Cell Transport

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Cell Transport

• Cell transport occurs across the cell membrane.
• Materials such as ions, water, molecules and
  nutrients are transported within cells as well as
  in and out of cells.
• This occurs via the processes of diffusion,
  osmosis, active transport, phagocytosis and
  pinocytosis.

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Diffusion

• Diffusion is the movement of particles from an
  area of high concentration to an area of low
  concentration.
• The particles can be in liquid or gas, and are
  said to be moving at random until both sides of
  the semi-permeable membrane are of equal
  concentration.
• It is known as moving DOWN the concentration or
  diffusion gradient.

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Diffusion
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Diffusion

• The process does not require any energy so is
  known as passive transport.
• Simple diffusion allows small particles to pass
  through the phospholipids
• Diffusion rates depend on
• Concentration gradient the greater the gradient
  the faster the rate of diffusion.
• Particle size small particles diffuse faster
  then large particles.
• State gases diffuse faster then liquids.
• Temperature warm particles move faster then cool
  particles.

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Diffusion

• Food and oxygen diffuse into cells, where waste
  products diffuse out.
• Cell membranes are semi-permeable. Only some
  molecules can diffuse directly through the lipid
  bilayer.
• Molecules such as Oxygen, fatty acids, Carbon
  dioxide and urea can do this.
• Lungs versus tissues

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Facilitated Diffusion

• A process by which substances are transported
  across cell membranes by means of protein channel
  molecules
• This can be faster then diffusion alone
• Proteins are within the cell membrane and
  specialise to diffuse certain particles or
  molecules that cannot cross the hydrophobic part
  of the lipid bilayer.
• e.g. polar molecules or ions
• This is known as facilitated diffusion.

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Questions

• Describe the process of diffusion.
• Explain some similarities and differences between
  simple diffusion and facilitated diffusion.
• What 4 variables effect the rate of diffusion and
  how? (increase or decrease)
• The plasma membrane is semi-permeable. What does
  this mean?

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Investigating the Effect of Temperature on
Diffusion rate

• An experiment was set up to test the hypothesis
  that increasing temperature increases diffusion
  rate. The time for a purple dye to diffuse set
  distances in water at different temperatures was
  recorded

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Results
Water Temp. 2 cm 4 cm 6 cm 8 cm 10 cm
10C 60 s 110 s 210 s 300 s 450 s
20C 30 s 66 s 108 s 282 s 222 s
30C 12 s 30 s 55 s 80 s 125 s

1. Plot the three sets of data on the same graph.
   Use a key.
2. Do these results support the hypothesis? Explain
   your answer

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Osmosis

• The Diffusion of water molecules across a
  semi-permeable membrane from the side with the
  highest water concentration of to the side with
  the lowest water concentration.
• Osmosis refers to water concentration.
• A high water concentration is therefore the same
  as a low solute concentration (and vice versa).

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Osmosis
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Osmosis
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Osmosis

• Osmosis requires no energy input (facilitated
  diffusion)
• When a cell is surrounded by pure water, water
  moves into the cell by osmosis.
• The cell then becomes turgid (swollen).
• In animal cells if it becomes to turgid it can
  burst the cell (lysed).
• In plant cells the cell wall is strong so the
  cell remains very turgid without bursting.
• The vacuole then absorbs water, maintaining
  osmoregularity.

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Osmosis
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Osmosis

• If the water concentration outside the cell is
  less than inside the cell, they lose water and
  become flaccid (limp).
• This causes the animal cell to shrivel
• Plant cells will plasmolyse if too much water is
  lost
• The cell membrane will detach from the cell wall.
• The vacuole then expels water to maintain
  osmoregularity.

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Osmosis
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Tonicity/osmoregulation

• Tonicity relates the osmotic pressure of a cell
  to its surroundings/environment.
• Osmoregulation is the way the cell maintains an
  optimal constant osmotic pressure.
• This requires the cell to deal with changes in
  tonicity of its environment

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Tonicity and Osmoregulation

• Tonicity can be grouped into 3 categories.
• Isotonic
• Hypertonic
• Hypotonic

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Tonicity
Tonicity

• Cells in an isotonic solution have the SAME
  concentration of impermeable solutes outside
  the cell as inside.
• Hypertonic cells have a greater concentration
  of impermeable solutes outside the cell.
• Hypotonic cells have a lower concentration of
  impermeable solutes outside the cell.

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Hypertonic environment
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Hypotonic environment
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Isotonic Environment
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