Investigation 2
Maddie Rita
Every day, humans go about their lives with a casual focus on their exterior
environment. Many people probably envision themselves as one brick in the large
structure of the Homosapien species, with little consideration of the fact that each
individual is composed of many separate little bricks – cells. All living things are
made of one or more cells; cells are the smallest unit of matter than can sustain all
the functions that vitality requires.
Though knowledge on our cellular composition is not regularly
acknowledged, this is not breaking news – back in 1665, Robert Hooke examined
cork, elder trees, carrots and ferns under a microscope. The tiny chambers he
observed reminded him of monks’ rooms, so he called them cells. We now know
today that he had seen dead plant cells. Anton van Leeuwenhoek was the first to
observe living cells.
These men were not the only key players in the early investigation of cells.
The Cell Theory, which contains three essential, consistent facts about cells, was
proven by evidence from Matthias Schleiden (1838), Theodor Schwann (1839),
and Rudolf Virchow (1855). The Cell Theory is as follows:
1) All living things are made of one or more cells
2) Cells are the basic units of structure and function in an organism
3) Cells are created only be the reproduction of preexisting cells
This theory is the simplest approach to the complex topic of cells. Even
cells themselves are not one bland particle – they have many parts, and can look
and behave very differently depending on their job in an organism. The human
body alone contains at least 200 different types of cells. It’s hard to imagine, since
most cells are only visible with a microscope, which is why a great chunk of the
general population does not regularly envision themselves in terms of skin cells,
blood cells, nerve cells, etc.
We can’t see these cells with the naked eye because their growth is limited
by the ratio between outer surface area and volume. The volume of a cell increases
with the cube of the side length, but the surface area only increases with the square
of the side length. If a cell remains the same shape throughout growth, the volume
will increase faster than the surface area, resulting in a surface area that cannot
absorb enough nutrients and oxygen to maintain cell health.
Maintaining that cell health is, in fact, one reason that cells are so complex.
While they have jobs to support the organism as a whole, they also must regulate
individual functionality. This explains the vast diversity of cells, including the
varying shapes – different forms are more helpful for certain jobs than others. For
example, skin cells are conveniently flat to more effectively cover the body, while
nerve cells have tentacle-like extensions that help transmit impulses.
No matter the shape, cells share some qualities. One common similarity is
internal organization. The arrangement of organelles, (specific components that
maintain and direct cell life), in a cell determines the classification. When an
organism is composed of cells with a membrane-bound nucleus, it is considered a
eukaryote. Single-cell organisms without a membrane-bound nucleus are called
prokaryotes.
Just what is a nucleus? To understand this essential component of a cell, it
helps to begin with the organelle that surrounds it – the cell membrane.
The cell membrane covers the cell, but it's much more than wrapping paper. Just
like in a larger organism, nutrients and waste must enter and exit the cell in order
for it to survive. The cell membrane regulates this activity, controlling what passes
easily and what doesn't. Since it doesn't just let any old thing enter the cell, this
characteristic is described as being selectively permeable. The structure of the
membrane varies depending on cell function; some destroy invaders while others
secrete materials. It is specialized for whatever the task may be, but all cell
membranes are mostly made of lipids and proteins.
The term membrane lipids may not be familiar, but humans are actually
well acquainted with them, just by another name. Lipids are fats, and in the case of
cell membranes, they are specifically phospholipids. This means the lipid has a
polar “head,” two nonpolar “tails,” and is hydrophollic – the head will always
orient itself as close to water as possible, while the tails will orient away. Cells are
bathed in a watery environment both internally and externally; in other words,
both sides of the membrane are surrounded by water. Because of this, the
phospholipids form two layers – a lipid bilayer. In cell membranes, steroids fit in
between the tails of phospholipids. A major membrane steroid in animal cells,
including humans, is cholesterol.
Proteins also are important in the membrane. Some are attached to the
surface, with peripheral proteins on both the interior and exterior. Weak bonds link
peripheral proteins to lipids or other proteins embedded in the lipid bilayer.
Membrane proteins play a significant role in transferring molecules through the
lipid bilayer – some form channels, while others bind to the molecule and carry it
to the other side.
The membrane does not just surround the nucleus, though. Cells have a
variety of parts. One such part, or organelle, is the endoplasmic reticulum, often
abbreviated as ER. The endoplasmic reticulum is a system of sacs and tubules that
functions as an “intercellular highway.” It is described as such because it is the
path that allows molecules to move throughout the cell. There are two types of ER.
One is covered with ribosome, which appear as dark dots, so it is known as rough
endoplasmic reticulum. Rough ER is common in cells that make lots of proteins.
