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Title: The Plasma Membrane http:staff'jccc'eduaalarabimemb_related_mov'htm


1
The Plasma Membranehttp//staff.jccc.edu/aalarabi
/memb_related_mov.htm
2
History of the Plasma Membrane
  • 1665 Robert Hooke
  • 1895 Charles Overton - composed of lipids
  • 1900-1920s must be a phospholipid
  • 1925 E. Gorter and G. Grendel - phospholipid
    bilayer
  • 1935 J.R. Danielli and H. Davson proteins also
    part, proposed the Sandwich Model
  • 1950s J.D. Robertson proposed the Unit
    Membrane Model
  • 1972 S.J. Singer and G.L. Nicolson proposed
    Fluid Mosaic Model

3
Plasma Membrane is made of Phospholipids
  • Gorter Grendel
  • Red Blood Cells analyzed
  • Enough for Phospholipid bilayer
  • Polar heads face out and
    Nonpolar tails face in
  • Does not explain why some
    nonlipids are permeable

4
Plasma Membrane Models
  • Sandwich Model
  • (Danielli Davson)
  • 2 layers of globular proteins with phospholipid
    inside to make a layer and then join 2 layers
    together to make a channel for molecules to pass
  • Unit Membrane Model
  • (Robertson)
  • Outer layer of protein with phospholipid bilayer
    inside, believed all cells same composition, does
    not explain how some molecules pass through or
    the use of proteins with nonpolar parts, used
    transmission electron microscopy
  • Fluid Mosaic Model
  • (Singer Nicolson)
  • Phospholipid bilayer with proteins partially or
    fully imbedded, electron micrographs of
    freeze-fractured membrane

5
Which membrane model is correct?
  • 1) Rapidly freeze specimen
  • 2) Use special knife to cut membrane in half
  • 3) Apply a carbon platinum coating to the
    surface
  • 4) Use scanning electron microscope to see the
    surface
  • According to the electron micrograph which
    membrane model is correct?
  • Why?
  • Fluid-Mosaic Model

6
Fluid-Mosaic Model
  • Fluid the plasma membrane is the consistency of
    olive oil at body temperature, due to unsaturated
    phospholipids. (cells differ in the amount of
    unsaturated to saturated fatty acid tails)
  • Most of the lipids and some proteins drift
    laterally on either side. Phospholipids do not
    switch from one layer to the next.
  • Cholesterol affects fluidity at body
    temperature it lessens fluidity by restraining
    the movement of phospholipids, at colder
    temperatures it adds fluidity by not allowing
    phospholipids to pack close together.
  • Mosaic membrane proteins form a collage that
    differs on either side of the membrane and from
    cell to cell (greater than 50 types of proteins),
    proteins span the membrane with hydrophilic
    portions facing out and hydrophobic portions
    facing in. Provides the functions of the membrane

7
Structure of the Plasma Membrane
8
Structure of the Plasma Membrane
  • Phospholipid bilayer
  • Phospholipid
  • Hydrophilic head
  • Hydrophobic tails
  • Cholesterol
  • Proteins
  • Transmembrane/
  • Intrinsic/Integral
  • Peripheral/Extrinsic
  • Cytoskeletal filaments
  • Carbohydrate chain
  • Glycoproteins

9
Proteins of the Plasma Membrane Provide 6
Membrane Functions
  • 1) Transport Proteins
  • 2) Receptor Proteins
  • 3) Enzymatic Proteins
  • 4) Cell Recognition Proteins
  • 5) Attachment Proteins
  • 6) Intercellular Junction
  • Proteins

10
1) Transport Proteins
  • Channel Proteins channel for lipid insoluble
    molecules and ions to pass freely through
  • Carrier Proteins bind to a substance and carry
    it across membrane, change shape in process

11
2) Receptor Proteins
  • Bind to chemical messengers (Ex. hormones)
    which sends a message into the cell causing
    cellular reaction

12
3) Enzymatic Proteins
  • Carry out enzymatic reactions right at the
    membrane when a substrate binds to the active
    site

13
4) Cell Recognition Proteins
  • Glycoproteins (and glycolipids) on
    extracellular surface serve as ID tags (which
    species, type of cell, individual). Carbohydrates
    are short branched chains of less than 15 sugars

14
5) Attachment Proteins
  • Attach to cytoskeleton (to maintain cell shape
    and stabilize proteins) and/or the extracellular
    matrix (integrins connect to both).
  • Extracellular Matrix protein fibers and
    carbohydrates secreted by cells and fills the
    spaces between cells and supports cells in a
    tissue.
  • Extracellular matrix can influence activity
    inside the cell and coordinate the behavior of
    all the cells in a tissue.

