Video Transcript
In this video, we’ll learn about
the structure and function of some specialized structures in plants. These structures are adapted to
interact with the environment as well as other parts of a plant to collect and
distribute resources as well as eliminate wastes using chemical and physical
processes. And we’ll work some example
problems as well.
Plants face a few different
challenges than animals do. Plants still need similar resources
like chemical energy that animals get in food, water, and gases from the air, but
they can’t move. So plants have to extract these
resources from the environment while staying in the same place almost their whole
life. Let’s start by taking a look at a
blackberry seed that’s fallen to the ground after sneakily catching a ride with a
bird to see how it can survive. If the seed lands in a place with
enough soil and water, a root and a shoot will start to sprout.
If we take a much closer look,
we’ll see that the cells at the tip of the roots and the tip of the shoot are
actively dividing, so that’s where the plant is growing. These areas are called meristems,
and they contain the stem cells of the plant. Stem cells divide to produce cells
that will later differentiate into more specialized types of cells. Meristems can occur in other areas
of plants as well. As the plant continues to grow,
more structures develop, such as leaves and some tiny hairlike protrusions along the
root. If we take a look through a
microscope, we can see that these protrusions extend from a type of specialized
cell. They’re called root hair cells, and
they’re very important for obtaining water and minerals from the soil.
Switching to a higher
magnification, we can see that root hair cells are specialized cells of the plant’s
epidermis. They also have several adaptations
that increase their ability to move water and minerals into the plant. The most obvious adaptation of a
root hair cell is the long hairlike structure that increases the surface area where
water and minerals can move in to the plant. Another important adaptation of a
root hair cell is its large vacuole that has a high concentration of solutes, such
as dissolved salt, minerals, and sugars, and a relatively low water
concentration. Osmosis is the diffusion of water
across a semipermeable membrane. And as long as the water
concentration in the soil is higher than the water concentration in the vacuum,
water will move into the root hair by osmosis.
In addition, most of a plant
surface is covered with a waxy substance known as the cuticle. And it prevents water loss but not
so with root hair cells because they need to take water in. Plants also need to take up
dissolved minerals from the soil. But since there’s a higher
concentration of solutes in the root hair than in the soil, these need to be taken
up by active transport. Active transport moves substances
across a membrane from a low concentration to a high concentration and requires
energy. So the main functions of root hair
cells are to take up water by osmosis and to take up minerals by active
transport. And their structure supports their
function.
Of course, as water and minerals
continue to move into the root hair cell, they must also move out towards other
parts of the plant. And that’s the job of another
specialized plant structure called xylem. Xylem is a system of narrow tubes
that branch throughout a plant. It’s made out of dead cells that
form narrow tubes. They’re strengthened with a
compound called lignin. The water in the xylem can only
move in one direction, up, from the soil towards the leaves. And it’s common for xylem tubes to
bundle together. If we take a closer look at one
xylem tube, we see it’s made out of dead cells that are stuck together that have no
end walls between them. And that’s why they form a hollow
tube.
Water molecules and dissolved
mineral salts adhere to the sides of the xylem tube as well as to each other as they
move up the tube towards the leaves. And they can’t move down in a xylem
tube. The flow is only from the roots
towards the leaves, and we’ll catch up with those water molecules in the leaves
shortly. And the cell walls that make up the
xylem tube are strengthened by a compound called lignin.
Next, let’s take a look what
happens once the water reaches the leaves. The outermost layer of cells around
a plant is called the epidermis, and the cells of the epidermis are called the
epidermal cells. These cells are covered on the
outside by a waxy layer that’s called the cuticle. It covers most of the plant, and it
keeps the plant from drying out. Epidermal cells are typically
transparent and provide protection. But specialized epidermal cells,
called guard cells, which we’ll review shortly, are responsible for gas
exchange. And as we’ve seen in the roots,
water and mineral absorption also occur through the epidermis.
