Chapter 3 Movement of Substances Across The Plasma Membrane

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Active Transport:




Passive Transport:








  • Excretory products such as carbon dioxide and urea are eliminated from the cells through the plasma membrane.
  • These substances need to move across the plasma membrane.
  • The plasma membrane is semi-permeable in nature and allows only the passage of water and specific dissolved substances across it.
  • Movement of substances across the cell is essential to enable the cell to receive substances required for cell respiration and to excrete waste products
  • The plasma membrane is composed of the phospholipid bilayer, carrier protein and pores
  • Movement of substances across the plasma membrane is through passive transport and active transport.
  • Passive transport involves the movement of dissolved substances from an area of higher concentration of the substance to an area of lower concentration of that subtance.
  • This type of movement follows a concentration gradient and does not require energy.
  • Example of passive transport are simple diffusion and osmosis.
  • Active transport involves the movement of dissolved substances against the concentration gradient.
  • Active transport requires energy.
  • Example of active transport is absorption of mineral salts by the root hairs.
  • Simple Diffusion involves the movement of molecules from a region of higher concentration of molecules to a region of lower concentration of the same molecules.
  • Osmosis is a process whereby solvent molecule (water molecule) moves from a region of low concentration (hypotonic) towards a region of higher concentration (hypertonic) via the semi-permeable membrane.


  • During active transport


(a) The substance to be transported attaches itself to the transport protein.

(b) With the assistance of the transport protein the substance moves across into the cell.

(c) This process requires energy from the ATP molecule which is broken into ADP molecule to release energy.

  • Water molecule enters the root hair by diffusion (osmosis)
  • Oxygen diffuses into the respiratory organs like the lungs, gills and moist skin.
  • The algae eliminate unwanted salt through active transport.

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