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**Why are most eukaryotic cells between 10 and 100 m in diameter?**

Eukaryotic Cell Size Why are most eukaryotic cells between 10 and 100 m in diameter? With audio

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**How big is that? Remember 1 mm is the smallest mark on a metric ruler.**

Remember it takes 1000 micrometers (m) to equal 1 millimeter (mm). 1 2 1000 m 1 2

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How big is that? Let’s use our metric conversions and figure out how big a typical cell is if measured in millimeters. 10 to 100 m = _______ to ______ mm

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**Eukaryotic cells are microscopic**

Answer: m = mm In other words, one cell is smaller than 1 millimeter. The largest of eukaryotic cells are 1/10 of a mm in diameter (0.1 mm)! 1 2

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Certainly we have small and large eukaryotic organisms (Think of an amoeba, a mouse and a redwood tree). Why don’t we find smaller and larger eukaryotic cells to match?

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**What keeps cells from getting smaller than 10 micrometers?**

This is what we call the LOWER LIMIT on cell size. It is related to the minimum amount of space it would take to hold all the essential cell structures. If you try to make a cell smaller than 10 m, something essential would not fit and therefore the cell would not survive.

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**Analogy There is a “lower limit” on suitcases.**

If you try to take a suitcase smaller than some minimum size on a vacation, you would not be able to fit all the essential things you’d need to “survive” into your bags.

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**But what about mammalian red blood cells?**

RBCs are only 5 m -- smaller than the usual lower limit. How is this possible??

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**RBCs don’t really violate the rule ….**

It is just than in their maturation process, the nucleus of RBCs is discarded. Leaving out such a prominent structure allows these cells to be unusually small. Thus mature RBCs are small enough enough to fit through the tiniest of blood vessels, the capillaries.

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**What keeps cells from getting larger than 100 micrometers?**

This is what we call the UPPER LIMIT on cell size. This is related to the ability of the cell to supply its metabolic needs.

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Metabolic Needs To survive a cell must have sufficient nutrients and gases for its size. A cell’s metabolic needs are defined by its volume*. The larger the cell the greater its metabolic needs will be. * Volume refers to the internal space of a cell. By analogy, a box’s volume refer to how much “stuff” it could hold. Volume Needs

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Supplying those Needs Everything that enters and leaves a cell must come through its cell membrane. This would include nutrients, gases and wastes. To supply its needs a cell must have enough surface area* to get those needed materials in and wastes out quickly. * Surface area refers to the covering of a cell. By analogy, a box’s surface area could be measured by the amount of wrapping paper it would take to cover it completely. Nutrients Gases Wastes

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**Surface Area to Volume Ratio**

To meet its metabolic needs, a cell must have sufficient surface area for its volume. This relationship is described as a “large surface area to volume (SA/V) ratio.” In other words, a cell must have enough cell membrane to be able to transport what it needs in and out at a fast enough rate to survive. The ratio of the two is critical.

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**What happens as a cell grows?**

V As a cell grows, its surface area (SA) increases. As a cell grows, its volume (V) increases.

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So what’s the problem? If both the SA (supply) and V (needs/demand) increase with increasing cell size, why can’t a cell grow as large as it wants? The problem is that while both SA and V increase, they don’t grow at the same rate. The volume (V = needs) increases faster than the surface area (SA= supply). V SA

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**Therefore, the SA/V ratio actually decreases with increasing cell size.**

Remember cells need a large SA/V ratio. And, the larger the cell the smaller that ratio will be.

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**The decreasing SA/V ratio limits cells from growing larger than 100 m.**

Without enough surface area for its size, such a large cell will not be able to supply its own needs.

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**It is all about supply and demand**

As cells grow larger, while both the supply and demand increase, the “needs” quickly outpace the “supply.” At that point, the cell must stop growing or divide into two smaller cells or it will die. Without the ability to supply its own needs, the cell would either have insufficient energy to live or poison itself with its own wastes.

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What about frog eggs? Frog eggs are single cells and are over 1 mm in size -- larger than the usual upper limit. How is this possible??

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**Unfertilized eggs are metabolically suppressed**

Therefore their needs are very low. And the limited surface area available is adequate to supply those needs.

