Membrane Composition and Structure

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Membrane Composition and Structure

• Cell membranes are bilayered, dynamic structures
  that
• Perform vital physiological roles
• Form boundaries between cells and their
  environments
• Regulate movement of molecules into and out of
  cells
• Lipids, proteins, and carbohydrates in various
  combinations make these tasks possible.

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Membrane Composition and Structure

• The lipid portion of a cellular membrane provides
  a barrier for water-soluble molecules.
• Lipids are like the water of a lake in which
  proteins float. This general design is called
  the fluid mosaic model.
• Membrane proteins are embedded in the lipid
  bilayer.
• Carbohydrates attach to lipid or protein
  molecules on the membrane, generally on the outer
  surface.

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Figure 5.1 The Fluid Mosaic Model
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Membrane Composition and Structure

• Most of the lipid molecules found in biological
  membranes are phospholipids.
• Each has a hydrophilic region, where the
  phosphate groups are located, and a hydrophobic
  region, the fatty acid tails.
• The phospholipids organize themselves into a
  bilayer.
• The interior of the membrane is fluid, which
  allows some molecules to move laterally in the
  membrane.
• http//www.johnkyrk.com/cellmembrane.html

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Figure 5.2 A Phospholipid Bilayer Separates Two
Aqueous Regions
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Membrane Composition and Structure

• .
• Cholesterol may decrease fluidity.
• For any given membrane, fluidity also decreases
  with declining temperature.
• Fluidity and permeability decreases with
  increased amounts of phospholipids with
  saturated fatty acid tails.

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Membrane Composition and Structure

• All biological membranes contain proteins.
• The ratio of protein to phospholipid molecules
  varies depending on membrane function.
• Many membrane proteins have hydrophilic and
  hydrophobic regions.

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Membrane Composition and Structure

• Integral membrane proteins have hydrophobic
  regions of amino acids that penetrate or entirely
  cross the phospholipid bilayer.
• Transmembrane proteins have a specific
  orientation, showing different faces on the two
  sides of the membrane.
• Peripheral membrane proteins lack hydrophobic
  regions and are not embedded in the bilayer.

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Figure 5.4 Interactions of Integral Membrane
Proteins
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Membrane Composition and Structure

• Markers
• Plasma membrane glycoproteins, glycolipids,
  lipoprotiens, enable cells to be recognized by
  other cells and proteins.

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Cell Recognition and Adhesion

• The plasma membrane also allows for cell adhesion
  and recognition.
• In cell recognition, one cell specifically binds
  to another cell of a certain type.
• In cell adhesion, the relationship between the
  two cells is cemented.

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Passive Processes of Membrane Transport

• Diffusion The movement of substances from an area
  of high concentration to an area of low
  concentration.
• .

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Passive Processes of Membrane Transport

• A biological membrane may be permeable to some
  molecules and impermeable to others.
• Unsaturated phospholipids higher permeability.
• Saturated phospholipids low permeability.

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Passive Processes of Membrane Transport

• Small molecules can move across the lipid bilayer
  by simple diffusion. (i.e. H2O, O2, CO2)
• The more lipid-soluble the molecule, the more
  rapidly it diffuses.
• An exception to this is water.
• Polar and charged molecules such as amino acids,
  sugars, and ions do not pass readily across the
  lipid bilayer.

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Passive Processes of Membrane Transport

• Osmosis is the diffusion of water from an area of
  high water concentration to an area of low water
  concentration.
• Osmosis is a completely passive process and
  requires no metabolic energy.

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Passive Processes of Membrane Transport

• Isotonic solutions have equal solute
  concentrations.
• A hypertonic solution has a greater total solute
  concentration than the solution to which it is
  being compared.
• A hypotonic solution has a lower total solute
  concentration than the solution to which it is
  compared.

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Figure 5.8 Osmosis Modifies the Shapes of Cells
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Passive Processes of Membrane Transport

• .
• One way for these important raw materials to
  enter cells is through the process of facilitated
  diffusion.
• Facilitated diffusion depends on two type of
  membrane proteins channel proteins and carrier
  proteins.

