Biological Membranes and Transport

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Biological Membranes and Transport

• Brooker Chapter 5

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Membranes of the Cell

• Plasma membrane (Cell membrane)
• Membranes that make-up organelles

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All cell membranes are made of phospholipids and
proteins

• Phospholipids have hydrophilic heads and
  hydrophobic carbon chain tails
• Phospholipids form a bilayer membrane

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The Phospholipid Bilayer

• Hydrophilic heads
• phosphate
• other charged groups

• Hydrophobic tails
• fatty acid side chains
• saturated
• unsaturated

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Sandwich and Fluid Mosaic Models
How are proteins and lipids arranged in cell
membranes?
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Proteins and lipids can move within the membrane

• Lateral movement of lipids/proteins is quick
• Lipids and proteins rarely flip across the lipid
  bilayer

Fluid Mosaic Model
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The Fluid Mosaic Model Today

• There are three types of membrane proteins
• Integral membrane proteins
• Transmembrane proteins
• Lipid anchored proteins
• Peripheral proteins
• Are attached to other proteins or lipids on the
  surface of the membrane

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ABO Blood Groups in Humans
FYI

• Cells that have A B blood types have slightly
  different versions of the enzyme galactosyl
  transferase. This enzyme adds a sugar chain to
  a membrane protein.
• Type O blood has a protein that is unable to add
  this sugar

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Did you know?
FYI

• Approximately 25 of all cellular proteins are
  membrane proteins
• Approximately 70 of all medications exert their
  effects by binding to membrane proteins
• Aspirin, ibuprofen, acetaminophen bind to an ER
  membrane protein that is necessary for the
  synthesis of chemicals that play a role in pain
  sensation and inflammation

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Membrane Permeability Getting things in and out
of the cell

• Biological membranes are selectively permeable
• Small molecules and lipids pass through the
  membrane
• Larger and charged/polar molecules cannot
• Proteins allow movement of charged/polar
  molecules

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Transport Across Membranes

• Diffusion and Osmosis
• Channel proteins
• Transport Proteins
• ATPase Pumps
• Bulk Transport
• Endocytosis and Exocytosis

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Diffusion

• Molecules in solution tend to slowly spread apart
  over time. This is diffusion.

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Diffusion is Concentration Dependant

• Concentration gradient a difference in
  concentration of a substance
• Diffusion is the net movement of particles from a
  region of higher to lower concentration down a
  concentration gradient.
• At equilibrium particles are uniformly distributed

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Concentration gradient
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Passive Diffusion
Movement of gases and small hydrophobic molecules

• Unaided by proteins
• No energy required
• Move across a concentration gradient (high-gt
  low)
• Dependant upon hydrophobicity of the molecule

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Osmosis

• ALL compounds undergo diffusion.
• Osmosis is the diffusion of water across a
  membrane.
• Since cells have membranes, osmosis is important
  to cells
• Water can pass directly through the membrane OR
• In special cells, it crosses the plasma membrane
  through proteins called aquaporins

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Osmotic Terminology
Solution Solute Solvent

• Hypertonic more concentrated
• Hypotonic less concentrated
• Isotonic equally concentrated

Refers to the solute concentration

• Hyper, hypo, and iso are used to compare two
  solutions to each other.

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Membranes and Osmosis

• Tank w/ semi-permeable membrane water may pass,
  solute (particles)cant
• At first the concentration of solute is very high
  on the left.

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• But over time, the water moves across the
  semi-permeable membrane, and dilutes the
  particles.

A
B
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Animal Cells and Osmotic Pressure
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Cells and Osmotic Pressure

• All cells have thin delicate membranes
• They respond to differences in external solute
  concentration and osmotic pressure
• If placed in an isotonic environment (usually
  0.9 w/vol NaCl), no change in shape
• If placed in an hypertonic environment (high
  salt) then the cell shrinks
• If placed in an hypotonic environment (low salt),
  then the cell expands and can lyse (burst)

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Plant Cells and Osmosis
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Plant Cells and Osmosis

• In plants, hypotonic solutions produce osmotic
  pressure also called turgor pressure
• Turgor means tight or stiff owing to being very
  full
• Keeps plant upright in hypertonic conditions
  plants wilt

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The exchange of CO2 between body cells and RBCs
FYI
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Movement of Molecules across a membrane

• Passive Diffusion
• Protein independent movement of molecules
• Simple diffusion of small molecules across the
  bilayers
• Protein dependant movement
• Facilitated diffusion
• Active transport
• Bulk Transport Endocytosis and Exocytosis

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Transport Across Membranes

• Facilitated Diffusion
• Channel Proteins
• Ion Channels
• membrane potential
• CFTR
• Transporters
• Uniporters
• Symporters
• Antiporters

• Active transport
• ATP Pumps
• Na/K Pump

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Facilitated diffusion

• Channel Proteins
• Protein channels in the plasma membrane
• Transport ions with the concentration gradient

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Facilitated diffusion

• Channel proteins
• A transport protein aids the movement of solutes
  down a concentration gradient.
• Movement is powered by a concentration gradient.

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Cystic Fibrosis

• Most common lethal autosomal recessive genetic
  disease of Caucasian population
• CFTR is the gene for a Cl- ion channel protein
• Channels are located in the epithelia of the
  lungs, pancreas, sweat glands, kidneys
• Chloride channels defective or absent
• Sodium channels effected too

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2. Transporters

• Uniporter
• one molecule/time down a concentration gradient
• Symporter/ Antiporter
• Simultaneous mvmt of two different molecules

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• Uniporters move a single solute in one direction.
• Uniporters are different from channels because
  they dont move just ions.

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• Symports move 2 solutes in same direction

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• Antiports move 2 solutes in opposite direction
• One must bind, then the other, to the same
  transmembrane protein, then both move

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Active Transport

• ATP-Pumps
• transport against concentration gradients
• depends upon ATP hydrolysis
• Organelles/Organs that use pumps to maintain
  acidic pH lysosomes, vacuoles, and the stomach

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Active Transport

• ATP-derived energy is used to move solutes across
  the membrane, against a gradient.
• Shown the Na/K pump
• Here, two gradients are formed, one for each ion.
• Gradients have opposite orientation
• Sodium is high outside, low inside
• Potassium is high inside and low outside

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The Na/K ATPase (Pump) in Action
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Other Pumps FYI
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Pumps, Channels, and Transporters work together
to maintain levels of ions and other molecules in
the cell
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Summary of Transport, Diffusion, and Osmosis
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Bulk Transport

• Exocytosis out of the cell
• Endocytosis into the cell
• Phagocytosis
• Receptor mediated endocytosis
• Pinocytosis intake of fluids

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Transport by Vesicles Exocytosis

• Very energy-expensive !!
• How are the vesicles getting from the Golgi to
  the Plasma membrane?

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Endocytosis
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Endocytosis Phagocytosis

• Uptake of particles
• Phagocytosis Cell eating (bacteria, viruses,
  etc)
• Food particles are digested by fusion of the
  uptake vesicle with lysosomes

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Receptor-Mediated Endocytosis

• Receptors on the surface of cell binds specific
  ligand
• Transport by clathrin coated vesicles
• Here, cholesterol uptake by the LDL receptor

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0.25µm
(b)