Transcript The Cell Membrane

The Plasma Membrane
The Fluid Mosaic Model
There were other models of the
plasma membrane
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Fluid mosaic model
Sandwich model
Phospholipid Bi-Layer
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Phospholipid
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Polar head- hydrophilic (Likes water)
Fatty acid tail- hydrophobic (Dislikes water)
Because of its hydrophilic and hydrophobic
properties, phospholipids spontaneously form
bilayers.
The Phospholipid
Discovering the Phospholipid
bilayer
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We did not know the structure of the cell membrane
until the advent of the electron microscope
Technique of Freeze Fracture Analysis exposed the
interior of the cell membrane and supported the Fluid
Mosaic Model. (Please read about freeze fracture analysis in your book.)
Other Components of the Cell
Membrane
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Glycerol: An alcohol, has up to 3 OH groups
and is soluble in water. Acts as a lubricant
allows the cell membrane to slide over itself.
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A bi-product of Bio-diesel production.
Cholesterol: Lipid related to the steroid
chemical group. Reduces the permeability of
the membrane to most biological molecules.
Also provide “structure” to fatty acid tails
Other compounds in the C.M.
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Glycolipid- Has similar structure to
phospholipids, but the hydrophilic head is a
chain of sugars.
Glycoproteins- Chain of polypeptides (part of
a protein) attached to a sugar. May play a role
in cell recognition.
Carbohydrate chains- chains of C,H,O that
play a role in cell identification/recognition.
Membrane Proteins
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Channel protein- allows a particular molecule or ion
to cross the plasma membrane freely. Chloride ions
cross the plasma membrane (p.m.) via a channel
protein.
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Cystic fibrous is a disorder caused by faulty channel
proteins.
Carrier protein- selectively interacts with a specific
molecule or ion so that it can cross the p.m. The
Na/K pump is a carrier protein.
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Some evidence suggests that obesity may be related to
ineffective Na/K pumps.
Membrane Proteins (cont.)
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Cell Recognition Protein- proteins in addition to
glycoproteins which allow cells to recognize other
cells.
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Major histocompatablity complex (MHC)- complex of
glycoproteins, and glycolipids which is how the body
recognizes organs as foreign. Makes organ donating
difficult
Receptor Proteins- only a specific molecule can bind
to it and enter the p.m. Receptor proteins rely on the
3-D shape of the protein and the molecule.
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Faulty receptor proteins limit the amount of growth
hormone that can enter cells and cause a type of dwarfism.
Membrane Proteins (cont.)
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Enzymatic Proteins- Catalyze a specific
reactions. Lowers the activation energy of a
reaction.
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Opportunity for points
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Study questions 1, 2, 3, 6 pg. 101
Objective questions 1-10 pg. 101
Crossing the Membrane
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Diffusion- Movement of molecules from an
area of high concentration to an area of low
concentration. Moves down a concentration
gradient.
Solute- The substance that gets dissolved
Solvent- The substance that does the dissolving.
Molecules can cross the p.m. two
ways
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Passive Transport Molecules cross the pm without the use
of energy
 Lipid soluble materials, water, gases,
 Two types of passive transport
 Diffusion / Osmosis
 Facilitated diffusion or transport
Passive Transport
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Osmosis- The diffusion of H2O across a semipermeable membrane.
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Types of Solutions
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Hypotonic- dissolved particles are higher in concentration
inside the cell than outside. H2O will rush into the cell
which is cause the cell to lysis (burst)
Hypertonic- dissolved particles are higher in
concentration outside the cell than inside. H2O will leave
the cell.
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Crenation- shriveling of red blood cells because of H2O loss
Plasmolysis- loss of H2O in plants.
Isotonic- Concentration of dissolved particles are the
same on both sides of the cell.
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Tonicity- strength of a solution in relationship to osmosis and
cell concentration.
Passive Transport
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Facilitative Diffusion-
molecules move
across the p.m. without the use of energy.
Requires a carrier protein and a
conformational change
in the protein.
(change in shape) see figure 6.10 pg 94.
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Moves material from high to low
concentration.
Moves small sugars and amino acids.
