Transpiration and Translocation

• Mass transport systems of plants involve the transport of sugars, known as translocation, and water, through transpiration.
• Plants make their own sugars through photosynthesis, which occurs in the leaves.
• Translocation is the process by which these sugars are transported to other parts of the plant for use or storage.
• Translocation is mainly achieved by phloem cells, which are arranged end to end to form long columns known as phloem tubes.
• Between the adjacent cells of the phloem tubes, there are a lot of small pores, or gaps, that enable the movement of cell sap, a liquid mixture of water and sugar.
• The sugars made in the leaves can be transported long distances through multiple cells.
• Once the sugars finally make it to whatever cells they're going to, they can be used for two things, either directly for energy, or they can be stored so that they can be used for energy later.
• The phloem can transport substances in either direction, up or down the plant.
• Xylem tubes are also made up of columns of cells, but these are dead xylem cells with no ends between them, forming one long hollow tube, strengthened with a material called lignin.
• Transpiration is the evaporation of water from the leaves, known as the transpiration stream.
• The rate of transpiration can vary due to four main factors: light intensity, temperature, air flow, and humidity.
• The brighter the light intensity, the more photosynthesis that can take place, which means more stomata will have to be open to let in the carbon dioxide required for photosynthesis.
• More water can evaporate, leading to a higher rate of transpiration.
• At nighttime, when there's no photosynthesis happening, the stomata will be closed, resulting in very little transpiration.
• The warmer it is, the higher the rate of transpiration, because the particles of water will have more energy, and so they're more likely to evaporate and diffuse out of the stomata.
• Airflow can affect the rate of transpiration, for example, if it's really windy, the water molecules that leave the leaf are quickly blown away, keeping the concentration gradient between the inside and outside high, increasing the rate of transpiration.
• Humidity, which is a measure of how much water vapor is in the air, can affect the rate of transpiration, for example, more humid air will decrease the rate of transpiration because the large amount of water in the moist air will decrease the concentration gradient, meaning that less water is going to diffuse out, and so we have less evaporation overall.
• The role of the xylem tubes is to transport both water and the mineral ions from the roots, up the stem, to the leaves, where the water can be used in photosynthesis.
• The movement of water through a plant is driven by the evaporation of water from the leaves.
• Every time water evaporates from the leaf out of the stomata, it drags the rest of the chain up slowly, pulling along another water molecule to replace it.
• As water is continually evaporating, there'll be a steady stream of water flowing up the plant.