The Plasma Membrane

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The Plasma Membrane -
Gateway to the Cell
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Photograph of a Cell Membrane
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Cell Membrane

• The cell membrane is flexible and allows a
  unicellular organism to move

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Functions of Plasma Membrane

• separates and protects the cell from its
  surroundings
• provides some shape
• Contains the cytoplasm (fluid in cell)
• Regulates transport in out of cell
  (selectively permeable)
• Maintains Homeostasis
• (Balanced internal condition of cells
• Also called equilibrium)

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Why is movement of molecules into and out of
cells necessary?

• All cells require nourishment for energy
  production
• All cells must eliminate toxic waste products

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Functions of Plasma MembraneMore

• Provide a binding site for enzymes
• Allows cell recognition (surface structures
  are like fingerprints)
• Provide attachment sites for filaments of
  Cytoskeleton
• Interlocking surfaces bind cells together
  (junctions)

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So how do molecules move ACROSS the cell membrane?

• To answer that we must first take a look at the
  structure of the cell membrane.

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Structure of the Cell Membrane
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PhospholipidsPrimary component of the membrane

• Make up the cell membrane

Contains 2 fatty acid chains that are nonpolar
Head is polar contains a PO4 group
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Cell Membrane
The cell membrane is made of 2 layers of
phospholipids called the lipid bilayer
Hydrophobic molecules pass easily hydrophillic
DO NOT
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Phospholipids

• glycerol backbone 2 fatty acids a phosphate
  group
• Phosphate Glycerol is hydrophilic
• Fatty acids are Hydrophobic

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Cell Membrane

• Polar heads are hydrophilic water loving
• Nonpolar tails are hydrophobic water fearing
• Makes membrane Selective in what crosses

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Solubility

• Materials that are soluble in lipids can pass
  through the cell membrane easily

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Semipermeable Membrane
Small molecules and larger hydrophobic molecules
move through easily. e.g. O2, CO2, H2O
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Fluid Mosaic Model

• http//www.susanahalpine.com/anim/Life/memb.htm

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Fluid mosaic model
FLUID MOSAIC MODEL

• FLUID- because individual phospholipids and
  proteins can move around freely within the layer,
  like its a liquid.
• MOSAIC- because of the pattern produced by the
  scattered protein molecules when the membrane is
  viewed from above.

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Membrane Components
Proteins(peripheral and integral)
Phospholipids
Cholesterol
Carbohydrates (glucose)
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Semipermeable Membrane
Ions, hydrophilic molecules larger than water,
and large molecules such as proteins do not move
through the membrane on their own.
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Membrane Proteins

• Control which particles can pass through the
  membrane.
• Act as markers so that cells can be recognized
  (the immune system).
• Serve as enzymes (may speed reactions).

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Membrane Proteins

• Thousands are found throughout the cell
  membrane.
• A special few cross the cell membrane.
• Each type of protein has a specific purpose.
• Two major types (Integral and peripheral)

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Integral Proteins

• Permanently connected to the cell membrane
• Large sections embedded in the middle layer of
  the membrane
• Known as the hard workers of the membrane
• pathway for ions and molecules (channels) and
  carriers)
• Very few integral proteins compared to peripheral

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Proteins Are Critical to Membrane Function
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Types of Transport Proteins

• Channel proteins are embedded in the cell
  membrane have a pore for materials to cross
• Carrier proteins can change shape to move
  material from one side of the membrane to the
  other

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A look at integral and peripheral proteins
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Peripheral Proteins

• Not bonded as strongly to the membrane
• Sit on the surface of the membrane, anchored by
  Hydrogen bonds

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Membrane Components
Proteins(peripheral and integral)
Phospholipids
Cholesterol
Carbohydrates (glucose)
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• Cell Processes
• Part I
• Transport Across Cell Membranes

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Transporting Molecules across the membrane

• There are two major ways that substances pass
  through a membrane.
• Passive transport- requires no energy
• Active transport- requires energy

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Passive Transport

• Type 1 Simple Diffusion
• Doesnt require energy
• Moves from high concentration
• to low concentration
• Example Oxygen or diffusing into a cell and
  carbon dioxide diffusing out.

