AP Biology Ch. 7 Membrane Structure

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AP Biology Ch. 7 Membrane Structure Function
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The Edge of Life

• The plasma membrane of a cell is the boundary
  that separates the living cell from its
  surroundings.
• It is only about 8nm thickit would take over
  8,000 of them to equal the thickness of a piece
  of paper.
• It is essential to maintain the internal
  environment of the cell.

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Selective Permeability

• The plasma membrane exhibits selective
  permeabilityit allows some substances to pass
  through more easily than others.
• What needs to come into a cell? (oxygen, water,
  nutrients, ions)
• What needs to leave a cell?(carbon dioxide,
  water, wastes, proteins)

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Cell membranes are fluid mosaics made of lipids
and proteins

• The most abundant molecules in cell membranes
  are phospholipids and proteins. Phospholipids are
  amphipathic. What does this mean?

Phospholipids make a bilayer around the cell with
water inside the cell kept separate from water
outside the cell.
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The Fluidity of Membranes

• The membrane is a fluid structure with a mosaic
  of various proteins embedded in or attached to
  the phospholipids.
• See video http//www.youtube.com/watch?vLKN5sq5d
  tW4featurerelated

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Fig. 7-7
Fibers of extracellular matrix (ECM)
Carbohydrate
Glyco- protein
Glycolipid
EXTRACELLULAR SIDE OF MEMBRANE
Cholesterol
Microfilaments of cytoskeleton
Peripheral proteins
Integral protein
CYTOPLASMIC SIDE OF MEMBRANE
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Movement of Molecules within the Plasma Membrane

• Membranes are not locked into place.
• Most of the lipids and some of the proteins can
  shift laterally side-to-side.
• It is rare for them to flip-flop transversely.
• Adjacent phospholipids move within the membrane
  rapidly107 times/second

Unsaturated fatty acid chains in the
phospholipids makes membranes more fluid since
the kinks in the tails dont pack together
tightly. This keeps it fluid even in cold
temperatures.
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Cholesterolimportant for cell membrane structure

• The steroid cholesterol is wedged between
  phospholipids in the plasma membrane of animal
  cells.
• At higher temperatures, cholesterol makes the
  membrane less fluid by restraining phospholipid
  movement.
• Because cholesterol keeps the membrane from
  packing tightly, it lowers the temperature
  required for the membrane to solidify (keeping
  animals from freezing)

Membranes must be fluid to work properly they
are usually about as fluid as salad oil.
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Membrane Proteins and Their Functions

• A mosaic of different proteins is embedded in the
  plasma membrane. Proteins determine most of the
  membranes functions.

Enzymes
Signal transduction
Active transport of materials
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Functions of Membrane Proteins
Inter-cellular Joining
Attachment to the cytoskeleton and the
extra-cellular matrix
Cell-Cell Recognition
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Two Major Kinds of Membrane Proteins

• Integral Proteins penetrate the hydrophobic core
  of the lipid bilayer

• Peripheral Proteins
• Not embedded in the lipid bilayer at all
• They are appendages

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The Structure of a trans-membrane protein
EXTRACELLULAR SIDE
N-terminus
C-terminus
CYTOPLASMIC SIDE
? Helix
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The Role of Membrane Carbohydrates in
Cell-to-Cell Recognition

• A cells ability to distinguish one type of
  neighboring cell from another is crucial.
• It helps sort cells into different tissues and
  helps the organism recognize foreign cells.

Membrane carbohydrates may be bonded to proteins
(Glyco-proteins) or to lipids (Glyco-lipids) Cell
s recognize other cells by binding to surface
molecules, often to carbohydrates.
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Synthesis and Sidedness of Membranes

• Membranes have distinctive inside and outside
  faces.
• When a vesicle fuses with the plasma membrane,
  the OUTSIDE of the vesicle membrane fuses with
  the INSIDE of the plasma membrane.
• Therefore, molecules that start on the INSIDE of
  the ER end up on the OUTSIDE of the plasma
  membrane.

