What is Transport in biology ?

What is meant by 'transport' in the context of biology ?

All living things 'organisms' need to move materials (e.g. very small particles such as molecules, including common biological molecules, or ions)

  • between themselves and their environment, and also
  • within the body of the organism itself.

The topic of 'transport' in biology deals with how organisms move substances they need and those they need to get rid of:

  1. into the organism
  2. around the organism, e.g. via a transport system, into a cell in a tissue

and how substances that are not required by the organism e.g. unwanted products of chemical reactions that have taken place inside the cells of the organism, e.g. to produce energy or food, are moved:

  1. around the organism, e.g. from a cell, into a transport system, then via the transport system to an organ that acts to remove the unwanted substance from the organism
  2. out of the organism e.g. in exhaled breath or in a fluid or solid released from the organism.

Examples of *types of materials transferred between organisms and their environments

  • Respiratory gases
    i.e. oxygen and carbon dioxide
  • Nutrients
    requirements vary between organisms, examples incl. glucose, fatty acids, amino acids, vitamins, minerals
  • Waste products produced by chemical reactions in the body, these are also called 'excretory products',
    e.g. carbon dioxide, urea and other nitrogenous waste. Can include excess salts, hormones, drugs and related by-products.
  • Heat (energy)

To understand how materials are moved around organisms it is helpful to know about types of movement across biological membranes.

Passive Transport Mechanisms

Active Transport Mechanisms

  1. Simple Diffusion
  2. Facilitated Diffusion
  3. Osmosis
  1. Active Transport
  2. Bulk Transport

They can be divided into two categories:

  • Passive (no energy required from the cell; movement depends on concentration gradients) and
  • Active (energy is required from the cell in order for these transport mechanisms to happen).

Membranes form surfaces.

Examples of surfaces include the cell membrane surrounding a unicellular organism, the total outer-surface (external surface) of a large plant or animal such as a tree or an elephant) and the many surfaces within organisms such as the alveoli in human lungs (which are an example of a gaseous exchange surface).

Some surfaces are important interfaces across which materials move from one side of the surface to the other.

Exchange surfaces are interfaces across which some different particles (e.g. of the types of materials transferred between organisms and their environments - see list above) can pass in both directions.

In the general case, "substance A" might move predominantly in one direction while "substance B" moves predominantly in the other direction.

Topics about transport in biology include:

Transport in biology, i.e. the ways in which materials move into, within, and out of organisms, varies according to the specific organism and its environment.

The surface area to volume ratio of an organism is a useful parameter for explaining the efficiency with which materials can be exchanged directly across the body surface of the organism. That is only efficient enough in very small organisms. Larger organisms need to have specialized exchange surfaces appropriate for the organism's size and environment, e.g. gills in fish, lungs in mammals. In addition to the exchange surfaces that enable materials such as respiratory gases to enter and leave the organism, transport systems are also needed to transport (move) the materials, e.g. molecules, from the surface by which they entered the organism to the cells throughout the body of the organism that require those substances.


For more about transport systems in mammals see compare transport systems in plants and animals (specifically, mammals vs flowering plants).

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