1.4 Membrane Transport

SUMMARY of what needs to be known


  • PASSIVE TRANSPORT across a membrane is dependent upon:
    • a diffusion gradient of molecules either side of the membrane
    • the kinetic energy of the molecules
  • ACTIVE TRANSPORT requires energy (ATP) from metabolism in the cell because:
    • molecules usually move against their concentration gradient
    • carrier proteins are often involved
    • the process is selective
  • BULK TRANSPORT is also a form of active transport, requiring metabolic ATP, but molecules or larger substances are transported within membrane vesicles.




Cells need to transport ions and molecules across membranes for several reasons:

  1. to obtain nutrients
  2. to excrete waste substances
  3. to secrete useful substances made within the cell
  4. to generate ionic gradients essential, eg. for nervous and muscle activity
  5. to maintain a suitable pH and ionic concentration within the cell for enzyme activity

READING (from Biology in Context for Cambridge International A Level by Glenn and Susan Toole)


Special case of FACILITATED DIFFUSION – structure and function of potassium channels in axons of neurons



Special case of PROTEIN PUMPS (ACTIVE TRANSPORT): structure and function of sodium/potassium pump in axons of neurons



OBLIGATORY PRACTICAL 2: Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions.

This is a practical investigation which should be completed in the lab. It is suggested by IB that this practical is exploited to demonstrate the importance of accurate quantitative (in this case mass and volume) measurements. Sources and margins of error should be investigated.

The inference is obvious for your exams! Paper 3 can carry questions about any of the obligatory practicals. If the syllabus suggests that this practical is used to demonstrate the importance of accurate, quantitative measurements, then you can be sure that, if this investigation is tested in the examination, there will be reference to accurate, quantitative measurements and how you obtained these in this practical investigation.

Here is how a worked example of a basic investigation might look:



All organisms must somehow manage to maintain their osmotic balance within ranges that permit survival. Some organisms live in environments where external water concentrations are forever changing. Such an environment is found in estuaries, where tides come in, bringing salt water from the sea, and go out again, leaving plants and animals surrounded by fresh, river water. Very capable osmo-regulation mechanisms are necessary in such difficult environments. Here is a nice investigation of such a problem, in the form of a data graphing and analysis question:


The kidneys are essentially a selective blood filter for the body, enabling the excretion of excess water and salts, in order to maintain the osmotic balance of the blood. (The kidneys also excrete urea and other waste products.) What happens if the kidneys are damaged or malfunction? Medicine can take over by artificially filtering the blood through semipermeable membranes (dialysis tubing). A patient is hooked up to a dialysis machine through an artery, usually in an arm, and the blood is passed through dialysis tubing in the machine. By artificially maintaining the correct solute/solvent concentrations outside the dialysis tubing, controlled diffusion will take place, to ‘clean’ the blood, which is then returned into a vein in the arm. Although there are some home machines now available to some patients, most who are in need of dialysis are required to check in to a clinic sometimes for several hours a week. Not a great scenario!



If scribd does not open or download, here is a Google Docs link: https://drive.google.com/file/d/0Byq5ornnlm2gdGtpMzhsQUJVNzg/view?usp=sharing


Eyyy! edited small