Diffusion In Cell: 11 Simple Facts (What, How, When, Where, Impact)


Diffusion in cell is the passive transport of solute down its concentration gradient. In biology, the concentration gradient exists across a semi permeable plasma membrane, such that the concentration of solutes is higher one side of the membrane than that of the other side.

The driving force for movement of solutes during diffusion is the difference in its concentration across a membrane which form a gradient of concentration. It directs the movement of the solute down the gradient until net flux of solutes across the membrane becomes equal.

What is the diffusion in a cell?

Diffusion in cell is the process in which solutes from a region of high concentration move towards the region of low concentration until equilibrium is achieved, without spending energy or ATPs on it. The solutes move through the plasma membrane by either simple diffusion or by facilitated diffusion, down the concentration gradient.

What does diffusion do to cell?

Diffusion allows different types of molecules to enter and exist the cells through their plasma membrane according to the needs of the cells or the metabolic and the regulatory reactions taking place inside the cell matrix.

  • Gases: Gases such as oxygen, carbon dioxide and carbon monoxide, can cross through the plasma membrane by simple diffusion.
  • Polar and charged molecules: Polar and charged molecules such ions, carbohydrates, amino acids, nucleosides can cross the plasma membrane only through the facilitative proteins present in the plasma membrane.
  • Non polar and uncharged molecules: Small, non polar and uncharged molecules along with some hydrophobic organic molecules can cross through the plasma membrane by simple diffusion.

Does diffusion affect cells?

Diffusion affects a cell in different ways. It can affect the concentration of solutes in the cell matrix, change cell volume and size if it undergoes osmosis and bring about conformational changes in the membrane proteins of the cell.

How does diffusion affect cells?

Diffusion can affect the cell size if the cell undergoes a special type of diffusion called osmosis. In osmosis, water which is a polar solvent crosses through the plasma membrane instead of the solutes dissolved in the water. As a result, absorption and release of these water molecules in response to the different in solute concentration across the membrane leads to an increase and decrease in the cell volume and size respectively.

How does diffusion affect cell size?

Osmosis which is a special type of diffusion in cell, affects the volume of a cell depending on the type of solution a cell is placed in or is surrounded with due to the exchange of solute and solvents that take place until equilibrium is reached. Then again, it’s affect is slightly different in animal and plant cells. These effects are reversible in nature and the cells can get back to their original dimensions within a few minutes by establishing an equilibrium between the solute concentration both inside and outside of the cell.

When two solutions of different solute concentration are separated by a semipermeable membrane, the water molecules in the hypertonic or hyperosmotic solution move through the semipermeable membrane towards the hypotonic or hypoosmotic solution. Hypertonic solutions have a comparatively higher concentration of solute in comparison to that of the cell, while the hypotonic solutions have a comparatively lower solute concentration than that of the cell. Isotonic or isoosmotic solutions, on the other hand have the same solute concentration as that of the cell.

Image credit: Wikimedia Commons. Tonicity in an animal cell.

When placed in a solution that is hypotonic in comparison to that of the intracellular solute concentration, a cell tends to gain water by endosmosis. Animal cells can imbibe water and increase in volume until it bursts whereas, in the plant cell, the cell wall helps to maintain a constant volume. Cells usually go back to a state of equilibrium by reducing their internal pressure which is achieved by losing the solutes. A plasmolyzed cell can be brought back to its original volume too by putting it into a hypotonic solution.

Image credit: Wikimedia Commons. Tonicity in a plant cell.

When placed in a hypertonic solution, a cell tends to lose all the water to the surrounding solution by the process of exosmosis because the intracellular solute concentration is hypotonic in comparison. Animal cells shrink in size due to loss of water and cell volume decreases whereas, the plant cells remain intact due to the presence of cell wall but its cell membrane detaches from the cell wall and matrix reduces in volume. A plant cell in which the cell matrix is greatly reduced, is called a plasmolyzed cell.

Where does diffusion occur in cell?

Diffusion in cell occurs at the plasma membrane of a cell, where the solutes or the solvent (as in case of osmosis), can diffuse in and out of the cell via different methods. Solutes can undergo simple diffusion or facilitated diffusion. In simple diffusion, small uncharged molecules, water and gasses can cross the lipid bilayer without the help of any carrier proteins. So, simple diffusion can occur almost anywhere in the plasma membrane of the cells.

