Transport Systems

Transport (the movement of substances within organisms) is more of a challenge for multicellular organisms than it is for unicellular organisms. This is because the distances that substances need to be moved are much greater in multicellular organisms.

Within unicellular organisms, substances are moved around by the processes of diffusion, osmosis and active transport (and some other short-distance transport processes that will not be covered in this course).

These processes are too slow to carry substances the long distances needed by multicellular organisms. Therefore, multicellular organisms have specialised transport systems to move substances around.

These transport systems carry substances to the cells that need them, or carry waste products away from the cells that produce them. Within the cells, substances move around by the same processes that they do in unicellular organisms.

Example: Oxygen transport in a yeast cell vs an elephant

As an example, let's consider the transport of oxygen in two organisms: a yeast cell (a unicellular organism) and an elephant (a multicellular organism). In both organisms, oxygen needs to reach the mitochondria where it is used in aerobic respiration to release energy from food molecules.

In a yeast cell, oxygen enters through the cell membrane and only needs to travel a short distance (perhaps a few micrometers or less) to reach the mitochondria. Since the distance is so short, the oxygen can simply move by diffusion.

In an elephant, oxygen enters the body through the lungs. It then needs to travel to all the cells that need it, enter those cells and then travel to the mitochondria.

Some of the elephant's cells are very close to the lungs, however most are further away. A cell in one of the elephant's back feet may be a couple of meters away from the lungs. Even over a distance of a few millimeters, diffusion is too slow to be effective.

To get around this problem, the elephant has a transport system: the circulatory system (consisting of the heart, blood vessels and blood).

Oxygen from the lungs only needs to diffuse a short distance to enter the blood (the lungs are covered in blood vessels). The blood then carries the oxygen around to all of the cells of the body, using the power generated by the heart's contraction. In every tissue that the blood passes through, some of the oxygen diffuses out of the blood, into the cells and through the cytoplasm to the mitochondria.

So the circulatory system is used to transport the oxygen almost all of the distance, with diffusion just being used to move the oxygen a short distance at either end.

Examples of transport systems

The main transport system in animals is the circulatory system. The circulatory system transports many substances, including:

  • Oxygen
  • Carbon dioxide
  • Glucose and other nutrients
  • Urea
  • Hormones
  • Antibodies

The main transport systems in plants are the xylem and phloem. The xylem transports water and mineral ions, and the phloem transports sucrose.


Flashcards help you memorise information quickly. Copy each question onto its own flashcard and then write the answer on the other side. Testing yourself on these regularly will enable you to learn much more quickly than just reading and making notes.


Why do multicellular organisms need transport systems?


What are the main transport systems in animals and plants?


What is transported in the circulatory system?


What is transported in the xylem?


What is transported in the phloem?


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