Membrane Chapter 7

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MembraneChapter 7
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Cell membrane
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Cell Membrane

• Plasma membrane
• Selective permeability
• Surrounds all living cells
• 2 molecules thick.

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Plasma Membrane

• Interacts with environment
• Exchanges information
• Manages the movement in and out of the cell

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Fluid mosaic model

• Revised in 1972 by Singer Nicolson
• Globular proteins imbedded in the membrane
• Not surrounding the lipid layer
• Proteins float in the membrane like boats in a
  pond.

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Plasma membrane

• Lipid membrane (phospholipids)
• Protein passageways
• Allow water-soluble substances to pass

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Membrane Structure

• 1. Phospholipid layer
• 2. Cholesterol
• 3. Transmembrane proteins
• 4. Interior protein network
• 5. Cell surface markers

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Membrane structure
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Triglycerides

• Water insoluble
• Non-polar

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Phospholipid

• Glycerol (3 carbon alcohol)
• 2 fatty acids
• Phosphate attached to 3rd carbon

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Phospholipid bilayer

• Orients polar head toward water molecules
• Non-polar tail away from water.
• Non-polar barrier to water soluble molecules

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Phospholipid bilayer

• Hydrophobic interactions hold membrane together
• Phospholipids unanchored proteins move freely
  within the membrane

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Cholesterol

• Cholesterol between lipids
• Increases or decreases fluidity
• Depends on temperature.

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Cholesterol
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Structures of membrane proteins

• 1. Peripheral proteins
• Attached to the surface membrane
• 2. Integral proteins
• Extend through the membrane
• Transmembrane proteins
• Partway

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Structures of membrane proteins

• Non-polar section of protein
• Contact with the non-polar interior (hydrophobic)
• Polar section of the protein
• Extends out of membrane

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Transmembrane proteins

• 1. Single-pass anchors
• 2. Multiple-pass channels and carriers
• 3. Pores
• Aquaporins

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Functions of membrane proteins

• 1. Transport
• Allow specific substances to enter or leave
• 2. Enzymes
• Carry out reactions on interior surface
• 3. Cell surface receptors
• Sensitive to chemical messages

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Functions of membrane proteins

• 4. Cell surface identity markers
• Cell recognition between cells
• Glycoproteins
• 5. Cell adhesion proteins
• Proteins stick to each other
• Temporary or more permanently
• 6. Attachments to the cytoskeleton

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Membrane Proteins
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Membrane structure
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Membrane formation

• Proteins lipids made in ER
• Processed in Gogi apparatus
• Vesicles
• Fuse with membrane
• Correct orientation

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Movement across the membrane

• 1. Passive transport
• 2. Bulk transport
• 3. Active transport

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Passive transport

• Diffusion
• High concentration to areas of lower
  concentration
• Concentrations are the same in all regions

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Passive transport

• Two conditions determine the direction of the
  flow of ions
• 1. Relative concentration of the ion on each side
  of the membrane
• 2. The voltage across the membrane

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Diffusion
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Osmosis

• Net movement of water across a membrane
• Moves towards an area of higher solute
  concentration
• Or lower water concentration
• Passive transport of water

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Osmosis

• Osmoregulation
• Control of water balance

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Osmosis

• Hyperosmostic
• Solution with higher concentration of solute
• Has less water
• Hypoosmostic
• Solution with the lower concentration of solute
• Has more water
• Isosmostic
• Solutions have equal concentration of solutes

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Solutions

• Solute
• Substance being dissolved
• Common solutes in the cell or cells cytoplasm?
• Solvent
• Substance doing the dissolving
• What is the most common solvent in the cell?

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Solutions

• Hypertonic
• Solution with higher concentration of solute
• Hypotonic
• Solution with lower concentration of solute
• Isotonic
• Solutions with equal solute concentrations

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Facilitated diffusion

• Carriers transport
• Ions, aa or sugars across the membrane
• Specific for type of solute
• Moves from high concentration to low
• The carrier facilitates the process

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Examples

• RBC diffusion of the bicarbonate ion
• RBC diffusion of glucose

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Facilitated diffusion

• 1. It is specific
• 2. It is passive
• 3. It saturate
• Carriers are saturated
• Transport rate cannot increase
• Despite the concentration gradient

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Bulk transport

• Endocytosis
• Membrane envelops food particles
• 1. Phagocytosis
• 2. Pinocytosis
• 3. Receptor-mediated endocytosis

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Bulk transport

• Phagocytosis
• The cell engulfs particles
• Pinocytosis
• The cell engulfs liquid
• Receptor-mediated
• Transported molecules bind specific receptors in
  the plasma membrane

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Phagocytosis
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Pinocytosis
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Receptor-mediated
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Exocytosis

• The reverse of endocytosis
• Discharge of materials from the cell
• Secretion of many hormones, neurotransmitters,
  digestive enzymes and other substances

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Active transport

• Movement of substances against or up the
  concentration gradient
• Low concentration to high
• Requires energy.
• Requires the use of ATP.
• Cell takes up substances into cytoplasm that are
  already hypertonic to the extracellular fluid.

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Sodium-potassium pump

• Protein transports Na and K ions across the
  membrane.
• Every cycle transports 3 Na ions out
• Exchange for 2 K ions into the cell
• Against concentration gradients.

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Coupled transport

• Molecule moves up its concentration gradient
• Uses the drive from another substance moving down
  its concentration gradient.
• Na ions and glucose

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Fig. 7-19


H
ATP
H


Proton pump
H
H


H
H


Diffusion of H
H
Sucrose-H cotransporter
H


Sucrose


Sucrose
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