For a deeper dive into this material, complete these tutorials: Topic 2.3: Surface Area, Volume, and Life
1. Watch this Video
2. Study this Summary
Why Are Cells Small?
Key Idea: Cells rely on diffusion for the exchange of nutrients, oxygen, and waste. A small cell has more surface area relative to its volume compared to a larger cell.
Mathematical Example:
A cube-shaped cell 1 micron on each side:
- Surface area: 1 × 1 × 6 = 6 units.
- Volume: 1 × 1 × 1 = 1 unit.
- Surface area-to-volume ratio: 6:1.
A cube-shaped cell 10 microns on each side:
- Surface area: 10 × 10 × 6 = 600 units.
- Volume: 10 × 10 × 10 = 1,000 units.
- Surface area-to-volume ratio: 0.6:1.
Conclusion: As a cell grows larger, its surface area-to-volume ratio decreases, limiting its ability to efficiently exchange substances through diffusion.
Why Do Organisms Increase Surface Area in Certain Tissues?
To enhance diffusion of molecules (e.g., oxygen, carbon dioxide) or heat exchange.
Examples of Adaptations:
- Thin Sheets: Fish gills increase oxygen intake and carbon dioxide release.
- Flat Structures: Elephant ears dissipate heat in hot environments.
- Highly Folded Surfaces:
- Inner mitochondrial membranes increase surface area for ATP production.
- Intestinal villi and microvilli maximize nutrient absorption.
Surface Area and Marine Mammals
Mammals are warm-blooded, and ocean water is cold, creating a challenge for heat retention.
Key Concept: Increased size decreases the surface area-to-volume ratio, reducing heat loss.
Examples:
- No Small Marine Mammals: A small mouse-sized marine mammal would lose too much heat to survive.
- Otters: Smallest marine mammals, maintain warmth with a thick fur coat.
- Whales: Largest marine mammals, evolved large size to minimize heat loss.
3. Master these Flashcards
[qdeck style=”width: 600px !important; min-height: 450px !important;” bold_text=”false” random=”true”]
[h] Cell Size and Surface area-to-volume ratios
[q json=”true” yy=”4″ unit=”2.Cell_Structure_and_Function” dataset_id=”AP_Bio_Flashcards_2022|1e5432c9c5110″ question_number=”53″ topic=”2.3.Cell_Size”] Use the relationship between surface area and volume to explain why cells are small.
In your explanation, compare surface area and volume for a cuboidal cell that’s one unit in length on a side to one that’s 10 units in length.
[a] Cells need to be small in order to have enough membrane surface area to allow for diffusion of substances in and out. That’s because as an object gets larger, its amount of surface area relative to its volume decreases.
The larger cell’s surface area to volume ratio is 1/10th that of the smaller cell. With such a small amount of surface area relative to its volume, a large cell can’t efficiently use diffusion to get the nutrients it needs in, and to release wastes.
[q json=”true” yy=”4″ unit=”2.Cell_Structure_and_Function” topic=”2.3.Cell_Size” dataset_id=”AP_Bio_Flashcards_2022|e7a82bf906c7e” question_number=”54″] Explain the structure of root hairs, fish gills (below, left), or the lining of intestine (below, right) in terms of surface area to volume ratios.
[a] Structures like root hairs (tiny extensions that project from the outer tissue layer of roots) or gills (which are organized as thin sheets of tissue) are adaptations for increasing the surface for the diffusion of molecules. Similarly, the epithelial cells that make up the lining of the gut have a highly folded shape to increase the surface area for the diffusion of molecules from the gut into the body.
[q json=”true” yy=”4″ unit=”2.Cell_Structure_and_Function” topic=”2.3.Cell_Size” dataset_id=”AP_Bio_Flashcards_2022|e79d67420847e” question_number=”55″] Explain the structure of the inner mitochondrial membrane in terms of surface area.
[a] The inner mitochondrial membrane (shown at 2) is an adaptation for increasing the working surface for enzymatic reactions. In the case of the mitochondria, more inner membrane allows for more electron transport chains and more ATP synthase channels, which increases the amount of ATP produced per mitochondrion.
[q json=”true” yy=”4″ unit=”2.Cell_Structure_and_Function” topic=”2.3.Cell_Size” dataset_id=”AP_Bio_Flashcards_2022|e79003fda947e” question_number=”56″] Explain the structure of the ears of elephants or jackrabbits in terms of surface area.
[a] The flat ears of elephants or the huge ears of jackrabbits are adaptations that increase surface area for radiating heat into the environment.
[q json=”true” yy=”4″ unit=”2.Cell_Structure_and_Function” topic=”2.3.Cell_Size” dataset_id=”AP_Bio_Flashcards_2022|f1c4732c9d511″ question_number=”57″] Why does an increased size decrease the surface area-to-volume ratio?
[a] As an object grows, its volume increases faster than its surface area because volume is a cubic function while surface area is a square function. This leads to less surface area per unit of volume, which limits diffusion efficiency.
