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Cell size

At 0.1 to 5.0 μm in diameter, prokaryotic cells are significantly smaller than eukaryotic cells, which have diameters ranging from 10 to 100 μm ( [link] ). The small size of prokaryotes allows ions and organic molecules that enter them to quickly diffuse to other parts of the cell. Similarly, any wastes produced within a prokaryotic cell can quickly diffuse out. This is not the case in eukaryotic cells, which have developed different structural adaptations to enhance intracellular transport.

Part a: Relative sizes on a logarithmic scale, from 0.1 nm to 1 m, are shown. Objects are shown from smallest to largest. The smallest object shown, an atom, is about 1 nm in size. The next largest objects shown are lipids and proteins; these molecules are between 1 and 10 nm. Bacteria are about 100 nm, and mitochondria are about 1 greek mu m. Plant and animal cells are both between 10 and 100 greek mu m. A human egg is between 100 greek mu m and 1 mm. A frog egg is about 1 mm, A chicken egg and an ostrich egg are both between 10 and 100 mm, but a chicken egg is larger. For comparison, a human is approximately 1 m tall.
This figure shows relative sizes of microbes on a logarithmic scale (recall that each unit of increase in a logarithmic scale represents a 10-fold increase in the quantity being measured).

Small size, in general, is necessary for all cells, whether prokaryotic or eukaryotic. Let’s examine why that is so. First, we’ll consider the area and volume of a typical cell. Not all cells are spherical in shape, but most tend to approximate a sphere. You may remember from your high school geometry course that the formula for the surface area of a sphere is 4πr 2 , while the formula for its volume is 4πr 3 /3. Thus, as the radius of a cell increases, its surface area increases as the square of its radius, but its volume increases as the cube of its radius (much more rapidly). Therefore, as a cell increases in size, its surface area-to-volume ratio decreases. This same principle would apply if the cell had the shape of a cube ( [link] ). If the cell grows too large, the plasma membrane will not have sufficient surface area to support the rate of diffusion required for the increased volume. In other words, as a cell grows, it becomes less efficient. One way to become more efficient is to divide; another way is to develop organelles that perform specific tasks. These adaptations lead to the development of more sophisticated cells called eukaryotic cells.

Art connection

On the left, a sphere 1 mm in diameter is encased in a box of the same width. On the right, the same sphere is encased in a box 2 mm in diameter.
Notice that as a cell increases in size, its surface area-to-volume ratio decreases. When there is insufficient surface area to support a cell’s increasing volume, a cell will either divide or die. The cell on the left has a volume of 1 mm 3 and a surface area of 6 mm 2 , with a surface area-to-volume ratio of 6 to 1, whereas the cell on the right has a volume of 8 mm 3 and a surface area of 24 mm 2 , with a surface area-to-volume ratio of 3 to 1.

Prokaryotic cells are much smaller than eukaryotic cells. What advantages might small cell size confer on a cell? What advantages might large cell size have?

Section summary

Prokaryotes are predominantly single-celled organisms of the domains Bacteria and Archaea. All prokaryotes have plasma membranes, cytoplasm, ribosomes, and DNA that is not membrane-bound. Most have peptidoglycan cell walls and many have polysaccharide capsules. Prokaryotic cells range in diameter from 0.1 to 5.0 μm.

As a cell increases in size, its surface area-to-volume ratio decreases. If the cell grows too large, the plasma membrane will not have sufficient surface area to support the rate of diffusion required for the increased volume.

Art connections

[link] Prokaryotic cells are much smaller than eukaryotic cells. What advantages might small cell size confer on a cell? What advantages might large cell size have?

[link] Substances can diffuse more quickly through small cells. Small cells have no need for organelles and therefore do not need to expend energy getting substances across organelle membranes. Large cells have organelles that can separate cellular processes, enabling them to build molecules that are more complex.

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Questions & Answers

what is chromosomes?
Mabiya Reply
it is a cell structure that contains DNA histones protein and other structural proteins
Ekechi
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Hassan Reply
liver is an organ in the body that meterbolise nutrients and produce bile.
Ekechi
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Gulfam
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muayad
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Ekechi
The mamalian endoskeleton has 206 bones
kaazim Reply
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Hannah
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206
Waneh
206
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206
anji
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ONELIA Reply
Name five parts of female reproductive system and state the function of each part
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ketchem
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Idriss Reply
the act or process of breathing .
Suzette
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Smiso
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ONELIA
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Shebas Reply
A female reproductive organs
Julius
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Henry Reply
Di= 2cotiledon mono=cotiledon
muayad
Mono = 1 cotiledon
muayad
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Shebas
A Virginia is a part of a female ,the passage of a child during child birth, receiver of the penis and the passage of menstral flow
Idriss
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Hassan Reply
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Hassan
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Gifty
is the study of life of living organisms
Chansa
study all about organisms like their reproduction, evolution , behaviour Physiology, etc
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okubasu
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Idriss
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ketchem
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ketchem
metabolism is the anabolic and catabolic process of how food are broken from large molecules into smaller soluble molecules.
Prince Reply
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Nakagolo Reply
Metabolism are all chemical reaction that takes place inside cells including those that takes energy and realise energy
Idriss
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Nakagolo
Way of animals to keep internal conditions
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balancing of water in the body
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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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