AICE Biology Chapter 7

7.1 The Transport Needs of Plants

• Plants vs. Animals

  • Animals consume food; plants produce their own nutrients through photosynthesis.

  • While plants absorb water and minerals from the soil, animals must actively seek food sources.

• Structure & Function

  • Leaves: The primary site for photosynthesis, requiring light; thus, they are typically positioned at the top of the plant to maximize light absorption.

  • Roots: Responsible for absorbing water and minerals from the soil; they extend deep underground to access essential nutrients.

  • Plants effectively spread above ground to capture sunlight and below to secure nutrients and water.

• Transport in Plants

  • Essential materials must travel long distances to reach all plant cells.

  • Sugars (e.g., glucose), generated in the leaves via photosynthesis, are transported to roots and other plant parts for respiration and cellulose synthesis.

  • Minerals (e.g., magnesium), absorbed by roots, are moved to leaves where they play a crucial role in chlorophyll production.

7.2 Vascular System: Xylem and Phloem

• Why Plants Need a Transport System

  • Plants require the movement of water, minerals, and nutrients throughout their structures.

  • These substances dissolve in water, allowing them to travel through specialized tubes known as the vascular system.

• Vascular System in Plants vs. Animals

  • Plants: Composed of Xylem and Phloem.

  • Animals: Consist of a blood vascular system (which carries blood) and a lymphatic system (which transports lymph).

• Xylem & Phloem (Vascular Tissue)

  • Xylem:

    • Responsible for transporting water and mineral ions from roots to aerial parts.

    • Features one-way flow (primarily upward).

    • The fluid transported is called xylem sap, which moves within xylem vessels.

  • Phloem:

    • Carries sugars and nutrients from leaves to various parts of the plant.

    • Exhibits two-way flow (upward or downward).

    • The fluid is referred to as phloem sap, which is transported in sieve tubes.

• Movement in Xylem vs. Phloem

  • Transport via xylem is generally slower compared to blood circulation in animals.

  • Unlike blood circulation, there is no pump equivalent to the heart in animals.

Key Terms

• Vascular system: A system of fluid-filled tubes that facilitates long-distance transport in living organisms.

• Vascular: Pertaining to the tubes or vessels (derived from the Latin word vascul, meaning vessel).

7.3 Structure of Stems, Roots, and Leaves & Distribution of Xylem and Phloem

• Main Transport Organs in Plants

  • The stems, roots, and leaves serve as the key organs for transportation.

  • Their structures are best examined using transverse sections (TS) under a microscope.

• Dicotyledons vs. Monocotyledons

  • Dicotyledons: Characterized by broad leaves that have a stalk (petiole).

  • Monocotyledons (e.g., grasses): Exhibit long, narrow leaves (not included in syllabus).

  • Both groups contain xylem and phloem, but their distribution varies significantly.

• Xylem & Phloem Distribution

  • In stems and leaves: Found within vascular bundles that include supporting tissues.

  • In roots: Centrally located.

• Key Tissues in Stems, Roots, and Leaves

  • Xylem: Transports water and minerals, made up of large vessels, typically stained red for identification.

  • Phloem: Responsible for transporting sugars and nutrients, contains small sieve tubes, usually stained green.

  • Epidermis: A protective outer layer, generally only one cell thick.

  • Parenchyma: Acts as packing tissue among structures, utilized for support and food storage.

  • Collenchyma: Provides strength with thicker walls, predominantly found in leaf midribs and stem edges.

  • Sclerenchyma: Composed of thick-walled support cells, often identified in vascular bundles, likewise stained red due to the presence of lignin.

  • Endodermis: An inner protective layer surrounding vascular tissue, crucial for transport (most observable in roots).

Key Terms

• Vascular bundle: A strand of vascular tissue present in plants.

• Parenchyma: The basic packing tissue in plants that aids in food storage and transport.

• Collenchyma: A strengthening tissue featuring extra cellulose in the cell walls.

• Epidermis: The outer protective layer of plants.

• Endodermis: An inner protective layer encircling vascular tissues, noticeably seen in roots.

• Sclerenchyma: Thick-walled cells that provide support and strength.

• Lignin: A robust material strengthening xylem and sclerenchyma.

7.4 Transport of Water in Plants – Study Notes

1. Overview of Water Transport

   • Water longitudinally moves from higher water potential (soil) to lower water potential (leaves).

