Series 4: A Transportation to the Plant Transport System
Why do plants need transport systems?
For most plants, the alternative means of transport, simple diffusion, is simply not feasible. Given most plants are highly complex organisms, they tend to have a relatively small surface area to volume ratio, large metabolic rate and need for complex compounds. Simple diffusion would simply be too slow to meet the demands of such an advanced and amazing life form!
The plant transport system is primarily composed of two types of vascular tissues: the xylem and the phloem. These tissues form a network of tubes that transport water, nutrients, and sugars throughout the plant. What are these tissues called? The xylem and the phloem of course!
The xylem’s main role is to transport water around the plant. It does this through its plethora of different cell types, including tracheids and vessel elements, which form hollow tubes that facilitate this upward movement. The phloem, on the other hand, transports the products of photosynthesis, mainly sugars, from the leaves (where they are produced) to other parts of the plant where they are needed for growth or storage. The phloem is comprised of sieve tube elements and companion cells that work symbiotically to move these nutrients.
Water is pulled up the plant via a mechanism called the transpiration stream. The loss of water vapor from the plant’s leaves through the stomata creates a negative pressure within the xylem vessels, pulling more water up from the roots. Like you sipping a drink from a straw, the loss of water from the leaves allows a stream of water to flow upwards within a tube. Water’s cohesive and adhesive properties allow the water to flow upwards and stick to the walls of the xylem vessels, which helps prevent the xylem from collapsing due to the negative pressure. In some instances, roots can create a positive pressure to push water up through the xylem. This happens when minerals are actively transported into the root cells, causing water to follow via osmosis and create pressure within the root xylem.
For sugars, they travel from a source to a sink. The source is any part of the plant that produces or stores nutrients, such as the leaves that participate in photosynthesis or storage organs. The sink is any part that consumes or stores these nutrients, such as growing tissues or storage organs like roots or fruits that do not photosynthesize.
Sugars are transported from the storage organ into the phloem via active transport. This increases the osmotic pressure within the phloem, creating a pressure gradient that drives the flow of the sugary sap towards the sink, where sugars are unloaded.
Limiting factors of transpiration such as carbon dioxide concentration, light intensity, humidity and wind speed will dictate the pore size of the stoma. This pore size is controlled by guard cells, which will alter their turgidity based on the water available within the environment and these limiting factors. Plant hormones like abscisic acid (ABA) also influence water uptake. ABA levels increase under drought conditions, signaling the stomata to close to reduce water loss. In waterlogged conditions, plants may develop aerenchyma, a type of tissue with large air spaces that helps to transport gases and avoid suffocation of roots.
Ultimately, with increasingly extreme weather conditions due to climate change, a plant’s ability to maintain homeostasis and control water levels is more pressing than ever. Therefore, further understanding of how a plant’s transport system works would only aid in our attempt to ensure that botanical life is able to survive the looming effects of a fluctuating climate.
Extra Resources:
Water Uptake and Transport in Vascular Plants | Learn Science at Scitable (nature.com)