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Photosynthesis and How Plants Use Water

Susette has a Masters degree in Sustainable Development. She leads her local Green Council and writes for The Sustainable Business Review.

Most people are aware of plants needing water, but not of all the different ways in which water helps a plant. This article should provide a new look for many people, with information about water and plants that plant biology lectures don't normally cover—at least, not from the point of view of water.

Water and plants––plants need water just as much as humans do . . . and for many of the same reasons.

Water and plants––plants need water just as much as humans do . . . and for many of the same reasons.

Here are some of the ways in which water contributes—inside and outside of a plant—to its overall health. We'll go over them in more detail later on.

Water inside the plant:

  • Transports nutrients to cells. As it transports food to them it also carries wastes away from them.
  • Helps create energy to fuel plant activities (photosynthesis).
  • Contributes to turgidity, keeping the plant straight and sturdy.
  • Makes nectar that attracts bees and birds to help pollinate.

Water outside the plant:

  • Cools the plant via transpiration (like sweating).
  • Moistens the air to help distribute scent.
  • Supplies extrafloral nectaries that protect the plant against insects.
  • Rainwater washes the outside of the plant clean.
  • Rain softens the soil so roots can penetrate down and out.
  • Groundwater softens the seed cover so a seed can take root.
The soil absorbs Irrigation water, which is then sucked up by the root system of plants.

The soil absorbs Irrigation water, which is then sucked up by the root system of plants.

Absorption & Transportation of Water by Plants

The first five functions above require a plant to be able to absorb water inside of itself. Plants obtain water through their root systems via osmosis. When the ground is saturated, the roots suck water in by absorbing it into the root cells, then transporting it up into its stems (or trunk), out into the leaves, and out the rest of the way into the air as transpiration (plant sweat). This process happens more often with some types of plants, and hardly at all with others, depending on the type of plant and the atmosphere in which it grows.

Here is another way to look at it. Ninety percent (90%) of a plant's water evaporates from the leaves, so the plant is continually being dried out. Dry cells pull water in from more saturated areas like wetter plants cells below and, further down, the wet soil. As a plant dries out, it creates a subtle suction that pulls fresh water up from the roots to slake its thirst, as long as water is available down there.

Cross section of the root of a young pine tree. The little holes are capillaries that suck water molecules up into the plant, which carry nutrients with them.

Cross section of the root of a young pine tree. The little holes are capillaries that suck water molecules up into the plant, which carry nutrients with them.

Here's a cross section of a pine tree stem. Note that it, also, has capillaries through which water and nutrients flow. This flow is called "capillary action."

Here's a cross section of a pine tree stem. Note that it, also, has capillaries through which water and nutrients flow. This flow is called "capillary action."

A Plant's Internal Watery Functions

Transportation: As water is subtly pushed and pulled into and out of a cell membrane it moves nutrients into the cell and pulls wastes out of it. The nutrients, including nitrogen, have come from the soil. Wastes are transported to the leaves to be discarded into the air, if they are gaseous, or they stay in the leaf structure to be discarded when the leaf grows old.

Photosynthesis: 6H2O + 6CO2 ----> C6H12O6 + 6O2 Water plus carbon dioxide - six molecules of each - make one molecule of sugar, with six of oxygen left over. The sugar is the plant's food, the oxygen its discards. This is the formula for photosynthesis. You can see the integral role that water plays (H2O), and also how plants produce the oxygen that we breathe.

The trigger for this conversion is sunlight. Sunlight is absorbed by the plant's chlorophyl - its green pigment - which starts the chemical conversion from water and carbon dioxide to sugar and oxygen. The plant creates its sugary energy in this two stage process, discarding leftover oxygen into the air.

Here's a simple diagram that shows the process of photosynthesis. Water  (carrying nutrients) travels up the roots of a plant and interacts with other elements to produce sugars (food). Oxygen is left over, which the plant breathes out.

Here's a simple diagram that shows the process of photosynthesis. Water (carrying nutrients) travels up the roots of a plant and interacts with other elements to produce sugars (food). Oxygen is left over, which the plant breathes out.

Nectar Production: Organs, called nectaries, inside flowers produce a sweet-tasting insect attractant called nectar. Nectar is a combination of water and sugars made by the plant. This nectar becomes food for insects, like bees, and tiny birds, like hummingbirds. The plant benefits from their accidental brush against plant pollens as they feed on the nectar. When they fly away they carry the pollen with them and deposit it on the next plant they feed on. The pollen is what plants use to fertilize other plants, so they can make seeds. Without this sweet nectar, most plants would not be able to reproduce.

Transpiration: Water that leaves the plant via transpiration serves the same purpose as sweat in a human body. It removes gaseous wastes (including oxygen) and helps cool the plant. Stomata are little openings in the plant leaf, like air pockets, that let used air and water evaporate out into the atmosphere and let carbon dioxide in to be utilized by the plant in photosynthesis. The stomata are flanked by neighboring "guard cells," which open and close, depending on how much water is in them.

When the plant is dry and there is no water in the guard cells, they automatically close, preventing the plant from transpiring, so it can retain its moisture. When the air is hot outside, the plant and its water heat up too, which makes the water travel to the surface, where some of it enters and opens the guard cells to let the rest of it out as cooling vapor.

