Transport of molecules by grapieroo6

VIEWS: 292 PAGES: 47

									Plasma Membrane and Transport of
          molecules
   How do things get in and out of the
                 cell?
     I. The Plasma Membrane
A. The fluid mosaic model describes the
  structure of the plasma membrane. Different
  kinds of cell membrane models have been
  proposed, and one of the most useful is the
  Fluid-mosaic model.
B. In this model the membrane is seen as a
  bilayer of phospholipids.
Figure 5.13 -- Phospholipids spontaneously form Lipid Bilayers
                                           Micelles are commonly
                                           formed by Fatty Acids
                                           (soaps) and Detergents;
                                           these have a single
                                           hydrophobic tail giving
                                           them a more conical shape




                                           Phospholipids have two
                                           hydrophobic tails and a
                                           more cylindrical shape that
                                           fits better in a bilayer.
                                       Figure 8.3

         Rotation




                                                        Unsaturated hydrocarbon tails have
                                                        kinks that inhibit molecules from
                                                        packing together enhancing membrane
                                                        fluidity




Cholesterol reduces membrane fluidity at moderate
temperatures and inhibits solidification at low temperatures
C. Structure of the plasma membrane
  1. The plasma membrane is a liquid (fat molecules)
  bi-layer structure. Each layer is consisted of lipid
  molecules with protein molecules embedded in the
  bi-layers.
2. Membrane lipids are phospholipids with
  polar, water-soluble heads and long,
  nonpolar, insoluble tails.
• Phospholipids have two fatty acids attached
  to glycerol instead of three. Because of their
  polarity, these molecules are attached to H2O
  molecules.
• Phospholipids align with the H2O soluble
  phosphate ends toward the outside of each
  layer and the nonpolar tails inside the bilayer.
Figure 5.12 -- Phospholipids contain 2 fatty acids, glycerol,
     phosphate, and an alcohol linked by ester bonds
          II. Cellular Transport
A. Diffusion
  1. Brownian motion is the random motion of
  molecules.

  2. Most materials in and around any cell are in
  H2O solution. Diffusion is the movement of
  particles from areas of high concentration to areas
  of low concentration. It is the result of Brownian
  motion.
3. Brownian Motion is continuous motion. When materials
   are evenly distributed in H2O and no further changes in
   concentration occur, dynamic equilibrium exists.
• Dynamic Equilibrium= random movement continues but
   there is no change in concentration. (dynamic = change;
   equilibrium = balance) Dynamic equilibrium is a
   characteristic of homeostasis in the cell.
4. Diffusion depends on concentration gradients.
• Concentration Gradients = difference in
   concentration between two areas (ex: Inside of a cell
   and outside of a cell).
• Ions and molecules automatically diffuse (move
   through a membrane) from an area of high
   concentration to an area of low concentration. This
   means they move with the gradient.
• Diffusion across a membrane continues until
   there is no concentration gradient. Dynamic
   equilibrium then exists because the concentration is
   the same on both sides of the membrane.
Figure 8.9 The diffusion of solutes across membranes
5. Selectivity of membrane- only H2O, oxygen, nitrogen,
carbon dioxide molecules, and a few other non-polar
molecules can diffuse directly across the plasma
membrane.
• Charged ions of polar molecules cannot automatically
diffuse across the plasma membrane.
B. Osmosis- Diffusion of water

  Osmosis=diffusion of water only through a
  selectively permeable membrane from an area of
  high concentration to an area of lower concentration.

1. No osmosis in an isotonic solution because the
   concentration of H2O is the same on either side of
   the plasma membrane(dynamic equilibrium).
   However, movement continues (Brownian motion).
Figure 8.10 Osmosis
2. Osmosis in a hypotonic solution- particulate
   concentration is lower outside the cell so H2O
   concentration is higher outside the cell, therefore,
   H2O diffuses into the cell.
• Turgor Pressure is the internal pressure of a
   plant cell. It aids in maintaining rigidity (fluid
   presses against the cell wall).
• Plasmolysis is loss of water from plant cells
   causes plant to wilt b/c of the loss of turgor
   pressure.
• Most animal cells burst if too much H2O enters
   the cell. Protists have contractile vacuoles to
   remove excess H2O.
3. Osmosis in a hypertonic solution- particulate
   concentration is higher outside the cell so H2O
   concentrations higher inside the cell, therefore,
   H2O diffuses out of the cell.
• This causes animal cells to shrivel up. In plant
   cells, it results in plasmolysis which is water loss
   from the central vacuole leading to loss of turgor
   pressure (wilting).
Figure 8.11 The water balance of living cells
C. Passive Transport (Diffusion and Osmosis)
• Passive transport is the movement of particles
  across the plasma membrane by diffusion
  requiring no expenditure of energy by the cell.

