
  • The later availability and use of radioactive Na
    and K ions showed that this was not true and that
    there was a metabolic pump that pumped Na out of
    the cell and K in the ratio being 3 Na pumped
    out of the cell for every 2 K pumped into the
    cell.

51
Is a Protein Involved ?
  • Experiments showed a dependency of both Na and K
    ions for pump to work
  • Pump was inhibited by ouabain (a drug)
  • 1957 an ATPase enzyme was found to be associated
    with Na/K pumping
  • Studies showed this ATPase capable of pumping
    Na/K ions
  • Text pg 90

52
Sodium/Potassium ATPase Protein
  • Made of 2 large and 2 small subunits
  • 2 large units span membrane
  • inside region contains ATP binding site
  • inside binding sites for Na
  • outside binding site for K
  • How does it work??

53
Sodium-Potassium Pump
  • Text pg. 90

http//www.cat.cc.md.us/courses/bio141/lecguide/un
it1/eustruct/sppump.html
54
Na-K Pump Model Part I
  • 3 Na bind to inner region of protein
  • Na binding triggers phosphorylation of protein.
    ATP ADP Pi
  • Phosphorylation causes conformation change and
    Na binding site faces outside
  • 3 Na released to outside

55
Na-K Active Pump Part II
  • 2 K ions on outside are able to bind
  • K binding causes dephosphorylation and new
    conformation change
  • 2K ions exposed to inside and released
  • Cyclic process uses ATP energy to drive Na K
    ion transport against conc. Gradient

56
Cell Junctions
  • Allow specific types of cells to stay together to
    perform special jobs
  • Layers of these types of cells…
  • Line body cavities
  • Cover body surfaces

57
3 Types of Cell Junctions
  • Tight Junctions
  • Desmosomes
  • Gap Junctions

58
Tight Junctions
  • Seal tissues and prevent leaks
  • Link epithelial cells together
  • Prevent things from moving through the
    intercellular space
  • Restrict migration of proteins and phospholipids

59
Desmosomes
  • Like spot welds!
  • Dense plaques with fibers attached- Anchor cells
    together from one side to the other.
  • These cells withstand lots of abuse!

60
Gap Junctions
  • Cell to cell communication.
  • Protein channels- (what type?) connexons
  • Plasmodesmata

61
Put Them All Together…
62
Membrane Structure
  • The cell is highly organized with many
    functional units or organelles inside. Most of
    these units are limited by one or more membranes.
    To perform the functions of an organelle, the
    membrane is specialized in that it contains
    specific proteins and lipid components that
    enable it to perform its unique roles. 
  • In essence membranes are essential for the
    integrity and function of the cell.
  • Membrane functions
  • be protective
  • regulate transport in and out of cell or
    organelle
  • allow selective receptivity and signal
    transduction by providing transmembrane receptors
    that bind signaling molecules
  • allow cell recognition
  • provide anchoring sites for cytoskeletal
    components. This allows the cell to maintain its
    shape and perhaps move to distant sites.
  • provide a stable site for the binding and
    catalysis of enzymes.
  • regulate the fusion of the membrane with other
    membranes in the cell via specialized junctions
  • provide a passageway across the membrane for
    certain molecules
  • allow directed cell or organelle motility
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