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Related Concept Videos

Diffusion01:21

Diffusion

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Diffusion is a type of passive transport. In passive transport, a substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across the space. For example, take the diffusion of substances through the air. When someone opens a perfume bottle in a room filled with people, the perfume is at its highest concentration in the bottle and is at its lowest at the edges of the room. The perfume vapor will diffuse, or spread away, from the...
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Diffusion is the passive movement of substances down their concentration gradients—requiring no expenditure of cellular energy. Substances, such as molecules or ions, diffuse from an area of high concentration to an area of low concentration in the cytosol or across membranes. Eventually, the concentration will even out, with the substance moving randomly but causing no net change in concentration. Such a state is called dynamic equilibrium, which is essential for maintaining overall...
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External respiration occurs in the lungs, and it is the first step in the journey of oxygen inside the body. When we inhale, oxygen enters our lungs and diffuses across the thin alveolar membrane. The alveoli are tiny, air-filled sacs that provide a vast surface area for gas exchange. Oxygen in the alveoli has a higher partial pressure (105 mmHg) than in the adjacent pulmonary capillaries (40 mmHg), establishing a pressure gradient. As a result, oxygen molecules move from the alveoli into the...
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Passive diffusion is a critical process that allows small lipophilic drugs to cross the cell membrane along a concentration gradient. This mechanism's efficiency depends on four primary factors: the membrane's surface area, the drug's lipid-water partition coefficient, the concentration gradient, and the membrane's thickness.
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Microorganisms exhibit diverse oxygen requirements and growth patterns driven by their metabolic strategies and environmental adaptations. Oxygen, while essential for many organisms, can also be toxic under certain conditions, shaping how microorganisms grow and survive.Oxygen Requirements of MicroorganismsMicroorganisms are classified based on their ability to use or tolerate oxygen:● Obligate aerobes like Mycobacterium tuberculosis need oxygen for energy production, as it serves as the...
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Related Experiment Video

Updated: Dec 13, 2025

Extraction of Structural Extracellular Polymeric Substances from Aerobic Granular Sludge
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Heterogeneous diffusion in aerobic granular sludge.

Lenno van den Berg1, Catherine M Kirkland2,3, Joseph D Seymour2,4

  • 1Department of Water Management, Delft University of Technology, Delft, The Netherlands.

Biotechnology and Bioengineering
|July 30, 2020
PubMed
Summary

Aerobic granular sludge (AGS) diffusion is key for wastewater treatment. This study found that water diffusion within AGS granules is about 70% of free water, with minimal impact from granule differences.

Keywords:
NMRaerobic granular sludgediffusiongranule structureheterogeneity

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Area of Science:

  • Environmental Engineering
  • Biotechnology
  • Chemical Engineering

Background:

  • Aerobic granular sludge (AGS) is crucial for compact wastewater treatment, enabling simultaneous removal of nitrogen, phosphorus, and carbon.
  • Understanding diffusive transport within AGS granules is essential for optimizing reactor operation, design, and modeling.

Purpose of the Study:

  • To characterize diffusive mass transfer within full-scale and lab-scale AGS using nuclear magnetic resonance (NMR) methods.
  • To evaluate the impact of heterogeneous diffusion on AGS granule performance using a reaction-diffusion model.

Main Methods:

  • Pulsed-field gradient NMR was used to determine the self-diffusion coefficients of water within AGS granules.
  • NMR imaging was employed to visualize the internal structure of the AGS granules.
  • A reaction-diffusion granule-scale model was developed to assess the influence of diffusion heterogeneity.

Main Results:

  • The self-diffusion coefficient of water in AGS was found to be approximately 70% of that in free water.
  • No significant differences in water self-diffusion were observed between AGS from full-scale and lab-scale reactors.
  • The reaction-diffusion model indicated that diffusional heterogeneity had a negligible impact (<1%) on the flux into the granules.

Conclusions:

  • Differences in granular sludge composition and internal heterogeneity have minimal effects on the kinetic properties of AGS.
  • A simplified approach is adequate for describing mass transport by diffusion into AGS granules.
  • These findings support the efficient design and modeling of AGS wastewater treatment systems.