plant tissues, organs, organ systems

• Tissue Types in Plants:

  • Epidermal Tissue
  • Palisade Mesophyll
  • Spongy Mesophyll
  • Xylem and Phloem
  • Meristem Tissue: Located at the TIPS of shoots and roots. These cells specialise through DIFFERENTIATION to become various specialised plant cell types.
• The Leaf


  Composed of several tissue types, including epidermal, mesophyll, xylem and phloem tissues, the leaf is a plant organ and is key for PHOTOSYNTHESIS and GAS EXCHANGE.
• UPPER EPIDERMIS:


  • Outer layer of cells on the leaf.
  • This is the outermost layer of cells on the leaf. It is TRANSPARENT so it allows sunlight to pass through to the chloroplast-rich layers beneath, which is vital for photosynthesis.
• PALISADE MESOPHYL:


  • Located just below the upper epidermis, this layer consists of CLOSELY PACKED CELLS that contain MANY CHLOROPLASTS to maximise light absorption for photosynthesis.
• SPONGY MESOPHYLL:

  • The cells here are arranged MORE LOOSELY, which creates air spaces to facilitate the efficient diffusion of gases like carbon dioxide (CO₂). This structure enhances the leaf's SURFACE AREA TO VOLUME RATIO.
• LOWER EPIDERMIS:


  • Contains STOMATA and GUARD CELLS that regulate gas exchange and water loss from the leaf.
• STOMATA:

  • HOLES primarily found on the lower epidermis that allow for the exchange of oxygen and carbon dioxide. They open during the day to facilitate photosynthesis and close at night to conserve water.
• GUARD CELLS:

  • Cells found either side of the stomata and can change shape to open or close them.
  • They REGULATE WATER AND GAS EXCHANGE by reacting to environmental conditions. 
• PHLOEM:

  • Transports SUGAR and other FOOD SUBSTANCES in the form of CELL SAP around the plant.
  • Made of ELONGATED LIVING CELLS that are joined together to create a continuous tube.
  • Cell sap can move from one phloem cell to the next through PORES in the end walls.
  • The sugar and nutrients can travel in BOTH DIRECTIONS of the phloem.
  • The process where sugar and nutrients move through the phloem is called TRANSLOCATION.
• XYLEM:

  • Transports WATER and MINERALS from the roots to the leaves.
  • They are made of DEAD CELLS that form continuous tubes and are strengthened by a substance known as LIGNIN.
  • The xylem is completely HOLLOW
  • The water and minerals can only move in ONE DIRECTION up the plant from the roots to the leaves.
  • The route that the xylem takes is known as the TRANSPIRATION STREAM.
• ROOT HAIR CELLS:

  • Found in the ROOTS and are specialised for ABSORBING WATER AND MINERALS.
  • Water is taken in by OSMOSIS and minerals are taken in by ACTIVE TRANSPORT.
  • Long hair like projections INCREASE SURFACE AREA for absorption from the soil.
• STOMATA: small openings on the underside of leaves, regulated by GUARD CELLS


  • In the day time when light intensity is HIGH, the guard cells become TURGID and they OPEN the stomata. This is because more PHOTOSYNTHESIS occurs in the daytime which means more CARBON DIOXIDE needs to be taken in. This also results in the release of more WATER and OXYGEN.
  • In the night time, the light intensity is LOW and the guard cells become FLACCID. This means the stomata CLOSE as less PHOTOSYNTHESIS occurs. This prevents the loss of excess WATER.
• TRANSPIRATION:

  • TRANSPIRATION is caused by the EVAPORATION and DIFFUSION of water from the plant's surface, primarily the leaves.
  • Most of the transpiration in a plant occurs through the STOMATA; small PORES found on the bottom of leaves.
  • This process helps draw water up from the roots through the xylem and is a key part of the plant's WATER MANAGEMENT and NUTRIENT DISTRIBUTION
• Process of transpiration:

  • Water from roots is taken in by root hair cells by osmosis
  • Water moves up xylem
  • Water escapes the leaves through the stomata by diffusion and evaporation.
• 4 factors affecting rate of transportation: 'washing on line'

  • Light intensity
  • Temperature
  • Air flow
  • Humidity
• Light intensity

  More light INCREASES transpiration. This is because the STOMATA OPEN UP in bright conditions to allow more gas exchange for photosynthesis.
• Temperature

  Higher temperatures INCREASE transpiration because they increase the energy of the water molecules, and makes them move FASTER. This means the rate of DIFFUSION and EVAPORATION of water molecules out of the stomata increases.
• Air flow

  Good air circulation around a leaf INCREASES transpiration. This is because the air removes water vapour from the surface of the leaf and keeps the concentration of water outside lower than the inside. This increase in CONCENTRATION GRADIENT makes DIFFUSION faster.
• Humidity

  Higher humidity levels outside the leaf DECREASES transpiration. This is because it means there is a high water concentration OUTSIDE the leaf, which results in a LOW CONCENTRATION GRADIENT which results in slower DIFFUSION.