Download Membrane Structure and Function Cell Membrane: a Phospholipid

Cell Membrane:
a Phospholipid Bilayer
Membrane
Structure
and
Function
Phospholipid
Chapter 5
Hydrophilic
“Head”
Hydrophobic
“Tail”
Lipid Bilayer
Fluid Mosaic Model
• Mixture of saturated and unsaturated
fatty acid tails create fluidity.
• Proteins in the membrane constantly shift
and flow within the fluid membrane.
Cell Membrane
• Isolates the cell’s contents from the
external environment.
• Regulates exchange of substance
across the membrane.
• Communicates with other cells.
• Creates attachments within and
between cells.
• Regulates many biochemical reactions.
Cholesterol Benefits
Membrane Structure
• Makes the bilayer
–
–
–
–
–
–
Stronger
More flexible
Less fluid at high temperatures
Less solid at low temperatures
Slows phospholipid movement
Less permeable to water-soluble substances
Isolate Cell Contents
• The cell is surrounded by an external aqueous
environment.
• The cell’s interior, the cytosol, is composed
mostly of water.
• Spontaneous arrangement of the lipid bilayer:
– Hydrophilic heads outside (interacting with water)
– Hydrophobic tails inside (interacting with
themselves)
• Most substances contacting the cell membrane
are hydrophilic and cannot penetrate the
hydrophobic interior of the membrane.
Protein Mosaic
within the Membrane
• Five major types of proteins:
– Receptor proteins
– Recognition proteins
– Enzymatic proteins
– Attachment proteins
– Transport proteins
Receptor Proteins
• Have a binding site, or receptor, for a
specific molecule.
• Binding to the receptor activates the protein.
• Activation leads to a
conformational change
in the protein, triggering
a sequence of reactions.
Recognition Proteins
• Glycoprotein, a membrane protein with
a carbohydrate group attached to the
extracellular side.
• The carbohydrate acts as an
identification tag for the cell.
• Examples:
Enzymatic Proteins
• Proteins attached to the inner surface of
membranes
• Promote chemical reactions
– Synthesize new molecules
– Break apart biological molecules
– Immune system and bacteria
– Red blood cells
Reactant
Product
Reactant
Enzyme
Enzyme
Enzyme
Enzymatic Proteins
Example: Dehydration Synthesis
Attachment Proteins
• Anchor the cell membrane by
– Binding to the cytoskeleton
– Binding to external structures
– Binding to other cells
Enzyme
An enzyme is a protein that
catalyzes the formation of a
bond between two molecules.
Enzyme
Transport Proteins
• Channel proteins: form channels to
allow specific ions or molecules to pass
through the membrane.
• Carrier proteins: bind substrates to move
them through the membrane.
• Movement through these proteins occurs
by both active and passive transport
Movement across the Membrane
Responds to Gradients
• Concentration: the number of molecules
of a substance in a given volume of fluid.
• Gradient: physical difference between
two different regions of space.
– Temperature, concentration, pressure, etc.
– Cells use energy and their cell membranes
to generate concentration gradients.
Diffusion
• Random movement of molecules from
regions of high to low concentration.
• Slow process.
• The greater the concentration gradient,
the faster the rate of diffusion.
Transport Across the Membrane
• The cell uses concentration gradients
across the cell membrane to move
molecules.
• Two types of movement:
– Passive Transport (no energy required)
• Simple Diffusion
• Facilitated Diffusion
• Osmosis
– Active Transport (energy required)
Simple Diffusion
• No energy needed
• Small molecules take advantage
of the selective permeability of
the membrane.
• O2, CO2, H2O
• Lipid-soluble
molecules
Facilitated Diffusion
• Small molecules and ions
diffuse across the membrane
with the help of channel and
carrier proteins.
– Channel proteins create
hydrophilic channels for ions.
– Carrier proteins have
receptors to recognize certain
small molecules
Aquaporin in the Membrane
Aquaporins
• Water channels within the
membrane.
• Respond to osmotic stress.
•Open when cells are in a
hypotonic environment.
•Close when cells are in a
hypertonic environment.
Osmosis
• Diffusion of water across the selectively
permeable membrane from high to low
concentration.
water
Osmosis in Plants
• Hypertonic: more
solute than solvent
outside the membrane.
• Water flows in by osmosis to fill
the central vacuole.
• A full central vacuole exerts
turgor pressure, pushing the
cytosol against the cell wall.
• Isotonic: equal
concentrations of solute
and solvent across the
membrane.
– Provides support and rigidity for
non-woody plants.
• Plants deprived of water shrink
the central vacuole.
• Hypotonic: more
solvent than solute
outside the membrane.
– The lack of turgor pressure causes
the plant to wilt - plasmolysis.
Active Transport
Transport Across the
Membrane
• Using a gated channel, cells use up
energy to move molecules against the
concentration gradient.
• Passive transport
– Down the concentration
gradient
– No energy required
• Active transport
– Against the concentration
gradient, from low to high
concentration.
– Energy required!
energy
Pinocytosis
Endocytosis
• “Cell drinking”
• The plasma
membrane
“dimples” inward
and pinches off into
the cytosol.
• Creates a vesicle
filled with
extracellular fluid.
• Acquiring extracellular fluid or particles
by engulfing in a membrane sac.
•Three types:
–Pinocytosis
–Receptor-mediated endocytosis
–Phagocytosis
Receptor-mediated Endocytosis
• Selectively concentrates certain molecules within
a vesicle.
• Receptors concentrate on
the extracellular
membrane in coated pits.
• Binding of the targeted
molecule causes the pit to
deepen and pinch off.
Phagocytosis
• “Cell Eating”
• Cells extend part
of their membrane,
creating
pseudopods, to
engulf large food
particles, prey, and
pathogens.
Exocytosis
• “Out of the Cell”
• Disposal of unwanted materials through
vesicle fusion with the extracellular
membrane.
Cell Communication
• Desmosomes: intermediate filaments link
the plasma membranes of adjacent cells.
Removal of:
• Products of
digestion
• Hormones
• Toxins
Cell Communication
Cell Communication
• Tight Junctions: attachment proteins link
cells to make them watertight.
• Gap Junctions: channel proteins link
cells to allow transfer of chemical signals.
Cell Communication
• Plasmodesmata: "holes" in the cell walls of
plants to allow transfer of water and nutrients.
Homework
Thinking Through the Concepts,
Review Question #2
Applying the Concepts #3