Membrane Structure and Function

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Membrane Structure and Function

• AP Biology Chapter 8

Campbell 6e
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The Cell Membrane
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The Plasma Membrane

• The plasma (cell) membrane is the edge of life,
  the boundary that separates the living cell from
  its nonliving surroundings.
• It is a remarkable film only about 8 nm thick- it
  would take over 8,000 layers to equal the
  thickness of a piece of paper.
• It controls what enters and exits the cell.

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Semipermeable Nature

• This bilipid layer is semipermeable, meaning that
  some molecules are allowed to pass freely
  (diffuse) through the membrane.
• The lipid bilayer is virtually impermeable to
  large molecules, relatively impermeable to
  molecules as small as charged ions, and quite
  permeable to lipid soluble low molecular weight
  molecules.

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Fluid Mosaic Model of Membrane Structure

• Mosaic an object comprised of bits and pieces
  embedded in a supporting structure.
• Membrane lipids form the supporting structure
• Membrane proteins provide the bits and pieces
• Both lipids and proteins may be mobile or 'fluid'
  .

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Structure

• Membrane structure has two main components,
  lipids and proteins.

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Lipids

• Lipids form the support structure.
• Three kinds of lipids make up this framework.
• Phospholipids
• Glycolipids
• Cholesterol

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Phospholipids

• Most plasma membrane lipids are phospholipids,
  which spontaneously arrange themselves into a
  bilayer.
• Nonpolar tails are hydrophobic and directed
  inward polar heads are hydrophilic and are
  directed outward to face extracellular and
  intracellular fluids.
• Amphipathic molecule- has hydrophobic and
  hydrophilic regions.

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Phospholipid Structure
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Glycolipids

• Glycolipids have a structure similar to
  phospholipids except the hydrophilic head is a
  variety of sugar.
• They are protective and assist in various
  functions.

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Cholesterol

• Cholesterol is a lipid found in animal plasma
  membranes.
• It reduces the permeability of membrane.

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Proteins

• Two types of proteins
• Integral
• Peripheral

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Integral Proteins

• Penetrate the bilayer or span the membrane
  entirely
• Can only be removed from membranes by disrupting
  the phospholipid bilayer

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Peripheral Proteins

• Do not penetrate the phospholipid bilayer
• Are not covalently linked to other membrane
  components.
• Form ionic links to membrane structures
• Located on both extracellular and intracellular
  sides of the membrane

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Peripheral Proteins
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Fluid Nature of the PM

• At body temperature, the phospholipid bilayer has
  consistency of olive oil.
• In each monolayer, the hydrocarbon tails move,
  and whole phospholipid molecules can move
  sideways at a rate of about 2 µmthe length of a
  prokaryotic cellper second.
• Phospholipid molecules rarely flip-flop from one
  layer to the other.
• Fluidity of the phospholipid bilayer allows cells
  to be pliable.

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Membrane Proteins
Transport, Enzymes, Signaling, Intracellular
joining, Recognition, Attachment
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Transport Proteins

• TRANSPORT PROTEINS are responsible for shipping
  and receiving. They move materials in and out of
  the cell.
• Some of them function as pores that allow
  substances to diffuse through the membrane.
• Others act as pumps that use energy to pull
  molecules across the membrane.

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Transport Proteins
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Enzymes

• Protein enzymes are built into the membrane.
• The active site is exposed to substances in the
  nearby solution.
• May be ordered as a team- (flashback-cooperativity
  )

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Signaling

• A membrane may have a binding site with a
  specific shape for a chemical messenger.
• The signaler may cause a shape change in the
  protein that sends a message to the interior of
  the cell.

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Cell to Cell Junctions

• Some of the proteins in the PM join cells
  together.

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Cell Recognition

• Some glycoproteins serve as ID tags or marker
  proteins so cells recognize each other.
• Each organism has its own unique marker proteins
  on its membranes.
• One of the functions of marker proteins is to
  enable a person's immune system to distinguish
  its cells from those of invading cells.

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Cytoskeletal Attachment

• Membrane proteins may be bonded to filaments.
• This helps the cell maintain its shape.
• Also can coordinate cellular changes.

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Cellular Transport
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Transport Proteins
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Diffusion

• Movement of substances from high concentration to
  low concentration.
• In its simplest form, diffusion is the transport
  of a material or chemical by molecular motion.
• If molecules of a chemical are present in an
  apparently motionless fluid, they will exhibit
  microscopic erratic motions due to being randomly
  struck by other molecules in the fluid.

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Passive Transport Animations

• http//zoology.okstate.edu/zoo_lrc/biol1114/tutori
  als/Flash/Osmosis_Animation.htm
• http//www.bbc.co.uk/education/asguru/biology/01ce
  llbiology/05pathways/06passivesimple/02passivesimp
  le_b/index.shtml
• http//programs.northlandcollege.edu/biology/Biolo
  gy1111/animations/transport1.html

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Osmosis

• Diffusion of water through a selectively
  permeable membrane.
• From high solute concentration to low solute
  concentration.
• From high water concentration to low water
  concentration.

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OSMOSIS IN EVERYDAY LIFE

• Preservation of food
• Eating of salty or sugary foods
• Salt on a slug
• Contractile vacuoles in plants
• Saltwater and Freshwater fish

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Hypertonic, Hypostonic, and Isotonic

• The concentration of solute in the solution can
  be equal to the concentration of solute in the
  cells. The cell is in an isotonic solution. (iso
  same as normal)
• The concentration of solute in the solution can
  be greater than the concentration of solute in
  the cells. The cell is in an hypertonic solution.
  (hyper more than normal)
• The concentration of solute in the solution can
  be less than the concentration of solute in the
  cells. The cell is in an hypotonic solution.
  (hypo less than normal)

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OSMOSIS IN PLANT CELLS
HYPERTONIC SOLUTION PLASMOLYSIS
HYPOTONIC SOLUTION NORMAL TURGOR PRESSURE
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Osmosis and Animal Cells
NO CHANGE
WILL LYSE
CRENATION
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OSMOSIS AND ANIMAL CELLS
WILL LYSE
CRENATE
NO CHANGE
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Osmosis Animations

• http//www2.nl.edu/jste/osmosis.htmOsmosis
• http//zoology.okstate.edu/zoo_lrc/biol1114/tutori
  als/Flash/Osmosis_Animation.htm
• http//www.colorado.edu/eeb/web_resources/osmosis/
  

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Facilitated Diffusion

• Movement from high concentration to low
  concentration through carrier proteins.
• Used to move ionic or large substances into or
  out of cells.
• Passive process
• Carrier proteins are specific. (will only move
  one substance)
• Important for moving sugars and amino acids into
  cells

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Active Transport

• Active transport involves the use of proteins
  that don't just passively facilitate the
  transport of substances across the cell membrane,
  but require the use of cellular energy (usually
  ATP) to actively pump substances into or out of
  the cell.

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Active Transport

• Movement of substances against a concentration
  gradient.
• From low concentration to high concentration.
• Requires energy.
• Basically like pumping a substance.
• ATP must be hydrolyzed to fuel this process.

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Endocytosis

• Process of engulfing large particles and
  enclosing them within invaginations of the cell
  membrane.

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Active and Passive Transport