In biology, transport refers to the act or the means by which a molecule or ion is moved across the cell membrane or via the bloodstream. There are two types of transport in this regard: (1) passive transport and (2) active transport. Passive transport is a kind of transport by which ions or molecules move along a concentration gradient, which means movement from an area of higher concentration to an area of lower concentration. The four major types of passive transport are diffusion, facilitated diffusion, filtration, and osmosis. Active transport is a kind of transport wherein ions or molecules move against a concentration gradient. This means the movement is from an area of lower concentration to an area of higher concentration. This type of transport requires expenditure of energy and the assistance of proteins (i.e. carrier protein).
In general, the term transport is the movement (of something) from one place to another. It can be used as an action word for carrying, moving, or conveying something from one location to another. In biology, transport is the act or the means by which molecules, ions, or substrates are moved across a biological membrane, such as the plasma membrane. It may also pertain to the moving of electrons in an electron transport chain. In this regard, a concentration gradient is necessary to incite them to be transported. It may either be along or against their respective concentration gradient. Transport may also be used to pertain to the transport activity of blood and other bodily fluids in the circulatory system.
The term transport came from Middle English, Old French transporter, meaning “to carry” or “convey across”. It is derived from the Latin transporto, from trans-, meaning “across” and porto, meaning “to carry”.
One of the major biological activities of a cell is the transport of biological molecules, ions, and substrates. The transport could occur inside the cell. For instance, the protein produced by the endoplasmic reticulum is transported or conveyed to the Golgi apparatus for further processing. This is an example of intracellular transport. Transport could also occur from the cell to the outside, such that occurs during secretion, or from the outside into the cell. There are substances that can easily move through the lipid bilayer component of the plasma membrane. For example, small nonpolar molecules can move across the membrane. Larger nonpolar molecules and polar molecules cannot enter or leave the cell because of their size and polarity, respectively. Nevertheless, they can still be moved across the membrane but they would need membrane proteins to shuttle or transport them across.
Biological transport at the cellular level may be passive or active. Both types need a concentration gradient to ensue. They differ though in the direction of the movement with respect to the concentration gradient. Passive transport is the transport of substances across the membrane from an area of high concentration to an area of low concentration. The movement is, therefore, along the concentration gradient. Conversely, active transport is a type of cellular transport where the movement is against the concentration gradient. The movement is from an area of low concentration to an area of greater concentration. Since the movement of substances in passive transport is downhill, kinetic energy is sufficient to drive the movement. In active transport, the movement is uphill and therefore needs greater source of energy to power up the process. Typically, it uses chemical energy in the form of adenosine triphosphate (ATP), which the cell generates metabolically, such as through glycolysis and citric acid cycle.
Passive transport vs. Active transport
|Passive transport||Active transport|
|Downhill movement of substances, , i.e. from higher to lower concentration||Uphill movement of substances, i.e. from lower to higher concentration|
|Along the concentration gradient||Against the concentration gradient|
|Does not require ATP||Mostly requires ATP|
|Types: ||Types: |
|May or may not utilize membrane transporters||Requires membrane transporters|
|Membrane transporters in assisted passive transport: ||Primary transporters |
|Examples: ||Primary active transport examples: |
Secondary active transport example:
Transport at the tissue level
At the tissue level, transport is the means by which substances are moved from the cell to the outside or to other parts of the body. The blood is the circulating fluid in the body of higher animals, including humans. It transports various molecules, such as oxygen (bound to hemoglobin), carbon dioxide and metabolic byproducts for excretion, hormones and other chemical signaling molecules, and nutrients (e.g. glucose, amino acids, and fatty acids).
In plants, transport of substances at the tissue level occurs at the vascular tissues, particularly phloem and xylem. The phloem tissues are responsible for the conduction of photosynthetic materials whereas the xylem tissues are the ones conducting water and nutrients from the roots to the different parts of the plant.
- Active transport
- Axonal transport
- Cholesterol ester transport proteins
- Cotranslational transport
- Coupled transport
- Electron transport
- Electron transport chain
- Facilitated transport
- Membrane Transport Protein
- Ovum transport
- Passive transport
- Respiratory transport
- Retrograde axonal transport
- Transport antibiotic
References and further readings
- TRANSPORT IN AND OUT OF CELLS. (2019). Retrieved from Estrellamountain.edu website: https://www2.estrellamountain.edu/faculty/farabee/biobk/BioBooktransp.html
- MEMBRANE TRANSPORT. (2019). Retrieved from Yvcc.edu website: http://www2.yvcc.edu/Biology/109Modules/Modules/MembraneTransport/membranetransport.htm
- Active Transport Across Cell Membranes. (2019). Retrieved from Gsu.edu website: http://hyperphysics.phy-astr.gsu.edu/hbase/Biology/actran.html
- MEMBRANE TRANSPORT. (2013). Retrieved from Byui.edu website: https://content.byui.edu/file/a236934c-3c60-4fe9-90aa-d343b3e3a640/1/module5/readings/membrane-transport.html
© Biology Online. Content provided and moderated by Biology Online Editors