biology essay on diffusion

Diffusion ( AQA A Level Biology )

Revision note.

Biology Lead

Diffusion & Facilitated Diffusion

• Diffusion is a type of transportation that occurs across the cell membrane
• It can be defined as:
• The molecules or ions move down a concentration gradient
• The random movement is caused by the natural kinetic energy of the molecules or ions

Diffusion across the cell membrane

• As a result of diffusion, molecules or ions tend to reach an equilibrium situation (given sufficient time), where they are evenly spread within a given volume of space
• The rate at which a substance diffuses across a membrane depends on several factors

Diffusion Factors Table

Facilitated diffusion

• Large polar molecules such as glucose and amino acids
• Ions such as sodium ions (Na + ) and chloride ions (Cl - )
• These substances can only cross the phospholipid bilayer with the help of certain proteins
• This form of diffusion is known as facilitated diffusion

Channel proteins

Carrier proteins.

• They are highly specific (they only allow one type of molecule or ion to pass through)
• Channel proteins are water-filled pores
• They allow charged substances (eg. ions) to diffuse through the cell membrane
• The diffusion of these ions does not occur freely, most channel proteins are ‘gated’, meaning that part of the channel protein on the inside surface of the membrane can move in order to close or open the pore
• This allows the channel protein to control the exchange of ions

A channel protein (open and closed)

• Unlike channel proteins which have a fixed shape, carrier proteins can switch between two shapes
• This causes the binding site of the carrier protein to be open to one side of the membrane first, and then open to the other side of the membrane when the carrier protein switches shape
• The direction of movement of molecules diffusing across the membrane depends on their relative concentration on each side of the membrane
• Net diffusion of molecules or ions into or out of a cell will occur down a concentration gradient (from an area containing many of that specific molecule to an area containing less of that molecule)

A carrier protein changing shape during facilitated diffusion

Remember – the movement of molecules from high concentration to low concentration is diffusion. If this movement requires the aid of a protein (for example because the molecule is charged and cannot pass directly through the phospholipid bilayer) this is facilitated diffusion.

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Diffusion Definition

Diffusion is a physical process that refers to the net movement of molecules from a region of high concentration to one of lower concentration. The material that diffuses could be a solid, liquid or gas. Similarly, the medium in which diffusion occurs could also be in one of the three physical states.

One of the main characteristics of diffusion is the movement of molecules along the concentration gradient. While this could be facilitated by other molecules, it does not directly involve high-energy molecules such as adenosine triphosphate (ATP) or guanosine triphosphate (GTP).

The rate of diffusion depends on the nature of interaction between the medium and material. For instance, a gas diffuses very quickly in another gas. An example of this is the way the noxious smell of ammonia gas spreads in air. Similarly, if a canister of liquid nitrogen leaks a little, nitrogen gas that escapes would quickly diffuse into the atmosphere. The same gas would diffuse slightly more slowly in a liquid such as water and slowest in a solid.

Similarly, two miscible liquids will also diffuse into each other to form a uniform solution. For instance, when water is mixed with glycerol, over time the two liquids diffuse radially into each other. This can even be observed visually by the addition of different colored dyes to each of the liquids. However, the same phenomenon is not seen when immiscible liquids like petrol and water are mixed together. Diffusion happens slowly and only across the small surface of interaction between the two fluids.

Examples of Diffusion

Diffusion is an important part of many biological and chemical processes. In biological systems, diffusion occurs at every moment, across membranes in every cell as well as through the body.

For example, oxygen is at a higher concentration inside arteries and arterioles, when compared with the oxygen levels in actively respiring cells. By the time blood flows into capillaries in the muscle or liver, for instance, there is only a single layer of cells separating this oxygen from hepatocytes or skeletal muscle fibers. Through a process of passive diffusion, without the active involvement of any other molecule oxygen passes through the capillary membrane and enters cells.

Cells utilize oxygen in the mitochondria for aerobic respiration, which generates carbon dioxide gas as a byproduct. Once again, as the concentration of this gas increases within the cell, it diffuses outwards towards capillaries where the force of flowing blood removes the excess gas from the tissue region. This way, the capillaries remain at a low carbon dioxide concentration, allowing the constant movement of the molecule away from cells.

