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The Plasma Membrane Gateway to the Cell Functions of Plasma

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Title: The Plasma Membrane Gateway to the Cell Functions of Plasma


1
The Plasma Membrane
Gateway to the Cell
2
Functions of Plasma Membrane
  • Protective barrier
  • Regulate transport in out of cell
    (selectively permeable)
  • Allow cell recognition
  • Provide anchoring sites for filaments of
    cytoskeleton

3
Functions of Plasma Membrane
  • Provide a binding site for enzymes
  • Interlocking surfaces bind cells together
    (junctions)
  • Contains the cytoplasm (fluid in cell)

4
Membrane Components
Proteins(peripheral and integral)
Phospholipids
Cholesterol
Carbohydrates
5
Composition of Membranes
  • Water
  • Lipid
  • many different types
  • Carbohydrate
  • on lipids and proteins
  • Simple to very complex
  • Protein
  • Many different types

6
Membrane lipids
  • Glycerophospholipids (phosphoglycerides)
  • Sphingolipids
  • sphingophospholipids
  • sphingoglycolipids
  • Cholesterol

7
Phosphoglycerides
  • Simplest glycerophos-pholipids is phosphatidic
    acid
  • Different molecules are added onto the phosphate
    group to make the other phosphoglycerides

Phosphate
Hydrophillic region
Glycerol
Ester linkage
Fatty acids (saturated and unsaturated)
Hydrophobic region
8
Phosphoglycerides (continued)
  • Polar groups attached to phosphatidic acid
  • Inositol Serine
  • Glycerol Choline
  • ethanolamine

9
Phosphoglycerides (continued)
Hydrophilic region
Hydrophobic region
10
Sphingolipids
  • Based on Sphingosine
  • Sphingosine convertrd to cermide with the
    condensation with a long chain fatty acid
  • Sphingomyelin derived from cerimides

11
Glycolipids
  • Carbohydrate attached to lipid by way of beta
    glycosidic bond instead of phosphate ester
  • Usually sphingosine derivative (by way of
    ceramide)
  • Cerebroside
  • Galactocerebroside
  • Glucocerobroside
  • Ganglioside
  • More complex carbohydrate structure
  • Contains sialic acid (an acidic carbohydrate)

12
Cholesterol
13
Membrane proteins
  • Varies from membrane to membrane
  • Amount
  • Diversity
  • Classes of membrane proteins
  • Integral
  • Peripheral
  • Lipid anchored

14
Proteins Are Critical to Membrane Function
15
Membrane protein function
  • transport
  • enzymatic activity
  • signal transduction
  • intercellular joining
  • cell-cell recognition
  • ECM attachment

16
Fluid mosaic model
FLUID MOSAIC MODEL
  • FLUID- because individual phospholipids and
    proteins can move around freely within the layer,
    like its a liquid.
  • MOSAIC- because of the pattern produced by the
    scattered protein molecules when the membrane is
    viewed from above.

17
Movement of lipids in membrane
  • Rotation
  • Flexing of fatty acyl chain
  • Lateral diffusion
  • Transverse (flip-flop) from one side to
    other

Frequently
Infrequently
18
RBC plasma membrane
19
Semipermeable Membrane
Small molecules and larger hydrophobic molecules
move through easily. e.g. O2, CO2, H2O Ions,
hydrophilic molecules larger than water, and
large molecules such as proteins do not move
through the membrane on their own.
20
Types of Transport Across Cell Membranes
21
Simple Diffusion
  • Diffusion is a PASSIVE process which means no
    energy is used to make the molecules move
  • Molecules move from area of HIGH to LOW
    concentration

22
Diffusion of H2O Across A Membrane
High H2O potentialLow solute concentration
Low H2O potentialHigh solute concentration
23
Isotonic Solution
Hypotonic Solution
Hypertonic Solution
NO NET MOVEMENT OF H2O (equal amounts entering
leaving)
CYTOLYSIS
PLASMOLYSIS
24
Three Forms of Transport Across the Membrane
25
Passive Transport
  • Simple Diffusion
  • Doesnt require energy
  • Moves high to low concentration
  • Example Oxygen or water diffusing into a cell
    and carbon dioxide diffusing out.

26
Passive Transport
  • Facilitated diffusion
  • Doesnt require energy
  • Uses transport proteins to move high to low
    concentration
  • Examples Glucose or amino acids moving from
    blood into a cell.

