Plasma Membrane Structure and Function

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• Plasma Membrane Structure and Function
• Separates the internal environment of the cell
  from its surroundings.
• Regulates what materials enter and leave a cell
• Made of 2 layers of phospholipids (phospholipid
  bilayer) with proteins embedded throughout.
• Fluid consistency (like a light oil) and a mosaic
  pattern of proteins.

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• Because of consistency and pattern of components,
  referred to as fluid-mosaic model of membrane
  structure

http//home.earthlink.net/shalpine/anim/Life/memb
.htm
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• Cells live in fluid environments, with water
  inside and outside the cell.
• Components of plasma membrane
• 2 layers of phosphlipids
• Polar head and nonpolar tail
• Proteins
• Cholesterol
• Carbohydrates

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• Phospholipid bilayer
• Polar heads (hydrophilic) (water-loving)
• Face outside and inside of cell.
• Nonpolar tails (hydrophobic) (water-fearing)
• Extend to the interior of the plasma membrane.

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• Proteins
• Some pass through entire membrane, others go
  part-way through
• Help large molecules pass through cell membrane

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• Cholesterol
• Strengthens the plasma membrane.
• Carbohydrates
• Help cells send messages and recognize one another

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• The Permeability of the Plasma Membrane
• Selectively permeable meaning only certain
  materials can cross the membrane.
• Two mechanisms of transport
• Active requires energy
• Passive does not require energy

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• How do materials get across the plasma membrane?
• Small, uncharged molecules pass through the
  membrane, following their concentration gradient
  -
• Gradual change in chemical concentration from one
  area to another
• Molecules tend to move from area of high to low
  concentration
• Larger macromolecules or tiny charged molecules
  rely on proteins to help them to get across.

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How molecules cross the plasma membrane
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• Diffusion and Osmosis
• Diffusion is the movement of molecules from a
  higher to a lower concentration until equilibrium
  is reached.
• Equilibrium
• State in which all materials are evenly
  concentrated
• Movement of molecules still occurs, but there is
  no NET movement of molecules
• Gases move through plasma membrane by diffusion.

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Osmosis

• Defined as the diffusion of water across a
  selectively permeable membrane due to
  concentration differences.
• Molecules always move from higher to lower
  concentration.

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Osmosis in cells

• A solution contains a solute (solid) and a
  solvent (liquid).
• Cells are normally isotonic (iso same) to their
  surroundings, and the solute concentration is the
  same inside and out of the cell.
• Cell is in equilibrium
• There is no net movement of water across the cell
  membrane.

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Isotonic solution
Solute Water
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Cells in isotonic solutions
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• Hypotonic (hypo means less than)
• Lower concentration of solute, higher
  concentration of water outside than inside cell.
• Net movement of water into the cell.
• Animal cells - may burst
• Plant cells
• Increased turgor pressure (makes plant cell
  rigid)
• Plant cells do not burst because they have a cell
  wall.

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Hypotonic solution
Solute Water
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Cells in a hypotonic solution
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• Hypertonic (hyper means more than)
• Higher concentration of solute, lower
  concentration of water outside than inside cell.
• Net movement of water out of the cell.
• Animal cells - shrivel
• Plant cells - shrink
• Turgor pressure is lost as plant cells shrink
  plant will wilt or feel soft and bendable.

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Hypertonic solution
Solute Water
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Cells in a hypertonic solution
http//ccollege.hccs.cc.tx.us/instru/Biology/AllSt
udyPages/Diffusion_Osmosis/Elodeagif.swf
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• Transport by Carrier Proteins
• Some materials cannot enter or leave cell due to
  their size and/or nature.
• Some of those molecules use the channel proteins
  and carrier proteins that span the membrane in
  order to enter or leave the cell.
• Carrier proteins are specific and combine with
  only a certain type of molecule.
• Facilitated transport and active transport both
  require carrier proteins.

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Facilitated transport

• Substances pass through a carrier protein
  following their concentration gradients (high ?
  low concentration).
• Does not require energy.
• Brings in materials such as glucose amino
  acids proteins are specific to molecules taken
  in.
• Some molecules can be taken in faster than others
  differential permeability.

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

• Ions or molecules are moved across the membrane
  against the concentration gradient from an area
  of lower to higher concentration.
• Energy in the form of ATP is required for the
  carrier protein to combine with the transported
  molecule.
• Occurs in cells such as kidney cells (taking
  sodium from urine), thyroid gland cells (taking
  in iodine), and cells in digestive tract
  (absorbing nutrients).

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Active transport
http//www.brookscole.com/chemistry_d/templates/st
udent_resources/shared_resources/animations/ion_pu
mp/ionpump.html
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• Exocytosis and Endocytosis
• For molecules that are too large to be
  transported with carrier proteins requires
  energy.
• Use vesicles small storage units
• Exocytosis - vesicles fuse with the plasma
  membrane for secretion.
• Causes cell membrane to enlarge process occurs
  during growth.

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Exocytosis
http//academic.brooklyn.cuny.edu/biology/bio4fv/p
age/exocy.htm
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• Endocytosis - cells take in substances when a
  portion of the plasma membrane folds in, and
  forms a vesicle around the substance.
• When vesicle fuses with lysosome, digestion
  occurs.
• Endocytosis occurs as
• Phagocytosis large particles (food or other
  cells)
• Pinocytosis small particles or liquids
• Receptor-mediated endocytosis form of
  pinocytosis for specific particles such as
  vitamins, hormones, or lipoproteins

http//www.stolaf.edu/people/giannini/flashanimat/
cellstructures/cell.swf
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Phagocytosis

• Seen in unicellular organisms like amoebas.
• White blood cells can use this process to take in
  bacteria worn out red blood cells

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Pinocytosis

• Used by root cells of plants.
• Seen in blood cells, cells that line kidneys and
  intestines.

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Receptor-mediated endocytosis
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• Seen during exchange of maternal fetal blood in
  placenta intake of cholesterol in body cells
  (failure to do so results in high blood pressure,
  blocked arteries, and heart attacks).