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ER
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Synthesis of membrane components and their
orientation in the resulting membrane. The plasma
membrane has distinct cytoplasmic (orange) and
extracellular (aqua) faces, with the
extracellular face arising from the inside face
of the ER, Golgi, and vesicle membranes.
Transmembrane glycoproteins
Secretory protein
Glycolipid
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Golgi apparatus
Vesicle
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Plasma membrane
Cytoplasmic face
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Extracellular face
Transmembrane glycoprotein
Secreted protein
Membrane glycolipid
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Membrane Structure Animations

• http//www.wisc-online.com/objects/ViewObject.aspx
  ?IDap1101

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Membrane structure results in selective
permeability

• Most important function of the cell membrane to
  regulate what goes in/out of the cell
• Form fits function the fluid mosaic model helps
  explain how membranes regulate the cells
  molecular traffic

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The Lipid Bilayer

• Nonpolar molecules such as hydrocarbons, CO2 and
  O2 are hydrophobic and can pass easily through
  the lipid bilayer. Why?

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Permeability of the Lipid Bilayer

• Ions and polar molecules cannot pass easily
  across the lipid bilayer. Why not?
• Polar molecules such as glucose and other sugars
  pass only slowly across the membrane.
• Even water passes more slowly than expected,
  since it is such a small molecule. Why?

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Transport Proteins

• Cell membranes are permeable to certain ions and
  other polar substances such as water.
• These hydrophilic substances can avoid contact
  with the Hydrophobic Zone by passing through
  transport proteins that span the membrane.

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Channel Proteins

• Channel proteins function by having a hydrophilic
  channel that certain molecules can use as a
  tunnel through the membrane

• Aquaporins help water molecules move through the
  membrane easily. Each one allows up to 3 billion
  water molecules/second!

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Carrier Proteins

• Carrier proteins hold onto their passengers and
  change shape in such a way that shuttles them
  across the membrane.
• A transport protein is specific for the substance
  it moves across.

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Passive Transport

• Passive transport is diffusion of a substance
  across a membrane with no energy investment
  required by the cell. Materials move by
  themselves! How?
• Diffusion the movement of a substance from
  where it is more concentrated to where it is less
  concentrated.

Molecules move DOWN a concentration gradient.
They move from HIGH concentration to LOW.
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Diffusion

• Molecules move by themselves due to kinetic
  energy, the energy of motion. This is also called
  thermal energy.
• Each molecule moves randomly, in all directions.
• Diffusion occurs as molecules move randomly, but
  the NET movement of a population of molecules is
  to move AWAY from high concentration to lower
  concentration until it reaches equilibrium.

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Osmosis

• Osmosis is a special kind of diffusion. It is the
  diffusion of water across a selectively permeable
  membrane.
• Water moves from where it is MORE concentrated to
  where it is LESS concentrated.

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Effect of Osmosis on Water Balance

• If there is a difference in the concentration of
  solutes in the water on either side of a
  membrane, the water level may change as it
  diffuses across the membrane.
• It is the difference in free water concentration
  that makes the difference.

Why does the level of water go up on the
right-hand side of the tube?
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3 Solutions a Cell Could be in

• Isotonic Solution a solution where the
  concentration of solutes is EQUAL inside and
  outside the cell
• Hypertonica solution where the concentration of
  solutes is GREATER in the water outside the cell.
  (More solutes less water)

• Hypotonica solution where the concentration of
  solutes is LESS in the water outside the cell.
  (Less solutes more water)

Hypertonic
Hypotonic
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What happens to cells in the 3 conditions?
Iso equal
Hyper Greater than
Hypo Less than
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Water Balance in Cells with Cell Walls ( mostly
Plants!)

• The cells of plants, prokaryotes, fungi and some
  protists have cell walls.
• When such a cell is placed in a hypotonic
  solution, the wall helps maintain the cells
  water balance.

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Facilitated Diffusion Passive Transport Aided by
Proteins

• Facilitated diffusion occurs when transport
  proteins embedded in the membrane aid the
  movement of substances. (Note it is still moving
  DOWN the concentration gradient!)
• Two types of transport proteins

Channel protein
Carrier Proteins
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Passive Transport Animation

• http//www.northland.cc.mn.us/biology/biology1111/
  animations/passive1.swf

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Active Transport

• Active transport uses the cells energy (ATP) to
  move materials across the membrane.
• Materials that must move AGAINST the
  concentration gradient must be moved with active
  transport.