Meanwhile, the second type of ER is not covered with ribosome, so it’s called
smooth endoplasmic reticulum. Smooth ER plays a role in the regulation of
calcium levels in muscle, the synthesis of steroids, and the breakdown of toxic
substances in liver.
Bound to the ER are yet another organelle – ribosomes. Ribosomes
assemble the proteins of a cell, and can exist in massive numbers if a cell has high
protein production levels. Ribosomes are also found in the cytosol (the aqueous
part of the cell).
Another component of a cell is the Golgi apparatus. This is the processing,
packaging and secreting organelle. Under a microscope, it looks like a series of
flat sacs with a convex shape. This organelle is responsible for modifying proteins
so the cell can export them.
Another type of organelles common in most cells, except plant cells, are
lysosomes. Lysosomes hold hydrolytic enzymes within membranes. It’s these
membranes that digest proteins, lipids, carbohydrates, DNA and RNA. Lysosomes
are essential to the development of an organism. For example, lysosomes assist in
the formation of human hands by breaking down membrane to form fingers.
With all this activity going on, it’s impressive that a cell can maintain its
shape and size. This is thanks to the cytoskeleton, a network of long proteins with
two major components – microfilaments and microtubules. Microfilaments are
made of a protein called actin; a polymer chain links the many actin molecules.
These are the smallest strands of a cytoskeleton. They contribute to cell movement
and muscle contraction. Meanwhile, microtubules are large, hollow strands that
extend from a central point to the cell membrane. Bundles of microtubules, called
spindle fibers, come together across a dividing cell. They assist in moving
chromosomes during cell division and disassemble when the division is complete.
As obvious by its name, the cytoskeleton serves a purpose similar to that of the
human skeletal structure.
There are other types of organelles that assist movement, such as the cilia
and flagella. These hair-like organelles extend from the surface of the cell. When
short and plentiful, they are called cilia. Many unicellular organisms are covered
in cilia, but they are found in multicellular organisms as well. For example, the
respiratory tract is covered in cilia that trap debris from inhaled air. Meanwhile,
when the organelles are longer and fewer, they are called flagella. For example, a
sperm’s tail is flagella. Both organelles are composed of nine pairs of microtubes
oriented around a central pair.
Now that the other organelles have been explained, it is logical to introduce
the organelle that is, perhaps, most essential to cell life. The nucleus, often the
most prominent structure in a eukaryotic cell, is the information super center in the
cell. It maintains its shape with a protein skeleton called the nuclear matrix; with
all the jobs it must complete, it is important for the nucleus to remain healthy and
functional. For this reason, a double membrane called the nuclear envelope
surrounds it. Inside are strands of chromatin, (DNA + protein), that become
chromosomes before cell division. The nucleus also stores hereditary DNA
information and is where RNA is copied from DNA. Additionally, most nuclei
have at least one spherical area called a nucleolus. The nucleolus is where
ribosome are synesthized and partly assembled before passing through nuclear
pores to the cytosol.
The aforementioned organelles make up the basic eukaryotic cell, but plant
cells, which are also eukaryotic, have several differences. In plants, there are three
additional structures: the cell wall, vacuoles and plastids.
The cell wall is a rigid wall containing long chains of cellulose that support
and protect the cell. There are two types: primary, which develops just outside the
cell membrane while the plant forms, and secondary, which develops between the
membrane and primary cell wall. The secondary cell wall is tough and woody, so
its completion prevents further plant growth.
Vacuoles are another organelle not found in animal cells. These fluid filled
organelles store enzymes and waste. Since they can account for up to 90% of a
cell’s volume, vacuoles tend to push other organelles against the cell membrane.
Since some stored waste is bad, it has to be contained from the rest of the cell.
Finally, plastids are organelles that store specific things, depending on the
type (there are three types). Leucoplasts store starch granules, chromoplasts store
pigment molecules, and chloroplasts serve as the site of photosynthesis.
Now that a basic explanation of a eukaryotic cell has been accomplished, a
comparison to prokaryotic cells is sensible. The main difference is in the
composition. While prokaryotes have a cell wall, chromosomes, simple cilia and
flagella, a cell membrane, and ribosomes, they lack the other organelles that make
up a eukaryotic cell.
Prokaryotes are single-celled organisms. A well-known example of such
cells is bacteria. These are often classified by staining and shape. As humans
know, bacteria can have a negative or positive effect on eukaryotic organisms.
Such impacts relate to natural selection, but that will be further investigated in the
future.
Overall, it is evident that cells are complex systems with unique parts and
roles. They are the building blocks of all life and vital to our very existence.