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

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http//www.tamu.edu/classes/bich/mullins/bich410/o
lprg/10-2a_FluidMosaic/FluidMosaic.htm
http//www.stolaf.edu/people/giannini/flashanimat/
lipids/membrane fluidity.swf
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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
• Integral proteins
• Peripheral proteins
• Cholesterol
• Carbohydrates

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• Phospholipid bilayer
• Hydrophilic (water-loving) polar heads
• Face outside and inside of cell.
• Hydrophobic (water-fearing) nonpolar tails
• Extend to the interior of the plasma membrane.

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• Proteins
• Peripheral proteins
• On inside surface
• Held in place by cytoskeletal filaments
• Integral proteins
• Embedded in membrane, can move laterally
• Most span the membrane, but some protrude only
  from one side

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• Cholesterol - strengthens the plasma membrane.
• Carbohydrates
• Glycoproteins proteins with carbohydrates
  attached
• Glycolipids phospholipids with carbohydrates
  attached

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Functions of membrane proteins
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• Some help to transport materials across the
  membrane.

• Channel Protein allows certain molecule or ion
  to cross membrane freely

• Carrier Protein interacts with certain molecule
  or ion to help move it across membrane

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• Others receive specific molecules, such as
  hormones.
• Receptor Protein has certain shape so that
  specific molecule, such as a hormone or some
  other signaling molecule, can attach
• Attachment can cause protein to change shape
  cause cell to perform certain action

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• Still other membrane proteins function as
  enzymes.
• Enzymatic Protein - Can carry out metabolic
  reactions

• Peripheral proteins stabilize and shape the
  membrane

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• Some proteins aid in cell recognition
• Glycocalyx - In animal cells, the carbohydrate
  chains of cell recognition proteins are
  collectively called this
• Can function in cell-to-cell recognition,
  adhesion between cells, and reception of signal
  molecules.
• The diversity of carbohydrate chains is enormous,
  providing each individual with a unique cellular
  fingerprint.

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Cell recognition protein

• Foreign carbohydrate chains are why transplanted
  tissue is often rejected by the body

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• The Permeability of the Plasma Membrane
• Differentially permeable (sometimes called
  selectively permeable) meaning only certain
  materials can cross the membrane.
• Two mechanisms of transport
• Active requires ATP (chemical energy)
• Passive does not require chemical energy

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• Examples of passive transport
• Diffusion (no carrier protein needed)
• Facilitated transport (which usually requires a
  carrier protein)
• Active transport
• Requires a carrier protein
• May sometimes involve the formation of vesicles
  to take in or get rid of materials (for example
  endocytosis and exocytosis).
• More on these types of transport later in
  chapter

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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 passive 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
  differentially permeable membrane due to
  concentration differences.
• Molecules always move from higher to lower
  concentration.
• Water enters cells due to osmotic pressure
  (pressure that develops in a system due to
  osmosis) within cells.

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Osmosis demonstration
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Osmosis in cells

• A solution contains a solute (solid) and a
  solvent (liquid).
• Cells are normally isotonic 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.

http//www.tvdsb.on.ca/westmin/science/sbi3a1/Cell
s/Osmosis.htm
http//www.linkpublishing.com/video-transport.htm
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Isotonic same
No net movement of water
X Solute X water
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Cells in isotonic solutions
http//www.linkpublishing.com/Videos/transport/blo
od_isotonic.wmv
http//www.linkpublishing.com/Videos/transport/elo
dea_normal.wmv
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• Hypotonic solutions cause cells to swell
  possibly burst hypo means less than.
• Lower concentration of solute, higher
  concentration of water outside than inside cell.
• Net movement of water from outside to inside the
  cell.
• Animal cells - may undergo lysis (cell is
  disrupted may even 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 less than
Water moves in (hypo/hippo swells like a hippo)
X Solute X water
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Cells in a hypotonic solution
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http//www.linkpublishing.com/Videos/transport/blo
od_hypo.wmv
http//www.linkpublishing.com/Videos/transport/elo
dea_hypo.wmv
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• Hypertonic solutions cause cells to lose water
  hyper means more than
• Higher concentration of solute, lower
  concentration of water outside than inside cell.
• Net movement of water from inside to outside of
  cell.
• Animal cells - undergo crenation (shrivel)
• Plant cells - undergo plasmolysis, the shrinking
  of the cytoplasm.
• Turgor pressure is lost as plant cells shrink.

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Hypertonic more than
Water moves out
X Solute X water
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Cells in a hypertonic solution
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http//www.linkpublishing.com/Videos/transport/blo
od_hyper.wmv
http//www.linkpublishing.com/Videos/transport/elo
dea_hyper.wmv
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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.coolschool.ca/lor/BI12/unit4/U04L03/fac
ilitated transport.swf
http//www.northland.cc.mn.us/biology/Biology1111/
animations/passive1.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
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• Carrier proteins involved in active transport are
  called pumps.
• The sodium-potassium pump is active in all animal
  cells (particularly nerve muscle cells), and
  moves sodium ions to the outside of the cell and
  potassium ions to the inside.
• The sodium-potassium pump carrier protein exists
  in two conformations one that moves sodium to
  the outside, and the other that moves potassium
  into the cell.

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The sodium-potassium pump
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 transported
  using vesicle formation requires energy.
• Exocytosis - vesicles fuse with the plasma
  membrane for secretion.
• Causes cell membrane to enlarge process occurs
  during growth.
• Molecules released become part of cell membrane,
  part of matrix surrounding cell, or nourish
  nearby cells.

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Exocytosis
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• Some cells are specialized to produce and release
  specific molecules.
• In some cases, release of molecules may occur
  only in the presence of signals received by the
  plasma membrane.
• Examples include release of digestive enzymes
  from cells of the pancreas, or secretion of the
  hormone insulin in response to rising blood
  glucose levels.

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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

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http//highered.mcgraw-hill.com/sites/0072437316/s
tudent_view0/chapter6/animations.html
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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).