|
The Human Cell / The Human Cell
Page: 2
Cells fall into one of two categories:
prokaryotic or eukaryotic.
In a prokaryotic cell, found only in bacteria and
archaebacteria, all the components, including the DNA, mingle freely in the
cell's interior, a single compartment. Eukaryotic cells, which make up
plants, animals, fungi, and all other life forms, contain numerous
compartments, or
organelles, within each cell. The DNA in eukaryotic cells is enclosed in
a special organelle
called the nucleus, which serves as the cell's command center and
information library. The term prokaryote comes from Greek words that mean
“before nucleus” or “prenucleus,” while eukaryote means “true nucleus.”
A. Prokaryotic Cells
Prokaryotic cells are among the tiniest of all
cells, ranging in size from 0.0001 to 0.003 mm (0.000004 to 0.0001 in) in
diameter. About a hundred typical prokaryotic cells lined up in a row would
match the thickness of a book page. These cells, which can be rodlike,
spherical, or spiral in shape, are surrounded by a protective cell wall.
Like most cells, prokaryotic cells live in a watery environment, whether it
is soil moisture, a pond, or the fluid surrounding cells in the human body.
Tiny pores in the cell wall enable water and the substances dissolved in it,
such as oxygen, to flow into the cell; these pores also allow wastes to flow
out.
Pushed up against the inner surface of the prokaryotic cell wall is a thin
membrane called the plasma membrane. The plasma membrane, composed of two
layers of flexible lipid molecules and interspersed with durable proteins,
is both supple and strong. Unlike the cell wall, whose open pores allow the
unregulated traffic of materials in and out of the cell, the plasma membrane
is selectively permeable, meaning it allows only certain substances to pass
through. Thus, the plasma membrane actively separates the cell's contents
from its surrounding fluids.
While small molecules such as water, oxygen, and carbon dioxide diffuse
freely across the plasma membrane, the passage of many larger molecules,
including amino acids (the building blocks of proteins) and sugars, is
carefully regulated. Specialized transport proteins accomplish this task.
The transport proteins span the plasma membrane, forming an intricate system
of pumps and channels through which traffic is conducted. Some substances
swirling in the fluid around the cell can enter it only if they bind to and
are escorted in by specific transport proteins. In this way, the cell
fine-tunes its internal environment.
The plasma membrane encloses the cytoplasm, the semifluid that fills the
cell. Composed of about 65 percent water, the cytoplasm is packed with up to
a billion molecules per cell, a rich storehouse that includes enzymes and
dissolved nutrients, such as sugars and amino acids. The water provides a
favorable environment for the thousands of biochemical reactions that take
place in the cell.
Within the cytoplasm of all prokaryotes is deoxyribonucleic acid (DNA), a
complex molecule in the form of a double helix, a shape similar to a spiral
staircase. The DNA is about 1,000 times the length of the cell, and to fit
inside, it repeatedly twists and folds to form a compact structure called a
chromosome. The chromosome in prokaryotes is circular, and is located in a
region of the cell called the nucleoid. Often, smaller chromosomes called
plasmids are located in the cytoplasm. The DNA is divided into units called
genes, just like a long train is divided into separate cars. Depending on
the species, the DNA contains several hundred or even thousands of genes.
Typically, one gene contains coded instructions for building all or part of
a single protein. Enzymes, which are specialized proteins, determine
virtually all the biochemical reactions that support and sustain the cell.
Also immersed in the cytoplasm are the only
organelles in
prokaryotic cells—tiny bead-like structures called ribosomes. These are the
cell's protein factories. Following the instructions encoded in the DNA,
ribosomes churn out proteins by the hundreds every minute, providing needed
enzymes, the replacements for worn-out transport proteins, or other proteins
required by the cell.
While relatively simple in construction, prokaryotic cells display extremely
complex activity. They have a greater range of biochemical reactions than
those found in their larger relatives, the eukaryotic cells. The
extraordinary biochemical diversity of prokaryotic cells is manifested in
the wide-ranging lifestyles of the archaebacteria and the bacteria, whose
habitats include polar ice, deserts, and hydrothermal vents—deep regions of
the ocean under great pressure where hot water geysers erupt from cracks in
the ocean floor.
B. Eukaryotic Animal Cells
Eukaryotic cells are typically about ten times
larger than prokaryotic cells. In animal cells, the plasma membrane, rather
than a cell wall, forms the cell's outer boundary. With a design similar to
the plasma membrane of prokaryotic cells, it separates the cell from its
surroundings and regulates the traffic across the membrane.
We have to start somewhere. Let's start on the
outside. Around every cell is a CELL MEMBRANE. The membrane is like a big
plastic bag with tiny holes in it. Scientists also call the cell membrane a
PLASMA MEMBRANE.
