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2008 Comprehensive Curriculum 4th Grade Science Links

Activity 2:  Food Factory! Photosynthesis (GLE:  1, 2, 3, 6, 7, 9, 10, 13, 16, 18, 40)

Materials List:  small potted plants with soft leaves, masking tape or aluminum foil, petroleum jelly, hand lens or microscope, resealable plastic bags, disposable gloves, safety goggles, science learning logs

Safety note:  Have students identify safety procedures or equipment to be used when handling plants.  (Wash hands, use disposable gloves, keep hands away from mouth, and identify allergies.)

Begin the activity by having students brainstorm (view literacy strategy descriptions) a list of things they need to survive.  Students should share their list and through discussion of what we cannot live without, narrow the list down to air, water, food, and shelter.  Have students brainstorm a list of things plants need to survive.  Have students share their list and discuss the basic needs of plants.   Help students remember the needs of plants with this acronym (LAWNS – Light, Air, Water, Nutrients, Space).  Students should compare and contrast the needs of plants and animals through class discussion. 

Ask students where their food comes from (grocery store, etc.).  Tell students that plants are special because they can make their own food through a process called photosynthesis.  Explain that photosynthesis is a process where plants combine carbon dioxide and water and in the presence of sunlight, they are able to produce water, oxygen, and sugar.  Using these terms, write the photosynthetic word formula on the board.  Explain that photosynthesis takes place primarily in the leaves of the plant and that the green pigment chlorophyll helps photosynthesis to take place. 

To demonstrate that photosynthesis occurs in the leaves, have students complete the following activity.  (Note: if supplies are limited, this may be done as a teacher demonstration.)

Place students in groups and give each group a plant with soft leaves, such as a geranium, to observe. Students should record all observations in their science learning logs (view literacy strategy descriptions)Direct students to place a piece of masking tape or aluminum foil over a small section of one leaf and place the plant in a sunny area of the classroom or under artificial light.  In three days, students should remove the masking tape and record their observations.  Discuss any changes that occurred in the leaf. (The color should have faded or turned yellow under the tape.)  Hold a class discussion to determine why the leaf changed colors and relate the changing of the colors to the plant’s need for sunlight during photosynthesis. 

Begin a discussion about ways to show that a plant is conducting photosynthesis. Prompt the students by having them think of what they would expect to see if a person said that he had cooked a meal (Pots on the stove, someone stirring, food served on plates). Next, ask students to consider what they would expect to find produced if a plant was conducting photosynthesis (Water, oxygen, sugar, starch). Ask students to recall the times that they were under a tree and felt drops of water fall on them, but it wasn’t raining. Explain that this water came from the tree as a by-product of photosynthesis which is occurring inside the leaf. Ask students to think of questions regarding the water falling off the tree and what is happening inside of the leaf. (Students may wonder how the water got on the outside of the leaf when it is being created inside the leaf.)

Ask the class, “How can we prove that water is released from the plant’s leaves during photosynthesis?”  One possible experiment is to have student groups place 3 plant leaves in a plastic resealable bag and place in a sunny window or under artificial light. As a control, place an empty resealable bag in the window.  Ask students why this was done. The next day, have the students make observations of the bags. Students should see condensation (water) on the bags with leaves. Ask the students where the condensation came from (the leaves)

Allow students to hypothesize how the leaf sets the water free. (They may think that there must be an opening in the leaf.)   Explain that if there are holes in the leaf allowing the water to escape, they may be able to plug them up to stop the water from escaping. To determine whether or not there are openings in the leaf, have students smear both sides of a leaf on a potted plant with petroleum jelly and place a clear plastic bag around the leaf, sealing it at the petiole. Explain to students that the petiole is the stalk of the leaf that attaches the blade to the stem of the plant.  Discuss the role of the petroleum jelly (to plug the holes).  Guide students to establish a control (a plant that has uncoated leaves sealed in a plastic bag).   Again, allow the bags to sit overnight and have the students make observations of the bags the next day.  The students will observe a collection of water in the bag of the uncoated leaves. Relate the formation of the water on the inside of the bags to the release of water (transpiration) from the leaves during photosynthesis. 

Introduce the term stomata as the opening in the leaves that allows the water to escape. Later students may wish to alternate the side of the leaf that is smeared with the petroleum jelly to explore further which side has the stomata. Allow students to observe the stomata of leaves using a hand lens or microscope, if available. Students should infer that the excess water created as a by-product of photosynthesis could escape from the plant through the stomata. Note: Aquatic plants have stomata on the top of the leaves, while most land plants have stomata on the bottom of the leaf.

Refer the students to the photosynthetic word formula and ask them to consider that if the stomata open to allow extra water out of the plant, what else could be escaping from the plant through the stomata (Oxygen)? Could the stomata open to let something else pass into the plant? Looking back at the photosynthetic word formula, the student will search for the matter for which they have not accounted (Carbon dioxide). The students should infer that carbon dioxide could enter the plant through the stomata. This provides a junction for a discussion about the interdependence of plants and animals for gas exchanges.

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