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Simulation of Diffusion

• http//www.biosci.ohiou.edu/introbioslab/Bios170/
  diffusion/Diffusion.html

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Diffusion through a Membrane

Cell membrane
Solute moves DOWN concentration gradient (HIGH to
LOW)
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Simple Diffusion

• Requires NO energy
• Molecules move from area of HIGH to LOW
  concentration

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DIFFUSION

• Why are the molecules moving?
• The molecules
• have a natural
• KINETIC ENERGY
• Brownian Motion molecules vibrating

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Diffusion of Liquids
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Diffusion across the membrane - various routes

• http//www.coolschool.ca/lor/BI12/unit4/U04L03/dif
  fusion.swf

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Passive transport type 2Osmosis A special
case of Diffusion Diffusion of Water across a
membrane
Diffusion across a membrane

• Water is moving from HIGH water potential (few
  solutes) to LOW water potential (many solutes)

Semipermeable membrane
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Diffusion of H2O Across A Membrane OSMOSIS
High H2O potentialLow solute concentration
Low H2O potentialHigh solute concentration
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Passive Transport Type 3

• Facilitated diffusion
• Doesnt require energy
• Uses transport proteins to move high to low
  concentration
• Examples Glucose or amino acids moving from
  blood into a cell.

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Facilitated Diffusion

• Substances diffuse across a cell membrane by
  using a special protein channel

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Facilitated diffusion animation

• http//highered.mcgraw-hill.com/sites/0072495855/s
  tudent_view0/chapter2/animation__how_facilitated_d
  iffusion_works.html

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• http//www.d.umn.edu/sdowning/Membranes/diffusion
  animation.html
• http//www.microbiologytext.com/index.php?moduleB
  ookfuncdisplayarticleart_id48
• http//www.coolschool.ca/lor/BI12/unit4/U04L03.htm
  
• http//highered.mcgraw-hill.com/sites/0072495855/s
  tudent_view0/chapter2/animation__how_facilitated_d
  iffusion_works.html

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Osmosis Close Up

• In any solution there is something dissolved and
  a fluid that the material is dissolved into.
• Solutes - A substance that is dissolved in a
  solvent (salt, sugar, protein etc)
• Solvent The liquid the solute dissolves in
• (water , alcohol, etc)
• In Osmosis, it is the solvent moves!

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• Hypertonic Solutions (Hyper more)
• high concentration of solute in a solution
  compares to another nearby solution (e.g. the
  cell's cytoplasm).

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Hypotonic vs. Hypertonic

• Hypotonic Solutions ( hypo low)
• low concentration of solute in a solution
  compares to another nearby solution (e.g. the
  cell's cytoplasm).

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Osmosis

• If solutes cannot move across a membrane, then
• Water will move across the membrane to balance
  the concentration of solute.

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Cell in Hypotonic Solution
What is the direction of water movement?
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Cell in Hypertonic Solution
What is the direction of water movement?
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Hypotonic and Hypertonic are Relative terms.

• If a solution is identified as hypertonic (
  having more dissolved solutes when compared to
  another solution),
• then there must be a hypotonic solution to
  compare it to.

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Cell in Isotonic Solution
10 NaCL90 H2O
ENVIRONMENT
CELL
NO NET MOVEMENT
10 NaCL 90 H2O
What is the direction of water movement?
equilibrium
The cell is at _______________.
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Isotonic

• Iso same
• Solute concentration is the same inside and out

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Cells in Solutions
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Isotonic Solution
Hypotonic Solution
Hypertonic Solution
NO NET MOVEMENT OF H2O (equal amounts entering
leaving)
CYTOLYSIS
PLASMOLYSIS
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Cytolysis Plasmolysis
Plasmolysis
Cytolysis
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Osmosis in Red Blood Cells
Isotonic
Hypertonic
Hypotonic
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• Types of Solutions
• HYPERTONIC High solute concentration compares
  to neighbor Water moves in
• HYPOTONIC Low solute concentration compared to
  neighbor Water moves out
• ISOTONIC concentration of water is same on each
  side
• No net (overall) movement of water

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hypotonic
hypertonic
isotonic
hypertonic
isotonic
hypotonic
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Osmotic Pressure

• the pressure created when water moves by osmosis
  
• without some means of controlling osmotic
  pressure, the cell will burst.
• Just like higher air pressure in a tire can cause
  it to burst

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Three (3) ways organisms control osmotic pressure

• 1) Cell walls- prevent cells from expanding
• 2 )Water pumps- single celled animals have
  contractile vacuoles that pump water out of the
  cell
• 3) Bathe cells- in fluid with nearly the same
  concentrations as the cells themselves( same
  solutes inside and out