Leaf structure

• Plants have different levels of organization with lots of similar cells combining to form tissues, different types of tissues combining to form organs, and multiple organs combining to form organ systems.
• Leaves are the site of photosynthesis which requires lots of carbon dioxide and water to make sugars (glucose).
• The water for photosynthesis comes from the soil and is transported to the leaves by the roots and xylem.
• Carbon dioxide diffuses into the leaf from the outside through thousands of tiny holes called stomata, which are scattered through the lower epidermis.
• Carbon dioxide moves up the leaf and enters the spongy mesophyll tissue, which has lots of air gaps between the cells so that the gas can easily diffuse through to the next layer, the palisade mesophyll layer.
• Most of the photosynthesis happens in the palisade cells of the palisade mesophyll layer.
• The upper epidermis, another layer of epidermal tissue, is almost transparent as the sunlight needs to be able to pass through it to get to the chloroplasts in the palisade cells.
• The water produced by photosynthesis is carried to the rest of the plant by the phloem.
• The main problem that leaves face is water loss, which is reduced by a waxy cuticle on top and a waterproof layer of guard cells at the bottom.
• Each stoma, or single stomata, is formed from the gap between two guard cells.
• When the plant has lots of water, the guard cells are well hydrated (turgid), making a gap between them larger, allowing more carbon dioxide to diffuse through.
• When the plant is short of water, the guard cells lose water due to osmosis, becoming flaccid, closing the stomata, and conserving water.
• The guard cells are sensitive to light, closing at night time when photosynthesis isn't taking place and they don't need carbon dioxide.
• Most of the stomata are on the underside of the leaves rather than the top because the lower surface is more shaded, making it cooler and reducing water evaporation.
• Stomata: little holes that opens and closes to allow gaseous exchange to take place. The stomata close to prevent water loss and open to let gases come in and out. When guard cells lose water, the stoma close (at night), while the stoma open when guard cells gain water & swell (during the day).

  
• Phloem: vessel which transports nutrients

  
• Xylem: vessel which transports water and dissolved minerals
• Spongy mesophyll layer: irregularly shaped cells which create air spaces to allow gaseous exchange to take place; do not contain many chloroplasts

  
• Palisade: found at the top of the cell and contains many chloroplasts which absorbs sunlight.

  
• Epidermis: transparent cell that allows sunlight to pass through to the palisade cell

  
• Cuticle: waxy layer that prevents water loss from top of the leaf