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Passive Processes of Membrane Transport

• Channel proteins are integral membrane proteins
  that form channels lined with polar amino acids.
• Nonpolar (hydrophobic) amino acids face the
  outside of the channel, toward the fatty acid
  tails of the lipid molecules.
• http//www.stolaf.edu/people/giannini/flashanimat/
  transport/channel.swf

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Figure 5.9 A Gate Channel Protein Opens in
Response to a Stimulus
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Figure 5.10 The K Channel
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Passive Processes of Membrane Transport

• Facilitated diffusion is diffusion of substances
  with the assistance of a carrier or channel
  protein.
• Carrier proteins allow diffusion in both
  directions.

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Figure 5.11 A Carrier Protein Facilitates
Diffusion (Part 1)
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Active Transport

• In contrast to diffusion, active transport
  requires the expenditure of energy.
• Ions or molecules are moved across the membrane
  against the concentration gradient.
• ATP is the energy currency used either directly
  or indirectly to achieve active transport.

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Active Transport

• Three different protein-driven systems are
  involved in active transport
• Uniport transporters move a single type of
  solute, such as calcium ions, in one direction.
• http//www.stolaf.edu/people/giannini/flashanimat/
  transport/caryprot.swf
• Symport transporters move two solutes in the same
  direction.
• http//www.stolaf.edu/people/giannini/flashanimat/
  transport/symport2.swf
• Antiport transporters move two solutes in
  opposite directions, one into the cell, and the
  other out of the cell.
• http//www.stolaf.edu/people/giannini/flashanimat/
  transport/antiport.swf

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Figure 5.12 Three Types of Proteins for Active
Transport
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Active Transport

• If ATP is used directly for the pumping system,
  as in the sodiumpotassium pump, the system is a
  primary active transport system.
• Only cations, such as sodium, potassium, and
  calcium, are transported directly by pumps that
  use a primary active transport system.
• http//www.stolaf.edu/people/giannini/flashanimat/
  transport/atpase.swf

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Figure 5.13 Primary Active Transport The
SodiumPotassium Pump
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Active Transport

• Secondary active transport systems use
  established gradients to move substances.
• This form of transport uses ATP indirectly. The
  ATP molecules are consumed to establish the ion
  gradient.
• The gradient is then used to move a substance, as
  described for the symport and antiport systems.
• http//www.stolaf.edu/people/giannini/flashanimat/
  transport/secondary20active20transport.swf

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Figure 5.14 Secondary Active Transport
31
Endocytosis and Exocytosis

• The group of processes called endocytosis brings
  macromolecules, large particles, small molecules,
  and even other cells into the eukaryotic cell.
• There are three types of endocytosis
  phagocytosis, pinocytosis, and receptor-mediated
  endocytosis.
• http//www.cellsalive.com/qtmovs/mac_mov.htm

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Figure 5.15 Endocytosis and Exocytosis
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Endocytosis and Exocytosis

• During phagocytosis is the engulfing of solid
  particles.
• Pinocytosis is cellular drinking. The engulfing
  of liquid droplets.

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Endocytosis and Exocytosis

• Receptor-mediated endocytosis is similar to
  pinocytosis, but it is highly specific.
• Receptor proteins are exposed on the outside of
  the cell in regions called coated pits. Clathrin
  molecules form the coat of the pits.
• Coated vesicles form with the macromolecules
  trapped inside.

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Figure 5.16 Formation of a Coated Vesicle (Part
1)
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Figure 5.16 Formation of a Coated Vesicle (Part
2)
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Endocytosis and Exocytosis

• Exocytosis is the process by which materials
  packaged in vesicles are secreted from the cell.

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Membranes Are Not Simply Barriers

• Membranes have many functions, including
• Information processing
• Energy transformation
• The inner mitochondrial membrane helps convert
  the energy of fuel molecules to the energy in
  ATP.
• The thylakoid membranes of chloroplasts are
  involved in the conversion of light energy in
  photosynthesis.
• Membranes are involved in organizing chemical
  reactions, allowing them to proceed rapidly and
  efficiently.

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Figure 5.17 More Membrane Functions (Part 1)