Chapter 7 Membrane Structure and Function

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Chapter 7 Membrane Structure and Function
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Plasma Membrane

• The membrane at the boundary of every cell.
• Functions as a selective barrier for the passage
  of materials in and out of cells.

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Membrane Composition

• phospholipids
• Proteins
• Question How are the
  materials arranged?

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Phosphophospholipid Bilayer

• Phospholipids
• Hydrophilic heads
• Hydrophobic tails

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Membrane Models
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Davson-Danielli Model 1935

• phospholipid bilayer.
• Proteins coat the surfaces.
• Sometimes called the sandwich model.

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Evidence

• Biochemical work.
• TEM pictures show the membrane as a double line.

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Problems

• Not all membranes in a cell were the same.
• How could the proteins stay in place?
• Result - the model was questioned and tested by
  scientific process.

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Fluid Mosaic Model 1972

• New model to fit the new evidence with membranes.
• Example of Science as a
  Process.

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Fluid Mosaic Model

• Refers to the way the phospholipids and proteins
  behave in a membrane.

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Fluid

• Refers to the phospholipid bilayer.
• Molecules are not bonded together, so are free to
  shift.
• Must remain "fluid" for membranes to function.

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Ways to keep the membrane fluid

• phospholipid changes or shifts
• Cold hardening of plants (shift to
  unsaturated fatty acids).
• Hibernating animals (Cholesterol increase).

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Mosaic

• Proteins float in a sea of phospholipids.
• Proteins form a collage or mosaic pattern that
  shifts over time.

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Evidence

• TEM pictures of fractured membranes.
• Cell fusion studies.
• Tagging of membrane proteins by antibodies.

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Protein Function in Membranes

• Transport.
• Enzymatic activity.
• Receptor sites for signals.
• Cell adhesion.
• Cell-cell recognition.
• Attachment to the cytoskeleton.

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Types of Membrane Proteins

• Integral - inserted into the phospholipid
  bilayer.
• Peripheral - not embedded in the phospholipid
  bilayer, but are attached to the membrane
  surface.

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Question?

• How do the integral proteins stick to the
  membrane?
• By the solubility of their amino acids.

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Hydrophilic Amino Acids
Hydrophobic Amino Acids
Hydrophilic Amino Acids
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Membranes are Bifacial

• The phospholipid composition of the two layers is
  different.
• The proteins have specific orientations.
• Carbohydrates are found only on the outer surface.

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Carbohydrates
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Membrane Carbohydrates

• Branched oligosaccharides form glycophospholipids
  and glycoproteins on external surface.
• Function - recognition of "self" vs "other.

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Question

• How do materials get across a cell's membrane?

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Problems

• phospholipid bilayer is hydrophobic. Hydrophilic
  materials don't cross easily.
• Large molecules don't cross easily. Too big to
  get through the membrane.

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Mechanisms

• 1. Passive Transport
• 2. Active Transport

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Passive Transport

• Movement across membranes that does NOT require
  cellular energy.

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Types of Passive Transport

• 1. Diffusion
• 2. Osmosis
• 3. Facilitated Diffusion

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Diffusion

• The net movement of atoms, ions or molecules down
  a concentration gradient.
• Movement is from High Low

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Equilibrium

• When the concentration is equal on both sides.
• There is no net movement of materials.

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Factors that Effect Diffusion

• 1. Concentration
• 2. Temperature
• 3. Pressure
• 4. Particle size
• 5. Mixing

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Osmosis

• Diffusion of water.
• Water moving from an area if its high
  concentration to an area of its low
  concentration.
• No cell energy is used.

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Tonicity

• The concentration of water relative to a cell.
• 1. Isotonic (same)
• 2. Hypotonic (below)
• 3. Hypertonic (above)

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Isotonic

• Isosmotic solution.
• Cell and water are equal in solute concentration.
• No net movement of water in or out of the cell.
• No change in cell size.

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Hypotonic

• Hypoosmotic solution
• Cell's water is lower than the outside water
  (more solutes).
• Water moves into the cell.
• Cell swells, may burst or the cell is turgid.

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Hypertonic

• Hyperosmotic solution
• Cell's water is higher than the outside water
  (less solutes)
• Water moves out of the cell.
• Cell shrinks or plasmolysis occurs.

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Facilitated Diffusion

• Transport protein that helps materials through
  the cell membrane.
• Doesn't require energy (ATP).
• Works on a downhill concentration gradient.

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Aquaporins

• Newly found channels for osmosis.

GFP labeled Aquaporins
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Active Transport

• Movement across membranes that DOES require
  cellular energy.

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Types of Active Transport

• 1. Carrier-Mediated
• 2. Endocytosis
• 3. Exocytosis

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Carrier-Mediated Transport

• General term for the active transport of
  materials into cells AGAINST the concentration
  gradient.
• Movement is low high

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Examples

• 1. Na- K pump
• 2. Electrogenic or H pumps
• 3. Cotransport

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Na- K pump

• Moves Na ions out of cells while moving K ions
  in.

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Electrogenic or H pumps

• Also called Proton pumps.
• Create voltages across membranes for other cell
  processes.
• Used by plants, fungi and bacteria.

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Cotransport

• Movement of H that allows other materials to be
  transported into the cell as the H diffuses back
  across the cell membrane.
• Example - Sucrose transport

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Exocytosis

• Moves bulk material out of cells.
• Example - secretion of enzymes.

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Endocytosis

• Moves bulk materials into cells.
• Several types known.

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Types

• 1. Pinocytosis - liquids
• 2. Phagocytosis - solids
• 3. Receptor Mediated - uses receptors to "catch"
  specific kinds of molecules.

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Carbohydrates
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Forming vesicles
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Summary

• Know membrane structure.
• Be able to discuss the various methods by which
  cells move materials through membranes.
• Be able to solve problems in osmosis.