   • This movement is primarily driven by transpiration (the evaporation of water from leaves).

   • The transpiration process creates a water potential gradient, pulling water from roots to leaves.

2. Transpiration – Loss of Water from Leaves

   • Definition: The process of losing water vapor from plant leaves, predominantly through stomata.

   • Process:

     1. Water evaporates from the walls of mesophyll cells into the air spaces.

     2. Water vapor diffuses out through open stomata.

     3. This reduction in water potential within the leaf necessitates the upward movement of more water.

   • Stomata typically open during the day, enhancing transpiration.

3. Water Movement Through a Plant

   • Water uptake: Occurs when water enters the roots near their tips.

   • Once in the root, water moves across root cells, entering the xylem.

   • Water travels upwards through the xylem vessels to reach the stem.

   • Upon arrival at the leaves, water moves into mesophyll cells.

   • Finally, transpiration occurs as water evaporates and exits through the stomata.

4. Pathways of Water Movement

   • Symplast Pathway: Water transfers through the cytoplasm via plasmodesmata (small channels between cells).

   • Apoplast Pathway: Water moves through the cell walls, avoiding the cytoplasm.

5. Xylem – Water Transport Tissue

   • Structure:

     • Composed of dead, hollow cells joined end to end, designed for optimal flow.

     • Reinforced with lignin to strengthen walls and prevent collapse under pressure.

     • Pits in xylem vessels allow for inter-vessel water movement.

   • Function:

     • Transports water through mass flow (the principles of cohesion and adhesion support this process).

     • Provides essential structural support for the entire plant.

6. Root Structure & Water Uptake

   • Root hairs: Tiny extensions that enhance the surface area for effective absorption.

   • Casparian Strip (Endodermis):

     • Acts as a waterproof barrier blocking the apoplast pathway, ensuring only selected substances enter the vascular system.

     • This forces water through cell membranes, controlling the entry of ions and nutrients.

7. Adaptations in Xerophytes

   • Xerophytes: Special plants that have evolved to thrive in dry environments.

   • Their adaptations include:

     • A thick cuticle that minimizes water loss.

     • Sunken stomata that trap moisture and diminish evaporation.

     • Hairy leaves which contribute to reducing water loss through transpiration.

Key Terms

• Transpiration: The process of losing water vapor from the plant surfaces.

• Mesophyll: The leaf tissue responsible for gas exchange and photosynthesis.

• Stomata: Pores on leaves that facilitate gas exchange, regulated by guard cells.

• Xylem: Tissue that fluidly transports water and minerals.

• Symplast Pathway: Water movement through living cells.

• Apoplast Pathway: The route water takes through cell walls.

• Casparian Strip: The impermeable barrier in root endodermis.

• Xerophyte: Plants specially adapted to arid conditions.

7.5 Transport of Assimilates

• What Are Assimilates?

  • Assimilates are organic compounds created by plants through a process known as assimilation.

  • Assimilation refers to the conversion of inorganic nutrients into organic substances that can be utilized by the plant.

  • Examples include:

    • Photosynthesis: This process converts CO₂ and H₂O into sugars.

    • Nitrate assimilation: The conversion of nitrates into amino acids.

  • Common assimilates transported in the phloem include:

    • Sucrose

    • Amino acids

• Source & Sink Concept

  • Source: The location where assimilates are produced or stored (e.g., leaves, tubers).

  • Sink: The destination where assimilates are consumed or stored (e.g., buds, flowers, roots, storage organs).

  • In phloem, transport occurs from source to sink.

• Phloem Structure

  • Main cells for transport:

    • Sieve tube elements: Form sieve tubes, characterized by minimal cytoplasm and lack of a nucleus.

    • Companion cells: These cells support sieve tubes and are rich in mitochondria and ribosomes.

  • Sieve tube elements:

    • Aligned end-to-end to create long sieve tubes necessary for efficient sap flow.

    • Features sieve plates (porous end walls) enabling the flow of sap.

    • Limited cytoplasm and absence of a nucleus, promoting efficient transport.

  • Companion cells:

    • Linked to sieve tube elements via plasmodesmata.

    • Provide vital metabolic support necessary for the functioning of sieve tubes.