15
6) Intercellular Junction Proteins
  • Bind cells together
  • Tight junctions
  • Gap junctions

16
Types of Cell Junctions
  • Tight Junctions
  • Desmosomes
  • Gap Junctions

17
Tight Junctions
  • Transmembrane Proteins of opposite cells attach
    in a tight zipper-like fashion
  • No leakage
  • Ex. Intestine, Kidneys, Epithelium of skin

18
Desmosomes
  • Cytoplasmic plaques of two cells bind with the
    aid of intermediate filaments of keratin
  • Allows for stretching
  • Ex. Stomach, Bladder, Heart

19
Gap Junctions
  • Channel proteins of opposite cells join together
    providing channels for ions, sugars, amino acids,
    and other small molecules to pass.
  • Allows communication between cells.
  • Ex. Heart muscle, animal embryos

20
How do materials move into and out of the cell?
  • Materials must move in and out of the cell
    through the plasma membrane.
  • Some materials move between the phospholipids.
  • Some materials move through the proteins.

21
Plasma Membrane Transport
  • Molecules move across the plasma membrane by

Active Transport
Passive Transport
22
What are three types of passive transport?
Passive Transport
  • Diffusion
  • Facilitated Diffusion
  • Osmosis

ATP energy is not needed to move the molecules
through.
23
Passive Transport 1 Diffusion
  • Molecules can move directly through the
    phospholipids of the plasma membrane
  • This is called

DIFFUSION
24
What is Diffusion?
  • Diffusion is the net movement of molecules from a
    high concentration to a low concentration until
    equally distributed.
  • Diffusion rate is related to temperature,
    pressure, state of matter, size of concentration
    gradient, and surface area of membrane.

http//www.biologycorner.com/resources/diffusion-a
nimated.gif
25
What molecules pass through the plasma membrane
by diffusion?
  • Gases (oxygen, carbon dioxide)
  • Water molecules (rate slow due to polarity)
  • Lipids (steroid hormones)
  • Lipid soluble molecules (hydrocarbons, alcohols,
    some vitamins)
  • Small noncharged molecules (NH3)

26
Why is diffusion important to cells and humans?
  • Cell respiration
  • Alveoli of lungs
  • Capillaries
  • Red Blood Cells
  • Medications time-release capsules

27
Passive Transport 2 Facilitated Diffusion
  • Molecules can move through the plasma membrane
    with the aid of transport proteins
  • This is called

FACILITATED DIFFUSION
28
What is Facilitated Diffusion?
  • Facilitated diffusion is the net movement of
    molecules from a high concentration to a low
    concentration with the aid of channel or carrier
    proteins.

29
What molecules move through the plasma membrane
by facilitated diffusion?
  • Ions
  • (Na, K, Cl-)
  • Sugars (Glucose)
  • Amino Acids
  • Small water soluble molecules
  • Water (faster rate)

30
How do molecules move through the plasma membrane
by facilitated diffusion?
  • Channel and Carrier proteins are specific
  • Channel Proteins allow ions, small solutes, and
    water to pass
  • Carrier Proteins move glucose and amino acids
  • Facilitated diffusion is rate limited, by the
    number of proteins channels/carriers present in
    the membrane.

31
Why is facilitated diffusion important to cells
and humans?
  • Cells obtain food for cell respiration
  • Neurons communicate
  • Small intestine cells transport food to
    bloodstream
  • Muscle cells contract

32
Passive Transport 3 Osmosis
  • Water Molecules can move directly through the
    phospholipids of the plasma membrane
  • This is called

OSMOSIS
33
What is Osmosis?
  • Osmosis is the diffusion of water through a
    semipermeable membrane. Water molecules bound to
    solutes cannot pass due to size, only unbound
    molecules. Free water molecules collide, bump
    into the membrane, and pass through.

34
Osmosis in action
  • What will happen in the U-tube if water freely
    moves through the membrane but glucose can not
    pass?
  • Water moves from side with high concentration of
    water to side with lower concentration of water.
    Movement stops when osmotic pressure equals
    hydrostatic pressure.

35
Why is osmosis important to cells and humans?
  • Cells remove water produced by cell respiration.
  • Large intestine cells transport water to
    bloodstream
  • Kidney cells form urine

36
Osmosis and Tonicity
  • Tonicity refers to the total solute concentration
    of the solution outside the cell.
  • What are the three types of tonicity?
  • Isotonic
  • Hypotonic
  • Hypertonic

37
Isotonic
  • Solutions that have the same concentration of
    solutes as the suspended cell.
  • What will happen to a cell placed in an Isotonic
    solution?
  • The cell will have no net movement of water and
    will stay the same size.
  • Ex. Blood plasma has high concentration of
    albumin molecules to make it isotonic to tissues.

