Plant carbohydrates, in the form of sugars are the energy source by which all plants carry out their major functions. All plants must photosynthesize, transpire and respire to survive. Sugar plays a vital role in all of these.
How They are Made
Simple sugars are made by plants through the process of photosynthesis. Plants take in light from the sun (or through artificial means) through openings in their leaves known as stomata and join together with water from the soil, carbon dioxide from the air and chlorophyll from within the plant, to create sugars in their leaves at sites known as chloroplasts. The same process occurs in aquatic plants, but they obtain their carbon dioxide from the water instead of the air. The by-product of this process is the oxygen that we breathe. Leaves in plants are akin to factories which create the vital components for plant survival. These factories create glucose. From there the sugar is mixed with water that the plant has absorbed through its roots and is transported throughout the plant via its vascular system.
Phloem, is like a botanical superhighway. The plant’s phloem transports the dissolved sugars from the leaves and takes them to various storage sites throughout the plants, like roots or tubers, known as “sinks”. The phloem off-loads its sugary cargo to these sinks across cell membranes through a process known as active transport. As much as 80 percent of the sugars created through the photosynthesis process are delivered to the plant’s sinks.
What They Do for the Plant
Sugars within the plant are responsible for affecting plant growth. The sugars produced in the leaves send the signal to trigger the transition between the juvenile phases of the plant to the adult one. Sugars are needed at all stages from seed, to cotyledon stage, to leaf development, stem development, fruit development and all stages in between. They (the sugars) further play a role in establishing the ratio of a plant’s below ground growth and above ground growth (roots to shoots). In addition to the regulation of growth, the sugars are also responsible for developing some of the plant’s structures. Plant cellulose, the fibrous materials in plants’ cell walls is made up of sugars as are the tubers of some plants, like potatoes.
Sugars are also used by the plant to regulate its time cycles. Plants, like all living things are subject to circadian rhythms which trigger when to “wake up” and when it’s time to “go to bed.” When needed, the simpler sugars (glucose) are converted to more complex sugars in the form of starches, made up of hundreds or thousands of sugar molecules, which a plant uses during the night when it is unable to undergo photosynthesis or later on when needed to form tissue or cell walls.
Plant sugars are also thought to regulate the time of year that a particular species flowers. Levels of sugar within the plant increase in response to the energy consumption required to flower, but some researchers believe that they may also be the cause of the species’ blooms and bloom times.
The process that allows for all of these actions is respiration. During respiration, the plant takes those sugars from the sinks and “burns” them to create the energy needed for growth and metabolism. This process happens independent of light, unlike photosynthesis.
In the third vital process of plant functions, transpiration, sugars are redistributed through the plant. To effect this, water that is absorbed through the plant’s roots, is mixed with the sugars, and then delivered throughout the plant before the excess water leaves the surface of the plant through evaporation. As much as 90 percent of all of the water a plant absorbs is dedicated to this process.
Plant Trickery Using Sugars
Plants use their sugars as a lure to animals to ensure their own survivability. This is done in a few different ways. Carnivorous plants often use not only their coloration, but also their sugars in the form of nectar to lure unsuspecting insects to their deaths. They then of course digest these insects to supplement their own nutritional needs. Plants also cleverly use their sugars to attract animals, including humans to aid in seed dispersal to survive. Fruit-bearing plants convert glucose into fructose, which is the natural sugar that gives most fruits their sweetness. This sweetness attracts a wide variety of animals to eat of their fruit, and through the animals’ natural digestive processes, they disperse the seeds in their manure which provides a protected site to germinate and provides some of the nutrition needed during development. Even if the animals don’t consume the fruit’s seeds, they are just as likely to scatter the seeds by dropping the fruit remains on the ground away from the site where the fruit was grown (think tossing an apple core or peach pit).
Sucrose is a more complex natural sugar that plants make combining both glucose and fructose. Common table sugar is made from the sucrose of sugar cane. This plant does not live in fear of being endangered any time in the near future as it produces one of mankind’s most sought after substances.