27
Types of Transport Proteins
  • Channel proteins are embedded in the cell
    membrane have a pore for materials to cross
  • Carrier proteins can change shape to move
    material from one side of the membrane to the
    other

28
Channels that are proteins
  • Cellular channels usually consist of large
    protein complexes with multiple transmembrane
    a-helices.
  • Control of channel gating is a form of allosteric
    regulation. Conformational changes associated
    with channel opening may be regulated by
  • Voltage
  • Binding of a ligand (a regulatory molecule)
  • Membrane stretch (e.g., via link to cytoskeleton)

29
Ion Channels
  • Channels cycle between open closed
    conformations.
  • When open, a channel provides a continuous
    pathway through the bilayer, allowing flux of
    many ions.
  • Gramicidin is an example of a channel.

30
ionophores
  • Antibiotics of bacterial origin facilitate the
    movement of ions across membranes called
    ionophores
  • Ionophores
  • carriers and channels.
  • valinomycin (a carrier)
  • gramicidin (a channel).

31
  • Valinomycin is a carrier for K.
  • It is a circular molecule, made up of 3 repeats
    of the sequence shown above.

32
  • Valinomycin is highly selective for K relative
    to Na.
  • The ionophore valinomycin is a uniport carrier.

33
Gramicidin A
34
Gating (opening closing) of a gramicidin
channel is thought to involve reversible
dimerization.
  • An open channel forms when two gramicidin
    molecules join end to end to span the membrane.

35
Classes of carrier proteins
  • Uniport (facilitated diffusion) carriers mediate
    transport of a single solute.
  • An example is the GLUT1 glucose carrier.

36
Symport (cotransport) carriers bind two
dissimilar solutes transport them together
across a membrane. Transport of the two solutes
is obligatorily coupled.
  • A gradient of one substrate, usually an ion, may
    drive uphill (against the gradient) transport of
    a co-substrate.
  • It is sometimes referred to as secondary active
    transport.
  • E.g ? glucose-Na symport, in plasma membranes
  • of some epithelial cells

37
Facilitated Diffusion
  • Some Carrier proteins do not extend through the
    membrane.
  • They bond and drag molecules through the lipid
    bilayer and release them on the opposite side

38
Carrier Proteins
  • Other carrier proteins change shape to move
    materials across the cell membrane

39
Active Transport
  • Requires energy or ATP
  • Moves materials from LOW to HIGH concentration
  • AGAINST concentration gradient

40
Active transport
  • Examples Pumping Na (sodium ions) out and K
    (potassium ions) in against strong concentration
    gradients.
  • Called Na-K Pump

41
Sodium-Potassium Pump
3 Na pumped in for every 2 K pumped out
creates a membrane potential
42
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43
Transport of macromolecules
  • Exocytosis secretion of macromolecules by the
    fusion of vesicles with the plasma membrane
  • Endocytosis import of macromolecules by forming
    new vesicles with the plasma membrane
  • phagocytosis
  • pinocytosis
  • receptor-mediated endocytosis (ligands)

44
Exocytosis
- moving things out.
Molecules are moved out of the cell by vesicles
that fuse with the plasma membrane.
This is how many hormones are secreted and how
nerve cells communicate with one another.
45
Pinocytosis
Most common form of endocytosis.
Takes in dissolved molecules as a vesicle.
46
Pinocytosis
  • Cell forms an invagination
  • Materials dissolve in water to be brought into
    cell
  • Called Cell Drinking

47
Receptor-Mediated Endocytosis
Some integral proteins have receptors on their
surface to recognize take in hormones,
cholesterol, etc.
48
Endocytosis Phagocytosis
Used to engulf large particles such as food,
bacteria, etc. into vesicles
Called Cell Eating
49
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50
Exocytosis The opposite of endocytosis is
exocytosis. Large molecules that are manufactured
in the cell are released through the cell
membrane.
Inside Cell
Cell environment
51
Specialized Transport
  • Transport proteins
  • Facilitated diffusion passage of molecules and
    ions with transport proteins across a membrane
    down the concentration gradient
  • Active transport movement of a substance against
    its concentration gradient with the help of
    cellular energy

52
Movement of compounds across membranes
  • Passive diffusion
  • Affected by
  • Size of molecule
  • Composition of molecule
  • Temperature
  • Lipid composition
  • Not mediated by proteins
  • Molecules move down a concentration gradient
  • Unimportant for most polar molecules

53
Pore (a.k.a. Channel) Proteins
  • Actively regulated to control flow across
    membranes
  • Very specific for molecules able to pass through
    channel
  • Rate of movement of solutes is usually greater
    than for transporters

Fig. 9.30
54
  • Barriers, Pores, pumps and gates.mht
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