• Facilitated diffusion- transport proteins embedded
  in the plasma membrane transport ions and
  molecules (that can’t get thru the membrane on
  their own) into and out of the cell as needed.
Figure 8.13 One model for facilitated diffusion
D. Active Transport
Active transport- diffusion goes against the
  concentration gradient meaning movement from
  an area of low concentration to an area of high
  concentration. The cell expends energy doing
  this.

1. How active transport occurs- the cell uses
   chemical energy to change the shape of transport
   proteins so that the particle to be moved is
   released on the other side of the membrane. The
   protein’s original shape in then restored.
Figure 8.15 Review: passive and active transport compared
Figure 8.16 An electrogenic pump
Cotransport:          Glucose-Na+   Pump
• Here is an example of
  two ways that cells
  can use ATP and
  manipulate
  concentration
  gradients to transport
  molecules inside.
  Cotransport:         Lactose-H + Pump


• Here, lactose enters
  the cell along
  hydrogen’s
  concentration gradient.
• Hydrogen is then
  pumped out using
  ATP, and the gradient
  is maintained.
Figure 8.14 The sodium-potassium pump: a specific case of active transport
Figure 8.17 Cotransport
E. Transport of large particles
Endocytosis- a cell surrounds material and
 takes it in from its environment by
 enclosing it in a newly formed vacuole.
Exocytosis- vacuole containing what the cell
 needs to dump, merges with the plasma
 membrane releasing the material outside the
 cell.
Pinocytosis and Phagocytosis
Figure 8.18 The three types of endocytosis in animal cells
Figure 8.18a Phagocytosis
Figure 8.18c Receptor-mediated endocytosis
      III. Endomembrane System
Q: How do things get around inside of the cell?
A: The endomembrane system is a collection of
 membranous structures involved in transport
 within the cell.
             III. Endomembrane System




                                                           Figure 7.16
The organelles of the Endomembrane System are are connected by transport vesicles.
     Figure 7.9

The Nucleus contains
the cellular DNA and
enzymes that copy the
DNA. It is surrounded
by a double membrane
called the Nuclear
Envelope, that is
continuous with the
Endoplasmic
Reticulum. Large
particles can pass
between the cytosol and
the nuclear lumen
through Nuclear Pore
Complexes.
                                     Figure 7.11


Endoplasmic Reticulum is the site
of synthesis of proteins and other
molecules destined for other
components of the Endomembrane
System and for secretion to the
extracellular space. The ER
membrane is continuous with the
Nuclear Membrane and has two
components:
(1) Rough ER contains many bound
ribosomes that synthesize proteins
(2) Smooth ER has no bound
ribosomes
                                   Figure 7.12




Golgi apparatus receives proteins, lipids, other molecules from the ER via
transport vesicles and modifies these molecules before passing them onto other
organelles or the plasma membrane via transport vesicles
Figure 7.14: Formation and Function of Lysosomes
       ***Non-Endomembrane Organelles:

The mitochondria and chloroplasts are surrounded by
double membranes; The intermembrane space and the
  matrix have different environments due to the two
  membranes, and we will later see how this allows
    certain processes to occur in these organelles
      IV. Cell Connections and
           Communication
• Desmosomes attach cells together.
• Tight junctions leakproof the cell.
• Gap junctions and plasmodesmata allow
  communications between cells.
•
  Figure 7.30


Intercellular Junctions are protein assemblies that connect neighboring cells
together. Gap Junctions allow neighboring cells to communicate by diffusion of
small molecules from one cell cytosol to the other.
      V. Diseases Associated with
     Difficulties in Transport across
               membranes.
      Diseases resulting from lack of functional
                          channels/pumps
•   Motor neuron problems -Na+ channel
•   Cystic fibrosis - Cl- channel
•   Bipolar disorder -Na+, K+, ATPase
•   Heart problems -Na+, K+, ATPase, Na+ channels
•   Resistance to chemotherapy - peptide transporter, p-
    Glycoprotein, (Multi-Drug Resistance)
•   Color Blindness, H+ gradient as pump (rhodopsin)
•   Some Food Poisoning - Ca+ channel

								
To top