Transport Systems in biology


1.0 What is a Transport System ?

Simple definition of a transport system in biology:

A transport system is a means by which materials are moved ('transported') from an exchange surface or exchange surfaces to cells* located throughout the organism.

Reminder re. transport and exchange surfaces:

  • Transport
    Organisms need to be able to move materials (such as respiratory gases, nutrients, waste products and heat) both into and out of, and within, themselves - that is called transport.
  • Exchange Surfaces
    Specialized exchange surfaces are biological structures whose features are such that they permit the highly efficient transfer of materials e.g. respiratory gases, across them (i.e. across the exchange surfaces) via mechanisms such as diffusion or active transport.

    Example:
    All living things need to breathe (perform respiration) so movement of respiratory gases, e.g. accepting oxygen into the organism, is an important part of transport in biology.
    Although many microorganisms accept oxygen directly through their cell membrane, organisms whose surface-area:volume ratio is larger, e.g. mammals, need specialized exchange surfaces through which to receive oxygen and release carbon dioxide. Such exchange surfaces are often part of organs, e.g. within the lungs of birds and mammals and within the gills of fish.

 

Multiple exchange surfaces?
Although gas exchange surfaces are important due to the need for respiration, they are not the only types of exchange surfaces. Another example in humans is the wall of the small intestine where digested (food) material is transported into the blood stream for distribution to cells throughout the body.

 

What is a mass transport system (in biology) ?

Explanation of terms: The terms transport system and mass transport system both appear in biology textbooks.
They are sometimes used interchangeably because the main transport systems included in introductory biology are mass transport systems:

  • In biology, mass flow is the movement of a fluid in one direction, usually through a system of tube-like vessels.
  • A mass transport system is a transport system (i.e. a means by which materials are moved from exchange surface/s that form part of an organism to all other locations within the organism where the materials from the exchange sufrace/s are required by cells) that involves mass flow.

So ... a definition of a mass transport system in biology is:

In biology a mass transport system is an arrangement of physical structures by which materials are moved in the form of a fluid containing particles of those materials travelling in one direction [through a system of tubes*] from one or more exchange surface(s) within an organism to cells located throughout the organism.

*If the fluid is contained within a system or network of vessels it forms a "closed" transport system. If fluid is not contained within a closed system, e.g. of blood vessels + heart, the system is described as "open", e.g. the open circulation system in insects. In that case the fluid is not constrained to flow all in the same direction, but can circulate with more freedom so may not count as a mass transport system (not if the fluid isn't all moving in the same direction). See the table below for open vs closed systems.

Examples of mass flow in mass transport systems include the movement of blood e.g. in the blood systems present in mammals and the movement of xylem and phloem through plants.

2.0 Features of Transport Systems

Transport systems in many different (and different types of) organisms have lots of common features, such as:

  • Contains a transport medium in which materials, including various sizes and shapes of molecules, can be conveyed. This medium is usually water-based. Water acts as a solvent for a wide range of substances and flows easily at the temperatures of living organisms. (See why water is important to life for more info.) Examples of transport media in animal transport systems include blood, lymph and hemolymph.
  • A structure or 'system' of vessels that contain (enclose) the transport medium and extend, via a branching network, to all locations to which materials carried in the transport medium must be transported.
    Note: This doesn't apply to 'Open Circulatory Systems', which may have some vessels but the transport medium is not retained within them at all times.
  • Enables substantial volumes* of the transport medium to be moved over large distances*.
    *Volumes appropriate to the size of the organism, e.g. the averge human adult body contains approx. 8-10 pints of blood. Distances are also large relative to the size of the organism, i.e. as necessary to supply materials to all parts of it, so larger distances in an elephant than in a mouse!
  • A mechanism for moving the transport medium through the system, e.g. through a network of vessels.
    Movement of a fluid through a system requires a difference in pressure between parts of the system.
    • Animal Transport Systems sometimes include a pumping organ such as the heart in mammals, birds and some other creatures. They may also use other mechanisms such as muscular contraction of muscle tissues as well as, or instead of, a heart.
    • Plant Transport Systems tend to rely on passive physical process e.g. evaporation of water.
  • Mechanism(s) to maintain the mass flow movement of the transport medium in one direction.
    The pressure difference that moves the transport medium through the system is helpful but not necessarily sufficient to prevent back-flow. Some of the vessels in the circulation systems of animals include valves that prevent black-flow of the fluid contained in the vessel.
  • Ability to control the flow of the transport medium, i.e. to adjust the flow of the transport medium (e.g. blood) according to the needs of the organism. An example of this is the blood circulation system in mammals reducing the flow of blood to extremities, such as fingers, in very cold conditions - which helps to conserve heat and blood supply to essential internal organs.

3.0 Types of Transport Systems

Although they have features in common, transport systems in plants and animals differ.

Examples of Transport Systems in Animals

Examples of Transport Systems in Plants

  • Open Circulatory Systems, e.g.
    • in insects - where hemolymph (so-called because it is equivalent to the combination of blood and lymph in organisms that have separate circulation systems for those two fluids) flows within body cavities, making direct contact with tissues and organs. There is no heart pumping hemolymph.
    • in humans - the lymphatic system includes a network of vessels that convey a clear fluid called lymph towards the heart from everywhere else in the body; as there is no pump the movement of lymph is slower than that of blood and discontinuous because it depends on other movements of and within the body.
  • Closed Circulatory Systems,
    of which there are two types:
    • Single circulation systems
      (consist of blood, blood vessels and a heart; blood passes through the heart once per cycle around the complete circuit)
    • Double circulation systems
      (consist of blood, blood vessels and a heart; blood passes through the heart twice per cycle around the complete circuit)
  • Xylem transports:
    • water
    • minerals (mineral ions dissolved in water)
  • Phloem transports assimilates, such as:
    • sucrose
    • amino acids

Note that closed circulatory systems, xylem and phloem are mass transport systems i.e. closed systems within the tubes (vessels) of which, fluid flows - all of it moving in the same direction at the same time.

See also comparison of transport systems in mammals vs flowering plants.

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