Plants Lesson 2, Activity 2: Modeling Photosynthesis

Answering the Three Questions for plants in the light.

Where are atoms moving from?

Where are atoms moving to?

The Movement Question

Which atoms and molecules are moving?

Water Minerals

Carbon dioxide

Oxygen

Leaf: Movement of atoms during photosynthesis

Question: Where did the sugar come from?

Leaf Cell: Microscopic ScaleMovement of atoms during photosynthesis

Question: How can sugar come out of leaf cells when carbon dioxide and water—but no organic materials—are going in?

What is Photosynthesis?

• Photosynthesis is the secret of plant growth.

• Plants do photosynthesis so they can grow (gain biomass).

The Carbon Question: What is happening to carbon atoms?

What molecules are carbon atoms in before photosynthesis? How are the atoms rearranged into new molecules during photosynthesis?

The Energy Question: What is happening to energy?

What forms of energy are involved in photosynthesis?How is energy changing from one form to another during photosynthesis?

Photosynthesis happens when light energy from the sun, carbon

dioxide, and water are used to make sugar and oxygen.

(the sugar is then used to build the plant’s mass; the O2 is released as

waste)

Using your poster and modeling kits, make a model of how matter and energy are transformed during photosynthesis.

Rules of Molecular Bonding Reminder:

• Atoms in stable molecules always have a certain number of bonds to other atoms:– Carbon: 4 bonds– Oxygen: 2 bonds– Hydrogen: 1 bond

• This means that if you have a carbon atom, for example, all 4 of its “prongs” should be attached to other atoms. There should not be empty prongs.

• Oxygen atoms do NOT bond to other oxygen atoms if they can bond to carbon or hydrogen instead.

Making the Reactant Molecules: Carbon Dioxide and Water

Photosynthesis occurs when plants combine carbon dioxide (CO2) and water (H2O) to produce glucose (C6H12O6) and oxygen (O2). Show how this can happen:1. Get the atoms you will need to make your molecules. Can you figure out

from the formula for sugar how many C, H, and O atoms you will need?a. You will need 6 carbon atoms to make one sugar molecule. How many CO2

molecules will you need to make for the reactants?b. You will need 12 hydrogen atoms to make one sugar molecule. How many H2O

molecules will you need to make for the reactants?

2. Use the bonds to make models of carbon dioxide (CO2) and water (H2O). Remember that CO2 has double bonds.

3. There are no high-energy bonds (C-C and C-H) in carbon dioxide or water. Add 12 twist ties to the reactants to represent light energy.

4. Compare your molecules to the pictures on the next slide. Are they the same?

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Photo of reactant molecules: CO2 (carbon dioxide) and H2O (water)Start by making the molecules of the reactants and energy units of light. Put them on the

reactants side, then rearrange the atoms and energy units to show the products.

Remember: Atoms last forever (so you can rearrange atoms into new molecules, but can’t add or subtract atoms). Energy lasts forever (so you can change forms of energy, but energy units can’t appear or go away).

Chemical change

Reactants Products

Water

Heat EnergyWork or Motion

Energy

Carbon dioxide

When you’re done, what will it look like? Arrange your atoms so that the water and carbon dioxide are transformed to glucose and oxygen.

Making the Product Molecules: Glucose and Oxygen

Photosynthesis occurs when carbon dioxide (CO2) and water (H2O) react to produce glucose (C6H12O6) and oxygen (O2). Show how this can happen:1. Break the bonds in the reactant molecules and recombine

the atoms into sugar (C6H12O6) and oxygen (O2). How many oxygen molecules can you make?

2. Identify the high-energy bonds (C-C and C-H) by putting twist ties on them. How many high energy bonds does a molecule of sugar have? Where does the energy for those bonds come from?

3. Compare your molecules to the pictures on the next slide. Are they the same?

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Photo of product molecules: H6C12O6 (sugar) and O2 (oxygen)Start by making the molecules and energy units of the reactants and putting them on the

reactants side, then rearrange the atoms and energy units to show the products..

Remember: Atoms last forever (so you can rearrange atoms into new molecules, but can’t add or subtract atoms). Energy lasts forever (so you can change forms of energy, but energy units can’t appear or go away).

Reactants Products

Glucose withchemical energy

Oxygen

Chemical change

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Comparing photos of reactant and product moleculesCompare the atoms and energy units on the reactant and products sides.

.

Remember: Atoms last forever (so you can rearrange atoms into new molecules, but can’t add or subtract atoms). Energy lasts forever (so you can change forms of energy, but energy units can’t appear or go away).

Reactants Products

Glucose withchemical energy

Oxygen

Chemical change

Water

Heat EnergyWork or Motion

Energy

Carbon dioxide

Writing a Chemical Equation• Writing in symbols: Chemists use an arrow to show how

reactants change into products:[reactant molecule formulas] [product molecule formulas]

• Saying in words: Chemists read the arrow as “yield” or “yields”:[reactant molecule names] yield(s) [product molecule names]

• Equations must be balanced: Atoms last forever, so reactant and product molecules must have the same number of each kind of atom.

• Try it: can you write a balanced chemical equation to show the chemical change for photosynthesis?

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Chemical Equation for Photosynthesis

6H2O + 6CO2 C6H12O6 + 6O2

(in words: water and carbon dioxide yield glucose and

oxygen)

Chemical changeReactantsGlucose

Oxygen

ProductsWater

Light energy

Carbon Dioxide

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What happens to atoms and energy in photosynthesis?

Chemical changeReactantsGlucose

Oxygen

ProductsWater

Light energy

Carbon Dioxide

What happens to carbon atoms

in photosynthesis?

Carbon atoms in carbon dioxide become part of

glucose molecules.

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Chemical changeReactantsGlucose

Oxygen

ProductsWater

Light energy

Carbon Dioxide

What happens to oxygen atoms

in photosynthesis?

Oxygen atoms become part of

glucose and oxygen molecules.

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Chemical changeReactantsGlucose

Oxygen

ProductsWater

Light energy

Carbon Dioxide

What happens to hydrogen atomsin photosynthesis?

Hydrogen atoms become part of

glucose molecules.

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Chemical changeReactantsGlucose

Oxygen

ProductsWater

Light energy

Carbon Dioxide

What happens to light energy

in photosynthesis?

Light energy is transformed into chemical energy.

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Chemical changeReactantsGlucose

Oxygen

ProductsWater

Light energy

Carbon Dioxide

What happens to atoms and energy in photosynthesis?

Atoms last forever! Energy lasts

forever!

How can we answer the Carbon Question and the Energy Questions now?

The Carbon Question: What is happening to carbon atoms?

What molecules are carbon atoms in before photosynthesis? How are the atoms rearranged into new molecules during photosynthesis?

The Energy Question: What is happening to chemical energy?

What forms of energy are involved in photosynthesis?How is energy changing from one form to another during photosynthesis?

The Carbon and Energy Questions• Carbon: carbon atoms

move from CO2 molecules in the air (inorganic) to C6H12O6 molecules in the plant (organic).

• Energy: energy is transformed from light energy into chemical energy in the high-energy (C-C and C-H) bonds of the glucose molecule.

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The Three Questions for plants in the light

Remember: Atoms last forever and Energy lasts forever

What forms of energy are in the reactants?

What molecules are carbon atoms in before the change?

What other molecules are involved?

Where are atoms moving from?

What forms of energy are in the products?

What molecules are carbon atoms in after the change?

What other molecules are produced?

Where are atoms moving to?

Chemical change