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**However as soon as they are fertilized, eggs become metabolically active**

If nothing changed, the SA/V ratio would be too low for survival. But within minutes after fertilization, the single large frog egg cell begins to divide and divide and divide. Soon the original volume of the egg has been divided into hundreds of cells no larger than 100 m.

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In summary --- Eukaryotic cells are typically larger than 10 m because you need at least 10 m to hold the minimum structures for survival. Eukaryotic cells are usually smaller than 100 m because a decreasing SA/V ratio limits a growing cell’s ability to supply its own needs.

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**Testing your understanding**

How is it possible for most prokaryotic cells to be smaller than 10 m?

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**This is related to their lower limit**

Prokaryotic cells lack a true nucleus and any membrane bound organelles. Thus less space is required to hold their essential components than is true for the more complex eukaryote.

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**References Section 4.2 (page 54) of text including Figure 4.2B**

Figure in study guide showing the effect of increasing cell size on surface area, volume and SA/V ratio

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**It would be unusual to find a eukaryotic cell larger than 100 µm because:**

a cell that size could not hold everything it needs to survive it would have too much surface area for its volume it wouldn’t be able to supply its metabolic needs its volume would be too small for its surface area it would exceed the lower limit for eukaryotic cell size

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**It would be unusual to find a eukaryotic cell larger than 100 µm because:**

a cell that size could not hold everything it needs to survive it would have too much surface area for its volume it wouldn’t be able to supply its metabolic needs its volume would be too small for its surface area it would exceed the lower limit for eukaryotic cell size Remember that how large a cell can grow is restricted by the surface area to volume ratio (balance of “supply and demand”). As cell grows, the surface area increases more slowly than the volume so that at some point (around 100 µm), the cell doesn’t have enough surface area to supply its growing metabolic needs.

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**It would be unusual to find a eukaryotic cell smaller than 10 µm because:**

a cell that size could not hold everything it needs to survive it would have too much surface area for its volume it would have too little surface area for its volume its volume would be too small for its surface area it would exceed the upper limit for eukaryotic cell size

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**It would be unusual to find a eukaryotic cell smaller than 10 µm because:**

a cell that size could not hold everything it needs to survive it would have too much surface area for its volume it would have too little surface area for its volume its volume would be too small for its surface area it would exceed the upper limit for eukaryotic cell size Remember that all the structures a cell needs to survive must be contained within its cell boundaries. As cells decrease in size there is less and less space inside to “pack” the minimum number of organelles. At some point (around 10 µm in diameter), cells simply run out of room.

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**Cell ____ has the greatest volume. **

B C Cell ____ has the greatest volume. Cell ____ has the greatest surface area Cell ____ has the greatest surface area to volume ratio.

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**Cell C has the greatest volume. Cell C has the greatest surface area **

B C Cell C has the greatest volume. Cell C has the greatest surface area Cell A has the greatest surface area to volume ratio. Remember that as a cell increases in size, its surface area and volume both increase, but the ratio of the two decreases. In this case, cell A would be able to supply its needs more easily than cell C.

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**Surface area to volume ratio limits a cell’s _______ limit. **

Fill in the blanks with either UPPER or LOWER. Surface area to volume ratio limits a cell’s _______ limit. Cells are not usually smaller than 10 µm because of the _____ limit on eukaryotic cell size. Finding a cell larger than 100 µm would be unusual, as this would exceed the _______ limit on eukaryotic cell size A eukaryotic cell 3 µm in diameter would be outside the typical _______ limit for cell size. (How might this be possible?)

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**Surface area to volume ratio limits a cell’s UPPER limit. **

Fill in the blanks with either UPPER or LOWER. Surface area to volume ratio limits a cell’s UPPER limit. Cells are not usually smaller than 10 µm because of the LOWER limit on eukaryotic cell size. Finding a cell larger than 100 µm would be unusual, as this would exceed the UPPER limit on eukaryotic cell size A eukaryotic cell 3 µm in diameter would be outside the typical LOWER limit for cell size. (One possible explanation is that the cell has found a way to survive with fewer cell structures than usual).

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