The purpose of the cell membrane is to
hold the cell together. It keeps all of the pieces, like the
organelles and
the CYTOPLASM, inside. The membrane also controls what goes in and out of
the cell. It acts like a crossing guard and says "You better stop right
there buddy. You aren't getting in here."
Scientists have a theory about the way a cell
membrane works. The theory is called the FLUID MOSAIC MODEL. The idea says
that there are two layers of MOLECULES, a BILAYER. These two layers are made
up of molecules called phospholipids. Take a look, it's like a sandwich with
two pieces of bread and some alfalfa on the inside.
Each phospholipid has an HYDROPHOBIC and HYDROPHILIC
end. They are big words, but they mean very simple things. HYDRO means
water. PHOBIC means afraid. PHILIC means loving. So one end of the molecule
is afraid of the water, and one end loves being in the water. Millions of
these molecules line up together to form a cell membrane.
Throughout the membrane are proteins stuck inside
the membrane. These proteins cross the bilayer and make the holes that let
ions and molecules in and out of the cell. (That crossing guard thing
again.)
When ions move through the cell membrane, it is called FACILITATED
DIFFUSION. Facilitated means helped. Diffusion means moving from one area to
another. So facilitated diffusion is a procedure where an ion is helped
across the membrane. (Like helping an old lady across the street.)
The eukaryotic cell cytoplasm is similar to that of
the prokaryote cell except for one major difference: Eukaryotic cells house
a nucleus and numerous other membrane-enclosed
organelles. Like
separate rooms of a house, these organelles enable specialized functions to
be carried out efficiently. The building of proteins and lipids, for
example, takes place in separate organelles where specialized enzymes geared
for each job are located.
 The
nucleus is the largest
organelle in an
animal cell. It contains numerous strands of
DNA, the length of each strand being many times the diameter of the
cell. Unlike the circular prokaryotic DNA, long sections of eukaryotic DNA
pack into the nucleus by wrapping around proteins. As a cell begins to
divide, each DNA strand folds over onto itself several times, forming a
rod-shaped chromosome.
The nucleus is surrounded by a double-layered membrane that protects the DNA
from potentially damaging chemical reactions that occur in the cytoplasm.
Messages pass between the cytoplasm and the nucleus through nuclear pores,
which are holes in the membrane of the nucleus. In each nuclear pore,
molecular signals flash back and forth as often as ten times per second. For
example, a signal to activate a specific gene comes in to the nucleus and
instructions for production of the necessary protein go out to the
cytoplasm.
Around the nucleus is another membrane (different
from the cell membrane). The nuclear membrane holds the nucleus together.
Scientists call the membrane the NUCLEAR ENVELOPE. Just like in the other
membranes and the cell wall, this one has tiny holes. Pieces of protein and
RNA pass through these holes.
 When
the cell is just sitting around, there is something called CHROMATIN in the
nucleus. We just talked about DNA. Well chromatin is made up of DNA, RNA and
proteins. When the cell is going to divide, the chromatin becomes very
compact, it condenses. When it comes together, you can see the things that
scientists call chromosomes.
If you see a picture of a nucleus, you might see a
small dark area inside the nucleus, almost like a tiny nucleus inside the
nucleus. This dark area is called the NUCLEOLUS. The nucleolus is made up of
protein and RNA with very little DNA.
 James Watson,
American, biologist, scientist, Nobel prize winner. He worked with a guy
named Crick and Wilkins and was one of the first scientists to come up with
the structure of a DNA molecule. They used something called
x-ray diffraction to see the shape of the molecule.
They took an X-ray of the DNA and when they looked at it, they saw it was in
the shape of a DOUBLE-HELIX. That's like having a ladder and then twisting
it. With their discovery, scientists were then able to figure out how DNA
replicates in the cell.
 Here is
Francis Crick. He worked with Watson and Wilkins on figuring out the
structure of DNA. He was born in England and then worked at Cambridge
University. For his work, he won a Nobel Prize in 1962.
The big deal about coming up with the structure was that scientists could
figure out how DNA duplicates. Crick helped discover that DNA uses something
called codons (sets of three nucleic acids) when it duplicates and when it
helps make proteins. Did you know that for every amino acid there is a
specific codon that binds to it and carries it to a ribosome to make a
protein?
|
 WITH
A MODEL OF DNA: At 35, the British Crick had not yet earned his
Ph.D., while the American Watson, 12 years younger, was awarded his
doctorate when he was just 22. |
|
JAMES
WATSON, RIGHT AND FRANCIS CRICK, LEFT, the young co-discoverers of
DNA's double-helix structure, in Cambridge, England, 1953. The brash
duo were impatient with authority, dismissive of prevailing opinions
-- and very eager to win the race to unravel the code. "A goodly
number of scientists," said Watson, "are not only narrow-minded and
dull, but also just stupid." |
|
Cell
Membrane Structure
