Cell Membranes

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Cell Membranes
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6 Cell Membranes

• 6.1 What Is the Structure of a Biological
  Membrane?
• 6.2 How Is the Plasma Membrane Involved in Cell
  Adhesion and Recognition?
• 6.3 What Are the Passive Processes of Membrane
  Transport?
• 6.4 What Are the Active Processes of Membrane
  Transport?
• 6.5 How Do Large Molecules Enter and Leave a Cell?

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6 Cell Membranes
The cell membrane regulates what enters and
leaves the cytoplasm. Some cell membranes have
pores called aquaporins that allow water to pass
freely.
Opening Question Water purity is a worldwide
problem. Can aquaporin membrane channels be used
in water purification?
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6.1 What Is the Structure of a Biological
Membrane?

• The general structure of biological membranes is
  known as the fluid mosaic model.
• Phospholipids form a bilayer, which is like a
  lake in which a variety of proteins float.

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Figure 6.1 The Fluid Mosaic Model
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6.1 What Is the Structure of a Biological
Membrane?

• Phospholipids have a polar, hydrophilic head
  and hydrophobic fatty acid tails.
• In an aqueous environment, phospholipids form a
  bilayer.

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Figure 3.22 Phospholipids (Part 1)
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Figure 6.2 A Phospholipid Bilayer
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6.1 What Is the Structure of a Biological
Membrane?

• Artificial bilayers can be made in the
  laboratory.
• Lipids maintain a bilayer organization
  spontaneously. This helps membranes fuse during
  phagocytosis, vesicle formation, etc.

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6.1 What Is the Structure of a Biological
Membrane?

• Lipid composition of membranes vary.
• Phospholipids vary in fatty acid chain length,
  degree of saturation, and phosphate groups.

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6.1 What Is the Structure of a Biological
Membrane?

• Animal cell membranes may be up to 25
  cholesterol, which is important for membrane
  integrity.

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6.1 What Is the Structure of a Biological
Membrane?

• The fatty acid tails make the interior somewhat
  fluid, allowing lateral movement of molecules.
• Fluidity depends on temperature and lipid
  composition.

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6.1 What Is the Structure of a Biological
Membrane?

• Cholesterol and long-chain, saturated fatty acids
  pack tightly, making a less-fluid membrane.
• As temperature decreases, movement of molecules
  and cellular processes slow. Some organisms
  change the lipid content of the cell membranes
  when they get cold.

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6.1 What Is the Structure of a Biological
Membrane?

• Membranes also contain proteins the number
  varies depending on membrane function.
• Peripheral membrane proteins lack exposed
  hydrophobic groups and do not penetrate the
  bilayer.

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6.1 What Is the Structure of a Biological
Membrane?

• Integral membrane proteins have hydrophobic and
  hydrophilic regions or domains.
• Some extend across the lipid bilayer others are
  partially embedded.

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Figure 6.3 Interactions of Integral Membrane
Proteins
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6.1 What Is the Structure of a Biological
Membrane?

• Freeze-fracturing is a technique that reveals
  proteins embedded in the phospholipid bilayers of
  cellular membranes.

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Figure 6.4 Membrane Proteins Revealed by the
Freeze-Fracture Technique
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6.1 What Is the Structure of a Biological
Membrane?

• The proteins and lipids interact noncovalently.
• But some membrane proteins have lipid groups
  covalently attached and are tethered to the lipid
  bilayer.

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6.1 What Is the Structure of a Biological
Membrane?

• Transmembrane proteins extend all the way through
  the phospholipid bilayer.
• They have one or more transmembrane domains, and
  the domains on the inner and outer sides of the
  membrane can have specific functions.
• Peripheral membrane proteins are located on one
  side of the membrane.

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6.1 What Is the Structure of a Biological
Membrane?

• Some membrane proteins can move freely within the
  bilayer, while some are anchored to a specific
  region.
• When cells are fused experimentally, some
  proteins from each cell distribute themselves
  uniformly around the membrane.