Palisade mesophyll tissue contains
cells with a large number of chloroplasts. And since chloroplasts are where
photosynthesis occurs, the palisade mesophyll tissue is the major site of
photosynthesis. So that’s why the epidermal cells
are largely transparent. Light can move through the
epidermis to the chloroplasts of the palisade mesophyll cells. There, the light powers
photosynthesis, which produces sugar that the plants can use as chemical energy. The cells of the palisade mesophyll
are very tightly packed together, which is another adaptation for increasing
photosynthesis and sugar production, which brings us back to the xylem. Another reactant required for
photosynthesis is water, and the xylem has brought water up from the soil for the
plant to use for photosynthesis.
The layer of cells beneath a
palisade mesophyll is called the spongy mesophyll. And it consists of loosely packed
cells that contain fewer chloroplasts that absorb any light that’s been transmitted
through the palisade mesophyll. The cells of the spongy mesophyll
absorb water from the xylem and evaporate excess water into the space between the
cells. Beneath the spongy mesophyll is
another layer of epidermis and waxy cuticle. This lower layer of epidermis
contains specialized structures called stoma, or plural stomata. When plants have plenty of water,
their stomata may be open, like this one here. That allows water that’s been
pulled up from the soil through the xylem to evaporate through the opening into the
atmosphere.
The movement of water through a
plant from the soil up through its leaves and back out to the air is called
transpiration. But if a plant is stressed for
water, it can close its stomata and stop this final step of transpiration, keeping
the water inside the leaf. So we followed the route of water
through the root hairs into the xylem of the plant through the xylem of the roots
and the stem up through the leaves or exits through the stomata back to the
atmosphere. And that’s called
transpiration.
And here’s a little more space to
see how plants transport the sugar that they make in their leaves during
photosynthesis to their other parts using a tissue called phloem. The ingredients or reactants of
photosynthesis are water and carbon dioxide, and we’ve already seen how the leaves
get water. But how do they get carbon
dioxide?
Remember how stomata open when a
plant has enough water? Stoma are made out of two guard
cells that expand and open when there’s enough water or they contract when there’s a
lack of water to close the stoma. This not only allows for the
transpiration of water from the leaves, it also allows the leaf to exchange gases
with the environment, including carbon dioxide. So as long as the stomata are open,
carbon dioxide will diffuse or move down its concentration gradient from the
atmosphere into the spongy mesophyll of the leaf, while water diffuses back out to
the atmosphere.
Now, the chloroplasts, which are
mostly contained in the palisade mesophyll cells, use the carbon dioxide and water
along with light energy from the Sun to produce sugar using the process of
photosynthesis. This sugar, along with other
substances, such as amino acids, is transported by the phloem to areas of the plant
that need it, such as the roots, flowers, or fruits. The chemical reactions of
photosynthesis not only produce sugar as a product, oxygen is produced as well. And it diffuses into the spaces of
the spongy mesophyll and then out through the stomata as long as there’s enough
water to keep them open.
Xylem and phloem are often bundled
closely together and are called vascular bundles. But the structure of phloem is
different from that of xylem. So let’s take a look at that
next. Like xylem, phloem contains long
tubes, but in phloem they’re called sieve tubes. And instead of facilitating
transpiration as does xylem, phloem enables a plant to translocate substances,
including the sugars made in photosynthesis, from where they’re made to where
they’re needed. The sieve tube is made out of
hollow living cells called sieve tube elements that have reduced sets of organelles
to allow for the transport of materials. Between each sieve tube element is
a porous plate through which materials can flow. And these are called the sieve tube
plates.
Since sieve tubes are hollow and
have a reduced set of organelles, they need companion cells for support. Companion cells have many
mitochondria to provide energy to actively transport materials from where they’re
made to where they’re needed. And in addition, they also provide
proteins that the sieve tube elements can’t make on their own. And that’s a lot of specialized
plant structures. So we’ll review them at the end of
the video. But before we do that, let’s work a
practice question.
Palisade cells near the top of the
leaf contain many chloroplasts. Which of the following best
explains why? (A) Palisade cells contain many
chloroplasts to capture the sunlight needed for respiration. (B) Palisade cells contain many
chloroplasts to allow the maximum diffusion of gases. (C) Palisade cells contain many
chloroplasts to expand the cell and increase the surface area available for water
uptake. (D) Palisade cells contain many
chloroplasts to capture the sunlight needed for photosynthesis.
