Plant-Tissues-Organs and Systems

• Important plant tissues:

  • Epidermal tissues
  • Palisade mesophyll
  • Spongy mesophyll
  • Xylem and phloem
  • Meristem tissue found at the growing tips of shoots and roots
• The structures of plant tissues are related to their functions
• Epidermal tissues are the outermost layer of cells in a plant, providing protection and regulating gas exchange
• Palisade mesophyll is a layer of elongated cells containing many chloroplasts, responsible for photosynthesis
• Spongy mesophyll is a layer of loosely packed cells with air spaces, facilitating gas exchange and photosynthesis
• Xylem and phloem are vascular tissues responsible for transporting water, minerals, and sugars throughout the plant
• Meristem tissue found at the growing tips of shoots and roots is responsible for plant growth and development
• Cuticle:

  • The cuticle is a waxy, waterproof layer that reduces water loss by evaporation
  • It also protects against parasitic fungi
• Upper Epidermis:

  • A single layer of transparent cells with no chloroplasts, allowing light to pass through
• Palisade Layer:

  • Made up of palisade cells containing chloroplasts
  • This is where most photosynthesis occurs
• Vein:

  • Contains xylem and phloem tubes
  • Xylem brings water and salts to the leaf for photosynthesis
  • Phloem transports dissolved foods away
• Spongy Layer:

  • Consists of irregularly shaped cells with large air spaces between them
  • Allows gas exchange (diffusion) between stomata and photosynthesizing cells
• Lower Epidermis:

  • Contains numerous tiny holes or pores called stomata at regular intervals
  • These stomata allow gases to diffuse in and out of the leaf
• Adaptations of the leaf:

  • Large surface area: to absorb more light
  • Thin: short distance for carbon dioxide to diffuse into leaf cells
  • Chlorophyll: absorbs sunlight to transfer energy into chemicals
  • Network of veins: to support the leaf and transport water, mineral ions and sucrose (sugar)
  • Stomata: allow carbon dioxide to diffuse into the leaf and oxygen to diffuse out
• Plant organs:

  • The structure of root hair cells, xylem, and phloem are adapted to their functions
  • The roots, stem, and leaves form a plant organ system for transport of substances around the plant
• Functions of xylem & phloem:

  • Plants contain two types of transport vessels: xylem and phloem
  • Xylem vessels transport water and minerals from the roots to the stem and leaves
  • Xylem is composed of hollow tubes strengthened by lignin, adapted for the transport of water in the transpiration stream
  • Phloem vessels transport food materials (mainly sucrose and amino acids) from photosynthesizing leaves to non-photosynthesizing regions in the roots and stem
  • Xylem and phloem vessels are arranged throughout the root, stem, and leaves in groups called vascular bundles
• Root hair cells:

  • Adapted for the efficient uptake of water by osmosis and mineral ions by active transport
  • Single-celled extensions of epidermis cells in the root that increase the surface area significantly for absorption of water and minerals
  • Grow between soil particles and absorb water and minerals from the soil
  • Water enters the root hair cells by osmosis due to soil water having a higher water potential than the cytoplasm of the root hair cell
• Osmosis pathway:

  • Water passes into the root hair cells, through the root cortex, and into the xylem vessels
  • Once in the xylem, water is carried up to the leaves where it enters mesophyll cells
  • Pathway: root hair cell → root cortex cells → xylem → leaf mesophyll cells
• Transpiration is the loss of water vapour from plant leaves by evaporation of water at the surfaces of the mesophyll cells followed by diffusion of water vapour through the stomata
• Water travels up xylem from the roots into the leaves of the plant to replace the water that has been lost due to transpiration
• Xylem is adapted in many ways:

  • A substance called lignin is deposited in the cell walls which causes the xylem cells to die
  • These cells then become hollow and join end-to-end to form a continuous tube for water and mineral ions to travel through from the roots
  • Lignin strengthens the plant to help it withstand the pressure of the water movement
• Movement in xylem only takes place in one direction - from roots to leaves
• • Increase enhances the rate of photosynthesis, causing stomata to open and allowing water to diffuse out of the leaf
• Factors affecting transpiration:

  • Temperature:
  • Increase leads to increased molecular movement, causing more water molecules to evaporate from cell surfaces
  • Increase in temperature also increases the rate of diffusion of water molecules from the leaf
  • Humidity:
  • Decrease reduces the concentration of water molecules outside the leaf, leading to an increase in the diffusion of water from the leaf
  • Air movement:
  • Increase removes water vapor from leaf surfaces, facilitating more water diffusion from the leaf
• • When water availability is low, guard cells lose water by osmosis, becoming flaccid, causing stomata closure and reducing water loss via transpiration
  • Stomata are mostly found on the leaf's underside, where it is cooler and shaded, resulting in lower light intensity, less transpiration, and reduced water loss
• Transpiration (stomata):

  • Stomata can be opened or closed based on plant conditions, controlled by guard cells
  • Stomata and guard cells, mainly located on the underside of the leaf, regulate gas exchange and water loss
  • Guard cells have cell walls with unevenly distributed cellulose, with a thicker inner wall and a thinner outer wall to aid stomata opening and closing
  • When water availability is high, guard cells become turgid due to osmosis, leading to open stomata and increased air circulation but also water loss via transpiration
• Soluble products of photosynthesis are sugars (mainly sucrose) and amino acids, collectively referred to as cell sap
• These products are transported around the plant in phloem tubes made of living, elongated cells
• Phloem tissue transports dissolved sugars from the leaves to the rest of the plant for immediate use or storage
• Phloem cells are joined end to end and contain pores in the end cell walls (sieve plates) allowing easy flow of substances from one cell to the next
• Transport of sucrose and amino acids in phloem, from regions of production to regions of storage or use, is called translocation
• Transport in the phloem can go in different directions depending on the plant's stage of development or time of year
• During winter, phloem tubes may transport dissolved sucrose and amino acids from storage organs to other parts of the plant for respiration
• During growth periods (e.g., spring), storage organs (e.g., roots) are the source and growing areas of the plant are the sinks
• After growth (usually summer), leaves are the source producing sugars, and roots are the sinks storing sucrose as starch until needed