This example also shows that the diffusion of any one material is independent of the diffusion of any other substances. When oxygen is moving towards tissues from capillaries, carbon dioxide is entering the bloodstream.

In chemical processes, diffusion is often the central principle driving many reactions. As a simple example, a few crystals of sugar in a glass of water will slowly dissolve over time. This occurs because there is a net movement of sugar molecules into the water medium. Even in large industrial reactions, when two liquids are mixed together, diffusion brings the reactants together and allows the reaction to proceed smoothly. For instance, one of the ways in which polyester is synthesized is by mixing the appropriate organic acid and alcohol in their liquid form. The reaction proceeds as the two reactants diffuse towards each other and undergo a chemical reaction to form esters.

Factors that Affect Diffusion

Diffusion is affected by temperature, area of interaction, steepness of the concentration gradient and particle size. Each of these factors, independently and collectively can alter the rate and extent of diffusion.

Temperature

In any system, molecules are moving with a certain amount of kinetic energy. This is usually not directed in any particular manner, and can appear random. When these molecules collide with one another, there is a change in the direction of movement as well as changes to momentum and velocity. For example, if a block of dry ice (carbon dioxide in solid form) is placed inside a box, carbon dioxide molecules in the center of the block mostly collide with each other and get retained within the solid mass. However, for molecules in the periphery, rapidly moving molecules in the air also influence their movement, allowing them to diffuse into the air. This creates a concentration gradient, with concentration of carbon dioxide gradually decreasing with distance from the lump of dry ice.

With increase in temperature, the kinetic energy of all particles in the system increases. This increases the rate at which solute and solvent molecules move, and increases collisions. This means that the dry ice (or even regular ice) will evaporate faster on a warmer day, simply because each molecule is moving with greater energy and is more likely to quickly escape the confines of a solid state.

Area of interaction

To extend the example given above, if the block of dry ice is broken into multiple pieces, the area that interacts with the atmosphere immediately increases. The number of molecules that only collide with other carbon dioxide particles within dry ice decreases. Therefore, the rate of diffusion of the gas into air also increases.

This property can be observed even better if the gas has an odor or color. For instance, when iodine is sublimated over a hot stove, purple fumes begin to appear and mix with air. If sublimation is carried out in a narrow crucible, the fumes diffuse slowly out towards the mouth of the container and then rapidly disappear. While they are confined to the smaller surface area within the crucible, the rate of diffusion remains low.

This is also seen when two liquid reactants are mixed with one another. Stirring increases the area of interaction between the two chemicals and allows these molecules to diffuse towards each other more quickly. The reaction proceeds towards completion at a faster rate. On a similar note, any solute that is broken into small pieces and stirred into the solvent dissolves rapidly – another indicator of molecules diffusing better when the area of interaction increases.

Steepness of the Concentration Gradient

Since diffusion is powered primarily by the probability of molecules moving away from a region of higher saturation, it immediately follows that when the medium (or solvent) has a very low concentration of the solute, the probability of a molecule diffusing away from the central area is higher. For instance, in the example about the diffusion of iodine gas, if the crucible is placed in another closed container and iodine crystals are heated for an extended period of time, the rate at which the purple gas seems to ‘disappear’ at the mouth of the crucible will reduce. This apparent slowing down is due to the fact that, over time, the larger container begins to have enough iodine gas that some of it will be moving ‘backwards’ towards the crucible. Even though this is random non-directed movement, with a large bulk, it can create a scenario where there is no net movement of gas from the container.

Particle Size

At any given temperature, the diffusion of a smaller particle will be more rapid than that of a larger-sized molecule. This is related to both the mass of the molecule and its surface area. A heavier molecule with a larger surface area will diffuse slowly, while smaller, lighter particles will diffuse more quickly. For example, while oxygen gas will diffuse slightly more quickly than carbon dioxide, both of them will move more quickly than iodine gas.

Functions of Diffusion

Diffusion in the human body is necessary for the absorption of digested nutrients, gas exchange, the propagation of nerve impulses, the movement of hormones and other metabolites towards their target organ and for nearly every event in embryonic development.