The facilitated diffusion is carried out with the help of carrier proteins or protein channels which are specialized integral membrane protein. These membrane proteins are highly selective and respond to only selected molecules which are allowed to pass through them, while they are unresponsive to others and do not allow them to cross through. Thus, facilitated diffusion of a given solute can only occurs at specific sites on the plasma membrane of the cell where the integral membrane proteins that are specific to the given solute are present.

When does diffusion occur in cell?

Diffusion in cell occurs across the plasma membrane to either uptake the solutes that are present outside and are needed for various functions inside the cell or for releasing the surplus solutes out of the cell as they are not needed inside the cell anymore or are needed outside the cell.

As, diffusion in cell occurs down the concentration gradient, the solutes can only cross the lipid bilayer if its concentration on one side of the membrane is higher than that of the other side. When such differences in the concentration gradient is formed or present, the solutes cross the plasma membrane from the side of higher solute concentration to that of the lower solute concentration. The exchange of solute occurs until the concentration on both the sides become equal.

How does diffusion occur in cell?

Diffusion in cell, of the different solutes depends on their size, charge, polarity, permeability, hydrophobicity and hydrophilicity. Hence, each types of solute follows a different method and are transported by the help of different facilitative transporters in case of facilitated transportation. Details of diffusion in cell of some of the solutes such as water, lipids, ions and charged molecules are mentioned here.

Water

  • Statistical pores: Water, having a relatively smaller size, is capable of readily crossing the lipid bilayer through statistical pores which are non- static structures. These pores with a diameter of around 4.2Å are formed when the adjacent phospholipid molecules in the cell membrane move laterally but in the opposite directions.
  • Kinks: Kinks are mobile structural defects in the acyl chains of the phospholipids which are caused by acyl chain melting. These kinks can allow water to pass through them.
  • Aquaporins: Aquaporins are a member of major intrinsic protein family that allow facilitated diffusion of water, in and out of the cell. In plants, aquaporins are present in both cell membrane and the membrane of vacuole which are called plasma membrane intrinsic protein and tonoplast intrinsic protein respectively. These are more common in cells that are heavily invested in water transport such as the cells of the kidney tubules in animals and the cell of roots in plants.
diffusion in cell
Image credit: Wikimedia Commons. Aquaporins.

Lipids

Lipid molecules can easily diffuse in and out of the cell because of their hydrophobic nature. Permeability of the lipids is directly proportional to its solubility. Faster penetration means the solute has higher lipid solubility.

  • Lipids take a comparatively longer time to diffuse out through the lipid bilayer because the actin- based cytoskeleton present on the cytoplasmic surface of the cell membrane, causes the it to form compartments.
  • Steric hindrance and high circumferential slowing around the transmembrane proteins which are anchored to the actin cytoskeleton network, leads to temporary confinement of the of the phospholipids present in the cell membrane.
  • Circumferential slowing is caused by the high amount of hydrodynamic friction that occurs when large quantity of phospholipids gets packed up near the transmembrane proteins.
  • These compartments mediate localization of the intracellular response at the regions where reception of extra cellular signal has occurred.

Ions and Charged Molecules

The lipid bilayer is highly impermeable to charged particles or even small ions such as H+, HCO3-, Na+, K+, Ca2+ and Cl-. Since these ions are critical for cellular activities of several multitudes including, reception and conduction of nerve impulse in animals, opening the pores of stomata in plants, muscle contraction in animals, regulate transport of other large proteins in and out of the cell membrane in both plants and animals.

Diffusion of ions in and out of the cells are facilitated by some integral membrane protein channels that are selective and allow only certain types of ions to pass through. Ion channels can allow the influx or outflux of ions in millions in per second, without any expense of energy.

  • Voltage gated ion channels: Membrane channel proteins that operates under the influence of charge difference across the lipid bilayer, are called voltage gated ion channels. Difference in the charge between the intra cellular and extra cellular matrix mediates the opening and closing of these ion channels. Neuron and muscle cells have such ion channels which help in impulse conduction and muscle movement respectively.
  • Ligand gated ion channels: These ion channels undergo conformational changes when the ligands bind to them on the outer or the inner/ cytosolic surface of the plasma membrane, thus, allowing the passage of solutes or ions in this case, across the membrane. Point to be noted here is that a ligand is neither the solute nor does it get transported across the membrane, instead, it is a part of the signaling system. When acetylcholine, a neurotransmitter, bind to the outer surface of the channels to facilitate nerve impulse conductance at synapses. Whereas, cAMP binds to the inner surface of some calcium ion channels.
  • Mechanically gated ion channels: Such channels change their conformation in response to mechanical forces. Hair cells in the inner ear open up ion channels by bending in response to sound waves. The transported ions then take part in the formation of the nerve impulse.
Image credit: Wikimedia Commons. Voltage gated channels.
Image credit: Wikimedia Commons. Ligand gated channels.
Image credit: Wikimedia Commons. Mechanical gated channels.