[q json=”true” yy=”4″ unit=”2.Cell_Structure_and_Function” topic=”2.3.Cell_Size” dataset_id=”AP_Bio_Flashcards_2022|f2c4732c9d611″ question_number=”58″] Why are there no small mouse-sized marine mammals?
[a] Marine mammals are warm-blooded, and small organisms have a high surface area-to-volume ratio. A small marine mammal would lose heat too rapidly in the cold ocean water to survive.
[q json=”true” yy=”4″ unit=”2.Cell_Structure_and_Function” topic=”2.3.Cell_Size” dataset_id=”AP_Bio_Flashcards_2022|f3c4732c9d711″ question_number=”59″] Why are whales the largest marine mammals?
[a] Whales are large because an increased size decreases the surface area-to-volume ratio. This reduces heat loss in cold water, allowing whales to maintain their body temperature more efficiently.
[/qdeck]
4. Tackle this Quiz
[qwiz random=”true”]
[h] Surface Area-to-Volume Ratios and Cell Size Quiz
[i]
[q] Why do cells need to be small?
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[q] As an object increases in size, what happens to its surface area-to-volume ratio?
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[q] Which of the following is an adaptation that increases surface area for diffusion in cells or tissues?
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[q] Which statement best explains why there are no small mouse-sized marine mammals?
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[q] How does the size of whales minimize heat loss in the ocean?
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[q] Which of the following is an example of an adaptation that increases diffusion efficiency in animals?
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[q] Why is the highly folded lining of the small intestine important for nutrient absorption?
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[q json=”true” xyz=”2″ multiple_choice=”true” dataset_id=”2019 AP Bio Dataset|206b115c761c5a” question_number=”64″ unit=”2.Cell Structure and Function” topic=”2.3.Cell_Size”] The body temperature of small ectothermic (also called “cold blooded”) animals, such as small fish, is dependent on the temperature of their environment. Large ectothermic animals, like sea turtles, can retain their body heat to remain warmer than their environment. Scientists believe large aquatic dinosaurs were also able to retain their body heat. Which of the following reasons best explains why large ectothermic animals can retain heat?
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What’s true of cubes is true of animals (only the calculations for finding area and volume are more complicated. Use that as a hint to think about what’s going on with larger organisms, and how that might relate to these organisms’ ability (or inability) to diffuse heat out of their bodies.
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[q json=”true” xyz=”2″ multiple_choice=”true” unit=”2.Cell Structure and Function” topic=”2.3.Cell_Size” dataset_id=”2019 AP Bio Dataset|13af80a891cd70″ question_number=”65″] The shapes below represent three cells that contain the same volume of cytoplasm. Which of the following statements is true?
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[q json=”true” xyz=”2″ multiple_choice=”true” unit=”2.Cell Structure and Function” topic=”2.3.Cell_Size” dataset_id=”2019 AP Bio Dataset|13aeb1706f8970″ question_number=”66″] The diagram below shows the relationship between the surface area:volume ratio and the volume for two rod-shaped bacteria with the same ratio of length to width (aspect ratio). Which of the following is true of the relationship shown?
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Cg==[Qq]
[q json=”true” xyz=”2″ multiple_choice=”true” unit=”2.Cell Structure and Function” topic=”2.3.Cell_Size” dataset_id=”2019 AP Bio Dataset|13adc89bca7970″ question_number=”67″] A cubical cell containing a volume of 1 μm3 measures 1 μm on each side. If the length of each side doubles, which of the following will be true?
[c]IFRoZSB2b2x1bWUgd2lsbCBkb3VibGU=[Qq]
[f]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Mw==ICg4KS4=
Cg==[Qq]
[c]IFRoZSBzdXJmYWNlIGFyZWEgd2lsbCBkb3VibGU=[Qq]
[f]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
Cg==[Qq]
[c]IFRoZSBzdXJmYWNlIGFyZWEgd2lsbCBpbmNyZWFzZSBtb3JlIHRoYW4gdGhlIHZvbHVtZS4=[Qq]
[f]IE5vLiBBcyBhIGN1YmUgZ2V0cyBsYXJnZXIsIHRoZSBzdXJmYWNlIGFyZWEgZ29lcyB1cCBieSA2eCB0aGUgc3F1YXJlIG9mIGEgc2lkZSB3aGlsZSB0aGUgdm9sdW1lIGdvZXMgdXAgYnkgdGhlIGN1YmUgb2YgYSBzaWRlLsKgIFRoZSBjZWxsIGRlc2NyaWJlZCB3aWxsIGdvIGZyb20gYSB2b2x1bWUgb2YgMSB0byA4LCB3aGljaCBpcyBhIGZhY3RvciBvZiA4LCB3aGlsZSB0aGUgc3VyZmFjZSB3aWxsIGdvIGZyb20gNiB0byAyNCwgd2hpY2ggaXMgYSBmYWN0b3Igb2Ygb25seSA0LsKgIFRoaXMgYW5zd2VyIGlzIGluY29ycmVjdCBiZWNhdXNlIGl0IGlzIHRoZSBvcHBvc2l0ZSBvZiB3aGF0IG11c3QgaGFwcGVuLg==
Cg==[Qq]
[c]IFRoZSB2b2x1bWUgd2lsbCBpbmNyZWFzZSBt b3JlIHRoYW4gdGhlIHN1cmZhY2UgYXJlYS4=[Qq]
[f]IFdheSB0byBnby4gQXMgYSBjdWJlIGdldHMgbGFyZ2VyLCB0aGUgc3VyZmFjZSBhcmVhIGdvZXMgdXAgYnkgNnggdGhlIHNxdWFyZSBvZiBhIHNpZGUgd2hpbGUgdGhlIHZvbHVtZSBnb2VzIHVwIGJ5IHRoZSBjdWJlIG9mIGEgc2lkZS7CoCBUaGUgY2VsbCBkZXNjcmliZWQgd2lsbCBnbyBmcm9tIGEgdm9sdW1lIG9mIDEgdG8gOCwgd2hpY2ggaXMgYSBmYWN0b3Igb2YgOCwgd2hpbGUgdGhlIHN1cmZhY2Ugd2lsbCBnbyBmcm9tIDYgdG8gMjQsIHdoaWNoIGlzIGEgZmFjdG9yIG9mIG9ubHkgNC4=
Cg==Cg==[Qq]
[q]As a cube gets bigger, its ratio of surface area to volume ____________.
