Plant Systems and Photosynthesis Review

Definition and Function: The root is the underground part of the plant. It serves two primary purposes:
- Anchoring the plant firmly in the soil.
- Absorbing water and essential minerals from the soil.
External Anatomy of the Root:
- Root Cap: This structure protects the apical meristem as the root pushes through the soil. It is critical for the downward growth of the root.
- Apical Meristem: Located directly behind the root cap, this region is responsible for producing new root cells.
- Zone of Elongation: This area follows the apical meristem, where newly formed cells increase in length, thereby elongating the root.
- Root Hairs: These are found in the zone of maturation. They help anchor the plant and are the primary site for the absorption of water and mineral nutrients from the soil.

Epidermis: The outermost layer of the root. It features root hairs that significantly increase the surface area for the absorption of water and minerals.
Cortex: Situated below the epidermis, the cortex is composed of large, thin-walled cells.
Endodermis: The innermost layer of the cortex. It is uniquely present in roots and functions to control the movement of absorbed materials into the vascular center.
Conducting Tissues: Located at the center of the root, these tissues facilitate transport:
- Xylem: Carries water and salts absorbed from the soil to the upper parts of the plant.
- Phloem: Responsible for transporting manufactured food within the various parts of the root.

Shoot System Overview: The shoot system consists of all plant parts that grow above the soil, including the stem, leaves, flowers, and buds.
The Stem:
- Functions: Provides structural support, holds leaves, flowers, and buds, and connects the roots to the leaves for the transport of water, minerals, and food.
- Variability: Stem length ranges from a few millimeters to hundreds of meters depending on the plant species.
- Location: While usually above ground, some plants like the potato have underground stems.
External Structure of the Stem:
- Nodes: The specific points on the stem from which leaves or buds grow.
- Internodes: The segment of the stem located between two consecutive nodes.
- Buds: Specialized structures on the stem that develop into flowers, leaves, or branches.

Epidermis: A single layer of cells covering the stem to protect internal tissues.
Cortex: A layer roughly $4$ to $5$ cells thick located just beneath the epidermis. This layer contains chloroplasts.
Endodermis: The innermost layer of the cortex, consisting of a single row of cells.
Vascular Components:
- Stele: Located below the endodermis, containing the xylem and phloem.
- Xylem: Transports water and minerals from the roots to the leaves.
- Phloem: Transports prepared food from the leaves to other plant parts.
- Cambium: A layer of tissue situated between the xylem and the phloem.

Function and Appearance: Leaves are thin, broad, flat, and green structures attached to the stem. They are the primary sites for photosynthesis.
External Parts of a Leaf:
- Petiole: The stalk-like structure that joins the leaf lamina to the stem.
- Leaf Lamina (Leaf Blade): The broad part of the leaf. It includes the midrib (the main vein) and smaller veins. The veins contain the leaf's vascular system.
Internal Anatomy of a Leaf:
- Epidermis: The outermost layer, divided into the upper and lower epidermis.
  - The lower epidermis contains many tiny openings called stomata (singular: stoma) for gas exchange.
  - Each stoma is bordered by two guard cells which regulate opening and closing.
  - Environmental Adaptation: While usually one cell thick, plants in very cold conditions may have a several-layered epidermis to protect against excessive water loss.
- Mesophyll: Layers of cells located between the upper and lower epidermis.
  - Can consist of one, two, or three layers.
  - Highly concentrated with chloroplasts containing chlorophyll, which traps light energy for photosynthesis.
- Vascular Bundle: Distributed through the veins within the mesophyll. It contains:
  - Xylem: Brings water and minerals from the roots.
  - Phloem: Transports synthesized sugars from the leaf to the rest of the plant.

Guard Cell Regulation:
- Daytime: Guard cells absorb water and swell, which results in the opening of the stomata.
- Nighttime: Guard cells release water and shrink, resulting in the closing of the stomata.
- Note: Guard cells are the only epidermal cells that contain chloroplasts.
Photosynthesis: The process of synthesizing glucose from carbon dioxide and water using light energy and chlorophyll. Oxygen is released as a byproduct.
Chemical Equation for Photosynthesis: $6CO_2 + 6H_2O \xrightarrow{\text{Light energy, Chlorophyll}} C_6H_{12}O_6 + 6O_2$

General Requirements: Plants need Carbon ( $C$ ), Hydrogen ( $H$ ), and Oxygen ( $O$ ) along with other minerals. $C$ and $O$ are absorbed from the air, while other minerals come from the soil.
Nitrogen ( $N$ ):
- A primary component of proteins, nucleic acids, vitamins, and chlorophyll.
- Absorbed as nitrates ( $NO_3^-$ ) through the roots.
- Required for healthy growth; deficiency leads to reduced chlorophyll, poor photosynthesis, and lower crop yields.
- Farmers often use fertilizers to boost soil nitrate levels.
Magnesium ( $Mg$ ):
- A structural component of the chlorophyll molecule.
- Necessary for the production of sugars and carbohydrates.
- Deficiency leads to the wilting and yellowing of leaves.

Question 1: What are the functions of root hairs?
Question 2: Define node and internode.
Question 3: What do xylem vessels carry?
Question 4: Which substance makes the cell wall of phloem vessels?
Question 5: State two ways in which phloem vessels differ from xylem vessels?