Plants come in all shapes and sizes and their leaves have many interior
structures that each have a specific function.
This video will cover the structure and function of the interior of a leaf
and connect the relevant parts back to the process of photosynthesis. If you'd like a
little extra review, be sure to check out the study guide in the video description.
During photosynthesis, plants use energy from sunlight to jumpstart the conversion
of carbon dioxide and water into glucose and oxygen. As we go through this video,
notice which structures are handling each of these reagents and products.
There are two major types of leaves; simple and compound. They both gather light from
the sun so plants can form sugars and other carbon-rich compounds through photosynthesis.
The uppermost layer of the leaf is called the waxy cuticle. This is a very thin hydrophobic
layer that's on the surface of most leaves. This thin coating helps to reduce water loss
via transpiration and it's why water will bead on the surface of leaves rather than soaking in.
The first true layer of cells is called the upper epidermis.
These cells are tightly packed in order to help prevent water loss.
In very hot or very cold climates sometimes this layer can be several layers of cells thick.
Because the upper epidermis is translucent it allows light to pass through for photosynthesis.
The next layer is called the palisade mesophyll, sometimes called the palisade parenchyma.
Palisade mesophyll cells are tightly packed together and column shaped. Because this is
the major site of photosynthesis, they are full of chloroplasts and there are often several layers
of this type of cell. Photons of light will shine down through the waxy cuticle and upper
epidermis in order to strike the chloroplasts that are located within this layer of tissue.
Beneath the palisade mesophyll is the spongy mesophyll.
The spongy mesophyll can also be called the spongy parenchyma.
These cells are more irregularly shaped relative to the palisade mesophyll but they do contain a
few chloroplasts so some photosynthesis does take place here. The space in between these
cells is called the intercellular space and this allows for gas exchange in and out of the leaf.
The next structure, called the vascular bundle, consists of several different types of tissues.
The first tissue is known as xylem. Xylem is a vascular tubule within the plant and its flow runs
upwards. It's used to transport water from the roots to the rest of the plant where it's needed.
Remember that water is a necessary reagent of photosynthesis.
Phloem is xylem's counterpart. These are also vascular tubules within the plant
but its flow runs both up and down. It's used to transport
the products of photosynthesis from the leaves to the rest of the plant.
Surrounding the xylem and phloem is a network of tightly packed cells called
the bundle sheath cells. In some plants bundle sheath cells allow for a specialized version
of photosynthesis that allow plants to survive in "arid" or dry environments. We can see the
role that bundle sheath cells play in this diagram of the C4 photosynthesis pathway.
The lower surface of the leaf also contains a layer of epidermal cells
called the lower epidermis. This is also meant to prevent water loss.
Within the lower epidermis we can sometimes see a small
opening called a stoma. This series of openings allows for gas exchange in and out of the leaf.
Ideally carbon dioxide will diffuse in and oxygen will diffuse out. Each stoma has two guard cells,
one on either side, which can open and close in response to the environment. If it's very hot,
the plant will likely keep its stomata closed in order to minimize water loss via transpiration.
Most of the stomata are located on the lower surface of the leaf rather than the upper.
In order to make glucose as well as a number of other plant products, plants need carbon.
To get the carbon that they need, plants have to extract it from the atmosphere in a process
called carbon fixation. During photosynthesis, they take atmospheric carbon from carbon dioxide
and convert it into organic carbon contained within molecules like glucose.
For plants that live in a hot environment, this is a delicate balancing act. They need to keep
their stomata open enough of the time to get carbon dioxide in, but not so often that they
allow themselves to dry out. C4 and CAM plants which live in dry environments have a number
of different ways of getting around this problem, including using a modified form of photosynthesis.
The lower portion of the leaf usually also has a waxy cuticle, much like the upper portion.
Many leaves have additional structures on their surface known as trichomes. These can perform a
variety of different functions but they often make the leaf appear a bit fuzzy. These tiny hair-like
structures can deter plant-eating organisms, and can also restrict insects from crawling over the
surface of the leaf. Some of them also store oily compounds with powerful chemical properties known
as terpenes. Lavender, pine, citrus fruits, and tetrahydrocannabinol all contain terpenes,
which is what gives each its distinctive smell. As you can see from these drawings, terpenes
tend to contain a lot of hydrocarbons. They're extremely non-polar and thus non-soluble in water.
This makes them an ideal defense mechanism for plants because they won't wash off when it rains.
To finish up examine this microscopic cross-section of an actual dicot leaf
and see how many structures you can identify.
In addition look at the reagents and products of photosynthesis
and see if you can remember which structures are associated with each one.
That wraps up this review of leaf structure and its cross section.
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