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Cells and Cell Theory
What advantages does small size give to a cell?
Many cellular processes occur by diffusion, which is efficient over short
distances, but less efficient over long distances. Since all materials going in
and out of a cell must pass through the plasma membrane, the greater the surface
area of this membrane, the faster a given quantity of molecules can pass
through. Smaller cells have a much greater surface-to-volume ratio than larger
cells and therefore can "feed" all areas of the cell in less time.
What is
"surface-to-volume ratio," and how does it affect cell size?
The
surface-to-volume ratio is a mathematical relationship between the volume of an
object and the amount of surface area it has. This ratio often plays an
important role in biological structures. Think of a cell as a sphere:
The
surface area of a sphere can be calculated by
4ð r2
where r is the
radius of the sphere.
Volume of a sphere can be calculated by
4/3 ð r3.
An increase in r will increase the surface area by a power of two, but
increase the volume by a power of three. This means that the volume will
increase much faster than the surface area. This puts an upper limit on the size
of a cell, because if the cell volume gets too big, there won't be enough
membrane to transport the amount of food in and wastes out to support that large
cell size.
What is the difference between prokaryotic and eukaryotic cells?
Prokaryotic cells are more simple: they are usually much smaller and don't
have a nucleus or any other membrane-bound organelles. Bacteria are prokaryotes.
Eukaryotic cells are much more complex, are usually larger, and have a nucleus
and several other membrane-bound organelles that allow them to compartmentalize
their functions. All multicellular plants and animals are eukaryotes. A helpful
trick to remember is that "you" are a "eu"karyote.
Are there any
single-celled eukaryotes?
Yes--yeast, for example. Yeast are single-celled
organisms, but they do contain a membrane-bound nucleus, mitochondria, and other
organelles.
What are the advantages and disadvantages of prokaryotic
compared to eukaryotic cells?
Although prokaryotes may seem more primitive
than eukaryotes, they are among the most successful species on our plant and
comprise a very large percentage of the total mass of all living things on
earth. Simple, small, and single-celled organisms can reproduce quickly and
evolve quickly. Prokaryotes can generate millions of progeny in a short period
of time. In addition, some have evolved to thrive in extreme conditions in which
no eukaryote can live. Some can also form a protective capsule enabling them to
survive periods of adverse conditions.
However, they have no membrane-bound
organelles, so they do not have some of the cellular functions that eukaryotes
do. Prokaryotes are also unable to join together to form a multicellular
organism with specialized tissues. The multicellular human body has a brain,
eyes, and an immune system that allow it to take in and process information
about its environment and protect itself from pathogens; prokaryotes don't have
these functions.
How did eukaryotic organelles evolve?
The endosymbiotic
theory suggests that eukaryotic cells may have arisen from prokaryotic cells
living in what is called mutualistic symbiosis. Mutualistic symbiosis is a
relationship in which two or more organisms live together and each benefit from
the partnership. According to the endosymbiotic theory, the first eukaryote
began to evolve when one ancient prokaryote lived inside another ancient
prokaryote, both benefiting from the arrangement. The inner cell might be able
to perform aerobic respiration and provide its host with an efficient way to
harness the chemical energy in food. The host could, in turn, provide the inner
cell with protection and a supply of resources. It is easy to imagine how such
an inner cell might have evolved into the mitochondria inside eukaryotic cells.
Other eukaryotic organelles could have evolved in similar ways.
Cells and
Cell Theory
cell
The basic unit of life. A living cell is composed of
cytoplasm (which contains organelles, such as ribosomes) and is surrounded by a
semi-permeable membrane. During part or all of its life, a cell also contains
genetic material (DNA). Organisms may be composed of one cell or many. A cell
may be eukaryotic, in which case its genetic material is enclosed by an internal
membrane to form a structure called a nucleus. A cell may be prokaryotic, in
which case its genetic material is not enclosed by a membrane.
cell theory
The theory that all living organisms are made of cells and all cells arise
from pre-existing cells.
cell wall
A rigid structure that surrounds the
plasma membrane of some cells (plant, fungal, and bacterial cells). Plant cell
walls contain fibers of the polysaccharide cellulose. Bacterial cell walls
contain a protein and carbohydrate compound called peptidoglycan. Fungal cell
walls contain the polysaccharide chitin. In both eukaryotes and prokaryotes, the
cell wall provides the cell with support and protection.
cellular work
Activities that occur in a cell which are necessary to the cell's survival.
Cellular work requires energy and includes activities such as growth,
reproduction, movement, and protein synthesis.
chloroplast
Organelle
that is the site of photosynthesis. Chloroplasts contain chlorophyll.
endosymbiotic theory
The theory that the first eukaryotic cells
originated from a symbiotic relationship between different types of ancient
prokaryotes.
eukaryotic cell
A cell that contains a membrane-bound
nucleus and membrane-bound organelles. Eukaryotic cells tend to be larger and
more complex than prokaryotic cells.
micron
The unit of measurement
often used to describe cellular structures. A micron is equal to one
one-millionth of a meter (1 micron = 0.000001 meters).
mitochondrion
(pl: mitochondria) Organelle that is the site of aerobic cellular
respiration. Mitochondria are the "powerhouses" of the cell.
nucleus
The
organelle that acts as the cell's control center. It contains most of the cell's
genetic information (DNA).
organelles
Structures within the cell that
carry out particular cellular tasks.
plasma membrane
The membrane
surrounding a cell that separates the cytoplasm from the cell's external
environment. It acts as a selective barrier, allowing only certain ions and
molecules to enter or leave the cell.
prokaryotic cell
A cell that lacks
a membrane-bound nucleus and other membrane-bound organelles. Prokaryotic cells
tend to be smaller and less complex than eukaryotic cells. Bacteria are examples
of prokaryotes.
ribosome
Organelle that is the site of protein
synthesis.
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