Astronaut in space

Energy from the sun

Green plants as primary food producers

What do the photovoltaic array and the tomato plant, shown in the images below, have in common?

Photovoltaic arrayTomato plant

ANSWER: They both convert sunlight, (radiant energy), into forms useful to humans. The photovoltaic array absorbs solar energy and stores it as chemical potential energy in batteries. The leaves of the tomato plant absorb solar energy which is (mainly) stored as chemical potential energy in compounds called carbohydrates.


Photovoltaic energy pathPhotovoltaic cells are well known for their ability to convert solar energy (1) into electrical energy.

Because sunlight is usually not available for 24 hours every day, electrical energy (2) needs to be stored to accommodate those times when there is no sunshine. Groups of voltaic cells, called batteries, are used to store the electrical energy in a chemical form (in molecular bonds).

When a light bulb is attached to the cells, the cells convert chemical energy back into electrical energy (3), and the electrical energy creates heat in the light bulb filament.

The hot light bulb filament radiates light energy (4).

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It is important to note that at each step in the energy transformation (along the energy path), only a little of the energy is actually transferred; most of the energy is lost at each step as heat.

Plant energy pathPlants also convert sunlight into other forms of energy. In this case plants convert light energy (1) into chemical energy, (in molecular bonds), through a process known as photosynthesis. Most of this energy is stored in compounds called carbohydrates.

The plants convert a tiny amount of the light they receive into food energy.

When animals eat green plants (2) they consume and absorb some of this energy, which stored as chemical energy in compounds known as fats and protein.

Milk from cows is a good source of solar energy (3), (also an excellent source of essential vitamins and minerals). When we drink milk our bodies convert the energy stored in the milk into energy for running, playing and important metabolic activities (4). We have become "solar powered"!

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Student activities

The energy trail

    Energy trail
  1. Each student writes the name of one food product on a small piece of paper.
  2. Fold each paper, place them in a container.
  3. Each student draws one paper out of the box.
  4. Arrange the students in groups of three.
  5. Give each group a copy of the template page.
  6. Each group will discuss and decide on the path that the energy from the Sun has taken to finally reside in the food product selected from the box. Not all steps in the template need to be filled but no transformation steps should be skipped.
  7. Award one point for each energy transformation step correctly shown. Deduct one point for each incorrect step. The team with the most total points wins.

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Students will sometimes choose a duplicate food to write down. For example, milk is popular and it may be selected twice by a group. Have on hand a box of extra food sheets from which they can draw another choice.

Many foods have multiple ingredients. Students should choose the main ingredient.

Some foods have very short energy paths, such as vegetables, whereas others, such as meat are somewhat longer. Students will discover this fact since it is "worth more points". This is a good introduction to the energy-pyramid in the food chain.

Sunlight to Food (How do plants do this?)

All green plants contain chlorophyll which absorbs sunlight. The chlorophyll is in small bodies called chloroplasts. The solar energy absorbed by the chlorophyll is used by the plant to activate a process called photosynthesis.

Photosynthesis is a process whereby the plant uses water and carbon dioxide to create compounds called carbohydrates.

Photosynthesis

Fruit bearing plants such as the tomato store carbohydrates in the fleshy part of the fruit as well as in all other parts of the plant.

Lights out

  1. Divide the class into small groups.
  2. Provide each group with a potted green plant, a sheet of aluminum foil, scissors and several paperclips.
  3. Students cut pieces of aluminum foil large enough to cover several leaves on their plant.
  4. Students attach the pieces of foil to leaves on their plant using the paperclips.
  5. Sudents label their plants and place them in classroom windows where they will receive plenty of light.
  6. After a week, students remove the pieces of aluminum foil and compare the appearance of the two groups of leaves.
  7. Propose an explanation for the changes they have observed.
  8. Discuss the significance of the presence or absence of green pigmentation in the plant leaves.