Plant cell organisation

• Plants are made of organs like stems, roots and leaves
• Epidermal tissue - covers the whole plant
• Palisade mesophyll tissue - the part of the leaf where most photosynthesis happens
• Spongy mesaphyll tissue - thin, spongy tissue that fills the spaces between the mesophyll cells. Also contains lots of air spaces to allows gasses to diffuse in and out of cells
• Xylem - transports water from soil up through the stem to other parts of the plant
• Phloem - transports sugars around the plant
• Meristem tissue - found in the tips of shoots and roots, and in the axils of leaves, is able to differentiate into lots of different tupes of plant cells, allowing the plant to grow
• The epidermal tissues are covered with a waxy cuticle, which helps to reduce water loss by evaporation
• The upper epidermis is transparent so that light can pass through it to the palisade layer
• The palisade layer has lots of chloroplats so they can get the most light for photosynthesis
• Guard cells control the opening and closing of stomata
• Stomata allow gas exchange but also cause water loss due to diffusion
• Air spaces in the spongey mesophyll tissue increase the rate of diffusion of gasses
• Transpiration is the loss of water in a plant
• Phloem transport food in both directions, this process is called translocation
• Xylem tubes transport water and mineral ions from the roots to the stem and leaves. The movement of water from the roots, through the xylem and out of the leaves is called the transpiration stream.
• Transporation is caused by the evaportation and diffusion of water from a plants surface.
• Transpiration rate is affected by four things:

  • Light Intensity - The brighter the light, the greater the transpiration rate. Stomata close as it gets darker, when they are closed very little water can escape.
• Temperature - As temperature increases so does the rate of transpiration because more energy means faster molecules which leads to increased rates of diffusion.
• Wind speed - Increased wind speeds mean that the stomata open wider allowing more water vapor to diffuse out of the leaf at a quicker rate.
• Humidity - When humidity decreases, there is less moisture in the air meaning that water will diffuse out of the leaf at a higher rate.
• More stomata are found on the underside of the leaf as the lower surface is shaded and cooler, so less water is lost