• Transport in Sieve Tubes (Mass Flow Hypothesis)

  • Mass flow refers to the movement of phloem sap driven by differences in pressure.

  • The process is active (unlike passive transport in xylem).

  • How it works:

    1. Loading sucrose at the source:

       • Sucrose is actively transported into the sieve tube, decreasing water potential.

       • Water then enters via osmosis, raising hydrostatic pressure.

    2. Mass flow from high to low pressure:

       • Sap moves from areas of high pressure (source) to low pressure (sink).

    3. Unloading at the sink:

       • At this location, sucrose is removed, which causes water to follow by osmosis, lowering the pressure.

  • Phloem transport is bidirectional, allowing movement both upwards and downwards in different tubes.

• Loading of Sucrose into Phloem

  • There are two pathways:

    • Symplast pathway: Moves through plasmodesmata directly from cell to cell.

    • Apoplast pathway: Moves along cell walls without crossing through the cytoplasm.

  • Active transport in companion cells:

    1. H⁺ ions are pumped out of companion cells into the cell wall using energy (ATP).

    2. H⁺ ions then diffuse back into the companion cell via a co-transporter, carrying sucrose along with them.

    3. Sucrose ultimately moves into the sieve tube through plasmodesmata.

Key Terms

• Source: The point at which food (e.g., sucrose) is produced or stored.

• Sink: The location where food is utilized or stored.

• Sieve tube element: A phloem cell featuring sieve plates, minimal cytoplasm, and devoid of a nucleus.

• Companion cell: A phloem cell that provides support for sieve tubes, high metabolic activity.

• Sieve tube: A structure formed by the alignment of sieve tube elements end-to-end.

• Phloem sap: The fluid within sieve tubes, containing substances such as sucrose and amino acids.

• Hydrostatic pressure: Pressure created by water movement within the phloem.

• Mass flow: The sap movement from areas of high pressure to low pressure within sieve tubes.

• Co-transporter: A protein facilitating the movement of H⁺ ions and sucrose into the companion cell.

Chapter 7 Summary:

1. Xylem: Water & Mineral Transport

   • Xylem is responsible for moving water and minerals from the roots upward to the leaves.

   • This transport is passive, requiring no energy expenditure.

   • Water moves down a gradient of water potential from soil → roots → stem → leaves → air, driven by solar energy fueling transpiration.

2. Transpiration

   • Defined as the loss of water vapor from leaves via stomata.

   • Water evaporates from mesophyll cell walls, transitions into air spaces, and exits through stomata.

   • This process establishes a water potential gradient that facilitates upward water movement.

3. Adaptations of Xerophytes (Plants in dry environments)

   • Xerophytes have developed several strategies to minimize water loss, including:

     • Thick cuticles to resist moisture evaporation.

     • Sunken stomata to conserve water.

     • A reduced number of stomata and hair-like structures to trap moisture.

4. Water Movement in the Leaf

   • Apoplast pathway: Involves the movement through cell walls.

   • Symplast pathway: Involves movement through the cytoplasm via plasmodesmata.

5. Water Transport in Xylem (Mass Flow)

   • Xylem sap rises due to:

     • Cohesion (water molecules' ability to stick to one another).

     • Adhesion (water molecules adhering to xylem walls).

     • Transpirational pull (the upward force generated by water loss from leaves).

   • Water enters roots by osmosis through root hairs, subsequently reaching the xylem.

6. Phloem: Transport of Organic Solutes

   • The phloem transports sucrose and amino acids from source to sink.

   • The source is where sucrose is synthesized (e.g., in leaves), and the sink is where sucrose is utilized or stored (e.g., in flowers, roots, tubers).

7. Phloem Transport (Mass Flow in Sieve Tubes)

   • Involves the active loading of sucrose at the source, which decreases water potential in the sieve tube.

   • Water enters the sieve tube through osmosis, generating high hydrostatic pressure.

   • Sap then flows from high pressure (source) to low pressure (sink).

   • Phloem transport is characterized as bidirectional, exhibiting movement in varying directions across different tubes.

8. Structural Adaptations

   • Xylem vessels possess thick walls, lack cytoplasm, and lack end walls, resulting in continuous tubes ideal for water flow.

   • Phloem sieve tubes are characterized by sieve plates, minimal cytoplasm, and companion cells that supply necessary energy and support.