How does surface area affect diffusion?

Q: How does surface area volume affect diffusion?

When a cells surface area increases, the amount of substances diffusing into the cell increases. As volume and surface area increase, the volume increases faster, so much so that the surface area available to allow substances in halves each time the cell volume doubles.

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Table of Contents Show

Limiting Effects of Diffusion on Size and Development
Does surface area affect the rate of diffusion?
How does surface area volume affect diffusion?
Why does smaller surface area affect diffusion?
Is smaller surface area better for diffusion?

Learning Objectives

Describe how diffusion limits cell size and development

Limiting Effects of Diffusion on Size and Development

The exchange of nutrients and wastes between a cell and its watery environment occurs through the process of diffusion. All living cells are bathed in liquid, whether they are in a single-celled organism or a multicellular one. Diffusion is effective over a specific distance and limits the size that an individual cell can attain. If a cell is a single-celled microorganism, such as an amoeba, it can satisfy all of its nutrient and waste needs through diffusion. If the cell is too large, then diffusion is ineffective at completing all of these tasks. The center of the cell does not receive adequate nutrients nor is it able to effectively dispel its waste.

An important concept in understanding the efficiency of diffusion as a transportation mechanism is the surface-to-volume ratio. Recall that any three-dimensional object has a surface area and volume; the ratio of these two quantities is the surface-to-volume ratio. Consider a cell shaped like a perfect sphere: it has a surface area of 4πr2, and a volume of (4/3)πr3. The surface-to-volume ratio of a sphere is 3/r; as the cell gets bigger, its surface-to-volume ratio decreases, making diffusion less efficient. The larger the size of the sphere, or animal, the less surface area for diffusion it possesses.

Figure \(\PageIndex{1}\): Surface-to-volume ratio: The image illustrates the comparison of spheres of one to one thousand volume units. The surface-to-volume ratio of a sphere decreases as the sphere gets bigger. The surface area of a sphere is 4πr2 and it has a volume of (4/3)πr3 which makes the surface-to-volume ratio 3/r. This has an effect on diffusion because it relies on the surface area of a cell: as a cell gets bigger, diffusion becomes less efficient.

The solution to producing larger organisms is for them to become multicellular. Specialization occurs in complex organisms, allowing cells to become less efficient at completing all tasks since they are now more efficient at doing fewer tasks. For example, circulatory systems bring nutrients and remove waste, while respiratory systems provide oxygen for the cells and remove carbon dioxide from them. Other organ systems have developed further specialization of cells and tissues and efficiently control body functions. Surface-to-volume ratio also applies to other areas of animal development, such as the relationship between muscle mass and cross-sectional surface area in supporting skeletons or in the relationship between muscle mass and the generation of dissipation of heat.

Key Points

Diffusion is effective over a specific distance, so it’s more efficient in small, single-celled microorganisms.
Diffusion becomes less efficient as the surface-to-volume ratio decreases, so diffusion is less effective in larger animals.
To overcome the limitations of diffusion, multicellular organisms have developed specialized tissues and systems that are responsible for completing a limited number of nutrient and waste tasks.

Key Terms

surface-to-volume ratio: the amount of surface area per unit volume of an object or collection of objects; decreases as volume increases

This page titled 33.4: Animal Form and Function - Limiting Effects of Diffusion on Size and Development is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.

==Diffusion and Cell Size== The larger a cell is, the greater the surface area available for diffusion. So why are cells so tiny? Although increasing the size of a cell would increase its surface area, it would also increase the cells volume and thus its demand for nutrients etc. In fact, increasing the size of the cell has a much greater effect on the cells volume than it does on its surface area. If a cell is too large, nutrients simply aren’t able to diffuse through the entire volume of the cell quickly enough. [image:http://i.imgur.com/GEc2npD.png?1] Materials must be able to reach all parts of a cell quickly, and when volume is too large relative to surface area, diffusion cannot occur at sufficiently high rates to ensure this. Smaller cells have a much greater surface area to volume ratio allowing material to diffuse throughout the entire volume of the cell quickly and efficiently. ==Surface Area : Volume== Surface area to volume ratio can also be used to explain the shape of many cells / cellular surfaces. For example the folds inside the mitochondria or the flat pan-cake like structures inside chloroplasts provide a greater surface area on which specific reactions can occur. The folds in the lining of our stomachs or the tiny cellular, finger-like projections that protrude from the wall of the intestine (villi) all act to increase the surface area without increasing the overall size or volume of the organ.

Does surface area affect the rate of diffusion?

The greater the surface area, the faster the rate of diffusion.

How does surface area volume affect diffusion?

When a cell's surface area increases, the amount of substances diffusing into the cell increases. As volume and surface area increase, the volume increases faster, so much so that the surface area available to allow substances in halves each time the cell volume doubles.

Why does smaller surface area affect diffusion?

Larger volume ratio to the smaller surface area, diffusion takes much time to occur and becomes less effective. This is because most of the surface is highly raised away from the diffusion media and needs much time to cover the entire volume, thus lower diffusion rates.