The
Dynamic Energy Budget (DEB) theory aims to identify simple quantitative rules for the organization of
metabolismMetabolism is the set of chemical reactions that occur in living organisms to maintain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories. Catabolism breaks down organic matter,...
of individual organisms that can be understood from basic first principles. The word "Dynamic" refers to the life cycle perspective of the theory, where the budget changes dynamically over time.
Cornerstones of the theory are:
- conservation of mass
A mass balance is an application of conservation of mass to the analysis of physical systems. By accounting for material entering and leaving a system, mass flows can be identified which might have been unknown, or difficult to measure without this technique...
, energyEnergy balance has the following meanings in several fields:* In physics, energy balance is a systematic presentation of energy flows and transformations in a system. Theoretical basis for an energy balance is the first law of thermodynamics according to which energy cannot be created or destroyed,...
and time,
- relationships between surface area and volume
- stoichiometric constraints on production
- organisational uncoupling of metabolic modules
Many organisms consist of modules, both anatomically and in their metabolism. Anatomical modules are usually segments or organs. When we look at illustrations of metabolic reactions, we find that they, too, are modular: we can clearly identify, for instance, the citric acid cycle as a complex...
- strong and weak homeostasis
Homeostasis is the property of a system, either open or closed, that regulates its internal environment and tends to maintain a stable, constant condition...
- partionability of reserve kinetics
They are essential to understand evolution of metabolic organisation since the origin of life.
DEB theory delineates
reserveReserve in the context of the Dynamic Energy Budget theory means the set of metabolites that the organism can use for metabolic purposes. These compounds can have active metabolic functions, however. They are not just "set apart for later use". Reserve differs from structure in the first place by...
s, as separate from structure.
Reserves are synthesised from environmental substrates (food) for use by the metabolism for the purpose of somatic
maintenanceMaintenance of an organism is the collection of processes to stay alive, excluding production processes.The Dynamic Energy Budget theory delineates two classes...
(including protein turnover, maintenance of concentration gradients across membranes, activity and other types of work), growth (increase of structural mass), maturity
maintenanceMaintenance of an organism is the collection of processes to stay alive, excluding production processes.The Dynamic Energy Budget theory delineates two classes...
(installation of regulation systems, preparation for reproduction, maintenance of defence systems, such as the
immune systemAn immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumour cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
),
maturationMaturation could refer to any of the following:* Fetal development* Developmental biology* Emotional development* Or physical maturation of any biological life form - see individual articles for maturation of different life forms....
(increase of the state of maturity) and reproduction.
The
Dynamic Energy Budget (DEB) theory aims to identify simple quantitative rules for the organization of
metabolismMetabolism is the set of chemical reactions that occur in living organisms to maintain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories. Catabolism breaks down organic matter,...
of individual organisms that can be understood from basic first principles. The word "Dynamic" refers to the life cycle perspective of the theory, where the budget changes dynamically over time.
Cornerstones of the theory are:
- conservation of mass
A mass balance is an application of conservation of mass to the analysis of physical systems. By accounting for material entering and leaving a system, mass flows can be identified which might have been unknown, or difficult to measure without this technique...
, energyEnergy balance has the following meanings in several fields:* In physics, energy balance is a systematic presentation of energy flows and transformations in a system. Theoretical basis for an energy balance is the first law of thermodynamics according to which energy cannot be created or destroyed,...
and time,
- relationships between surface area and volume
- stoichiometric constraints on production
- organisational uncoupling of metabolic modules
Many organisms consist of modules, both anatomically and in their metabolism. Anatomical modules are usually segments or organs. When we look at illustrations of metabolic reactions, we find that they, too, are modular: we can clearly identify, for instance, the citric acid cycle as a complex...
- strong and weak homeostasis
Homeostasis is the property of a system, either open or closed, that regulates its internal environment and tends to maintain a stable, constant condition...
- partionability of reserve kinetics
They are essential to understand evolution of metabolic organisation since the origin of life.
DEB theory delineates
reserveReserve in the context of the Dynamic Energy Budget theory means the set of metabolites that the organism can use for metabolic purposes. These compounds can have active metabolic functions, however. They are not just "set apart for later use". Reserve differs from structure in the first place by...
s, as separate from structure.
Reserves are synthesised from environmental substrates (food) for use by the metabolism for the purpose of somatic
maintenanceMaintenance of an organism is the collection of processes to stay alive, excluding production processes.The Dynamic Energy Budget theory delineates two classes...
(including protein turnover, maintenance of concentration gradients across membranes, activity and other types of work), growth (increase of structural mass), maturity
maintenanceMaintenance of an organism is the collection of processes to stay alive, excluding production processes.The Dynamic Energy Budget theory delineates two classes...
(installation of regulation systems, preparation for reproduction, maintenance of defence systems, such as the
immune systemAn immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumour cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
),
maturationMaturation could refer to any of the following:* Fetal development* Developmental biology* Emotional development* Or physical maturation of any biological life form - see individual articles for maturation of different life forms....
