Cell Transport

• The Cell Membrane/Plasma Membrane

  •Two layers (bi-layer) of phospholipid molecules.

  •One polar hydrophilic head.

  •Two non-polar hydrophobic tails.

  •Two types of proteins:

  •Integral proteins – always in the bilayer.

  •Peripheral proteins – can move temporarily.

  •Cholesterol (in animal cells only).

  •Glycoproteins – assist in body immunity.

  •Glycolipids – serve as markers for cell recognition.

  •Fluid like properties.
• Integral Proteins

  • Protein channel does not require any energy
  • Carrier proteins use some energy
• Cells Require Energy 


  •Autotrophs: Are able to produce their own food (glucose) from inorganic material (carbon dioxide and water).

  •Photo-autotrophs: use light energy from the sun via photosynthesis (most plants).

  •Chemo-autotrophs: use chemical energy from the breakdown of inorganic substances (e.g. hydrogen sulphide) to synthesise organic molecules.

  •Certain types of prokaryotes

  •Some of these organisms live around volcanic crack at the bottom of the ocean (hydrothermal vents)
• Cells Require Energy

  • Heterotrophs: Cannot manufacture their own food.
  • Need to consume other organisms or their products in order to obtain chemical energy.
  • Mostly animals, but some plants and protists fall under this category
  • Cells cannot convert inorganic molecules into organic molecules, so must consume organisms that provide these organic molecules (e.g. carbohydrates, lipids, proteins & nucleic acids).
• Metabolic Wastes

  Cells also need to be able to remove metabolic wastes from cells.

  • Metabolic wastes are products that cells create through chemical reactions.
  • High concentrations these wastes can be toxic to cells.

  Examples include:

  • Carbon dioxide
  • Water vapour
  • Urea
  • Ammonia
• Controlled Exchange of Material

  • Size – only very small molecules can freely move across the cell membrane.
  • Solubility – water and hydrophilic molecules (soluble in water) can only be transported through specific membrane transport proteins.
  • Charge – ions (atoms/molecules with a charge) can only be transported through membrane transport proteins.
• Membrane permeabilities

  Permeability – how easily substances will move across the membrane.

  High permeability = easy to move across.

  Low permeability = hard to move across.
• Passive Transport

  • Kinetic model for matter – all atoms and molecules are in a state of constant, random motion.
  • In gases and liquids, these molecules collide frequently, and move away from an area of high concentration, to an area of low concentration (of molecules).
  • Passive transport doesn’t require energy.
• Diffusion

  • Small, uncharged molecules are continuously exchanged across the membrane by diffusing across the cell membrane.
  • Moves down the concentration gradient (from high to low concentration).
  • The steeper the gradient, the greater the rate of diffusion.
  • Examples are carbon dioxide (CO2) or oxygen (O2) molecules.
  • Energy is not required for this.