As a cube gets smaller, its ratio of surface area to volume ___________.
As objects get ________, the volume increases so much more rapidly than surface area because volume is a _______ function, while area is a _________ function.
[l]bigger
[fx] No, that’s not correct. Please try again.
[f*] Great!
[l]cubic
[fx] No. Please try again.
[f*] Great!
[l]increases
[fx] No, that’s not correct. Please try again.
[f*] Great!
[l]square
[fx] No, that’s not correct. Please try again.
[f*] Excellent!
[l]decreases
[fx] No, that’s not correct. Please try again.
[f*] Great!
[q]The movement of a substance from where it is more concentrated to where it is less _________________ is called ____________.
The image on the left shows the result of a diffusion demonstration using agar cubes soaked in vinegar. Vinegar is a weak acid that changes the color of the cubes from magenta to white. In this demonstration, the vinegar entered the agar cubes by ___________.
For the vinegar to enter the cubes, it had to go through the cube’s outer ___________.
[l]concentrated
[fx] No. Please try again.
[f*] Excellent!
[l]diffusion
[fx] No. Please try again.
[f*] Good!
[l]surface
[fx] No. Please try again.
[f*] Excellent!
[q labels = “top”]The image on the left shows the result of a diffusion demonstration using agar cubes soaked in vinegar. Vinegar is a weak acid that changes the color of the cubes from magenta to white. Pretend that the cubes are actually cells, and the vinegar was food. In this analogy, the surface of the cube would be the _____________ of the cell, and the inside of the cube would be the ____________. The part of the “cell” that remained pink-ish would be ___________ because little or no food would have been able to _________ into it.
[l]cytoplasm
[fx] No, that’s not correct. Please try again.
[f*] Correct!
[l]diffuse
[fx] No. Please try again.
[f*] Great!
[l]membrane
[fx] No, that’s not correct. Please try again.
[f*] Excellent!
[l]suffering
[fx] No. Please try again.
[f*] Excellent!
[q labels = “top”]Cells are small for the following reason: compared to its _______, anything that’s _______ has a lot of surface area. This makes it _______ for things to diffuse in and out of cells
Cells in your intestine are very wavy on top. This gives them a particularly high surface area to __________ ratio, making it easy for food to __________ into them.
[l]diffuse
[fx] No. Please try again.
[f*] Good!
[l]easy
[fx] No, that’s not correct. Please try again.
[f*] Excellent!
[l]small
[fx] No, that’s not correct. Please try again.
[f*] Good!
[l]volume
[fx] No. Please try again.
[f*] Excellent!
[q labels = “top”]Gills in fish consist of lots of thin flaps of tissue. These thin flaps provide lots of ______________ for absorbing ___________ from the water.
Heat also diffuses in and out through an animal’s surface. Because they’re so big, whales have a very _____ surface area to volume ratio. This makes it __________ for heat to escape from their bodies, which helps them survive in _______ water.
[l]cold
[fx] No, that’s not correct. Please try again.
[f*] Great!
[l]difficult
[fx] No, that’s not correct. Please try again.
[f*] Good!
[l]low
[fx] No. Please try again.
[f*] Excellent!
[l]oxygen
[fx] No, that’s not correct. Please try again.
[f*] Correct!
[l]surface area
[fx] No, that’s not correct. Please try again.
[f*] Great!
[x] If you want to take this quiz again, please click the button below.
[restart]
[/qwiz]
What’s next?
Please proceed to this next tutorial: Cellular Compartmentalization and Endosymbiosis