Plants use a process called photosynthesis to make food. During photosynthesis, plants trap light energy with their leaves. Plants use the energy of the sun to change water and carbon dioxide into a sugar called glucose.
Glucose is used by plants for energy and to make other substances like cellulose and starch. Plants produce sugar in their leaves, but these leaves do not taste sweet. That is because the sugar does not remain in the leaves. Plants use sugar to grow, and it may also be transported to the roots, seeds, stalks, or fruits for storage. We learnt already that sugar may exist as simple sugars, like glucose or fructose, or as a double sugar, like sucrose.
But there are also sugars in which thousands of sugar molecules are linked together to form an enormous unit, like starch. All these different sugars are a type of nutrients called carbohydrates Table 1.
Many plants store their sugar in the form of starch, like potatoes. Others store sugar in the form of fructose or glucose in their fruits, like apples and oranges. Some plants store high concentrations of sucrose.
High-sucrose plants are cultivated for the table sugar we use to make our delicious sweets. Worldwide, sugar beets and sugar cane are the main crops that produce sugar.
For photosynthesis, the green parts of plants are important. Leaves are green because they contain structures called chloroplasts , which have a green pigment, called chlorophyll Figure 1.
Photosynthesis has two main steps. In the first step, sunlight shines on the leaves and the light energy is collected by the chlorophyll in the chloroplasts.
The collected light energy gets converted into chemical energy and stored in energy-storing molecules. Oxygen is also produced during this process, and is released by the plants [ 1 ]. In the second step of photosynthesis, carbon dioxide from the air enters the leaves through very small openings. Using the previously stored chemical energy, the chloroplasts convert carbon dioxide into glucose [ 1 ]. Fructose is also produced during this step.
Glucose is then combined with fructose to create sucrose. People have always found substances to sweeten food. But, in cool climates, sugar was luxury product for many years. Light travels as electromagnetic waves. The wavelength — distance between waves — determines energy level.
Some of those wavelengths are visible to us as the colors we see. If a molecule, such as chlorophyll, has the right shape, it can absorb the energy from some wavelengths of light. Chlorophyll can absorb light we see as blue and red. Green is the wavelength plants reflect, not the color they absorb. While light travels as a wave, it also can be a particle called a photon. Photons have no mass. They do, however, have a small amount of light energy.
When a photon of light from the sun bounces into a leaf, its energy excites a chlorophyll molecule. That photon starts a process that splits a molecule of water. The oxygen atom that splits off from the water instantly bonds with another, creating a molecule of oxygen, or O 2.
Both of these allow a cell to store energy. Notice that the light reaction makes no sugar. This is where sugar is made. But the light reaction does produce something we use: oxygen. Appears in. Food macromolecules The purpose of this animation is to show visually how glucose molecules can be assembled to form simple sugars and large macromolecular carbohydrates such as starch and cellulose.
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