Turgidity: Water provides volume that bolsters the cells and walls of a plant's stems, helping them to stand up straight. Water flows up a series of tubules called "xylem vessels" on its way to the leaves. It fills all the plant's cells, creating pressure against cell walls and making them stiff. Without water most plants become flaccid and wilt.

Orchid Roots (Note the turgidity of some roots, while others are flaccid.)

Orchid Roots (Note the turgidity of some roots, while others are flaccid.)

A Painted Lady Butterfly feeds on nectar produced by the flower. She found it originally by following its scent carried through the air on water vapor molecules.

A Painted Lady Butterfly feeds on nectar produced by the flower. She found it originally by following its scent carried through the air on water vapor molecules.

External Functions of Water for Plants

Leaf Cleaning: Rainwater and irrigation water falling on a plant cleans the leaves and stems. Without this service, the plant would have a hard time carrying out photosynthesis. Similar to solar panels (which were designed from a plant's ability to photosynthesize), the plant needs its surfaces to be clean in order to function optimally.

Air Humidifier: Humid air around a plant provides two helpful functions. One is to help prevent the plant's sensitive leaves from drying out. The other is to carry scent to foraging insects and birds. Scent carries much further in moist air, which is why roses smell stronger right after it rains. Because plants produce some of their own humidity through transpiration, they are often used as small humidifiers indoors. Outdoors, of course, the cumulative transpiration of plants is part of the earth's water cycle.

Indoor plants help moisten and clean the air inside your home.

Indoor plants help moisten and clean the air inside your home.

Extrafloral Nectary Supply: Extrafloral nectaries do the same thing as nectaries in flowers, but serve a different purpose. In this case, nectaries are positioned on leaves, stems, and sometimes on the outsides of flowers to attract predatory insects that will eat the plant's enemy bugs. Again, water is used to produce the nectar in these outdoor nectaries.

Soil Softening: Water softens and cools hard soil. Softer soil makes it easier for plants to push their roots down and out further, giving them greater access to nutrients and providing a more stable base to hold them up. This is why soil should be soaked before planting baby plants. In turn, roots provide underground tunnels through which water can flow down into the aquifer when it rains. Water stored in the aquifer (underground) becomes future food for trees and larger bushes.

Seed Penetration: The last main service water provides to plants (exterior to itself) is to soften and penetrate the protective covering of its seeds. This starts the germination process. Many plants produce seeds that can last years and even decades under very dry conditions. When water finally arrives it soaks the seed, softens its walls, and the plant sprouts, thereby starting the next generation of that type of plant.

Water helps soften the soil prior to planting. It also softens protective seed covers, so the little sprouts inside the seed can push through and grow.

Water helps soften the soil prior to planting. It also softens protective seed covers, so the little sprouts inside the seed can push through and grow.

Growing Food with Hydroponics

Hydroponics is a way of growing plants in water, without soil, to which nutrients have been added. The Hanging Gardens of Babylon are an example of a well known hydroponic garden in the old days. Hydroponic plants grow faster and yield more than their "ensoiled" brethren. Nutrients are delivered several times a day, so the plant doesn't have to search for them, and the energy it saves can be used for plant growth. Growers need to be careful to use healthy liquids to fertilize with, because they are replacing the nutrients plants normally get from soil.

Hydroponically grown plants are usually pretty vigorous, so they have fewer problems with bugs, fungus, and disease. This results in less need for herbicides and fungicides, which can hurt the plant as well. Watch the video below to learn how cities can use hydroponics to feed their residents.

Comments

Simone Haruko Smith from San Francisco on June 28, 2012:

I have never read such a thorough look at all the roles water plays in a plant's life! I usually only hear about the internal chemical process. What a great overview this is!

Susette Horspool (author) from Pasadena CA on June 23, 2012:

Thanks everyone. The more I write about water, the more I see how integral it is to life on earth - we couldn't have life without it. Hydroponics is almost a physical depiction of it's power, since plants are grown with just water alone. You've heard that the ocean is a primordial soup that we all came from? Water filled with nutrients. That's what hydroponics is. So cool.

Leah Lefler from Western New York on June 22, 2012:

Hydroponics is so cool! I once went on a ride at Epcot (years and years ago) and they had a giant display of growing plants using only water. It was really fascinating! Also, I have been wanting to use the words xylem and phloem in conversation for SO long, and now have a chance to do so! Great hub, watergeek!

Joy M from Sumner, Washington on June 22, 2012:

Interesting hub - particularly the bit about hydroponics. I live in the suburbs so I have space to garden, but the maximizing of space and the health of the plants looked good. I'll have to read more about it. Thanks for the info.

whonunuwho from United States on June 21, 2012:

An interesting hub and well done. Few people realize the need for clean water and abundant green plants to renew and continue the basic life processes involved in photosynthesis and the mutual relationship of water and plants, as well as the oxygen that is produced on the planet by green things. Deforestation and poisoning waters with dangerous chemicals as well as heavy air pollution, create an unhealthy environment for all living things. Thanks for your insightful hub.

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