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Figure 6.5 Rapid Diffusion of Membrane Proteins
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Figure 6.5 Rapid Diffusion of Membrane Proteins
(Part 1)
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Figure 6.5 Rapid Diffusion of Membrane Proteins
(Part 2)
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6.1 What Is the Structure of a Biological
Membrane?

• Membranes are dynamic and are constantly forming,
  transforming, fusing, and breaking down.

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Figure 5.9 The Endomembrane System (Part 2)
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6.1 What Is the Structure of a Biological
Membrane?

• Membranes also have carbohydrates on the outer
  surface that serve as recognition sites for other
  cells and molecules.
• Glycolipidscarbohydrate lipid
• Glycoproteinscarbohydrate protein

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Working with Data

• A key experiment providing evidence for the fluid
  mosaic model used the technique of cell fusion to
  show that membrane proteins rapidly diffuse
  within the cell membrane.

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Working with Data 6.1, Table 1
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Working with Data 6.1 Rapid Diffusion of
Membrane Proteins

• Question 1
• Plot the percentage of fully mixed cells over
  time.
• How long did it take for complete mixing?

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Working with Data 6.1 Rapid Diffusion of
Membrane Proteins

• Question 2
• What does your answer to Question 1 indicate
  about the rate of diffusion of the mouse and
  human proteins?

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6.2 How Is the Plasma Membrane Involved In Cell
Adhesion and Recognition?

• Cells arrange themselves in groups by cell
  recognition and cell adhesion.
• These processes can be studied in sponge
  cellsthe cells are easily separated and will
  come back together again.

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Figure 6.6 Cell Recognition and Adhesion
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6.2 How Is the Plasma Membrane Involved In Cell
Adhesion and Recognition?

• Molecules involved in cell recognition and
  binding are glycoproteins.
• Binding of cells is usually homotypic The same
  molecule sticks out from both cells and forms a
  bond.
• Some binding is heterotypic The cells have
  different proteins.

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6.2 How Is the Plasma Membrane Involved In Cell
Adhesion and Recognition?

• Cell junctions are specialized structures that
  hold cells together
• Tight junctions
• Desmosomes
• Gap junctions

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Figure 6.7 Junctions Link Animal Cells Together
(Part 1)

• Tight junctions help ensure directional movement
  of materials.

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Figure 6.7 Junctions Link Animal Cells Together
(Part 2)

• Desmosomes are like spot welds.

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Figure 6.7 Junctions Link Animal Cells Together
(Part 3)

• Gap junctions allow communication.

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6.2 How Is the Plasma Membrane Involved In Cell
Adhesion and Recognition?

• Cell membranes also adhere to the extracellular
  matrix.
• The transmembrane protein integrin binds to the
  matrix outside epithelial cells, and to actin
  filaments inside the cells.
• The binding is noncovalent and reversible.

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6.2 How Is the Plasma Membrane Involved In Cell
Adhesion and Recognition?

• Cells can move within a tissue by the binding and
  reattaching of integrin to the extracellular
  matrix.
• This is important for cell movement within
  developing embryos and for the spread of cancer
  cells.

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Figure 6.8 Integrins and the Extracellular Matrix
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6.3 What Are the Passive Processes of Membrane
Transport?

• Membranes have selective permeabilitysome
  substances can pass through, but not others.
• Passive transportno outside energy required
  (diffusion).
• Active transportenergy required.

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6.3 What Are the Passive Processes of Membrane
Transport?

• Energy for passive transport comes from the
  concentration gradient the difference in
  concentration between one side of the membrane
  and the other.

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6.3 What Are the Passive Processes of Membrane
Transport?

• Particles in a solution move randomly until they
  are evenly distributed.
• At equilibrium, the particles continue to move,
  but there is no net change in distribution.

