The Cell Membrane

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The Cell Membrane
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Purpose of the membrane

• 1) Transport raw materials into the cell.
• 2) Transport manufactured products and wastes out
  of the cell.
• 3) Prevent the entry of unwanted matter into the
  cell.
• 4) Prevent the escape of matter needed for
  cellular functions.

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Composition

• The cell membrane is composed of phospholipid
  molecules. A Phosphate group and two fatty acids
  bonded to a glycerol backbone.
• The phosphate group (head) is hydrophillic
  (likes water) the two fatty acid tails are
  hydrophobic (water hating).
• (Pg 51 Fig 2.22)

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Importance of their dual nature

• When phospholipids are spread throughout
• water, a curious thing happens. The attractions
  and repulsions of the heads and tails cause them
  to form a cage-like spherical layer. (Pg 51, fig
  2.23).
• -The water attracting heads face in and out of
  the sphere.
• -The non-polar tails face each other.
• -This phospholipid bilayer forms the basis of the
  cell membrane.

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Fluid-Mosaic Model

• There are numerous proteins, carbohydrates and
  other lipids spread and moving throughout the
  membrane creating a fluid-like consistency.

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Benefits of the Fluid Mosaic Model

• Cholesterols found in animal cell membranes.
  These keep the membrane fluid at lower
  temperatures. They also prevent some other
  molecules from passing through.
• Protein and carbohydrate arrangements in the cell
  membrane that allow the cell to be recognized
  by other cells. (Pg 52, fig 2.24)

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Outside the cell

• In multicellular organisms, the cells are bathed
  in a thin layer of extracellular fluid, which is
  made of water, mineral and some cell wastes that
  have been discarded.

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Transport Through the Membrane

• Passive and Active

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Crossing the Membrane

• The membrane is a cells primary means for it to
  maintain homeostasis (the constant steady state
  of conditions achieved to remain healthy).
• The plasma membrane is selectively permeable. It
  allows some materials to pass.

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Transport

• Diffusion- a passive way that molecules can move
  across the membrane from an area of high to
  low ( means concentration). - materials
  that pass easily are gases, small uncharged
  molecules, and fat soluble molecules.

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Motion

• Any atom above absolute zero (-273 oC) is in
  motion. The random motion within a liquid is
  called Brownian motion. (Think of dye spreading
  through a liquid).
• The difference in concentrations of a substance
  across a membrane is called the concentration
  gradient.

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OSMOSIS

• The diffusion of water is called Osmosis. Where
  water moves from a high to low. There are
  different types of environments that will lead to
  a net direction of movement
• Isotonic condition when the concentrations of
  solutes on both sides of the membrane are the
  same causing no net movement of water.

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• Hypotonic condition when the water is higher
  (lower solute) outside the cell. Water will
  move into the cell to balance out concentrations,
  causing the cell to swell and possibly burst.
• Hypertonic condition when the water is higher
  (lower solute) inside the cell. Water will move
  out of the cell, causing the cell to shrivel.
• No energy is required to diffuse water, runs on
  concentration gradient.

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Facilitated Diffusion

• Facilitated Diffusion molecules that do not
  readily pass, due to size or charge etc.., may
  need assistance to cross the membrane.Specialized
  transport proteins help substances move in this
  case.
• Transport proteins are selective and will only
  move the molecule/ion it was designed to move.

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Carrier Proteins

• Numerous carrier proteins lie within the membrane
  to facilitate the movement of glucose (sugar).
  These carriers still operate by concentration
  gradients and do not require energy.

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Channel Proteins

• Channel Proteins are transport proteins that
  carry charged particles. They have a tunnel-like
  shape and have their own charge. They attract
  the ion like attracting a magnet. Size is a
  limiting factor for using these channels.

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• http//www.stolaf.edu/people/giannini/flashanimat/
  transport/channel.swf

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Active transport

• Certain cells require materials that may not be
  present in great amounts. In this case, a cell
  may need to pump these materials in or out of the
  cell against the normal concentration gradient.
• This moving materials from low to high
  requires the cell to spend energy, and is
  therefore called Active transport.

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Examples

• 4 Important examples of Active Transport -
  kidney cells pump glucose and Amino acids out of
  the urine and back into the blood. - Intestinal
  cells pump in nutrients from the gut. - Root
  cells pump in nutrients from the soil - Gill
  cells in fish pump out sodium ions.

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• Active transport pumps are protein pumps in the
  membrane that grab ions and other materials and
  pump them across the membrane against the
  gradient.- one of the best understood is the
  Na/K pump.
• In some cases the unequal distribution of Na (or
  other molecule) can allow another molecule to
  piggyback into the cell when regular diffusion
  continues.

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• http//www.northland.cc.mn.us/biology/Biology1111/
  animations/active1.swf
• Once you enter this site click on MAIN at the
  bottom.

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Bulk Membrane Transport
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Bulk Transport

• Material too big or material that is charged can
  not freely cross the cell membrane.
• The cell must accommodate these substances by
  folding in on itself creating a vesicle to take
  in or expel these large molecules.

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2 Types of Bulk transport

• Endocytosis (Endo into cyto cell) the
  membrane folds inward, trapping matter from the
  extracellular fluid and enters the cell as a
  vesicle.

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3 Types of Endocytosis

• Phagocytosis (phago eating) the membrane
  folds inward, trapping solid matter from the
  extracellular fluid.
• Pinocytosis (pino drinking) the membrane
  folds inward trapping fluids from the
  extracellular fluid.
• Receptor-mediated Endocytosis the membrane
  takes in particles after they bind to specialized
  receptor proteins embedded in the membrane.
  Cholesterol is brought in this way.

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• Exocytosis (exo exit) the reverse of
  endocytosis. A vesicle moves out towards the
  plasma membrane, fuses with it and opens up its
  contents into the extracellular fluid.- This is
  important in the body secreting manufactured
  products from within the cells.

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Endocytosis and Exocytosis

• http//highered.mcgraw-hill.com/olc/dl/120068/bio0
  2.swf