Plants are living organisms, so like any other
organism on the planet, we know that they must be made of cells. And more importantly, plants are rather large
multicellular organisms, so we should say they are made specifically of eukaryotic cells,
just as all multicellular organisms are. As we learned in the biology series, eukaryotic
cells have a variety of organelles inside, and most of these organelles are quite similar
whether the cells belong to plants or animals. But we can identify plant cells very easily
under a microscope because they have a few key differences. These would include the presence of chloroplasts,
a central vacuole, and a cell wall, all of which are structures that are absent in animal cells. If you need to review general information
about plant cells, visit my biology tutorial on the subject now, as well as the one on
animal cells, if you’re rusty on organelle function and organization. Now, with the basics regarding plant cell
structure understood, we need to get more specific. With animals such as ourselves, we have all
kinds of different cell types that allow for the wide variety of organs and tissues in
our bodies, as we learned in our study of human anatomy and physiology. Plants are precisely the same way, in that
not all the cells in a plant are exactly the same. We can verify this just by looking at a tree. The roots, trunk, branches, and leaves are
all made out of cells, but those cells look very different under a microscope and they
have very different jobs, just like our own nerve cells look very different from muscle
cells, which look very different from skin cells, and so forth. Let’s name the different types of plant
cells and describe them now. First, there are the cells that plants use to grow. These are called meristematic cells, and they’re
very much like the stem cells we have in our own bodies. What this means is that meristematic cells
are undifferentiated, or without a specific job assigned to them when they are first created
through mitosis. So when meristematic cells divide and replicate,
they can produce daughter cells belonging to any other kind of plant cell. The important difference between meristematic
cells in plants and stem cells in animals is that meristematic cells don’t get used
up, so they can continue dividing and helping the plant grow indefinitely. Meristematic cells can be found in the tips
of the roots, which are the parts of a plant that grow down into the soil, and in the tips
of the shoots, which are the parts of a plant that grow up into the air. This means meristematic cells allow the roots
of a plant to grow deeper into the soil and the branches of a plant to grow taller into the air. More specifically, they can be present in
the apical or farthest position at the tips of roots and shoots, the lateral or side position
within the vascular or transportation tissues of the plant, or in the intercalary position
where branches intersect and where leaves attach to branches. The second type of plant cells, parenchyma
cells, do most of the work within a plant, being that they’re a sort of general-use cell. Parenchyma cells are responsible for most
of the photosynthesis that occurs, but they also do most of the energy and nutrient storage
for the plant, as well as much of the nutrient transport. These cells have thin walls, no specialized
structure, and come in a variety of shapes to support their diverse functions. In leaves, parenchyma cells form the two layers
of mesophyll where photosynthesis and gas exchange take place. In roots and seeds, parenchyma cells are responsible
for storing starch, fat, and water. Parenchyma cells also make up most of the
structure of a fruit, and they create new structures to heal areas where a plant has
been wounded. Collenchyma cells are the third type of plant
cell, and they’re a kind of back-up system for the plant. These cells can contribute to photosynthesis
and nutrient storage. But the most important job for collenchyma
cells is providing flexible structure to the plant. Collenchyma are long cells that have thickened
cell walls, meaning that when they’re in a group they act to make that part of the
plant stiffer. You’ve probably eaten collenchyma cells before. In fact, the “strings” in celery are collenchyma cells. Groups of collenchyma cells can keep leaves
from tearing, allow petioles, which are the little stems on the ends of leaves, to bend
and flex in the breeze, and essentially give the plant some room to stretch without breaking. What’s really amazing is that plants which
experience a lot of bending due to wind, or even artificial disturbance from a scientific
researcher, have collenchyma cells that can be up to twice as thick as normal. Even though collenchyma cells are pretty stiff
in their structure, they’re also very flexible and able to grow and change as the plant grows. However, collenchyma cells don’t last forever. In woody plants, the collenchyma cells are
only needed to do their job of stiffening and strengthening the plant until the sclerenchyma
cells take over. The final type of plant cell, as we just mentioned,
are the sclerenchyma, and these are a little different from the first three. Though sclerenchyma also have thickened cell
walls like the collenchyma, the real difference is that sclerenchyma cells are dead and found
in parts of the plant that are no longer growing. These sclerenchyma cells provide the most
support for the plant by creating woody tissue in stems and trunks. Sclerenchyma cell walls contain lots of cellulose
and lignin, which are both complex biopolymers that are difficult to break down, so they
last a long time. These structural cells can be arranged in
two basic ways. Sclerenchyma fibers are stretched lengthwise
in a plant stem and provide most of a plant’s support. These fibers are also what people use to make
rope and fabric out of plants, such as flax, jute, and hemp. The other group of sclerenchyma cells are
the sclereids, and they’re much more versatile. These sclereid cells are what make up the
shells of nuts, the hard coatings of seeds like those found in peaches and plums, and
many other hardened structures in a plant. So now we know about the four types of plant
cells, those being meristematic, parenchyma, collenchyma, and sclerenchyma. But cells don’t really function individually
inside a plant. Instead, they clump together to form tissues
with different jobs. This means that tissues are what we will investigate next.