Types of Diffusion

Diffusion can either be simple diffusion and be facilitated by another molecule

Simple Diffusion

Simple diffusion is merely the movement of molecules along their concentration gradient without the direct involvement of any other molecules. It can involve either the spreading of a material through a medium or the transport of a particle across a membrane. All the examples given above were instances of simple diffusion.

Simple diffusion is relevant in chemical reactions, in many physical phenomena, and can even influence global weather patterns and geological events. In most biological systems, diffusion occurs across a semi-permeable membrane made of a lipid bilayer. The membrane has pores and openings to allow the passage of specific molecules.

Facilitated Diffusion

On the other hand, facilitated diffusion, as the term indicates, requires the presence of another molecule (the facilitator) in order for diffusion to occur. Facilitated diffusion is necessary for the movement of large or polar molecules across the hydrophobic lipid bilayer. Facilitated diffusion is necessary for the biochemical processes of every cell since there is communication between various subcellular organelles. As an example, while gases and small molecules like methane or water can diffuse freely across a plasma membrane, larger charged molecules like carbohydrates or nucleic acids need the help of transmembrane proteins forming pores or channels.

Since they are relatively large openings in the plasma membrane, these integral membrane proteins also have high specificity. For instance, the channel protein that transports potassium ions has a much higher affinity for that ion than a very similar sodium ion, with nearly the same size and charge.

Related Biology Terms

• Concentration Gradient – Gradual decrease in the concentration of a substance, often a solute in a solution. Within living systems, this gradient is usually seen on two sides of a semi-permeable lipid membrane.
• Hepatocytes – Cells in the inner parenchymal region of the liver, making up a large proportion of liver mass. Involved in the digestion and metabolism of proteins, lipids and carbohydrates. They also play a crucial role in the detoxification of the body.
• Integral Membrane Protein – Proteins that span the width of a membrane and are important structural and functional parts of biological membranes.
• Sublimation – The conversion of a material in its solid phase directly into the gaseous state, without an intervening transition into the liquid state.
• Mehrer, H. and Stolwijk, N. A. (2005). “Heroes and Highlights in the History of Diffusion”. Diffusion Fundamentals 2, 1.1–1.10.
• Philibert, J. (2009). “One and a Half Century of Diffusion: Fick, Einstein, before and beyond”. Diffusion Fundamentals 11 (1):1-32.
• Spaeth, E. E., and Friedlander, S.K. (1967). ” The Diffusion of Oxygen, Carbon Dioxide, and Inert Gas in Flowing Blood”. Biophys J. 7(6): 827–851.

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• Biology Article

Table of Contents

What is Diffusion?

Types of diffusion, factors affecting diffusion, examples of diffusion, causes of diffusion, significance of diffusion, diffusion definition.

“Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration down the concentration gradient.”

Read on to explore what is diffusion and the different types of diffusion.

Diffusion is the process of movement of molecules under a concentration gradient. It is an important process occurring in all living beings. Diffusion helps in the movement of substances in and out of the cells. The molecules move from a region of higher concentration to a region of lower concentration until the concentration becomes equal throughout.

Liquid and gases undergo diffusion as the molecules are able to move randomly.

Take water in a beaker. Add a few copper sulfate crystals in one place and leave it as it is for some time without disturbing it. After some time we can see that the beaker contains a uniformly coloured solution. Here, both water and copper sulfate diffuse independently. With this experiment, we can infer that solutes move from a higher concentration to a lower concentration in a solution.

Also Read:  Diffusion in Plants

Recommended Video:

Diffusion is widely used in various fields such as biology, physics, chemistry, etc. Diffusion can be classified into two main types: Simple diffusion and facilitated diffusion.

Simple diffusion

A process in which the substance moves through a semipermeable membrane or in a solution without any help from transport proteins.  For example, bacteria deliver small nutrients, water and oxygen into the cytoplasm through simple diffusion.

Facilitated diffusion

Facilitated diffusion is a passive movement of molecules across the cell membrane from the region of higher concentration to the region of lower concentration by means of a carrier molecule.

Dialysis:  It is the diffusion of solutes across a selectively permeable membrane. A selectively permeable membrane is one that allows only specific ions and molecules to pass through, while it obstructs the movement of others.

