Life science Gr 10

• Tissues


  Group of similar cells that are structurally adapted to perform a particular function
• Cell differentiation


  Process of cells adapting for specific functions
• Examples of plant tissues

  • Xylem
  • Phloem
  • Parenchyma
  • Collenchyma
  • Sclerenchyma
  • Epidermis
  • Meristematic tissue

  
• Examples of animal tissues

  • Epithelial tissue
  • Connective tissue
  • Muscle tissue
  • Nerve tissue
• Various plant tissues are important ingredients in traditional medicine
• Biotechnology involves manipulating the properties of tissues and cells
• Organ


  Structure made up of a number of tissues that collectively enable a specific role in an organism
• Meristematic tissue


  Undifferentiated tissue containing actively dividing cells that result in formation of other tissue types
• Types of meristematic tissue

  • Apical meristem
  • Lateral meristem

  
• Apical meristem


  • Found in buds and growing tips, generally makes plants grow taller or longer
• Lateral meristem


  • Makes plants grow thicker, found in woody trees and plants, examples include vascular cambium and cork cambium
• Structure of meristematic tissue


  • Cells are small, spherical or polygonal, with small or absent vacuoles, large amount of cytoplasm and nucleus

  
• Permanent tissue


  Tissues that have developed a specific function and lost the ability to divide
• Types of permanent tissue

  • Epidermis
  • Ground tissue (parenchyma, sclerenchyma, collenchyma)
  • Vascular tissue (xylem, phloem)
• Epidermis


  • Single layer of brick-shaped cells covering plant surface, acts as barrier, transparent to allow light, contains root hairs and trichomes, covered in waxy cuticle
• Guard cells


  • Contain chloroplasts and control opening and closing of stomata
• Root hair cells


  • Specialised epidermal cells with thin-walled, hair-like outgrowths to increase surface area for water and nutrient absorption
• Stomata


  • Pores formed by two bean-shaped guard cells, allow gas exchange
• Parenchyma


  • Thin-walled cells with intercellular spaces, large central vacuoles, some retain ability to divide
• Collenchyma


  • Cells with unevenly thickened corners, provide mechanical strength and flexibility
• Parenchyma cells


  • Specialised parenchyma cells known as chlorenchyma found in plant leaves and stems contain chloroplasts
  • This allows them to perform a photosynthetic function and responsible for storage of starch
  • Some parenchyma cells retain the ability to divide
  • Allows replacement of damaged cells
• Ground tissues


  Located in the region between epidermal and vascular tissue
• Collenchyma


  • Cells are spherical, oval or polygonal in shape with no intercellular spaces
  • Corners of cell wall are unevenly thickened, with cellulose and pectin deposits
  • Cells are thin-walled on most sides
• Collenchyma


  • Provides mechanical strength
  • Provides flexibility, allowing plant to bend in the wind
  • Typically found in the shoots and leaves of plants just below the surface of the epidermal layer
• Sclerenchyma


  • Mature cells are dead and have hardened secondary cell walls
  • Cell walls are evenly thickened with lignin
  • Cells possess a small lumen (air space)
  • Two types: Sclereids are more rounded, while fibres are more elongated
  • No intercellular spaces between the cells
• Sclerenchyma


  • Provides mechanical strength and structural support
  • The lignin provides a 'wire-like' strength to prevent it from tearing easily
• Complex permanent tissues are made up of more than one cell type that combine to perform a particular function
• Xylem


  • Has the dual function of supporting the plant and transporting water and dissolved mineral salts from the roots to the stems and leaves
  • Consists of vessels, tracheids, fibres and parenchyma cells
  • The vessels and tracheids are non-living at maturity and are hollow to allow the transport of water
• Xylem vessels


  • Composed of a long chain of straight, elongated, tough, dead cells known as vessel elements
  • The vessel elements are long and hollow with perforated or completely dissolved end walls
  • The role of xylem vessels is to transport water from roots to leaves
  • Xylem vessels often have patterns of thickening in their secondary walls
  • Secondary wall thickening can be in the form of spirals, rings or nets
  • Pits occur in the thickened walls which allow lateral transport
• Xylem tracheids


  • Have thick secondary cell walls and are tapered at the ends
  • The thick walls of the tracheids provide support and tracheids do not have end openings like the vessels
  • The tracheids' ends overlap with one another, with pairs of pits present which allow water to pass through horizontally from cell to cell
  • Water moves more slowly in the tracheids than in the xylem vessels
• Xylem


  Water and dissolved mineral salts are transported upwards from the roots to the rest of the plant
• Phloem


  • Sieve elements: conducting cells which transport sucrose
  • Parenchyma cells: store food for transport in phloem
  • Companion cells: associated with parenchyma cells and control the activities of sieve tube elements, since the latter have no nuclei
  • Companion cells are responsible for providing energy to the sieve elements to allow for the transport of sucrose
  • Companion cells play an important role in loading sieve tubes with sucrose produced during photosynthesis
  • Companion cells and sieve tube elements are connected via connecting strands of cytoplasm called plasmodesmata
  • Fibres: unspecialised cells and supportive cells
• Phloem


  The living tissue responsible for the transport of organic nutrients produced during photosynthesis (mainly as the carbohydrate sucrose) to all parts of the plant where it is needed
• Phloem sieve tubes


  • Elongated cells with cellulose cell walls
  • Form good conducting tubes over long distances, allowing transfer over a large area
  • They are living cells without a nucleus or organelles like vacuoles and ribosomes
  • Allows more space for sap transport, which is why sieve elements need companion cells to perform all cellular functions
• Phloem companion cells


  • Contain a large number of ribosomes and mitochondria, but no nuclei
  • Due to absence of organelles or nuclei in sieve tubes, companion cells perform cellular functions of the sieve tube
  • Has many plasmodesmata (intercellular connections) in the wall attached to the sieve tube
  • Allows transfer of sucrose-containing sap over a large area
• Absorption of water and mineral salts

  1. Movement of water through the dicotyledonous root

  2. Movement of water to the xylem of the root

  3. Transport of water and minerals to leaves

  
• Root hair cells


  • Located at the tips of plant roots
  • Specialized in absorbing water and minerals
  • Do not perform photosynthesis and lack chloroplasts
  • Feature large vacuoles for water and mineral storage
  • Elongated shape and small diameter ensure a large surface area for absorption

  
• Root hair


  • Numerous elongated root hairs increase the total root surface area for water absorption
  • Thin walls facilitate rapid intake of water by osmosis
  • Large vacuoles allow for quick water absorption and transport to adjacent cells
  • Salts within vacuoles enhance water absorption from soil water
  • Lack of cuticles prevents hindrance to water absorption

  
• Movement of water to the xylem of the root

  1. Some water passes through the cells themselves by osmosis

  2. Most water travels within or between the cell walls of the parenchyma cells through simple diffusion

  3. To enter the xylem, water must pass through the endodermis

  
• Symplastic route


  The main route that water takes from cell to cell by osmosis through the selectively permeable membranes and plasmodesmata of each cell. This movement occurs slowly.