Cell Transport

Cell Membrane

The Cell membrane is the most important organelle in an organism’s cell. It controls how substances move in and out of the cell.

5 methods for movement in and out of cell:

  • 1) lipid diffusion
  • 2) osmosis
  • 3) passive transport
  • 4) active transport
  • 5) vesicles

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Lipid Diffusion

  • Carried out by very small molecules – H2O, O2, CO2
  • No energy is involved
  • Substances move down concentration gradient
  • Cannot be controlled by cell

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  • Diffusion of water (abundant in cells) across a membrane
  • Higher concentration of solution = less water molecules
  • Net flow of water diffuses down concentration gradient
  • Water moves from dilute to concentrated solution

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Solution Types

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Passive Transport

  • Transport of substances across a membrane by a trans-membrane protein molecule
  • Transport proteins are often specific to one molecule
  • No energy is involved
  • Substances move down concentration gradient

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Channel proteins

  • form channel in membrane which is filled with water
  • ions diffuse across the membrane
  • many channels can be opened or closed

Carrier proteins

  • flip around so that only one side of membrane is open
  • high concentration: substance binds
  • low concentration: substance is released

Active Transport

  • Protein pump molecule “pumps” substances across membrane
  • Molecule of substance binds to protein and is then released on opposite face
  • Each molecule fits a particular protein pump
  • Energy is required; substances move up concentration gradient


Move large molecules in and out of cells e.g. proteins, polysaccharides, nucleotides

  • In to cell: endocytosis
  • Out of cell: exocytosis

Cell Transport Summary

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