The Building Blocks of Life
Every living thing on Earth is composed of cells – from floating phytoplankton, to a peach tree, to
an elephant, to you. Cells are the building blocks, where everything starts, yet most are
microscopic. A large organism, like an elephant, isn’t just a single cell; it’s composed of billions
of microscopic cells. All these teeny little cells are vital to the functioning of life.
Types of Cells
Cells are cells, right? Well, sort of. Scientists divide cells into two basic categories: prokaryotic
cells (which are very simple, have few organelles and aren’t capable of much complex
functioning) and eukaryotic cells (which are complex have more organelles, and can perform all
sorts of functions). You are composed of eukaryotic cells.
Cell Structure and Function
Cell complexity varies from simple to intricately complex. But in a few basic areas, all cells are
alike.
All cells have a plasma membrane that surrounds an inner region. The plasma membrane is the
cells outermost layer. It separates the internal events from the environment so that they can
proceed in organized, controlled ways. If you think of a cell as a blown-up balloon, the plasma
membrane would be the balloon itself; it contains the inner workings of the cell.
The inner region of the cell is called the cytoplasm. The cytoplasm is nearly everything that is
enclosed by the plasma membrane. Going back to the balloon analogy, you could say that the air
in the balloon represents the cytoplasm.
A cell’s cytoplasm is full of all sorts of things, most notably organelles. An organelle is a
membrane-bound sac. A cell might have many organelles in its cytoplasm or almost none. There
are also different types of organelles in the cytoplasm, each one performing a different function
for the cell. For instance, some organelles enable the cell to produce proteins, while others help
metabolize nutrients.
Within the cytoplasm, the cell’s genetic material is concentrated in one area, called the nucleus.
The nucleus is the membrane-bound compartment that houses all the cell’s genetic material. The
nucleus contains the cell’s DNA. Although the nucleus could technically be considered an
organelle (because it is indeed a membrane-bound sac), because it’s so important, scientists refer
to it as the nucleus, not just an organelle. It’s hard to say that one part of a cell is more valuable
than another because all parts of a cell function together, but a cell’s nucleus is definitely
important, the way your own brain is.
The nucleus is surrounded by the nuclear envelope. This structure is similar to the plasma
membrane in that it contains many small openings that allow materials to pass in and out of the
nucleus.
The Basics of Animal and Plant Eukaryotes
From an evolutionary standpoint, eukaryotic cells are more advanced than prokaryotic cells. Both
possess a plasma membrane and cytoplasm, but after that, differences emerge. The cytoplasm of
eukaryotic cells is quite complex. Both animals and plants are composed of eukaryotic cells.
Cytoplasm is a jelly-like material. It surrounds the nucleus and organelles, and contains water,
salts, and organic molecules. The cytoplasm of is in constant motion as the organelles and
particles jostle and move about. Scientists refer to this constant motion as cytoplasmic streaming.
Get Those Organelles to Work, Work, Work!
The organelles are where all the action is in the cell, and animal cells have a number of different
kinds of these mini-organs, each with their own set of functions.
One type is the endoplasmic reticulum (ER). The ER is a series of membranes that extends
throughout the cytoplasm of eukaryotic cells. In some places, the ER is studded with ribosomes,
which makes the cell’s proteins. In those instances, the ER is referred to as “rough ER.” In places
where there are no ribosomes attached, the endoplasmic reticulum is referred to as “smooth ER”.
Smooth ER acts like an intercellular highway, moving molecules from one end of the cell to the
other. The ER is primarily the site of protein synthesis.
Ribosomes are the structures that are responsible for building all the proteins that your cells use to
build structures or perform cellular functions. As mentioned above, they are found attached to the
surface of the Rough ER. They are also found floating freely in the cytoplasm
Another type of organelle is called the Golgi body or Golgi apparatus. The Golgi body looks like
a series of flattened sacs. It’s the processing and packaging plant of the cell. The Golgi body
processes the cell’s proteins and lipids before sending them out to other areas.
