2.3 - plant tissues, organs and systems

• The epidermis is the outermost layer of cells that protects plants from water loss.
• Guard cells control the opening and closing of stomata based on environmental conditions such as light intensity and humidity.
• Stomata are small openings on the underside of leaves that allow gases to enter or exit the leaf.
• Xylem transports water and minerals upwards through the stem towards the leaves.
• Transpiration is the process by which water evaporates through the stomata, carrying with it dissolved minerals and nutrients.
• Water moves up the xylem vessels due to transpiration pull, whereby water molecules move towards areas of lower concentration (i.e., the air outside the leaf).
• Phloem transports sugars downwards from the leaves to other parts of the plant.
• Xylem vessels have thick cell walls made of lignin, making them rigid and strong enough to support the weight of the plant.
• Root hair cells increase surface area for absorption of water and mineral ions.
• Meristems are undifferentiated tissue found at growing tips (apical meristems) and between xylem and phloem (lateral meristems).
• The upper epidermis is transparent so that light can pass through it. This means that as much light as possible can reach the palisade mesophyll cells below, and be used in photosynthesis
• If there is plenty of water in the leaf, guard cells become turgid, which opens the stomata. This allows carbon dioxide to enter the leaf, but also allows water to escape
• At night time there is no light for photosynthesis, so plants don't need to absorb carbon dioxide. This means they can close their stomata and conserve water.
• Meristem tissue is made up of plant stems cells. It's found in the growing parts of the plant such as the tips of shoots and roots. 
• Levels of organization in plants

  • Cells
  • Tissues
  • Organs
  • Organ systems
• Leaf


  An organ that, along with a stem and roots, forms an organ system for transport of substances around the plant
• Photosynthesis

  1. Requires carbon dioxide and water

  2. Water transported from soil by roots and xylem

  3. Carbon dioxide diffuses into leaf through stomata

  4. Photosynthesis occurs in palisade mesophyll layer
• Leaf structure

  • Upper epidermis
  • Palisade mesophyll
  • Spongy mesophyll
  • Lower epidermis with stomata
• Stomata open


  Allows carbon dioxide diffusion but increases water loss
• Stomata closed

  Reduces water loss but limits carbon dioxide intake
• Guard cells


  Control opening and closing of stomata based on water status
• Most stomata are on the underside of leaves to reduce water evaporation
• Meristem tissue


  Plant form of stem cells, found at growing tips, can differentiate into many cell types
• Meristems are covered in the video on plant stem cells
• Root hair cells and active transport are covered in the video on root structure and function
• Translocation

  Plants transport sugars made through photosynthesis in leaves to the rest of the plant
• Phloem cells

  Arranged end to end to form long columns called phloem tubes, enable movement of cell sap (water and sugar)
• Transpiration

  Movement of water from roots to leaves, driven by evaporation of water from leaves
• Xylem tubes


  Columns of dead xylem cells with no ends, strengthened with lignin, transport water and mineral ions from roots to leaves
• Stomata open

  Allows carbon dioxide in for photosynthesis, but also allows water evaporation (transpiration)
• Factors affecting rate of transpiration

  • Light intensity
  • Temperature
  • Air flow
  • Humidity
• High light intensity

  Increases photosynthesis, opens more stomata, increases transpiration
• High temperature

  Increases rate of transpiration
• High air flow


  Increases rate of transpiration by quickly removing water molecules from leaf surface
• High humidity


  Decreases rate of transpiration by reducing water concentration gradient between leaf and air
• high wind speed


  The wind blows away water vapour as soon as it diffuses out of the leaf. This maintains a strong concentration gradient between the inside and outside of the leaf, which increases the rate at which water diffuses out.