Function of Membranes

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Function of Membranes

• Compartmentalisation/ Dynamic boundary
• Isolation of cell from external world
• Alter cell form as necessary
• Division cell contents

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Functions of Membranes

• Selective permeability
• Control entry/exit in cells
• Control over organelle contents
• .Lysosome pH 5
• Electrical isolation
• Electrochemical gradient maintained
• Neuronal function

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Functions of Membranes

• Localisation of Chemical reactions e.g.
• Mitochondrial cristae - development of H ion
  gradients required for ATP generation
• Contain electron transport chain proteins
• Chloroplast membranes - light gathering proteins
• Lysosomes contain digestive enzymes

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Functions of Membranes

• Transport
• Contain proteins for transport
• Active transport processes
• ATP dependent transport
• co transporters
• Pinocytosis, endocytosis
• Including generation of gradients
• Use of these gradients to provide energy for
  active transport

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Functions of Membranes

• Signal transduction
• Receptors e.g.
• Insulin receptor
• Transduction mechanism
• e.g. cAMP cascade requires enzymes adenylate
  cyclase

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Functions of Membranes

• Cell-cell recognition
• glycoproteins
• Cell-cell adhesion
• Stick cells together

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STRUCTURE

• Phospholipid bilayer
• FLUID MOSAIC MODEL
• Proteins embedded in a sea of lipid
• Cholseterol reduces fluidity
• Proteins can be anchored by cytoskeleton
• Proteins can span the membrane (transmembrane),
• Extra/intracellular surface

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REGULATION OF FLUIDITY
Fatty acids are crucial regulators of fluidity
determined by chain length and degree of
saturation
Short chain fatty acids reduce the tendency of
hydrocarbon chains to interact and hence increase
fluidity
The kinks in unsaturated fatty acids result in
less stable van der Waals interactions with other
lipids and hence increase fluidity
High cholesterol content restricts the random
movement of polar heads and decreases fluidity.
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IMPAIRED FLUIDITY CAN DAMAGE CELLS
Increased cholesterol content of red blood cell
membranes is associated with severe liver disease
eg. Cirrhosis
Cholesterol content of red blood cell membranes
is increased by 20-60, leading to decreased
fluidity
Alters cell shape, impairs O2 transport,
destruction of red blood cells and anaemia
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PROTEINS

• Key to (most) membrane functions are the proteins
  embedded/ attached in the membrane

Pore forming proteins
Carrier protein active transport
Receptor protein
Membrane bound enzyme
Cell-cell adhesion
Cell-cell recognition
Cytoskeleton
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Channel Proteins

• Pore forming proteins
• Transmembrane
• Membrane spanning regions contain hydrophobic
  amino acids
• Allow diffusion of polar molecules across cell
  membrane e.g. sodium ions
• Na channel in cell membrane voltage gated
  (action potential)

• Tend to show some selectivity

• Water will also flow through a channel

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Carrier Proteins

• 3 methods
• Facilitated diffusion solute assisted by
  carrier protein to diffuse down concentration
  gradient, no additional energy supply needed.
• Active transport - ATP hydrolysis provides the
  energy. e.g. Na K pump
• Co transport of e.g. Na or H down their
  electrochemical gradient provides energy for a
  second solute to be moved against its
  concentration gradient e.g. Na are co
  transported with sugar molecules in the gut

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Membrane bound enzyme

• Enzymes attached to the membrane
• Can be external e.g.
• cell wall synthesis in plants
• Acetylcholinesterase (neuromuscular junctions)
• Breaks down acetylcholine released by nerve cells
  to cause muscular contraction.

• Poisoned by
• Organophosphorous insecticides (OPs),
• malathion (specific for insects - insecticide)

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• Poisoning causes muscle spasms, spasticity
• Internally bound enzymes are often involved in
  cell signalling cascades (see cell signalling
  section for details)

• sarin (nerve gas),

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Receptor Protein generating an intracellular
response

• Receptor protein specifically binds a ligand e.g
  histamine binds to a receptor on blood vessels.

• Binding induces conformational change, on the
  receptors intracellular surface.
• Variety of intracellular proteins activated
• Production of second messengers or opening of an
  ion channel.
• The second messengers (or ions) change cell
  function
• e.g. Causes vasodilation/ increases permeability
  of blood vessel

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Cell adhesion molecules

• Responsible for connecting cells to cells
• Often glycoproteins
• Bind to proteins on neighbouring cells or
  extracellular matrix
• Maintain tissue structure

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Cellcell recognition via a glycoprotein

• Cell surface glycoproteins specific for each
  species
• Determine e.g. blood group (causes difficulty for
  blood transplants/ xenografts)

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Membrane protein attached to the cytoskeleton

• Membranes are fluid and proteins can move around
  in them
• But some proteins may need to be localised e.g.
• Underneath a synaptic cleft or neuromuscular
  junction
• Specific localisation on cells to give sidedness
• e.g. intestinal lumen
• Cytoskeletal proteins can anchor membrane
  proteins in a specific location

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Cytoskeleton

• Intricate network of thread-like filaments
• Microfilaments (or Actin filaments)
• Intermediate filaments
• Microtubules
• Support the interior, produce movements, shape
  changes

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Microfilaments

• Actin filaments (microfilaments)
• Two actin strands twisted together
• rope approx. 7nm diameter
• Concentrated under the cell membrane
• Important for cell movements
• Dynamic

