Transport in Plants: Xylem


Water is then transported through the xylem tissue and diffuses into other tissues of the plant. Entering the leaf, the xylem's conducting vessels branch into numerous veins. At the end of each vein, water and minerals can diffuse not the leaf's cells.  Though much of the water that reaches the leaf, 90% of it, evaporates through the stomata. This process is called transpiration. 
The movement of water in plants seems as a mystery. How can plants transport water at a long distance if they have no muscle? In veins humans have one way valves in veins to prevent the back flow of blood due to gravity, but how is this accomplished in plants?


Long distance transport in the xylem is accomplished by two processes: root pressure (positive pressure), and transpirational pull (negative pressure). Both positive pressure (pushing) and negative pressure (pulling) contribute to the movement of water upwards through the xylem. 


Root Pressure: mechanism by which positive pressure in roots moves water and minerals upward in plants. Water entering the roots creates a positive pressure, pushing water upwards. Minerals from the soil is moved to the xylem against the concentration gradient.

Transpirational Pull: also known as the cohesion-tension model. Recall that most of the water that reaches leaves is evaporated through transpiration. This loss of water creates a negative pressure and pulls the water up to replace the lost water. 
There are three main factors that play a role:
1. Transpiration: evaporation of water through the stomata. Negative pressure exerts tension on the water confined in the xylem's tube down to the roots. 
2. Cohesion: force of attraction between water molecules. This attraction keeps the water column unbroken while being pulled upwards.
3. Adhesion: force of attraction between water molecule and the walls of the xylem. 
Both attractions keep the water column unbroken while being pulled upwards.


The following video explains and illustrates the process of transpiration pull:

Plants have been abundant in the Earth for several years. Seed plants first appeared in the fossil record approximately 400 million years ago, while flowering plants appeared in the fossil record approximately 135 million years ago. Over many generations humans developed various uses of plants; food, shelter, medicine, fuel, etc. Plants are significant and vital for our survival.

Issues

Today, we continue to use plants for our various needs. But as we continue to consume, we end up significantly damaging the environment, Through pollution, deforestation, and over consumption, humans are using up more sources on Earth than can be replenished.

Why does this matter?

Ever heard the quote, "If the bee disappears from the surface of the earth, man would have no more than four years to live"? Bees play an important role in pollination. Pollination enables the cycle of fertilization to occur. It is a necessary step for the reproduction of flowering plants. Without pollinators, certain plants won't be able to go through pollination. Fruit, nuts and vegetable crops would be significantly affected by this and food would end up being scarce. Just like humans, animals need plants to sustain life. With food scarce, it will be harder for animals to survive. With less animals and food, it will be harder for humans to survive. And the chain goes on. There is an equilibrium within the ecosystem, it is a cycle that sustains life. Breaking this cycle can cause an ecosystem to be off balance.

Plants are often disregarded in our busy modern world. We begin to interfere with the cycle and destroy the habitats of plants and animals. The damage we've inflicted upon our environment is a current issue and some are trying to accommodate.

What Can I Do?

The little things count. A simple action can go a long way. It is important to be aware about current environmental issues and what you can do to help. Educate yourself and have an understanding of plants and their significance to the environment. This site is for informative purposes, but there are ways to get involve. You can start by clicking the "get involved" section.