The Cell Membrane and Cellular Transport

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The Cell Membrane and Cellular Transport
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• The cell membrane is a thin structure that
  surrounds the cell. It is mainly made of
  phospholipids (type of organic molecule) with
  proteins embedded throughout.

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Functions of Membranes

• 1. Protect cell
• 2. Control incoming and outgoing substances
• 3. Maintain ion concentrations of various
  substances
• 4. Selectively permeable - allows some molecules
  in, others are kept out

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Phospholipid Bilayer
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Fluid Mosaic Model
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Methods of Transport Across Membranes

• 1. Diffusion
• 2. Osmosis
• 3. Facilitated Diffusion
• 4. Active Transport

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Methods of Transport Across Membranes

• 1. Diffusion -passive transport no energy
  expended
• 2. Osmosis - Passive transport of water across
  membrane
• 3. Facilitated Diffusion - Use of proteins to
  carry polar molecules or ions across
• 4. Active Transport- requires energy to
  transport molecules against a concentration
  gradient energy is in the form of ATP

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Diffusion
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Diffusion is the net movement of molecules (or
ions) from a region of their high concentration
to a region of their lower concentration.The
molecules move down a concentration
gradient.Molecules have kinetic energy, which
makes them move about randomly. As a result of
diffusion molecules reach an equilibrium where
they are evenly spread out.This is when there is
no net movement of molecules from either side.
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DIFFUSION
Diffusion is a PASSIVE process which means no
energy is used to make the molecules move, they
have a natural kinetic energy.
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Diffusion through a membrane
Cell membrane
Outside cell
Inside cell
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Diffusion through a membrane
Cell membrane
diffusion
Outside cell
Inside cell
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Diffusion through a membrane
Cell membrane
Outside cell
Inside cell
EQUILIBRIUM
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What determines the rate of diffusion?There 4
factors

• The steepness of the concentration gradient. The
  bigger the difference between the two sides of
  the membrane the quicker the rate of diffusion.
• Temperature. Higher temperatures give molecules
  or ions more kinetic energy. Molecules move
  around faster, so diffusion is faster.

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• 3. The surface area. The greater the surface
  area the faster the diffusion can take place.
  This is because the more molecules or ions can
  cross the membrane at any one moment.
• 4. The type of molecule or ion diffusing. Large
  molecules need more energy to get them to move so
  they tend to diffuse more slowly. Non-polar
  molecules diffuse more easily than polar
  molecules because they are soluble in the non
  polar phospholipid tails.

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Molecules that diffuse through cell membranes

• 1. Oxygen Non-polar so diffuses very
  quickly.
• 2. Carbon dioxide Polar but very small
  so diffuses quickly.
• 3. Water Polar but also very small so
  diffuses quickly.

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Facilitated diffusion

• Large polar molecules such as glucose and amino
  acids, cannot diffuse across the phospholipid
  bilayer. Also ions such as Na or Cl- cannot
  pass.
• These molecules pass through protein channels
  instead. Diffusion through these channels is
  called FACILITATED DIFFUSION.
• Movement of molecules is still PASSIVE just like
  ordinary diffusion, the only difference is, the
  molecules go through a protein channel instead of
  passing between the phospholipids.

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Facilitated Diffusion through a membrane
Cell membrane
Protein channel
Outside cell
Inside cell
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Facilitated Diffusion through a membrane
Cell membrane
diffusion
Protein channel
Outside cell
Inside cell
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Facilitated Diffusion through a membrane
Cell membrane
diffusion
Protein channel
Outside cell
Inside cell
EQUILIBRIUM
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Facilitated DiffusionMolecules will randomly
move through the opening like pore, by diffusion.
This requires no energy, it is a PASSIVE process.
Molecules move from an area of high
concentration to an area of low conc.

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Facilitated diffusion
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Cotransport also uses the process of diffusion.
In this case a molecule that is moving naturally
into the cell through diffusion is used to drag
another molecule into the cell. In this example
glucose hitches a ridewith sodium.
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Receptor Proteins These proteins are used in
intercellular communication. In this animation
you can see the a hormone binding to the
receptor. This causes the receptor protein
release a signal to perform some action.
Essential to hormones and nervous system
communication
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Osmosis
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Osmosis

• The diffusion of water from an area of high
  concentration of water molecules (high water
  potential) to an area of low concentration of
  water (low water potential) across a partially
  permeable membrane.

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Osmosis
CONCENTRATED SOLUTION
DILUTE SOLUTION
Cell membrane partially permeable.
Sugar molecule
VERY Low conc. of water molecules. High water
potential.
VERY High conc. of water molecules. High water
potential.
Outside cell
Inside cell
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Osmosis
Cell membrane partially permeable.
Low conc. of water molecules. High water
potential.
OSMOSIS
High conc. of water molecules. High water
potential.
Outside cell
Inside cell
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Osmosis
Cell membrane partially permeable.
OSMOSIS
Outside cell
Inside cell
EQUILIBRIUM. Equal water concentration on each
side. Equal water potential has been reached.
There is no net movement of water
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Tonicity is a relative term

• Hypotonic Solution - One solution has a lower
  concentration of solute than another.
• Hypertonic Solution - one solution has a higher
  concentration of solute than another.
• Isotonic Solution - both solutions have same
  concentrations of solute.

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Plant and Animal Cells put into various
solutions
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• Ultimately, water moves from an area where there
  is a high concentration to an area where there is
  a low concentration.
• THERE IS NO MOVEMENT OF ANY OTHER MOLECULES

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Active Transport

• Movement of molecules against a concentration
  gradient from an area of LOW concentration to
  an area of HIGH concentration. This requires the
  input of energy. Occurs in desert plants where
  the concentration of water is higher inside the
  plant than in the soil - this also occurs in
  areas where mineral concentration is low in the
  soil

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Types of Transport