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In-Text Art, Ch. 6, p. 113
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6.3 What Are the Passive Processes of Membrane
Transport?

• Diffusion the process of random movement toward
  equilibrium.
• Net movement is directional until equilibrium is
  reached.
• Diffusion is the net movement from regions of
  greater concentration to regions of lesser
  concentration.

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6.3 What Are the Passive Processes of Membrane
Transport?

• Diffusion rate depends on
• Diameter of the molecules or ions
• Temperature of the solution
• Concentration gradient

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6.3 What Are the Passive Processes of Membrane
Transport?

• Diffusion works very well over short distances
  (e.g., within a cell).
• Membrane properties affect the diffusion of
  solutes.
• A membrane is permeable to solutes that move
  easily across it impermeable to those that
  cannot.

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6.3 What Are the Passive Processes of Membrane
Transport?

• Simple diffusion Small molecules pass through
  the lipid bilayer.
• Water and lipid-soluble molecules can diffuse
  across the membrane.
• Electrically charged and polar molecules can not
  pass through easily.

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6.3 What Are the Passive Processes of Membrane
Transport?

• Osmosis the diffusion of water.
• It depends on the relative concentrations of
  water molecules on each side of the membrane.
• Hypertonic higher solute concentration
• Isotonic equal solute concentrations
• Hypotonic lower solute concentration

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Figure 6.9 Osmosis Can Modify the Shapes of
Cells (Part 1)
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Figure 6.9 Osmosis Can Modify the Shapes of
Cells (Part 2)
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Figure 6.9 Osmosis Can Modify the Shapes of
Cells (Part 3)
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6.3 What Are the Passive Processes of Membrane
Transport?

• If two solutions are separated by a membrane that
  allows water, but not solutes, to pass through
• Water will diffuse from the region of higher
  water concentration (lower solute concentration)
  to the region of lower water concentration
  (higher solute concentration).

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6.3 What Are the Passive Processes of Membrane
Transport?

• Water will diffuse (net movement) from a
  hypotonic solution across a membrane to a
  hypertonic solution.
• Animal cells may burst when placed in a hypotonic
  solution.
• Plant cells with rigid cell walls build up
  internal pressure that keeps more water from
  enteringturgor pressure.

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6.3 What Are the Passive Processes of Membrane
Transport?

• Facilitated diffusion of polar molecules
  (passive)
• Channel proteinsintegral membrane proteins that
  form a channel.
• Carrier proteinsmembrane proteins that bind some
  substances and speed their diffusion through the
  bilayer.

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6.3 What Are the Passive Processes of Membrane
Transport?

• Ion channels Channel proteins with hydrophilic
  pores.
• Most are gatedcan be closed or open to ion
  passage.
• Gate opens when protein is stimulated to change
  shape by a chemical signal (ligand) or an
  electrical charge difference (voltage-gated).

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Figure 6.10 A Gated Channel Protein Opens in
Response to a Stimulus
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6.3 What Are the Passive Processes of Membrane
Transport?

• The potassium channel allows K in the unhydrated
  state to pass through, but hydrated Na is too
  large to pass.

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In-Text Art, Ch. 6, p. 116
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6.3 What Are the Passive Processes of Membrane
Transport?

• Water can cross a membrane by moving through
  special water channels called aquaporins.
• The function of these proteins was determined by
  injecting the aquaporin proteins into a frog
  oocyte.

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Figure 6.11 Aquaporins Increase Membrane
Permeability to Water
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6.3 What Are the Passive Processes of Membrane
Transport?

• Carrier proteins transport polar molecules such
  as glucose across membranes in both directions.
• Glucose binds to the protein, causing it to
  change shape and release the glucose on the other
  side.

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Figure 6.12 A Carrier Protein Facilitates
Diffusion (Part 1)
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6.3 What Are the Passive Processes of Membrane
Transport?