Osmosis:  It is the movement of solvent molecules from the region of lower concentration to the region of higher concentration through a semipermeable membrane.    Since water is solvent in every living being, biologists define osmosis as the diffusion of water across a selectively permeable membrane. For example, plants take water and minerals from roots with the help of osmosis.

Also Read:  Facilitated Diffusion

There are a few factors that affect the process of diffusion, which individually and collectively alters the rate and extent of diffusion. These factors include:

• Temperature.
• Area of Interaction.
• Size of the Particle.
• The steepness of the concentration gradient.
• A tea bag immersed in a cup of hot water will diffuse into the water and change its colour.
• A spray of perfume or room freshener will get diffused into the air by which we can sense the odour.
• Sugar gets dissolved evenly and sweetens the water without having to stir it.
• As we light the incense stick, its smoke gets diffused into the air and spreads throughout the room.
• By adding boiling water to the dried noodles, the water diffuses causing rehydration and making dried noodles plumper and saturated.

Diffusion is a natural and physical process, which happens on its own, without stirring or shaking the solutions. Liquid and gases undergo diffusion as the molecules are able to move randomly. The molecules collide with each other and change their direction.

Diffusion is an important process, which is involved in the different life processes . As mentioned above, it is the net movement of particles, ions, molecules, solution, etc. In all living species, diffusion plays an important role in the movement of the molecules during the metabolic process in the cells.

Diffusion is important for the following reasons:

• During the process of respiration, this process helps in diffusing the carbon dioxide gas out through the cell membrane into the blood.
• Diffusion also occurs in plant cells. In all green plants, water present in the soil diffuses into plants through their root hair cells.
• The movement of ions across the neurons that generates electrical charge is due to diffusion.

Also Read:  Difference between diffusion and osmosis

Frequently Asked Questions

1. what is diffusion.

Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration down the concentration gradient.

2. List the types of diffusion.

Diffusion can be divided into two main types, namely, simple diffusion and facilitated diffusion.

3. What is simple diffusion?

Simple diffusion is defined as the process in which a substance moves through a semipermeable membrane or in a solution without any help from transport proteins.

4. State an example of simple diffusion.

In a cell, water, oxygen and carbon dioxide molecules can pass directly through the cell membrane without requiring any energy along the concentration gradient. This is a form of simple diffusion.

5. What is facilitated diffusion?

Facilitated diffusion can be defined as the passive movement of molecules across the cell membrane from a region of higher concentration to a region of lower concentration by means of a carrier molecule.

6. Provide an example of facilitated diffusion.

In the human body, glucose molecules, sodium and potassium ions use carrier proteins to pass through the cell membranes.

7. How does dialysis work?

Dialysis works through the diffusion of solutes across a selectively permeable membrane. A selectively permeable membrane is the one that allows only specific ions and molecules to pass through while obstructing the movement of other molecules.

8. What are the factors affecting diffusion?

Temperature, area of interaction, size of the particle and the steepness of the concentration gradient are all factors that affect the process of diffusion.

9. State the significance of diffusion.

Diffusion is a very important process occurring in all living beings. All living organisms exhibit one or the other form of diffusion, allowing the movement of the molecules during various metabolic or cellular processes.

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What is the purpose of diffusion?

Diffusion is the way of passive transport of substances in and out of the cell across the cell membrane. Diffusion occurs down the concentration gradient, i.e. from higher concentration to lower concentration.

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1.6: Diffusion and Osmosis

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• Susan Burran and David DesRochers
• Dalton State College via GALILEO Open Learning Materials

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(Adapted from biologycorner.com)

Introduction:

Diffusion is the process by which molecules spread from areas of high concentration to areas of low concentration. This movement, down the concentration gradient , continues until molecules are evenly distributed. Osmosis is a special type of diffusion: the diffusion of water through a semipermeable membrane . The concentration of water is inversely related to the concentration of solute: more solute corresponds to less water and less solute corresponds to more water. This is important because osmotic vocabulary describes the solute and not the water. Hypertonic solutions contain a high concentration of solute and little water, relative to hypotonic solutions that have a low concentration of solute and therefore a higher concentration of water. The term “ isotonic ” is used when two areas have an equal concentration of solute: no net osmosis is occurring.

