SPEAK OUR LANGUAGE

• CELL – Core Experience Learning Lab
• SDR – Scientist Data Record

ASK WHY

Great scientists question the world around them. We encourage our LabLearner students to do the same. In anticipation of this, we explain the importance of learning the concepts in the Ask Why section within the CELL. Our hope is that these explanations help students understand why science matters.

BRANCH OUT

Each Investigation introduces students to a different branch of science or STEM (Science, Technology, Engineering, Mathematics) career that utilizes the scientific concepts of the CELL. These real-world connections will help students see the relevance of what they are learning. STEM connections are also integrated into each Performance Assessment.

GET FOCUSED

The Focus Questions in each Investigation are designed to help teachers and students focus on the important concepts. By the end of the CELL, students should be able to answer the following questions:

Investigation 1:

• Which pigments or colors are present in spinach leaves? The leaves contain yellow and yellow-green pigments.

Investigation 2:

• How do light and photosynthesis affect carbon dioxide levels? In the presence of light, carbon dioxide levels decreased in water containing Elodea but not in water alone.

• How do light and photosynthesis affect oxygen levels? In the presence of light, oxygen levels increased in water containing Elodea but not in water alone.

Investigation 3:

• What is the importance of light in photosynthesis? Light is necessary for photosynthesis to take place.

• In which parts of the plant does photosynthesis occur? Both the Elodea stem and the Elodea leaf contain chloroplasts; therefore, photosynthesis occurs in both stem and leaf.

Investigation 4:

• In which parts of a Coleus leaf does photosynthesis occur? The experiment illustrated that photosynthesis takes place in the chloroplasts, the parts of the leaves which contain chlorophyll, often indicated by their green color.

• Which pigment is required for photosynthesis? The green pigment chlorophyll is required for photosynthesis.

Note: These are succinct responses to the Focus Questions and are placed here for easy reference. Fully developed responses to the Focus Questions can be found on each PostLab page.

Note: Some questions may be revisited as the CELL progresses. As students acquire additional knowledge, their responses should reflect this.

LEARN THE LabLearner LINGO

The following list includes Key Terms that the teacher should introduce, as appropriate, within the CELL. These terms should be used, as appropriate, by teachers and students during everyday classroom discourse.

Note: Additional words may be bolded within the Backgrounds. These words are not Key Terms and are strictly emphasized for exposure at this time.

Investigation 1:

• Photosynthesis: process carried out by bacteria, algae, and higher-order plants in which energy derived from light is converted into the energy of chemical bonds. It consists of light reactions and dark reactions.
• Chloroplasts: cytoplasmic organelles present in plant cells
• Chlorophyll: the green pigment found in the chloroplasts of plants
• Carbon dioxide: a gas that is necessary for photosynthetic reactions
• Oxygen: a gas that is a product of photosynthetic reactions and is necessary for the life of animals and other living organisms

Investigation 2:

• pH: a property of matter which describes the acidity or alkalinity of a substance. A pH of 7 indicates a neutral substance, lower than 7 indicates that the substance has acidic properties, and greater than 7 indicates that the substance has basic properties.

Investigation 3:

• There are no Key Terms introduced in Investigation 3.

Investigation 4:

• There are no Key Terms introduced in Investigation 4.

BE PREPARED

An overview of the materials for each lab is placed here for easy reference. Specific teacher preparation for the labs is placed at the beginning of each Lab page.

EXTEND YOUR THINKING

The following information is included so that teachers have additional background knowledge pertaining to the concepts introduced in the CELL. Teachers may choose to use this information enrich students during instruction, but this is optional and not necessary for the intended students’ learning outcomes.

Photosynthesis is a process carried out by bacteria, algae, and higher-order plants in which energy derived from light is converted into the energy of chemical bonds. Within plants, the process of photosynthesis reduces carbon dioxide to three-carbon sugars and oxidizes water to oxygen. The general equation describing the reactants and products of photosynthesis is as follows:

Within plants, photosynthesis occurs inside cytoplasmic organelles called chloroplasts and consists of two distinct series of reactions, the light reactions, and the dark reactions, or the Calvin Cycle.

Each chloroplast in a cell is surrounded by a membrane, including an outer membrane and an inner membrane. Within the membrane are disc-shaped sacs called thylakoids. The membrane which surrounds each thylakoid, the thylakoid membrane, is the site of photosynthesis. The thylakoid membrane contains pigments that are essential to photosynthesis, including the chlorophylls and the carotenoids, among others. The most common types of chlorophylls are chlorophyll a and chlorophyll b, chlorophyll a being the pigment primarily responsible for photosynthesis.

Photosynthesis begins as pigments in the thylakoid membrane absorb photons of light. These pigments are clustered together in structures called photosystems. The thylakoid membrane of most plants contains two photosystems, referred to as photosystem I and photosystem II. Although similar, these two photosystems have specific functions within the photosynthetic process.

As photosynthesis is initiated pigments such as chlorophyll a, chlorophyll b, and carotenoids in photosystems absorb photons of light. Due to differences in molecular structure, each pigment absorbs specific wavelengths of light. As a result, photosystems within plant cells are able to absorb photons of different wavelengths and harness the energy within different wavelengths of light. The pigments located in the outer regions of the cluster are called antennae molecules. Within the center of photosystems are two special chlorophyll a molecules, called reaction center chlorophylls. As photons are absorbed by antenna molecules, the increase in energy associated with the photons is transferred from one antenna molecule to the next until it reaches the two chlorophyll a molecules in the reaction center.