(increase of the state of maturity) and reproduction. This organisational position of reserve creates a rather constant internal chemical environment, with only an indirect coupling with the extra-organismal environment. Reserves as well as structure are taken to be
generalised compoundA generalized compound is a mixture of chemical compounds of constant composition, despite possible changes in the total amount. The concept is used in the Dynamic Energy Budget theory, where biomass is partitioned into a limited set of generalised compounds, which contain a high percentage of...
s, i.e. mixtures of a large number of compounds, which do not change in composition. The latter requirement is called the strong
homeostasisHomeostasis is the property of a system, either open or closed, that regulates its internal environment and tends to maintain a stable, constant condition...
assumption. Polymers (
carbohydrateCarbohydrates
[Means "hydrates of carbon"] or saccharides
[The word comes from the Greek σάκχαρον, sákcharon, meaning "sugar").] are the most abundant of the four major classes of biomolecules...
s,
proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
s,
ribosomal RNARibosomal RNA is the central component of the ribosome, the protein manufacturing machinery of all living cells. The function of the rRNA is to provide a mechanism for decoding mRNA into amino acids and to interact with the tRNAs during translation by providing peptidyl transferase activity.The...
) and
lipidLipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, phospholipids, and others...
s form the main bulk of reserves and of structure.
Some reasons for including
reserveReserve in the context of the Dynamic Energy Budget theory means the set of metabolites that the organism can use for metabolic purposes. These compounds can have active metabolic functions, however. They are not just "set apart for later use". Reserve differs from structure in the first place by...
are to give an explanation for:
- the metabolic memory; changes in food (substrate) availability affect production (growth or reproduction) with some delay. Growth continues for some time during starvation
Starvation is a severe reduction in vitamin, nutrient, and energy intake. It is the most extreme form of malnutrition. In humans, prolonged starvation can cause permanent organ damage, and eventually death...
; embryo development is fuelled by reserves
- the composition of biomass
Biomass, a renewable energy source, is biological material derived from living, or recently living organisms, such as wood, waste, and alcohol fuels. Biomass is commonly plant matter grown to generate electricity or produce heat. For example, forest residues , yard clippings and wood chips may be...
depends on growth rate. With two components (reserves and structure) particular changes in composition can be captured. More complex changes require several reserves, as is required for autotrophAn autotroph is an organism that produces complex organic compounds from simple inorganic molecules using energy from light or inorganic chemical reactions....
s.
- the body size scaling of life history parameters. The specific respiration rate decreases with (maximum) body size between species because large bodied species have relatively more reserve. Many other life history parameters directly or indirectly relate to respiration.
- the observed respiration
In animal physiology, respiration is the transport of oxygen from the outside air to the cells within tissues, and the transport of carbon dioxide in the opposite direction...
patterns, which reflect the use of energy. Freshly laid eggsIn most birds and reptiles, an egg is the zygote, resulting from fertilization of the ovum. To enable incubation the egg is usually kept within a favourable temperature range as it nourishes and protects the growing embryo. When the embryo is adequately developed it breaks out of the egg in the...
hardly respire, but their respiratory rateRespiratory rate is the number of breaths a living being, such as a human, takes within a certain amount of time ....
s increase during development while egg weight decreases. After hatching, however, the respiration rate further increases, while the weight now also increases
- all mass fluxes are linear combinations of assimilation, dissipation and growth. If reserves are omitted, there is not enough flexibility to capture product formation and explain indirect calorimetry
Calorimetry is the science of measuring the heat of chemical...
.
The standard model quantifies the metabolism of
isomorphAn isomorph is an organism that does not change in shape during growth. The implication is that its volume is proportional to its cubed length, and its surface area to its squared length...
s with 1
reserve and 1 structure that feeds on one type of food with a constant
composition. The rules for the standard model for reproducing multicellulars, and modified for dividing unicellulars, are:
Assumptions of the standard model
The state variables of the individual are structural mass and reserve; they have a constant composition (strong homeostasis).
Food is transformed into reserve, which fuels all other metabolic processes
The reserve density at birth equals that of the mother at egg formation. Foetuses develop similarly, but receive unrestricted amount of reserve from the mother during development.
Stage transitions occur if the cumulated investment into maturationMaturation could refer to any of the following:* Fetal development* Developmental biology* Emotional development* Or physical maturation of any biological life form - see individual articles for maturation of different life forms....
exceeds threshold values. These stages typically are: embryo, juvenile and adult.
Somatic maintenance is proportional to structural body volume, and maturity maintenance to maturity but maturity does not increase in the adult stage. Heating costs for endotherms and osmostic work (for fresh water organsism) are proportional to surface area.
The feeding rate is proportional to the surface area; food handling time and the transformation efficiency from food to reserve are independent of food density.
The reserves must be partitionable (i.e. mergable), such that the dynamics is not affected and weak homeostasisHomeostasis is the property of a system, either open or closed, that regulates its internal environment and tends to maintain a stable, constant condition...
applies.
