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Transport

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).

Transport definition

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.

Etymology

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”.

Cellular transport

An illustration to show how passive transport occurs. Water-soluble molecules move down the concentration gradient across the membrane via a channel protein (an example of facilitated diffusion). Fat-soluble molecules easily move across the lipid-bilayer membrane (an example of unassisted diffusion).

Active transport: ATP is coupled to the movement of sodium ions across a biological membrane.

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:

  • Simple diffusion
  • Facilitated diffusion
  • Filtration
  • Osmosis
 Types:

  • Primary active transport
  • Secondary active transport
May or may not utilize membrane transporters Requires membrane transporters
Membrane transporters in assisted passive transport:

  • Glucose transporters
  • Ion channels
  • Aquaporins
 Primary transporters

  • Ion pumps
  • Ion channels
  • ATPases (e.g. sodium potassium pump, calcium pump, proton pump, mitochondrial ATP synthase, chloroplast ATP synthase, vacuolar ATPase, ABC transporters)

Secondary transporters

  • Antiporters
  • Symporters
Examples:

  • Simple diffusion of gases, e.g. CO2 and oxygen
  • Assisted passive transport of polar ions such as Na+ and Cl–
  • Assisted passive transport of polar molecules, such as glucose and amino acids
  • Assisted passive transport of large nonpolar molecules, such as retinol
  • Assisted passive transport of water molecules via aquaporins during osmosis
 Primary active transport examples:

  • Active transport using ATP via sodium-potassium pump to move 3 Na+ ions out while moving 2 K+ ions into the cell
  • Active transport using Redox energy (of NADH) to generate a proton gradient in the inner mitochondrial membrane
  • Active transport using photon energy (light) to generate a proton gradient during photosynthesis

Secondary active transport example:

  • Active transport of a second substrate while another ion, typically Na+, K+, or H+ ions, move down the concentration gradient

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.

Related terms

See also

References and further readings

  1. TRANSPORT IN AND OUT OF CELLS. (2019). Retrieved from Estrellamountain.edu website: https://www2.estrellamountain.edu/faculty/farabee/biobk/BioBooktransp.html
  2. MEMBRANE TRANSPORT. (2019). Retrieved from Yvcc.edu website: http://www2.yvcc.edu/Biology/109Modules/Modules/MembraneTransport/membranetransport.htm
  3. Active Transport Across Cell Membranes. (2019). Retrieved from Gsu.edu website: http://hyperphysics.phy-astr.gsu.edu/hbase/Biology/actran.html
  4. MEMBRANE TRANSPORT. (2013). Retrieved from Byui.edu website: https://content.byui.edu/file/a236934c-3c60-4fe9-90aa-d343b3e3a640/1/module5/readings/membrane-transport.html

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Last updated on July 7th, 2021

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