- Diffusion is the passive movement of molecules from a high concentration to a lower concentration
- Diffusion can also be facilitated by membrane proteins such as channels or carriers
- Osmosis is a water-specific type of diffusion, where water moves from a high to a low concentration across a selectively-permeable membrane
- Larger molecules are transported into and out of the cell by endocytosis or exocytosis, respectively.
Cells have various mechanisms of transporting particles within and outside the cell. Some of these are passive and do not require energy (such as diffusion, facilitated diffusion, and osmosis), and some are active processes requiring energy (such as endocytosis and exocytosis).
Diffusion is the movement of molecules from a higher concentration to a lower concentration without the use of energy. In the cell, this can occur when small molecules diffuse through the pore of a cell membrane, or when substances move within the cytosol. Diffusion occurs when the concentration of a particular molecule is greater in one area than another, establishing a concentration gradient. The diffusion of molecules across a concentration gradient does not require a transport protein, and is an example of passive transport.
During diffusion, molecules move from a higher to a lower concentration until the concentrations are equal between the spaces. The stronger the concentration gradient (and assuming if there is a barrier present that it is permeable), the faster the diffusion will occur. Substances such as water and gases can enter the cell by this type of transport. A good example is if you imagine dropping a sugar cube in a glass of water without stirring it. The sugar will slowly dissolve and disperse throughout the water by he process of diffusion.
Diffusion occurs because in an area where there is a high concentration of a particular substance, it is likely that one of them will move to the empty space, but unlikely that the reverse will occur since there are so few in the less concentrated area. Therefore, the total movement of molecules will be out of the more concentrated area and into the less concentrated area. This does not use energy, and in fact, a concentration gradient can generate potential energy.
Facilitated diffusion describes the process of passive transport of molecules across a membrane, with the help of transmembrane proteins. However, facilitated diffusion is still an example of passive transport and does not require energy expenditure. Facilitated diffusion usually occurs when a molecule cannot pass through the plasma membrane because of their polarity or charge. Proteins that aid in facilitated diffusion shield these molecules from the hydrophobic core of the membrane, allowing molecules to cross the membrane through a pore or channel.
Channel proteins are transmembrane proteins that make hydrophilic tunnels across it. This provides a favourable environment for molecules to pass through by diffusion. These channels are highly selective of the molecules they allow across, and will usually only aid in the transport of one molecule or a group of similar molecules. For example, aquaporins are a special kind of channel proteins that allow water to rapidly pass through the cell membrane. Some channel proteins are always open, others only open in response to signals from within or outside the cell.
Carrier proteins are another class of membrane-spanning proteins that permit facilitated diffusion. Carrier proteins are able to change their shape to allow a molecule to diffuse across the cell membrane. These proteins are also very selective. They are different from pumps involved in active transport, because they simply provide the correct environment for molecules to pass the cell membrane, but do not actively push them through. These proteins are generally slower at transporting molecules than channel proteins.
Osmosis is a term describing the movement of water from across a selectively permeable membrane as a result of a concentration gradient. I.e. it is a special type of diffusion concerned only with water. The water moves towards a high concentration of a solute, with the effect of equalising the solute concentration across a permeable membrane. From the point of view of the water, it undergoes diffusion, moving from a high concentration of a lower concentration of itself. Osmosis is also a form of passive transport, as it does not require the expense of energy.
There are three terms to describe the concentration of solutes in a solution outside of a cell. An isotonic solution has an equal solute concentration to that within the cell. Therefore, water moves in both directions inside and outside of cell. A hypertonic solution is one which has a higher solute concentration outside the cell than inside. Water moves out of the cell into the extracellular environment, shrinking the cell and making it shrivel up. This process is known as plasmolysis in plant cells, and the cell is said to be plasmolysed. A hypotonic solution has a lower solute concentration inside of the cell than outside, so that cell swells up, and can potentially burst. In plant cells, the cell wall prevents the cell from bursting, and maintains it in a turgid state.
Active transport across a cell membrane requires a transporter protein and a supply of energy for the transport of molecules the membrane. Its requirement for energy distinguishes it from passive transport. This process is very important to transport molecules across the cell membrane which are present at a very low concentration in the extracellular environment.
To transfer large particles such as proteins and sugars into the cell, the cell relies on different mechanisms. Usually, these types of transport involve the cell engulfing the macromolecule, enclosing it with the plasma membrane and taking it into the cell through vesicular transport. This process is called endocytosis, of which there are various types. The reverse is called exocytosis, where molecules are transported out of the cell.
Endocytosis is a general term for a ctive transport that involves the movement of particles into a cell through vesicles. The plasma membrane first forms a pocket around the molecule, bringing it into the cell. The pocket is then pinched off, trapping it in a vesicle inside the cell. Three types of endocytosis are phagocytosis, pinocytosis, and receptor-mediated endocytosis.
Phagocytosis is a type of endocytosis in which very large particles, such as cells or cell debris, are transported inside the cell. Cells such as the macrophage cells of the immune system take advantage of phagocytosis to ‘eat’ bacteria
Pinocytosis (“cell drinking”) is a form of endocytosis in which liquid droplets are taken up by the cell.
Receptor-mediated endocytosis is a form of endocytosis in which transmembrane receptor proteins across the cell membrane are used to take up a target molecule. When the molecules are bound to the receptors, the receptors and their attached molecules are taken into the cell in a vesicle. This process allows cells to take up large amounts of molecules that are present in low concentrations in the extracellular fluid.
Exocytosis is the process by which cells are transported from the inside of the cell to the outside. This occurs through vesicles which fuse to the cell membrane and allow the release of the particles.
Further reading and references:
. https://socratic.org/questions/active-transport-vs-passive-transport-how-to-determine (Image facilitated diffusion)
. http://leavingbio.net/osmosis-diffusion/ (Image osmosis)
. https://en.wikipedia.org/wiki/Endocytosis#/media/File:Endocytosis_types.svg (Image endocytosis)
. Campbell, Neil A., and Jane B. Reece. Biology (8th Edition). San Francisco: Benjamin Cummings, 2007.