Plants: tissue, systems and organs

• • The leaf is a plant organ adapted specifically for photosynthesis
  • You should be able to observe and draw of a transverse section of a leaf
• Wax Cuticle:

  • Protective layer on top of the leaf prevents water from evaporating
• Upper Epidermis:

  • it is thin and transparent to allow light to enter the palisade mesophyll
• Palisade mesophyll:

  • column shaped cells tightly packed with chloroplasts to absorb more light, maximising photosynthesis
• Spongy Mesophyll:

  • contains internal air space that increases surface area to volume ratio for the diffusion of gases ( mainly carbon dioxide)
• Lower Epidermis:

  • Contains Guard cells and Stomata
• Guard Cells:

  • absorbs and loses water to open and close the stomata, to allow carbon dioxide diffuse in and oxygen to diffuse out
• Stomata:

  • where gas exchange takes place; opening during the day and closing during the night. Evaporation of water also takes place here. In most plants, its found a great concentration under the leaf to reduce water loss
• Vascular Bundle:

  • contains xylem and phloem to transport substances to and from the leaf
• Xylem:

  • Transports water into the leaf for mesophyll cells to use in photosynthesis and for transpiration from the stomata
• Phloem:

  • transports sucrose and amino acids around the leaf
• Root hair cells

  • Adapted for the efficient uptake of water by osmosis
  • Adapted for the efficient uptake of mineral ions by active transport
• Root hairs

  Single-celled extensions of epidermis cells in the root which increase the surface area of the cells significantly
• Increased surface area of root hairs

  Increases the rate of the absorption of water by osmosis and mineral ions by active transport
• Structure of a root

  • Specifically allows it to maximise absorption of water by osmosis
  • Specifically allows it to maximise absorption of mineral ions by active transport
• Pathway of water into and across a root

  1. Root hair cell

  2. Root cortex cells

  3. Xylem

  4. Leaf mesophyll cells
• Transpiration

  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
• Xylem

  • Substance called lignin is deposited in the cell walls
  • Xylem cells die and become hollow
  • Xylem cells join end-to-end to form a continuous tube
  • 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
• Movement in xylem only takes place in one direction - from roots to leaves (unlike phloem where movement takes place in different directions)
• • Air movement, humidity, temperature and light intensity all have an effect on the rate at which transpiration occurs
  • The table below explains how these four factors affect the rate of transpiration when they are all high; the opposite effect would be observed if they were low
• Stomata and guard cells

  • Control gas exchange and water loss
  • Found predominantly on the underside of the leaf
• Guard cells

  Have cell walls with unevenly distributed cellulose - the inner wall is thicker and the outer wall is thinner to aid opening and closing of the stomata
• Guard cells are flaccid

  They pull together, closing the stomata and reducing water loss via transpiration
• Stomata distribution

  • Predominantly on the underside of the leaf where it is cooler and shaded (lower light intensity) - this leads to less transpiration and therefore less water loss
• Translocation

  The transport of soluble products of photosynthesis (sugars and amino acids) around the plant in the phloem tubes
• Phloem tubes

  • Made of living, elongated cells
  • Allow easy flow of substances from one cell to the next through pores in the end cell walls (sieve plates)