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The light is absorbed in the leaves of the green plants so that they can convert it to energy by a process known as photosynthesis. Carbon dioxide concentration, light intensity, and temperature are the limiting factors of photosynthesis. In this article, we will discuss how the structure of the leaf is adapted to carry out the process efficiently and how xylem and phloem tissues play a role in the transportation of substances in the plant. So, let us get started.
Leaves and Photosynthesis
The leaf structure is specially adapted to carry out the process of photosynthesis effectively. To photosynthesize, a leaf needs the following things:
- A mechanism to carry water to the leaf and glucose to the remaining parts of the plant
- A mechanism to exchange oxygen and carbon dioxide
- A capacity for efficient absorption of light energy
Transportation
Water is delivered from the roots to the leaf through a xylem tissue and phloem tissue is responsible for transporting glucose away from the leaf. Both xylem and phloem tissues create vascular bundles in plants.
Gas exchange
Gas exchange occurs in the spongy mesophyll tissue of the leaf. Spongy mesophyll cells are loosely arranged and are covered by a thin layer. During the day, when the plant is photosynthesizing, these characteristics enable carbon dioxide to diffuse into the spongy mesophyll cells, and oxygen to diffuse out of it.
Gasses diffuse through tiny pores referred to as stomata to reach the spongy mesophyll cells inside the leaf. They control the loss of water from the leaf by opening or closing through a process known as transpiration. Stomata are spaces present between two guard cells mainly on the leaf’s lower epidermis. The guard cells help to open or close the stomatal pore which regulates water loss and exchange of gases.
Absorbing light energy
Light absorption occurs in the palisade mesophyll tissue of the leaf. The column-shaped palisade cells are packed with several chloroplasts. They are tightly packed or arranged closely together so that plenty of light energy can be absorbed.
Leaf Adaptations For Photosynthesis
- Larger surface area: Leaves have a large surface area to volume ratio that allows for maximum absorption of light energy
- Thin: They are thinner to provide a shorter distance for carbon dioxide to diffuse into the leaf cells.
- Thin waxy cuticle: They have a thin waxy cuticle that acts as a protection for the leaf without blocking out the light
- Thin transparent epidermis: Leaves have a thin transparent epidermis that allows the light to reach palisade cells.
Other adaptations of the leaf structure include:
- Air spaces: The air spaces allow the movement of gas around the mesophyll cells (loosely arranged).
- Stomata: Several stomata in lower epidermis open in sunlight to enable movement of gas in and out of the leaf
- Moist air: Moist air allows gases to dissolve so that they can easily move in and out of the cells.
- Close contact: The close contact between the air spaces and the cells enables the efficient exchange of gases for reparation and photosynthesis.
Xylem and Phloem for Transportation
The two different types of vascular tissues are the xylem and phloem. These two tissues play a major role in the transportation process. Both xylem and phloem create a vascular bundle and work as the unit. Xylem is characterized by its unidirectional movement, whereas the phloem by its bidirectional movement.
In this section, we will discuss xylem and phloem for transportation in detail.
Xylem
Xylem tissue in plants transports water and minerals from the roots up to the stem and into the leaves of the plants. Xylem is composed of dead cells. The cells that make up the xylem tissue are specially adapted to perform specific functions:
- Xylem forms a continuous hollow tube as they lose their end walls.
- A substance known as lignin strengthens this tissue. Lignin not only strengthens but also supports the plant. The lignified cells are known as wood.
Location
Xylem tissue is present along with the phloem and other tissues in vascular bundles. The location of vascular bundles depends on the type of organ in which they are present.
- Roots: In the roots, the vascular bundle is present in the center and the xylem tissue tp the center core. This assists the roots to bear the pulling stress they are subjected to because the plant carries water upwards and grows.
- Stems: In the stems, the vascular bundles are present around the outside and the xylem tissue is present on the inside (nearest to the stem’s center) to support the plant
- Leaves: In the leaves, the vascular bundles create the veins and midrib. Hence, they spread from the leaf’s center in a parallel line. The xylem tissue is present on the upper area of the bundles (nearest to the upper epidermis).
Functions
Xylem tissues in plants perform the following functions in plants:
- These vascular tissues transport dissolved minerals and water up the plant
- They provide structural support to the plant
- They help in food storage
Phloem
It is a complex tissue composed of different types of cells. Its bulk is composed of sieve tube elements which are the primary conducting and companion cells. Parenchyma is other cells type of phloem.
Plants produce their food through photosynthesis. The phloem tissue in plants moves food substances created by the plant to the parts where they are required for certain processes like:
- Growing regions of the plant for instant use
- Storage organs like tubers and bulbs
- Seeds that are developing
Transportation in phloem occurs both up and down the stem. The transport sugar in the phloem is sucrose. The substances are transported in the phloem through a process known as translocation. Translocation is an active process as it needs energy.
Location
The location of vascular bundles depends on the kind of organ they are located in because different organs are under different strains:
- Roots: In the roots, the vascular bundle is present in the center and the phloem tissue on the edges of the center core.
- Stems: In the stems, the vascular bundles are present around the outside, while the phloem tissue is present on the outside (nearest to the epidermis).
- Leaves: In the leaves, the vascular bundles create the veins and midrib. Hence, they stretch from the center of the leaf in a parallel line. The phloem tissue is present on the lower side of the bundles (nearest to the lower epidermis).
Functions
In the plants, the phloem tissues perform the following functions:
- They carry organic compounds, especially sucrose from the source (for instance leaf) to the sink (for instance roots). These compounds can be transported up and down the plant.
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