• In carrier-mediated transport, the rate of
  diffusion is limited by the number of carrier
  proteins in the cell membrane.
• When all carriers are loaded with solute, the
  diffusion system is saturated.
• Cells that need lots of energy (e.g., muscle
  cells) have many glucose transporters.

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Figure 6.12 A Carrier Protein Facilitates
Diffusion (Part 2)
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6.4 What Are the Active Processes of Membrane
Transport?

• Active transport moves substances against a
  concentration and/or electrical gradient.
  Requires energy.
• The energy source is often adenosine triphosphate
  (ATP).

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Table 6.1
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6.4 What Are the Active Processes of Membrane
Transport?

• Active transport is directional. It involves
  three kinds of proteins
• Uniportermoves one substance in one direction
• Symportermoves two substances in one direction
• Antiportermoves two substances in opposite
  directions

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Figure 6.13 Three Types of Proteins for Active
Transport
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6.4 What Are the Active Processes of Membrane
Transport?

• Primary active transport requires direct
  hydrolysis of ATP.
• Secondary active transport energy comes from an
  ion concentration gradient that is established by
  primary active transport.

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6.4 What Are the Active Processes of Membrane
Transport?

• The sodiumpotassium (NaK) pump is primary
  active transport.
• Found in all animal cells.
• The pump is an integral membrane glycoprotein (an
  antiporter).

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Figure 6.14 Primary Active Transport The
SodiumPotassium Pump
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6.4 What Are the Active Processes of Membrane
Transport?

• In secondary active transport, energy can be
  regained by letting ions move across a membrane
  with the concentration gradient.
• Aids in uptake of amino acids and sugars.
• Uses symporters and antiporters.

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Figure 6.15 Secondary Active Transport
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6.5 How Do Large Molecules Enter and Leave a Cell?

• Macromolecules (proteins, polysaccharides,
  nucleic acids) are too large to cross the
  membrane.
• They can be taken in or secreted by means of
  membrane vesicles.

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6.5 How Do Large Molecules Enter and Leave a Cell?

• Endocytosis processes that brings molecules and
  cells into a eukaryotic cell.
• The plasma membrane folds in or invaginates
  around the material, forming a vesicle.

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Figure 6.16 Endocytosis and Exocytosis
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6.5 How Do Large Molecules Enter and Leave a Cell?

• Phagocytosis molecules or entire cells are
  engulfed. Some protists feed in this way. Some
  white blood cells engulf foreign substances in
  this way.
• A food vacuole or phagosome forms, which fuses
  with a lysosome.

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6.5 How Do Large Molecules Enter and Leave a Cell?

• Pinocytosis a vesicle forms to bring small
  dissolved substances or fluids into a cell.
  Vesicles are much smaller than in phagocytosis.
• Pinocytosis is constant in endothelial
  (capillary) cells.

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6.5 How Do Large Molecules Enter and Leave a Cell?

• Receptor mediated endocytosis is highly specific
• Depends on receptor proteinsintegral membrane
  proteinsto bind to specific substances.
• Sites are called coated pitscoated with other
  proteins such as clathrin.

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Figure 6.17 Receptor-Mediated Endocytosis
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6.5 How Do Large Molecules Enter and Leave a Cell?

• Mammalian cells take in cholesterol by
  receptor-mediated endocytosis.
• In the liver, cholesterol is packaged into
  low-density lipoprotein, or LDL, and secreted to
  the bloodstream.
• Cells that need cholesterol have receptors for
  the LDLs in clathrin-coated pits.

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6.5 How Do Large Molecules Enter and Leave a Cell?

• Exocytosis material in vesicles is expelled from
  a cell.
• Indigestible materials are expelled.
• Other materials leave cells such as digestive
  enzymes and neurotransmitters.

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Table 6.2
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6 Answer to Opening Question

• Aquaporins in both animals and plants are similar
  in structure.
• Aquaporins are being inserted into synthetic
  membranes to purify drinking waterthey allow
  only water to pass through, not solutes.