Since the question uses the term
leaf, we know that we’re dealing with a plant. And it mentions a type of cell
that’s found in the leaf. So we’ll need to know about some
plant cells as well. Another key term in the question is
chloroplasts. Chloroplasts are organelles, and
organelles are structures and cells that are responsible for specific functions. Let’s take a look at the terms and
the solution options to see what else we need to know about these plant cells.
Option (A) contains the term
respiration, and respiration occurs in an organelle called the mitochondrion. Option (B) mentions the diffusion
of gases between the interior of a leaf and the environment, which occurs through a
structure called a stoma that’s formed by two guard cells. Option (C) contains the term water
uptake, but that actually occurs in the roots, especially through some tiny
structures called root hairs that increase the surface area where water uptake
occurs. Option (D) mentions the process of
photosynthesis. And photosynthesis can be
summarized by the equation below, where carbon dioxide and water react using the
energy in sunlight to form sugar and oxygen.
So let’s make some connections
between these structures on our diagram. First, water is taken up by the
root hairs. It moves through a tissue called
the xylem, where it finally gets to the leaf. As long as there’s enough water in
the guard cells of the stoma, they’ll stay expanded, which keeps the stoma open for
gas exchange, including the diffusion of CO2 from the atmosphere down its
concentration gradient into the area of the leaf known as the spongy mesophyll. Plants use the carbon dioxide and
water in the process of photosynthesis to make sugar and oxygen. The sugar can be used by the
mitochondria in the process of cellular respiration. The oxygen can be used by the
mitochondria in cellular respiration, or it can exit into the atmosphere by
diffusing down its concentration gradient.
So option (A) must be incorrect
because cellular respiration occurs in the mitochondria and not in the
chloroplasts. Option (B) is incorrect because the
diffusion of gases takes place in the plants’ stomata located on the underside of
their leaves. And option (C) is incorrect because
water uptake occurs in the root hairs, not in the chloroplasts. Option (D) says that chloroplasts
capture sunlight that’s needed for the process of photosynthesis. And that is indeed true. That’s what happens in the
chloroplast.
It’s interesting to note though
that we didn’t even have to know what a palisade cell was to be able to answer this
question correctly. So if you don’t know a term in a
question, don’t let it scare you away. Look at the rest of the question
and try to figure out what you can. On the other hand, it’s even better
to know what a palisade cell is. Palisade cells make up the palisade
mesophyll tissue, which is directly beneath the epidermis and cuticle on the top of
the leaf. And they contain many
chloroplasts. And the tissue beneath the palisade
mesophyll is called the spongy mesophyll. It’s called spongy because there’s
a lot of air pockets, and that allows the gases to diffuse through.
So the answer to the question
“Palisade cells near the top of the leaf contain many chloroplasts. Which of the following best
explains why?” is option (D). Palisade cells contain many
chloroplasts to capture the sunlight needed for photosynthesis.
Here’s some key points from the
video. The main theme is that plant
structures have specialized adaptations that allow them to carry out their
function. Root hairs have a large surface
area to increase water uptake, and they have a high concentration of solutes to
promote osmosis into the roots. Water and minerals are transported
from the roots up towards the top of the plant through a tissue called xylem. Xylem is made out of cells that
have died and are hollow and they form tubes.
If we look at a cross section of a
leaf through the microscope, the first layer would be the waxy cuticle, which helps
keep the plant from drying out. The cells beneath the waxy cuticle
are called the epidermal cells. And they again are protective and
transparent so that they can allow light through. The tightly packed palisade cells
of the palisade mesophyll have many chloroplasts, and the main function of this
tissue is photosynthesis. Spongy mesophyll tissue is composed
of loosely packed cells to allow for gas exchange.
Closable pores on the underside of
leaves are called stomata. Each stoma is made out of two guard
cells that control the diffusion of water, carbon dioxide, and oxygen in and out of
the leaf. The sugar that’s made in
photosynthesis is transported from the leaves to other areas of the plant in a
tissue called phloem. While xylem is made out of dead
cells that have very thick walls that are strengthened by a compound called lignin,
phloem is made out of living sieve tube element cells supported by companion
cells.
