Leaf and Stomata 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.
• 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, 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 and become flaccid, closing the stomata and conserving water.
• The guard cells are sensitive to light and close at night time when photosynthesis isn't taking place.
• 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.
• Meristem tissue, which is the plant form of stem cells, is found at the growing tips of the roots and shoots and can differentiate into loads of different cell types so that the plant can grow.