Type of diffusion in cell

Diffusion or passive diffusion of solutes across a semi permeable membrane can to broadly classified on two types depending on the need for the protein channels for transportation.

Simple passive diffusion

Solute diffuses through the lipid bilayer by simply diffusing in and out of the cell. Molecules that can cross the cell membrane by simple passive diffusion are gasses, water and small non- charged solutes. The process of simple passive diffusion does require energy to cross a bilayer membrane but not in the form of ATP. The energy for crossing the membrane comes from the water of hydration, that is the number of hydroxyl (-OH) groups attached to a solute molecule. The process occurs in three steps-

  1. The solute loses the waters of hydration.
  2. The dehydrated solute crosses through phospholipid bilayer.
  3. It regains the water of hydration on the other side of the cell membrane.
Image credit: Wikimedia Commons. Simple diffusion across plasma membrane.

The activation energy for diffusion through the lipid bilayer is directly proportional to the water of hydration present on the solute. The only exception to this relationship being water as it has an extremely high permeability.

Facilitated passive diffusion

Solute diffuses in and out of the lipid bilayers at particular locations with the help of solute specific integral membrane proteins which are also called facilitative transporters. There are two types of proteins involved the facilitated diffusion, carrier proteins and channel proteins.

When the solute binds to the carrier proteins on one side, it brings about a conformational change in them. This changed conformation of the protein facilitates movement of the solute particles through it and across the lipid bilayer in both the direction, from a region of high solute concentration to the lower solute concentration region, until equilibrium is achieved. One the other hand, channel proteins allow free diffusion of any molecule that has an appropriate size and can fit into the pore that runs through the channel protein.

Image credit: Wikimedia Commons. Facilitated passive diffusion.

Diffusion in cell metabolism

Diffusion in cell has a vital role to play in cell metabolism as it has the ability to regulate the metabolic reactions that occurs in a cell, but diffusion can get affected by several factors as well.

  • Concentration gradient: The extent of solute transport is dependent on the difference between the concentration of the solute on both the sides of the membrane. Larger the difference, higher and faster will be the rate of diffusion of the solutes across the membrane.
  • Size and mass of solute: Rate of diffusion is indirectly proportional to the size of the molecules. Solutes having larger size or higher mass, take longer time to diffuse through the lipid bilayer.
  • Cell size: Size of the cell or diameter of the cell is also inversely proportional to the rate of diffusion. A cell that is smaller in size will have faster rate of diffusion as the distance to be traversed is less whereas, in a cell with a larger diameter, the distance travelled by the solute will be greater and so the rate of diffusion will become slow.
  • Interaction with other molecules: If the solutes of smaller size interact with larger proteins, its rate of diffusion gets highly reduced in comparison to when it was not involved in any such interaction. This happens because the smaller solutes are now not as free to diffuse because their interaction with the larger molecules hinders their free movement.
  • Temperature: Temperature is directedly proportional to the movement of the solute in a solution and is thus also directly proportional to the rate of diffusion. When a solution is at a high temperature, its solutes gain a lot of energy and their random movement becomes very rapid. As a result, such molecules diffuse faster too.

Role of diffusion in cell metabolism

  • Glucose undergoes facilitated diffusion when its concentration in the blood is more than that of the cells of skeletal muscles and adipose tissues, by the help of GLUT protein transporters. Of the many proteins in the GLUT family, GLUT4 is activated by the insulin. When insulin binds to the GLUT4 transporter protein, it allows facilitated diffusion at a very high rate in insulin sensitive tissues. Thus, insulin maintains the blood glucose level at normal.
  • As, glucose plays a vital role in cellular metabolism, it is required in huge amount inside the cell matrix but the concentration gradient can prohibit the influx of glucose in the cell when equilibrium is reached. In order to keep the concentration gradient from reaching the equilibrium, the glucose molecules undergo phosphorylation when they enter through the cell membrane. Phosphorylation lowers the intracellular concentration of glucose and allows steady influx of glucose into the cell.
  • Neurons maintain a state of polarization or a resting membrane potential by having an excess of sodium ions outside the cell membrane and having an excess of potassium ions on the inside of it. When the nerve receives a stimulus, the sodium-potassium ion gated channels open up to allow one sodium ion to enter the cell and two potassium ions to exit the cell. This leads to depolarization of the membrane and the action potential gets shifted along the membrane leading to transmission of nerve impulse.
  • The hemoglobin present in the blood usually binds with oxygen at some particular sites and undergoes facilitated diffusion. Apart from that, oxygen also get exchanged by simple diffusion. Carbon dioxide and carbon monoxide are also exchanged between alveoli the blood capillaries by diffusion in a similar way.
Image credit: Wikimedia Commons.

