The Cell Membrane

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The Cell Membrane Cell Transport

• Packet 12

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Fluid Mosaic Model
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Properties of Phospholipids

• Molecules with both hydrophilic and hydrophobic
  properties are termed amphipathic
• Other examples
• Sterols
• Cholesterol
• Glycolipids
• Hydrophilic (sugar) head
• The aqueous environment inside and outside the
  cell prevent membrane lipids from escaping the
  bilayer

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Fluidity of the Membrane

• Depends on Two Main Features
• Saturated vs. Unsaturated Fatty Acid tails
  (phopsholipids)
• Unsaturated more fluid
• Kinks prevent molecules from packing together
• Cholesterol
• Absent in plants, yeast and bacteria
• Fill the holes produced by kinks
• Stiffens bilayer and makes it less fluid and
  permeable.

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Functions of Membrane Proteins

• Cell Membrane Proteins
• Six different functions
• See Figure 7.9 for details

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Integral vs. Peripheral Proteins

• Integral Proteins
• A protein that is firmly anchored in the plasma
  membrane via interactions between its hydrophobic
  domains and the membrane phospholipids
• Directly attached to the membrane

• Peripheral Proteins
• Not embedded in the lipid bilayer
• Can be released from the membrane by relatively
  gentle extraction procedures

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Transmembrane Protein

• Protein that spans the entire membrane
• Have both hydrophobic and hydrophilic regions
• Alpha helical secondary structure is normally the
  hydrophobic regions of the protein

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Transport of Materials Into and Out of Cells
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Permeability of the Lipid Bilayer

• Permeable

• Non-Permeable

• Small hydrophobic molecules
• O2
• CO2
• N2
• Benzene
• Small Uncharged Polar Molecules
• H2O
• Glycerol
• Ethanol

• Larger Uncharged Polar Molecules
• Amino Acids
• Glucose
• Nucleotides
• Ions
• H
• Na
• HCO3-
• K
• Ca2
• Cl-
• Mg2

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Introduction

• There are 5 ways of transporting materials across
  the cell membrane
• Diffusion
• Regular Facilitated
• Passive Transport
• Active transport
• Osmosis
• Phagocytosis
• Pinocytosis

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Diffusion

• The movement of a substance from an area of high
  concentration to an area of low concentration
• The difference in concentration between the two
  regions is known as the concentration gradient

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Diffusion
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Rate of Diffusion

• The rate of diffusion depends on
• The difference in concentration
• The greater the concentration gradient, the
  faster the process
• The distance between the two regions
• Smaller distance means faster process
• The area
• If the total area is increased, the faster the
  process
• The size of the molecules
• Small and fat-soluble molecules will diffuse
  faster

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Regular vs Facilitated Diffusion

• Regular Diffusion
• Movement of molecules is from high concentration
  to low concentration
• No proteins are used
• No energy (ATP) is required
• Facilitated Diffusion
• Movement of molecules is from high concentration
  to low concentration
• Proteins are used
• No energy (ATP) is required

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Facilitated Transport
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Glucose Carrier

• Carrier protein
• Found in the plasma membrane of liver cells
• Glucose, an uncharged molecule, is plentiful,
  outside the cell, after a meal
• Glucose is moved from a high concentration to an
  area of low concentration
• Which form of glucose can cells only use in
  Glycolysis?
• D-glucose and not the mirror image L-glucose

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Glucose Carrier II

• Hungry?
• Blood glucose is low
• Hormone Glucagon glycogen to glucose
• Glucose concentration higher inside the liver
  cell than outside
• Glucose moves from high concentration to low
  concentration

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Electrogenic Pump

• Electrically charged molecules
• Small organic or inorganic ions
• MOST cell membranes have a voltage across them
• Difference in electric potential on each side is
  called the membrane potential.

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Electrogenic Pump

• Exerts a force on any molecule that carries an
  electrical charge
• Cytoplasmic side is USUALLY at a negative
  potential relative to the outside and this tends
  to pull positively charged solutes into the cell
  and drive negative charged ones outside the cell
• Net driving force electrochemical gradient

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The Sodium Potassium Pump

• For some, ions, voltage and concentration
  gradients work in the same direction
• Sodium Potassium Pump

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Active Transport

• Materials are moved against the concentration
  gradient
• Molecules move from an area of low concentration
  to an area of high concentration
• Proteins are used to move materials across the
  membrane

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Active Transport II

• Energy is used
• Because energy is used, cells carrying out active
  transport have
• A high respiratory rate
• Many mitochondria
• A high concentration/reserve of ATP
• Any factor which reduces or stops cell
  respiration will stop active transport
• Cyanide

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Active Transport III

• Cells carry our active transport in three ways
• Coupled transport (co-transport)
• ATP driven pumps
• Couple uphill transport with hydrolysis of ATP
• Light driven pumps
• Found mainly in bacterial cells
• Input of energy from light
• Bacteriohodopsin

