In the
life sciences
This list of life sciences comprises the branches of science that involve the scientific study of life – such as microorganisms, plants, and animals including human beings. This science is one of the two major branches of natural science, ...
, mass flow, also known as mass transfer and bulk flow, is the movement of fluids down a pressure or temperature gradient.
[Moyes & Schulte (2008). Principles of Animal Physiology. Pearson Benjamin Cummings. San Francisco, California] As such, mass flow is a subject of study in both fluid dynamics and biology. Examples of mass flow include
blood circulation
In vertebrates, the circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the body. It includes the cardiovascular system, or vascular system, that consists of the heart an ...
and transport of
water
Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
in vascular plant tissues. Mass flow is not to be confused with diffusion which depends on concentration gradients within a medium rather than pressure gradients of the medium itself.
Plant biology
In general, bulk flow in plant biology typically refers to the movement of water from the soil up through the plant to the leaf tissue through
xylem
Xylem is one of the two types of transport tissue (biology), tissue in vascular plants, the other being phloem; both of these are part of the vascular bundle. The basic function of the xylem is to transport water upward from the roots to parts o ...
, but can also be applied to the transport of larger solutes (e.g. sucrose) through the
phloem
Phloem (, ) is the living tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to the rest of the plant. This transport process is ...
.
Xylem
According to
cohesion-tension theory
Xylem is one of the two types of transport tissue in vascular plants, the other being phloem; both of these are part of the vascular bundle. The basic function of the xylem is to transport water upward from the roots to parts of the plants such ...
, water transport in xylem relies upon the cohesion of water molecules to each other and adhesion to the vessel's wall via
hydrogen bonding
In chemistry, a hydrogen bond (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. It occurs when a hydrogen (H) atom, Covalent bond, covalently b ...
combined with the high water pressure of the plant's substrate and low pressure of the extreme tissues (usually leaves).
[Taiz, Lincoln; Zeiger, Eduardo; Moller, Ian Max; Murphy, Angus (2015). Plant Physiology and Development. Sunderland, MA: Sinauer Associates, Inc. p. 66. .]
As in blood circulation in animals,
(gas) embolisms may form within one or more xylem vessels of a plant. If an air bubble forms, the upward flow of xylem water will stop because the pressure difference in the vessel cannot be transmitted. Once these embolisms are
nucleated
The cell nucleus (; : nuclei) is a membrane-bound organelle found in eukaryotic cells. Eukaryotic cells usually have a single nucleus, but a few cell types, such as mammalian red blood cells, have no nuclei, and a few others including osteo ...
, the remaining water in the capillaries begins to turn to water vapor. When these bubbles form rapidly by
cavitation
Cavitation in fluid mechanics and engineering normally is the phenomenon in which the static pressure of a liquid reduces to below the liquid's vapor pressure, leading to the formation of small vapor-filled cavities in the liquid. When sub ...
, the "snapping" sound can be used to measure the rate of cavitation within the plant .
[Pockman, W.T., Sperry, J.S., & O'Leary, J.W. 1995. Sustained and significant negative water pressure in xylem. 'Nature' 378: 715-716] Plants do, however, have physiological mechanisms to reestablish the capillary action within their cells .
Phloem
Solute flow is driven by a difference in hydraulic pressure created from the unloading of solutes in the sink tissues.
[Lambers, Hans (2008). Plant Physiological Ecology. 233 Spring Street, New York, NY: Springer Science+Business Media, LLC. p. 153. .] That is, as solutes are off-loaded into sink cells (by active or passive transport), the density of the phloem liquid decreases locally, creating a pressure gradient.
See also
*
Countercurrent exchange
Countercurrent exchange is a mechanism between two flowing bodies flowing in opposite directions to each other, in which there is a transfer of some property, usually heat or some chemical. The flowing bodies can be liquids, gases, or even solid ...
*
Pounds Per Hour
Pound per hour is a mass flow unit based on the international avoirdupois pound, which is used in both the British imperial and, being a former colony of Britain, the United States customary systems of measurement. It is abbreviated as PPH, ...
*
Fluid Dynamics
In physics, physical chemistry and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids – liquids and gases. It has several subdisciplines, including (the study of air and other gases in motion ...
*
Mass flow rate
In physics and engineering, mass flow rate is the Temporal rate, rate at which mass of a substance changes over time. Its unit of measurement, unit is kilogram per second (kg/s) in SI units, and Slug (unit), slug per second or pound (mass), pou ...
*
Hemorheology
Hemorheology, also spelled haemorheology (''haemo'' from Greek ‘αἷμα, ''haima'' 'blood'; and ''rheology'', from Greek ῥέω ''rhéō'', ' flow' and -λoγία, ''-logia'' 'study of'), or blood rheology, is the study of flow properties o ...
*
Flying and gliding animals
A number of animals are capable of aerial locomotion, either by powered flight or by gliding (flight), gliding. This trait has appeared by evolution many times, without any single common ancestor. Flight has evolved at least four times in separ ...
References
{{DEFAULTSORT:Mass Flow
Fluid dynamics