B2.3 - Plant tissues, organs, and systems

• Palisade Mesophyll function and adaptation?

  The palisade mesophyll layer of the leaf is adapted to absorb light efficiently. The cells:

  • are packed with many chloroplasts
  • are column-shaped and arranged closely together
  • towards the upper surface of the leaf
• Spongy Mesophyll function and adaptation?

  Spongy mesophyll tissue is packed loosely for efficient gas exchange. The spongy mesophyll cells are covered by a thin layer of water. Gases dissolve in this water as they move into and out of the cells.

  When the plant is photosynthesising during the day, these features allow carbon dioxide to diffuse into the spongy mesophyll cells, and oxygen to diffuse out of them. To enter the leaf, gases diffuse through small pores called stomata. As the stomata open, water is lost by the process of transpiration. Closing the stomata helps to control water loss.
• The xylem transports water and minerals from the roots up the plant stem and into the leaves.

  In a mature flowering plant or tree, most of the cells that make up the xylem are specialised cells called vessels.

  Vessels

  • Lose their end walls so the xylem forms a continuous, hollow tube.
  • Become strengthened by a chemical called lignin. The cells are no longer alive. Lignin gives strength and support to the plant. We call lignified cells wood.

  Transport in the xylem is a physical process. It does not require energy.
• The phloem moves food substances that the plant has produced by photosynthesis to where they are needed for processes such as:

  • growing parts of the plant for immediate use
  • storage organs such as bulbs and tubers
  • developing seeds.

  Transport in the phloem is therefore both up and down the stem. Transport of substances in the phloem is called translocation.

  Phloem consists of living cells.
• Adaptations of cells within phloem cells

  • Sieve tubes - specialised for transport and have no nuclei. Each sieve tube has a perforated end so its cytoplasm connects one cell to the next.
  • Companion cells - transport of substances in the phloem requires energy. One or more companion cells attached to each sieve tube provide this energy. A sieve tube is completely dependent on its companion cell(s).
• Epidermis function

  The plant epidermis is a protective tissue that covers the entire surface of the plant. The epidermis protects the plant from infection and water loss. It regulates the gas exchange in plant cells. The epidermis regulates the secretion of metabolic substances.
• Meristem function

  Meristems at the tips of roots provide a constant supply of new cells allowing roots to grow longer. Meristems at the tips of shoots allow a plant to keep growing taller and spread wider. Meristems between the xylem and phloem increase the width of a plant. These result in the annual growth rings in trees.
• Transpiration stream

  1. Water evaporates from the leaf cells into air spaces in the spongy Mesophyll.
  2. The water has now changed state into water vapour.
  3. The concentration of water vapour in the air spaces increases.
  4. The water vapour diffuses through the stomata into the air,
• Factors affecting transpiration

  Light intensity, temperature, humidity, wind
• Role of stomata

  Stomata are tiny holes found in the underside of leaves. They control water loss and gas exchange by opening and closing. They allow water vapour and oxygen out of the leaf and carbon dioxide into the leaf.
• Role of guard cells

  Guard cells are cells surrounding each stoma. They help to regulate the rate of transpiration by opening and closing the stomata. Light is the main trigger for the opening or closing. Each guard cell has a relatively thick and thinner cuticle on the pore-side and a thin one opposite it.
• Translocation:

  Photosynthesis produces glucose in the green parts of plants, which are often leaves, which is converted into sucrose. The sucrose is transported around the plant in phloem vessels. It needs to be able to reach all cells in the plant so that the sucrose can be converted back into glucose for respiration. The movement of sucrose and other substances like amino acids around a plant is called translocation. In general, this happens between where these substances are made (the sources) and where they are used or stored (the sinks).