A fixed fraction (called kappa) of mobilized reserve is allocated to somatic maintenance plus growth, the rest on maturity maintenance plus maturation or reproduction.
During starvation, individuals always give priority to somatic maintenance and follow one of two possible strategies:
- they do not change the mobilization of reserve (so continue to invest in maturation or reproduction)
- cease energy investment in maturation and reproduction (thus changing mobilization of reserve).
These assumptions quantify all energy and mass fluxes in an organism (including
heatIn physics and thermodynamics, heat is the process of energy transfer from one body or system due to thermal contact, which in turn is defined as an energy transfer to a body in any other way than due to work performed on the body....
, dioxygen,
carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
,
ammoniaAmmonia is a compound of nitrogen and hydrogen with the formula NH
3. It is normally encountered as a gas with a characteristic pungent odor. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to foodstuffs and fertilizers...
) and imply rules for the covariation of parameter values across species (body size scaling relationships).
Extensions of the standard model
DEB theory has been extended into many directions, such as
- effects of changes in shape during growth (e.g. V1-morph
An V1-morph is an organism that changes in shape during growth such that its surface area is proportional to its volume to the power 1, so to its volume...
s and V0-morphAn V0-morph is an organism that changes in shape during growth such that its surface area is proportional to its volume to the power 0, so its surface area is constant....
s)
- inclusion of more types of food (substrate), which requires Synthesizing Unit
Synthesizing Units are generalized enzymes that follow the rules of classic enzyme kinetics with two modifications:*product formation is not taken to be a function of substrate concentrations but of substrate fluxes that arrive at the SUs...
s to model
- inclusion of more reserves (which is necessary for organisms that do not feed on other organisms) and more structures (which is necessary to deal with plants)
- the formation and excretion of metabolic products (which is a basis for syntrophic
Syntrophy, Cross-feeding, or Cross feeding is the phenomenon that one species lives off the products of another species.For example house dust mites live off human skin flakes, of which a healthy human being produces about 1 gram per day...
relationships, and useful in biotechnologyBiotechnology is technology based on biology, agriculture, food science, and medicine. Modern use of the term usually refers to genetic engineering as well as cell- and tissue culture technologies...
)
- the production of free radicals (linked to size and nutritional status) and their effect on survival (aging)
- the growth of body parts (including tumours)
- effects of chemical compounds (toxicant
A toxicant is a chemical compound that has an effect on organisms. Toxicants are typically introduced into the environment by human activity....
s) on parameter values and the hazard rate (which is useful to establish no effect concentrationMeasures of pollutant concentration are used to determine risk assessment in public health.Industry is continually synthesizing new chemicals, the regulation of which requires evaluation of the potential danger for human health and the environment...
s for environmental risk assessmentRisk assessment is a step in a risk management process. Risk assessment is the determination of quantitative or qualitative value of risk related to a concrete situation and a recognized threat...
): the DEBtoxThe DEBtox method for the evaluation of effects of toxicants makes use of the Dynamic Energy Budget theory to quantify the effect. See the Organisation for Economic Co-operation and Development report, below, for a description of the method....
method
- processes of adaptation (gene expression) to the availability of substrates (important in biodegradation
Biodegradation is the material breakdown of chemicals by a physiological environment. The term is often used in relation to ecology, waste management and environmental remediation . Organic material can be degraded aerobically with oxygen, or anaerobically, without oxygen...
)
DEB theory provides constraints on the metabolic organisation of
sub-cellular processes. Together with rules for interaction between
individuals (competition,
syntrophySyntrophy, Cross-feeding, or Cross feeding is the phenomenon that one species lives off the products of another species.For example house dust mites live off human skin flakes, of which a healthy human being produces about 1 gram per day...
, prey-predator relationships), it
also provides a basis to understand population and ecosystem
dynamics. The theory, therefore, links various levels of biological
organisation (
cellThe cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos...
s,
organismIn biology, an organism is any living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homeostasis as a stable whole...
s and
populationIn biology, a population is the collection of inter-breeding organisms of a particular species; in sociology, a collection of human beings. Individuals within a population share a factor may be reduced by statistical means, but such a generalization may be too vague to imply anything...
s).
A considerable number of popular empirical models turn
out to be special cases of the DEB model, or very close numerical
approximations.
Dynamic energy budget theory and body size
The explanation of certain body size relationships differs for intra- and inter-species comparisons in the context of the Dynamic Energy Budget (DEB) theory. Young (small) organisms behave different from old (large) ones of the same species because they typically do different things (grow fast and don't reproduce). Adults of small-bodied species, however, are expected to behave similarly to adults of large-bodied species. The reason the parameters of the DEB theory vary between species may thus follow naturally from the structure of the theory.
Maximum body size itself can be written as a function of body size parameter values, because it results from the maximum food intake (which is linked to the surface area of the organism) and the maintenance costs (which are linked to the volume of the organism). Each of these parameters are thus either independent on maximum length, or proportional to maximum length. Any eco-physiological quantity that can be written as function of size parameters can for this reason also be written as function of the maximum body size.