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The Need for Energy in Active Transport

• To pump a solute across a membrane against its
  gradient requires work the cell must expend
  energy.
• Carrier proteins embedded in the membrane can
  move these solutes.

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One Example Sodium-Potassium Pump

• The sodium-potassium pump exchanges sodium ions
  (Na) for potassium (K) across the cell
  membrane.
• Each ion is in high concentration on one side of
  the membrane. The cell needs to maintain that
  higher concentration.

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Sodium-Potassium Pump Animations

• http//highered.mcgraw-hill.com/sites/0072495855/s
  tudent_view0/chapter2/animation__how_the_sodium_po
  tassium_pump_works.html
• http//www.brookscole.com/chemistry_d/templates/st
  udent_resources/shared_resources/animations/ion_pu
  mp/ionpump.html

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How Ion Pumps Maintain Membrane Potential

• All cells have voltages across their plasma
  membranes.
• Voltage is electrical potential energya
  separation of opposite charges.
• The cytoplasm is negative (-) the extracellular
  fluid is positive ().

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Membrane Potential

• The voltage across a membrane is called membrane
  potential. It ranges from about 50 to 200
  millivolts.
• The membrane potential acts like a battery, an
  energy source that affects the traffic of all
  charged substances (either or -).
• The membrane potential favors charged ions.
  Why?

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Membrane Potential

• Two forces drive the diffusion of ions across the
  membrane
• 1)a chemical force (the concentration gradient)
• 2)an electrical force (the membrane potential)
• Together, these are called the electrochemical
  gradient

In the above example, there are more negative
charges inside the cell than outside. Ions such
as Na are more likely to move inside. Why? (2
reasons!)
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Electrogenic Pumps

• Some membrane proteins that actively pump ions
  contribute to the membrane potential.
• The Sodium-Potassium pump is an example because
  it pumps out 3 Na ions for every 2 K ions,
  creating a difference in charge.

An electrogenic pump creates voltage across a
membrane. With each crank of the pump, there
is a net transfer of one charge, a process that
stores energy as voltage.
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Proton Pumps

• The main electrogenic pump of plants, fungi, and
  bacteria is a proton pump.
• A proton pump actively pumps out hydrogen ions
  (protons, H).
• The pumping of H transfers charge from the
  cytoplasm, generating voltage.

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Cotransport

• A single ATP-powered pump that transports a
  specific solute can indirectly drive the active
  transport of other solutes as well.
• A substance that has been pumped across the
  membrane can do work as it moves back across by
  diffusion.

An example of cotransport A carrier protein such
as this sucrose-H transporter is able to use the
diffusion of H down its electrochemical gradient
to drive the uptake of sucrose.
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Exocytosis

• Exocytosis the cell secretes (transports out
  of the cell) certain biological molecules by
  fusing vesicles with the plasma membrane.
• The contents of the vesicle then spill to the
  outside of the cell and the vesicle membrane
  becomes part of the cell membrane.

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Endocytosis

• In endocytosis, the cell takes in biological
  molecules (such as food) by forming new vesicles
  from the plasma membrane.
• A small area of the plasma membrane sinks inward
  to form a pocket.
• As the pocket deepens, it pinches in, forming a
  vesicle.

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Phagocytosis Cell Eating

• In Phagocytosis, a cell engulfs a particle by
  wrapping pseudopodia around it and packaging it
  within a membrane-enclosed sac that can be large
  enough to be called a vacuole.
• The particle is then digested by a lysosome.

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Pinocytosis Cell Drinking

• In pinocytosis, a cell gulps in droplets of
  extracellular fluid into tiny vesicles.
• It is not the fluid itself that is needed by the
  cell, but the molecules dissolved in the
  droplets.

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Receptor-Mediated Endocytosis

• Receptor-mediated endocytosis enables the cell to
  bring in bulk quantities of specific substances,
  even if they are not in high concentrations
  outside the cell.

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Coated Pits

• Embedded in the membrane are proteins with
  specific receptor sites exposed to the
  extracellular fluid.
• The receptor proteins are usually clustered in
  regions of the membrane called coated pits.
• These coated pits are lined on the inside of the
  cell with fuzzy proteins.