Ch 5 Membrane Structure and Function

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Ch 5 Membrane Structure and Function

• Control the movement of materials into and out of
  the cell.

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Membrane Structure

• Phospholipids arranged in a bilayer due to
  polarity of molecule.
• Glycolipids Similar to phospholipids mono or
  polysaccharides instead of phosphates. Cell to
  cell communication
• Cholesterol lipid found in animal plasma
  membranes, reduces permeability and provides
  rigidity.

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Membrane Structure Contd

• Membrane Proteins Integral proteins largely
  determine the membranes function(s).
• Channel proteins, Carrier proteins, Receptor
  proteins, Enzymatic proteins, Glycoproteins.

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Membrane structure

• Fluid Mosaic Model The composition of
  phospholipids will contribute to the degree of
  permeability, and the proteins are free to move
  laterally w/in the plasma membrane.

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QOD

• 1. How would you expect the saturation levels of
  membrane fatty acids to differ in plants adapted
  to cold environments and plants adapted to hot
  environments?

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Answers

• Plants adapted to cold environments would be
  expected to have more unsaturated fatty acids in
  their membranes, since those remain fluid at
  lower temperatures. Plants adapted to hot
  environments would be expected to have more
  saturated fatty acids, which would allow the
  fatty acids to stack more closely, making the
  membranes less fluid and therefore helping them
  to stay intact at higher temperatures.

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Permeability of the Plasma Membrane
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Selective Permeability

• Some substances can freely move across the
  membrane whereas others cannot.
• Hydrophobic (nonpolar) molecules, such as
  hydrocarbons, carbon dioxide, and oxygen, can
  cross with ease.
• Small Polar molecules (H2O) can pass between the
  phospholipids

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Selective Permeability contd

• Macromolecules cannot pass through membrane
• Transported by vesicle formation
• Ions and charged molecules cannot
• Carrier and channel proteins transport these.
• Transport across the plasma membrane includes
• Passive transport
• Facilitated transport
• Active transport
• Membrane assisted

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Passive Transport

• Diffusion The spontaneous tendency of a
  substance to move down its concentration gradient
  from a more concentrated to a less concentrated
  area.
• The diffusion of a substance across a biological
  membrane passive transport

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Note that each substance diffuses down its own
concentration gradient, unaffected by the
concentration differences of other substances
What affects the rate of diffusion?
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Passive Transport contd

• Osmosis The diffusion of water across the plasma
  membrane.
• Solutions a liquid that is a homogeneous mixture
  of two or more substances. Has 2 parts
• Solvent The dissolving agent of a solution
  (liquid portion) . Water is the most versatile
  solvent known.
• Solute A substance that is dissolved in a
  solution.

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Passive Transport contd

• Tonicity The ability of a solution to cause a
  cell within it to gain or lose water. 3 types
• Hypertonic A solution with a higher
  concentration of solute than inside the cell.
• Water diffuses out.
• Hypotonic A solution with a lower concentration
  of solute than inside the cell.
• Water diffuses into.
• Isotonic A solution with equal concentrations of
  solute on both sides of the membrane.
• No net movement of water into or out of.

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Classify the tonicity of the blood that these red
blood cells are in? Explain your prediction using
the glucose(blood sugar) and water levels both
inside and outside of each cell.
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• Turgid A walled cell becomes turgid if it has a
  greater solute concentration than its
  surroundings, resulting in entry of water.
  (turgor pressure) (Tonicity?)
• Plasmolysis A phenomenon in walled cells in
  which the cytoplasm shrivels and the plasma
  membrane pulls away from the cell wall when the
  cell loses water to a hypertonic environment.

Crenate
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Transport by Proteins

• Molecules that cannot diffuse across the plasma
  membrane, can be transported by integral membrane
  proteins.
• Channel Carrier Proteins Specific to the
  molecule they transport
• Required for facilitated and active transport.

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Facilitated Transport

• An integral protein (channel or carrier) assists
  the movement of a molecule down its concentration
  gradient.

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Facilitated Diffusion

• Substances too big for passive, but too small for
  Memb Ass. Trans.
• Specific to molecule they trans.
• Increases the rate at which the solute crosses
  the plasma membrane

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Active Transport

• The movement of molecules against their
  concentration gradient

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Active Transport

• Movement from low concentration to high
  concentration.
• Requirements
• Carrier protein (specific to molecule) aka pumps
• Energy (ATP)

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• The sodiumpotassium pump a specific case of
  active transport. 
• -pumps ions against steep concentration gradients
• -Sodium ion concentration (represented as Na)
  is high outside the cell and low inside, while
  potassium ion concentration (K) is low outside
  the cell and high inside.
• -The pump oscillates between two conformational
  states in a pumping cycle that moves three sodium
  ions out of the cell for every two potassium ions
  pumped into the cell.
• - ATP powers the changes in conformation by
  phosphorylating the transport protein (that is,
  by transferring a phosphate group to the
  protein).
• Fig 5.11

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Proton pumps, the main electrogenic pumps of
plants, animals, fungi, and bacteria, are
membrane proteins that store energy by generating
voltage (charge separation) across membranes of
mitochondria and chloroplasts.
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Membrane Assisted Transport

• The formation of vesicles by the plasma membrane
  to enable the transport of macromolecules (too
  big for transport proteins)
• - Bulk transport

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Membrane Assisted Transport

• Exocytosis out of cell Vesicles (formed by
  Golgi) fuse with plasma membrane to secrete
  specific molecules to external environment.

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Membrane Assisted Transport

• Endocytosis into cell invagination and
  pinching off of plasma membrane to form a vesicle
  to take in certain substances into cell.
• Types
• Phagocytosis cell eating solids such as food
  particles or other cells.
• Pinocytosis cell drinking liquids.

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