38
Hypotonic
  • Solutions that have a lower solute concentration
    than the suspended cell.
  • What will happen to a cell placed in a Hypotonic
    solution?
  • The cell will gain water and swell.
  • If the cell bursts, then we call this lysis. (Red
    blood cells hemolysis)
  • In plant cells with rigid cell walls, this
    creates turgor pressure.

39
Hypertonic
  • Solutions that have a higher solute concentration
    than a suspended cell.
  • What will happen to a cell placed in a Hypertonic
    solution?
  • The cell will lose water and shrink. (Red blood
    cells crenation)
  • In plant cells, the central vacuole will shrink
    and the plasma membrane will pull away from the
    cell wall causing the cytoplasm to shrink called
    plasmolysis.

40
Review Passive Transport
  • Diffusion O2 moves in and CO2 moves out during
    cell respiration
  • Facilitated Diffusion glucose and amino acids
    enter cell for cell respiration
  • Osmosis cell removal or addition of water

41
Review Tonicity
  • What will happen to a red blood cell in a
    hypertonic solution?
  • What will happen to a red blood cell in an
    isotonic solution?
  • What will happen to a red blood cell in a
    hypotonic solution?

42
What are three types of Active transport?
  • Active Transport
  • Primary
  • Secondary (no ATP)
  • Bulk Transport
  • Exocytosis
  • Endocytosis
  • Phagocytosis
  • Pinocytosis
  • Receptor-Mediated endocytosis

Active Transport
ATP energy is required to move the molecules
through.
43
Active Transport
  • Molecules move from areas of low concentration to
    areas of high concentration with the aid of ATP
    energy.
  • Requires protein carriers called Pumps.

44
The Importance of Active Transport
  • Bring in essential molecules ions, amino acids,
    glucose, nucleotides
  • Rid cell of unwanted molecules (Ex. sodium from
    urine in kidneys)
  • Maintain internal conditions different from the
    environment
  • Regulate the volume of cells by controlling
    osmotic potential
  • Control cellular pH
  • Re-establish concentration gradients to run
    facilitated diffusion. (Ex. Sodium-Potassium
    pump and Proton pumps)

45
The Sodium-Potassium Pump
  • 3 Sodium ions move out of the cell and then 2
    Potassium ions move into the cell.
  • Driven by the splitting of ATP to provide energy
    and conformational change to proteins by adding
    and then taking away a phosphate group.
  • Used to establish an electrochemical gradient
    across neuron cell membranes.

http//www.biologie.uni-hamburg.de/b-online/librar
y/biology107/bi107vc/fa99/terry/images/ATPpumA.gif
46
Secondary Active TransportVia Facilitated
Diffusion of Na
  • Counter Transport the transport of two
    substances at the same time in opposite
    directions, without ATP. Protein carriers are
    called Antiports.
  • Co-transport the transport of two substances at
    the same time in the same direction, without ATP.
    Protein carriers are called Symports.
  • Gated Channels receptors combined with channel
    proteins. When a chemical messenger binds to a
    receptor, a gate opens to allow ions to flow
    through the channel.

47
Bulk Transport Exocytosis
  • Movement of large molecules bound in vesicles out
    of the cell with the aid of ATP energy. Vesicle
    fuses with the plasma membrane to eject
    macromolecules.
  • Ex. Proteins, polysaccharides, polynucleotides,
    whole cells, hormones, mucus, neurotransmitters,
    waste

48
Bulk Transport Endocytosis
  • Movement of large molecules into the cell by
    engulfing them in vesicles, using ATP energy.
  • Three types of Endocytosis
  • Phagocytosis
  • Pinocytosis
  • Receptor-mediated endocytosis

49
Phagocytosis
  • Cellular Eating engulfing large molecules,
    whole cells, bacteria
  • Ex. Macrophages ingesting bacteria or worn out
    red blood cells.
  • Ex. Unicellular organisms engulfing food
    particles.

50
Pinocytosis
  • Cellular Drinking engulfing liquids and small
    molecules dissolved in liquids unspecific what
    enters.
  • Ex. Intestinal cells, Kidney cells, Plant root
    cells

51
Receptor-Mediated Endocytosis
  • Movement of very specific molecules into the cell
    with the use of vesicles coated with the protein
    clathrin.
  • Coated pits are specific locations coated with
    clathrin and receptors. When specific molecules
    (ligands) bind to the receptors, then this
    stimulates the molecules to be engulfed into a
    coated vesicle.
  • Ex. Uptake of cholesterol (LDL) by animal cells

52
Review Types of Endocytosis
  • What is phagocytosis?
  • What is pinocytosis?
  • What is receptor-mediated endocytosis?
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