Video Transcript
In this video, we will learn how to
describe the structure and function of different tissues found in plants. We will discover how to distinguish
between simple and compound plant tissues, specifically looking into the structure
and function of all three simple plant tissues parenchyma, collenchyma, and
sclerenchyma. We will also learn about the
structure and function of the compound tissues that make up the plant’s vascular
system, the xylem and the phloem.
Have you ever wondered why celery
is so crunchy or why pears are a little gritty in their texture? Different plant tissues contain
various types of cells with diverse chemical and structural components that give
them their unique properties, the specifics of which we will discover in this
video.
A tissue is a group of specialized
cells that carry out a particular function. There are two main types of tissues
in plants, simple tissues and compound tissues, which are otherwise known as complex
tissues. Let’s look at simple tissues
first. Simple tissues are generally
composed of one type of cell, or at least the majority of their cells have very
similar structures and functions. For example, this micrograph shows
some cells in one type of simple tissue in a plant leaf. And as you can see, all the cells
have a similar large rectangular shape and contain small green circles, which are
their chloroplast organelles.
Compound issues, otherwise known as
complex tissues, consist of multiple different types of cell, which vary not only in
their structure but also in their function. Compound tissues are mainly found
in the vascular regions of the plant, called the vascular bundles. The vascular bundles are
responsible for transporting substances such as water or the products of
photosynthesis to the parts of the plant that require them. You can see several vascular
bundles in this micrograph of the cross section of a sunflower stem, one of which
has been circled in pink. Each vascular bundle consists
primarily of just two compound tissues, the xylem and the phloem.
Each of these compound tissues are
made up of different cells with very different structures. For comparison with these compound
tissues, we can also see the three simple tissues found in plant parenchyma,
collenchyma, and sclerenchyma, which sits on the outside of each vascular
bundle. Each of these simple tissues
contain cells that are very similar in structure and function, though their size
does vary slightly. While parenchyma tissues make up
the bulk of the central region of plant stems, leaves, and roots, collenchyma
tissues mainly provide structural support and some flexibility and are usually found
just beneath the outer layer of plant cells, called the epidermis. Sclerenchyma tissues also provide
strong mechanical support, mainly to the vascular bundles.
Let’s have a look at the structure
and functions of these different tissues in a little more detail, starting with
simple parenchyma tissues. Parenchyma cells make up the soft,
fleshy tissues inside various parts of nonwoody plants, such as the leaves, stem,
and roots. This diagram represents some
parenchyma cells that have been magnified within a tuber of a potato plant. Tubers such as these make up part
of the root system of the potato plant, responsible for storing sugars. And they are the part that we,
humans, tend to consume. These sugars are stored as starch
within these parenchyma cells, providing us with the first function of parenchyma
tissue, nutrient storage.
Let’s keep a list of the
distinguishing structural and functional characteristics of parenchyma cells as we
observe them. You can see that parenchyma cells
are all round or oval in their shape. Most parenchyma cells have not yet
specialized. So they have the potential to
differentiate into most types of plant cell. But they all share similar basic
characteristics. For example, most parenchyma cells
have a large number of chloroplasts and thin cell walls made of cellulose. Parenchyma cells are living and
have many functions in addition to nutrient storage. They can store water in their large
permanent vacuoles. And many parenchyma cells secrete
sap that is stored in these vacuoles. Parenchyma cells can use the stored
water and carbon dioxide from the air to carry out photosynthesis to produce their
own food. Parenchyma cells also tend to have
other varied roles, such as gas exchange, to obtain this carbon dioxide that they
require for photosynthesis.
Let’s look at another simple
tissue, collenchyma, next. Collenchyma tissues consist of long
cells with thick cell walls that provide structure, support, and some elasticity to
a plant, particularly to young stems, and are therefore essential in the growing
regions of the plant. This diagram represents a cross
section of collenchyma cells that would be found just below the epidermis of a
celery stem. Celery is crunchy as the main role
of collenchyma is to provide mechanical support to these growing regions. They also provide some
flexibility. Collenchyma tissues, like
parenchyma, usually consists of living cells. As you can see, collenchyma cells
are typically rectangular in shape, and they tend to be longer than parenchyma
cells. And they have thick cell walls
where the cellulose is reinforced with another substance called pectin, which adds
this mechanical support to the stem.
Let’s look at our final simple
plant tissue next, sclerenchyma. Sclerenchyma cells are the toughest
of the three simple tissue types. Their mechanical strength is
provided by their thick cell walls that provide support in stems and leaves, for
example, around the vascular bundles. This diagram represents the
structure of sclerenchyma cells found in the fruit of a pear plant. Sclerenchyma cells vary more in
their size and shape than the other two simple tissue types. But they are typically recognizable
by their thicker cell walls. Plants do not have a skeleton as we
humans do, but these strong sclerenchyma cells allow them to stand upright and
remain rigid. The cell walls and sclerenchyma
tissues are made of cellulose, hemicelluloses, and a chemical called lignin. Lignin kills cells as it
waterproofs them. So lignified sclerenchyma tissues
are technically dead. It is also this lignin which adds
the gritty texture to the pear fruit.
Let’s look at the compound plant
tissues next, starting with xylem. As we mentioned earlier, xylem is
an example of a plant vascular tissue, which means it is involved in transport. The xylem is responsible for
transporting water and dissolved mineral ions, which are absorbed from the soil into
the roots of a plant, where they are then transported through the xylem to all the
parts of the plant that might require them. For example, the parenchyma cells
in the leaves will need water to photosynthesize. Xylem tissue consists of two main
types of cell: xylem vessels, otherwise known as tracheids, and xylem fibers, both
of which can be seen in this diagram.
