Botany Lesson Nine
Light
In this unit, we will:
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Learn about photosynthesis
Discover why leaves turn brown in the autumn
Investigate some of the differences between deciduous trees and gymnosperms
Explore plant hormones
Harnessing the sun
What if I told you that plants use electrical current to make sugar? In the last unit, I mentioned
photosynthesis as a process that produces the sugars needed by mitochondria to produce energy for the
plant. In the diagram, I indicated (and you probably learned this as a child) that plants need carbon
dioxide, water, and sunlight to carry on photosynthesis. What does the plant actually do with these
substances? Why is light so important? Get ready for some rather technical, but very cool, information.
Below is a diagram of a chloroplast, the site of photosynthesis within cells of the leaves.
The most important structure within the chloroplast is the thylakoid, which are stacked on top of each
other like plates. Each stack of thylakoids is called a granum. The thylakoids contain the light trapping
pigment, chlorophyll, along with other light trapping pigments, enzymes, and additional substances
needed for photosynthesis.
There are two phases in photosynthesis. The first phase results in ATP (adenosine triphosphate, the
energy currency of cells) and NADPH (nicotinamide adenine dinucleotide phosphate, which provides the
hydrogen atoms for glucose production). The second phase results in glucose for energy production.
In the first phase, the chlorophyll and other pigment molecules are excited by red and blue wavelengths
of light (accessory pigments capture yellow and orange light). This excitement causes the electrons in
these pigments to boost up and out of their orbits for a fraction of a second before they fall back into
place again. This releases a vibrating energy that passes along from one chlorophyll molecule to the next
very rapidly until the energy reaches a special molecule that causes electrons themselves to be captured
and passed along. At the end of this chain can be either adenosine diphosphate (ADP) or NADP. ADP
uses electrons that have been passed along to add another phosphate molecule, becoming ATP, while
NADP uses electrons that have been passed along to combine with hydrogen (H+) that is needed for the
second phase of photosynthesis. The H+ atoms come from the splitting of water by the moving electrons
and enzyme activity. In other words, plants use a tiny electric current that began with electromagnetic
wavelengths generated by the sun to eventually produce sugar.
The second phase of photosynthesis occurs in the stroma, the aqueous fluid around the thylakoids that
contain chlorophyll. This phase does not require light, but occurs in the presence of both light and dark.
It is also temperature dependent. Carbon dioxide is needed and is dispersed in the stroma where it is
fixated by ribulose biphosphate (RuBP). Through several steps all controlled by enzymes, the carbon,
hydrogen, and oxygen are recombined into glucose molecules.
To help you understand this process, please draw a diagram or flow chart on your worksheet of what
you just read about photosynthesis.
Changing Seasons
As mentioned in the last unit, a plant uses light for many purposes besides photosynthesis. A plant is
able to tell how long the day is, and is consequently able to tell when the days are growing shorter. Since
phase two of photosynthesis is also somewhat
temperature dependent, these two factors combine to tell
the plant (or tree) when it is time to stop making glucose.
The chlorophyll molecules atrophy and the leaves no longer
appear green. Instead, the other leaf pigments that were
hidden now show through and leaves appear yellow, red,
brown, and orange. These other pigments are carotenes
(orange), xanthophylls (yellow), and anthocyanins (red,
purple, and blue). The varying blends of these pigments, along with the remnants of the atrophied
chlorophyll, give deciduous trees their brilliant fall colors.
The cooler temperatures, longer nights, and reduced availability of water signal the tree that it is time to
cut off energy to the leaves. A separation layer of cells forms where the leaf joins the branch and this
layer releases enzymes that cause the cells in that region to break down. Eventually, the leaf falls off.
Cork cells rapidly divide to protect the branch.
But, not all leaves fall off in the autumn. What about gymnosperms, or evergreens? These leaves are not
affected by seasonal changes. They have several characteristics that allow the process of photosynthesis
to continue all year long. First, the leaves are covered with a thick, waxy cuticle instead of a thin one.
This, coupled with the greatly reduced surface area of the leaves, allows them to retain more water and
be more resistant to temperature changes. Also, these trees have thick, resinous, sap instead of the
watery sap of deciduous trees. This, too, allows them to survive cold temperatures and retain moisture.
They do not need to shed their leaves because they are not affected by low temperatures and reduced
availability of liquid water.
Please fill out the appropriate spaces on your Gymnosperms versus Angiosperms worksheet.
Plant hormones
Just as hormones regulate growth and development in your body, plants have hormones that do the
same things in them. These hormones are often controlled by light and temperature, and that is why I
have listed them here. There are four main types of hormones in plants.
 Auxins: produced in the shoot buds, they stimulate root growth, direction of shoot growth, and
when fruits will fall.
 Gibberellins: produced in the root tips, they stimulate canopy growth and flowering, increasing
the size of leaves and fruits, and controlling dormancy in seeds and buds
 Cytokinins: these are at work mostly at the meristems, promoting cell division and
differentiation, and influence aging of leaves
 Abscisic acid: this hormone inhibits other hormones when the plant is under stress
Horticulturists use this information to prune and shape plants the way that they want them to look and
gardeners use this information to control where the plant directs its energy so that there is a higher
produce yield.
You may have heard of phytoestrogens or isoflavones, plant compounds that have a hormonal effect on
humans and animals and seem to protect against breast cancer in women. These compounds are not
plant hormones, but rather chemicals formed by the plant to protect it from pathogenic fungi and to
stimulate nitrogen fixing bacteria. We will be talking more about these chemicals when we talk about
plant defenses.