Unit 6: Investigative Project
A: Undertake a literature search and review to produce an investigative project proposal
Project proposal
Introduction
Diffusion is defined as the movement of molecules from an area of high concentration to an area of low
concentration, whilst moving across a concentration gradient (this exists when a membrane separates two different
concentrations of molecules) [LibreTexts, 2018]. Certain factors can affect how efficiently the diffusion of substances
is carried out, one of which is concentration, which will be investigated as part of my project. This assignment will
provide an outline of the project including its objectives, hypothesis, justification and limitations.
This investigation is crucial to carry out because the diffusion of substances is an important process to several
biological processes such as the urinary and respiratory systems [Markings, S. 2018] (see ‘Background science’ for
further detail on this). Measuring diffusion rate is critical to cell function as the transport of molecules helps to
maintain the balance of substances in the cell [StudyMind, n.d.]. If there are changes in the rate of diffusion, the
regulation of substances may be disrupted and cause changes to overall cell function [StudyMind, n.d.].
Background science
Types of diffusion
There are two types of diffusion: simple diffusion and facilitated diffusion. Both facilitated diffusion and simple
diffusion are forms of passive transport, which means that they do not require energy (in the form of ATP) to move
molecules across the concentration gradient [Biology Online, 2019]. Simple diffusion is used to transport small and
nonpolar ions that are able to diffuse across the plasma membrane (e.g. oxygen, carbon dioxide) but it cannot be
used to transport large and charged ions (e.g. sodium ions, glucose, amino acids) across the plasma membrane
[Biology Online, 2019]. Instead, these ions are transported through facilitated diffusion, which involves the aid of a
channel protein or carrier protein to cross the membrane [Annets, F. 2017].
Figure 1 - Molecules moving across the plasma membrane in simple diffusion [CK-12 Foundation, 2019]
Figure 2 - Molecules moving across the plasma membrane in facilitated diffusion [CK-12 Foundation, 2019]
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, Unit 6: Investigative Project
A: Undertake a literature search and review to produce an investigative project proposal
Factors affecting the rate of diffusion
Several factors can have an effect on the rate of diffusion such as concentration, temperature, solubility, surface area
and the size of molecules. A greater difference in concentration leads to a faster diffusion rate [Annets, F. 2016] and
the rate of diffusion slows as the distribution of the substance reaches equilibrium [LibreTexts, 2018]. Higher
temperatures increase the energy and the movement of molecules, resulting in a faster rate of diffusion [Annets, F.
2016]. Nonpolar or lipid-soluble molecules move through plasma membranes more easily than polar materials,
resulting in quicker diffusion [LibreTexts, 2018]. A larger surface area enhances the rate of diffusion, while a thicker
membrane decreases it [LibreTexts, 2018]. If molecules have to travel long distances, the rate of diffusion will be
slower, whereas the rate of diffusion is faster over short distances. [Annets, F. 2016].
Fick’s laws of diffusion
Fick's first and second laws of diffusion were established in 1855 by Adolf Fick to mathematically represent the rate
of dissolved gas diffusion over membranes [Science Facts, 2020]. They were inspired by Thomas Graham's early
work and dealt with detecting salt concentrations and fluxes diffusing between two reservoirs via water tubes
[Science Facts, 2020]. Although he primarily experimented with fluids, both laws are still used today to define
diffusion in solids, liquids and gases [SimScale, 2023]. Fick's laws serve as the foundation for the majority of our
understanding of solid, liquid and gas diffusion [SimScale, 2023]. A diffusion process that does not follow Fick's rules
is referred to as "non-Fickian diffusion" [SimScale, 2023].
Fick’s first law of diffusion
Fick’s first law of diffusion explains the diffusive flux within a medium to the gradient of the concentration [SimScale,
2023]: the diffusion flux (movement of molecules) from a region of high concentration to a region of low
concentration is directly proportional to the magnitude of the concentration gradient of a substance [Science Facts,
2020]. Fick’s first law is demonstrated with the following equation:
𝑑𝜑
J = -D 𝑑𝑥
The diffusion flux (J) measures the amount of substance that flows through a unit area during a unit time interval
[Science Facts, 2020]. It is measured in g/m2 [Science Facts, 2020]. The diffusion coefficient (D) is proportional to the
squared velocity of the diffusing particles, which depends on the temperature, viscosity of the fluid and the size of
the particles [Science Facts, 2020]. It is measured in m2/s [Science Facts, 2020]. ‘d𝜑’ represents the change in
concentration and ‘dx’ reprevents the change in position [Science Facts, 2020]. The negative sign of the equation
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