Role of diffusion in Cell

  • GLUT4 proteins: Binding of insulin hormone to the GLUT4 transporter proteins leads to influx of surplus glucose molecules inside the cells by facilitated diffusion.
  • Secondary messenger signaling: Secondary messengers are usually small molecules or ions that are involved in relaying the signals from the receptors present on the plasma membrane to several other intra cellular proteins and start a signaling cascade. The influx of the secondary messengers inside the cell matrix occurs via facilitated diffusion.
  • Osmosis as a special type of diffusion: osmosis is important in the cells both animals and plants. These cells are involved in water absorption such as, absorption of water from soil by the root hairs, reabsorption of water by the proximal and distal convoluted tubules present in the kidneys of animals, reabsorption of tissue fluid into the blood capillaries and absorption of water in the digestive tract of the animals.
  • Blood filtration: Kidneys in animals are involved in removal of waste products by the process of both simple and facilitated diffusion.
  • Respiration: Diffusion in cell also allows the exchange of gases in between the alveoli of the lungs and the capillaries of the blood to allow respiration to occur. Exchange of gases in aquatic animals follow a similar mechanism too. As, the gases exchanged are uncharged, they undergo simple diffusion.
  • Food absorption: The food that gets digested in the digestive tracts, contain some amount of lipid soluble nutrients such as vitamin A, D, R and K, which can easily pass through the cells present in the villi, into the blood vessel by the process of simple diffusion.

Diffusion in Cellular Transport

Diffusion in cell is primarily responsible for cellular transport of oxygen, carbon dioxide, glucose, amino acids and fats.

  • Blood filtration: In animals, the cells of kidneys are responsible for removal of waste products like urea, creatinine and extra fluid from blood by the process of diffusion. The same principle of diffusion is applied during the dialysis of blood of the patients who haven kidney function disorders.
  • Respiration: The hemoglobin present in the blood usually binds with oxygen at some particular sites and undergoes facilitated diffusion. Apart from that, oxygen also get exchanged by simple diffusion. Carbon dioxide and carbon monoxide are also exchanged between alveoli the blood capillaries by diffusion in a similar way.
  • Diffusion in plants: Diffusion is responsible for the survival of plants as they allow the cells of the roots to absorb water from the surround soil. Plants require a huge amount of water to maintain their turgidity at both cellular and organ level. In case of loss of excess water, which again also occurs by diffusion, plants wilt and can dry up if the loss of water is not made up for.

Diffusion in Cell Membrane

Diffusion in a cell occurs at the cell surface interface that interacts with both the extracellular matrix and the cellular matrix. Under the direction of the concentration gradient, solutes from a region of high concentration move towards the region of low concentration. Solutes can cross through the plasma membrane by either simple diffusion or by facilitated diffusion in which carrier proteins and protein channel are involved. The type of diffusion a solute undergoes depends on its size, mass, charge, polarity, hydrophilicity or hydrophobicity and the temperature.

Facilitated Diffusion in Cell

Facilitated diffusion is a type of passive diffusion in which the movement of solute, specifically the polar solutes, occurs with the help of highly specific membrane proteins which are also called facilitative transporters. Binding of solute to the surface of these integral proteins channels leads to a change in their conformation, allowing the solute to pass through them and across the lipid bilayer, down the concentration gradient. The difference in concentration of the solutes on the two sides of the plasma membrane decides the direction of the net flux.

Facilitated transporters can get saturated if the influx or outflux of solutes goes above the maximum capacity. They can allow a few hundreds to thousands of solutes to pass through, every second.

Conclusion

Diffusion plays a vital role in cell metabolism and cell signaling by either simple or facilitated diffusions or by osmosis. The understanding of mechanism of diffusion at every interface can give us a great insight into how a cell completes its activities.

Tulika Priyadarshini

Hi ...I am Tulika Priyadarshini, a Biotechnology fresher working in the field of research. Writing gives me mental peace and satisfaction. Sharing the knowledge that I gain in the process is a cherry on the cake. My articles are related to Lifesciences, Biology and Biotechnology. Lets connect through LinkedIn- https://www.linkedin.com/in/tulika-priyadarshini/

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