Exercise 1: Diffusion Through a Gel

One factor that can affect the rate of diffusion is the size of the molecule. Larger molecules tend to move more slowly than smaller molecules. In this experiment, students will compare the diffusion rates of two dyes traveling through agar.

• Pre-punched agar plates
• Potassium permanganate
• Janus green
• Using the dropper, drop a single drop of potassium permanganate into one of the wells on the plate.
• Repeat with Janus green.
• Allow the plates to sit undisturbed for 30 minutes.
• Which dye do you think will have a faster diffusion rate? _____________________
• After 30 minutes, measure the radius of the dye front from the middle of the well and record your results.
• Calculate the diffusion rate (mm/hr) by dividing the dye front radius by 0.5.

1. Did your outcome match your expectation? Provide an explanation for your results.

2. What are other factors that can affect the rate of diffusion?

Exercise 2: Observation of Osmosis in a Plant Cell

Plants have cell walls that can prevent lysis if too much water flows into the cell. Plant cytoplasm tends to be hypertonic to the outside environment, which results in an inflow of water and a high amount of pressure ( turgor pressure ) inside the cell. When a plant is placed in a hypertonic environment, the water will leave the cell. This causes the cell to shrink and detaches the plasma membrane from the cell wall ( plasmolysis ). Turgor pressure can hold plants upright, while plasmolysis can cause plants to wilt.

Observe the two Elodea leaves under the microscope. One slide is a leaf in isotonic solution: you should be able to identify the chloroplasts and an empty space in the middle of the cells which is the vacuole. The next leaf has been soaked in a salt water solution; compare the cells to the first slide.

1. What is the difference between a hypertonic solution and a hypotonic solution?

2. What will happen to plant cells that are placed in a hypertonic solution?

3. What will happen to animal cells placed in hypotonic solution? Why should this be different from plant cells?

4. Why are dehydrated patients given saline intravenously instead of water?

Exercise 3: Osmosis Across a Membrane

• Dialysis bags (4 per group)
• Dental floss
• 15% sucrose solution
• 30% sucrose solution
• Triple beam balance
• Beakers (4 per bench)
• Graduated cylinder

1. Obtain 4 strips of dialysis tubing and tie a knot in one end of each using the dental floss.

2. Pour approximately 10ml of each solution into separate bags (see table below).

3. Remove most of the air from the bag (but leave a little bit of space) and tie the bag.

4. Blot the bags to remove any sugar that may have spilled; check the bags for leaks.

5. Record the weight of each baggie in the data table.

6. Place a bag in each beaker (be sure to keep track of which bag is in which beaker!). Fill the beakers with enough of the appropriate solution to cover your bags (refer to the above table).

7. Predict what you think will happen during the experiment.

8. Record weight every 10 minutes in the data table below.

9. After 30 minutes, remove the bags from the solution and record the final weight.

1. Did your results match your predictions? Propose an explanation for why your results (either overall or an individual bag) may have differed from what you were expecting.

2. Based on what you have observed, are the dialysis bags permeable to sucrose?

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The importance of diffusion essay

Subject: Biology

Age range: 16+

Resource type: Other

Last updated

24 May 2021

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Importance of diffusion to living organisms

Diffusion is basically the movement of chemical species (ions or molecules) under the influence of concentration difference. The species will move from the high concentration area to the low concentration area till the concentration is consistent in the whole system. Diffusion mostly occurs in gases and liquids as these can move freely.

The main features of an efficient diffusion system would be that it has a large surface area, thin membrane and a continuous supply of substances. A large surface area is needed so that high amount of substances can be exchanged at a time while the thin membrane means that the diffusion pathway would be short so that it is more efficient. The continuous supply would help in maintaining a concentration gradient which is essential for diffusion to take place.

The 2 main types of diffusion are simple and facilitated. Simple diffusion is when a small, non-polar molecule passes through a lipid bilayer. In this type of diffusion, a hydrophobic molecule moved into the hydrophobic region of the membrane without getting rejected. A key feature is that it does not  need a carrier protein to take place. An example of simple diffusion is osmosis.

Facilitated diffusion on the other hand is dependant on carrier proteins to transport it across the membrane.