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Intermediate filaments

• Provide mechanical strength (important for animal
  cells)

• Act like a scaffold

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Microtubules

• Hollow tubes made from tubulin
• Heterodimers (one ? and one ?) arrange to form
  13 protofilaments
• Important in cell division forming spindle fibre
• Also involved in intracellular transport

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Centrosome (Microtubule organising Centre)

• Area in the cell which controls polymerisation,
  depolymerisation of microtubules
• Centrioles are found there (animal cells only)
• Plant cells have MOC, but no centrioles

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The cytoskeleton
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• CytoskeletonCytoskeleton and its major
  functionsProvides internal support and
  streanghtDifferent types of fibers all supply the
  same need The life a cytoskeleon is never
  complete. Its chief functions include movement
  for the cell, movement of material through the
  cell, maintaining the shape of the cell. But
  keeping the cell from getting smashed by other
  cells and moving the cell to where it need to be
  are its main roles. Early on, when not much was
  known about the structure of the cell, scientist
  believed that the cytoplasm that surrounds the
  organelle was completely random. As the years
  went on, better microscopes were invented, and
  better techniques were improved upon for making a
  slide, "a lacy network of fibers was revealed."
  These fibers look similar to girders that hold up
  a bridge, so it was hypothesized that they would
  do the same for the cell, hold its shape. These
  fibers can be broken down into three main groups
  Microfilaments, microtubules, and intermediate
  filaments, all of which can be recognized by
  there structure and their protein makeup.
  Regardless of their differences all three of them
  serve the same goal in the cell, to make the cell
  more ridged.MicrofilamentsThey, along with other
  proteins and ions, are responsible for every
  muscle contraction The microtubules and the
  microfilaments play a role in whole cell
  activities, including cell division, contraction
  of cell, and the crawling of the cell to a new
  location. Also they help in movement of vesicles,
  small sacs used for holding molecules, within the
  cell. The microfilaments resemble a string of
  beads of the protein actin, thus earning them the
  name of filaments. These filaments are the
  smallest cytoskeletal component, ranging in at 6
  nanometers. These actin proteins play a large
  role in muscle contraction. A model was made by
  Hugh Huxley that shows that the actin proteins
  are in alternating rows that alternate with
  myosin. The contraction of the muscle is caused
  by a calcium atom that excites the myosin, thus
  grabbing the actin and causing the muscle to
  shorten. After more studying of this event, it
  was learned that ATP was required to grab the
  actin.MicrotubulesThese together wiht centrioles
  aid in cell division by pulling apart
  chromosomesMovement is a result of the breaking
  down and building up of these internal stuctors
  of the cytoskeleton The thickest the components
  are the microtubules, at 22 nanometers. These
  were noticed to be in the cell since the 1950s
  but could not be studied until 1963. Each tube is
  filled with the protein tubulins. With this
  protein, the microtubules can shrink and grow.
  One of the most important jobs of these tubes is
  to aid in cell division It releases centrioles,
  which or composed of microtubules, which migrate
  to the cells poles. After doing this the
  chromosomes as move to the poles as result of the
  centrioles, this proves that cell division does
  not require ATP. When the cell is not dividing
  the microtubules carry vesicles to new
  locations.          The ability of the
  microtubules and microfilaments to breakdown and
  build up aid in the movement of a cell. Although
  there is the intermediate filaments that are very
  stable. They play important roles with cells that
  do mechanical stress. Research is being done to
  see whether these also play a role in cancerous
  cells. As we have just learned, the cytoskeleton
  plays a major role in almost every function of
  the cell.

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• The Cytoskeleton
• Every cell contains specialized cytoplasmic
  proteins which serve as a support and contractile
  system, maintaining or changing cell shape. The
  cytoskeletal structures include the microtubules,
  microfilaments, and intermediate filaments.
• Microtubules
• Cytoplasmic microtubules appear as 25 nanometer
  tubular structures and are readily assembled and
  disassembled from cytoplasmic pools of the
  protein tubulin. Microtubules are fairly rigid
  and play a role in the maintenance of cell shape
  (Microtubules 1). They are associated with cilia
  formation (from basal body) (Microtubules 2), and
  spindle apparatus formation (from the centriole)
  during cell division (Microtubules 3).
• Microfilaments
• Microfilaments are fine, thread like structures
  about 6 to 7 nanometers in diameter. An analogy
  sometimes employed to indicate function is that
  the microtubules act as the "bones" of the cell,
  whereas the microfilaments act as the "muscles"
  since they provide for movement and shape change.
  Microfilaments are composed of the contractile
  protein actin and represent a primitive
  contractile system, forming large bundles called
  stress fibers (Microfilaments 1). In most cells,
  the microfilaments are found in a band just under
  the plasma membrane (Microfilaments 2).
• Intermediate Filaments
• Intermediate filaments are generally 8 to 12
  nanometers in diameter and biochemically are a
  heterogeneous group (keratan, dermatan, desmin,
  and vimentin are a few of the biochemical species
  of intermediate filaments). The intermediate
  filaments are not contractile and serve
  exclusively in a supportive role. They are
  frequently grouped into delicate bundles
  (fibrils) in the cytoplasm, suited to meet stress
  and provide an overall girder-like support
  system.