Facilitative diffusion
Active transport
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Requires ATP (energy)
Requires a conformational change
Moves material from an area of low
concentration to an area of high
concentration.
Moves sugar, amino acids and ions.
Sodium potassium pump is an example.
See fig. 6.11 on pg 95
Active Transport
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Active transport Movement of molecules across the pm requires
energy
 Sugars, proteins, macromolecules, small cells
 Types of Active transport
 Active transport
 Membrane assisted transport (Exocytosis,
Endocytosis, pinocytosis, phagocytosis)
Membrane assisted transport
Molecules too large to fit through the p.m. or
carried by proteins enter and exit via vesicles
formation.
Requires energy!
Exocytosis- vesicles formed by the golgi bodies
fuse with the p.m. secreting vesicle contents
outside the cell.
Endocytosis- Portion of the p.m. envelopes a
substance and pinches off to form an
intracellular vesicle. There are three types.
Three Types of Endocytosis
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Phagocytosis- cell eating (amoeba)
Pinocytosis- cell drinking. Vesicles
form around liquids or VERY small
particles.
Receptor mediated endocytosis- A form
of pinocytosis.
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Receptor proteins bind with a specific
substance
The receptors then bunch together
The cell membrane folds in and forms a
vesicle.
Vocab for Receptor mediated
endocytosis
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Ligand- substance that binds to
the receptor (macromolecule)
Coated pit- Location where the
receptors gather. A protein coats
the interior of the p.m. at the pit
Clathrin- the fibrous protein that
coats the p.m.
Cellular Communication
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Cells come in contact with each
other. Especially when they
interact with each other as
tissues. We see three different
types of junctions in animal cells.
The type of junction affects the
function of the cell.
Types of Junctions
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Adhesion junctions- hold cells together. The
cytoskeletons of each cell are firmly
connected via filaments. Cells in the heart,
bladder, stomach, or organs that are
stretched have adhesion junctions.
Tight Junctions- Plasma membrane proteins
from adjacent cells actually attach to each
other. Serve as barriers. Found in the
intestines and kidneys. Keep digestive
juices out of the body cavity, and prevent
urine from leaving the tubules in the kidneys.
Types of Junctions (cont.)
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Gap junctions- allows cells to communicate.
Occur when two different plasma membrane
channels join. Allows for the flow of ions and
molecules.
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Which organs would you expect to find Gap Junctions,
why?
Heart and smooth muscle
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Plasmodesmata- strands of cytoplasm that
are found in the narrow channels that pass
through the cell wall of plant cells. Function
in the same manner as Gap junctions do in
animals.
Intercellular Junctions
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Neighboring cells in tissues, organs, or organ
systems often adhere, interact, and communicate
through direct physical contact
Intercellular junctions facilitate this contact
There are several types of intercellular junctions
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Plasmodesmata
Tight junctions
Desmosomes
Gap junctions
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Plasmodesmata in Plant Cells
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Plasmodesmata are channels that perforate
plant cell walls
Through plasmodesmata, water and small
solutes (and sometimes proteins and RNA) can
pass from cell to cell
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 6-31
Cell walls
Interior
of cell
Interior
of cell
0.5 µm
Plasmodesmata Plasma membranes
Tight Junctions, Desmosomes, and
Gap Junctions in Animal Cells
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At tight junctions, membranes of neighboring cells
are pressed together, preventing leakage of
extracellular fluid
Desmosomes (anchoring junctions) fasten cells
together into strong sheets
Gap junctions (communicating junctions) provide
cytoplasmic channels between adjacent cells
Animation: Tight Junctions
Animation: Desmosomes
Animation: Gap Junctions
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 6-32
Tight junction
Tight junctions prevent
fluid from moving
across a layer of cells
0.5 µm
Tight junction
Intermediate
filaments
Desmosome
Gap
junctions
Space
between
cells
Plasma membranes
of adjacent cells
Desmosome
1 µm
Extracellular
matrix
Gap junction
0.1 µm
Fig. 6-32a
Tight junctions prevent
fluid from moving
across a layer of cells
Tight junction
Intermediate
filaments
Desmosome
Gap
junctions
Space
between
cells
Plasma membranes
of adjacent cells
Extracellular
matrix