Water Absorption & Movement in Plants

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  • 0:04 Vascular vs Non-Vascular
  • 1:47 Absorption & Transport…
  • 6:25 Water Absorption
  • 7:36 Lesson Summary
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Lesson Transcript
Instructor: Sujata Archer
In this lesson, you'll learn about the complex process involved in the transport and absorption of water from the roots to the leaves of different types of plants.

Vascular vs. Non-Vascular

The simple plant sitting in a pot on a window sill is occasionally watered by you. The process seems rather boring, but you do not realize what amazing things start happening when you water that plant. It's a gravity defying feat which allows the water to even climb up a tree that is 30 stories high!

To understand how water is transported and absorbed in plants you must remember there are two types of plants: vascular, which is a plant that has connections within it that move water and minerals around, and non-vascular, which is a plant that doesn't have those things - like algae. Water uptake and absorption in both these plants differ. The table below shows the differences between these two types of plants:

Vascular Plants Non-vascular Plants
Well developed root, stem, and leaf system Root, stem, and leaf system not well developed
Have xylem and phloem No xylem or phloem
Water absorbed passively Relies on osmosis for water movement
Can transport water for long a distance Poor transport of water
Tolerate drought due to water storage Cannot tolerate drought
Do not have to grow near water source Can only grow near water source
Some names: Redwood, Honeysuckle, Hibiscus Some names: Liverworts, moss, Hornwort

As you can see, vascular plants have a well-developed root, stem, and leaf system while non-vascular plants don't. That's their major difference. Because of this, as you can see in the third comparison, water is absorbed passively through rain - or your watering can - in vascular plants, while non-vascular plants rely on osmosis to get their water. This is why vascular plants don't have to grow near a water source (rain, human cultivation), while non-vascular plants like algae do. This is also why vascular plants can tolerate drought - some better than others, like cacti, and non-vascular plants can't (see also dried seaweed).

Water Transport in Plants

So, as you can see, there are many differences between vascular and non-vascular plants, mostly having to do with water. The process of transportation is much more developed in the vascular plants with specialized systems called xylem and phloem. The non-vascular plants do not have specialized cells so the process in inefficient.

While plants can absorb water from many sources, the best place for this is the root system, which is in contact with the soil where most of the water is found. The root system is usually extensive and has root hairs which help increase the surface area for water absorption.

Water enters the root hair cells and then moves from cell to cell till it reaches the root cortex. It goes on to the xylem vessels to be transported to the leaves. The xylem system is like a bunch of drinking straws tucked between fibrous tissues. Water travels up in those straws. It's your plant on the window sill taking in a sip of water! Interestingly, the xylem is made of dead cells. It's a tubular type of structure and is well reinforced for its function. The process that the xylem uses is called transpiration. It's a fascinating process because it involves a tissue made of dead cells through which water defies gravity and moves upwards! Transport in the xylem only occurs in one direction: up.

So what is phloem? Phloem also makes up the vascular bundles which move food from the leaves to the rest of the plant. The phloem is made of living cells, and the process it uses is called translocation. The phloem takes substances such as sugars made during photosynthesis from the leaves to growing tissues such as root tips and shoots. Phloem also takes nutrients to be stored in the roots. Transport by the phloem occurs in two directions: up and down.

Non-Vascular Plants

If you have ever gone to the Pacific Northwest where a lot of rain falls, even the sidewalks sometimes are covered with moss. These are the non-vascular plants trying to exist where they can get water. Non-vascular plants tend to be small due to the lack of a vascular tissue to transport water. These plants depend on diffusion and osmosis for their water supply. This means that they are restricted to grow in moist environments. Due to the lack of an internal transport system, they do not develop true roots, stems, and leaves.

Symplast and Apoplast Pathways

Once the water enters the root hair cells it can go down two paths: symplast or apoplast. In the apoplast path, the water goes through the cell walls till it reaches a waxy strip, the casparian strip. Water cannot go through this so it makes its way to the cytoplasm. In the symplast path, the water moves through the cytoplasm directly into the xylem.

Absorption and Transport Processes

There are three key processes involved when water is transported in plants: imbibition, diffusion, and osmosis. All three processes are important for the plant to grow, and it all starts with the seed.

Imbibition: this is where the water transport system starts from the seed. When you take a seed that is non-dormant and put it in that flower pot on the window sill and add a little water to it, you have started imbibition. The seed imbibes or uptakes the water and germination starts.

When the seed uptakes water it gets hydrated. This hydration causes enzymes to become active and start working with metabolic processes. The metabolic processes produce energy which are necessary for growth. The water also increases the turgor pressure in the cells and the seeds enlarge. Eventually, the radicle (embryonic root) emerges. The seed is the imbibant and the process is imbibition.

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