The walls of xylem vessels are made
of sclerenchyma cells, which are lignified and therefore dead. The lignin in their cell walls
waterproofs them and provides extra structural support to the plant. The cells in a xylem vessel are
stacked end to end with their end walls broken down to form a hollow tube. And this allows water and dissolved
minerals to be pulled up through them as if through a straw. Xylem fibers are also lignified and
nonliving, and their main role is to provide mechanical support.
Let’s look at another type of
compound tissue now, phloem tissue. Phloem is another type of vascular
plant tissue that transports primarily the products of photosynthesis to the cells
of a plant. These products of photosynthesis,
sometimes called assimilates, are organic solutes in the form of dissolved sugars
and amino acids, which are needed in various regions of the plant. These substances are moved
primarily from the leaves as this is where most photosynthesis will occur, both up
and down the stem to all tissues in the plant, as sugars and amino acids are needed
in almost all of the plant cells.
Phloem tissue consists of four main
types of cell that you can see in this diagram: sieve tube members, otherwise known
as sieve tube elements, companion cells, and fibers and sclereids, which are
represented here as just a singular black cell. Sieve tube members are long, hollow
columns of cells fused end to end. But unlike xylem vessels, their end
walls are not completely broken down. Instead, there are sieve tube
plates between each adjacent sieve tube member, which, much like a sieve, have holes
to allow solutes to pass through them. In order to make the sieve tube
members hollow, the majority of their organelles break down and mature cells have no
nucleus.
Companion cells are linked to sieve
tube members by channels in their cell walls called plasmodesmata, which link
together the cytoplasm of the two cells. Companion cells are examples of
specialized parenchyma cells, so these cells are living. Fibers and sclereids are examples
of sclerenchyma cells, and they have thick cell walls to provide structural support
to the phloem vessels. Now that we know some more
information about plant tissues, let’s review what we’ve learned by having a go at a
practice question.
Which type of simple tissue in
plants is being outlined in the following description? The tissue has cells that are oval
or round in shape, surrounded by thin cellulose cell walls, and may contain
chloroplasts.
We are asked to determine which
type of simple tissue has these structural features. So to figure this out, let’s
compare the three simple tissues in plants using a table and define some key terms
along the way. Simple tissues are generally
composed of one type of cell. Or at least the majority of cells
have very similar structures and functions. There are three different types of
simple tissues in plants: parenchyma, collenchyma, and sclerenchyma. The drawings below the table
correspond to the typical shape and structure of cells in each type of simple
tissue.
Parenchyma cells are typically oval
or round in shape, while collenchyma cells usually look like elongated
rectangles. And as you can see, sclerenchyma
cells vary widely in their shape and size. You might notice that parenchyma
cells have the thinnest cell walls of all three tissue types. And these walls are made of
cellulose. Collenchyma cells have thicker cell
walls to provide mechanical support and are reinforced with a substance called
pectin. Sclerenchyma cells have the
thickest cell walls of all three types to structurally support and provide strength
to the plant’s transport vessels, for example. These cell walls are reinforced
with strong substances, such as lignin, to provide them with this strength.
Parenchyma cells also contain
chloroplasts, as one of their many functions is to carry out photosynthesis. Collenchyma cells might also
contain chloroplasts, but they are unlikely to be so many as in a parenchyma
cell. Sclerenchyma cells are technically
nonliving, partly due to the waterproofing lignin in their walls. Therefore, there is no point in
sclerenchyma cells containing chloroplasts as they would not be able to obtain the
water needed for photosynthesis across their walls. Let’s look back to the information
in the question to discover which simple tissue type it is describing.
The question describes the cells’
overall shape as oval or round. This suggests that the cells might
be found in parenchyma tissues rather than the long rectangular cells of
collenchyma. As collenchyma cells vary in their
shape, the cells in the question being round does not necessarily rule out
sclerenchyma. So let’s continue to the other
features. The question also states that the
cell walls of these cells are thin and made of cellulose. This provides further evidence that
they may be found in parenchyma tissues as both collenchyma and sclerenchyma cells
have thicker cell walls that are reinforced with other substances, such as pectin
and lignin.
The question tells us that the
cells in this tissue may contain chloroplasts. We know that these organelles may
be found either in parenchyma or in collenchyma cells, but not in sclerenchyma
cells. Based on this evidence, we can
deduce that these cells are unlikely to be found in collenchyma or in sclerenchyma
tissues, which means that as they meet all the necessary criteria, these cells are
most likely to be found in simple parenchyma tissues.
Let’s review some of the key points
that we’ve covered in this video. Simple tissues consist of cells
that are very similar in their structure and function, for example, in plants, the
three simple tissues of parenchyma, collenchyma, and sclerenchyma. While parenchyma tissues have
varied functions that include photosynthesis and water and nutrient storage,
collenchyma and sclerenchyma both function to provide structural support to
different regions of the plant, with collenchyma also providing some elasticity to
the growing regions.
Compound issues are made up of
different cells with varied structures and functions, for example, the xylem and
phloem tissues that make up the vascular bundle in plants. Xylem tissues function to transport
water and dissolve minerals from the roots to the other parts of the plant. And phloem tissues function to
transport sugars and amino acids from the site of photosynthesis, usually in the
leaves, to the parts of the plant that require them.