Eukaryotic Membranes: Plasma Membrane

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Eukaryotic Membranes Plasma Membrane

• As we discussed when we first mentioned the
  plasma membrane it is not as simple as pictures
  make it look.
• These membranes are complex, heterogeneous
  structures with different parts performing very
  distinct functions, and they change dynamically
  in response to their surroundings.

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Eukaryotic Membranes Plasma Membrane

• One theory states that the plasma membrane is a
  fluid mosaic (fluid mosaic model)
• An everchanging image of proteins and
  phospholipids

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Eukaryotic Membranes Plasma Membrane

• The liquid portion of this mosaic is the
  phospholipid bilayer, which we have discussed.
• This is made up of lipids with two distinct parts
• A polar hydrophilic head
• A pair of nonpolar, hydrophobic tails

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Eukaryotic Membranes Plasma Membrane

• This barrier encloses the cytoplasm
• Which includes all of a cells internal contents,
  including organelles (except the nucleus)
• The watery medium inside and the watery medium
  outside cause the spontaneous formation of the
  lipid bilayer

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Eukaryotic Membranes Plasma Membrane

• In addition to the phospholipids most membranes
  contain some if not many cholesterol molecules
• These make the lipid bilayer stronger
• Less fluid
• Less permeable to water soluble substances

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Eukaryotic Membranes Plasma Membrane

• The diagram below shows a representation of the
  cholesterol molecules intersperses in the
  phospholipid bilayer

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Eukaryotic Membranes Plasma Membrane

• Beyond the cholesterol thousands of proteins are
  embedded with in or attached to the surface of a
  membranes phospholipid bilayer.
• These proteins come in many forms and perform
  many functions

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Eukaryotic Membranes Plasma Membrane

• Transport Proteins regulate the movement of
  hydrophilic molecules through the plasma membrane
• Receptor Proteins trigger cellular responses
  when specific molecules in the extracellular
  fluid, such as hormones or nutrients, bind to
  them
• Recognition Proteins many of which are
  glycoproteins, serve as identification tags and
  cell-surface attachment sites

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Eukaryotic Membranes Plasma Membrane

• The logistics of how these proteins operate to
  allow and control transportation will be dealt
  with later in this course.
• For now just understand that they are there.

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Eukaryote Cells Cell Wall

• The outer surfaces of the cells of bacteria,
  plants, fungi and some protists are covered with
  stiff, nonliving coatings calls cell walls.
• Plant Cells cellulose (polysacharide)
• Fungal Cells chitin (polysacharide)
• Bacterial Cells chitin like coating

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Eukaryote Cells Cell Wall

• The two cells then secrete cellulose through
  their plasma membranes, underneath the middle
  lamella, forming the primary cell wall.
• This process may be repeated to create the
  secondary cell wall, which could become thicker
  than the whole cell (this will be inside the
  primary cell wall).

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Eukaryote Cells Cell Wall

• What are the functions of cell walls?
• Support and protect otherwise fragile cells.
• Thus allowing plants and mushrooms to resist the
  power of wind, gravity and rain and stand
  straight up
• Tree trunks
• Though strong cell walls tend to be porous,
  preventing cell suffication

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Eukaryotic Membranes Endoplasmic Reticulum

• Endoplasmic reticulum (ER) is a series of
  interconnected membranous tubes and channels in
  the cytoplasm.

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Eukaryotic Membranes Endoplasmic Reticulum

• Most eukaryotic cells have two forms of ER
  rough and smooth.
• Numberous ribosomes stud the outside of the rough
  and smooth ER.
• The different structures of smooth and rough ER
  reflect different functions.

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Eukaryotic Membranes Endoplasmic Reticulum

• Enzymes embedded in the membranes of the smooth
  ER are the major site of lipid synthesis,
  including the phospholipids of the ER and other
  membranes.
• In other cells smooth ER might synthesize steroid
  hormones testoterone and estrogen.

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Eukaryotic Membranes Endoplasmic Reticulum

• The ribossomes on the outside of rough ER
  synthesize proteins, including membrane proteins.
• There for the ER can produce itself.

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Eukaryotic Membranes Endoplasmic Reticulum

• These membrane proteins have three directions to
  move.
• Outward to replace membrane of the Golgi complex,
  lysosomes, or plasma membrane.
• Inward to replace nuclear envelope
• Or within the ER to replace itself

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Eukaryotic Membranes Endoplasmic Reticulum

• Aswell these ribosomes on the outside of the
  rough ER produce proteins that some secretory
  cells export into their surroundings.
• Digestive enzymes
• Protein Hormones

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Eukaryotic Membranes Endoplasmic Reticulum

• Once these enzymes are produced they are uptakend
  by the ER and transported up tubes to the end of
  the ER
• Here they bud off as membrane bound vescicles and
  are transported to the Golgi complex.