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Co-transport
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Co-transport

• Hydrogen gradients are used to drive membrane
  transport in plants, fungi and bacteria
• They do not have sodium-potassium pumps
• Hydrogen pumps, found in the plasma membrane,
  pump H out of the cell
• Setting up an electrochemical gradient

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Co-transport

• Pump creates an acid pH in the medium surrounding
  the cell
• The uptake of sugars and amino acids into
  bacterial cells, for example, are driven by H
  pumps

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H Pumps in Bacteria

• In some photosynthetic bacteria, the H gradient
  is created by the activity of light driven H
  pumps such as bacteriorhodopsin.
• In plants and fungi and many other bacteria, the
  gradient is set up by ATPases in their plasma
  membrane

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Co-transport

• The Na gradient generated by the
  sodium-potassium pump can be used to drive active
  transport of a 2nd molecule.
• The downhill movement of the first solute down
  provides the energy to drive the uphill transport
  of the second.

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Glucose Absorption II

• Plasma membrane of kidney cells and intestinal
  cells
• Active export of Na and import of K
• Glucose is actively transported into the cell by
  Na and is released from the cell down the
  concentration gradient via passive transport at
  the basal and lateral surfaces.

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Review
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Osmosis

• Transfer of a liquid solvent through a semi
  permeable membrane, that does not allow dissolved
  solids (solutes) to pass from an area of high
  concentration to an area of low concentration
• Review of solvents vs solutes

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Osmosis
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Osmosis and Animal Cells

• Osmotic Pressure
• The driving force for the water is the difference
  in water pressure
• Osmotic pressure
• The pressure exerted by the flow of water through
  a semi-permeable membrane separating two
  solutions with different concentrations of solute
• If a solution is separated from pure water by a
  semi-permeable membrane, the pressure which must
  be applied to prevent osmosis is called the
  osmotic pressure
• Hypothetical situation
• Solution does not actually exert any pressure in
  normal circumstances, the term osmotic potential
  is preferred

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Osmosis and Animal Cells II

• Osmotic Potential
• Difference in osmotic pressure that draws water
  from an area of less osmotic pressure to an area
  of greater osmotic pressure.
• The potential of a solution to pull in water
• Value is always negative
• The more concentrated the solution, the more
  negative its osmotic potential

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Osmosis and Animal Cells III

• Isotonic
• When two solutions have the same osmotic
  potential
• Hypertonic vs. Hypotonic
• Hypertonic
• When one solution has a greater osmotic potential
  than another
• Contains a higher concentration of solute
• More concentrated
• Hypotonic
• When one solution has a lower osmotic potential
  than the other
• Contains a lower concentration of solute in
  comparison to the other
• Less concentrated

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Osmosis
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Osmosis and Plant Cells I

• Although the osmotic principles apply equally to
  plant and animal cells, a different set of terms
  is currently applied to the osmotic relationship
  of plant cells

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Osmosis and Plants II

• Water potential
• Represented by the Greek letter psi
• ?
• A measure of the tendency of water to leave a
  solution
• Pure water has a water potential of zero
• Solute molecules tend to prevent water from
  leaving the solution. Therefore, as the amount of
  solute increases, the water potential decreases
• Giving the solution a lower water potential than
  pure water
• The more concentrated the solution, the less the
  water potential

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Osmosis Plant Cells III

• Plant cell
• Can be considered as a solution of salts and
  sugars in the vacuole surrounded by a
  semi-permeable membrane
• Tonoplast
• Cytoplasm
• Plasma membrane
• And a slightly elastic but completely permeable
  cell wall.
• A plant cell therefore has more water potential
  than pure water and will draw in water when
  surrounded by it

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Osmosis and Plants IV

• This entry of water forces the living part of the
  cell, known as the protoplast, against the cell
  wall.
• This pressure is known as pressure potential
• ?p
• In a turgid plant cell that has a positive value,
  although the xylem of a transpiring plant(which
  is under tension) it is negative
• The water potential of a cell is changed by the
  presence of the solute. The change in water
  potential, as a result of the solute, is referred
  to as the solute potential

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Osmosis and Plants V

• Solute Potential
• ?s
• As the solute molecules invariably lower the
  water potential, its value is always negative.
  Here is the relationship between the three terms
• ? ?s ?p

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Phagocytosis

• The take up of large particles by cells via
  vesicles formed in the plasma membrane
• The cell invaginates to form a depression in
  which particles are contained
• This then pinches off to form a vacuole
• White blood cells
• Neutrophils
• Monocytes

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Pinocytosis

• The take up of liquids rather than solids
• Vacuoles are smaller than those used during
  phagocytosis

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Endocytosis vs. Exocytosis

• Both phagocytosis and pinocytosis involve the
  taking of materials into the cell in bulk.
• These are examples of endocytosis
• The removal of materials from the cell in bulk is
  called exocytosis.

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