As the energy reaches the reaction center molecules, it excites the electrons within the chlorophyll a molecules such that the electrons are transferred from the chlorophyll a molecules to other molecules within the thylakoid membrane. This movement of electrons is called electron transfer.

The light reactions of photosynthesis are responsible for a series of electron transfers from one molecule in the thylakoid membrane to another. The entire process is referred to as an electron transport chain. Within an electron transport chain, electrons are always transported from states of higher potential energy to lower potential energy. Energy is released as electron transport occurs and can be used to drive other processes within the thylakoid.

In the thylakoid, it is easiest to understand the electron transport chain by focusing on events of one photosystem at a time. The convention is to describe the events at photosystem II before describing photosystem I. As the light energy is absorbed in photosystem II, electrons from the reaction center chlorophyll a molecules within the photosystem are excited and are transferred to a molecule called plastoquinone, or PQ. Because the chlorophyll a molecules, in their excited state, give up electrons they must regain electrons. An enzyme present in the cell replaces these lost electrons by splitting water molecules. Through this splitting of water, two electrons are transferred from water to chlorophyll a, producing oxygen. This reaction accounts for the release of oxygen in photosynthesis.

From PQ, electrons are transferred to another molecule called the cytochrome b/f complex and then to a molecule called plastocyanin (PC). As this occurs and energy is released, it is used to transport hydrogen ions from the stroma, the solution outside the thylakoid membrane, into the thylakoid space enclosed by the membrane. This transportation of protons (hydrogen ions) creates a concentration gradient, with more protons held within the thylakoid space than outside the thylakoid membrane. This concentration gradient then provides the energy which fuels the production of ATP. ATP synthase enzyme complexes in the thylakoid membrane provide pathways through which protons can flow from the highly concentrated thylakoid space into the less concentrated stroma. As hydrogen ions flow down the concentration gradient, the gradient provides the energy required to produce ATP. This process of creating ATP as protons move from the thylakoid space into the stroma is called photophosphorylation.

The movement of energy and electrons continues as the electrons from PC are transferred to photosystem I. Recall that as light interacts with photosystem I, photons are absorbed and the energy is transferred from antenna molecules to two chlorophyll a molecules. As this energy is transferred to chlorophyll a, electrons are released from the chlorophyll a molecules. The electrons lost from the chlorophyll a molecules, however, must be replaced. The replacement comes as electrons are transferred from plastocyanin to photosystem I, rather than by the splitting of water in photosystem II.

What happens to the electrons released from Photosystem I? These electrons are part of a final electron transport chain that ends at the enzyme NADP reductase. This enzyme transfers the electrons that have passed through the transport chain to NADP, which leads to the storage of the electrons in the form of NADPH.

To summarize what we have learned thus far, during the light reactions, photons are absorbed by photosystem II. As this occurs, the pigments within that photosystem become excited and transfer energy to chlorophyll a, which then passes electrons along an electron transport chain. The release of this energy as electrons are passed down the chain provides the necessary energy to pull protons from the stroma into the thylakoid space. These protons then pass down the concentration gradient, through the ATP synthase complex, forming ATP. Meanwhile, the electrons continue through the electron transport chain until they reach photosystem I. In photosystem I, light energy is also absorbed, and electrons of chlorophyll a molecules become excited and pass through a second electron transport chain, ending with the enzyme NADP reductase. This enzyme leads to the storage of electrons in NADPH. Thus, in the light reactions of photosynthesis, chloroplasts are responsible for forming ATP and NADPH.

From the light reactions, we have accounted for part of the general equation for photosynthesis, the oxidation of water to oxygen, and the transfer of electrons. However, sugar has not yet been produced. The synthesis of sugar occurs during the Calvin Cycle, otherwise known as the dark reactions. The Calvin Cycle consists of three stages, carbon fixation, reduction, and regeneration of the CO2 acceptor. In the first of the three stages, carbon dioxide enters the cycle and an enzyme called ribulose bisphosphate carboxylase (rubisco) combines it with or fixes it to a five-carbon sugar known as ribulose bisphosphate, or RuBP. The resulting six-carbon compound is very unstable and immediately splits into two molecules of 3-phosphoglycerate (PGA).

In the second stage of the cycle, PGA is converted into the sugar glyceraldehyde 3-phosphate, or G3P through the completion of a two-step process. First, each PGA receives a phosphate group from a molecule of ATP to form the molecule 1, 3-bisphosphoglycerate. The second part of the process involves using NADPH to convert 1, 3-bisphosphoglycerate into G3P. Through further enzymatic reactions, G3P is converted to glucose. It is through this set of reactions that the electrons from the light reactions which were stored as NADPH are transferred to G3P. Thus a carbon dioxide derivative is reduced or receives electrons.

During the third and final stage of the Calvin Cycle, G3P is converted to RuBP, the original five-carbon sugar used in the first stage of the cycle and the cycle is ready to begin again. ATP is required for the regeneration of G3P. Thus, nine ATP molecules and six NADPH molecules from the light reactions are used to produce one molecule of G3P through the Calvin Cycle. The G3P is in turn converted to glucose. The final result is that electrons are added to carbon dioxide forming glucose molecules.

When we look at the process of photosynthesis as a whole we see that the light reactions generate ATP and NADPH. During these reactions, water is split and oxidized, as electrons are transferred to chlorophyll a molecules and then down an electron transport chain to NADPH. The splitting of water and the release of electrons produces oxygen. The Calvin Cycle then uses ATP and electrons from NADPH to form sugar. The combination of both of these reactions describes and defines the general equation for photosynthesis:

Photosynthesis, therefore, consists of two sets of reactions, each equally important.