Yet another organelle is the lysosome. The lysosome looks like a sac, and it’s full of enzymes that
are used by the cell for digestion. The lysosomes break down particles of food and make them
available for use by the cell. Depending on the organism, the lysosome could be breaking down a
banana, some grass, or whatever the organism uses for nourishment. Once nourishment gets
delivered to a cell (and, in the case of humans, your bloodstream delivers nutrients out to every
single cell in your body), the lysosomes go to work breaking down the nourishment into a form
the cell can use.
Now we come to the real powerhouse of the cell, the mitochondria. The mitochondria are where
energy is stored and released. The released energy is used to form adenosine triphosphate.
Adenosine triphosphate, or ATP, is just a storage vehicle for energy until it’s used. It provides the
chemical energy needed to drive the chemical reactions of the cell.
Mitochondria are not just your typical organelles. They have their own DNA, which allow them
to divide and produce new mitochondria. In fact, scientists think that mitochondria developed
from prokaryotic cells that moved into eukaryotic cells. According to the theory, the prokaryotic
cells (that is, the fledgling mitochondria), sought to protect themselves from outside forces. In
return for the protection they received, they began supplying energy to eukaryotes.
The nucleus is where the cell’s nucleic acids are synthesized. It directs all the activities of the
cell. Within the cytoplasm, the nucleus is surrounded by a double membrane, called the nuclear
envelope. Inside the nuclear envelope, the nucleus has its own version of cytoplasm: a dense
protein rich, jelly-like substance called nucleoplasm. The nucleoplasm contains fine strands of
chromatin, a combination of DNA and proteins, and the nucleus.
The cell is very much like you in that it too has a skeleton. Just like your skeleton, the cell
skeleton, or cytoskeleton, performs the same functions as a human skeleton, movement and
protection. The cytoskeleton is used to create two important structures, the cilia and the flagella.
These structures protrude from the cell surface and enable the cell to swim rapidly through
liquids. The cytoskeleton is also used to generate structures that are vital to cell division.
Other things can be found in a cell’s cytoplasm, but these organelles are the main “inhabitants.”
The Basics of Plant Eukaryotes
Plants are also composed of eukaryotic cells. Nearly all the parts of the animal cell described
previously can be found in plants. But plants also possess some additional structures.
In plants, a cell wall surrounds the plasma membrane and helps support and protect the plant.
A plant’s cell wall is made up of long chains of cellulose (a tough fiber that humans use to make
paper and textiles; essentially, wood is cellulose), embedded with hardening compounds such as
pectin (a carbohydrate that is usually found in fruits). Plants produce two types of cell walls:
primary cell walls are formed cell growth and secondary cell walls are formed after growth has
ceased. Both types strengthen the cell. This strengthening through cellulose and pectin (neither of
which is found in the cells of eukaryotic animals) is a distinguishing feature of plant cells. The
strong layer offers protection to the plant.
Plants use the sun as a source of energy. In a process called photosynthesis, plastids convert solar
energy into chemical energy that the plant can use. Photosynthesis is a series of chemical
reactions that convert radiant energy from the sun into chemical energy that the plant can use.
The energy is then stored in the bonds of organic compounds, usually carbohydrates. Plastids
really set plant eukaryotes apart; if eukaryotic animal cells had plastids, we wouldn’t have to
eat—we’d just engage in photosynthesis.
Three types of plastids work together in the photosynthesis process: Chloroplasts contain
chlorophyll, a green pigment that absorbs sunlight. Chromoplasts synthesize and store pigments,
such as orange carotenes, red pigments, and yellow xanthophylls. Like chloroplasts, some of the
chromoplasts also trap sunlight for energy. Leucoplasts store food, including lipids, proteins, and
starches. Each of these plastids plays a vital role in the process of photosynthesis.
An important ingredient that plants need in order to perform photosynthesis is water. Since water
is not always immediately available, plants have structures that store water when it is available.
These structures are call vacuoles. This water storage is not only used for photosynthesis, but is
also used to create turgor pressure which provides the rigidity required to keep a plant standing
up.