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:

3. 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
4. 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)

*Specific substances vary between organisms, e.g. re. carbohydrates, in mammals glucose is transported in blood plasma while in flowering plants sucrose is transported phloem sap.

 

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

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.

For example, the alveoli in human lungs enable both oxygen and carbon dioxide to diffuse across their very thin walls - which consist of just a single layer of squamous epithelial cells no more than 0.5um (= 0.0005mm see scientific numbers) thick. The direction of diffusion of these particles depends on their concentration gradients. Air breathed into the lungs is present on one side of the surface. The oxygen-poor ('deoxygenated') yet carbon-dioxide-rich blood that has been returned to the lungs by the systemic circulation is on the other side of the surface. Therefore the overall flow across this exchange surface results in oxygen entering the body and carbon dioxide leaving the body, in the case of the CO₂ by moving from the blood returned to the lungs into air in the lungs that is then breathed out of the body. In this example the exchange surface is a 'gaseous exchange surface' because the substances that move across it are gases - 'free' gases in the air, dissolved in the blood when contained within the blood vessels.

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 example, oxygen that enters a rhino via its lungs is needed to supply cells all over its body, including in its feet. That is achieved via blood circulation, many of the functions of blood being the movement of resources, including oxygen, around the body.

 

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