Diffusion is essential for many organisms as it is a feature of a number of processes which control and supply vital substances to the body in order for basic survival. A few of these are discussed below.

This is a preview of the whole essay

Gas exchange is one of these processes. It is when much needed oxygen is obtained by the body in order for respiration to take place and the waste CO 2  is taken out of the body. In us mammals, the exchange takes place in the lungs which contain a large number of alveoli. These are sponge-like structures in which the diffusion takes place. They are highly adapted to diffuse the gases as they give a large surface area for exchange of the gases. Also, there is only a thin layer of cells between the alveoli and the blood capillaries meaning there is a short diffusion pathway so diffusion takes place more efficiently. There is a ventilation system which means there is continuous oxygen supply to the alveoli which again makes diffusion more efficient.

Fish are adapted to exchange gases as the gills have many lamellae which are covered with plates. These give a large surface area while also they are very thin to give a short diffusion pathway. There is a counter flow blood system meaning it flows in the opposite direction of the water. This maintains a concentration gradient in order for diffusion to take place.

The main feature of the mechanism in insects is that they have an extensive tracheole system that gives a large surface area. These are permeable to CO 2 and O 2  in order to allow the exchange of gases.

Plants require CO 2  for the purpose of photosynthesis. Therefore they have thin cell walls and membranes to allow gases to diffuse through. There are stomata which control the entry and exit of gases while there are large air spaces for circulation so that diffusion is efficient.

The transport of glucose to cells is by facilitated diffusion and this is needed as all cells need to respire therefore glucose is vital.

Diffusion also plays a part in the human digestive system as after the carbohydrates and other substances are processed through digestion, the substances that are wanted and are small enough are reabsorbed through the gut wall through diffusion. These can then be used for energy and growth purposes.

The process of osmosis (or osmoregulation) is classed as a type of diffusion. This is due to the fact that the substance (which is water in the case of osmosis) moves from an area of high concentration to an area with lower concentration.

One example of when osmosis takes place is when water is taken up by the roots of plants. Firstly, the water enters the root hair from an area of high water potential. As the water builds up in the cell, it has a higher potential than that of the next cell up, meaning that once again the water moves to the next cell by osmosis. This process carries on transporting the water up through the roots. The system of the roots is called the symplast pathway.

An instance of osmosis in mammals is the reabsorption of water from the kidney tubules back into the blood. A function of the kidney is to control the water level in the blood. The water is absorbed in the descending limb of the loop of henle. This is the part which is permeable to water. The water potential in the descending limb is higher than in the surrounding tissues meaning that water diffuses through into the tissues by osmosis. This is vital for mammals as it controls water loss to a degree by keeping it to a minimum (although water is lost by other means such as perspiration).

The occurrences of diffusion discussed above give us a basic insight into the importance of diffusion in organisms. The majority of living organisms require diffusion to take place in order for main systems to function. These key systems such as gaseous exchange and water control would be unable to function without diffusion taking place. . Without diffusion there would be no respiration of the cells which is the central process in all the biological systems, thus we can say safely state that it is probably the most important process for organisms as without it, there would be no chance of survival whatsoever.

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*** This piece of writing discusses diffusion in the right way with a good range of relevant examples. However, it does not gain a higher star rating because of a lack of detail. Examiners want to see evidence that candidates have learned new content at AS and A level and that they can use A level terminology correctly and in the right context. If particular terms are used it is important that a candidate shows that they really understand what those terms mean.

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Article Versions Notes

Zeng, J.; Zhu, Q. NSVDNet: Normalized Spatial-Variant Diffusion Network for Robust Image-Guided Depth Completion. Electronics 2024 , 13 , 2418. https://doi.org/10.3390/electronics13122418

Zeng J, Zhu Q. NSVDNet: Normalized Spatial-Variant Diffusion Network for Robust Image-Guided Depth Completion. Electronics . 2024; 13(12):2418. https://doi.org/10.3390/electronics13122418

Zeng, Jin, and Qingpeng Zhu. 2024. "NSVDNet: Normalized Spatial-Variant Diffusion Network for Robust Image-Guided Depth Completion" Electronics 13, no. 12: 2418. https://doi.org/10.3390/electronics13122418

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